AC Motor

An AC motor is an electric machine that converts alternating current into mechanical rotation. AC motors’ applications range from industrial bulk power conversion from electrical to mechanical to household small power conversion. In this article, let us briefly discuss the various traits and working of AC motors.

Synchronous Motor
Induction Motor

Construction of an AC motor

An AC motor is driven by alternating current. The stationary stator and the rotating rotor are the important parts of the AC motors. In this section, let us study about the different parts of an AC motor.

AC Motor Diagram

Following are the different parts of an AC motor:


The stator is the stationary part of the motor that delivers a rotating magnetic field to interact with the rotor.

Stator of an AC Motor

Stator Core

The stator core is made of thin metal sheets, which are known as laminations. Laminations are used to reduce energy loss.

Stator Windings

Stator windings are stacked together, forming a hollow cylinder. The slots of the stator core coils of insulated wires are insulated.

When the assembled motor is in operation, the stator windings are connected to a power source. Each group of coils, along with the steel core, becomes an electromagnet when the current is applied.


A rotor is a central component of a motor that is fixed to the shaft. The most common type of rotor used in an AC motor is the squirrel cage rotor. A squirrel-cage rotor is cylindrical and is made by stacking thin steel laminations.

Squirrel Cage Rotor

Instead of inserting coils of wire between the slots, conductor bars are die-cast into the evenly spaced slots around the cylinder. Once the conductor bars are die-casted, they are electrically and mechanically connected to the end rings.

Motor Shaft

The rotor is pressed onto a steel shaft to form a rotor assembly. The shaft extends outside the motor casing allowing connection to an external system to transmit the rotational power.


Bearings hold the motor shaft in place. The bearings minimize the shaft’s friction connected to the casing, which increases the motor’s efficiency.


The enclosure protects the internal parts of the motor from water and other environmental elements. The enclosure consists of a frame and two end brackets.

Classification of AC Motor

The AC motor is classified into several types based on various types. Below, we have discussed the different types of AC motors in detail.

Based on the principle of operation, AC motors are classified as:

  • Synchronous Motor
  • Induction Motor

Classification of AC Motors

Synchronous Motor

The motor that runs at synchronous speed is known as the synchronous motor. The constant speed at which the motor generates the electromotive force is known as synchronous speed. An electromagnet in the rotating magnetic field magnetically locks itself with the rotating magnetic field and rotates simultaneously as the rotating field. This is where the name synchronous motor arrives from. This also means that synchronous motors have fixed speed. The synchronous speed can be calculated using the following formula:

AC Motor Synchronous Speed Formula



ns is the synchronous speed

f is the line voltage frequency in Hz

P is the number of poles

Working Principle of a Synchronous Motor

  • The synchronous motor works with two electrical inputs provided to it.
  • The stator is equipped with a 3 phase AC supply, while the rotor is provided with the DC supply.
  • The stator winding supplied with 3 phase AC supply generates 3 phase rotating magnetic flux. The rotor carrying DC supply produces a constant flux.
  • At a particular instant, the rotor and the stator poles might be of the same polarity (N-N or S-S), causing a repulsive force and the very next second it will be N-S causing an attractive force.
  • Due to this attractive and repulsive force, the motor cannot rotate in any direction and remains in a standstill position.
  • To overcome this resistance to motion, the rotor is initially fed mechanical input that rotates it in the same direction as the magnetic field. After some time, magnetic locking occurs, and the synchronous motor rotates in synchronism.

Induction Motor

Induction motors are the most commonly used motors. Induction motors are also known as asynchronous motors because they always run lower than synchronous speed.

Based on the type of rotor construction, they are divided into two types as follows:

  • Squirrel Cage Motor
  • Slip Ring Motor

Working Principle of Induction Motors

  • In an induction motor, the stator winding is fed with an AC supply. This causes the stator winding to develop an alternating flux. We call this rotating flux “Rotating Magnetic Field (RMF).”
  • According to Faraday’s Law of Electromagnetic Induction, the relative speed between the stator RMF and the rotor RMF causes an induced emf in the rotor conductors. Rotor conductors are short-circuited, and a rotor current is produced due to induced emf.
  • This induced current produces alternating flux around it. It should be noted that the stator flux lags behind the rotor flux.
  • Due to the relative velocity between the rotating stator flux and the rotor, the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. This is the basic working principle of the induction motor.
  • The difference between the synchronous speed (Ns) and the actual speed (N) of the rotor is known as the slip.
    \(\%s=\frac{N_s-N}{N_s}\times 100\)

Applications of AC Motors

AC motors are a preferred source of supply due to the following reasons:

  • Longevity: With only a few moving parts, AC motors have the potential to last for years. The durability of AC motors makes them a preferred solution for field applications such as agricultural equipment and commercial applications such as vending machines.
  • Efficiency: The speed-to-torque characteristics of AC motors allow them to provide excellent performance in many applications without overheating, degeneration or braking. This is why an AC motor is chosen for high demand applications such as pumps and packaging equipment.
  • Quiet Operation: Producing less noise, AC motors are ideal for applications in stores, hospitals and restaurants.
  • Availability: AC motors are available in a wide range of sizes and power outputs. This wide range makes it ideal for many applications.


  • AC Motor Definition – An AC is an electric motor that converts alternating current into mechanical power.
  • AC Motor Working Principle – An AC motor works on the principle of electromagnetic induction.
  • AC Motor Types – Based on the operation principle, they are classified into two types as synchronous motors and induction motors.

Frequently Asked Questions on AC Motor

When was the AC motor invented?

Nikola Tesla invented the first AC induction motor in 1887.

What are the advantages of an AC motor over a DC motor?

AC motors develop higher torque and are brushless. The speed and the power of an AC motor can be varied using a controller. An AC motor requires no commutator and brushes; hence there is no sparking.

What is the working principle of an AC motor?

AC motors generate magnetic flux and induced current inside the motor by the actions of the stator and the rotor and obtain rotational force.

What is a synchronous speed?

The constant speed at which the motor generates the electromotive force is known as synchronous speed.

How can you differentiate an AC motor from a DC motor?

If the motor has no commutator and brushes, then it is an AC motor. On the other hand, if the motor has a commutator and brushes, it is a DC motor.

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