ed_manual

DSP Based Electric Drives Laboratory

USER MANUAL

Department of Electrical and Computer Engineering University of Minnesota

Revised: July 3, 2007

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CONTENT EXPERIMENT – 1 INTRODUCTION TO THE DSP-BASED ELECTRIC-DRIVES SYSTEM ..................................................................................................................................1

1.1 INTRODUCTION ................................................................................................................1 1.2 DSP-BASED ELECTRIC-DRIVES SYSTEM...........................................................................1 1.3 DEMONSTRATION OF SPEED CONTROL OF A DC MOTOR.................................................3 1.4 LAB REPORT AND READING ASSIGNMENT.....................................................................11

EXPERIMENT – 2 SIMULATION AND REAL-TIME IMPLEMENTATION OF A SWITCH-MODE DC CONVERTER................................................................................12

2.1 INTRODUCTION ..............................................................................................................12 2.2 THEORETICAL BACKGROUND OF DC SWITCH-MODE CONVERTER................................12 2.3 SIMULATION OF DC SWITCH-MODE CONVERTER..........................................................14 2.4 REAL-TIME IMPLEMENTATION OF DC SWITCH-MODE CONVERTER..............................19 2.5 LAB REPORT ..................................................................................................................24

EXPERIMENT – 3 NO-LOAD DC MOTOR TEST .......................................................25 3.1 INTRODUCTION ..............................................................................................................25 3.2 CONTROL OF A DC MOTOR UNDER NO-LOAD CONDITION IN OPEN LOOP ...................25 3.3 MAKING THE CASE FOR OPEN-LOOP SPEED CONTROL AND CHARACTERIZATION OF DC-MOTOR..........................................................................................................................30 3.4 LAB REPORT ..................................................................................................................31

EXPERIMENT – 4 CHARACTERIZATION OF DC MOTOR ...................................32 4.1 INTRODUCTION ..............................................................................................................32 4.2 OPEN LOOP CONTROL OF DC-MOTOR WITH LOAD .........................................................32

EXPERIMENT – 5 DC MOTOR SPEED CONTROL ...................................................50 5.1 INTRODUCTION ..............................................................................................................50 5.2 SIMULINK MODEL OF THE DC-MOTOR ..........................................................................50 5.3 CONTROLLER DESIGN ....................................................................................................51 5.4 REAL-TIME IMPLEMENTATION OF FEEDBACK CONTROL ................................................55 5.5 LAB REPORT ..................................................................................................................58 5.6 REFERENCES ..................................................................................................................58

EXPERIMENT – 6 FOUR-QUADRANT OPERATION OF DC-MOTOR ................59

6.1 INTRODUCTION ..............................................................................................................59 6.2 REAL-TIME IMPLEMENTATION .......................................................................................59 6.3 CREATING THE CONTROL-DESK LAYOUT......................................................................63 6.4 RUNNING THE EXPERIMENT...........................................................................................63 6.5 LAB REPORT ..................................................................................................................64

EXPERIMENT – 7 CHARACTERIZATION OF INDUCTION MOTOR .................65 7.1 INTRODUCTION ..............................................................................................................65 7.2 REAL-TIME IMPLEMENTATION .......................................................................................65 7.3 CREATING THE CONTROL-DESK LAYOUT......................................................................68

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7.4 RUNNING THE EXPERIMENT ...........................................................................................69 7.5 LAB REPORT ..................................................................................................................72 7.6 REFERENCES ..................................................................................................................72

EXPERIMENT – 8 V/F SPEED-CONTROL OF A THREE-PHASE INDUCTION MOTOR .................................................................................................................................73

8.1 INTRODUCTION ..............................................................................................................73 8.2 REAL-TIME IMPLEMENTATION.......................................................................................74 8.3 CREATING THE LAYOUT .................................................................................................75 8.4 RUNNING THE EXPERIMENT ...........................................................................................76 8.5 LAB REPORT ..................................................................................................................78

EXPERIMENT – 9 PERMANENT MAGNET AC (PMAC) MOTOR ........................79 9.1 INTRODUCTION ..............................................................................................................79 9.2 REAL-TIME IMPLEMENTATION.......................................................................................79 9.3 RUNNING THE EXPERIMENT...........................................................................................86 9.4 LAB REPORT ..................................................................................................................88

APPENDIX – A SAFETY PRECAUTIONS AND POWER-ELECTRONICS-DRIVES-BOARD FAMILIARIZATION .........................................................................89

1.1 WHY IS SAFETY IMPORTANT? ........................................................................................89 1.2 POTENTIAL PROBLEMS PRESENTED BY POWER ELECTRONIC CIRCUITS.........................89 1.3 SAFETY PRECAUTIONS TO MINIMIZE THESE HAZARDS ...................................................90 1.4 POWER-ELECTRONICS-DRIVES-BOARD FAMILIARIZATION ..........................................93

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E x p e r i m e n t - 1 I n t r o d u c t i o n t o t h e D S P - b a s e d E l e c t r i c - D r i v e s S y s t e m

1.1 Introduction

There are four major components of the DSP-based electric-drives system, which will be

used to perform all the experiments in this course. They are as follows: 1) Motor coupling

system, 2) Power Electronics Drive Board, 3) DSP based DS1104 R&D controller card and

CP 1104 I/O board and 4) MATLAB Simulink and Control-desk. In this experiment, you

will be briefly introduced to the role of abovementioned four components in the DSP-based

electric-drives system. An example of speed-control of a DC-motor will be demonstrated.

The Simulink file and Control-desk layout will be supplied to perform this experiment. The

communication between the four components will be explained while controlling the speed

of the motor. Section 1.2 details the DSP-based electric-drives system vis-à-vis the role of

the four components listed above. In Section 1.3 a step-by-step procedure to run the DC

motor speed-control will be performed.

1.2 DSP-based electric-drives system

Fig. 1.1 shows the block diagram of the DSP-based electric-drives system.

• Motor coupling system: This system contains the motor that needs to be

characterized or controlled. The system has a mechanical coupling arrangement to

couple two electric machines. The motor under test or whose speed/torque needs to

be controlled, could be either a DC motor or Three-phase induction motor or Three-

phase permanent-magnet AC (PMAC) motor. The system also has an encoder

mounted on the machine which is used to measure the speed of the machine. This

can be used for close loop feedback speed-control of the motor. The motor demands

a controlled pulse-width-modulated (PWM) voltage to run at controlled speed or

torque. The PWM voltage is generated by Power Electronics Drive Board (briefed

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next); the voltage source thus generated is connected to the motor coupling system

as shown in Fig 1.1.

CURR B1CURR A1 CURR A2 CURR B2

GND

+42 V

ENCODER

To INC 1/INC 2(on CP 1104)

CP 1104 I/O board

42 V

DC

Pow

er

Supp

lyA 1 B 1 C 1 C 2B 2A 2

Digital I/O

Input +12 V

VOLT DC

Power Electronics Drive Board

Motor coupling system

ADC and DAC interface, Digitial I/O, Encoder interface and RS232

Computer

MATLAB-Simulink

DS1104 R&D Controller Card

Control-desk

Communication between Control-desk and Hardware in real-time

Motor Current, DC Voltage etc. for feedback control

Fig 1.1 DSP-based electric-drives laboratory system

• Power Electronics Drive Board: This board has the capability to generate two

independent PWM voltage sources (A1B1C1 and A2B2C2) from a constant DC

voltage source (see Fig). Hence two machines can be controlled independently for

independent control variables, at the same time. This board also provides the motor

phase currents, dc-bus voltage etc. to control the motor for a desired speed or

torque. To generate the controlled PWM voltage source, this board requires various

digital control signals. These control signals dictates the magnitude and phase of the

PWM voltage source. They are generated by the DS1104 R&D Controller board

inside the computer.

• DS1104 R&D controller Board and CP 1104 I/O board: In each discrete-time-step,

the DS1104 controller board takes some action to generate the digital control

signals. The type of action is governed by what we have programmed in this board

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with the help of MATLAB-Simulink real-time interface. This board monitors the

input (i.e. motor current, speed, voltage etc) with the help of CP1104 I/O board in

each discrete-time step. Based on the inputs and the variables that need to be

controlled (i.e. motor speed or torque); it takes the programmed action to generate

the controlled digital signals. The CP1104 I/O board is an input-output interface

board between the Power Electronics Drive Board and DS1104 controller board. It

takes the motor current, dc-voltage etc. from the Power Electronics Drive Board

and also, speed signal (from encoder) from motor coupling system, to the DS1104

controller board. In turn, the controlled digital signals supplied by DS1104

controller board are taken to the Power Electronics Drive Board by CP1104.

• MATLAB Simulink and Control-desk (Programming DS1104 and control in real-

time): Simulink is a software program with which one can do model-based design

such as designing a control system for a DC motor speed-control. The I/O ports of

CP 1104 are accessible from inside the Simulink library browser. Creating a

program in Simulink and procedure to use the I/O port of CP 1104 will be detailed

in future experiments. At this stage, let us assume that we have created a control-

system inside the Simulink that can control the speed of a DC motor. When you

build the Simulink control-system (CTRL+B) by using real-time option, it

implements the whole system inside the DSP of DS1104 board, i.e. the control-

system that was earlier in software (Simulink) gets converted into a real-time

system on hardware (DS1104). Simulink generates a *.sdf file when you build

(CTRL+B) the control-system. This file gives access to the variables of control-

system (like reference speed, gain, tuning the controller etc) to separate software

called Control-desk. In this software a control panel (see Fig 1.2) can be created

that can change the variables of control-system in real time to communicate with

DS1104 and hence change the reference quantities such as the speed or torque of

the motor.

1.3 Demonstration of Speed Control of a DC motor

The system for the speed-control of a DC motor is shown in Fig 1.2. Note that the currA2

(i.e. phase-current of DC motor) and encoder signal (speed of DC motor) is fed back to the

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DS1104 board via CP 1104. The requirement of feeding back phase-current and speed of

the motor will be studied in experiment-4. For now, assume that theses two quantities are

required to control the speed of DC motor. Perform the following steps to run the

experiment. The communication between the four components (explained in section 1.2) is

detailed in each step, wherever necessary.

Computer

MATLAB-Simulink

DS1104 R&D Controller Card

Control-desk

CURR A2

ADC 5

INC 1

GND

+42 V

ENCODER

To INC 1(on CP 1104)

FromENCODER

CP 110442

V D

C P

ower

Su

pply

B 2A 2

Digital I/O

+

_

+

_

Input +12 V

*.sdf Control-desk provides a bi-directional communication

between its control panel and DS1104

Power Electronics Drive Board

Fig 1.2 Demonstration of DC motor speed-control

• Connect the circuit as shown in Fig 1.2. You are given with files Exp1.mdl

(Simulink control-system file) and Exp1.lay (control panel in Control-desk).

Create a new folder on desktop as Exp1 and bring these two files into that folder.

Open MATLAB Simulink and set the folder Exp1 as the path of the current

working directory. Verify in the command window for the correct path (Fig

1.3).Open the Simulink file Exp1.mdl as shown in Fig 1.3.

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Fig 1.3 Opening the Simulink file Exp1.mdl, changing the path of current working directory

• The Simulink file Exp1.mdl will look as shown in Fig 1.4. Open the simulation

parameters from the tools menu and set the parameters as shown in Fig 1.4. The

fixed step size is the same as the discrete-step, which will be used by the DSP

DS1104. This means that in every “discrete-step” the whole program (i.e. control-

system in this case) will be executed, I/O data will be exchanged and the decision

making will be done inside DS1104.

• Press CTRL+B to build the control-system in real-time now. Refer to Fig 1.5, note

the sequence: 1) Compilation of C-code that is generated by Simulink, which will

be used to implement control-system in hardware DS1104, 2) Generation of

Exp1.sdf file, which will be used later on by Control-desk, to access the variables of

control-system.

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Fig 1.4 Simulink file Exp1.mdl, setting the simulation parameter

Fig 1.5 Building the Simulink program, real-time implementation

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• Check the folder Exp1; this should contain the file Exp1.sdf in addition to the other

system files. Leave them as such for the proper operation of Control-desk. Open

Control-desk by double clicking on the dSPACE Controldesk icon. The icon

should be located on the desktop of the computer; the opened control-desk is shown

in Fig 1.6 (ignore the lower panel window shown in Fig. 1.6 at this moment, and

also the pop-up window showing Exp1.lay). Click File->Open Variable File. A

pop-up window will appear, locate the Exp1.sdf file in the directory Exp1 and click

Open. Now you should be able to see the lower panel window shown in Fig 1.6.

