Operation of Large D-C Motors From Controlled Rectifiers

The d-c output voltage of a typical power 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 retard. The voltage ripple affects the performance of d-c motors in the following respects: 1. Heating. 2. Commutation. 3. Speed regulation, particularly when current is discontinuous. D-c motors constitute the major part of the load of general purpose power rectifiers. It is a matter of experience that d-c motors in the fractional horsepower and small integral horsepower ranges must be derated when they are operated over a wide speed range from rectifiers with voltage control. The factors which result in derating of these motors are present to a very limited extent in general purpose rectifiers, which usually have six or more phases and are seldom operated with more than 15 per cent phase control. Hence little consideration has been given to the derating of motors connected to such rectifiers. 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.