Variable-voltage diode convertor rolling-stock transformers to control DC traction motors

The first generation of traction transformers used on railway vehicles in the UK, which went into service in the early 1960s, transformed the 50 Hz overhead supply voltage of 25 kV or 6.25 kV down to an acceptable voltage for diode convertors supplying DC traction motors. They also had to vary the output voltage by providing voltage tappings on the transformer windings, selected by on-load tapchangers. They were originally required to supply mercury arc rectifiers but these were replaced by silicon rectifiers, which made transformer operating conditions less arduous. After more than 25 years service they still perform reliably, provided that their on-load tapchangers are maintained regularly and simple routine maintenance checks are carried out