What Is an Induction Electric Motor?

All electric motors convert electrical energy into mechanical energy, to power larger machines, appliances and vehicles. There are many different types of machines which use electricity to produce mechanical energy, with distinctions being made between differing sources of electrical energy, internal construction, application and the type of motion given.

 

Induction electric motors are classified according to the source of electrical energy and particular construction. Induction models, also known as asynchronous motors, are powered by an alternating current (AC) by the means of electromagnetic induction, instead of the commutators that are used in other AC motors. Induction motors are commonly used across industry as well as in domestic appliances such as fridges, washing machines, dishwashers and clothes driers.

 

Induction electric motors were the first type of AC machine to be invented, by Nikola Tesla in 1883. As such, induction motors follow a simple principle of construction and operation compared to more modern designs, but are still very rugged, quiet and long lasting. Induction motors are distinctive because they use an induced current in the rotor to produce rotational motion.

 

In induction motors, the stators are arranged so that when fed with a polyphase supply, they create a rotating magnetic field that induces a current. This current interacts with the rotating magnetic field, causing motion. In order for the current to be induced in the rotor, the speed of the physical rotor must be less than the rotating magnetic field, causing a motion that is asynchronous.

 

In synchronous electric motors, to compare, the speed of the rotor matches the speed of the rotating magnetic field at a steady state. Synchronous models work by imposing a torque on the magnetic field of the rotor through the rotating magnetic field of the stator, causing steady rotation.

 

Induction motors consist of two main parts, the wound stator and the rotor assembly. The stator windings are distributed in slots around the stator, with an equal number of north and south poles. The rotor assembly is available in three variations: squirrel cage rotors, slip ring rotors and solid core rotors.

 

Modern squirrel cage rotors are made up of skewed bars of cast-aluminium conductors and short-circuiting end rings. They are named for their abstract resemblance to a rodent’s rotating exercise cage; although in reality the rotor is solidly constructed. Generally, these motors run on single phase or three-phase power, although some two-phase models are available. In theory induction electric motors could run on any number of phases.

First and foremost you will need to know what voltage source is available in your application. Electric motors can be classified as either AC (Alternating Current) or DC (Direct Current). Alternating current types only run on AC Voltage and direct current types only run on DC Voltage. There is also a universal motor that can run on both AC and DC voltages.

 

Once you have established which power source you have you will need to determine which style will work for your application. AC motors can be sub-divided into the following: Single Phase Induction, Three Phase Induction, Two Phase Servo, and Hysteresis Synchronous. DC motors can be sub-divided into: Brushless DC, Brush DC, and Stepper types.

 

Next we need to understand the different characteristics of each type in order to properly match a motor to its application.

 

A single phase induction motor is connected to a single voltage line. An external capacitor is required to make this motor operate. The different types of single phase induction motors are distinguished by which method they are started. The four basic types are: split phase, capacitor start, permanent split capacitor, and capacitor start/capacitor run.

 

A split phase motor uses a switching device to disconnect the start winding once the motor gets to 75% of its rated speed. Although this type has a simple design which makes it less expensive for commercial use, it also has low starting torques and high starting currents.