Operating D-C motors from controlled rectifiers

THE D-C output voltage of a typical rectifier has more ripple than the output of a d-c generator. The ripple increases as the number of rectifier phases is decreased. It also increases as the rectifier voltage is reduced by phase control. The voltage ripple affects the performance of d-c motors in a number of respects including heating, commutation, and speed regulation, particularly at light loads and reduced voltage. In recent times the extension of controlled-rectifier drives to d-c motors in the range of 100 horsepower or more has raised the question of modifying the rating of these larger motors when operated from rectifiers with phase control. In order to evaluate the effect of a polyphase rectifier supply on a medium or large sized motor, tests were made on a 400-horsepower 375/750-rpm 230-volt d-c stabilized shunt-wound force-ventilated motor. Tests were made with both 6-phase and 3-phase rectifier supplies, and the results were compared with similar tests using a d-c generator as a source.