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Consider the following statements regarding synchronous machines: 1. When a synchronous motor is over-excited, its back emf is greater than the supply voltage 2. When a synchronous motor is over-excited, its power factor is leading 3. Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impractical Which of these statements are correct?

Consider the following statements regarding synchronous machines: 1. When a synchronous motor is over-excited, its back emf is greater than the supply voltage 2. When a synchronous motor is over-excited, its power factor is leading 3. Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impractical Which of these statements are correct? Correct Answer 1, 2 and 3

Over Excited Synchronous Motor:

  • If DC field excitation of a synchronous motor is such that back EMF Eb is greater than applied voltage V, then the motor is said to be over-excited.
  • An over-excited synchronous motor draws leading current. Therefore, if the field of the synchronous motor is overexcited then its power factor will be leading.
  • An over-excited synchronous motor running at no load is known as the synchronous capacitor or synchronous condenser. Synchronous capacitors are always totally enclosed. The shaft does not extend beyond the case of the motor. They are used for correcting the power factor of a lagging load such as transformers and induction motors in an installation.
  • Power factor correction means raising the power factor of the load from its low value to a higher value. Some industrial loads run at very low power factors. It is advantageous to raise this power factor to unity or near about.
  • Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impractical
  • Any increase in power factor, increases supply capacity, efficiency and improves the operating characteristics of the system.
  • Synchronous capacitors cancel the lagging KVARs of the installation with their leading KVARs. They are usually connected in parallel with the incoming power lines to the plant.

 

Additional Information

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Excitation Power factor
Under excitation Lagging power factor
Normal excitation Unity power factor
Over excitation Leading power factor

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