Click File->Open, the pop-up window will appear again asking for layout file this

time. Locate the file Exp1.lay as shown in Fig 1.6 and click Open. Click yes if a

pop-up box opens asking for data-connection. The layout thus opened should look

as shown in Fig. 1.7. At this stage the control panel of the control-desk is ready to

communicate or transfer data with DS1104 via CP1104. Click Start (PLAY) and

switch to the Animation mode. The motor will start running. It will rotate in the

positive direction for some-time and in the negative direction for some time, i.e. the

direction of rotation alternates. Right click on the graph and select edit capture

setting, set the capture setting as shown in Fig 1.8.

Fig. 1.6 Control-desk panel, opening the layout

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Fig. 1.7 Control panel layout

Fig. 1.8 Various control buttons, starting and stopping the system

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• You should be able to observe the speed of the motor on control panel (Fig. 1.8).

The waveform for motor phase current, reference speed and actual speed is also

shown. Press the button Edit mode and then press STOP. Take the waveform as

shown in Fig 1.9. Study the explanations detailed next. The control-desk is able to

access the following data with the help of Exp1.sdf file:

o Actual Speed of the motor from W_mech subsystem which is inside the

Simulink control-system. You can open the subsystem W_mech; you will

observe how the input port INC1 (DS1104ENC_POS_C1) of CP 1104 is

utilized to read the actual speed of the motor. In the actual system, this port

is connected (hardwired) to DS1104 though. But, this port is also a part of

Simulink control-system; hence it will be listed as a variable inside the

Exp1.sdf. Since the control-desk has access to the variables of control-

system through Exp1.sdf, hence the control-desk can read the port INC1,

modify the data and send it back to any of the output port of CP 1104, if

necessary.

o Motor current, reference speed and actual speed can be observed in the

same manner, the communication among components is same as explained

above.

o Note that, in this demonstration, we are only observing the variables of

control-system such as speed of the motor and current. It is also possible to

change the variables of control-system in real-time from the control panel.

In the future experiments, this will be done to give a reference speed

command to run the motor at a desired speed. This reference speed

command will be changed in real-time to change the speed of the motor.

• Sequence of events, when Start button is pressed on the control panel:

o DS1104 will be commanded to generate the controlled digital signals as per

the speed and phase-current of the motor in real-time. The information

about the speed and current of the motor is available to DS1104 via CP

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1104, which is connected to the Power-Electronics-Drives-Board (for

current feedback) and Motor coupling system (for speed feedback).

o The controlled digital signals will be received by CP 1104 from DS1104.

Digital I/O of CP 1104 is connected to the Power-Electronics-Drives-Board;

hence the board will start generating the PWM voltage source.

o The motor will receive the PWM voltage at its terminals and hence start

rotating. It will speed up; the current in the winding will increase. Since the

speed and the current of the motor thus increased are fed back to DS1104 in

real-time, the DS1104 will take the next action as per control-system.

DS1104 will change the pattern of digital signal to change the speed of the

motor such that the motor will achieve the speed as commanded in the

control-system (block Wref_4quad). Note that, the speed command in

control-system is alternating; hence the motor alternates its direction of

rotation. The instantaneous motor speed and current is shown in Fig 1.8.

Fig. 1.9 Motor actual speed (red), reference speed (green) and phase current (blue)

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1.4 Lab Report and Reading assignment

• List the sequence of events i.e. the communication between the four major

components when STOP button is pressed in the control panel.

• Draw a flow-chart indicating step-by-step procedure to create a real-time model in

Simulink, which is followed by controlling a DC motor from control-desk.

• Study thoroughly appendix-A, pay special attention to the Power-Electronics-

Drives-Board features. Make a note of voltage and current scaling in the drive

board.

• Draw a block diagram of Power-Electronics-Drives-Board indicating the inputs

(like power supplies, digital input, resets etc) and outputs (like PWM voltage, motor

current, dc-bus voltage etc).

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E x p e r i m e n t – 2 S i m u l a t i o n a n d R e a l - t i m e I m p l e m e n t a t i o n o f a S w i t c h -m o d e D C C o n v e r t e r

2.1 Introduction

In the previous experiment, a demonstration highlighting various components of the

electric drives laboratory was performed. Real-time simulation file (*.mdl) and a Control-

desk layout file (*.lay) were provided.

In this experiment, a Simulink model (*.mdl) of a DC switch-mode power converter will be

built. After verifying the simulation results with Simulink model, the model will be

modified to control the output voltage of the converter in real-time. A control panel using

dSPACE Control-desk will be designed (*.lay) that will serve as a user-interface to regulate

the output voltage of the converter.

In section 2.2, theoretical background to implement DC switch-mode power converter in

Simulink is briefed. Section 2.3 gives step-by-step instructions to simulate the converter in

Simulink. In section 2.4, the Simulink model is modified for real-time implementation and

step-by-step instructions to design the control panel using Control-desk are given.

2.2 Theoretical Background of DC Switch-mode converter

2.2.1 Switching Power-Pole Building Block

The switching power-pole building block has been explained in Section 1-6-1 of [1].

Depending on the position of the bi-positional switch, the output pole-voltage Av is either

inV or 0. The output pole-voltage of the power-pole is a switching waveform whose value

alternates between inV and 0 depending on the pole switching function Aq . The average

output voltage Av of the power-pole can be controlled by controlling the pulse width of the

pole switching function Aq .

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(1)

pulse-width of

switching time period

upA in A in

s

up A

s

Tv V d V

TT q

T

= =

→

→

2.2.2 PWM of the Switching Power-Pole

As seen in section 2.2.1, in order to control the average output voltage of the switching

power-pole, the pulse width of the pole switching function Aq needs to be controlled. This

is achieved using a technique called Pulse-Width Modulation (PWM). This technique is

explained in section 12-2-1 of [1]. To obtain the switching function aq , a control voltage

,cntrl av is compared with a triangular waveform triv of time period sT . Switching signal

1aq = if ,cntrl a triv v> ; 0 otherwise. As in [1],

,ˆ (2)cntrl a a triv d V=

Using equations (1) & (2) and assuming ˆ 1VtriV = ,

, (3)

where average pole-output voltage with respect to negative DC-bus voltage.

aNcntrl a

d

aN A

vvV

v v

=

= =

2.2.3 Two-pole DC Converter

The two-pole switch-mode DC converter utilizes two switching power-poles as described

in the previous sections. The output voltage of the two-pole converter is the difference

between the individual pole-voltages of the two switching power-poles. The average output

voltage o abv v= can range from to +d dV V− depending on the individual average pole-

voltages.

(4)o ab aN bNv v v v= = −

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To achieve both positive and negative values of ov , a common-mode voltage equal in

magnitude to / 2dV is injected in the individual pole-voltages. The pole-voltages are then

given by:

(5)2 2

(6)2 2

d oaN

d obN

V vv

V vv

= +

= −

Solving equation (1) to (6),

,

,

1 1 (7)2 21 1 (8)2 2

oa cntrl a

d

ob cntrl b

d

vd vVvd vV

= = +

= = −

The above equations will be implemented in Simulink.

2.3 Simulation of DC Switch-mode Converter

2.3.1 Triangular waveform

As explained in section 2.2.2, to modulate the pulse-width of the switching signal in a

power converter, a control voltage has to be compared with a triangular waveform signal.

This triangular waveform will be generated in Simulink, using the Repeating Sequence

block.

• Create a new directory for the experiment (say Expt03).

• Start Matlab and set the path to this directory.

• Type “simulink” at the command prompt and create a new model from File menu.

• Access the Simulink library by clicking View > Library Browser.

• In the Library Browser expand the Simulink tree and click on Sources. Drag and

drop the Repeating Sequence block into your model.

• Simulink blocks usually have properties that can be modified by double-clicking on

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the blocks. Double click on the Repeating Sequence block and edit the properties as:

o Time values: [0 0.5/fsw 1/fsw]

o Output values: [0 1 0]

Where “fsw” is the switching frequency set as a global variable in the Matlab prompt.

• Add a Scope to the model from Simulink→Sinks.

• Connect the output of Repeating Sequence block to the input of the Scope.

• Type the value of “fsw” at the Matlab prompt, fsw = 10000 (Switching frequency: 10 kHz)

• The simulation model is now ready. However before running the simulation

parameters need to be changed. Go to Simulation menu and select Simulation

Parameters. Set the parameters to the following values:

o Stop time : 0.002

o Fixed step size : 0.000001

o Solver Options: fixed step, ode1 (Euler)

• Run the simulation by clicking on the triangular button on the top. Double click on the

scope after the simulation finishes. The result should look similar to the one shown in

Fig. 2.1.

2.3.2 Duty Ratio and Switching Function

For a desired average output pole-voltage aNv , the control voltage ,cntrl av is given by

equation (3). Equation (3) is implemented in Simulink and the control voltage thus

generated is compared with the triangular signal generated in the last part.

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Fig. 2.1 Triangular Waveform with 10 kHz frequency

The desired voltage aNv is set by a Constant block with value one, and can be varied with

a Slider gain from ‘0’ to the maximum DC-bus voltage dV ( dV = 42V in the model). The

control voltage is generated by dividing aNv by dV . This is done by using a Gain block (of

value1/ dV ) at the output of the Slider gain.

Comparison of the triangular signal and the control voltage is done using a Relay block.

The triangular signal is subtracted from the control voltage. The Relay block output is then

set to ‘1’, when the difference is positive and ‘0’ when the difference is negative.

To create the model, follow the steps below:

• Open Simulink and create a new model.

• Copy and paste the model of triangular waveform generator from section 2.3.1 (Fig

2.1).

• Add these parts to the model

o Constant block from Simulink→Sources.

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o Slider Gain from Simulink→Math Operations.

o Gain from Simulink→Math Operations.

o Sum from Simulink→ Math Operations.

o Relay from Simulink→ Discontinuities.

• Now change the properties of these blocks as follows:

o Change the Slider Gain limits as shown in Fig. 2.2.

o Change the value of Gain to 1/ dV ( dV will be set to 42V from the command

prompt later).

o Change Sum block signs to| −+ .

Fig. 2.2 Switching Function generation for single pole converter

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• Rename the blocks and connect them as shown in Fig. 2.2.

• In the Matlab prompt, type: fsw = 10000, Vd = 42.

• Set the simulation parameters as in section 2.3.1 and save the model.

• Run the simulation and save the waveform for the switching function. (Fig. 2.2)

2.3.3 Two-pole Converter Model

Equations (7) and (8) describe the control voltages of the two poles A & B depending on

the desired output voltage o abv v= . These equations will be implemented in Simulink (Fig.

2.3). Also, the switching power-poles will be modeled using a Switch. The Relay blocks

provide the switching functions for the poles and a bq q . Depending on the value of the

switching function, the Switch outputs the pole-voltage as follows:

For 1aq = , switch output (Pole A) = aNv = dV

For 0aq = , switch output (Pole A) = aNv = 0

Create the Simulink model as shown in Fig. 2.3. The Switch block can be found in

Simulink→Signal Routing.

• Set the simulation parameters and values of fsw and Vd as in section 2.3.2. Run the

simulation.

• Collect the following results:

o Switching function q(t) for pole ‘A’ of the two-pole converter.

o Simulation results of two pole converter model for two different values of

V_ab, one positive and one negative.

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Fig 2.3 Two Pole Switch-Mode Converter Model in Simulink

2.4 Real-time Implementation of DC Switch-mode Converter

Having simulated the two-pole DC switch-mode converter, it will now be implemented in

real-time on DS1104. This means that the converter will now be implemented in hardware

and its output voltage amplitude will be controlled in real-time using an interface (made

possible by the use of dSPACE Control-desk). As explained in experiment-1, real-time

implementation involves exchange of signals between the dSPACE Control-desk interface,

DS1104 and the Power-Electronics-Drives-Board. In this experiment, the output voltage

reference will be set from the Control-desk interface. The duty ratios for the two poles will

be calculated from this output voltage reference inside DS1104. PWM will be internally

performed and the switching signals thus generated will be sent to the power electronics

drives board through the CP1104 I/O interface.

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dSPACE provides a block called DS1104SL_DSP_PWM3, which embeds the triangular

waveform generator and the comparator for all converter poles. The inputs for

DS1104SL_DSP_PWM3 are the duty-ratios for the poles. In Fig. 2.3, the lower part of the

model is called the Duty Ratio Calculator. This part of the model will again be used in the

real-time model to generate the pole duty ratios. The triangular wave generator and

comparison using relays will be replaced by DS1104SL_DSP_PWM3, as these functions

are internal to the block. Two legs of the drives board (refer appendix ‘A’) will replace the

two poles (modeled using the Switches in Simulink).

