class choppers

CLASSIFICATION OF CHOPPERSChoppers are classified as follows

Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper

CLASS A CHOPPER

V

C hopper

FW D

+

v 0

v 0

i0

i0

LOAD

V

Fig. 2.14: Class A Chopper and Characteristic

Figure 2.14 shows a Class A Chopper circuit with inductive load and free-wheeling diode. When chopper is ON, supply voltage V is connected across the load i.e.,

and current i0 flows as shown in figure. When chopper is OFF, v0 = 0 and the

load current continues to flow in the same direction through the free wheeling diode.

Therefore the average values of output voltage and current i.e., and are always positive. Hence, Class A Chopper is a first quadrant chopper (or single quadrant chopper). Figure 2.15 shows output voltage and current waveforms for a continuous load current.

O utpu t cu rren t

T hyristo rgate pu lse

O utpu t vo ltage

ig

i0

v 0

t

t

ttO N

T

C H O N

FW D C onduc ts

Fig. 2.15: First quadrant Chopper - Output Voltage and Current Waveforms

Class A Chopper is a step-down chopper in which power always flows from source to load. It is used to control the speed of dc motor. The output current equations obtained in step down chopper with R-L load can be used to study the performance of Class A Chopper.

CLASS B CHOPPER

V

C hopper

+

v 0

v 0

i0

i0

L

E

R

D

Fig. 2.16: Class B ChopperFig. 2.16 shows a Class B Chopper circuit. When chopper is ON, and E

drives a current through L and R in a direction opposite to that shown in figure 2.16. During the ON period of the chopper, the inductance L stores energy. When Chopper is

OFF, diode D conducts, and part of the energy stored in inductor L is returned to

the supply. Also the current continues to flow from the load to source. Hence the average output voltage is positive and average output current is negative. Therefore Class B Chopper operates in second quadrant. In this chopper, power flows from load to source. Class B Chopper is used for regenerative braking of dc motor. Figure 2.17 shows the output voltage and current waveforms of a Class B Chopper.

The output current equations can be obtained as follows. During the interval diode ‘D’ conducts (chopper is off) voltage equation is given by

V

i0

V 0

R

L

E

+

-

D C onduc ting

For the initial condition i.e., at .

The solution of the above equation is obtained along similar lines as in step-down chopper with R-L load

Therefore

At

During the interval chopper is ON voltage equation is given by

i0

V 0

R

L

E

+

-

C hopperO N

Redefining the time origin, at .

The solution for the stated initial condition is

At

Therefore

O utpu t cu rren t

D con d u cts C h o pp e r

con d u cts

T hyristo rgate pu lse

O utpu t vo ltage

ig

i0

v 0

t

t

t

Im in

Im ax

T

tO NtO F F

Fig. 2.17: Class B Chopper - Output Voltage and Current Waveforms

CLASS C CHOPPERClass C Chopper is a combination of Class A and Class B Choppers. Figure 2.18

shows a Class C two quadrant Chopper circuit. For first quadrant operation, is ON

or conducts and for second quadrant operation, is ON or conducts. When

is ON, the load current is positive. i.e., flows in the direction as shown in figure 2.18.

The output voltage is equal to and the load receives power from the

source.

V

C hopper

+

v 0

D 1

D 2C H 2

C H 1

v 0i0

i0

L

E

R

Fig. 2.18: Class C Chopper

When is turned OFF, energy stored in inductance L forces current to flow

through the diode and the output voltage , but continues to flow in positive

direction. When is triggered, the voltage E forces to flow in opposite direction

through L and . The output voltage . On turning OFF , the energy stored

in the inductance drives current through diode and the supply; output voltage the input current becomes negative and power flows from load to source.

Thus the average output voltage is positive but the average output current

can take both positive and negative values. Choppers and should not be turned ON simultaneously as it would result in short circuiting the supply. Class C Chopper can be used both for dc motor control and regenerative braking of dc motor. Figure 2.19 shows the output voltage and current waveforms.

G ate pu lseof C H 2


O utput cu rren t

O utpu t vo ltage

ig 1

ig 2

i0

V 0

t

t

t

t

D 1 D 1D 2 D 2C H 1 C H 2 C H 1 C H 2

O N O N O N O N

Fig. 2.19: Class C Chopper - Output Voltage and Current WaveformsCLASS D CHOPPER

V+ v 0

D 2

D 1 C H 2

C H 1

v 0

i0

L ER i0

Fig. 2.20: Class D ChopperFigure 2.20 shows a class D two quadrant chopper circuit. When both and

are triggered simultaneously, the output voltage and output current flows

through the load in the direction shown in figure 2.20. When and are turned

OFF, the load current continues to flow in the same direction through load, and

, due to the energy stored in the inductor L, but output voltage . The average load

voltage is positive if chopper ON-time is more than their OFF-time and

average output voltage becomes negative if . Hence the direction of load current is always positive but load voltage can be positive or negative. Waveforms are shown in figures 2.21 and 2.22.



O utpu t curren t

O utpu t vo ltage

Average v 0

ig 1

ig 2

i0

v 0

V

t

t

t

t

C H ,C HO N1 2 D 1,D 2 Conducting

Fig. 2.21: Output Voltage and Current Waveforms for



O utpu t curren t

O utpu t vo ltage

Average v 0

ig 1

ig 2

i0

v 0

V

t

t

t

t

C HC H

1

2

D , D1 2

Fig. 2.22: Output Voltage and Current Waveforms for

CLASS E CHOPPER

V

v 0

i0L ER

C H 2 C H 4D 2 D 4

D 1 D 3C H 1 C H 3

+

Fig. 2.23: Class E Chopper

v 0

i0

C H - C H O NC H - D C onduc ts

1 4

4 2

D D2 3 - C onductsC H - D C onduc ts4 2

C H - C H O NC H - D C onduc ts

3 2

2 4

C H - D C onduc tsD - D C onducts

2 4

1 4

Fig. 2.23(a): Four Quadrant Operation

Figure 2.23 shows a class E 4 quadrant chopper circuit. When and are

triggered, output current flows in positive direction as shown in figure 2.23 through

and , with output voltage . This gives the first quadrant operation.

When both and are OFF, the energy stored in the inductor L drives through

and in the same direction, but output voltage . Therefore the chopper operates in the fourth quadrant. For fourth quadrant operation the direction of battery must be reversed. When and are triggered, the load current flows in

opposite direction and output voltage .

Since both and are negative, the chopper operates in third quadrant. When

both and are OFF, the load current continues to flow in the same direction

through and and the output voltage . Therefore the chopper operates in

second quadrant as is positive but is negative. Figure 2.23(a) shows the devices which are operative in different quadrants.

EFFECT OF SOURCE AND LOAD INDUCTANCEIn choppers, the source inductance should be as small as possible to limit the

transient voltage. Usually an input filter is used to overcome the problem of source inductance. Also source inductance may cause commutation problem for the chopper. The load ripple current is inversely proportional to load inductance and chopping frequency. Therefore the peak load current depends on load inductance. To limit the load ripple current, a smoothing inductor is connected in series with the load.

class choppers

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