Working principle of 3-Phase induction motor

Definition

A 3-phase induction motor is an AC motor in which the rotor current is induced by the electromagnetic action of a rotating magnetic field produced by a 3-phase stator supply, and the rotor rotates at a speed less than synchronous speed.

Main Content

1. Rotating Magnetic Field in the Stator

• When a 3-phase AC supply is given to the stator windings, three currents of equal magnitude and frequency flow through the three windings, each displaced by 120° electrical.
• These three alternating currents produce three magnetic fluxes that are also 120° apart in phase. Their resultant is a magnetic field of constant magnitude that rotates around the stator at synchronous speed.

The concept of the rotating magnetic field is the foundation of motor action. If the stator has windings placed 120° apart in space and connected to a 3-phase supply, the flux produced by each phase combines vectorially. At every instant, the resultant magnetic field does not remain stationary; instead, it rotates smoothly. The synchronous speed of this rotating field is given by:

$$N_s = \frac{120f}{P}$$

where:

• $N_s$ = synchronous speed in rpm
• $f$ = supply frequency in Hz
• $P$ = number of poles

For example, in a 50 Hz, 4-pole motor, the synchronous speed is 1500 rpm. The rotating field is always present in the air gap between stator and rotor, and its speed depends only on supply frequency and number of poles, not on load.

2. Electromagnetic Induction in the Rotor

• The rotating magnetic field cuts the rotor conductors, so by Faraday’s law of electromagnetic induction, an emf is induced in the rotor bars.
• Since the rotor circuit is closed, this induced emf causes current to flow in the rotor, and the interaction between rotor current and stator magnetic field produces torque.

The rotor of an induction motor may be squirrel-cage type or wound type, but in both cases the principle remains the same. The rotor does not receive electrical supply directly; instead, it works on induction. When the rotating field sweeps past the stationary rotor at starting, the relative speed is maximum, so the induced emf and rotor current are high. As the rotor begins to move, the relative speed decreases, but induction continues as long as there is slip between the rotor and the rotating field.

The induced rotor current is proportional to the relative speed between the rotating field and the rotor. The greater the slip, the larger the induced emf. This is why the motor draws current in the rotor only when there is relative motion between the field and rotor conductors.

3. Torque Production and Slip

• The interaction between the stator rotating magnetic field and the rotor currents produces electromagnetic torque, causing the rotor to rotate in the same direction as the rotating field.
• The rotor never reaches synchronous speed; it always runs slightly below it so that induction can continue. This speed difference is called slip.

Slip is essential for the working of an induction motor. It is defined as:

$$s = \frac{N_s - N_r}{N_s}$$

where:

• $s$ = slip
• $N_s$ = synchronous speed
• $N_r$ = rotor speed

If the rotor were to reach synchronous speed, there would be no relative motion between the rotor and rotating magnetic field, so no emf would be induced in the rotor, no current would flow, and no torque would be produced. Therefore, the rotor must run slightly below synchronous speed to maintain the induction process.

At starting, slip is 1 or 100% because the rotor is stationary. During running, slip becomes small, usually 2% to 6% depending on the load and motor design. As the load increases, rotor speed decreases slightly, slip increases, more rotor current is induced, and greater torque is produced to meet the load demand.

Working / Process

1. A 3-phase AC supply is applied to the stator windings, producing a rotating magnetic field at synchronous speed.
2. The rotating magnetic field cuts the stationary rotor conductors and induces emf and current in the rotor circuit.
3. The interaction between the rotor current and the rotating magnetic field generates torque, so the rotor starts rotating in the same direction as the field, but at a speed slightly less than synchronous speed.

Advantages / Applications

• Simple and robust construction with no brushes or commutator in squirrel-cage type, making maintenance low and reliability high.
• Self-starting operation and efficient performance, especially suitable for constant-speed industrial drives.
• Widely used in fans, pumps, compressors, conveyors, machine tools, blowers, and many manufacturing systems.

Summary

• A 3-phase induction motor works on the principle of electromagnetic induction.
• A 3-phase supply produces a rotating magnetic field in the stator.

• The rotating field induces current in the rotor, and the interaction of field and rotor current produces torque.

• Important terms to remember

• Rotating magnetic field
• Synchronous speed
• Slip
• Electromagnetic induction
• Rotor current
• Torque