Type C Chopper working Tutorial

In this post let us see the operation, working of the Class C chopper. Before proceeding further it is highly recommended to read about the four quadrant operation of the motor and principle of Chopper Operation posts.

Type-C or Class-C Chopper: An Introduction

The type C chopper is the combination of Class A chopper and Class-B chopper.
It operates in two quadrants (First and Second Quadrant) of the V_O- I_O plane.
The following figure shows the quadrants of operation of class C chopper.
Class C chopper can perform the rectifier operation and inverter operation.
In the first quadrant forward motoring takes place and in the second quadrant forward regenerative braking operation takes place.
Average load voltage is always positive but average load current may be positive or negative.
So power flow may be from source to load (first quadrant) or from load to source (2^nd quadrant).
Class C chopper configuration is used for motoring and regenerative braking of DC motors.

The chopper circuit diagram is shown in below figure.
Here
When T1 or D2 conduct, the output current and output voltage will be always positive and the chopper operates in the first quadrant.
When the T2 or D1 conduct, the load current I_O is negative and the chopper operates in the second quadrant.
In this chopper switches T1 and T2 are turned ON and OFF alternatively.
But the Switches(T1, T2) should not be simultaneously ON at any instant, as this would lead to direct short-circuit of the supply V_S.
The working of type C chopper can be understood by considering four modes.

Mode-1: Switch T1 turned ON

When T1 is turned ON the load current and load voltage are positive and the motor receives power from the supply.

Mode-II: Diode D2 ON
Class_C_Mode_2_Operation

When the switch T1 is turned ON, the energy stored in the inductor L is Ldi/dt.
The inductance L forces the load current to flow through diode D2.
It will happen till the value of Ldi/dt becomes equal to the back EMF of the motor and the load current becomes zero.

Mode-III: Switch T2 ON
Class_C_Mode_3_Operation

At this point, if the gate signal of T2 is available, the back emf of the motor forces current in the opposite direction through L and T2.
This continues until T2 is turned OFF and T1 ON.

Mode-IV: Diode D1 ON

Class_C_Mode_4_Operation

When T2 is OFF, the energy stored in the inductance forces current through diode D1 to the supply Vs.
The input current becomes negative.
Thus energy flows from load to source(forward regenerative braking operation).
During this period, switch T1 can not conduct due to reverse bias condition.
But, when the input current reduces to zero, the switch T1 starts to conduct (provided the gate signal of T1 is still available).
Now both the load and input current becomes positive. ie Mode-I repeats...

Points to remember about the Type C Chopper:

When both the average load voltage and average load current are positive, the motor receives power from the supply.
When the average load voltage is positive and average load current negative, the motor operates in the braking mode.
For higher duty ratio of switch T1 the average output voltage is greater than the back emf(Eb) of the motor. Thus the chopper operates in the driving mode.
For lower duty ratio, the average output voltage becomes less than the motor back emf(Eb). Switch T1 and diode D2 can not conduct. Switch T2 and diode D1 conduct forcing the load energy back to the supply. Thus the motor operates in the regenerative mode.

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