Switching Transients in Wound Rotor Induction Motors [includes discussion]

This paper presents an analysis of the transients that occur when voltage is suddenly applied across the terminals of an induction motor, with and without a connected capacitor. In wound rotor motors, the transient currents are reduced by connecting resistance in the rotor circuit, but transient voltages then appear across the slip rings. Both current and voltage transients are increased when capacitors are connected across the stator terminals. Calculated curves and test oscillograms are given for the transient currents when voltage was applied to the terminals of a 4-pole 3,000-horsepower wound rotor induction motor, with an external resistance across the slip rings, with and without a capacitor. The calculations and tests confirm the wellknown rule that, when there is an appreciable reactance between the power source and the motor, and when voltage is suddenly applied, the peak transient voltage on the motor capacitor unit will rise above the steady-state voltage. If the capacitor kilovolt-amperes (kva) exceed the motor magnetizing kva, the steady-state voltage will be higher than the source voltage, and the motor voltage will also rise above its steady-state value when the motor is disconnected. It is shown that the capacitor increases the time constant of voltage decay by the factor v/1-Kv2, approximately, where K is the ratio of capacitor to magnetizing kva, and v is the per unit motor speed. If Kv2>1, E builds up instead of decaying.