Working Principle of AC Motor

AC motors are widely used in industries for various industrial applications as well as in some home appliances. So, in this blog let's see what are they and how they work. Stay tuned and let's get started!

What is an AC Motor?

AC motor is an electrical machine that takes in AC and produces rotary motion. The difference between an AC motor and a DC motor is that an AC motor uses an AC power supply to work. Also in DC motor power passes/conducts through armature with the help of brushes and commutator that is why this motor is also called conduction motor. Whereas in an AC motor, instead of receiving power from conduction to the armature coil, here the power is received from the armature coil with the help of induction. Imagine we have a transformer. Similar to the power transferred from primary to secondary in a transformer in an AC motor the same phenomenon takes place.

Construction of AC Motor

Working Principle of AC Motor

It consists of two main parts:

Stator: It is the static or not moving part of the motor

Stator frame: provides stator core

Stator core: Laminated and slotted to carry the 3 phase windings

Rotor: It is the moving part of the motor

Two types based on rotor construction

Squirrel cage rotor and slip ring rotor.

Working principle of an AC Motor

When 3 Phase current flows in a 3-phase winding, a rotating magnetic field is created which has constant magnitude, but rotates as a function of the supply frequency. Here rotating magnetic field means its phase angle is changing(similar to how the phase angle of ac supply changes) due to the sinusoidal nature of the power supply.

Working principle of an AC Motor

This generated rotating magnetic field gets cut by the rotor conductor. So according to the faradays law change in magnetic field generates emf in the rotor which gives rise to the current.

This force F acts on the rotor and it rotates in the same direction as that of magnetic field.

Also, the current generated according to lenz law generates a opposite magnetic field which opposes the cause which has generated it. So, in order to reduce these eddy current losses lamination is done.

Types of an AC Motor

Types of an AC Motor

As we have seen above the rotor rotates in the same direction as that of the magnetic field. So depending on the speed the synchronous speed whether its equal to or greater than rotor speed motor’s are classified:

Synchronous Motor

When synchronous speed is equal to the rotor speed then the torque is zero and during this time motor is neither driving the load nor braking. So, a synchronous motor is a motor in which the speed of the rotor is equal to the rate at which the magnetic field is rotating.

Note: Synchronous speed is the rate at which the magnetic field is moving and not the stator. It is always greater than the rotor speed since it determines the rotor speed(slip between 0 and 1). If rotor speed is greater a generator action is produced and not motoring.

Asynchronous Motor

Speed of rotor is not equal to the rate at which the magnetic field is moving.

S is slip, Ns is synchronous speed and N is rotor speed.

Slip is the measure of difference between synchronous speed and shaft speed.

Asynchronous motors are again classified into single-phase and three-phase ac motors.

Asynchronous Motor

Single-phase AC motors

They work on a single-phase power supply. Single-phase power is the power supply which is used in most of the homes since power requirement is less.

Three-phase AC motors

They work on a three-phase power supply. The three-phase power supply is used in commercial and industrial areas since the electricity requirement is more. It splits the power supply for a load into three phases which decreases the load in one phase.

Example : If you have 3 machines running generally on a single-phase u can make each machine run on one phase in case of 3 phase power supply thus reducing the load effect on single phase.

Advantages of AC Motor

Design simple, AC source can be directly connected to the motor, Maintenance less, good for applications where high motor speed is required.

Disadvantages of AC Motor

Starting torque is less, it is difficult to operate at low speed, and eddy current loss can be high

Application of AC Motor

Following are the applications of the AC motor:

  1. Hydraulic and irrigation pumps
  2. Conveyor systems
  3. Fans and air conditioners.
  4. Compressor drives and systems.
  5. Water pumps.
  6. Refrigerator.


In this article, we have seen what is AC motor, its working principle, different types of AC motor, and their applications.

Hope this article helped you. Feel free to comment if you have any doubts or questions. 

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