• Create the real-time model as shown in Fig. 2.4. Use the Duty Ratio Calculator from

section 2.3.3.

• For the DS1104SL_DSP_PWM3 block, set the switching frequency as 10000 Hz and

the dead-band to ‘0’.

• Open the simulation parameters. Change the stop-time to inf, fixed step size to 0.0001

and turn block reduction OFF.

• Set Vd = 42V in the Matlab command prompt.

Fig. 2.4 Control of a two-pole switch-mode converter in real-time

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Once the real-time model is ready, it can be implemented on the DSP of DS1104 by

building the model. As explained in experiment-1 building the model will broadly cause:

1. Compilation of C-code (generated by Simulink) and its hardware implementation

on DS1104.

2. Generation of a variable file (with extension .sdf) that allows access to the variables

and signals in the real-time Simulink model.

• Build the Simulink model by pressing (CTRL+B). Observe the sequence of events in

the Matlab command window.

• Once the real-time model is successfully built, open Control Desk (icon on PC

Desktop).

• Using the File menu, create a New Experiment and save it in the same working root

as the real-time Simulink model. Create a New Layout using the File menu again.

Two new windows will appear in the Control Desk workspace. The one called

Layout1, will contain the instruments used for managing the experiment. The

second window is a library which will let us drag and drop the necessary controls for

the experiment into the Layout.

• Now, select File>Open Variable File. Browse to the directory containing the real-

time Simulink model. Open the .sdf file (E.g. For Simulink model named

twopole.mdl, the variable file will be twopole.sdf).

• After opening the variable file, notice that a new tab in the lower window called

Variable Manager appears below the layout (Fig 2.5). The variables of the real-time

Simulink model file are under the tree Model Root. Expand Model Root, observe

the variables and relate them with the real-time Simulink model.

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Fig 2.5 New Layout Window for Instrumentation and Control

• Now a user-interface that allows us to change input variables & system parameters

(in real-time) and also observe signals will be created. The input variable in this

experiment is the output voltage of the switch-mode converter. The duty ratios

generated by the Duty Ratio Calculator will be the signals that will be observed in

the layout. The actual pole voltages will be observed directly from the power

electronics drives board using an oscilloscope.

• In order to change the reference output voltage and observe the duty ratios, suitable

parts need to be added to the layout. These parts are available in the window to the

right of the layout. The output voltage reference V_AB will be set using a Slider and

a Numerical Input. Both these parts are found under Virtual Instruments. Drag

23

and drop these parts in the layout. The duty ratios will be observed in a Plotter

available in Data Acquisition. Select Plotter and draw it in the layout.

• Now appropriate variables will be assigned to the parts. Under Model Root, locate

V_AB and select it. It will have a parameter called Value (right side panel) which

corresponds to the value of the Constant block V_AB in the real-time Simulink

model. Drag and drop V_AB/Value into the Slider and also on Numerical Input

one-by-one. Now, the value of V_AB can be changed in real-time using these two

parts. Similarly, to observe the duty ratios in real-time, assign the two outputs (Out1

and Out2) of the De-mux (the one following the Gain2 block) to the plotter. The

experiment is now ready, it should look as shown in Fig 2.6. Start the experiment by

clicking the Start button and select the animation mode (Fig 1.8)

• Turn the power supply ON and observe the pole-voltages on the oscilloscope. Vary

the output voltage reference (V_AB) using the Slider or the Numerical Input.

Observe the changing duty-ratios and pulse-widths of the pole-voltages.

Fig 2.6 Control Desk layout for Switchmode DC Converter

24

2.5 Lab Report

• Record the output voltage waveform on the oscilloscope for the values of V_AB set

in section 2.4.

• Record the corresponding duty ratio waveforms for the above values.

• Measure the output voltage frequency and comment on the result obtained (Hint:

relate the frequency set in the PWM block to the frequency of the voltage observed

on the oscilloscope).

2.6 References

[1] “FIRST COURSE ON Power Electronics” by Ned Mohan, 2005, MNPERE.

25

E x p e r i m e n t – 3 N o - L o a d D C M o t o r T e s t

3.1 Introduction

The output voltage control of a two-pole DC-Switch-mode-converter was implemented in

real-time, in the last experiment. The purpose of the real-time implementation was to obtain

a variable DC-voltage at the output of the power converter, while controlling its amplitude

with a dSPACE-based Control-desk user interface. In this experiment, a DC-motor will be

connected to the output of the power converter. With this arrangement, a variable voltage

can be applied to the terminals of the DC-motor. We will observe that by changing the

magnitude of the applied voltage, the speed of the motor can be varied. This is also referred

as open-loop voltage controlled DC-motor. At the end of the experiment we will make a

case for open-loop control and also for the characterization of DC-motor, which will be

done in the next experiment.

3.2 Control of a DC Motor under No-Load Condition in Open Loop

Before starting the implementation of the model, for control of the DC Motor in open-loop,

connect the armature of the DC-motor under test to the output of two converter poles A and

B. Connect the CURR. A1 (phase-current measurement port) on the drives board to the

Channel ADCH5 of CP 1104 I/O board. Also, connect the encoder output (mounted on

the DC-motor) to the INC1 9-pin DSUB connector on CP 1104 I/O board.

The speed of a DC-motor can be changed by varying its supply voltage. The model of

output voltage control of the switch-mode dc converter was discussed in experiment – 2

and the same will be used.

• In Simulink, open a new model.

• Set the simulation parameters as described in experiment-2.

• Copy the model from experiment-2 and save the new model.

26

• Change the name of the Constant block from V_ab to V_motor; this will be the

input voltage of the DC-motor.

• The current and speed measurement blocks need to be added to the existing model.

3.2.1 Adding current measurement blocks

For measuring the current, Channel 5 of the A/D converter (ADCH5) on CP 1104 will be

used. Remember that the data have to be scaled by a factor of 10. In addition, the current

sensor outputs 1V for 2 amps of current; therefore it actually needs to be scaled by 20

(shown in Fig. 3.2).

• Drag and drop the DS1104ADC C5 block from the dSPACE library.

• Connect a Gain block at its output and set its value at 20.

• Connect a Terminator at the output of the Gain block and label the signal as aI .

3.2.2 Adding speed measurement blocks

To measure speed we shall use the DS1104ENC_POS_C1 block from the dSPACE

library. This block provides read access to the delta-position and position of the first

encoder interface input channel. The delta position represents the scaled difference of two

successive position values of a channel. To receive the radian angle from the encoder the

result has to be multiplied with 2 2_ 1000encoder linesπ π

= ; where encoder_lines is 1000 for

the encoders used in the laboratory setup.

The delta-position scaled to a radian-angle has to be divided by the sampling time to obtain

the speed, as in:

1

d= (1)dt k k st t Tθ θ θω

+

∆ ∆= =

−

27

Drag and drop the DS1104ENC_POS_C1 block from the dSPACE library. In addition the

encoder set-up block DS1104ENC_SETUP is to be added to the model. Connect a

Terminator block to the Enc position. Connect a Gain block at Channel 1 output (i.e. Enc

delta position) and set its value as 21000 sT

π where sT is the sampling time set in the

simulation parameters under the fixed-step box. The output of this block is the motor speed

in rad/s.

However at low speeds, there will be oscillations in the measured speed values. Hence an

averaging to get more accurate readings is needed. For preparing the averaging block,

• Select the Unit Delay block from Simulink→Discrete and drag the required

number of blocks into the model (say 10 for 11 point averaging).

• Select the input port In from Simulink→Source library and drag it into your

model.

• Drag a sum block and double click on the sum block and add the required number

of ‘+’ signs in list of signs.

• Drag a Gain block and connect it at the output of Sum block. Set the value to 1/11

for 11 point averaging.

28

Fig. 3.1: Averaging model in Simulink

• Select the output port Out from Simulink→Sink and drag it into the model.

• Rename and connect all the blocks as shown in Fig. 3.1.

• Select the unit delays, input port, output port, gain and sum blocks together and

create a subsystem. For creating a subsystem, go to Edit menu of your model and

then click Create Subsystem.

• The system obtained is an 11-point averaging system. Change the subsystem name

to Averaging Block.

• Connect a Terminator at the output of this 11 point averaging of speed

measurement and label the signal: wm.

• The model obtained should look like the one shown in Fig. 3.2. Before building the

real-time model, don’t forget to define and s dT V as global variables at Matlab

prompt.

3.2.3 Creating Control Desk Layout

• Build (CTRL+B) the Simulink model.

• Start Control-desk and create a new experiment in the same working directory as

that of Simulink file. Load the variable file (*.sdf) into the Control-desk (Refer

experiment-1).

• Create a layout and drag a slider gain control and two plotters.

• Drag and drop V_motor into slider gain control.

• Assign one of the plotters for Ia and another one for wm.

29

• In order to record the numerical values of current and speed, add two Displays into

the layout. Drag and drop Ia in one and wm in another.

The experiment should look similar to the one shown in Fig. 3.3.

Fig. 3.2 Real-time model for no-load motor test

30

Fig. 3.3 Control Desk Layout

3.2.4 Results of No Load test of DC-motor

• Record the values of current and speed for different set of readings for the

corresponding voltage values specified in Table. 3.1

• Plot the voltage vs. speed curve.

• Find the slope of the above graph

3.3 Making the Case for Open-Loop Speed Control and Characterization of DC-

Motor

In steady state, with voltage aV applied to the armature terminal of a DC-motor, following

equation can be written:

31

(2)a a a a a a EV I R E I R k ω= + = +

From equation (2), the armature voltage can be calculated in real-time to run the DC-motor

at a desired speed ω (rad/s). Note that, there is no feedback here, we are calculating the

equivalent amount of voltage that need to be applied, to run the motor at a desired speed.

Hence this type of speed control can be termed as open-loop voltage control. To calculate

the value of aV from equation (2), the value of armature resistance aR and the back-EMF

constant Ek should to be known beforehand. The measurement of aR and Ek will always

carry some error and also, the right hand side of equation (2) is load dependent ( aI ). Hence

in open-loop the accuracy of speed control will be a function of load and measurement

error. In the next experiment, we will characterize the DC-motor to find the value of aR

and Ek , in addition to other parameters. In experiment-5, we will implement a close-loop

control of DC-motor, which can maintain the desired speed independent of load and

measurement errors.

3.4 Lab Report

The lab report should be short with all the details asked below.

• Plot of voltage vs. speed curve for the no-load operation of DC machine.

• Specify the slope of the curve obtained.

• Assume 1aR = Ω and 0.1V/rad/sEk = . Now using (2), calculate the speed (rad/s) for

each measurement in Table 3.1. Add a new row speed_cal in Table 3.1 and fill the

calculated speeds. By using (3), report the % error for each measurement (add

another row) in Table 3.1.

speed_cal - speed%error = 100 (3)speed_cal

×

32

E x p e r i m e n t – 4 C h a r a c t e r i z a t i o n o f D C M o t o r

4.1 Introduction

In the earlier experiment, you designed and implemented the switch mode control and no

load measurement of DC-motor along with the current and speed measurement. In this

experiment characterization of a DC-motor will be done, which will be helpful in designing

the closed loop control of DC-motor.

We will be using the Simulink model prepared in the earlier experiment and modify the

same to perform the characterization of DC-motor.

4.2 Open loop control of DC-motor with load

We will use the same Simulink model used in the earlier experiment (to do the no load

testing of DC-motor).

• Create a new folder Expt 4.

• Start Matlab and change the directory path to Expt 4

• Open the Simulink model used in the last experiment (Section 3.4).

To this model, appropriate blocks for controlling the active load (a DC-generator whose

electromagnetic torque will be varied) need to be added.

4.2.1 Adding a DC-load (LOAD) to the DC-motor (MOTOR)

To determine the DC-motor steady state characteristics, a second DC-motor will be axially

coupled to the motor under test (MUT). The second motor will be open-loop voltage-

controlled, similar to the MUT.

• The terminals of the load (DC-motor) should be connected to PHASE A2 and

PHASE B2 terminals on the power board. Set the Bus Voltage, 42VdV = .

33

Fig. 4.1 Real-time Simulink model for motor and load control

• Ensure a firm mechanical coupling between the motors.

• Add a second set of voltage-control blocks in the Simulink model and connect the

duty-cycles to the 2 and 3 inputs of DS1104SL_DSP_PWM.

• Set the switching frequency as 10000 Hz in the PWM block.

