Question

When starting up the motor (turning up the variac and the 3-phase voltage) what happens to the input current?  What is the per unit value of the highest current? What is the condition of the motor before it begins to turn (open circuit, short circuit)? What element in the circuit model is affected?

Answer 1

At starting: The speed is zero at the time of starting and the Back emf is zero , Due to this motor takes a very high amount of current from the supply. So that to limit this current, reduced voltage is applied to the motor by variac or autotransformer. Due to this low voltage motor takes low current until the rated is voltage applied(at varaic switched on and moving from its intitilal position ,motor takes high current but this value gradually reduced.)

Per unit value of the current depends on the actual motor current current and base value of current. and mostly its maximum value is 1pu

At starting simply the motor conditions is short circuit because the stator voltage is Zero

motor reactance is affected because at starting frequency is zero and the reactance is depends on frequency.