Three-Phase AC machines THREE-PHASE CAGE ROTOR INDUCTION MOTORS

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

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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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

1. Principle of Operation 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