Many advances have been made in the design of control circuits associated with on-load tap changing. Mention has already been made of driving mechanisms and the fundamental circuits associated with the starting of the motor for carrying out a tap change. While these vary from one maker to another they are comparatively simple. In general, the motor is run up in one direction for a ‘raise’ tap change and in the reverse direction for a ‘lower’ tap change.
In some cases a brake is employed to bring the motor to rest while in others clutching and declutching are carried out electrically or mechanically. It is, however, the initiation of the tap change and the control of transformers operating in parallel where the main interest lies and where operational problems can arise if the tap changers are ‘out of step’.