Selection of Ratings of Devices of a Chopper Converter:
Chopper Converter – The waveforms of currents and voltages are useful in deciding the ratings of devices. The main thyristor carries the load current. Superimposed on this there is a current pulse during the recharging process of the capacitor through the resonating circuit. Therefore the peak value of the main thyristor current is roughly thrice the load current. The maximum conduction time of the current of the main thyristor occurs when a = 1. Based on these facts, the current rating of the main thyristor can be decided. Besides, it must be an inverter grade thyristor with a small turn off time, which helps while using a small capacitor.
The free wheeling diode conducts load current. Its period of conduction is a maximum and is equal to the chopper period when the time ratio a = 0. It must also be a fast diode, since normal diodes cause increased switching losses. The auxiliary thyristor conducts only for a short period which is independent of chopper frequency (80 to 200 μs). The peak value of the auxiliary thyristor current is twice the load current.
A reactor may be placed in series with the thyristor for protection against high values of di/dt. In case the line inductance is not sufficient for this purpose it should be supplemented with an additional inductance. The thyristor T3 (replacing the diode D2) need not be of inverter grade. Its conduction time is 70 to 100 μa and it carries a current of peak value equal to the main thyristor current minus the load current. The diode D4 conducts only pulses of current, for the same period as the auxiliary thyristors. The devices T2, T3 and T4 are loaded for short intervals of time.
Application of DC Chopper
Choppers are used in drives requiring loss free, efficient speed control with the possibility of regenerative braking. They can be used with reversible drives. Chopper fed dc motors are used in traction. They enjoy smooth starting, acceleration and braking. They can also be used to obtain a variable resistance from a fixed resistance, which is called a chopper controlled resistance. We shall now discuss the principles of chopper controlled resistance.