Working Principle and Types of Induction Motors

An induction motor is a type of AC motors in which the rotor current is not supplied from outside. It is induced in the rotor by the rotating (or pulsating) magnetic field of the stator winding. That is why only the stator is connected to the supply. This article explains the types, the working principle of induction motors, synchronous speed and slip, with a worked example and practice questions.

Induction motors are robust, simple and low in cost. That is why they are used in industry and in homes, from pumps and fans to conveyors and compressors.

Table of Contents

• Main Parts and construction
• Working principle
• Types of induction motors
• Key terms and formulas
• Why the rotor runs below synchronous speed
• Worked example
• Common mistakes
• Practice questions
• Key takeaways
• Frequently Asked Questions

Main parts and construction of an Induction Motor

The above cutaway image illustrates a squirrel cage induction motor, with key components numbered and described below. Note that slip-ring (wound-rotor) motors feature an insulated winding on the rotor instead of short-circuited bars

1. Shaft: Transfers mechanical power to the Load.
2. Bearings: Support the shaft and allow smooth rotation.
3. Stator: Stationary part with laminated iron core and stator windings
4. Rotor (Squirrel Cage): Rotating part with aluminum/copper bars short circuited by end rings.
   In slip-ring type induction motors, rotor has windings instead of bars.
5. Frame (Yoke): Supports all parts and provides mechanical strength.
6. Mounting base: To securely anchor an induction motor to a floor, frame, or equipment chassis
7. Terminal box: For electrical connections of stator windings.
8. Colling Fan: Circulates air for cooling.

Please check out Three-Phase Induction Motor Construction: Interactive 3D Model

Working principle

Think of the motor as a transformer that is free to turn. The stator is like the primary. The rotor is like the secondary. Energy passes from stator to rotor by induction, and the rotor converts it into mechanical torque. In a DC motor, by contrast, the armature is connected to the supply through the commutator. In an induction motor, the rotor has no supply connection.

Here is how an induction motor works:

1. The three-phase supply feeds the stator winding. The three currents are 120° apart in time, so together they produce a rotating magnetic field (RMF).
2. The RMF turns at synchronous speed N_s. It cuts the rotor conductors, and an emf is induced in them (Faraday's law).
3. The rotor circuit is closed (short-circuited), so the induced emf drives a current through the rotor conductors.
4. By Lenz's law, this current acts to oppose the cause that produces it. The cause is the relative motion between the RMF and the rotor. The rotor therefore tries to follow the field and reduce that relative motion.
5. The rotor field interacts with the stator field and produces torque. The rotor accelerates in the direction of the RMF.
6. The rotor settles at a speed N slightly below N_s. The difference is called slip.

Scope note: A single-phase winding produces a pulsating field, not a rotating one. A single-phase induction motor therefore needs a separate starting arrangement. The steps above describe the three-phase motor.

Also Read: Difference between synchronous and induction motor

Types of induction motors

Induction motors are classified in two ways.

1. Based on input supply:
   • Single-phase induction motor
   • Three-phase induction motor
2. Based on rotor construction:
   • Squirrel cage induction motor: rotor bars are permanently short-circuited by end rings.
   • Slip-ring (wound rotor) induction motor: rotor winding is brought out to slip rings, so external resistance can be added.

Key terms and formulas

Synchronous speed (N_s)

This is the speed of the stator's rotating field.

N_s = 120f / P   (rpm)

• f = supply frequency (Hz)
• P = number of poles (not pole pairs)

At 50 Hz:

Poles (P)	Synchronous speed Ns (rpm)
2	3000
4	1500
6	1000
8	750

Slip (s)

Slip tells you how far the rotor lags behind the field, as a fraction of synchronous speed.

s = (N_s − N) / N_s   (per unit)
% slip = s × 100
N = N_s(1 − s)

Here N is the rotor speed in rpm. Slip has no unit.

Rotor frequency (f_r)

The rotor sees the field moving past it at the relative speed N_s − N, not at N_s. So the emf and current in the rotor have a lower frequency than the supply:

f_r = s f

At standstill s = 1, so f_r = f. As the rotor speeds up, f_r falls.

