A large induction motor utilizes a star-delta starter to reduce inrush current. During the start sequence, the motor starts in star but trips the main breaker the exact moment it transitions to delta. What is the most likely cause?
Answers
AThe motor requires a soft starter instead of star-delta
BThe timer is set too long, overheating the star contactor
CThe phase sequencing in the delta contactor is reversed, causing a dead short or massive torque spike
DThe thermal overloads are set too low
Correct answer: C — The phase sequencing in the delta contactor is reversed, causing a dead short or massive torque spike
If the wiring on the delta contactor is not phased correctly matching the motor rotation, the transition creates a massive phase-to-phase short or instant torque reversal, tripping the breaker immediately.
MIFOTRA exam practicealigned to NFPA 70 / IEC 60364·4 marks·Hard