AC Motor

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

What is an AC Motor?

An AC motor is a motor that converts alternating current into mechanical power. The stator and the rotor are important parts of AC motors. The stator is the stationary part of the motor, and the rotor is the rotating part of the motor. The AC motor may be single-phase or three-phase. Nikola Tesla invented the first AC induction motor in 1887.

Construction of an AC motor

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

AC Motor Diagram

Following are the different parts of an AC motor:

Stator

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

Stator Core

The stator core is made of thin metal sheets 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 operates, 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.

Rotor

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.

Instead of inserting wire coils 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

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

Enclosure

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

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

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 with the rotating field. This is where the name synchronous motor comes from. This also means that synchronous motors have fixed speeds. The synchronous speed can be calculated using the following formula:

AC Motor Synchronous Speed Formula

where n_s is the synchronous speed, f is the line voltage frequency in Hz, and 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 slower 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 the stator flux to minimize the relative velocity. This is the basic working principle of the induction motor.
• The difference between the synchronous speed (N_s) and the actual speed (N) of the rotor is known as the slip.
  
  \(\begin{array}{l}\%s=\frac{N_s-N}{N_s}\times 100\end{array} \)

Watch the video and understand the concept of electromagnetism with the help of animations.

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.

Overview of AC Motor