SPEED CONTROL OF INDUCTION MACHINE WITH REDUCTION IN...

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International Journal of Advanced Research in Engineering and Technology

(IJARET) Volume 7, Issue 2, March-April 2016, pp. 78–90, Article ID: IJARET_07_02_008

Available online at

http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=7&IType=2

ISSN Print: 0976-6480 and ISSN Online: 0976-6499

© IAEME Publication

___________________________________________________________________________

SPEED CONTROL OF INDUCTION

MACHINE WITH REDUCTION IN TORQUE

RIPPLE USING ROBUST SPACE-VECTOR

MODULATION DTC SCHEME

Yeshoda Harish Kumar

M.Tech, Power Electronics, Manipal University Jaipur, India

Vishnu Goyal

Assistant Professor, EEE Department,

Manipal University Jaipur, India

ABSTRACT

In this paper a novel and simple algorithm for three-phase induction

motor(IM) under Direct Torque Control (DTC) scheme using Classic DTC

switching table for dynamic torque ripple reduction and space-vector

modulation scheme for steady state torque and flux control is proposed. The

proposed scheme having the advantages of low torque ripples as well as

constant switching frequency.

Simulation results are given to prove the ability of the proposed method

obtaining good speed control bandwidth while overcoming classic DTC and

DTC-SVM drawbacks.

Key words: Direct Torque Control, Induction Motor, Space-Vector

Modulation, Torque Ripple Minimization.

Cite this Article: Yeshoda Harish Kumar and Vishnu Goyal. Speed Control

of Induction Machine with Reduction In Torque Ripple Using Robust Space-

Vector Modulation DTC Scheme. International Journal of Advanced

Research in Engineering and Technology, 7(2), 2016, pp. 78–90.

http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=7&IType=2

1. INTRODUCTION

The basic concept of DTC of AC motor drives is to control both stator flux linkage

and the electromagnetic torque of the machine simultaneously. Since a DTC-based

drive system select the inverter switching states using switching table, neither current

controllers nor pulse-width modulation (PWM) modulator is required, as shown in

Figure.1, thereby introducing fast torque dynamics response in comparison with the

Speed Control of Induction Machine with Reduction In Torque Ripple Using Robust Space-

Vector Modulation DTC Scheme


field oriented vector control technique. However, this conventional DTC approach has

some disadvantages such as; high torque ripple and variable switching frequency,

which is varying with speed, load torque and the selected hysteresis bands. On the

other hand to reduce the torque ripple; the hysteresis torque and flux controller bands

must be reduced to match the required torque performance, which requires reduction

of the system sampling time and it is necessary to use a very fast processing

controller. Although the system sampling frequency can be increased in the

conventional DTC the inverter switching frequency is still low, approximately less

than one third of the sampling frequency [1, 2]. The inverter switching frequency can

be increased using a dithering signal, by adding a limited amplitude high frequency

signal to the torque and flux error signals [3]. Although the switching frequency is

increased it is still variable for small error bands. Other research concerns with these

disadvantages using multilevel inverter, there are more voltage space vectors

available to control the flux and torque [4]. However, these approaches require more

power devices, which increase the cost of the system and make it more complex. In

[5, 6] discrete space vector modulation DTC approach is used to reduce the torque

ripple. However, there is a complexity of selecting the additional hysteresis

controllers. On the other hand, space vector modulation (SVM) modulator is

incorporated with direct torque control for Induction Motor drives to provide a

constant inverter switching frequency and low torque ripple, a predictive PI controller

is used to calculate the command voltage vector [7].

In this paper, a new and simple DTC algorithm with fixed switching frequency for

Induction Motor is proposed to reduce the torque ripples. The well-developed SVM

technique is applied to inverter control in the proposed DTC-based Induction Motor

drive system, thereby dramatically reducing the torque ripples. The simulation results

are carried out by modelling the drive systems using a Matlab/Simulink.

2. CLASSICAL DIRECT TORQUE CONTROL (DTC)

OPERATION.

The block diagram of classical DTC proposed by I. Takahashi and T. Nogouchi is

presented in Figure.1.

Figure 1 Block Diagram of Classical-DTC

Yeshoda Harish Kumar and Vishnu Goyal


The stator flux amplitude and the electromagnetic torque

are the reference

signals which are compared with the estimated values respectively. The flux and torque errors are delivered to the hysteresis controllers. The digitized output

variables and the stator flux position sector selects the appropriate voltage vector from

the switching table. Thus, the selection table generates pulses to control the power

switches in the inverter.

The stator flux vector and the torque produced by the motor can be estimated

respectively. These equations only require the knowledge of the previously applied

voltage vector Vs, measured stator current Is, stator resistance Rs, and the motor poles

number p [5].

The components of stator flux are given by:

(1)

(2)

The magnitude of the stator flux can be estimated by:

(3)

The flux vector zone can be obtained using the stator flux components. The

electromagnetic torque can be calculated by:

(4)

For the flux is defined two-level hysteresis controller and for the torque three-

level hysteresis controller is used.

