AC Electrics - AC Motors 15

The Induction Motor

The induction motor gets its name from the fact that an alternating current is induced in the rotor by the rotating magnetic field in the stator.

It is the most commonly used because of its simplicity, its robustness and because it is relatively cheap to produce.

This relative cheapness is mainly because of the fact that the rotor is a self-contained unit and not connected to the supply.

Figure 15.2 Squirrel cage induction motor

The Squirrel Cage Rotor

The rotor consists of a cylindrical laminated iron core which has a number of longitudinal bars of copper evenly spaced around the circumference. These bars are joined at either end by rings of the same material to form a composite structure called a Squirrel Cage. The rotor bars are of very low resistance material so that a large current can flow through them.

The Induction Motor Stator

The stator consists of windings, the number of which is related to the number of poles and also to the number of phases of the power supply. The rotating magnetic field produced in the stator cuts through the bars of the rotor which is basically a closed circuit of low resistance.

The resultant induced voltage creates a relatively large current flow in the squirrel cage. This current flow sets up its own magnetic field which interacts with the rotating field of the stator to produce a torque. If a three phase motor has two phases of its supply reversed, then its direction of rotation will be reversed also.

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15 AC Electrics -AC Motors

Slip Speed

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The speed of the motor is determined by the frequency of the supply and the load on the motor. The rotor never quite reaches true synchronous speed, if it did then the squirrel cage bars would not be cut by any lines of force and thus would not produce the induced voltage. The difference between synchronous speed and rotor speed is called the slip speed or rotor slip. A typical value of slip would be 5%. Because of the difference in speed between the stator field and rotor, the induction motor is sometimes referred to as being asynchronous.

Starting Single Phase Induction Motors

Single phase induction motors are not self-starting. Different methods are used to assist in making them self-starting. The most common method is the use of what is called a Split Phase Winding.

If the current in the split phase winding can be made to lead or lag the current in the main winding by 90° then a rotating field can be produced.

The lead or lag can be produced by the following methods:

•Resistance starting

•Inductance starting

•Resistance / inductance starting

•Capacitance starting

The application of each method depends on the power output of the motor, e.g. capacitance started motors are usually of less than 2 HP output.

Fault Operation

Occasionally the failure of one phase of the supply to a three phase induction motor does happen. If the motor is lightly loaded then it will probably continue to run at about half of its normal speed. This will create a humming noise in the motor which, because of the usually remote locations in which the motors are mounted, will probably not make itself apparent. The fault usually becomes apparent the next time an attempt is made to run the motor, when it will not start.

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Questions

Questions

1.Synchronous motors are usually supplied by:

a.three phase AC

b.single phase AC

c.DC to the stator

d.DC to the stator and AC to the rotor

2.Reversing two phases to a three phase motor will:

a.blow the phase fuses

b.cause the motor to run in reverse

c.overheat the stator windings

d.stall the motor

3.A synchronous motor runs at a speed that depends upon the supply:

a.voltage

b.current

c.reactance

d.frequency

4.If one phase of the supply to a three phase motor fails, then:

a.the motor will continue to run at the same speed

b.will slow down and stop

c.will stop immediately

d.will run at about half speed but will not start on its next selection

5.The basic principle of operation of a 3 phase induction motor is:

a.a rotating field created in the rotor

b.a rotating field created in the stator

c.a stationary field created in the stator

d.a stationary field created in the rotor

6.In an induction motor:

a.the rotor is star connected

b.magnetic fields blend evenly with one another

c.AC is induced in the rotor

d.a DC supply produces DC in the rotor

7.In a synchronous motor, the rotor is:

a.energized by DC and it lines up with the magnetic field in the stator

b.wave wound

c.both AC and DC energized

d.impeded by the AC induced into it

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15 Questions

8.An induction motor has:

a.slip rings and brushes

b.a commutator

c.no slip ring or brushes

d.slip rings but no brushes

9.A squirrel cage rotor:

a.is not connected to the supply

b.is expensive to produce

c.rotates at exactly synchronous speed

d.is a closed circuit of high resistance

10.A starting circuit for a powerful single phase induction motor might be:

a.a capacitance starter

b.a resistance / inductance starter

c.a cartridge starter

d.a bump starter

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Questions 15

Questions 15

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