The effect of magnetic saturation on the d.c. dynamic braking characteristics of a.c. motors

Abstract

It is shown that the effect of magnetic saturation on the d.c. dynamic braking characteristics of a.c. motors can be determined by a mathematical method of analysis which uses the net value of the equivalent alternating magnetizing current, rather than the fractional speed, as the independent variable of functions expressing the values of the braking torque and secondary-circuit current. The method is applicable to both asynchronous and synchronous polyphase machines, and to asynchronous single-phase motors. It is noted that the deep-bar type of squirrel-cage induction motor, in which the rotor resistance varies appreciably with the rotor speed, does not present any special difficulty in the application of the method. A quantitative relationship is derived for the maximum-torque conditions. The variation of the maximum torque, and of the criticalspeed/secondary-resistance ratio, with the direct primary current is determined for values of the latter up to four times the rated primary current, and with zero, normal and excessive values of secondary reactance. It is shown that even when the existence of the normal reactance only is neglected, the maximum torque may be in error to the extent of 16%. Still greater errors arise from the neglect of magnetic saturation, even with the direct primary current limited to the rated value. It is shown, also, that the torque/resistance characteristics may be derived directly from the torque/speed characteristics.