Synchronous Motor Interview Question Answer - 1

Describe the meaning of synchronous.

Synchronous

• The term ‘syn’ means to ‘with’ and ‘chronous’ means to ‘in time’. Therefore the synchronous motor operates ‘in time with’ the power supply.

At what principle the synchronous motor operates?

Principle of synchronous motor

• When the three phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced in it. 
• When the DC excitation is given to the field winding, rotor interlock magnetically through the air gap and revolves synchronism with the rotating magnetic poles of the stator.

Describe the function of the exciter.

Function of exciter

• The function of the exciter is to supply magnetizing current to the field winding of the synchronous motor.

Describe the difference between Synchronous motor and Induction motor.

Difference between Synchronous motor and Induction motor

• The synchronous motor always runs as synchronous speed whereas the speed of the induction motor always less than the synchronous speed.
• The power factor of the induction motor is always lagging whereas the synchronous motor runs at lagging, leading as well as unity power factor.
• The DC supply is required in the synchronous motor not in the induction motor.
• The cost of the synchronous motor is much higher than of the induction motor of the same rating.
• The breakdown torque in the induction motor varies as the square of the supply voltage whereas its value directly proportional to the supply voltage.

Explain the terms: Normal excitation, over excitation and under excitation

Normal excitation

• If the DC excitation in the synchronous motor is such that the back emf E_b is equal to supply voltage V, it is called as normal excitation.

Over excitation

• If the DC excitation in the synchronous motor is such that the back emf E_b is greater than the supply voltage V, it is called as over excitation.

Under excitation

• If the DC excitation in the synchronous motor is such that the back emf E_b is less than the supply voltage V, it is called as under excitation.

Explain the term: Load angle or Coupling angle

Load angle or Coupling angle

• The angle between stator pole and rotor pole of opposite polarity is called as load angle.

Which parameters greatly affect the value of back emf in the DC motor and synchronous motor?

• The value of the back emf in the synchronous motor depends upon rotor excitation only whereas its value depends upon flux per pole and speed of the motor in the DC motor.

Which parameter is responsible for copper losses in the synchronous motor?

• The copper loss in the synchronous motor depends upon rotor excitation or direct voltage supplied to the field winding.

Explain the terms: Starting torque, running torque or Synchronous torque, Pull - in torque and Pull - out torque

Starting torque

• It is the torque developed by the synchronous motor when voltage is applied to the stator terminal as per name plate rating (or at the instant of starting at zero speed).

Running torque (synchronous torque)

• It is the torque developed by the synchronous motor under running conditions.

Pull – in torque

• The synchronous motor starts as induction motor and runs 4 – 5% below the synchronous speed. When the excitation is switched on, the synchronous motor rotor pulls into step with the synchronous rotating stator field. 
• The amount of torque at which synchronous motor pulls into steps is called as pull – in torque.

Pull – out torque

• The maximum torque which can be developed by synchronous motor without losing synchronism is called as pull – out torque.

                    OR

• It is defined as maximum steady state torque developed by the synchronous motor, for one minute, before it pulls out of step due to overload.

What is meaning of the slipping poles?

Slipping poles

• When the angular displacement of the rotor exceeds the stability limit, the rotor slips a pole pitch or rotor flux slips with respect to the stator flux. This condition is called pole slipping.

What is condition for maximum power developed in the synchronous motor?

• The condition for the maximum power developed in the synchronous motor is internal angle θ is equal to load angle α.

Describe the effect on armature current of the field current in the synchronous motor

• The armature current increasing for both lagging and leading power factor and its value becomes minimum at unity power factor.

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