34

• Double click the DS1104SL_DSP_PWM block, go to PWM Stop and

Termination, uncheck “Set all Ch” and then click on “Set all”.

• Connect the CURR. A2 channel to ADCH6 on the controller box

• Make the load current (CURR. A2) available in the Simulink model by copying the

first current measurement blocks, and then double clicking on DS1104ADC_C5

and changing it to channel 6.

• Save the model as Step1_04.mdl. The model should like the one shown in Fig. 4.1.

4.2.2 Creating the Control Desk Interface

• Build (CTRL+B) the Simulink model, Step1_04.mdl.

• Open Control Desk and create a new experiment in the same working root as the

Simulink file.

• Create a new layout and drag two Slider Gain controls and two Plotters.

• Drag and drop the V_motor and V_load variables to the Slider gains.

• Assign one plotter to display aI and LI (Im and IL in Fig. 4.1) currents and one

plotter to display the speed mω .

• In order to record the numerical values of currents and speed, add three Displays to

the layout and assign them the speed and current variables. The control desk layout

should look like as shown in Fig. 4.2.

35

4.3 Steady-state characteristics of dc-motor drives

In this experiment, you will derive the characteristics of a DC-motor, using the second

motor as load. For a constant V_motor voltage, the load is varied using V_load. The

MOTOR current and speed are recorded in a table. A set of measurements are obtained for

different supply voltages. The characteristics will be drawn using Matlab.

Fig. 4.2 Control-desk layout

4.3.1 Theoretical background

The steady-state mechanical characteristics of a DC-motor are the dependency between the

electromagnetic torque (N-m) and the electrical speed (rad/s). Since the dependency is

linear, the characteristics will be straight lines for the entire voltage range 0 - ratedV and is

independent of the load. The motor equations reflect this linearity:

36

(1)(2)

motor a a E

e T a

V R I kT k I

ω= +=

From equation (1), one can obtain the steady-state motor characteristic:

( ) (3)

where

and

a motora a a

E E

a motor

E E

R VI I mI nk k

R Vm nk k

ω = − + = +

= − =

The steady-state model for the load can be approximated with a friction-type model, where

the torque is proportional to the speed, and a constant friction torque is always present:

(4)e L frictionT T B Tω= + +

where all terms in the right hand side are load related. For our setup, where the load is a

voltage controlled DC-machine, the load torque LT is, in fact, the electromagnetic torque

developed by the second DC-motor. The procedure to determine the parameter for the

steady-state model uses the current, voltage and speed measurements to obtain a linear

approximation for both equation (3) and (4). Once the slope and the interception point are

found, the parameters can be easily calculated.

4.3.2 Steady-state parameters determination

According to the theoretical description from section 4.3.1, the motors will be driven in

several steady-state operating points.

37

Estimation of Ek :

• At the rated armature voltage supplying the MOTOR, control the LOAD in the

active region, such that the MOTOR current would become zero ( aI = 0). Measure

the speed ( ) rad/sω . Substituting value of aI , equation (1) becomes:

__( 42 ) (5)a rated

E a rated

Vk V V

ω= =

• To perform the previous step, increase the V_motor slider gain to 42 V. Now

adjust V_load voltage such that the active torque will decrease the MOTOR current

Im (same as aI ) until it becomes zero.

• Record the values for speed, at Im = 0 and calculate Ek as per equation (5).

Drawing the torque-speed characteristics:

• Maintain the MOTOR voltage at constant levels 42; 21; 10; 3 V. Adjust the

LOAD voltage reference such that the MOTOR current takes the following values

at each voltage: 0; 1.0; 2.0; 3.0; 4.0; 5.0 A.

• Record the speed ( mω ), the LOAD current (IL) and LOAD voltage (V_load)

required to obtain the specified MOTOR currents. All current measurements will be

multiplied with Ek to obtain the corresponding torque values as per equation (2).

• Record the measured data in TABLE 4.1.

• Draw the MOTOR and LOAD characteristics using the data acquired during the

measurement process.

38

• Find the slope (m) and intercept (n) using MATLAB:

o The slope and the intercept of the linearized characteristic could be determined

with the following instructions:

p = polyfit(ia_ data, w_ data,1)

m = p(1)

n = p(2)

Table 4.1: Steady-state Experimental Data V Motor[V] V Load[V] I Motor[A] I Load[A] Speed[rad/s]

42 0.0

42 1.0

42 2.042 3.0

42 4.0

42 5.0

21 0.0

21 1.0

21 2.0

21 3.0

21 4.0

21 5.0

10 0.0

10 1.010 2.0

10 3.0

10 4.0

10 5.0

3 0.0

3 1.0

3 2.0

3 3.0

3 4.0

3 5.0

39

o w_data is the array of speed data, while ia_data is the corresponding current

data for the speed.

o Two set of parameters (m, n) are determined for each dc-machine (MOTOR and

LOAD).

• Determine Ra, Ek : Using the corresponding m and n, the machine parameters can be

determined using the following equations

__

a ie i

Vk

n=

where i = 1...4, the number of voltage levels considered previously.

_ E e i

a e

k avg k

R k m

=

= −

A table for final values of aR and Ek is obtained by averaging the data in TABLE 4.2

Determination of the friction model parameters

For determining the friction parameters, the MOTOR will be run under no-load conditions

as done in the previous experiment. Since the MOTOR has to overcome only friction

( 0LT = ), the electromagnetic torque ( e E aT k I= ) will follow the linear friction model (see

equation (4)) in steady-state. Fill in the TABLE 4.3 using the values obtained in the earlier

experiment. Linearize the dependency of ( )eT ω by determining the coefficients (m, n)

40

depicted in equation (4) (Use the same procedure as for drawing the torque speed

characteristics).

• Determine B and frictionT : Using equation (4), the friction parameters will be

B = m; frictionT = n; Fill in the Table 4.4 provided.

4.4 Dynamics of DC-motor

In this section the dynamic characteristics of the DC-motor will be derived. The dynamics can

be divided into electrical and mechanical dynamics. By independently studying both transients,

two motor parameters can be determined: armature inductance ( )aL and moment of inertia (J).

For this section the same Simulink model and dSPACE layout as in earlier section will be used.

To analyze the dynamics of a DC-motor some theoretical background will be presented, then

using the already determined steady-state parameters, the experiment steps will be described.

The setup consists of two DC-motors, axially coupled and supplied from two converters. One

motor is current controlled, such that it will act as an active load. This motor will be named as

LOAD. The second motor (MUT) is open-loop voltage controlled. This motor will be named as

41

MOTOR. In this experiment two parameters need to be determined: aL - armature inductance

[H] and J -moment of inertia [ 2kg m− ]

4.4.1 Dynamic Model Characteristics

There are several ways to determine the inductance and the inertia. All methods involve the

dynamic analysis of the machines in transient operation. The dynamic equations of a dc-motor

are:

(6)aa a a E a

diV R i k Ldt

ω= + +

(7)e L frictiondT T T B Jdtωω= + + +

The system of two first order differential equations shows that the DC-motor is a second order

system. The two state variables, armature current ( )ai and angular speed ( )ω , are not

independent. Therefore, the inductance ( )aL and the moment of inertia (J) would both

contribute to the variation of each of the two state variables. It is convenient to “isolate” the

state variables described in equations (6) and (7), thus only a first-order differential equation

has to be solved for each variable. Two sets of experiments are then required to determine aL

and J, while keeping the speed and the current zero respectively.

Inductance Determination:

To estimate the armature inductance, the motor must be held a standstill ( )0ω = . If the rotor is

blocked and a step voltage is then applied to the armature terminals, the current increases

exponentially in time and equation (6) becomes:

(8)a a a adiV R i Ldt

= +

42

Solving equation (8) for current,

1 (9)at

aa

a

Vi eR

τ− = −

where aa

a

LR

τ =

The current increases exponentially to the final value equal to a

a

VR

. The slope of this exponential

curve, measured at t = 0, is dependent on the value of inductance aL as given below:

.0

0

(10)a

a

RLa a a a

t a a a

di V R Vedt R L L

−

=

= =

A graphical determination of the slope, at a given voltage, would lead to the determination of

the motor inductance ( )aL .

Determination of Inertia

The motor is brought to a no-load steady-state 0dJdtω =

speed 0ω , by disconnecting the

load ( )0LT = . At this point ( 0 )t −= ,

0 ; where (0 )

(0 ) MOTOR current at steady-state no-load speede friction e T a

a

T T B T k I

I

ω −

−

= + =

→

To make electrical torque ( )eT equal to zero in the mechanical dynamics equation (7), a

complete shutdown of the motor supply is required. At ( 0 )t −= the whole system is shutdown.

This implies that the electromagnetic torques in the MOTOR ( )eT becomes zero. The dynamic

equation will be:

43

0 (11)frictiondT B Jdtωω= + +

At ( 0 )t += i.e. just after shutting down the system, the equation (11) can be written as

00

0 (12)frictiont

dT B Jdtωω

+=

= + +

Thus,

( )0

0 0

(0 ) (13)friction T a

t t

T B k IJd ddt dt

ωω ω

+ +

−

= =

− + −= =

By knowing oω , (0 )aI − and graphically determining the slope 0t

ddtω

+=

of the speed curve

at ( 0 )t += , the system inertia J can be calculated using equation (13).

4.4.2 Simulink model for dynamic parameter determination

The model presented in section 4.3 will be used for this section. However, one additional block

has to be inserted, such that a step command in voltage is possible.

Both, electrical and mechanical dynamic parameters require either a positive (from 0 to aV ) or a

negative (from aV to 0) step change in supply voltage. Here, aV =V_motor.

The aV = 0 condition implies also that the motors have no armature current 0ai = . To achieve

open-circuit of the motors armature winding ( 0ai = ) while aV = 0, the converters need to be

shutdown. This operation is possible by using the SHUTDOWN signal on the drives board.

The SHUTDOWN signals are controlled by the digital I/O channels 11 and 12. When IO11/12

is 0 (OFF state) the switching signals are inhibited and the switches are opened. Setting

IO11/12 to 1 (ON state) and resetting (IO10) resumes the regular operation of the converters.

44

The IO10/11/12 digital channels will be added as slave bit out blocks for our model from the

slave library. In addition two constant blocks and two Boolean conversion blocks should be

added with SD1 and SD2 using the same signal. The model should like the one shown in Fig.

4.3.

Fig. 4.3 Simulink model for dynamic parameter characterization

4.4.3 dSPACE Experiment Layout for dynamic model determination

• A new experiment will be created in the same directory containing the Step1_04.mdl

file.

45

• Copy the Layout from the previous experiment and rename it.

• Add 2 CheckButtons controls to the Layout.

• Link all variables from the model with the controls in the Layout, as in earlier section.

• Link Reset and SD to the CheckButtons control.

The layout of the experiment is shown in Fig. 4.4.

Fig. 4.4 Control Desk Layout for Dynamic Parameter Characterization

46

4.4.4 Dynamical parameters determination

Inductance determination

• Using the blocking device, block the rotors firmly.

• Uncheck and then recheck the SD control. This button works as a switch to connect and

disconnect the machines from the power supply.

• Set the V_motor to a low value (around 3 V) and uncheck Reset to give a step input

voltage. The current should increase exponentially (as shown in Fig. 4.6) and reach a

constant steady state value.

• To save the current response, open View/Control bars/Capture Settings Window. Drag

the Reset signal from the Tool Window into the gray box situated below the Level -

Delay set boxes. Check the box called On/Off, check the edge direction, and set the

Level value to 0.5.

• Now, you will observe that every time you uncheck the Reset control in the layout, the

plot area will display the current and it will stop when it reaches the maximum

measurement time. Set the Length to 0.2 (see Fig.4.5). This will set the data capture

time as 0.2s which is large enough to observe the whole transient process in current.

• Check and uncheck SD and Reset to make some measurements. The current waveform

will look similar to the one shown in Fig. 4.6. After you are satisfied with the data

displayed go to the Capture Settings Window and press the SAVE button. The dialog

box will ask you to name the .mat file that will contain the graphic data in all plot areas.

• Plot the .mat file by using MATLAB command “plot” (refer to help by typing help plot

in the command window, if required); determine the inductance value as explained in

section 4.4.1.

47

Inertia determination

• In the Capture Settings Window two modifications must be made: Increase the display

length to 2 seconds and change the Trigger Signal to SD.

• Check the SD control and increase the voltage on the MOTOR to 42 V. Make sure that

the LOAD terminals are disconnected from the Power-Electronics-Drive-Board.

• Record the speed 0ω and armature current Im = (0 )aI − value at this operating point.

