What Is the Difference Between Induction Motor and Synchronous Motor

by Starlight Generator dieselgeneratortech

The basic difference is that an induction motor is an asynchronous machine whereas the other one, as the name suggests is a synchronous machine.

Induction Motor:

An induction motor consists of a stator with conductors placed at specific positions depending on the phase requirement. For instance, in a three phase motor, the conductors are placed at a space angle of 120 deg..The rotor in an induction motor consists of conductors usually shorted using end rings or has an external winding which can be connected through an external resistance. Consider the case of a three phase induction motor. When the supply is switched on, a rotating magnetic field is produced, called the stator field. This field rotates at a speed Ns, called the synchronous speed. This field induces an emf in the rotor conductors, which in turn causes a current flow in the shorted rotor conductors. The flow of this current produces the rotor magnetic field, which follows the stator field. The rotor rotates at a speed Nr. The relative speed between the rotor and the stator is Ns-Nr, called the slip speed. The speed of the rotor field with respect to the stator field will be (Ns-Nr)+Nr. [Relative speed between stator and rotor + the speed of the rotor].Thus the speed of the rotor field will also be Ns. The two fields rotate at the same speed in the same direction but the speed of the rotor can never be equal to the synchronous speed. If the speed of the rotor is equal to synchronous speed, there is relative motion between the stator field and the rotor conductors, hence there is bo induced emf and current, and hence there is no torque. Thus, an induction motor can never run at the synchronous speed and hence is the name asynchronous motor.

Synchronous motor:

The stator of a synchronous motor is similar to that of an induction motor. The rotor of the synchronous motor consists of an electromagnet, produced by a dc excitation. This creates the rotor field. The three phase supply at the stator produces a rotating magnetic field at the synchronous speed. The rotor field now follows the stator field with an angle lag but rotates at the synchronous speed. A synchronous motor can run only at the synchronous speed.

In a nutshell, the torque in an induction motor is produced due to the relative speed whereas in a synchronous motor, the torque production is due to the angle lag between the two fields.

The basic difference is that an induction motor is an asynchronous machine whereas the other one, as the name suggests is a synchronous machine.

Induction Motor:

Speed: The speed of induction motor is always less than the synchronous speed and it is dependent on the load since the speed decreases with the increase in load.

Starting Torque: This kind of motor has its own self starting torque.

Excitation: It is a singly excited machine where its stator windings are excited using an AC source. Contrary to the case of synchronous motor, induction motor works only under lagging power factor.

Efficiency: It is comparatively less efficient.

Cost: The cost of induction machine is less when compared with a synchronous motor of same rating.

Synchronous Motor:

Speed: Starting from its name ‘synchronous’, this motor runs at synchronous speed whatever the amount of load it may be. The speed of this kind of motor is not dependent on the load.

Starting Torque: This motor does not have any self starting torque, so some other auxiliary means have to be provided for starting the synchronous machine.

Excitation: Synchronous motor is a doubly excited machine. The field winding that is rotor is excited using DC source and its stator that is armature winding is excited using AC source. In addition to that it can be made to operate at leading power factor from lagging power factor just by changing its excitation.

Efficiency: This is comparatively more efficient than induction motor.

Cost: This kind of motor is much costlier that a similar rating induction motor.

Thus, we can summarize the main differences between an induction machine and a synchronous machine :

Induction machine is self starting, but synchronous machine is not.

Induction machine works on the principle of electromagnetic induction, similar to the three phase transformers. The short circuited rotor winding acts as a current carrying loop inside a varying magnetic field, where the magnetic field varies due to the difference in the speed between the rotor and stator magnetic field. The synchronous machine works on the principle on exciting the rotor using DC and making it behave as an electromagnet, so that it is made to lock with the synchronous speed of the magnetic field.

As we see, the induction motor rotates and gives output at non synchronous speeds (where there is relative flux cutting the rotor), while synchronous machines are destined to run on synchronous speeds.

In case of induction motor, any speed other than the synchronous one will be fine, and offer you an output, while for a synchronous machine, the only way to change speed is to change the frequency. In other words, a synchronous machine is a constant speed (synchronous speed) machine.

Effect of load : As load increases in an induction motor, it tends to reduce the speed. However, in the case of synchronous machine, this change of speed is not negotiable, instead, the system tries to rotate with the synchronous speed itself, but with a difference between the aligned positions of stator and rotor pole axis called load angle.

A synchronous motor experiences the phenomenon of “hunting” when the load is suddenly changed. This means that the rotor, due to the inertia, is not able to instantly finalize its equilibrium, and swings about the position. Anyway, this is an undesirable phenomenon from motor point of view (since it causes large fluctuations in drawn currents) It is to be noted that hunting is absent in induction machines.

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