Rotor part of 3 phase induction motor

How a 3 Phase AC Induction Motor Works

This article and video will focus on the basics of a 3 phase AC induction motor, one of today’s most common types of industrial electric motors. This overview will explain what 3 phase power is, how Faraday’s law works, understanding the main components of an induction motor, and the effect of the number of stator poles on the rated speed and torque of a motor.


You can also watch the video below for an overview of 3 phase AC induction motors.

Mike explains the basics of 3 phase industrial induction motors


What is 3 Phase Power?

The first concept we need to understand about a 3 phase induction motor is the first part of its name – 3 phase power.  A single phase power supply uses two wires to provide a sinusoidal voltage.  In a three phase system, three wires are used to provide the same sinusoidal voltage, but each phase is shifted by 120°.  At any point in time if you were to add up the voltage of each phase, the sum would be constant.  Single phase power is fine for residential or other low power applications, but three phase [JS2] power is typically required for industrial or higher power applications.  This is because it can transmit three times as much power while only using 1.5 times as much wire.  This makes for a more efficient and economical power supply.


magnetic rotation for three phase induction motor

What is Faraday’s Law?

Another underlying principle of AC induction motors comes from Faraday’s Law. The British scientist, Michael Faraday, discovered that a changing magnetic field can induce a current and conversely, a current can induce a magnetic field. By using the right hand rule, you can predict the direction of the magnetic field.  To do so, imagine grabbing a straight wire with your thumb pointing in the direction of the current. Your fingers would wrap around in the direction of the magnetic flux lines.


Right Hand rule of Faraday's Law
Mike gripping a marker to demonstrate the right hand rule

Components of a Induction Motor

An induction, or asynchronous, motor is composed of two main components: the stator and rotor.  The stator consists of the outer windings or magnets and is stationary.  The stator is stationary.  The rotor is the inner core and is what actually rotates in the motor.  The rotor rotates.

3 phase induction motor – rotor inside the stator

The squirrel cage design is the most common type of induction motor because they are self-starting, reliable, and economical.  In this design, the rotor looks similar to a hamster wheel or ‘squirrel cage,’ thus the name.  The rotor consists of an outer cylinder of metal bars that are shorted on the ends.  The interior consists of the shaft and a solid core built of steel laminations. 

Rotor part of 3 phase induction motor

How it works

To actually achieve torque at the motor shaft, a current is applied across the stator.  This creates a rotating magnetic field which in turn induces a current in the rotor.  Because of this induced current, the rotor also creates a magnetic field and starts to follow the stator due to magnetic attraction.  The rotor will turn slower than the stator field, and this is referred to as ‘slip.’  If the rotor were to turn at the same speed as the stator, no current would be induced, thus no torque.  The difference in speed ranges from 0.5-5% depending on the motor winding.


Windings and Poles

Three phase motors are available in configurations of 2, 4, 6, 8, and up poles.  The number of poles in the windings defines the motor’s ideal speed.  A motor with a higher number of poles will have a slower rated speed but a higher rated torque.  Because of this, high pole motors are sometimes referred to as torque motors and can be used to replace a motor using a gearbox.  The ideal relationship between the number of poles, frequency, and speed is defined by the following:

Relationship between the number of poles and the RPM of an induction motor.

Conclusion

3-phase AC induction motors are comprised of the stator and rotor.  During operation, a current is applied through the stator, which induces a magnetic field and leads to the rotation of the rotor.  The rotational speed of the shaft and the applied torque is dependent on the operating frequency and the number of pole pairs in the motor’s windings. If you are interested in our product line of induction motors, gearmotors or even servo motors, contact a KEB application engineer with the contact form below.


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