• Uncheck the Shutdown button. This will initiate the display process and, after two

seconds, the speed plot will stop and a decreasing exponential curve will be obtained

(see Fig. 4.7).

• Press again the SAVE button in the Capture Settings Window and store the data in

another .mat file. Plot the .mat file using MATLAB command “plot”. Calculate the

value of J as explained in section 4.4.1.

Fig. 4.5 Capture Settings

48

Fig. 4.6 Current Waveform

Fig 4.7 Speed Wavefrom

49

4.5 Lab Report

The lab report should contain the details asked below:

• Complete all the tables.

• Provide all the machine parameters which you obtained using the above procedure.

• Provide all the graphs you obtained while finding the machine parameters.

50

E x p e r i m e n t – 5 D C m o t o r S p e e d C o n t r o l

5.1 Introduction

In experiment-3, speed of the DC-motor was controlled by using an open-loop voltage control.

The purpose of this experiment is to design and implement a close-loop speed control of a DC-

motor drive. We shall use the same DC-motor for which the parameters were calculated in the

previous experiment. At first, the controllers will be designed and tested on a simulation model

of the DC-motor. Once the parameters are tuned, the model of the DC-motor will be replaced

with the real motor. The tuned controllers will be implemented in real-time on DS1104 to

perform the close-loop speed control of the DC-motor.

5.2 Simulink Model of the DC-motor

The model for a DC-motor in frequency domain is derived in Chapter 8 [1].

( ) ( )( ) ; ( ) . ( ) (1)

( ) ( )( ) ; ( ) ( ); (2)

a aa a E m

a a

em Lm em T a T E

eq

V s E sI s E s k sR sL

T s T ss T s k I s k ksJ

ω

ω

−= =

+−

= = =

Equations (1) and (2) can be easily implemented in Simulink using standard blocks as shown in

Fig. 5.1

• Create a new Simulink model and drag all blocks as shown in Fig. 5.1.

• Make the connections and define the parameters as shown in Fig. 5.1.

• The representation in Fig.5.1 uses integrators instead of transfer functions. This allows

setting the initial conditions for the current and speed state variables. The model also

includes the friction coefficient B. However, during simulations, B can be considered

zero, and the model will be similar to the one described by equations (1) and (2). Note

51

that the torque constant is replaced with the voltage constant in the Gain block at the top

of the figure. Create a subsystem by selecting component shown in Fig 5.1 and name it

as DC Machine. The system will look as shown in Fig 5.3 (a).

Fig 5.1 Simulink model of DC-motor

• Now enter the values of DC-motor parameters which were evaluated in experiment - 4.

You can either built a *.m file and run it, or access the various blocks in Fig 5.1 in the

actual model and replace with the parameter values.

5.3 Controller design

Once the DC-motor model is built, the controllers can be added and tuned. Start with the

current loop for which a PI controller is required.

• The model for a PI controller is first created - see Fig.5.2. Double click the integrator

block and enable limit output. Then set the Upper and Lower saturation limits to +lim/-

lim. The lim value should be set to 1 as the absolute maximum value of control voltage

is 1, which is the input to Kpwm block. The resultant maximum value of voltage

applied to the DC motor will be ±42 which is the rating of the DC motor.

• The armature current is fed back to the controller input.

52

Fig 5.2 PI controller model

• The parameters of the PI controller (namely Kp_i and Ki_i) are computed using the

motor parameters, which were evaluated in the earlier experiment. This procedure is

described in section 8-7-1 [1].

• The Saturation block sets the maximum and minimum limits for the control voltage (in

our case ±1 ).

• Set the value of Kp_i and ki_i in Matlab prompt. Also set the values of lim = 1, Kpwm

= 42, Ia_ref=1 in Matlab prompt.

Instead of setting the values of various variables in Matlab prompt, create an m-file which

contains the values of all the variables. Run the m-file before running the simulation, which

will load the values of all the variables.

• Running the Simulink model for the current controller with reference current as 1A,

results similar to the Fig.5.3 (b) and Fig. 5.3(c) will be obtained.

• Once the response in current is considered optimal (low overshoot, fast rise-time, zero

steady state error), the speed controller can be designed.

• A similar PI controller for the speed loop will be added to the Simulink model.

• Follow the procedure described in 8-7-2 [1] to design the speed control loop, using the

motor parameters determined in earlier experiment.

53

(a) Simulink model for current control loop

(b) Current waveform for 1A current reference (c) Speed waveform for 1A current reference

Fig 5.3 Simulink model and result for current control loop

54

(a) Simulink model for cascade control

(b) Current waveform for a step change in speed (c) Speed waveform for a step change in speed

Fig 5.4 Simulink model and result for cascade control

55

The Simulink model for the cascade control and the waveforms for speed and current are

shown in Fig.5.4. The Speed PI controller has a current limit output of ±5A, necessary to limit

the current during transients (both in simulation and real-time systems). To check the controller

design, we will give a step change in the speed reference. In the example of Fig.5.4, the speed

is commanded to a step change to 200rad/s at t=0s, then at t=5s it is changed to 450 rad/s. The

above reference speed command is implemented using a constant and step source blocks. The

results of cascade control are shown in Fig. 5.4(b) and Fig. 5.4(c). If the controller parameters

were correctly tuned, then it’s time to go on for the next step, and implement the controllers in a

real-time system.

5.4 Real-time implementation of feedback control

For dSPACE implementation, the dc-motor model will be replaced with the real motor and

Kpwm block will be replaced by power converter with 42V dc supply. The control voltage to

duty cycle conversion was already discussed and implemented in experiment – 3.

• Add the reset block used in the previous experiment.

• Modify the Speed Control block as shown in Fig. 5.5. Change the integrator block

parameters by double clicking on it and changing its external reset to either. Open the

Current Controller and change its integrator’s reset as was done in the Speed Control.

Connect the reset inputs of speed controller and current controller as shown in Fig. 5.5.

These changes allow the integrators to start up correctly in the real-time environment.

Fig 5.5 Speed Controller

56

• Remove the DC-motor mask model and gain Kpwm block.

• Copy and paste the duty-cycle calculator from the Simulink model used in previous

experiments.

• The current and the speed are to be measured. For measurements use the blocks already

designed in previous experiments.

• Replace the speed ref, wm_ref_step and sum block with a constant block for setting the

speed reference.

• At the Matlab prompt, set the sampling time Ts=0.0001 and the dc-bus voltage at

Vd=42V. Also set the values of various variables you have defined in the model.

• Change the switching frequency in the DS1104SL DSP PWM3 to be 50000Hz.

• Set the simulation parameters with Ts as sampling time and inf as total simulation time.

Now, the model looks like in Fig.5.6.

• Build (CTRL+B) the model and start Control Desk.

• Create a new Experiment and set the working root the same as the path for the Simulink

model.

• Create a new Layout and add controls as shown in Fig.5.7.

• Run the experiment and compare the real-time results with the simulations.

57

Fig 5.6 Simulink model for real-time implementation of DC motor control

58

Fig 5.7 Control-desk interface for DC motor control

5.5 Lab Report

• Why do you need a close-loop speed-controller? (Hint: Compare with open-loop

control)

• Why is the bandwidth of the current-control loop much higher than that of the speed-

control loop?

• Comment on the experimental results as compared to the simulation results.

5.6 References

[1] “ELECTRIC DRIVES an integrative approach” by Ned Mohan, 2000, MNPERE.

59

E x p e r i m e n t - 6 F o u r - Q u a d r a n t O p e r a t i o n o f D C -m o t o r

6.1 Introduction

In this experiment, the four-quadrant operation of DC-motor will be studied. The four-quadrant

operation is performed by giving an alternating reference-speed command to the DC-motor,

from positive speed (500 r/min) to negative speed (-500 r/min) with a constant ramp. The

speed controller designed in the previous experiment is used to track the instantaneous

reference-speed command. Fig. 6.1 shows the circuit diagram of the system. To keep the same

inertia constant of the system, which was used to design the speed-control-loop in the

experiment-5, another DC-motor is coupled to the DC-motor whose four-quadrant operation is

desired.

Fig. 6.1 Circuit diagram, four quadrant opeartion of DC-motor

6.2 Real-time implementation

The real-time Simulink model of four-quadrant operation of DC-motor is shown in Fig. 6.2.

The block Wref_4quad is used to generate alternating reference speed command. The selector

42 V

DC

Pow

er

Supp

ly

60

Switch takes the speed command as per the value of start_4quad_op. Initially, when the

system is ready to run, the value of start_4quad_op is zero. Hence it takes the reference speed

from Wref_start, i.e. at this time the motor is operating in the first quadrant only. The four-

quadrant operation is started by changing the value of start_4quad_op to 1. Follow the steps

below to make the real-time Simulink model.

• Alternating reference speed command (Wref_4quad): From the Simulink library

browser drag and drop the “repeating sequence” and rename it as Wref_4quad. The

“repeating sequence” is located at Simulink → sources → repeating sequence.

Double-click on the block and change the parameter as shown in Fig. 6.3. Also, drag

and drop the “constant source” from Simulink → sources → constant; rename and

connect them as shown in Fig. 6.3 to the Switch. Switch can be found at Simulink →

signal routing → switch. Double-click on the Switch block and change the parameter

as follows: “Criteria for passing first input” → u2>Threshold and “Threshold” →0.1.

Fig 6.2 Real-time Simulink block, four quadrant-operation of DC-motor

61

Fig 6.3 Alternating reference speed command

• Speed Measurement: The speed measurement is done in the same way as done in the

previous experiments. Connect a gain block (60/(2*pi)) at the output of averaging block

(see Fig. 6.4) to convert the speed in rad/sec to r/min. Create a sub-system and name it

as Speed Measurement as shown in Fig. 6.4. Also, connect a gain block (2*pi/60)

before feeding the error signal (i.e. the difference of reference speed and actual speed)

to the speed-controller PI_speed.

Fig 6.4 Speed Measurement

• PI controllers for current and speed loop: The design of the speed and current

controller was explained in the last experiment. Follow the same procedure to

determine the values of pk and ik . The bandwidth of current control loop should be 1.5

kHz and that of speed loop should be 150 Hz. Add a reset input to the integrator (by

62

choosing the external reset option as “level”, see Fig. 6.5) and check the “limit output”

option in the integrator block. The values of the limit would be the rated-current of the

motor. Also, connect a “saturation” block (the limit would be same as the rated current

of the motor; the block is located at Simulink → discontinuities → saturation) at the

output of the controller. Follow the same procedure for current controller PI_current.

Note that the output limits in this controller would be the rated-voltage of the motor.

Fig 6.5 PI controller for speed loop

• Motor current and dc-bus voltage measurement: Analog input channel

DS1104ADC_C5 and DS1104ADC_C7 are used to sense the motor current and dc-bus

voltage respectively. Motor current is fed back and connected to the negative input of

the sum block for negative-feedback current-control-loop (PI_Current). Connect a

filter 1/(s+1) for monitoring the ripple-free motor current and dc-bus voltage on the

control-desk as shown in Fig. 6.2. Note the gain blocks used with analog input channel.

These are used to convert the sensed signals into the actual currents and voltages that

are applied to the motor.

• Connect the Duty cycle generation block and DS1104SL_DSP_PWM block as done

in the previous experiments. The whole system should look as shown in Fig. 6.2.

• At the Matlab prompt, set the sampling time Ts = 0.0001 and the dc-bus voltage at Vd =

42V. Also set the values of various variables (like pk and ik ) you have defined in the

63

model. Set the simulation parameters with Ts as sampling time and “inf” as total

simulation time. Build (CTRL+B) the model.

6.3 Creating the Control-desk Layout

• Start Control Desk and create a new experiment in the same working directory as that

of Simulink file. Open the variable file (*.sdf) for the Simulink model that you built.

Create a new layout and drag the following parts in it: Plotter, Slider, Display and two

Check Boxes. Assign the variables to the parts in the layout as shown in Fig. 6.7.

• Change the Slider properties such that it ranges from 0 to 500 r/min. Right click on the

Plotter and click on “Edit Capture Settings”. Change the capture settings parameter

length to 40.

6.4 Running the Experiment

• In the edit mode, press the play button. Select the Animation Mode and verify that the

Reset and Wref_start Check Boxes are unchecked. Check and uncheck the Reset

Check Box so that the PI integrators are reset.

• Slowly increase the speed reference (Wref_start) to its maximum (500 r/min). Notice

that the DC-motors will start turning. Verify that the Display for Speed Measurement

displays a speed about 500 r/min.

