Types of AC Motors

Different Types of Motors

Electric motors play a significant role in various industrial sectors and also have wide-ranging applications in several commercial and domestic areas. There are many varieties of electric motors available in the market, each of which differs in terms of operation, voltage, and use cases. 


In general, every motor has two crucial components – the armature winding and the field winding. The field winding is responsible for producing a fixed magnetic field, whereas the armature winding has the appearance of a conductor and is arranged inside the magnetic field. Due to the magnetic field, the armature winding utilises energy and generates a torque that makes the motor shaft turn.


The different types of electric motors are classified into three main segments – DC motor, AC motor, and motors for particular purposes. 


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AC Motor Working Principle

Electric motors use the current flowing through electromagnets for the conversion of electrical energy into rotational mechanical energy. The electromagnets are present within nested rings, and the magnets' polarities alternate in the rings. Rotor magnets are moving, while stator magnets are stationary. The north-south polarity of the electromagnets reverses constantly.


An AC motor has stator windings to which an alternating current is applied for producing a rotating magnetic field. Due to the rotation of the magnetic field, an AC motor does not require any mechanical aid or power to be put on the rotor. The rotor rotates via the magnetic field and creates a torque on the motor's driveshaft. The stator has a fixed number of magnetic poles which decide the speed of rotation, known as the synchronous speed. However, AC induction motors operate with a slip or lag to allow the rotor current to flow.


Construction of an AC Motor:

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Classification of AC Motor

So how many types of motor classifications are there? The different types of AC motor are differentiated based on the following four criteria:

1. Principle of Operation

  1. Synchronous Motors: Synchronous motors mainly work on a three-phase supply. In such motors, the stator rotates at a stable speed dictated by the AC frequency and is responsible for generating the field current. The rotor is connected to the load and turns at the same speed as that of the stator current. Thus, there is no air gap or slip between the rotor and the speed of the stator current. Based on how the rotor is magnetized, synchronous motors are of two types:

  • Non-excited and

  • Direct current-excited

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  1. Induction Motors: Also known as asynchronous motors, induction motors have an asynchronous speed that slips with respect to the stator current. Such motors utilise electromagnetic induction to switch from electrical energy to mechanical energy. Based on the construction of the rotor, induction motors are of the following types:

  • Squirrel cage and

  • Slip-ring

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  1. Another type of asynchronous motor is Commutator motor which has the following models:

  • Series

  • Compensated

  • Shunt 

  • Repulsion

  • Repulsion-start induction

  • Repulsion induction

2. Type of Current

Based on the supply phase of the current, AC motors are of the following types:

  • Single-phase and

  • Three-phase

3. Speed of Operation

In this classification, AC motors are of three types:

  • Variable speed

  • Constant speed

  • Adjustable speed

4. Structural Features

Based on structural features, AC motors are divided into the following categories:

  • Open

  • Semi-enclosed

  • Enclosed

  • Ventilated

  • Pipe-ventilated

  • Riveted frame-eye


Applications of AC Motor

  • Synchronous motors are ideal for automation processes such as robotics and positioning devices that require high precision.

  • Asynchronous or induction motors are mainly used for pumping water to overhead tanks, boiler feed pumps in power plants, and compressors.

  • Single-phase motors are suitable for applications that demand low power, such as domestic appliances (including mixer grinders, ceiling fans, portable power tools, and more). The three-phase supply motors cater to high power requirements such as hydraulic pumps, power drives of compressors, irrigation pumps, compressors of air-conditioners, etc.

  • Adjustable and variable speed motors have applications in seawater cooling pumps and cargo pumps in ships. 

FAQ (Frequently Asked Questions)

1. How are AC Motors Different from DC Motors?

Ans:

  • Nature of the Input Current: For AC motors, the primary input power is alternating current. But for DC motors, direct current provides the input power.

  • Commutator: AC motors do not have commutators. DC motors are equipped with commutators.

  • Supply Phase: Single, as well as three-phase supply, is used for AC motors. DC motors operate only with single-phase supply.

  • Input Terminals: While AC motors have three input terminals (RYB), DC motors have only two input terminals (+, -).

  • Brushes: There are no carbon brushes in an AC motor. DC motors have carbon brushes for spark-free commutation.

  • Armature Position: In AC motors, the magnetic field is rotating, and the armature is static. In DC motors, the armature rotates, and the magnetic field is stationary.

  • Starting: AC motors require external equipment to start. DC motors do not need external help for the start-up operation.

2. What are the Advantages and Disadvantages of Using AC Motors?

Ans:

Advantages

  • Cost-Effective: Due to its simple design, AC motors are a cost-effective option for various fixed speed applications in the industrial, domestic, and commercial sectors where AC power is readily available. AC motors are frequently found in industrial machinery, air-conditioners, washers, dryers, etc.

  • Speed Variation: AC motors have a fixed number of built-in winding sets that determine its base speed. The motor speed can be altered by changing the frequency of the AC voltage.

  • High Power Factor: AC motors have a high power factor. Thus, it draws a lesser current, reducing the energy loss in the distribution system. 

  • Reliability: An AC motor can operate for years without any frequent needs for replacement.

Disadvantages

  • AC motors cannot operate at speeds lower than 1/3rd of its base speed.

  • AC motors have a crude and expensive positioning control.

  • Eddy currents slow down AC motors.