Three Phase Induction Motor Notes for SSC JE Electrical Engineering

The three phase induction motor is the most widely used electric motor in industry, because it is simple, rugged and cheap. It is a very important chapter of Electrical Machines in the SSC JE paper, with questions on the synchronous speed, the slip, the torque, the types and the starters. These three phase induction motor notes for SSC JE explain every point in simple tables with solved examples and practice questions.

Working Principle

• When a three phase supply is given to the stator winding, a rotating magnetic field is produced.
• The rotating field cuts the rotor conductors and induces an EMF and a current in them (like in a transformer, so it is called an induction motor).
• The rotor current in the magnetic field produces a torque, and the rotor starts to rotate in the direction of the rotating field.
• The rotor can never reach the synchronous speed, because then there would be no relative motion and no induced EMF.

Synchronous Speed and Slip

Example. A 4-pole motor runs on a 50 Hz supply with a slip of 4 per cent. Ns = 120 × 50 / 4 = 1500 rpm. Rotor speed = 1500 × (1 − 0.04) = 1440 rpm, and rotor frequency = 0.04 × 50 = 2 Hz.

Types of Three Phase Induction Motors

Torque and Slip

• Rotor power flow: Rotor input : Rotor copper loss : Mechanical power developed = 1 : s : (1 − s).
• The torque is proportional to s E2² R2 / (R2² + (s X2)²).
• The maximum torque occurs at the slip s = R2 / X2.
• The value of the maximum torque does not depend on the rotor resistance, but the slip at which it occurs does.
• The starting torque is maximum when the rotor resistance equals the rotor reactance (R2 = X2).
• The starting current of a motor is high, about 5 to 7 times the full load current.

Starters

Speed Control

Other Facts

• The direction of rotation is reversed by interchanging any two of the three supply leads.
• The power factor of an induction motor is low at no load (about 0.1 to 0.2) and improves with the load.
• The speed of an induction motor falls a little as the load increases.
• The synchronous motor, by comparison, runs exactly at the synchronous speed and is not self-starting.
• Single phase induction motors (split phase, capacitor start and shaded pole) are used in homes, such as in fans and washing machines.

Quick Tips

• Learn Ns = 120 f / P and the slip formula with the units in rpm.
• Remember that the slip is 1 at the start and almost zero at no load.
• Remember that a slip ring motor has a higher starting torque.
• Remember that the star-delta starter reduces the starting current to one-third.

Quick Revision Points

• Induction motor works on the rotating magnetic field and electromagnetic induction.
• Ns = 120 f / P.
• Slip = (Ns − Nr) / Ns.
• Maximum torque occurs at s = R2 / X2.
• Reversal of direction needs the interchange of any two supply leads.

Practice Questions With Answers

1. What is the formula of synchronous speed?
   Answer: Ns = 120 f / P
2. What is the synchronous speed of a 6-pole motor on a 50 Hz supply?
   Answer: 1000 rpm
3. What is the slip at the instant of starting?
   Answer: 1 (100 per cent)
4. A 4-pole, 50 Hz motor runs at 1440 rpm. Find the slip.
   Answer: 4 per cent
5. Which type of induction motor has a higher starting torque?
   Answer: Slip ring (wound rotor) motor
6. Which starter reduces the starting current to 1/3?
   Answer: Star-delta starter
7. What is the frequency of the rotor current at the start?
   Answer: Equal to the supply frequency
8. How can the direction of an induction motor be reversed?
   Answer: By interchanging any two supply leads
9. At which slip does the maximum torque occur?
   Answer: s = R2 / X2
10. Which induction motor is mostly used in industry?
    Answer: Squirrel cage induction motor

How to Practise This Chapter

Solve three numericals on speed and slip daily and revise the table of starters and speed control. Then solve questions from our practice papers, the free SSC quizzes and the previous year papers page. Read the previous chapters, Network Theorems and Transformer, and the next chapter, Electrical Measuring Instruments. See all topics on the SSC JE Electrical Engineering notes page.

Frequently Asked Questions

What is an induction motor?

An induction motor is an AC motor in which the rotor current is produced by induction from the rotating magnetic field of the stator.

What is the principle of an induction motor?

It works on the principle of a rotating magnetic field and electromagnetic induction.

What is synchronous speed?

Synchronous speed is the speed of the rotating magnetic field, equal to 120 times the frequency divided by the number of poles.

What is slip in an induction motor?

Slip is the difference between the synchronous speed and the rotor speed, divided by the synchronous speed.

Why can an induction motor not run at the synchronous speed?

At the synchronous speed there is no relative motion between the field and the rotor, so no EMF and no torque would be produced.

What is the difference between squirrel cage and slip ring motors?

A squirrel cage motor has a simple short-circuited rotor and low starting torque, while a slip ring motor has a wound rotor with external resistance and high starting torque.

What is the use of a starter?

A starter reduces the high starting current of the motor and protects the supply and the motor.

How does a star-delta starter work?

The motor is started in star connection, which reduces the voltage and the current, and then it is changed to delta for running.

How is the speed of an induction motor controlled?

By changing the supply frequency, the number of poles, the stator voltage or the rotor resistance in a slip ring motor.

How can I reverse the direction of an induction motor?

Interchange any two of the three supply leads of the stator.

Is the induction motor important for SSC JE Electrical?

Yes. It is a key topic of Electrical Machines and gives numerical and theory questions every year.

How many questions come from induction motors in SSC JE?

Usually three to five questions come from induction motors, but the number changes from year to year.