An induction motor is simply an electric transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and the other the secondary winding. Alternating current supplied to the primary winding from an electric power system induces an opposing current in the secondary winding, when latter is short-circuited or closed through an external impedance. Relative motion between the primary and secondary structures is produced by the electromagnetic forces corresponding to the power thus transferred across the air gap of induction.
The essential feature which distinguishes the induction machine from other types of electric motors is that the secondary currents are created solely by induction, as in a transformer instead of being supplied by a D.C. exciter or other external power source, as in synchronous and D.C. machines.
Advantages. Three-phase induction motor is the most commonly used motor in industrial applications because of the advantages listed below:
- Simple design
- Rugged construction
- Reliable operation
- Low initial cost
- Easy operation and simple maintenance
- High efficiency
- Simple control gear for starting and speed control.
Induction motors are available with torque characteristics suitable for a wide variety of loads:
(i) The standard motor has a starting torque of about 120 to 150 per cent of full-load torque. Such motors are suitable for most applications.
(ii) For starting loads such as small refrigerating machines or plunger pumps operating against full pressure or belt conveyors, high torque motors with a starting torque of twice normal full-load torque, or move, are used.
(iii) For driving machines that use large flywheels to carry peak loads, such as punch presses and shears, a high-torque motor with a slip at full-load up to 10 per cent is available. The high slip permits enough change in speed to make possible the proper functioning of the flywheel.
(iv) By the use of a wound-rotor with suitable controller and external resistances connected in series with the motor winding, it is possible to obtain any value of starting torque up to the maximum breakdown torque. Such motors are well adapted as constant-speed drives for loads that have large friction loads to overcome at starting.