Q3Electrical Machine Design
Question
Q.3. Define short circuit current of induction motor.
Answer
Short circuit current of an induction motor is the very high transient current drawn from the supply at the instant of a sudden short circuit (or equivalently, at starting/locked-rotor condition), limited primarily by the motor's leakage reactance.
The short circuit current of an induction motor refers to the large current drawn by the motor when its terminals are suddenly short-circuited, or equivalently and more commonly used in design practice, the locked-rotor (starting) current drawn when the motor is energized at standstill (s=1) — since at standstill the rotor circuit is effectively analogous to a short-circuited transformer secondary. This current is limited essentially only by the combined stator and rotor leakage impedance (since at starting there is no significant back-EMF/motional voltage to limit current, unlike normal running condition), and is typically 5 to 7 times the rated full-load current for standard induction motors, which is why starting methods (star-delta starters, autotransformer starters, soft starters) are commonly employed for larger motors to limit the impact of this high starting current on the supply system.