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Class B Chopper
Class B Chopper is a
step-up chopper
• When
chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure.
• 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 i
s returned to the
supply.
• Average output voltage is positive. 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 brak
ing of dc motor.
Figure: 3.12 circuit diagram and quadrant operation of Type B chopper
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Figure: 3.13 Output voltage and current waveforms of type B chopper
Class C chopper
Class C Chopper can be used as a step-up
or step-down chopper
• Class C Chopper
is a combination of Class A and Class B Choppers.
• For first quadrant operation, CH1 is ON or D2 conducts.
•
For second quadrant operation, CH2 is ON or D1 conducts.
• When CH1 is ON, the load current is positive.
• The output voltage is equal to ‘V’ & th
e load receives power from the source.
• When CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2
and the output voltage is zero.
• Current continues to flow in positive direction.
• When CH2 is triggered, the volta
ge E forces current to flow in opposite direction through L and CH2 .
• The output voltage is zero.
• On turning OFF CH2 , the energy stored in the inductance drives current through diode D1 and the
supply
• Output voltage is V, the
input current becomes n
egative and power flows from load to source.
• Average output voltage is positive
• Average output current can take both positive and negative values.
• Choppers CH1 & CH2 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.
