INDUCTION MOTORS

The induction motor differs from other types of motor in that there is no electrical connection from the rotor winding to any source of electrical energy. The necessary voltage and current in the rotor circuit are produced by induction from the stator winding. So, its name is the induction motor. Like other electrical machines, the induction motor can operate as both a motor and a generator. The three-phase induction motor was invented by Dobrowolsky of Russia in 1890. Since then it has been improved and is in use in all countries.

Normally an induction motor consists of an annular core (the stator), which carries the primary winding in slots on its inner periphery. The principal winding is commonly arranged for three-phase power supply. The stator encircles a cylindrical rotor carrying the secondary winding in slots on its outer periphery.

Both the stator and the rotor are assembled of silicon steel punchings 0.35 mm and 0.5 mm thick, respectively. The individual punchings are given a cast of varnish in order to reduce eddy-current losses. The air gap between the stator and rotor is made as small as practicable (0.3. to 0.35 mm in small machines and 1 to 1.5 mm in high-power machines).

The rotor winding may be one of two types: squirrel cage or wound rotor (slip-ring) in British terminology). In the case of a squirrel – cage machine, the rotor winding consists simply of copper or aluminium bars embedded into slots in the iron core of the rotor and connected at each end by means of copper or aluminium rings; the rotor winding thus forms a complete closed circuit in itself, resembling a squirrel cage. The rotor winding of the wound-rotor (or slip-ring) machine is similar to the armature winding of the revolving armature type of a-c generator. The free ends of the winding are brought out to slip-rings. The rotor circuit is completed only when the slip-rings are connected either directly together or through some resistance external to the machine.

There are induction motors in which the relative positions of the primary and secondary windings are reversed relative to the ordinary induction motor. In addition to the primary winding, the rotor carries a second winding, which is connected to a commutator in the same manner as in a d-c motor. So the nameof this type of induction motor- the commutator type.

Two ends of each phase of the secondary winding (which is stationary) are connected to two brushes bearing upon the commutator. The position of the brushes can be varied at will. In this type the speed can be varied within broad limits in both single-phase and three-phase operation.

N. Translate the sentences from English into Russian, using the word stock of this lesson.

1. The squirrel-cage induction motor is one of the most widely used types of motors.

2. It consists of a stationary part or stator, carrying the primary winding, usually arranged for three-phase power supply.

3. Since the stator winding carries an alternating current, an alternating magnetic flux will thread through the stator core.

4. The advantages of squirrel-cage motor are:

a) nearly constant speed at various loads;

b) high overload capacity;

c) simplicity and robustness of construction;

d) simplicity of starting.

5. The squirrel-cage motors disadvantages may be summed up thus:

a) their speed can hardly be regulated;

b) the starting current is very high;

c) the power factor is very low at underload;

d) they are very sensitive to fluctuations in supply voltage.

6. The advantages of slip-ring motors are:

a) high starting torque;

b) high overload capacity;

c) constant speed at various loads;

d) low starting current in comparison with the squirrel-cage type;

e) adaptability to automatic starting.

7. The disadvantages are:

a) sensitivity to fluctuations in the supply voltage;

b) lower power factor and efficiency in comparison with the squirrel-cage type;

c) low power factor at under load.

O. Translate this text and say what this text is about, think of 10 questions covering the

text.

Date: 2014-12-28; view: 853