Performance analysis of self-controlled synchronous motors considering the effects of magnetic saturation

Abstract

A self-controlled synchronous motor, which has similar characteristics to a conventional dc motor, is being used in various industrial fields as an adjustable speed motor. The inductances in the motor are changed with the change in the air-gap flux due to magnetic saturation as in the field-weakening region. A new and simple formula representing the inductances (i.e. reactances) in which the effect of magnetic saturation is taken into account is proposed in this paper, and the validity of the formula is shown for the transient performances of a tested motor when the field voltage was stepped down. The effects of the magnetic saturation on the steady-state and dynamic performances are then clarified for a motor having average constants of synchronous machines. It is shown that the steady-state motor speed for small values of dc input current when the magnetic saturation is considered is larger than that for constant parameters. It is also clarified that the field-weakening speed control regions for constant-power driving become narrow if the motor has a property of considerable magnetic saturation. The transient responses when dc input and field voltages are changed, respectively, are finally investigated, and it is shown that if magnetic saturation is considered, the commutation limit of inverter thyristors for a step-down change in the field voltage is widened when compared with the case for constant parameters.