A Comparative Investigation of a Three-Phase Induction Machine Drive with a Component Minimized Voltage-Fed Inverter under Different Control Options

Possibilities for minimizing component cost in variable- speed drives with frequency control by solid-state converter are investigated. It is shown that in a three-phase system voltage and frequency control of a three-phase squirrel-cage machine may be attained by PWM of the output of a simplified bridge inverter with only four switches. As the phase voltages are in principle asymmetrical, this technique is called phase asymmetric PWM (PAPWM). The operational region of PAPWM control is investigated theoretically in the frequency plane, compared to PWM control of a six-element bridge, and shown to have application possibilities. The system was experimentally tested on a 1O-kVA four-transistor inverter and experimentally compared to the operation of a six-transistor inverter of the same rating. The entire signal electronics subsystem was also minimized regarding components by using a specially dedicated IC circuit developed for PWM applications in three-phase systems. These results prove that a PAPWM system with a B4 bridge can be operated to give the same operational drive characteristics as a B6 bridge with PWM. The price to be paid for using a bridge with only four switches lies in the higher pulse frequencies required to give the PAPWM the same characteristics as a conventional system. Since transistor switches may be operated at high switching frequencies with low switching losses, the system is well suited to transistor inverter control of induction machine drives.