Evaluation of the short circuit current in induction motors with deep bars

Large induction motors can contribute a significant portion of the fault current in a system, even though the duration of the fault is normally short. An accurate prediction of this current is important to properly design the protection of the system, and to rate the circuit breakers. Many of the induction motors in use exhibit `deep bar´ behaviour, that is the referred rotor resistance and inductance change with changing frequency of the rotor currents. This paper examines the effect that the deep bar property has on the evaluation of the three phase symmetrical short circuit current. The deep bar model is verified against measured results of a short circuit applied to a 75 kW induction motor