Introduction to Induction Machine | Use of Induction Motor:
The Introduction to Induction Machine is an important class of electric machines which finds wide applicability as a motor in industry and in its single-phase form in several domestic applications. More than 85% of industrial motors in use today are in fact induction motors. It is substantially a constant-speed motor with a shunt characteristic; a few per cent speed drop from no-load to full-load. It is a singly-fed motor (stator-fed), unlike the synchronous motor which requires ac supply on the stator side and dc excitation on the rotor.
The torque developed in this motor has its origin in current induction in the rotor which is only possible at non-synchronous speed; hence the name asynchronous machine.
Torque in a synchronous machine on the other hand, is developed only at synchronous speed when the “locking” of the two fields takes place. Therefore, the induction motor is not plagued by the stability problem inherent in the synchronous motor. Since it is a singly-fed machine, it draws its excitation current from the mains to set up the rotating field in the air-gap which is essential for its operation.
As a consequence it inherently has a power factor less than unity which usually must be corrected by means of shunt capacitors at motor terminals. There is no simple and inexpensive method of controlling the induction motor speed as is possible in a dc shunt motor.
A wide range of speed control is possible only by expensive circuitry using silicon-controlled rectifiers (SCRs). It still finds stiff competition from the dc shunt motor in such applications.