AC motors can be categorized into two types: asynchronous motor and synchronous motor. An asynchronous motor is also called as induction motor. The two types are very different from each other. This paper is focused on the differences between an induction motor and a synchronous motor.
Difference on Construction
Induction motor: the stator winding is similar to that of a synchronous motor, which is wound for a specific number of poles. A squirrel cage rotor or a wound rotor can be used in an induction motor. As for both the squirrel cage rotor and wound rotor, the rotor bars are permanently short-circuited with end rings, hence no slip rings are needed.
Synchronous motor: Stator has axial slots consisting of stator winding wound for a specific number of poles. Generally a salient pole rotor is applied on which rotor winding is mounted. Rotor winding is fed with a DC supply with the help of slip rings. A rotor with permanent magnets can also be used.
Difference In Working
Induction motor: a Rotating Magnetic Field shorted as RMF is generated when the stator is fed with two or three phase AC supply. The relative speed between stator’s rotating magnetic field and the rotor will lead to an induced current in the rotor conductors. And the rotor current will cause the rotor flux. According to Lenz’s law, the direction of this induced current tends to oppose the cause of its production, i.e. relative speed between stator’s RMF and the rotor. Thus, the rotor will try to catch up with the RMF and reduce the relative speed.
Induction motor always runs at a speed less than that of synchronous .
Synchronous motor: Stator poles rotate at the synchronous speed (Ns) when they are fed with a three phase supply and the rotor is fed with a DC source. The rotor has to be rotated at a speed near to that of synchronous during starting stage. If it is done so, the rotor poles get magnetically coupled with the rotating stator poles, and hence the rotor starts to rotate at the synchronous speed.
Synchronous motor usually operates at a speed equal to its synchronous speed.
i.e. Actual speed = Synchronous speed
or N = Ns = 120f/P
1.Synchronous motors require an additional DC power to energizing rotor winding while Induction motors do not.
2.synchronous motors contain slip rings and brushes, but induction motors do not. (except wound type induction motor in which slip ring motors are used to add external resistance to the rotor winding).
3.Synchronous motors need additional starting mechanism to initially rotate the rotor near to the synchronous speed, but induction motors need no starting mechanism .
4.The power factor of a synchronous motor can be adjusted to lagging, unity or leading by varying the excitation, while an induction motor always runs at lagging power factor.
5.Synchronous motors are more efficient than induction motors.
6.The cost of synchronous motors are higher than induction motor.