The output signals are defined as:

Where is the total hysteresis bandwidth of the controller. The actual stator

flux is constrained within the hysteresis band and tracks the command flux. The

torque control loop has three levels of digital output represented by the following

conditions.

The above considerations allow construction of the selection table as presented in

below Table-1.

Table I Switching Scheme of Basic-DTC

S(1) S(2) S(3) S(4) S(5) S(6)

1

1

0

-1

-1

1

0

-1




The classical DTC method can be characterized as follows:

Advantages: simple structure, no coordinate transformation, no separate voltage

modulation block, no current control loops, very good flux and torque dynamic

performance,

Disadvantages: Variable switching frequency, problems during starting and low

speed operation, high torque ripples, flux and current distortion caused by stator flux

vector sector position change and high sampling frequency is required for digital

implementation.

3. DTC-SVM SCHEME

In the DTC-SVM system, the same flux and torque estimator which is used for the

conventional DTC is still used in the proposed DTC scheme as shown in Figure.2.

Instead of the switching table and hysteresis controllers, an optimal voltage vector

calculator is used to calculate the reference voltage vector as a function of the torque

and flux errors. The applied voltage vectors and their duration times are selected and

calculated using the SVM modulator. From the block diagram of the DTC-SVM

shown in Fig.4, it is seen that this scheme have the most important advantage over

conventional DTC, such as at steady state time of the torque response the SVM is

applied to insure a ripple reduction occurring at the steady state torque response[7].

Figure 2 Block Diagram of DTC-SVM Scheme

The procedure for implementing a two-level space vector PWM can be summarized

as follows:

1. Calculate the angle and reference voltage vector based on the input voltage

components.

2. Find the sector in which lies and the adjacent space vectors of Vk and Vk + 1

based on the sector angle .

3. Find the time intervals , and the angle .



3.1. Voltage Vector Calculation

In this modulation technique the three phase quantities can be transformed to their

equivalent 2-phase quantity in stationary frame. A reference voltage vector V that

rotates with angular speed in the plane represents three sinusoidal waveforms

with angular frequency in the abc coordinate system. The magnitude of reference vector used for modulating inverter output is,

(5)

The phase angle is evaluated from,

(6)

3.2. Sector Determination

It is necessary to know in which sector the reference output lies in order to determine

the switching time and sequence. The identification of the sector where the reference

vector is located is straightforward. The phase voltages correspond to eight switching

states: six non-zero vectors and two zero vectors at the origin. Depending on the

reference voltages and , the angle of the reference vector can be used to

determine the sector as per Table.2.

Table II Voltage Vector Sector Determination

Sector Angle (Degrees)

1

2

3

4

5

6

3.3. Time Durations T0, T1, T2

Times T1 and T2 denote the required on-time of the active-state vectors during each sample period, and k is the sector number denoting the reference location.

The calculated times T1 and T2 are applied to the switches to produce space vector

PWM switching patterns based on each sector. The switching time is arranged

according to the first half of the switching period while the other half is a reflection

forming a symmetrical pattern. T0 denote time of the null state vectors.

Assuming that the reference voltage and the voltage vectors and are constant during each pulse-width modulation period Ts and splitting the reference

voltage into its real and imaginary components (V or and V) gives the following

result:

(7)




The inverse matrix of above is used to calculate as

(8)

4. PROPOSED DTC BY MEANS OF BOTH SWITHCHING

SCHEME AND SVM

From the block diagram of the proposed DTC shown in Figure.3, it is seen that the

proposed scheme have the most important advantage of the conventional DTC, such

as fast torque dynamics, by applying the conventional DTC during the step change in

the torque command; then at steady state time of the torque response the SVM is

applied to insure a ripple reduction occurring at the steady state torque response.

The proposed DTC scheme can be further explained as follows; at starting up or at

the instance of the load change, almost when the torque error is large, the switching

state selector selects the switching states of the conventional DTC switching table to

get a fast torque response at starting and at a step change in the load torque command.

When the torque error is within the hysteresis band limits, the switching state selector,

select the switching states generated by the SVM which guaranty that the switching

frequency is fixed and the ripples at the steady state torque response are reduced.

Figure 3 Block Diagram of Proposed-DTC Scheme



5. SIMULATION RESULTS

The system performance under the proposed DTC strategy is evaluated, and the

results are compared with the results obtained by the conventional DTC and DTC-

SVM schemes. MATLAB/SIMULINK models were developed to examine the

conventional and the proposed DTC algorithms. In simulation, the sampling time is

20 µs for both the conventional and proposed DTC schemes and the switching time

for the proposed DTC is 100 µs. The switching table used in [1] is employed for the

conventional DTC. The switching delays and the forward drop of the power switches,

the dead time of the inverter and the non-ideal effects of the Induction motor are

neglected in the models. The torque dynamics performances of the conventional and

proposed DTC schemes at 700 rpm are compared as shown in Figure.5 under the

same operation conditions. It is seen that the torque ripple is reduced by more than 50

% at no-load and load conditions in the proposed DTC.

