A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have_____________?
Correct answer: D. leading power factor with over-excitation
- A. no change of power factor
- B. lagging power factor with over-excitation
- C. leading power factor with under-excitation
- D. leading power factor with over-excitation
Explanation
With load and speed fixed, increasing excitation above the unity-power-factor value makes the synchronous motor over-excited. Its armature current then leads the supply voltage, so the power factor becomes leading.
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