Synchronous Motor Interview Questions

Synchronous Motor Interview Questions

Q: Why synchronous motor is not self starting?

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux.

The rotor carrying DC supply also produces a constant flux. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

Q: What are the methods of starting synchronous motor?

  1. Pony motor starting( using ac or dc motor)

  2. Starting as squirrel cage Induction motor or using damper winding

  3. Starting as slip ring Induction motor or synchronous motor

  4. Using dc machine coupled to it.

Q: What is pony motor starting?

In this method the rotor is made to rotate by some external device called pony motors. Once the rotor attains synchronous speed, the DC excitation to the rotor is switched on. Once synchronism is established the pony motor is decoupled. The synchronous motor continuously runs at synchronous speed.

Q: Why damper windings are used in synchronous motor? Or How synchronous motor is started as squirrel cage motor?

The damper winding consists of short circuited copper bars embedded in the the face of the rotor poles. When an ac supply is provided to stator of a 3-phase synchronous motor, stator winding produces rotating magnetic field. Due to the damper winding present in the rotor winding of the synchronous motor, emf is induced in damper winding and hence currents starts to flow. Hence torque acts on rotor. Damper windings in synchronous motor will carryout the same task of induction motor rotor windings. Therefore due to damper windings synchronous motor starts as induction motor and continue to accelerate. When the motor attains about 95% of the synchronous speed, the rotor windings is connected to exciter terminals and the rotor is magnetically locked by the rotating magnetic field of stator and it runs as a synchronous motor. Now as the rotor rotates at synchronous speed the relative motion between rotating magnetic field and damper winding is zero. Hence there will be no emf induced in damper winding.

Q: What is the function of damper winding?

To provide starting torque

To prevent or minimize hunting

What is hunting?

The sudden changes of load on synchronous motor set up oscillations in the rotor. Such oscillation of rotor about its new equilibrium position is called hunting.

Q: What are the effects of hunting?

Loses synchronism

Develops mechanical stress on rotor shaft

Increase machine losses

Increase temperature of machine

Q: What is pull in torque?

The torque which is required to pull the motor into synchronism when it is running as induction motor is called pull in torque.

Q: What is pull out torque?

The maximum torque developed by the motor without pulling out of synchronism is called pull out torque. Its value varies from 1.5 to 3.5 times full load torque.

Q. What is the effect on speed if the load is increased on a 3 phase synchronous motor?

The speed of operation remains constant from no load to maximum load in

the motor operating at constant frequency bus bars.

Q.Why a synchronous motor is a constant speed motor?

When load increase in synchronous motor the load angle
δ also increases but speed remains constant. The further increase in load cause further increase in load angle. When load angle reaches 90 degree electrical then the motor comes out of synchronism. Hence the motor rotates at synchronous speed otherwise it comes out of synchronism.

Q: What is normal excitation?

The excitation for which power factor of the synchronous motor is lagging and the back emf is equal to supply voltage.

Q: What is under excitation?

The excitation for which power factor of the synchronous motor is lagging and the back emf is less than supply voltage.

Q: What is over excitation?

The excitation for which power factor of the synchronous motor is leading and the back emf is greater than supply voltage.

Q: What is critical excitation?

The excitation for which power factor of the synchronous motor is unity and the back emf is equal to supply voltage.

Q. What is synchronous condenser?

An over-excited synchronous motor under no load , used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.

Q. Distinguish between synchronous phase modifier and synchronous condenser

A synchronous motor used to change the power factor or power factor in

the supply lines is called synchronous phase modifier.

A synchronous motor operated at no load with over excitation condition to

draw large leading reactive current and power is called a synchronous


Q. How the synchronous motor can be used as s synchronous condenser?

Synchronous motor is operated on over excitation so as to draw leading

reactive current and power from the supply lines. This compensates the

lagging current and power requirement of the load making the system

power factor to become unity. The motor does the job of capacitors and

hence called as synchronous condenser.

Q. What are V and inverted V curves of synchronous motor ?

The variation of magnitude of line current with respect to the field current

is called V curve .

The variation of power factor with respect to the field current is called inverted V curve.

Q Write the applications of synchronous motor.

a. Used for power factor improvement in sub-stations and in industries.

b. As synchronous condenser

c. Used for constant speed applications

Q. State the characteristic features of synchronous motor.

a. the motor is not inherently self starting

b. The speed of operation is always in synchronous with the supply frequency

irrespective of load conditions

c. The motor is capable of operating at any power factor.

Q. In what way synchronous motor is different from other motors?

All dc and ac motors work on the same principle. Synchronous motor operates

due to magnetic locking taking place between stator and rotor magnetic fields.

Q A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?

a. Exciter voltage may be too low.

b. Field spool may be reversed

c. There may be either open-circuit or short-circuit in the field.

Q What could be the reasons if a 3-phase synchronous motor fails to start?

It is usually due to the following reasons

a. Voltage may be too low.

b. Too much starting load.

c. Open circuit in one phase or short circuit.

d. Field excitation may be excessive

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