Synchronous Motor Interview Question Answer - 2

What is synchronous condenser?

Synchronous condenser

  • When the synchronous motor runs at over excitation and no load conditions, then it is working as synchronous condenser.

What is V – curve in the synchronous motor?

V curve

  • The variation of the armature current (y – axis) with respect of field current (x – axis) at constant power is known as V – curve.

Describe the significance of the V – curve.

Significance of V – curve

  • The V – curve represents the relation of different armature current at different value of excitation at constant power. 
  • The armature current becomes minimum at unity power factor. As the field current decreases from its normal value, the line current increases and motor operates at lagging power factor of the load. 
  • If the field current increases from unity power factor condition, the line current increases, but now the synchronous motor will operate at leading power factor. 
  • The armature current increases for both lagging and leading power factor and its value becomes minimum at unity power factor.

For which type of load the hunting effect is produced in the synchronous motor?

  • When the synchronous is used for driving variable load (compressors, punching press etc.) hunting effect is produced in it.

Describe the effect of hunting in the synchronous motor.

Effect of hunting

  • The load current has significant effect on the rotor in the synchronous motor. The synchronous motor rotor falls back and advances with increasing and decreasing of load. 
  • The rotor of the synchronous motor starts to oscillate due to sudden change of load. This is called as hunting effect in the synchronous motor.

How to reduce hunting or oscillating effect in the synchronous motor?


Describe the function of damper winding or amortisseur winding?

Function of damper winding

  • The damper windings are the bars which are placed on the rotor pole faces. These bars are short circuited similar to the rotor winding of the squirrel cage induction motor.
  • The function of the damper winding is to dampen the oscillation of the rotor due to variation of the load.
  • Another function of the damper winding is to start ( to accelerate ) the synchronous motor during starting as the rotating field move past the winding, which induced current in them and produced necessary accelerating torque.
  • It prevents rotor over – heating when negative sequence current flow through the rotor winding of the synchronous motor during short circuit condition. The negative sequence current is induced in the rotor of the generator during this condition. The damper winding provides path for the negative sequence current and prevent the flow of current from forging and also overheating.

How damper winding provides the starting torque in the synchronous motor?

Damper winding – Starting torque

  • When the magnetic lines of flux produced by the armature winding swept air gap and cut the damper winding bars, the voltage is generated in it and causing current to flow at given frequency in them. 
  • The interaction of the current and magnetic flux developed starting torque and therefore the rotor turns the same direction as the direction of the magnetic field.

What is procedure to start synchronous motor?

Starting of synchronous motor

  • At start, reduce voltage is applied to stator winding with the condition that field winding is unexcited. 
  • When the rotor start to rotate at steady speed, DC excitation is applied such that rotor pulls into synchronism.
  • Finally the rated supply voltage is applied to the stator winding.

Why rotor winding is insulated higher voltage than rated voltage rating of the field winding in the synchronous motor?

Field winding

  • The rotor is stationary at starting and rotating field of the stator winding induces a very large emf in the rotor at the starting period. 
  • The field winding is insulated higher voltage than the voltage rating of the field winding due to this large emf particularly at starting. 
  • However this rotor emf goes on decreasing as rotor gathers speed.

Is it possible to start synchronous motor at as low as 20 – 30% of rated voltage at starting in the synchronous motor?

To start synchronous motor

  • The synchronous motor does not start as low as 20 – 30% of the rated voltage because the starting torque in the synchronous motor varies directly the supply voltage( Tst α V) whereas in the induction motor the starting torque is directly proportional to the supply voltage ( Tst α V2 ).

State the applications of the synchronous motor.

Application of synchronous motor

  • To improve the voltage regulation of the long transmission line
  • Constant speed application such as centrifugal pumps blowers, line shafts, paper mills etc.
  • Synchronous condenser (to improve the power factor of the lagging power factor load such as welding machine and fluorescent lights)

How synchronous motors control the voltage the receiving end of the transmission line?

The transmission line consists of parameters such as resistance, inductance and capacitance.

  • When the transmission line is disconnected from inductive load, the receiving end voltage rise suddenly due to line capacitance. 
  • The synchronous motor is installed as the receiving end of the transmission line to control the receiving end voltage. 
  • By varying the excitation voltage the receiving end voltage and hence voltage regulation can be controlled.

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