Why the rotor runs below synchronous speed

Suppose the rotor reached N_s. Then the rotor and the field would move together. There would be no relative motion, so no emf would be induced in the rotor. With no emf there is no rotor current, and with no current there is no torque.

Friction and load would then slow the rotor a little. Relative motion returns, emf and current return, and torque returns. The rotor settles at the speed where its torque exactly balances the load torque. That speed is always a little below N_s, so slip is never zero in a loaded motor.

Scope note: This applies to motoring operation. If a prime mover drives the rotor above synchronous speed, slip becomes negative and the machine acts as an induction generator.

Worked example

A 4-pole, 50 Hz, three-phase induction motor runs at 1440 rpm at full load. Find (a) the synchronous speed, (b) the slip, (c) the rotor frequency, and (d) the speed of the rotating field relative to the rotor.

(a) Synchronous speed:
N_s = 120f / P = 120 × 50 / 4 = 1500 rpm

(b) Slip:
s = (1500 − 1440) / 1500 = 60 / 1500 = 0.04, which is 4 %

(c) Rotor frequency:
f_r = s f = 0.04 × 50 = 2 Hz

(d) Relative speed:
N_s − N = 1500 − 1440 = 60 rpm. This equals 1 rev/s, which matches 2 Hz in a 4-pole motor: 2 Hz × 60 × 2 / 4 = 60 rpm.

Check: N = N_s(1 − s) = 1500 × 0.96 = 1440 rpm. It matches.

What this shows: The rotor field slips past the rotor at only 60 rpm, so the rotor emf changes slowly (2 Hz). This is why rotor iron losses in a running induction motor are small.

Common mistakes

1. Using pole pairs in 120f/P. P is the number of poles. A 4-pole motor has P = 4, not 2.
2. Calling slip an rpm value. Slip is a fraction (or percentage) of N_s. The rpm difference N_s − N is the slip speed.
3. Thinking the rotor is connected to the supply. Only the stator is. The rotor current is induced.
4. Saying the rotor can reach N_s. At s = 0 there is no induced emf and no torque.
5. Assuming a single-phase winding gives a rotating field by itself. It gives a pulsating field, so a single-phase motor needs a starting arrangement.

Practice questions

1. A 6-pole, 50 Hz induction motor runs at 960 rpm. Find the slip.
2. What is the synchronous speed of a 2-pole motor on a 50 Hz supply?
3. A 4-pole, 50 Hz motor has 3 % slip. Find its speed.
4. A 50 Hz motor has a slip of 0.05. Find the rotor frequency.
5. Why can an induction motor not run at synchronous speed?

Answers:

1. N_s = 120 × 50 / 6 = 1000 rpm. s = (1000 − 960) / 1000 = 0.04 (4 %).
2. N_s = 120 × 50 / 2 = 3000 rpm.
3. N_s = 1500 rpm. N = 1500 × (1 − 0.03) = 1455 rpm.
4. f_r = s f = 0.05 × 50 = 2.5 Hz.
5. At N_s there is no relative motion between field and rotor, so no emf, no current and no torque.

Also Read: Torque-Slip Calculator: Plot the Torque-Speed Curve and Torque equation of three phase induction motor

Key takeaways

• In an induction motor only the stator is supplied. The rotor current is induced.
• A three-phase supply produces a rotating field at N_s = 120f/P.
• Slip s = (N_s − N)/N_s, and rotor frequency is f_r = sf.
• Without slip there is no emf, no current and no torque, so the motor cannot run at N_s.
• Rotor types: squirrel cage and slip-ring (wound rotor).

Also Read: Starting methods of three phase induction motors and Speed control methods of induction motor

FAQ on induction motors

Why is it called an induction motor?

Because the rotor current is produced by electromagnetic induction from the stator field, not by a supply connected to the rotor.

Why is it also called an asynchronous motor?

Because the rotor runs at a speed different from the synchronous speed of the rotating field.

What is the difference between synchronous speed and rotor speed?

Synchronous speed is the speed of the stator's rotating field, N_s = 120f/P. Rotor speed N is always a little lower in motoring operation. Their difference gives the slip.

Does an induction motor have zero slip at no load?

No. Slip is very small at no load, but not zero. A small torque is still needed to overcome friction and windage.