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Which of the following is a characteristic of a salient pole synchronous machine?


1. Uniform air gap
2. Flux produced by armature wave is independent of spatial orientation of the wave with respect to field poles
3. Constant reluctance
4. Reluctance along the direct axis is smaller than that along the quadrature axis

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Correct Answer - Option 4 : Reluctance along the direct axis is smaller than that along the quadrature axis

Synchronous Machine :

Synchronous Machine constitutes of both synchronous motors as well as synchronous generators. An AC system has some advantages over a DC system. Therefore, the AC system is exclusively used for the generation, transmission, and distribution of electric power. The machine which converts mechanical power into AC electrical power is called a Synchronous Generator or Alternator. However, if the same machine can be operated as a motor is known as Synchronous Motor.

Salient Pole Rotor

  • The Field winding is placed on pole core,
  • The Air gap is nonuniform i.e minimum air gap along the d-axis and maximum air gap along the Q-axis.
  • The Air gap is proportional to reluctance. If the air gap is increased, then flux will decrease.
  • Flux distribution in the salient pole machine can be improved by shaping the salient pole such that having a minimum air gap along the pole center and increasing towards pole ends. These are said to be chamfered salient poles.
  • Air gap length under the pole shoe is proportional to sin θ ∝ 1/cos θ here θ = Electrical degree measured from the pole center.

 

Important Points :

Salient Pole Synchronous Alternator:

  • Salient pole Generators will have a large diameter and short axial length
  • Pole shoes cover 2/3 of the pitch
  • Salient Poles are laminated to reduce eddy currents
  • They are used in hydraulic turbines or diesel engines
  • Salient pole generators will have a typical speed of about 50 to 100 rpm.
  • As the speed of the water turbine is slow, hence more number of poles are required to attain the frequency. Therefore Salient pole machines will have a typical number of poles that will be 4 to 60.
  • Cheaper compared to cylindrical rotor machines for speeds below 1000 rpm.
  • Causes excessive windage losses
  • Flux distribution is not uniform due to the presence of salient poles, hence emf waveform generated is not good compared to the cylindrical machine
  • Salient Pole Synchronous Generators are employed in Hydro-Power plants.

 

Non-Salient Pole Synchronous Alternator:

  • Non-Salient pole generators will have a smaller diameter and longer axial length
  • They are used for High-speed operation (typically speed will be 1500 and 3000 rpm)
  • Better in dynamic balancing because of an absence of salient poles.
  • Less windage loss
  • Robust construction and noiseless operation
  • Nearly sinusoidal flux distribution around the periphery, therefore, gives a better emf waveform than salient pole machine
  • No need to provide damper windings because the field poles themselves act as efficient dampers.
  • Non-Salient pole generators are used in Thermal, Gas, and  Nuclear Power plants

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