• Observe the Wref_4quad waveform in the Plotter. It should change between -500 to

500 r/min. When this waveform is constant at +500 r/min, check the Wref_start Check

Box. This will make the speed reference for the DC-motor to be the same as the

Wref_4quad waveform. You will now observe that the speed of the DC-motor varies

like the Wref_4quad waveform. When one complete cycle of Wref_4quad is in the

plotter, select the Edit Mode and copy the screen. An example is shown in Fig. 6.7.

64

Fig. 6.7 Output Example

6.5 Lab Report

Include the following in your lab report:

• Copy of the waveforms you observed. In the waveforms, indicate the regions of:

1. Forward Motoring

2. Reverse Motoring

3. Forward Regenerative Breaking

4. Reverse Regenerative Breaking

• Give reasons for the nature of the DC Bus Voltage waveform that you observed. (Hint:

Look for change in slopes)

65

E x p e r i m e n t – 7 C h a r a c t e r i z a t i o n o f I n d u c t i o n m o t o r

7.1 Introduction

In this experiment, a three-phase induction motor will be characterized to determine the various

parameters used in its per-phase equivalent circuit (Fig. 7.1). The circuit diagram for this

experiment is shown in Fig 7.2, where a DC motor is coupled to the induction motor under test.

DC resistance test will be done to determine the value of sR . The magnetizing

inductance ( )m lsL L will be calculated by running the induction motor at synchronous speed,

at rated-voltage and rated-frequency. Speed of the DC motor (coupled to the induction motor)

will be controlled, to run the induction motor at synchronous speed. The rotor circuit

parameters i.e. and 'lr rL R will be calculated by blocked-rotor test while injecting slip

frequency at the stator terminals.

sR

sV

lsL

mL'rR

s

lrL

7.1 Per-phase equivalent circuit of a three-phase induction motor

7.2 Real-time implementation

The real-time Simulink model is shown in Fig. 7.3. Perform the following steps to build the

real-time model in Simulink.

• Create the variable magnitude and variable frequency voltage source: A three-phase

balanced voltage source of variable-magnitude and frequency is required, to run the

induction motor at synchronous and slip-frequency respectively. The duty ratios for the

66

three poles A, B and C to generate this type of voltage source are given in section 4-6-

2[1]. Equations (1-3) are modified form of equations given in [1] which are suitable for

real-time implementation. Create a subsystem in Simulink as shown in Fig. 7.4. Drag

and drop Fcn block from Simulink → user-defined-function → Fcn, to create these

sources. Rename them as dA, dB and dC as shown. Write the equations inside the Fcn

block as shown, which actually corresponds to equations (1-3).

CURR B1CURR A1

ADC 5

INC 1

CURR A2 CURR B2

GND

+42 V

ENCODER

To INC 1(on CP 1104)

FromENCODER

CP 1104

42 V

DC

Pow

er

Sup

ply

A 1 B 1 C 1 C 2B 2A 2

Digital I/O

3-PHASE INDUCTION MOTOR

DAC 1

To SCOPE

To SCOPE

DC MOTOR

+

-

Fig. 7.2 Circuit diagram for the characterization of a three-phase induction motor

,,

,,

,,

( ) 1ˆ( ) 0.5 ; ( ) sin( [2]) (1); [2] 2 2

( ) ˆ( ) 0.5 ; ( ) sin( [2] 2 / 3) (2); s Laplace operator

( ) ˆ ˆ( ) 0.5 ; ( ) sin( [2] 4 / 3) (3); [1] ;

c AA c A c

d

c BB c B c

d

c C mC c C c c m

d d

v td t v t V u u ft f

V sv t

d t v t V uV

v t Vd t v t V u u V VV V

π π

π

π

= + = = = ×

= + = − →

= + = − = =

67

Fig. 7-3 Simulink block-set for charcterization of a three-phase induction motor

Fig. 7.4 Three-phase variable magnitude and variable frequency source

• Add constant sources and rename them as

o Vslip→ Voltage source, used when performing blocked-rotor operation

68

o Select_slip_op →Used to select the reference for the magnitude of three phase

voltage, either from Vslip or from a proportional voltage reference (V/f block).

V/f block generates the magnitude, proportional to the required frequency of the

three-phase voltage, which is applied to the induction motor. This reference is

used by duty cycle generation IM block to generate three-phase balanced

voltage source. V/f block also ensure proper starting of induction motor. The

value of gain used in V/f block would be the ratio of rated-peak-phase voltage

to the rated-frequency (Hz) of the induction motor.

o Ref_frequency →To generate the desired frequency of the three-phase voltage

o Tref →To load the induction motor by increasing the electromagnetic torque of

DC-motor. This is used when calculating the rated slip of the motor.

• Connect the duty ratio for pole ‘A’ ( Ad ) to DAC channel DS1104DAC_C1. The wave-

form of Ad and phase ‘A’ voltage will be same, this in conjunction with the input phase

‘A’ current Ai , will be used to calculate the power factor and input power to the

induction motor drive.

• Connect the rest of the blocks as shown in Fig. 7.3. Refer to experiment-6 for any

clarification in connecting the remaining blocks. Set the simulation parameter as done

in the previous experiments and build the model (CTRL+B). Create a new experiment

and set the working root the same as the model, create a new Layout and add controls as

explained in section 7.3.

7.3 Creating the Control-desk Layout

• Start Control-desk and create a new experiment in the same working root as your

Simulink model file. Open the variable (*.sdf) file and create a new layout. In the

layout, drag the following parts: Four Sliders, Two Numerical Inputs, Three Check

Boxes and One Display. Assign the variables to the parts in the layout as shown in Fig.

7.5

69

Fig. 7.5 Control-desk

7.4 Running the experiment

• Determination of sR : Measure the line-line resistance of the induction motor with the

help of a multi-meter. ( )Measured line-line resistance value / 2sR =

• Run the experiment and select the animation mode. Verify that the Reset,

select_slip_op and Torque_SpeedControl Check Boxes are all unchecked. Check and

uncheck the Reset Check Box.

• Determination of mL

o Since the Torque_SpeedControl Check Box is unchecked, the DC motor is

operating under speed control. Slowly increase the DC motor reference speed to

rated mechanical synchronous speed ((rated frequency in r/min)/number of

poles) using the Slider. Use the Numerical Input to accurately set the speed

reference.

70

o Now, increase the reference frequency for the induction machine to rated

frequency in Hz. Use the Numerical Input to accurately set the frequency

reference.

o Observe the waveforms for Ad and Ai on the oscilloscope. Set the channel

displaying Ad to AC coupling. Take the readings for the rms values of these

variables using the Measurement option on the oscilloscope. Also measure the

phase difference between the two waveforms using the cursors.

o Now set the frequency reference of induction motor to zero and subsequently

the speed reference of DC-motor to zero so that the machines come to rest. Stop

the experiment in the edit mode.

o The scaling factor for Ad and Ai are 10 and 0.5 respectively. Actual rms values

of the phase ‘A’ voltage, phase ‘A’ current and the per phase reactive power

drawn by the three-phase induction motor can be calculated as follows:

, ,

, (measured on scope), ,

, , (measured)

42 4210

2

sin ; where cos power factorA rms A rms

A rmsA rms A rms

A rms A rms

v V V

i i

Q v i

dd

θ θ

× ×

= ×

= × × →

= =

o This reactive power is consumed only by the magnetizing inductance mL

(assuming that the stator leakage inductance is small). Thus,

2, . mA rmsiQ X= ,

mX and thus mL can be calculated from the above equation.

71

• Determination of rotor parameters ( ' ,r lrR L )

o Run the experiment and select the animation mode. Verify that the Reset,

select_slip_op and Torque_SpeedControl Check Boxes are all unchecked.

Check and uncheck the Reset Check Box. Check the Torque_SpeedControl

Check Box. The DC-motor will now operate under torque-control.

o Slowly increase the reference frequency of the induction motor to the rated

frequency in Hz. Now, increase the DC-motor torque reference while observing

the induction motor phase ‘A’ current waveform. Increase the torque reference

till the peak phase ‘A’ current of the induction motor reaches 4.6 A. This is the

rated current of the machine and it is now operating at rated conditions.

o Record the speed from the display. This is the speed of the induction motor at

rated load. The rated rotor slip speed and slip frequency can now be calculated

as,

( / min) ( / min) ( / min)

( / sec)

2

slip r syn r m r

slip radslipf

π

ω ω ωω

= −

=

o Decrease slowly the reference torque of DC-motor to zero and subsequently the

rated frequency of induction motor to zero. Shut down the experiment into edit-

mode.

o Run the experiment again and select the animation mode, Verify that the Reset,

select_slip_op and Torque_SpeedControl Check Boxes are all unchecked.

Check and uncheck the Reset Check Box. Check the select_slip_op Check

Box. The DC-motor operates under speed control. Increase the speed reference

of the DC-motor to the rated slip speed calculated in the previous section.

72

o Now increase the reference frequency of the induction motor to the slip

frequency calculated in the previous section. Increase the value of Vslip using

the slider and observe the phase ‘A’ current of the induction motor. Adjust

Vslip such the induction motor draws the rated magnetizing current (same as the

current drawn while determining mL .

o Now bring the speed reference of the DC-motor to zero. Verify that the motors

come to rest. Take readings for the rms phase ‘A’ voltage & current and the

phase difference between the two. The per-phase equivalent circuit of a three-

phase induction motor, under such condition will be same as shown in Fig. 7.1

with s=1.

o At this stage, the values of mL and sR are known. Assuming 23ls lrL L= , we have

two unknowns, i.e. 'andr lrR L . Calculate the impedance seen by the voltage

source, in the per-phase equivalent circuit, in terms of the two unknown

quantities. Having known the voltage and current vectors (previous step), the

two unknown quantities can be solved for.

7.5 Lab Report

In your lab report include the following:

• All the readings taken in various sections.

• Calculation of , , , 'andm s ls r lrL R L R L .

• During determination of mL , the mechanical speed was set to be same as the

synchronous speed of the induction motor. Under such conditions, explain why only the

magnetizing current flows through the per-phase equivalent circuit.

7.6 References

[1] “ELECTRIC DRIVES an integrative approach” by Ned Mohan, 2000, MNPERE.

73

E x p e r i m e n t – 8 V / f s p e e d - c o n t r o l o f a t h r e e -p h a s e i n d u c t i o n m o t o r

8.1 Introduction

In a three-phase induction motor, the frequency of the applied voltage ( )ref Hzf and the

mechanical speed _ ( / min)m ref rω in r/min are related by equation (1). The slip frequency ( )slip Hzf is

a load dependent quantity, which can be calculated in real-time, if the value of load-torque and

the slope of the motor torque-speed characteristic are known. Having known the value of

( )slip Hzf and the desired mechanical speed _ ( / min)m ref rω , the frequency ( )ref Hzf of the three-phase

voltage can be calculated by equation (1). A proportional-amount of magnitude of the three-

phase voltage ( )ref ref HzV k f= × is applied, such that, the induction motor can produce the rated-

torque at the rated-current for each values of ( )ref Hzf . The constant k is known as V/f-constant

which is given by equation (2). In the first part of this experiment, by using the motor torque-

speed characteristic, we will derive a relation between the load torque and ( )slip Hzf . Also, we

will observe that in low frequency range, to maintain the rated-torque at rated-current a voltage

boost is required, i.e. the proportional relation ( )ref ref HzV k f= × seizes to exist. Having known

the relation between load torque and ( )slip Hzf , the second part of the experiment uses equation

(1) to implement V/f speed-control. The circuit diagram for this experiment is same as that of

experiment-7.

( ) _ ( / min) ( )

( )

( )

(1)120

where, number of poles

(2)

where, rated phase voltage and rated frequency in Hz

ref Hz m ref r slip Hz

rated

rated Hz

rated rated Hz

pf f

p

Vkf

V f

ω= × +

→

=

→ →

74

8.2 Real-time Implementation

The real-time Simulink model is shown in Fig. 8.2. This model is similar to the model in

experiment-7. Open the model used in experiment-7; delete the blocks which are not shown in

Fig. 8.2. Perform the following steps to add new blocks.

Fig. 8.2 Simulink block-set for V/f speed-control of induction motor

• Add constant source Wref_r_min and fref. Wref_r_min will be used to do V/f speed-

control of the motor, while f_ref will be used when determining the motor torque-speed

characteristic.

• Adding f_ref_slip_comp: This block is used to determine ( )ref Hzf for a desired speed

reference, by calculating ( )slip Hzf . Create a subsystem as shown in Fig. 8.3 and name it

as f_ref_slip_comp. Detailed equations used to calculate slip-frequency ( )slip Hzf is given

in section 8.4.2.

