At what operating condition is a synchronous motor's power factor unity_____________?
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Answer: B. the armature current is minimum
Explanation
At unity power factor, the real power is maximized for a given applied voltage. Reactive current component drops to zero, leaving only the active current component flowing. Armature current reaches its absolute minimum value on the V-curve characteristic plot. Unity power factor means the phase angle between voltage and current is exactly zero degrees. Minimum armature current corresponds to the bottom vertex of the motor V-curve graph. Active power equals apparent power when operating at a perfect unity power factor.
Plant operators tune motor excitation to achieve minimum armature current for optimal energy efficiency. Running at unity power factor eliminates reactive power charges imposed by electrical utility companies. Minimizing armature current significantly reduces copper resistive heating losses within the motor windings. Maximum armature current occurs at high or low excitation levels with poor power factors. Armature current is never zero because the motor must always draw active power to overcome losses. Stating none of the above is wrong because minimum armature current correctly identifies unity power factor.
- Subject
- Engineering
- Category
- Electrical Engineering
- Topic
- —
- Difficulty
- medium
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