Control of DC Motors by Choppers · PRINCIPLE OF CHOPPER OPERATION • A chopper is a high speed...

Control of DC Motors by Choppers

Dr. D G Padhan PSD 1

DC Chopper

• a static power electronic device that converts fixed dc

input voltage to a variable dc output voltage

• considered as dc equivalent of an ac transformer since

they behave in an identical manner

• used all over the world for rapid transit systems

• used in trolley cars, marine hoist, forklift trucks and mine

haulers

• offer smooth control, high efficiency, faster response and

regeneration facility

• The power semiconductor devices used for a chopper

circuit can be force commutated thyristor, power BJT,

MOSFET and IGBT 2 Dr. D G Padhan PSD

• GTO based chopper are also used

• These devices are generally represented by a switch.

• When the switch is off, no current can flow.

• Current flows through the load when switch is “on”.

• The power semiconductor devices have on-state voltage

drop of 0.5V to 2.5V across them.

• For the sake of simplicity, this voltage drop across these

devices is generally neglected

3 Dr. D G Padhan PSD

PRINCIPLE OF CHOPPER OPERATION

• A chopper is a high speed “on" or “off” semiconductor

switch

• It connects source to load and disconnect the load from

source at a fast speed.

• In this manner, a chopped load voltage is obtained from

a dc supply of constant magnitude



• During the period Ton, chopper is on and load voltage is

equal to source voltage Vs.

• During the period Toff, chopper is off, load voltage is

zero. In this manner, a chopped dc voltage is produced

at the load terminals



CONTROL STRATEGIES

• The average value of output voltage Vo can be

controlled through duty cycle by opening and closing the

semiconductor switch periodically. The various control

strategies for varying duty cycle are as following:

1. Time ratio Control (TRC)

2. Current-Limit Control.


Time ratio Control (TRC)

• Time ratio Ton/T(duty ratio) is varied.

• 1. CONSTANT FREQUENCY SYSTEM

on-time is varied but chopping frequency f is kept constant.

Variation of Ton means adjustment of pulse width, as such

this scheme is also called pulse-width-modulation scheme.

• 2. VARIABLE FREQUENCY SYSTEM

the chopping frequency f is varied and either (i) on-time

Ton is kept constant or (ii) off-time Toff is kept constant.

This method of controlling duty ratio is also called

Frequency-modulation scheme.


CURRENT- LIMIT CONTROL

• the on and off of chopper circuit is decided by the

previous set value of load current. The two set values

are maximum load current and minimum load current.

• When the load current reaches the upper limit, chopper

is switched off. When the load current falls below lower

limit, the chopper is switched on. Switching frequency of

chopper can be controlled by setting maximum and

minimum level of current.



Types of choppers


Step down chopper


When S is ON, e0 is equal to Edc. When S is OFF, eo is equal to

zero.

Step up chopper





• The choppers are also classified based on

their regions of operation

• Single Quadrant chopper

1. Type A Chopper

2. Type B Chopper

• Two Quadrant Chopper 1. Type C Chopper

2. Type D Chopper

• Four Quadrant Chopper 1. Type E Chopper


First quadrant chopper or Type A chopper


• The equivalent circuit is shown below





Second quadrant or type-B chopper


• If the chopper (or switch S) is turned ON and turned OFF

at regular intervals the average voltage E0 is +ve and I0

is –ve.

• The stored energy in the rotor is converted into electrical

energy and fed back into the system.

• This is equivalent to regenerative braking


• During the period TON the switch S ON and hence e0 =0.

The equivalent circuit is shown in Fig.4.6(a)






Two-quadrant chopper or Type C chopper

T1 conducts va = Vdc

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc


Two-quadrant converter

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

D2 conducts va = 0 T1 conducts va = Vdc

Quadrant 1 The average voltage is made larger than the back emf Dr. D G Padhan PSD 35


Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

T2 conducts va = 0

Quadrant 2 The average voltage is made smaller than the back emf, thus

forcing the current to flow in the reverse direction



Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

D1 conducts va = Vdc


Two quadrant or Type C Chopper









Two quadrant chopper- Type D Chopper














59

Chopper fed DC drive

Four-quadrant Chopper

+ Va Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q1 & Q4 is ON

Current Flow : Vdc + _ Q1 _Motor_Q4_Vdc

-

Current Ia & Va are positive

Operates in First Quadrant

Forward Motoring

Va

Ia

60



+ Va Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q1 is OFF & Q4 is ON. Inductor current has to flow in the

same Direction.

Diode D2 is FB

Inductor Current freewheels through D2 & Q4

Output Voltage is Zero

61



+ Va Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q4 is OFF. Q2 is ON. Load is not connected with the source.

Back Emf drives the current through Q2 & D4

62



+ Va Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q2 is OFF. Diode D1 is FB

Current flows through D4 & D1

Current Ia is negative & Va is positive.

Operates in second quadrant.

Va

Ia

Forward Braking

63



- Va + Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q3 & Q2 is ON

Current Flow : Vdc + _ Q3 _Motor_Q2_Vdc

-

Current Ia & Va are negative

Operates in third Quadrant

64



- Va + Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q3 is OFF. Q2 is ON. Current has to be continuous.

Diode D4 is FB

Current flows through Q2,D4 & (Eb,La,Ra)

65



- Va + Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q4 Is ON & Q2 is OFF.

Back emf Drives the current through Q4 – D2 - MOTOR

66



- Va + Q1

Q2

Q3

Q4

D1 D3

D4 D2

+

Vdc

Q4 IS Turned Off,

D3 is FB

Current Flows through Va+ - D3 – D2 – Va-

Va is negative. But current Ia is positive

Operates in fourth quadrant

67

Four Quadrant Chopper or Type E Chopper

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70

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72

73

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75

76

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Control of DC Motors by Choppers · PRINCIPLE OF CHOPPER OPERATION • A chopper is a high speed...

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