Refrigeration & Air Conditioning Mechanic Red Seal · Question
A technician is troubleshooting a three-phase motor that hums loudly but does not turn. An ammeter shows high current draw on two phases and very low or zero current on the third phase. What is the most probable cause?
A humming motor with high current on two phases and low/zero on the third is characteristic of a single-phasing condition. This occurs when one of the three sup
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Question: A technician is troubleshooting a three-phase motor that hums loudly but does not turn. An ammeter shows high current draw on two phases and very low or zero current on the third phase. What is the most probable cause?
Answer options:
- An internal short circuit in one winding.
- A grounded winding in the motor. ✅ An open circuit in one of the supply phases.
- The motor bearings are seized.
Correct answer: An open circuit in one of the supply phases.
Explanation: A humming motor with high current on two phases and low/zero on the third is characteristic of a single-phasing condition. This occurs when one of the three supply phases to the motor is lost (open circuit), causing unbalanced currents and potential motor damage if allowed to continue.
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