• Set the simulation parameter (as done in previous experiments) and build the model

(CTRL+B)

75

Fig. 8.3 Generation of reference frequency

8.3 Creating the layout

• Start Control Desk and create a new experiment in the same working directory as that

of Simulink file. Open the variable file (*.sdf) for the Simulink model that you built.

Create a new layout and drag the following parts in it: three Sliders, two Displays, two

Numerical Inputs and a Check Box.

• Drag and drop variables into appropriate parts as shown in Fig. 8.4, Change the slider

properties as indicated.

Fig. 8.4 Control-desk

76

8.4 Running the experiment

8.4.1 Torque-Speed Characteristics

• To derive the torque-speed characteristics of the induction motor, it will be operated at a

fixed input frequency and a proportional input voltage (defined by the V/f ratio). The

DC-motor will act as a variable load and the value of the load-torque will be varied by

Tref (Fig. 8.4). In addition to the DC-motor torque, the induction motor will also

experience a Coulomb friction torque frT and viscous friction torque Bω . Thus,

The values of and were calculated in Experiment - 4.IM DC fr

fr

T T T B

T B

ω= + +

• In the Simulink model file, verify that the frequency input of the Duty Ratio Gen.

subsystem is from the constant f_ref. If not, make necessary changes and rebuild the

model. In the Control Desk layout, run the experiment and select the animation mode.

Check and uncheck the Reset Check Box.

• Take the following readings (Table 8.1) for the specified values of Tref (varied using

Slider). Prepare the sets of readings for the following frequencies applied to the

induction motor: 30, 40, 60, 80 and 100 Hz (set using Slider and Numerical Input). Care

must be taken to use proper units while calculating IMT .

TABLE 8.1 Torque Vs Speed for a constant input frequency

Speed (r/min)

0

0.025

0.050

0.075

0.100

0.125

0.150

(N-m)IM DC frT T T Bω= + +(N-m)DCTref T=

• The torque-speed characteristics can now be plotted for various input frequencies.

77

8.4.2 Speed Control at Constant V/f Ratio

• Disconnect the DC-motor terminals from the drives board. The system now consists of

an induction motor coupled with a DC-motor which does not produce any torque. So

the load-torque in the systems is frT T Bω= + . Since frT and B are known, torque for

a given mechanical speed can be calculated.

• The objective is to apply such a frequency reference to the induction motor so that the

system rotates at the reference mechanical speed. This frequency reference can be

calculated from the torque-speed characteristics derived in the previous section as

follows:

Let the mechanical speed reference in rad/sec be mechω . The torque at this speed

will be IM fr mechT T Bω= + . Also let the equation of the torque-speed

characteristics be T m cω= + where ' 'm is the slope of the characteristics in N-

m/r/min and ' 'c is a constant. Thus,

_ ( / min)_ ( / min)

_ ( / min)

260

m ref rfr mech m ref r mech

fr mech m ref r

T B m c

c T B m

πωω ω ω

ω ω

+ = + =

= + −

The synchronous speed in r/min corresponding to the characteristic with

mechanical speed mechω and torque IMT can be given by

_ ( / min)_ ( / min)

( ) ( )120 120;where, and

fr mech m ref rsyn m ref r slip

fr mech ref Hz slip Hzslip syn slip

T B mcm mT B f f

m p p

ω ωω ω ω

ωω ω ω

+ −= − = − = +

+= − = =

78

• Having known synω , the frequency to be applied to the induction motor can be

calculated for a 4-pole motor as ( ) 120 30syn syn

ref Hz

pf

ω ω×= =

• Calculate the slope ' 'm (N-m/r/min) of the torque-speed characteristics that was

derived in the previous section. Enter this value in the appropriate block of the

Simulink model.

• In the Simulink model, change the frequency input of the Duty Ratio Gen. subsystem

to the output of f_ref_slip_comp subsystem. Build (ctrl+B) this model.

• In the Control Desk layout, switch to the animation mode.

• Increase the reference speed and note the actual speed from the Display. Make

observations for the following values of reference speed: 500 rpm, 1200 rpm, 2000

rpm, and 3000 rpm.

8.5 Lab Report

• Tabulate the readings as per instructed in various sections. Draw the torque-speed

characteristic.

• Comment on the torque-speed characteristic in low frequency range (30-40Hz); why is

the voltage boost needed?

• Why does the V/f ratio need to be kept constant?

• Why there is an error in the reference speed and actual speed (you measured in 8.4.2) in

V/f speed-control? (Hint: open loop speed-control)

79

E x p e r i m e n t - 9 P e r m a n e n t M a g n e t A C ( P M A C ) M o t o r

9.1 Introduction

In this experiment, the vector control of a three-phase permanent magnet AC (PMAC) motor

will be studied. Real-time Simulink and layout files are given to perform the experiment. The

objective would be to understand how maximum electromagnetic torque from the motor is

achieved, when stator current space vector is maintained perpendicular to the rotor flux vector.

This angle is changed in real-time to verify the decrease in electromagnetic torque. Also to

verify the angle itself, first, the PMAC would be characterized to determine the value of

inductance, resistance of the motor and the back-EMF constant. These values would then be

used in the per-phase equation of motor to calculate and hence verify the angle between the

back-EMF and phase “A” current of the motor. The preliminary steps for the preparation of

vector control are given for reference. This is an advance material that can be used to

understand the Simulink file and layout file given, to perform the experiment.

9.2 Real-time Implementation

The objective is to run a PMAC motor (torque control) by using vector control. The encoder

(1000 line) attached to the PMAC motor gives an index pulse in each revolution. The index

pulse is used to maintain the rotor flux vector rB perpendicular to current space vector sI . The

angle θ between rB and sI can be changed in real-time; this allows the variation in

electromagnetic torque that can be observed by observing the speed of the motor at constant

load. The MATLAB Simulink file and the Control Desk files for this experiment will be

provided. The following sub-sections explain the operation of PMAC motor under vector

control.

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9.2.1 Preliminary steps to prepare the vector control of PMAC motor

1. Observation of index pulse: Run the motor as a generator; observe the output line

voltages. In the present case the index pulse comes when the back-emf Eab peaks. The

PMAC motor has 10 poles; hence the next index pulse will come after 5 electrical

cycles (i.e. one complete rotation of shaft), exactly when Eab peaks. This index pulse

will be used to maintain the value of 90θ °= in each rotation.

Eab

Index Pulse from Encoder

T=0

Position of Locking, when line AB is excited with DC

Fig. 9.1 Back-emf and Encoder Index Pulse

2. Determination of initial position of rB : At T=0, the position of rB is unknown. To

calculate the position of rB , a dc voltage of very low magnitude is injected in line AB.

This will result into the alignment of rB and stator flux space vector sψ . This is defined

as initial position (lock-position) T=0 shown in Fig. 9.1 and 9.2. It can be observed

from Fig. 9.2 that in the lock-position the flux linkage to the fictitious winding AB

peaks (positive) and hence the back-emf Eab will cross the origin from positive value to

a negative value. At this position, the d-axis or rB (in vector control the d-axis is

aligned with rB ) is / 6π− away (Fig. 9.3) from the physical a-axis winding. Hence at

T=0, [1] / 6daθ π= − . After locking the motor at known

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EabΨab

Ψm = Ψab = Ψs or,

Br vector

Position of Locking, when line AB is excited with DC

T=0

Fig. 9.2 Rotor Locking Position

O A

C

Eab=0, orposition of Br. orPosition of Ψm

When locking with Vab = 2V

T=0, Θda=0- π/6

q-axis

d-axis

B

Fig 9.3 Rotor Locking Position (Space Vector)

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position, at T=0+ current space vector sI is calculated using the value of Isq [1] and daθ

such that 90θ °= . In fact, this is done through a PI controller that generates the d-axis

and q-axis voltage magnitude. The magnitude and phasor of the required phase voltages

are calculated by using daθ , Vsd and Vsq [1].

3. Maintaining 90θ °= in each rotation of shaft: Refer to Fig. 9.1, the index pulse comes

after 360n + 270 (where n = 0-5) electrical degrees from T=0. Hence, when index pulse

is received, rB has moved in anti-clockwise direction by 270 electrical degrees or 270-

30 = 240 electrical degrees from physical a-axis winding. To align the d-axis with rB ,

the value of daθ at each index pulse is reset to 240 degrees, hence 90θ °= is maintained.

4. Assuring the correct direction of rotation and phase sequence: Before finding the

position of Eab in Fig. 9.3, the correct phase sequence ABC (anticlock) need to be

checked. This is the assumed direction of positive speed of rotation. The phase

sequence can be verified by injecting DC voltages in line AB, BC and CA, one-by-one.

The rotor should move forward in positive direction, each time the DC voltage is

applied to the line AB, line BC and line CA respectively.

9.2.2 On various blocks of real-time Simulink file

Subsystem: This contains the logic to maintain 90θ °= at starting and at each rotation of

shaft. The logic action is divided into two parts, 1) From T=0 to reception of first index

pulse and 2) After reception of first index pulse. In part-1, the position information is

calculated by integrating the frequency (from port-1). Part-1 uses the following blocks:

Integrator2 (initial condition -30 electrical degrees, as calculated before) and port-1.

The output of “Triggered Subsystem” dictates the end of part-1. When the first index

pulse is received, the system “Triggered Subsystem” is triggered, which changes the

initial output of “Triggered Subsystem”=0 to “Triggered Subsystem”=1 (see how to set

the initial output in Fig 9.4). Hence “theta_da” is taken from the top port of the

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“switch”, which is a continuous position information form encoder with reset value as

240 degrees.

(Note: Encoder outputs the encoder lines from 0-1000 as it makes a full rotation; Here 0

and 1000 is converted into 0 and 360 electrical degrees respectively. When the index

pulse is received, the encoder output is 0, this is added with reset angle value of

“Angle”=240 degrees to give the value of “theta_da”)

Fig 9.4 Maintaining 90θ °=

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Fig. 9.5 Simulink block-diagram

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1. Using “Enc position” from DS1104ENC_POS_C1:

o Connect the DS1104ENC_POS_C1, DS1104ENC_SETUP,

DS1104ENC_HW_INDEX_C1 and DS1104ENC_SET_POS_C1 as shown in

Fig. 9.5.

o Select “reset count at every pulse” in DS1104ENC_HW_INDEX_C1, this will

result in the output of “Enc position” of DS1104ENC_POS_C1 from 0-1000 in

every rotation.

o DS1104ENC_SET_POS_C1 enables the counter “Enc position” and reset the

counter to zero at every index pulse

o Note that, while debugging a gain of 1 should be connected to the output of

DS1104ENC_HW_INDEX_C1 and “Enc position”. This is a requirement to

enable these outputs while debugging the logic when these signals are not

connected to any other point in the system.

2. Current sensing: At the input of the current sensors put a first order filter of bandwidth

4 kHz. This is done to remove noise and proper starting of the system.

3. PI controller: While designing the controller, take into account the filter used in step-3.

The specification of current loop is as follows: Phase margin=82 degrees and

bandwidth = 450 Hz. Lower phase margin with high bandwidth has resulted into

significant oscillation at higher speed.

4. Switching frequency: The inductance and resistance of the motor under test is very low.

Switching frequency of 10 kHz (as used in previous experiments) will result into a very

high ripple in the motor current. This has resulted into significant oscillation in the

controller behavior. The switching frequency was increased to 50 kHz for this

experiment.

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5. Locking and starting the motor: When Lock=1 the output of the inverter terminal AB

will be a 2 V dc. This will bring the rotor to a known position at starting.

6. Activate_index: This deactivates the index pulse while locking the motor to a known

position. The deactivation is actually a pseudo deactivation, i.e. the input “Reset” of

“Subsystem” will not see a transition if an index pulse will come during the locking of

motor. This is required, otherwise, the starting angle will not be correct and the reset

logic will jump directly into part-2.

9.3 Running the Experiment

The circuit diagram for this experiment is the same as experiment-8 with the PMAC motor

replacing the induction motor.

9.3.1 Determination of sR and sL

• Determination of sR : Measure the line-line resistance of the induction motor with the

help of a multi-meter. ( )Measured line-line resistance value / 2sR =

• Determination of sL : sL can be determined by blocking the rotor using a dc-motor

operated under speed-control and applying a voltage across the synchronous motor

terminals. Determine sL as done in experiment-7.

