1. Three-Phase AC machines
Resource 2
THREE-PHASE CAGE ROTOR INDUCTION MOTORS

2. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
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

3. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction 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

4. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Simple stator made of 3 pole pairs of coils around iron pole pieces A’
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 C
Iron Stator Ring B B’
Phase Coils C’ A
Phase connections

5. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Changing which coils are energised alters direction of magnetic flux C B’ C’ A B A’ C B’ C’ A B A’ C B’ C’ A B A’ N S N N S S
AA’ energised
CC’ energised
BB’ energised

6. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Energising two sets of coils together in sequence
Compass settles half way between poles

7. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Sequence 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

8. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Three-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

9. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Three-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

10. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
Rotor cage added in the centre of the stator field
Cage rotor
Copper end ring
Cage conductors

11. Three-Phase AC Machines Resource 2
Three-phase Cage Rotor
Induction Motors
1. Principle of Operation
As magnetic field rotates, cage conductors cut lines of magnetic flux
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