Difference Between Induction Motor and Synchronous Motor

Main Difference – Induction Motor vs. Synchronous Motor

Induction motors and synchronous motors are two different types of AC motors. They both contain a stator which creates a rotating magnetic field, and a rotor that rotates in response. The main difference between induction motor and synchronous motor is that, in synchronous motors, the rotors rotate at the same speed at which the magnetic field rotates, whereas the rotors of induction motors rotate at a speed slower than that of the rotating magnetic field.

What is a Synchronous Motor

A synchronous motor consists of a stator (a non-moving part), which has windings that are supplied with a 3-phase alternating power supply. The windings are connected to the power supply in such a way that as the phases alternating currents varies, a rotating magnetic field forms around the stator. The rotor (the rotating part) of the synchronous  motor is supplied with a direct current so that it forms an electromagnet, whose magnetic field does not change with time. When the motor is working, the rotor’s magnetic field interacts with the stator’s rotating magnetic field and the rotor itself is rotating so that its magnetic poles are “locked” with an attracting magnetic pole in the stator.

However, at the beginning, the magnetic field produced by the stator rotates so quickly that the rotor is not able to keep up with the rotation due to its own inertia. In other words, synchronous motors are not self-starting. To overcome this issue, squirrel-cage rotors can be used. When these rotors are placed inside a rotating magnetic field, currents are induced on the squirrel-cage structure. These currents create their own magnetic field which interact with the rotating magnetic field, causing the cage to experience a force. The result is that the “squirrel-cage” also starts rotating. Since the rotor is attached to the squirrel-cage, the rotor also now starts rotating. When the rotor starts rotating at a speed closer to the speed at which the magnetic field is rotating, the direct current on the stator is switched on. Now, the rotor is moving at a speed fast enough that allows for its magnetic field to get locked with the magnetic field from the stator. Once they are locked, the rotor can continue to rotate along with the rotating magnetic field.

Another method to get the rotor  rotating at a speed closer to that of the magnetic field is to connect the rotor to an external motor. Once again, when the rotor reaches a speed that is close enough, its current is switched on so that its magnetic field can get locked with the stator’s rotating magnetic field.

The rotor in the synchronous motor rotates at the same speed as the speed of the rotating magnetic field, and this is why the motor is called synchronous. The number of rounds that the magnetic field rotates per minute is called the synchronous speed (N_s), and it is given in terms of the frequency of the AC current f and the number of poles of the stator connected to one of the three phases p by:

N_s=120\frac{f}{p}

The video below provides a good explanation for how a synchronous motor works.

What is an Induction Motor

The setup of an induction motor have some similarities to the setup of a synchronous motor. Just like the synchronous motors, induction motors also consist of a set of stator windings that are connected to a 3-phase alternating power supply. As we mentioned earlier, this would produce a rotating magnetic field.

The rotor of an induction motor is of the squirrel-cage type. As mentioned before, when a squirrel-cage rotor is placed inside a rotating magnetic field, it produces a current across the cage. The current produces its own magnetic field, which, in turn, interacts with the rotating magnetic field. As a result, the squirrel-cage rotor also starts rotating.

Difference Between Induction Motor and Synchronous Motor - Induction_motors

Induction motors

In contrast to the synchronous motor, the rotor of an induction motor rotates at a slower speed than the speed at which the magnetic field is rotating. This is because, if the rotor rotated at the same speed as that of the magnetic field, the magnetic flux across the rotor would stop changing and so according to Faraday’s law, there would no longer be a current flowing within the rotor. Therefore, when the rotor begins to rotate at a speed closer to the speed of rotation of the magnetic field, the force on it would reduce and it would begin to slow down. When it begins to slow down, the magnetic flux across it would change at a larger rate, so now it would experience a larger force. In this way, the rotor never comes to a stop, but never reaches the speed of the rotating magnetic field, either. Due to this reason, induction motors are said to be a type of asynchronous motor

The difference between the rotor’s speed and the speed of the rotating magnetic field is called the slip. The amount of slip is greater when a larger load is connected to the rotor. The video below gives an explanation to how an induction motor works.

Difference Between Induction Motor and Synchronous Motor

Rotor Speed

Rotors of a synchronous motor rotate at the same speed at which the magnetic field formed by the stator is rotating.

Rotor of an induction motor rotates slower compared to the magnetic fields produced by stators.

Self Starting

Synchronous motors are not self-starting.

Induction motors are self-starting.

Current Requirement

Synchronous motors require a DC current in order to create a static magnetic field across the rotor. This is usually produced from alternating current using slip rings and brushes. 

Induction motors do not require the rotor to be provided with a direct current.

Image Courtesy:

“3-phase electric induction motors (delta connection)…” by Zureks (Own work) [], via

About the Author: Nipun


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