U16 APPLICATIONS OF POWERELECTRONICSU16 – 2 Control of direct current (dc) motorsLecture - 08

SUB UNIT OUTLINE

Speed control of dc motor: armature voltage; field weakening; dc choppers; controlled rectifiers; closed loop; tachogenerator; reversal; braking; waveforms

Torque control of dc motor: armature current control loops; speed reversal; braking; single quadrant; four quadrant operation; regeneration

Industrial dc motor controller: preparation and interpretation of circuit and block diagrams; setting of parameters (eg min/max speed, ramp up/down time, current/torque limits)

applications (eg process control, mills, pumps, CNC machinery); specification and selection

EQUIVALENT CIRCUIT OF DC MOTOR

(01)

(02)

Fig 01

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FOUR-QUADRANTS OF THE TORQUE-SPEEDPLANE

Assumption,• Flux φf is kept constant.• The motor is initially driving a load at a

speed of ωm.

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FOUR-QUADRANTS OF THE TORQUE-SPEEDPLANE CONT...

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FOUR-QUADRANTS OF THE TORQUE-SPEEDPLANE CONT... Generated energy must be somehow absorbed by

the source of vt or dissipated in a resistor. During breaking operation, (please go next slide)

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FOUR-QUADRANTS OF THE TORQUE-SPEEDPLANE CONT...

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FOUR-QUADRANTS OF THE TORQUE-SPEEDPLANE CONT... By changing the terminal voltage polarity we can

reverse the direction of rotation. Therefore, a dc motor can be operated in either

direction and its electromagnetic torque can be reversed for breaking, as shown in four quadrants of the torque-speed plane.

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EFFECT OF THE ARMATURE CURRENTWAVEFORM

Output of power converter contain an ac ripple voltage superimpose on the desired dc voltage.

Ripple terminal voltage ripple in the armature current

Therefore form factor of the armature current,

Input power to armature , because Loss in the resistance of armature winding Because

(03)

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EFFECT OF THE ARMATURE CURRENTWAVEFORM CONT... When power factor loss in the motor Motor

efficiency , because (with the assumption

of Фf is constant. If there ripple in current there will be ripple in

instantaneous electromagnetic torque hence fluctuation in speed. Se we need to reduce ripple in armature current.

High frequency ripple in torque will result in smaller speed fluctuation, as compare with a low frequency torque ripple of the same magnitude.

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CLOSED LOOP POSITION/SPEED DC SERVODRIVERS

They have feedback path for controlling decision making.

Commonly used in servo applications.

Closed-loop position/speed dc servo drive11

TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE

If there exist small deviation around their steady-state values;

(04)

(05)

(06)

(07)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT... Take the Laplace transform of equation (04), (05),

(06) and (07)

(08)

(09)

(10)

(11)

(12)13

TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT... By combining equation (08), (09), (10), (11), and

(12);

(13)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT...

Block diagram representation of the motor and load (without any feedbac)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT... Equation (13) results in two closed loop transfer

functions:

(14)

(15)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT... Simplification,

• Usually friction is small. It will be neglected by setting B=0 in equation (14)

• Analyze motor without load. Then ‘J’ term in equation (14) will consist only the motor inertia Jm.

(16)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT...

Lets define the terms,• Mechanical time constant

• Electrical time constant

(17)

(19)

(20)

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TRANSFER FUNCTION MODEL FOR SMALL-SIGNAL DYNAMIC PERFORMANCE CONT...

Generally , approximation; by

Therefore,

(21)

(22)

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ELECTRICAL TIME CONSTANT ( ) determines how quickly the armature current

built up, in response to a step change in the terminal voltage.

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MECHANICAL TIME CONSTANT ( ) determines how quickly the speed built up, in

response to a step change in the terminal voltage.

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POWER ELECTRONIC CONVERTER

Power electronic converter supplying a dc motor should have following characteristics.

• Should allow both its output voltage and current to reverse in order to allow four-quadrant operation.

• Should be able to operate in a current-controlled mode by holding the current at its maximum acceptable value during fast acceleration and deceleration.

• To ensure the accurate control of position, the average output of them should vary with its control input, independent of the load on the motor. 22

POWER ELECTRONIC CONVERTER CONT...• To minimize the fluctuation in torque and

speed of the motor, converter should have good form factor.

• The converter output should respond as quickly as possible to its control input.

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RIPPLE IN ARMATURE CURRENT

If the inertia of the motor is sufficient enough, we can assume that, instantaneous speed at the steady-sate operation is constant.

Where,

(23)

(24)

(25)

(26)24

RIPPLE IN ARMATURE CURRENT CONT...

As we know Ra, is very law. So La primarily determine the armature ripple current.

