Induction Motor

an electrical asynchronous machine for converting electrical energy into mechanical. It operates on the principle of an interaction between a rotating magnetic field, resulting from the passage of three-phase alternating current through windings on the stator, and the current induced by the stator in windings on the rotor. The interaction develops mechanical forces which make the rotor turn in the direction of rotation of the magnetic field, provided that the rate of rotation for the rotor n is less than the rate of rotation of the field n_1. Thus the rotor performs asynchronous rotation with respect to the field.

The phenomenon called magnetism of rotation was first demonstrated by the French physicist D.-F.-J. Arago (1824). He showed that a copper disk fastened to a vertical axis begins to rotate if a permanent magnet is rotated above it. Fifty-five years later, on June 28, 1879, the English scientist W. Bailey obtained a rotating magnetic field by successively connecting the windings of four bar electromagnets to a direct current source. In articles by M. Deprez (France, 1880–1883), E. Thomson (USA, 1887), and others, devices are described which are also based on the properties of a rotating magnetic field. However, an accurate scientific exposition of this phenomenon was given for the first time, almost simultaneously and independently of each other, in 1888 by the Italian physicist G. Ferraris and the Croatian engineer and scientist N. Tesla.

The two-phase induction motor was invented by N. Tesla in 1887 (English patent no. 6,481), and he reported it publicly in 1888. This type of induction motor did not become popular, mainly because of the poor starting characteristics. In 1889, M. O. Dolivo-Dobrovol’skii designed and tested the world’s first three-phase induction motor in which a “squirrel-cage” type of rotor was used (German patent no. 51,083), and the stator winding was placed in slots around the entire periphery of the stator. In 1890, Dolivo-Dobrovol’skii invented the phase-wound rotor, with slip rings and starting devices (English patent no. 20,425 and German patent no. 75,361). After two years he proposed a rotor design known as the “double squirrel cage” which, however, was used extensively only after 1898 thanks to the work of the French engineer P. Boucherot, who introduced an induction motor with a rotor like the motor with special starting characteristics.

The structural design of induction motors, their power, and their dimensions depend on the purpose and operating conditions. For example, there are motors with air and water cooling (general application); hermetically sealed, oil-Med (for electrodrills) and explosion-proof motors (for use in mines, dangerously explosive locations, and the like); dust-proof and spray-proof types (for use under marine conditions and in a tropical climate); and so forth. Some types of induction motors (such as servomotors for servo systems, circuits in automation and telemechanics with graded control of speed, and others) are developed and manufactured complete with control units, starter-protection apparatus, and built-in reduction gears. By comparison with single-phase induction motors the three-phase type has better starting and operating characteristics. The principal structural elements are a stator (the immovable part) and a rotor (the rotating part). According to the design of the rotor winding, induction motors are divided into wound-rotor and squirrel-cage motors. The air gap between stator and rotor is made as small as possible (down to 0.25 mm). The rotation rate of the rotor is a function of the rotation rate for the magnetic field of the stator, which is controlled by the power frequency and the number of pole pairs.

When a squirrel-cage induction motor is started, the starting current is between four and seven times greater than the nominal value. Therefore, direct connection to the power line is only used for motors up to a 200 kW power rating. The more powerful squirrel-cage induction motors are connected at first to a lower voltage so that the starting current is reduced by a factor of three or four. For the same reason, use is made of an autotransformer which is switched in series with the stator winding during starting. The starting current of phase-wound motors is limited by means of a starting resistance in the rotor circuit which is gradually reduced as the rotor picks up speed. After the start the rotor winding is short-circuited. In order to reduce frictional losses and brush wear, the brushes are usually raised by a brush-lifting mechanism which short-circuits the rotor winding beforehand through a ring.

The rotation rate of an induction motor is controlled primarily by a change in the number of pole pairs, by the resistance connected in the rotor circuit, by variation in the power supply frequency, and also by a cascade connection of several machines. The direction of rotation can be altered by changing over any two phases of the stator windings.

Because it is simple to manufacture and reliable to operate, the induction motor is widely employed in electric drives. Its chief drawbacks are the limited speed control range and the substantial reactive power drain under low load conditions. The creation of static, controlled, semiconductor frequency converters substantially broadens the field of application for induction motors in automatically controlled electric drives.