Synchronous motor


Synchronous motor are used in radar and gyroscope.

window wiper, viscotherm, synchroscope, shaft generator, electric propulsion


Stator has 3phase armature winding in the slots of the stator core. Rotor has a set of salient poles to which DC supply is provided. Both the rotor and stator has same no of poles.


When 3phase supply is given to stator, and DC source applied to rotor winding. The stator produces a rotating magnetic field by which the pole changes its polarity at synchronous speed. Now the rotor will oscillate due to change in polarity of the flux and change in flux  happens so fast that rotor oscillates and practically is constant due to its inertia.

Thus to make it rotate, the rotor has to be brought to synchronous speed by external means. Then it locks with the flux and rotates at synchronous speed and thus called so.


i)             Motor starting with an external PONY MOTOR: Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor. DC excitation is not fed initially. It is rotated at speed very close to its synchronous speed and after that DC excitation is given. After some time when magnetic locking takes place supply to the external motor is cut off.

ii)            Damper winding is provided on the Rotor. Copper Bars are  embedded in the pole faces of the salient poles of the rotor and they are short circuited at the ends to form in effect of a partial squirrel cage winding.

 During the start a 3phase supply is given to the state of winding due to which the magnetic flux cuts the copper bar in the Squirrel cage winding producing current and it due to which the rotor rotates as an induction motor. As the motor approaches the synchronous speed the router is excited with direct current. Now the rotor gets locked with the rotating magnetic field of the stator and that rotates at the synchronous speed.

Power factor of synchronous Motors

In an induction motor only the stator winding produces the necessary flux. The stator winding provides reactive power from the supply to setup the flux. Consequently induction motor must operated lagging power factor.

But in a synchronous motor, the flux may be produced by the stator or rotor or both. If the rotor exciting current is of such magnitude that it producers all the required flux then no magnetizing current is used which means that the motor will operate at Unity power factor.

If the rotor exciting current is less that is under excited, the deficit in flux is made up by the stator. Consequently the Motor provides reactive power to provide the remaining flux hence the motor will operated lagging power factor.

Now if the motor is over excited, the the excess flux must be counterbalanced in the stator. Now the stator instead of absorbing reactive power actually delivers reactive power to the three phase line. Thus the motor is behaving like a capacitor supplying reactive power.

When such machine is connected in parallel with induction motors or other loads that operate at lagging power factor, the leading KVAR supplied by the synchronous capacitor partly neutralizes the lagging reactive KVAR of the loads. Consequently power factor of the system is improved.