Working of Type C chopper

Two Quadrant Choppers Type C

Q. Classify chopper based on output voltage and quadrants of operation. List any four applications of chopper.

Q. Classify choppers based on quadrants of operation. Sketch only circuit diagram of each.

Q. With the help of neat circuit diagram and necessary waveforms, explain class C chopper.

Q. Draw and explain the operations of class C chopper.

Type C chopper

• The two quadrant chopper configurations can be obtained by connecting the single quadrant choppers of type A and type B in parallel. Figure (a) shows type C chopper.

  

  Figure (a)

Operation of type C chopper

   Refer to Figure (a). SCR S₁ and D₁ form a type A chopper whereas SCR S₂ and D₂ form a type B chopper.

1. Mode I (S₁ turned ON)

• As soon as switch S₁ is turned on, the DC supply gets connected across the load making the load voltage positive (V₀ = + V).
• The load current flows from the dc supply to the load. Thus load voltage and load current both are positive and load receives power from the source.

2. Mode II (S₁ turned OFF/ D₁ starts conducting)

• As soon as S₁ is turned off, due to sudden interruption of load current, a negative self induced emf (L di / dt) appears across the load inductance.
• This will forward bias diode D₁ and freewheeling takes place through it.
• After sometime the freewheeling current reduces to zero, and D₁ turns off. Load voltage is zero and load current is positive in this mode of operation.

  

  Table

3. Mode III (D₁ turns OFF/ S₂ turned ON)

• S₂ can be turned on only after D₁ goes out of conduction. S₂ can be turned on due to the presence of the back emf E_b in the circuit. [Refer Table].
• The load voltage is zero and load current becomes negative in this mode of operation.

4. Mode IV (S₂ turned OFF/ D₂ starts conducting)

• D₂ comes into conduction as soon as S₂ is turned off.
• The stored energy is returned back. The motor acts as a generator. The load voltage is positive (v_o = V) but load current is negative in this mode of operation.
• Table summarizes the operation of type C chopper.
• Care must be taken to ensure that the two switches S₁ and S₂ are not turned on simultaneously, otherwise the dc supply V will be short circuited.
• A class C chopper can either work in first or second quadrant as shown in Figure (b).

  

  Figure (b)

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