Designs of asynchronous electromechanics converters

Asynchronous EEC are concerned to the electric machines of induction type, that is to such which are excited by an alternating current.

An prevalent sector of the asynchronous machines use is electric motors of alternating current. The three-phase asynchronous motors are most widespread. Two-phase and especially single-phase motors meet rarely.

Stator design of asynchronous EEC has no differences from design of synchronous EEC stator. At connecting of three-phase source of alternating current of “f” frequency to the three-phase stator winding of asynchronous EEC a magnetic flux is appeared in air-gap between stator and rotor, that is revolved with the synchronous speed n₁:

n₁ = 60∙f / p, round / min

where p - quantity of poles pairs of stator winding.

This flux induces EMF in the phases of rotor winding, thanks to which the currents flow in phases. As a result of interaction of rotor winding currents with the stator magnetic flux the rotor is revolved in the direction of stator magnetic flux rotation. But the rotor rotation with the synchronous frequency is impossible, because in this case a rotor will be immobile relative to the stator magnetic flux and EMF in the rotor winding will not arise up. This the "asynchronous" name is conditioned.

There are two rotors designs of asynchronous EEC: phase rotor and short-circuited rotor.

The phase rotor has an ordinary three-phase winding, phases of which, as a rule, are connected by "star", and the free outputs of phases are connected with the contact rings which the brushes are imposed on. Thus, the phase rotor allows to change its parameters by introduction of additional resistances or EMF in the contour of rotor, that ensures possibility of, for example, control rotation frequency.

The sliding contact complicates asynchronous machine design with phase rotor and diminishes its reliability Therefore the asynchronous motors with the phase rotor find the limited application.

Asynchronous EEC with the short-circuited rotor or with the squirrel-cage rotor are most widespread (fig. 1.7).

The rotor of asynchronous EEC with the short-circuited winding has a laminated package 1 of the electrical engineering steel, rigid on the shaft 3. Bars 4 of a squirrel cage are placed in slots 2. Bars are connected by rings 5 on the butt ends of rotor package.

Rotor, represented on fig. 1.7, has bars Fig. 1.7 of round cross-section, but different shapes

of bar cross-sections happen: rectangular, oval, trapezoidal and more complicated configurations.

There are also short-circuited rotors with the double squirrel-cage. The short-circuited cages of rotors of industrial asynchronous motors usually outpour of the aluminium alloys. The squirrel cages of aviation asynchronous EEC are made copper, and the bars and rings weld between itself.

In the aircraft equipment asynchronous motors with the short-circuited rotors are used only.

The inverted design of asynchronous motors are used very rarely. There are also asynchronous motors with the aluminium hollow rotor. Such motors are used in some devices of automation. The asynchronous motor with the hollow rotor has two stators: outside and inside. The stator two-phase winding of such motors is located in the slots of only one stator. Second stator has not slots and needed for reduction of magnetic resistance only.