Harmonic reduction in adjustable-speed synchronous motors

Adjustable-speed synchronous motors (ASSM) are very popular in high-power h igh-speed applications. The Chilean mining industry uses them in milling op erations in ranges of several thousands hp. Although a high pulse number fo r the inverter would be beneficial, it never goes beyond 12, due to complic ated connections. Thus, a distorted harmonic-rich current feeds the synchro nous motor, increasing the inverter commutation angle and causing additiona l rotor-heating and pulsating electric torque. Harmonic suppression by filt ers is not viable due to the variable frequency nature of the motor. In thi s paper a new design for the inverter, without complicated circuitry, is pr oposed, so that it works with 36 pulses and then the motor is fed with a hi ghly sinusoidal current. Also, the same technique is applied to the rectifi er end, so that an effective harmonic reduction is carried out at both ends of the system, i.e., the motor as well as the ac supply system. A 20-KVA l aboratory development drive system has been built and experimental waveform s for the conventional (12-pulse) and proposed (36-pulse) configurations ar e included.