Important Questions and Answers of Synchronous Motors

1. What does hunting of Synchronous motor mean?

The rotor Oscillates about its Synchronous position with respect to the stator field when the load applied to the Synchronous motor is Abruptly Increased or Decreased. This is referred to as hunting.

2. 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

3. What is Synchronous Condenser?

Because it takes a leading current, like a Capacitor, an Over-excited Synchronous motor used to improve power factor is known as a Synchronous Condenser.

4. Write the applications of synchronous motors.

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

c. Used for constant speed drives such as motor-generator set, pumps and compressors.

5. What is an inverted ‘V’ curve?

If the power factor is plotted against different values of field exciting current for a constant load, an inverted V-shaped curve, also known as a “inverted V curve,” results. The graph is also drawn.

6. 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.

7. What are the two types of 3-phase induction motor?

a. Squirrel cage induction motor.

b. Slip ring induction motor.

8. Write the two extra features of slip ring induction motors.

a. Rotor is having 3-phase winding.

b. Extra resistance can be added in the rotor circuit by connecting through the help of three slip rings for improving the power factor, increasing Starting Torque, limiting the starting current.

9. Can we add extra resistance in series with squirrel cage rotor? State the reason?

Because copper end rings short circuit all of the rotor’s copper bars on either side to create a closed circuit, we are unable to add additional resistance in series with the rotor.

10. Why an induction motor is called rotating transformer?

Similar to how the secondary of a two winding transformer receives its power from the primary, the rotor receives electrical power in this manner. Since the secondary winding of an induction motor is free to rotate while the primary winding is stationary, it can be referred to as a rotating transformer.

11. Why an induction motor will never run at its synchronous speed?

Since there would be no related speed if it runs at synchronous speed, there would be no rotor emf, no rotor current, and no rotor torques to keep the rotors rotating. The rotor runs at its synchronous speed as a result.

12. Define SCR?

Short circuit ratio (SCR) is defined as the ratio of field current required to produce rated voltage on open-circuit to field current required to produce rated armature current with the terminals shorted, while the machine runs at synchronous speed.

13. When does a synchronous motor get over excited?

If the field excitation of the motor is increased, the field flux will become strong and Eb will increase. As a result Eb will exceed V and the motor will be called an over excited motor.

14. Define pullout torque?

The pullout torque is the torque, beyond which the synchronous link between field poles and resultant flux wave is severed and the machine falls out-of-slip.

15. 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

The motor is capable of operating at any power factor.

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

The same principle governs both dc and ac motor operation. Magnetic locking between the magnetic fields of the stator and rotor is what allows synchronised motors to operate.

17. Name any two methods of starting a synchronous motors

1. By an extra 3 phase cage induction motor
2.  By providing damper winding in pole phases
3.  By operating the pilot excitor as a dc motor

18. 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.

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

Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed. Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed.

20. What is the phasor relation between induced emf and terminal voltage of a 3 phase Synchronous motor?

The rotating magnetic field is initially established by the prime source of supply V. The main field then causes an emf to get induced in the 3phase winding. Hence when the machine operates as asynchronous motor the emf phasor always lags the terminal voltage phasor by the load/torque

21. 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 in verted V curve.

22. What happens when the field current of asynchronous motor is increased beyond the normal value at constant input?

Increase in emf causes the motor to have reactive current in the leading direction. The additional leading reactive current causes the magnitude of line current, accompanied by the decrease in power factor.

23. Distinguish between Synchronous phase Modifier and Synchronous Condenser.

Asynchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier. Asynchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called Asynchronous condenser.

24. How the Synchronous motor can be used as s Synchronous Condenser?

Synchronous motors are run on Overexcitation in order to draw the most power and Reactive current from the supply lines. This makes up for the load’s lagging current and power Requirements, bringing the system power factor to unity. The motor is referred to as a Synchronous Condenser because it performs the function of Capacitors.