An operator reports that a 3-phase conveyor motor is "humming loudly and getting extremely hot" but not turning. Voltage measurements at the motor peckerhead show 480V between L1-L2, 0V between L2-L3, and 0V between L1-L3. What is happening?
Answers
AThe motor bearings have seized completely
BThe VFD frequency is set to zero
CThe motor is single-phasing due to a blown fuse or broken wire on L3
DThe stator windings are shorted to ground
Correct answer: C — The motor is single-phasing due to a blown fuse or broken wire on L3
With 0V across two phase pairs, one of the incoming lines (L3) is dead. The motor is experiencing a single-phasing condition, causing high current, heat, and humming without rotation.
MIFOTRA exam practicealigned to NFPA 70 / IEC 60364·3 marks·Medium