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

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