three-phase cage rotor induction motors resource 2 three-phase ac machines

THREE-PHASE CAGE ROTOR INDUCTION MOTORS

Resource 2

Three-Phase AC machines

Aims

1. To understand the principle of the operation of the motor

2. To describe the construction of a cage rotor induction commercial motor

3. To understand the factors that affect the performance of the motor

Three-Phase AC Machines Resource 2

Three-phase Cage RotorInduction Motors

Objectives

At the end of this lesson you should be able to:

• Describe the principle of the rotating magnetic field• Describe the principle of rotor cage induction• Describe the construction of a commercial stator• Describe the construction of a commercial cage rotor• Explain and calculate synchronous speed• Explain and calculate fractional slip • Describe the effect upon the performance of the motor by

connecting the stator in star and delta• Performance simple calculations involving electrical and

mechanical parameters that describe the performance of the motor

• Describe the performance curve for a cage rotor induction motor



Iron Pole Pieces

Phase connections

A’

A

B

B’

C

C’

Iron Stator Ring

Phase Coils

Simple stator made of 3 pole pairs of coils around iron pole pieces

• Current enters coil A and leaves coils A’

• Magnetic flux set up in coils with North Pole at the bottom and South Pole at the top



1. Principle of Operation


Changing which coils are energised alters direction of magnetic flux

AA’ energised CC’ energised BB’ energised



N

S

C

B’

C’

A

B

A’

N

S N

S

C

B’

C’

A

B

A’

C

B’

C’

A

B

A’

Energising two sets of coils together in sequence

Compass settles half way between poles




1. Principle of OperationSequence produces one complete rotation of the magnetic field

1. CC’ & B’B

2. AA’ & B’B

3. AA’ & C’C

4. BB’ & C’C

5. BB’ & A’A

6. CC’ & A’A

7. CC’ & B’B



1. Principle of OperationThree-Phase supply provides the correct sequence for stator coils



1. CC’ & B’B

2. AA’ & B’B

3. AA’ & C’C

4. BB’ & C’C

5. BB’ & A’A

6. CC’ & A’A

7. CC’ & B’B



1. Principle of OperationThree-Phase supply provides the correct sequence for stator coils

1. CC’ & B’B

2. AA’ & B’B

3. AA’ & C’C

4. BB’ & C’C

5. BB’ & A’A

6. CC’ & A’A

7. CC’ & B’B

Rotor cage added in the centre of the stator field1. Principle of Operation

Cage conductors

Cage rotor

Copper end ring



As magnetic field rotates, cage conductors cut lines of magnetic flux1. Principle of Operation

This induces currents in cage conductors

Induced currents produce magnetic field in rotor

Magnetic field in rotor attracted towards rotating stator field

Rotor turns in same direction as stator field but slower



three-phase cage rotor induction motors resource 2 three-phase ac machines

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