Hi friends, in this article, I am going to describe advantages and disadvantages of synchronous motor and very interesting features of this machine. Let us start now.
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 Motor Objective Type question Answers | MCQ



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#1 The maximum power developed in the synchronous motor will depend on

rotor excitation, maximum value of coupling angle and supply voltage.

#2 A synchronous motor switched on to supply with its field winding shorted on themselves. It will

start as an induction motor

#3 The back EMF set up in the stator of a synchronous motor will depend upon

rotor excitation only

#4 With the increase in the excitation current of synchronous motor the power factor of the motor will


#5 The armature current of a synchronus motor has large values for

both low and high excitation

#6 A 3 phse 400 V, 50 Hz salient pole synchronus motor is fed from an infinite bus bars and is running at no load. Now if the field current of the motor is reduced to zero

the motor will run at synchronous speed

#7 A 3 phase, 400 V, 50 Hz 4 pole synchronous motor has a load angle of 10 degrees electrical. The equivallent mechnical degrees will be

5 degrees

#8 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#9 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#10 The break down torque of synchronous motor varies as

applied voltage

#11 The name plate of an induction motor reads 3 phase, 400 V, pf 0.8 lagging, 1440 RPM. On similar lines a name plate of synchronous motor should read

3 Phase, 400 V, 50 Hz, 0.8 pf leading, 1500 RPM

#12 In a 3 phase synchronous motor, the magnitude of field flux

remains constant at all loads

#13 A four pole synchronous machine has 48 slots. A coil having one coil side in slot number 1 and the other coil side in slot number 13 will be termed as

full pitch coil

#14 A three phase synchronous motor is running clockwise. In case, the direction of its field current is reversed

the motor will continue to run in the same direction

#15 In a synchronous motor out of the following losses, which one will have the highest proportion ?

Iron losses

#16 When a synchronous motor is connected to an infinite bus, while operating on leading power factor

the excitation voltage will be more than the supply voltage

#17 In a synchronous motor hunting can be minimised

by any of the above method

#18 While starting a synchronus motor by induction motor action, very high EMF is induced in the field. This induced EMF may damage the insulation of the field winding and of the slip rings. This insulation damage can be prevented by

either short-circuiting the field winding by field discharge resistance or splitting the field winding into several sections

#19 Synchronous motors are gererally of

salient pole type machines

#20 The fact that a synchronous motor with salient poles will operate, even if the field current is reduced to zero, can be explained by

magnetization of rotor poles by stator magnetic field

#21 Hunting of a synchronous motor may be due to

any of the above.


Synchronous Machines | All Posts


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