Power converters

Although AC drives have almost completely taken over from choppers for propulsion duties in the field of railway traction, there still remain important applications - rheostatic braking, refurbishment of camshaft equipment and in auxiliary systems - where the chopper continues to be an essential technology at the disposal of the designer . Four-quadrant PWM converters, despite their complexity and drawbacks, offer such good power-factor and posphometric current control that they have become the preferred type of AC-to-DC converter on 15 kV and 25 kV railways