(27)

(28)

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RIPPLE IN ARMATURE CURRENT CONT... For a PWM bipolar voltage switching;

• Ripple voltage is maximum when the average output voltage is zero.

• All switches operate at equal duty ratios.

(29)

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RIPPLE IN ARMATURE CURRENT CONT... For a PWM unipolar voltage switching;

• Ripple voltage is maximum when the average output voltage is half of input dc voltage.

(30)

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A DC MOTOR SERVO DRIVE; 4-QUADRANTOPERATION

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A DC MOTOR SERVO DRIVE; 4-QUADRANTOPERATION CONT... Diode rectifier: To rectify line frequency ac input Filter capacitor: To smooth output of rectifier

bridge Energy absorption circuit: To prevent the filter

capacitor voltage from becoming large in case of braking of the dc motor.

If we ignore the effect of blanking time, the average voltage output of the converter varies linearly with the input control voltage, independent of the load.

(31) 29

RIPPLE IN ARMATURE CURRENT CONT... From equation (29) and (30),

Therefore La and fs must be selected very carefully.

La can be increased by adding an external inductor in the series with the motor armature.

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EXERCISE

1. Discuss the following block diagram which is used to control of servo drives.

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EXERCISE

2. Discuss the following block diagram which is used to control of servo drives.

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EXERCISE

3. Discuss the difference between two block diagram given earlier.

4. Define the term blanking time and discuss the effect of blanking time.

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SELECTION OF SERVO DRIVE PARAMETERS

Ripple in the armature current, which causes torque ripple and additional armature heating, is proportional to La/fs.

The dead zone in the transfer function of the converter, which degrades the servo performance, is proportional to .

Switching loss in the converter are proportional to .

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ADJUSTABLE-SPEED DC DRIVES

Response time to speed and torque command is not critical.

Can be used either;1. Switch-mode dc-dc converters2. Line frequency controlled converters

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SWITCH-MODE DC-DC CONVERTER

If we need four-quadrant operation, then full-bridge converter is used.

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SWITCH-MODE DC-DC CONVERTER CONT... If the speed does not have to reverse but breaking is

needed, then the two-quadrant converter can be used.

Only single switch is on at a given time. Which keep the output voltage independent of the

direction of ia. We can reverse the armature current. That negative

Ia corresponding to breaking mode of operation. At the breaking mode, power flows from the dc

motor to Vd.37

SWITCH-MODE DC-DC CONVERTER CONT... We can control magnitude of the output voltage

Vt. But it always remaining unipolar. Since ia flow in either direction, it will not

become discontinuous for two-quadrant operation.

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SWITCH-MODE DC-DC CONVERTER CONT... We can use single-quardrant operation where the

speed remaining unidirectional and breaking is not required.

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LINE-FREQUENCY CONTROLLEDCONVERTERS

Also called phase-controlled converters. There exist a armature ripple that is a multiple

of 50 Hz line frequency. Disadvantage: the longer dead time in

responding to the changes in the speed control signal.

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LINE-FREQUENCY CONTROLLEDCONVERTERS CONT... Once a thyristor or pair of thyristors is triggered on

bellow circuit, the delay angle α that controls the controller output voltage applied to the motor terminal cannot be increased for a potion of 50Hz cycle.

Although output of the power converter can reverse polarity , the current through the converter is unidirectional.

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LINE-FREQUENCY CONTROLLEDCONVERTERS CONT... When we need regenerative breaking, two back-

to-back connected thyriastor converter can be used.

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LINE-FREQUENCY CONTROLLEDCONVERTERS CONT... We can make four-quadrant operation using

single phase control converter together with two pair of contactors.

Motor operation: M1 and M2 are closed and R1 and R2 are opened.

Inverter mode: M1 and M2 are opened and R1 and R2 are closed.

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EFFECT OF DISCONTINUOUS ARMATURECURRENT

Output current of the motor can be discontinuous at light on the motor if we use following power converter to control the motor,

1. The line-frequency phase-controlled converter,

2. The single-quadrant step-down switch-mode dc-dc motor.

Because of the discontinuous in output current; output voltage increases then motor speed increases for the fixed control voltage vcontrol and delay angle α.

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EFFECT OF DISCONTINUOUS ARMATURECURRENT CONT...

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OPEN LOOP CONTROL

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OPEN LOOP CONTROL CONT... No feedback path. Control command generated by comparing the driver

output with its desired value. e.g.: Temperature in capacity modulated heat pump. d/dt limiter;

• Allows speed command to change slowly.• So rotor current does not exceed its rating.• Work as protective device, when current exceed

its rated value, the controller shut the drive off.• d/dt limit can be adjustable to match the motor-

load inertia.47

HOMEWORK

1. Find ten applications of dc motors.2. What are the specification of dc motors?3. How we select the motor for specific application

and what are the major factors we need to consider?

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