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Consider the following statements with respect to synchronous machines. State whether these statements are true or false. (a) Armature reaction is demagnetising when a generating machine supplies zero power factor lagging current. (b) Armature reaction is magnetising when a generating machine supplies zero power factor leading current. (c) Armature reaction is mostly cross-magnetising, though it has a small demagnetising component, when a generating machine supplies unity power factor current.

Consider the following statements with respect to synchronous machines. State whether these statements are true or false. (a) Armature reaction is demagnetising when a generating machine supplies zero power factor lagging current. (b) Armature reaction is magnetising when a generating machine supplies zero power factor leading current. (c) Armature reaction is mostly cross-magnetising, though it has a small demagnetising component, when a generating machine supplies unity power factor current. Correct Answer (a) True, (b) True and (c) True

Armature reaction:

The effect of armature (stator) flux on the flux produced by the rotor field poles is called armature reaction. It depends on the power factor i.e. the phase relationship between the terminal voltage and armature current.

  • The armature reaction flux is constant in magnitude and rotates at synchronous speed
  • At unity power factor load, the armature reaction is cross-magnetizing (distortional)
  • At lagging power factor load, the armature reaction is partly demagnetizing and partly cross-magnetizing; At zero power factor lagging load, it is purely demagnetizing and the induced emf will get decreased
  • When the generator supplies a load at leading power factor the armature reaction is partly magnetizing and partly cross-magnetizing; At zero power factor leading load, it is purely magnetizing and the induced emf will get increased

Important Points

Power factor

Alternator

Synchronous motor

Unity power factor (UPF)

Purely cross magnetization

Purely cross magnetization

Zero power factor (ZPF) lagging

Purely demagnetization

Purely magnetization

Zero power factor (ZPF) leading

Purely magnetization

Purely demagnetization

0.8 lagging

Partly demagnetizing and partly cross magnetization

Partly magnetization and partly cross magnetization

0.8 leading

Partly magnetization and partly cross magnetization

Partly demagnetizing and partly cross magnetization

 

Related Questions

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