Harmonic reduction in adjustable-speed synchronous motors

Adjustable-speed synchronous motors (ASSM) are very popular in high-power high-speed applications. The Chilean mining industry uses them in milling operations in ranges of several thousands hp. Although a high pulse number for the inverter would be beneficial, it never goes beyond 12, due to complicated connections. Thus, a distorted harmonic-rich current feeds the synchronous motor, increasing the inverter commutation angle and causing additional rotor-heating and pulsating electric torque. Harmonic suppression by filters is not viable due to the variable frequency nature of the motor. In this paper, a new design for the inverter, without complicated circuitry, is proposed, so that it works with 36 pulses and then the motor is fed with a highly sinusoidal current. Also, the same technique is applied to the rectifier 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 laboratory development drive system has been built and experimental waveforms for the conventional (12-pulse) and proposed (36-pulse) configurations are included.