Voltage Commutated Chopper Explained

Voltage commutated chopper is one of the simplest and earliest chopper circuit. This chopper is generally used in high power circuit where load fluctuation is not very large. This chopper is also known as Parallel Capacitor Turn Off Chopper, Impulse Commutated Chopper or Classical Chopper.

Working Principle of Voltage Commutated Chopper:

The working principle of voltage commutated chopper is explained using the circuit diagram shown below. This circuit comprises of one main thyristor T1 and one auxiliary thyristor TA. Auxiliary thyristor TA, capacitor C, diode D and inductor L basically forms the commutation circuit. Free-wheeling diode FD is connected across the RLE type load.

Working of this chopper can only start if the capacitor C is charged with polarities as marked in Figure-1. This can be achieved in one of the tow ways as mentioned below:

• By closing switch S so that capacitor C gets directly connected to source V_s. Once the switch S is closed, capacitor gets charged up to source voltage Vs through R_c.
• Auxiliary thyristor TA is triggered so that C gets charged through Vs, C, TA and load. The charging current through C decays and as it reaches ZERO, v_c = V_s and TA is turned off.

With capacitor C charged with the polarities as marked in Figure-1, the chopper circuit is ready for its operation. The current i_c, i_T1, i_fd and i_o are taken positive in the arrow direction marked. Similarly, the voltages v_c, v_T1, v_TA and v_o across C, T1, TA and load are taken as positive with the polarities marked.

Simplifying assumption for this chopper are:

• Load current is constant, and
• Thyristors and diodes are ideal elements

The chopper operation is divided into four modes for the sake of convenience and better understanding.

Mode-I:

In this mode, the main thyristor T1 is triggered at t=0 and RLE load gets connected to the source V_s so that load voltage v_o = V_s. During this mode, there are two current paths as shown in figure-2 below.

Load current I_o, assumed constant, constitute one path and commutation current i_c the other path. Load current i_o flows through source V_s, main thyristor T1 and load whereas the capacitor current i_c flows through oscillatory circuit formed by C, T1, L and D. The capacitor current first rises from zero to a maximum value when the voltage across the capacitor is zero. As i_c decreases to zero, the voltage across C becomes equal to the source voltage with reverse polarity i.e. v_c = -V_s. This voltage across the capacitor is held constant at (-V_s) by diode D.

The voltage across auxiliary thyristor TA is equal to the negative of voltage across capacitor. Therefore, when voltage across capacitor is zero, the same becomes zero across TA. However, when v_c = -V_s, the voltage across TA becomes V_s.

Mode-II:

During Mode-II, the voltage across capacitor C is (-Vs) and that across TA is V_s. The capacitor charging current i_c is zero. Only main thyristor T1 continues to conduct during this mode and hence the chopper output voltage is equal to source voltage V_s. This simply means that chopper is ON during this mode. This mode is shown in figure-3.

Mode-III:

When main thyristor T1 is to be turned off, auxiliary thyristor TA is triggered. Once TA is triggered, the voltage across the main thyristor becomes equal to (-V_s) and current drops to zero as it begins to flow through TA. Thus, the main thyristor T1 gets turned off. Since, capacitor voltage is used here for commutation of main thyristor, it is called voltage commutated chopper.

After T1 is turned off, load current begins to flow through C and TA. This is shown in figure-4 below.

The load voltage is sum of source voltage and voltage across the capacitor i.e. V_o = 2V_s. However, it decreases linearly as the voltage across capacitor, stars decreasing. When capacitor gets charged to V_s, the load voltage becomes ZERO and load current drops to zero.

Mode-IV:

During this mode, the load current is zero, capacitor and auxiliary thyristor current are zero and the voltage across main thyristor T1 is Vs. Auxiliary thyristor TA is reversed biased due to voltage across capacitor and current through TA is zero. Therefore, TA gets naturally commutated. As capacitor is slightly overcharged, free-wheeling diode gets forward biased and load current free-wheels through FD and load as shown in figure-5 below.

During this mode, i_c = 0, i_T1 = 0, i_fd = I_o, v_T1 = V_s, v_o = 0 and i_TA = 0. In this mode, the chopper is OFF.

After this mode, the main thyristor T1 is triggered again and the flour modes as explained above repeats.

Disadvantage:

Voltage commutated chopper is simple, it has therefore been used extensively. It, however, suffers from flowing disadvantages:

• A starting circuit is required to charge the capacitor.
• Load voltage at once rises to 2V_s at the instant commutation of main SCR is initiated. Free-wheeling diode is therefore subjected to twice the supply voltage.
• This chopper cannot work on no load. This is because at no load, capacitor would not get charged from (-V_s) to V_s when auxiliary thyristor is triggered for commutation of main thyristor.