9.3.2 Closed loop operation of PMAC motor

Operation of Permanent Magnet Synchronous Motor

• Start Control-desk and create a new experiment in the same working root as your

Simulink model file. Open the layout (*.lay) file provided and select “yes” to make data

connections. The layout should look as shown in Fig. 9.6.

• Uncheck Lock, Act index and Resetinteg. The functions of these variables have been

explained above.

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Fig. 9.6 Control-desk Layout

• Observe that the numerical display for Angle displays 4.189 which equals / 3π π+ , the

value to which the integrator resets every rotation.

• Now increase Isq_ref to 1.5 A. Under vector control, the torque is proportional to Isq.

Note down the speed of the machine. Observe the phase ‘A’ current and voltage

(derived from phase ‘A’ duty ratio, as in experiment-7).

• Calculate the back-emf of phase ‘A’ using the per-phase equivalent circuit [1]. Verify

that the phase ‘A’ current and back-emf are in phase.

• Change the angle value by 10o± using the numerical input Angle. Note down the speed

of the motor for the two cases.

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9.4 Lab Report

Include the following in your lab report:

• Values of sR and sL calculated in 9.3.1.

• Phasor diagram showing the phase ‘A’ voltage, current and back-emf.

• Give reasons for the change in speed of the motor when Angle is changed in 9.3.2.

9.5 References

[1] “ADVANCED ELECTRIC DRIVES Analysis, Control and Modeling using Simulink” by

Ned Mohan, 2001, MNPERE.

89

A p p e n d i x – A S a f e t y P r e c a u t i o n s a n d P o w e r -E l e c t r o n i c s - D r i v e s - B o a r d f a m i l i a r i z a t i o n

1.1 Why is safety important?

Attention and adherence to safety considerations is even more important in a power electronics

laboratory than it’s required in any other undergraduate electrical engineering laboratories.

Power electronic circuits can involve voltages of several hundred volts and currents of

several tens of amperes. By comparison the voltages in all other teaching laboratories rarely

exceed 20V and the currents hardly ever exceed a few hundred milliamps.

In order to minimize the potential hazards, we will use dc power supplies that never exceed

voltages above 40-50V and will have maximum current ratings of 20A or less. Most of the time

we will use dc supplies of 20V or less and 1 A or less output current capability. However in

spite of this precaution, power electronics circuits on which the student will work may involve

substantially larger voltages (up to hundreds of volts) due to the presence of large inductances

in the circuits and the rapid switching on and off of amperes of current in the inductances. For

example a boost converter can have an output voltage that can theoretically go to

infinite values if it is operating without load. Moreover the currents in portions of some

converter circuits may be many times larger than the currents supplied by the dc supplies

powering the converter circuits. A simple buck converter is an example of a power electronics

circuit in which the output current may be much larger than the dc supply current.

1.2 Potential problems presented by Power Electronic circuits

• Electrical shock may take a life.

• Exploding components (especially electrolytic capacitors) and arcing circuits can cause

blind- ness and severe burns.

90

• Burning components and arcing can lead to fire.

1.3 Safety precautions to minimize these hazards

1.3.1 General Precautions

• Be calm and relaxed, while working in Lab.

• When working with voltages over 40V or with currents over 10A, there must be at least

two people in the lab at all times.

• Keep the work area neat and clean.

• No paper lying on table or nearby circuits.

• Always wear safety glasses when working with the circuit at high power or high

voltage.

• Use rubber floor mats (if available) to insulate yourself from ground, when working in

the Lab.

• Be sure about the locations of fire extinguishers and first aid kits in lab.

• A switch should be included in each supply circuit so that when opened, these switches

will de-energize the entire setup. Place these switches so that you can reach them

quickly in case of emergency, and without reaching across hot or high voltage

components.

1.3.2 Precautions to be taken when preparing a circuit

• Use only isolated power sources (either isolated power supplies or AC power through

isolation power transformers). This helps using a grounded oscilloscope and reduces the

possibility of risk of completing a circuit through your body or destroying the test

equipment.

91

1.3.3 Precautions to be taken before powering the circuit

• Check for all the connections of the circuit and scope connections before powering the

circuit, to avoid shorting or any ground looping that may lead to electrical shocks or

damage of equipment.

• Check any connections for shorting two different voltage levels.

• Check if you have connected load at the output.

• Double check your wiring and circuit connections. It is a good idea to use a point-to-

point wiring diagram to review when making these checks.

1.3.4 Precautions while switching ON the circuit

• Apply low voltages or low power to check proper functionality of circuits.

• Once functionality is proven, increase voltages or power, stopping at frequent levels to

check for proper functioning of circuit or for any components is hot or for any electrical

noise that can affect the circuit’s operation.

1.3.5 Precautions while switching off or shutting down the circuit

• Reduce the voltage or power slowly till it comes to zero.

• Switch of all the power supplies and remove the power supply connections.

• Let the load be connected at the output for some time, so that it helps to discharge

capacitor or inductor if any, completely.

1.3.6 Precautions while modifying the circuit

• Switch Off the circuit as per the steps in section 3.5.

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• Modify the connections as per your requirement.

• Again check the circuit as per steps in section 3.3, and switch ON as per steps in section

3.4.

1.3.7 Other Precautions

• No loose wires or metal pieces should be lying on table or near the circuit, to cause

shorts and sparking.

• Avoid using long wires, that may get in your way while making adjustments or

changing leads.

• Keep high voltage parts and connections out of the way from accidental touching and

from any contacts to test equipment or any parts, connected to other voltage levels

• When working with inductive circuits, reduce voltages or currents to near zero before

switching open the circuits.

• BE AWARE of bracelets, rings, metal watch bands, and loose necklace (if you are

wearing any of them), they conduct electricity and can cause burns. Do not wear them

near an energized circuit.

• Learn CPR and keep up to date. You can save a life.

• When working with energized circuits (while operating switches, adjusting controls,

adjusting test equipment), use only one hand while keeping the rest of your body away

from conducting surfaces.

93

1.4 Power-Electronics-Drives-Board familiarization

The drives board which we use in the Electric Drives Laboratory has been designed to enable

us to perform a variety of experiments on AC/DC machines. The main features of the board

are:

• Two completely independent 3-phase PWM inverters for complete simultaneous

control of two machines

• 42 V dc-bus voltage to reduce electrical hazards

• Digital PWM input channels for real-time digital control

• Complete digital/analog interface with dSPACE board

The basic block diagram of drives board is shown in Fig. 1 and the actual drives board is shown

in Fig. 2. Please note that various components on the board are indicated in Table. 1.

1.4.1 Inverters

Each 3-phase inverter uses MOSFETs as switching devices. The 3-phase outputs of the first

inverter are marked A1 (D-6 in Fig. 2), B1 (E-6 in Fig. 2), C1 (F-6 in Fig. 2) and those of the

second inverter are marked A2 (I-6 in Fig. 2), B2 (K-6 in Fig. 2), C2 (L-6 in Fig. 2).

1.4.2 Signal Supply

±12 volts signal supply is required for the isolated analog signals output form the drives board.

This is obtained from a wall-mounted isolated power supply, which plugs into the DIN

connector J90 (B-2 in Fig. 2). Switch S90 (C-2 in Fig. 2) controls the signal power to the board.

The green LED D70 (C-2 in Fig. 2) indicates if the signal supply is available to the board.

Fuses F90 (C-2 in Fig. 2) and F95 (B-2 in Fig. 2) provide protection for the +12 V and −12 V

supplies respectively. Please note that the green LED indicates the presence of only the +12 V

supply. Please note that turning off S90 will not stop the PWM signals from being gated to the

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inverters. The power supply for the 3-phase bridge drivers for the inverters is derived from the

DC Bus through a flyback converter (A-2 in Fig. 2).

Table 1: Locations of components on drives board No. Component Ref. Des. Location in Fig. 2

1 Terminal +42 J1 A-4

2 Terminal GND J2 A-3

3 Terminal PHASE A1 J3 D-6

4 Terminal PHASE B1 J4 E-6

5 Terminal PHASE C1 J5 G-6

6 Terminal PHASE A2 J6 J-6

7 Terminal PHASE B2 J7 K-6

8 Terminal PHASE C2 J8 L-6

9 DIN connector for ±12 V signal supply J90 B-2

10 Signal supply switch S90 C-2

11 Signal supply +12 V fuse F90 C-2

12 Signal supply-12 V fuse F95 B-2

13 Signal supply LED D70 C-2

14 MOTOR1 FAULT LED D66 D-2

15 MOTOR2 FAULT LED D67 L-2

16 DIGITAL POWER LED D68 I-2

17 MAIN POWER LED D69 B-3

18 Inverter 1 D-3 to G-4

19 Inverter 1 I-3toL-4

20 DC Link capacitor of Inverter 1 C1 B-5

21 DC Link capacitor of Inverter 2 C2 G-5

22 Driver IC IR2133 for Inverter 1 U1 E-2

23 Driver IC IR2133 for Inverter 2 U3 J-2

24 Digital Supply Fuse F2 G-1

25 dSPACE Input Connector P1 H-1 and I-1

26 RESET switch S1 L-1

27 Phase A1 current sensor (LEM) CS2 C-5

28 Phase B1 current sensor (LEM) CS3 D-5

29 Phase A2 current sensor (LEM) CS5 H-5

30 Phase B2 current sensor (LEM) CS6 J-5

31 DC link current sensor (LEM) CS1 L-5

32 VOLT DC BNC5 B-4

33 CURR A1 BNC1 B-3

34 CURR B1 BNC2 C-3

35 CURR A2 BNC3 H-3

36 CURR B2 BNC4 I-3

95

Fig 1 Block Diagram of Electric Drives Board

1.4.3 Voltage Measurement

Test points are provided to observe the inverter output voltages. BNC connector VOLT DC (B-

4 in Fig. 2) has been provided to sense the DC bus voltage. To measure the DC bus voltage,

• Connect a BNC cable to VOLT DC BNC connector.

• The scaling factor of input voltage is 1/10.

1.4.4 Current Measurement

LEM sensors are used to measure the output current of the inverters. Only A and B phase

currents are sensed. The C phase current can then be calculated using the current relationship

Ia+Ib+Ic = 0, assuming that there is no neutral connection for the machines. The calibration of

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the current sensor is such that for 1 A current flowing through the current sensor, output is 0.5

V. To measure the output current of phase A of inverter 1,

• Connect BNC connector to CURR A1 (B-3 in Fig. 2).

• To measure the output current of phase B of inverter 1,

• Connect BNC connector to CURR B1 (C-3 in Fig. 2).

To measure the output current of phase A of inverter 2,

• Connect BNC connector to CURR A2 (H-3 in Fig. 2),

• To measure the output current of phase B of inverter 2,

• Connect BNC connector to CURR B2 (I-3 in Fig. 2).

1.4.5 Inverter Drive Circuit

The inverters are driven by 3-phase bridge drivers (IR2133). The PWM inputs are isolated

before being fed to the drivers.

1.4.6 PWM/Digital Signals

PWM and other digital signals for the board are to be given to the 37-pin DSUB connector (H-

1 in Fig. 2). For pin-out of the connector, see Table 2.

1.4.7 Fault Protection

The Drives Board consists of over-current protection for each inverter. An over-current fault

occurring on inverter 1 is indicated by red LED ”MOTOR FAULT 1” (D-2 in Fig. 2), while

that of inverter 2 is indicated by red LED ”MOTOR FAULT 2” (L-2 in Fig. 2). Each time a

fault occurs, reset the fault using the ”RESET” switch (L-1 in Fig. 2) on the board. All the

faults are reset by this switch.

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Table 2: 37-pin DSUB Connector No. Pin Number Description

1 GND Digital Digital ground

2 FAULT 1 Inverter 1 Fault output. Fault Signal high

3 NC Not Connected


5 NC Not Connected


7 PWM A1 A phase PWM signal of Inverter 1

8 PWM B1 B phase PWM signal of Inverter 1

9 PWM C1 CphasePWMsignalofInverter1

10 PWM A2 A phase PWM signal of Inverter 2

11 PWM C2 C phase PWM signal of Inverter 2





16 SD1 Shutdown signal for Inverter 1

17 FLTCLR-IN Clear Fault signal

18 VCC

19 VCC


21 FAULT 2 Inverter 2 Fault output. Fault Signal high

22 NC Not Connected

23 NC Not Connected

24 NC Not Connected


26 NC Not Connected

27 NC Not Connected

28 NC Not Connected

29 PWM B2 CphasePWMsignalforInverter2





34 NC Not Connected

35 SD2 Shutdown signal for Inverter 2



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Fig 2 Power Electronics Drives Board

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