Zero-Switching of Transformers

When inductive loads such as transformers or motors are switched on, a sudden rush of current into the winding is observed. This is known as the inrush current. Typically, the inrush current is around 5 to 6 times of the rated value. This is due to the absence of the back-emf when the winding when the power is first applied. This sudden surge of current causes disturbances in the system voltage and sometimes spurious operation of relays. The high current also causes stress on the windings of the machines.

There are a number of methods to address this problem. Adding resistors in series to the winding and then gradually taking them out of circuit is one option. Another option is the use of softstarters which raise the terminal voltage of the machines gradually. Zero crossover switching is one method of addressing the issue of high inrush current when switching on inductive equipment. The method involves the use of Static Relays consisting of devices such as SCR or TRIACs. These devices switch on when the sinewave crosses the zero point so that the voltage is gradually increased. This, in some circumstances, reduce the inrush current.

However, this methods has its downsides too. There have been reports that if the zero cross over switching is carried out on a core that is already saturated from previous operation, extremely high currents can result.

Apart from inductive loads, inrush currents is also observed in resistive loads such as filament lamps. In filament lamps the resistance when the filament is in the cold, that is, switched off condition is lesser than the resistance when the lamp is in the switched on condition. This is due to the positive temperature co-efficient of resistance. When the lamp is switched on from the cold condition, there is a high surge of current which continues till the temperature of the resistance increases. Zero-crossover switching in resistive loads can ensure a smoother increase in current value to the steady-state condition.

This ensures a smoother increase in the current to the steady-state value.