The speed and torque response for the classical DTC algorithm is given in Fig. 4

(b) and the Fig. 4 (c) show the torque response for the DTC algorithm with DTC-

SVM Scheme, and finally the proposed DTC algorithm torque response is given in

Fig. 4 (d). It can be seen from the figure that the proposed DTC has achieved a fast

speed response than that for the conventional DTC. The speed dynamic response and

their corresponding torque response for three algorithms more declaration is shown in

Figure.5 and Figure.7. It is seen that the torque ripple reduction is still valid at low

and high speed ranges. A speed reverse dynamics is carried out at two different speed

references, +700 rpm and -700 rpm, to indicate the validity of the controller operation

at both speed directions. The simulation results for the speed reversing dynamics are

shown in Figure.7.

Also a comparative simulation results between the proposed DTC algorithm and

other algorithms are shown in Figure.5 and Figure.6, where a reference speed of 700

rpm is applied to the motor then, a load torque of 7 Nm is applied at t = 0.75 sec.

4(a) Reference Speed and Load Torque




4(b) Classical-DTC Scheme

4(c) DTC-SVM Scheme

4(d) Proposed DTC Scheme

Figure 4 Speed and Torque Responses for Three Different DTC Schemes



5(a) Classical-DTC scheme

5(b) DTC-SVM Scheme

5(c) Proposed-DTC Scheme

Figure 5 Speed-Torque Dynamics with Change in Speed (0 to 700 rpm)

6(a) Classical-DTC Scheme




6(b) DTC-SVM Scheme


Figure 6 Speed-Torque Response with Applied Load (7N-m)

7(a) Classic-DTC Scheme



7(b) DTC-SVM Scheme


Figure 7 Speed-Torque Dynamics with Speed Reversal (0 to -700rpm)

The Stator-Flux trajectory of Classic-DTC scheme is shown in Figure.8 (a) and

Figure.8 (b) shows the Stator-Flux trajectory of DTC-SVM scheme, finally Stator-

Flux trajectory of proposed-DTC scheme is shown in Figure.8 (c). It clearly shows

that the Proposed-DTC has better Flux control over Classical and SVM DTC

schemes.

8(a) Classic-DTC 8(b) DTC-SVM 8 (c) Proposed-DTC

Figure 8 Stator-Flux Circular Trajectory of Three DTC Schemes




The Stator-Current characteristic with change in load (7 Nm to -7Nm) is shown in

Figure.9. It clearly shows that the Proposed-DTC scheme has fast and better stator

current response due to change in load.

Figure 9 Stator Current Characteristics with Load Reversal (7Nm to-7Nm)

6. CONCLUSION

In this paper a new DTC algorithm for the 3-Phase Induction Machine has been

developed. The effectiveness of the proposed DTC algorithm in reducing the torque

ripples was verified by computer simulations on a 3-phase Induction Motor rated at 4

kW. The presented results have demonstrated that the problems which were

associated with conventional DTC schemes such as, high torque ripples and variable

switching frequency have been completely avoided. It is also interesting that the

proposed torque controller has been shown to be robust to control the machine flux,

allowing satisfactory overall performance of the machine under study. The developed

algorithm also offers the prospect for optimizing the machine performance in a

manner similar to conventional vector controllers with better toque dynamics.



APPENDIX

Parameters of the used Induction Machine are given in below Table.3.

Table III Induction Motor Parameters

Nominal Power 4000W

Frequency 50Hz

Number of Pole Pairs 2

Stator Resistance 1.405ohm

Rotor Resistance 1.395ohm

Stator Inductance 5.839mH

Rotor Inductance 5.839mH

Mutual Inductance 172.2mH

REFERENCES

[1] L.Zhong, and M.F.Rahman, Analysis of direct torque control in Permanent

magnet synchronous motor drives, IEEE Trans. Power Elec., 12(3), May

1997.

[2] S. Dan, F.Weizhong, and H.Yikang, Study on the direct torque control of

permanent magnet synchronous motor drives, 5th

International Conference on

Electrical Machines, ICEMS, 2001.

[3] T. Noguchi, M. Yamamoto, S. Kondo, I. Takahashi, Enlarging switching

frequency in direct torque controlled inverter by means of dithering, IEEE

Transaction on Industry Applications,1999.

[4] L.X.Tang, M.F.Rahman, Y.Zhiqing, and T.Cun, In-depth research on direct

torque control of permanent magnet synchronous motor, IECON 02

Industrial Electronics Society, IEEE 28th annual Conference, 2002.

[5] Brahim Metidji, Lotfi Baghli, and Seddik Bacha, Low-Cost Direct Torque

Control Algorithm for Induction Motor without AC Phase Current Sensors,

IEEE Transactions on Power Electronics, 27(9), September 2012.

[6] M.Ramaprasad Reddy, T.Brahmananda Reddy and B.Brahmaiah, Discrete

Space Vector Modulation Algorithm Based Vector Controlled Induction

Motor Drive, Journal of Electrical Engineering.

[7] Anjana Manuel and Jebin Francis, Simulation of Direct Torque Controlled

Induction Motor Drive by Using Space Vector Pulse width Modulation for

Torque Ripple Reduction, IJAREEIE, 2(9), September 2013.

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