Voltage commutated chopper - Engineering notes

Voltage Commutated Chopper

Voltage Commutated Chopper circuit diagram

• Similar to step down chopper.
• T1 = Main thyristor, TA = Auxiliary thyristor, L,C = commutating components, Rc = charging resistor
• Assume output current is constant.
• Close the switch, initially capacitor short circuited, after 4 - 5 time constants, Vc = Vs.
• At t = 0, T1 is on, load is connected across the supply Vo = Vs.
• Tank circuit starts conduction ( diode forward bias).
• After conduction polarities across capacitor are changed.
• D is reverse biased polarities across capacitor are changed.
• Upto t2 we completed now we have to turn off the main thyristor.

• Make TA on, T1 to be off (applying reverse voltage).
• To make the conduction continues use free wheeling diode.
• In order to make the output  continuous, the existing path will be changed as Vs, C, TA and the load.
• Voltage across the capacitor changes.
• Now make the voltage across capacitor > Vs.
• Free wheeling diode conducts, output voltage becomes zero.
• To start next cycle, no need to close switch 's'.
• A reverse voltage is applied across conducting SCR due to which current through SCR becomes zero and it is getting off. Hence it is called voltage commutation.
• Other name of this is impulse commutation. It is because a high reverse voltage will turn off the SCR.

Limitations of voltage commutated chopper:

1. A starting circuit is required.
2. load voltage at once rises to 2Vs at the instant commutation of main SCR is initiated.
3. It can't work at no load. It is because at no load, capacitor would not get charged frm -Vs to Vs when auxiliary SCR is triggered for commutating the main SCr.
4. Main thyristor is required to carry current more than load current. So, it is to be over rated.

Voltage Commutated Chopper Waveform

• The values of commutating components C and L can be obtained.
• The values depend upon turn off time of main thyristor T1. during tc capacitor voltage changes from -Vs to zero linearly.

ic = C dV / dt  for a constant load current Io.

Io = C . Vs / tc

C = Io . tc / Vs

• The commutation circuit turn off time tc must be greater than thyristor turn off time.

• Load current should not be too large.