D-G motor losses with rectifier operation

THE USE OF grid-controlled rectifiers for the supply of controlled power to d-c machinery has raised the problem of the increased losses in motors used in this way. The output of a grid-controlled rectifier has a large ripple voltage, the waveform of which varies with the firing delay caused by the grid control. This ripple causes alternating currents to flow in the motor which do not add to the rotational power produced but which cause additional copper loss. In addition, another component of flux is created in the magnetic circuit which causes additional hysteresis and eddy current losses.