Synchronous Motor and its basic concept


Synchronous Motor

A synchronous motor is an Electrical Machine which converts electrical energy into mechanical energy. As the name implies synchronous, it always runs as synchronous speed. Synchronous motors are doubly excited machine which means excitation is given to both stator as well as rotor. In Synchronous motor armature winding is mounted on the stator whereas field winding is mounted on rotor.

Synchronous Speed

Synchronous speed of motor is that speed at which revolving magnetic field rotates. Synchronous speed of motor is always constant because it depends upon supply frequency and poles and these two parameters are always constant for a synchronous machine. Synchronous speed of motor is given as

                                 Synchronous motor

From above equation we have seen that

Ns  α   F              (Frequency)

Ns  α   1/P         (P = number of poles)


As we know that frequency is always constant i.e. 50Hz. So we can say synchronous speed of motor is inversely proportional to number of poles.

From above discussion we can conclude that for desired speed of motor number of poles should maintain.

Construction of Synchronous Motor

As we studied in dc motor, Synchronous motor contains two main parts one is stationary part which is known as stator and other is rotatry part which is known as rotor. The stator and rotor are both made of ferromagnetic material (steel) which conducts the magnetic flux upon which depends the process of energy conversionThe armature winding of synchronous motor is main winding which carries main working current is mounted on stator whereas field winding which is mounted on rotor provides support so that energy transfer takes place.

Generally two types of rotor used in synchronous machine i.e. cylindrical rotor and salient pole rotor.

Salient pole are also called projected poles because in salient pole rotor poles are projected towards the stator (armature winding). In salient pole synchronous machine the field winding is concentric and concentrated. Salient pole rotor is used for low speed synchronous machines.


Synchronous Motor

Cylindrical rotor is completely solid round shaped rotor without laminations. Cylindrical rotor is called round rotor and used in high speed machines. In cylindrical rotor slots are cut on outer periphery of the rotor. In cylindrical rotor synchronous machine air gap is uniform so for sinusoidal flux density distribution distributed field winding are used.


Synchronous Motor

Synchronous Motor Working principle

The basic principle of operation of synchronous motor is, if an electromagnet placed in rotating magnetic flied than this electromagnet get magnetically locked with rotating magnetic field and rotates at the speed of rotating magnetic field.

When a three phase supply is given to three phase armature winding which is balanced in space and mounted on stator then a revolving magnetic field is produced which revolves at synchronous speed in the direction of leading phase axis to lagging phase axis. This revolving magnetic field (flux) is sinusoidally distributed in space. It develops alternate north south poles on stator.

As we know that rotor is excited by dc excitation this excitation also produces poles on rotor. As the dc excitation remains constant so that it produces fixed poles.

As stator is ac excited which produce sinusoidal flux. The polarity of this flux is change in every half cycle of given time period i.e. positive for first half and negative for second half simultaneously north and south poles develops.

So from above discussion we have seen that rotor has fixed poles and stator has alternate changing poles. When stator north (south) pole and rotor north (south) pole are on same side then they repel each other and rotor wants to rotates in opposite direction but when stator north (south) pole and rotor south (north) pole are same side then they attract each other and rotor tends to rotate in direction of revolving magnetic field. So we can say rotor wants to move in both directions in one time period but it is unable to overcome its inertia and vibrates its position. So we can say ac synchronous motor is not self-starting.

synchronous motor

synchronous motor

Some Specifications of Synchronous Motor

  • Synchronous motor is not self-starting it requires some external means to start
  • An overexcited synchronous motor supply lagging VARs (Reactive Power)
  • An under-excited synchronous motor takes reactive power (Lagging VARs)
  • So by Controlling the excitation of synchronous motor its reactive power demand can be maintained.

Synchronous Motor Applications


Synchronous motor is used for power factor improvement in power transmission system. An overexcited Synchronous motor on no load supply reactive power and this is also called as synchronous condenser.

Synchronous motors are mainly used in constant speed applications like rolling mills, printing machine, compressor, tape recorder etc.

Unlike induction motor synchronous motor is not widely used in industries because of their doubly excited nature.

More terms in Electrical Machine :-

#_Induction Motor

#_DC Motor


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