Advantages | Disadvantages of Synchronous Motor

Some characteristic features of synchronous motor are as follows:

  • It runs at a constant speed (synchronous speed). The only way to change its speed is to vary the supply frequency (because Ns = 120f/P).
  • It is not inherently self-starting.
  • At no-load it draws a very small current from the mains to meet the internal losses of the motor. With the increase in load torque, the torque angle δ increases and the motor draws more current from the mains. After the input current reaches maximum (at torque angle δ nearly equal to 90o) no further increase in the load is possible. If the motor is further loaded it goes out of synchronism and stops.

Advantages and Disadvantages of Synchronous Motor

The advantages and disadvantages of synchronous motor are as under:

Advantages of Synchronous Motor


  • It can be operated under a wide range of power factors both lagging and leading. Hence, it can be used for power factor correction, in addition to supplying torque to drive loads.
  • Most synchronous motors are rated between 150 kW and 15 kW and run at speeds from 150 to 1800 rpm.
  • It is less costly in certain kW and speed ranges i.e. for 35 to 350 kW rating at speeds less than 500 rpm.
  • It usually runs at higher efficiencies, especially in low-speed unity power factor
  • It can be constructed with wider air gaps than induction motors, which makes it batter mechanically.

Disadvantages of Synchronous Motor


  • It is not self-starting. Special methods are adopted to make it self-starting.
  • It needs frequent maintenance.
  • External DC source is necessary for providing excitation.
  • Additional damper winding is necessary.
  • Hunting takes place if the load is changed suddenly.

Application of Synchronous Motor

Synchronous motors are mainly used in constant speed applications, some of the applications are:

  • An overexcited synchronous motor operates at leading power factor and takes a leading current from the bus bars, so it can be used to raise overall the power factor of the installation. When a synchronous motor is run without load with over-excitation for improving the power factor of an installation it is called as the synchronous capacitor or synchronous condenser.

  • Such motors are also used to regulate the voltage at the end of transmission lines.
  • Because of the higher efficiency possible with synchronous motors, they can be used for loads where constant speed is required. Typical applications of a high-speed synchronous motor (above 500 rpm) are fans, blowers, pumps and compressors.

  • Since synchronous motors can be built for speeds as low as 120 rpm. They are well-suited for direct connection to reciprocating compressors.
  • Pulp beaters and grinders, steel and metal rolling mills, rock and ore crushers are generally coupled or geared to the motor.


Advantages of Rotating Field System in Synchronous Machines

Only in small synchronous machines (i.e. synchronous motors and synchronous generators) the field system is placed on stator and armature winding on rotor but in higher rating machines the field winding is placed on the rotor and armature winding is placed on the stator. Following are the important advantages of rotating field system over stationary field system:

  • The armature winding is more complex than the field winding. Therefore, it is easy to place armature winding on a stationary structure.
  • The size of the armature conductors is much more to carry heavy currents, therefore, high centrifugal stresses are developed. Thus it is preferred to place them on a stationary structure.

  • In the modern alternators (synchronous generators), a high voltage is generated, therefore, heavy insulation is provided and it is easy to insulate the high voltage winding when it is placed on the stationary structure.
  • The size of slip-rings depend upon the magnitude of flow of current, therefore, it is easy to deliver small current for excitation through slip rings of a smaller size when the rotating field system is used.
  • It is easier to build a properly balanced high speed rotor when they carry the field system.
  • The weight of the rotor is small when the field system is provided on the rotor and as such friction losses are reduced.
  • Batter cooling system can be provided when the armature is kept stationary.

Thanks for reading about advantages and disadvantages of synchronous motor.

Synchronous Motors | Objective Type Question Answers


1. In a synchronous motor, damper winding is provided in order to

2. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

3. An electric motor in which both the rotor and stator fields rotates with the same speed is called a/an ........motor.

4. While running, a synchronous motor is compelled to run at synchronous speed because of

5. The direction of rotation of a synchronous motor can be reversed by reversing

6. When running under no-load condition and with normal excitation, armature current Ia drawn by a synchronous motor

7. The angle between the synchronously-rotating stator flux and rotor poles of a synchronous motor is called........ angle.

8. If load angle of a 4-pole synchronous motor is 8º (elect), its value in mechanical degrees is

9. The maximum value of torque angle a in a synchronous motor is ........degrees electrical.

10. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in its

11. When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because

12. When load on a normally-excited synchronous motor is increased, its power factor tends to

13. The effect of increasing load on a synchronous motor running with normal excitation is to

14. Synchronous capacitor is

15. If the field of a synchronous motor is underexcited, the power factor will be

16. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value, it leads to

17. A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity p.f. On changing the excitation only, the armature current will have

18. The V-curves of a synchronous motor show relationship between

19. When load on a synchronous motor is increased, its armature currents is increased provided it is

20. If main field current of a salient-pole synchronous motor fed from an infinite bus and running at no-load is reduced to zero, it would

21. In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop

22. In a synchronous motor, the rotor Cu losses are met by

23. A synchronous machine is called a doubly-excited machine because


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