Adjustable speed synchronous motors. I. System harmonic reduction

Abstract

Adjustable speed synchronous motors are very popular in large power applications. The Chilean mining industry use 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 transformer connections. Thus, a highly-distorted harmonic-rich current feeds the synchronous motor, increasing the inverter commutation angle and causing additional rotor-heating and pulsating electric torque. 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 system.