When does negative phase sequence occur in a 3-phase synchronous motor_______________?
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Answer: A. supplied with unbalanced voltage
Explanation
Negative phase sequence currents appear in a synchronous motor when supply voltages are unbalanced. Unbalanced three-phase supply voltages contain symmetrical components including negative sequence voltages. Negative sequence voltages drive reverse-rotating magnetic fields that induce double-frequency currents in the rotor. Symmetrical components theory breaks unbalanced polyphase systems into positive, negative, and zero sequences. A perfectly balanced three-phase supply generates only positive sequence magnetic rotating fields. Negative sequence fields rotate in the opposite direction to the normal mechanical rotor rotation.
Power system faults like single-phase short circuits create severe voltage unbalance across motor terminals. Negative phase sequence currents cause rapid localized overheating in rotor damping and field windings. Protection engineers install phase unbalance relays to trip motors before thermal damage occurs. Under-loaded operating conditions affect current magnitude but do not introduce negative phase sequences. Over-loaded conditions increase positive sequence armature current without creating negative sequence voltages. Stating none of the above is incorrect because supply voltage unbalance is the exact cause.
- Subject
- Engineering
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- Electrical Engineering
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- Difficulty
- medium
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