Performance Of A Separately Excited DC Motor Fed From A Multiphase Chopper

A dc chopper is normally employed for the speed control of a dc motor (separately excited or series) by varying the armature voltage. The output voltage and current of chopper possess harmonics, which affect the performance of the motor to a large extent. Current Limit Control when employed requires high frequency switching to limit the ripple to a desired extent. This switching introduces switching losses and requires the thyristors of very small turn off time. A Time Ratio Control on the other hand requires a large inductance to reduce harmonics and also to reduce the discontinuous conduction. A Multiphase Chopper where several choppers are operated in parallel with a phase shift is very effective in reducing the ripple content of voltage waveform and improving the performance of the dc motor. Further the system will be advantageous for large power applications with good reliability and without the need for operating the devices in parallel thereby increasing the device capability. The present paper deals with the analysis of a separately excited dc motor when fed from two choppers with a phase shift, operating from a dc supply. A Simple and eminent approach based on the state Space model of the do motor is presented. The effect of variation of the phase shift between the choppers on the performance of the dc motor is investigated in detail. The improvement in the performance is found to be very good even at small switching frequency.