Performance evaluation of three-phase variable-speed DC drive systems with uniform PWM control

Abstract

Pulse width modulation (PWM) control of DC drive systems is considered as a challenge to the conventional phase-controlled systems. In this paper, the performance characteristics of three-phase single-quadrant variable-speed DC motor drive systems with uniform PWM (UPWM) control are evaluated over the entire range of operation under the popular fan-type load characteristics. This includes the harmonic contents of the load and line currents. An experimentally verified mathematical model is used for this purpose. The solution procedure of the system is arranged for given loading conditions, which is highly necessary from the practical point of view. The impacts of the chopping frequency on the performance characteristics of the system, including the harmonic contents of the load and line currents, are thoroughly investigated and compared with those achieved by increasing the armature time-constant. It is shown that almost all the performance parameters of the load and line sides are improved as the chopping frequency increases. Also, the chopping frequency, when increased, is shown to have advantageous effects on both the amplitudes and the orders of the load and line harmonics. It reduces the amplitudes of the lower-order harmonics and simplifies the filtration requirements of the dominant higher-order harmonics. However, when the chopping frequency increases beyond a certain level, which can be considered as a function of the motor armature time-constant, it is shown that its influence becomes almost negligible and hence practically unjustified.