Synchronous Machines – Question and Answer

1. What is the principle of an alternator?

The alternator works on the principle on the Faraday’s law of electromagnetic induction. Whenever a conductor links with a magnetic field, either the conductor is moving, or the field is moving, an emf is induced in the conductor.

2. What are the different types of alternators? Which is in common use?

Alternators are of two types

  1. Rotating armature type.
  2. Rotating field and stationary armature type.

Rotating field type is commonly used.

3. What are the main parts of an alternator?

The main parts of an alternator are i) Rotor (Salient pole type or cylindrical type), ii) Stator (Frame, core and Armature conductors) and iii) Exciter.

4. Does the change of the number of conductors have any effect on the frequency?

No, the frequency does not depend upon the number of conductors.

5. What do you mean by the salient – pole type rotor?

Salient – pole type rotor means a low and moderate speed rotor having large diameter and small axial length with projected poles coming out of the rotor frame the outer surface of which almost follows the inner cylindrical surface of the stator frame.

6. How will you adjust the frequency of an alternator?

Frequency of an alternator can be adjusted by varying the speed of the prime mover (as f is proportional to Ns).

7. What are the advantages of having rotating field system?
  1. Better insulation
  2. Ease of current collection
  3. Increased armature tooth strength.
  4. ‘More rigid construction
  5. Reduced armature leakage reactance.
  6. Lesser number of sliprings.
  7. Lesser rotor weight and inertia
  8. Improved ventilation and heat dissipation.
8. What are the different voltage drops in an alternator?
  1. Voltage drop due to resistance of winding.
  2. Drop due to leakage reactance.
  3. Voltage drop due to armature reaction.
9. What is armature reaction?

The effect of armature flux on main flux is called armature reaction.

10. Why is EMF method called Pessimistic method?

he value of voltage regulation obtained by EMF method is always more than the actual value and therefore it is called Pessimistic method.

11. Why is MMF method called Optimistic method?

The value of voltage regulation obtained by MMF method is always less than the actual value, therefore it is called Optimistic method.

12. What do you mean by hunting in alternators?

Due to the fluctuation of power input to the prime mover if the driving torque applied to the alternator becomes pulsating, the rotor during rotation will oscillate from its normal speed; this oscillation is called hunting.

13. Differentiate salient pole rotor and smooth cylindrical type synchronous machines.
Salient Pole Rotor Cylindrical Rotor
Large diameter and short axial length Small diameter and long axial length.
Used for low speed alternators Used for high- speed turbo alternators
Has projecting poles No projecting poles
Needs damper windings Does not need damper windings.
Windage loss is more. Windage loss is less.
14. How is the armature winding in alternators different from those used in dc machines?

The armature winding of the alternator is placed in the stator, but in the case of dc machines, armature winding is placed in the rotor.

15. What are squirrel-cage windings of alternators? How and why are they used?

Damper windings are squirrel cage windings of the alternators. This winding is placed in rotor pole shoes.

16. What is hunting? How is hunting minimized?

When a synchronous motor is used for driving a fluctuating load, the rotor starts oscillating about its new position of equilibrium corresponding to the new load. This is called hunting or phase swinging.

To prevent hunting, dampers (or) damping grids are employed. Damper windings are short circuited, copper bars are embedded in the faces of the field poles of the motor.

17. What are the inherent disadvantages of synchronous motor?
  1. Higher cost
  2. Necessity of a dc excitation source
  3. High initial cost
  4. High maintenance cost
18. Mention four applications of synchronous motor?
  1. Power factor correction
  2. Constant speed, constant load drives
  3. Voltage regulation of transmission lines.
19. What is the role of synchronous motor in a transmission line? How?

Synchronous motor acts as a voltage regulator in a transmission line.

When line voltage decreases due to inductive load, motor excitation is increased thereby increasing its power factor which compensates for the line voltage drop.

When the line voltage increases due to line capacitive effect, synchronous motor excitation is decreased, thereby making its power factor lagging which helps to the maintain the transmission line voltage at its normal value.

20. Define pullout torque in synchronous motor.

The maximum torque which the motor can develop without pulling out of step or synchronism is called the pull out torque.

21. What is synchronous condenser?

Synchronous motor is operating at an over excited condition is called synchronous condenser. The synchronous condensers having leading power factor are widely used for improving power factor of those power systems which employ a large number of induction motors and other lagging power factor loads.

22. Define pull in torque in synchronous motor.

It pertains to the ability of the machine to pull into synchronism when changing: from induction to synchronous motor operation.

23. What is meant by V curves of synchronous motor?

The V-curves show the relation that exists between the :armature current and field current for different constant power input.

24. List any two methods of starting of a synchronous motor.
  1. Starting with the help of damper winding.
  2. Starting with the help of separate small induction motor.
  3. Starting by using an ac motor coupled to the synchronous motor.
25. Why does the synchronous motor always run at synchronous speed?

A synchronous motor always runs at synchronous speed because of the magnetic locking between the stator and rotor poles.

26. Explain why a synchronous motor has no starting torque.

A synchronous motor is a constant speed motor. It develops torque only when running at synchronous speed.

27. What are the two classification of synchronous machines?

The two classifications of synchronous machines are:
i) Cylindrical rotor type
ii) Salient pole type

28. What are the effects of load variation in synchronous motor?

Speed of synchronous motor remains the same from no load to full load, but upto some extent. If the motor is over loaded beyond the limit, then its speed will reduce, the torque will become zero and the motor will stop.

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Santhakumar Raja

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