1. Three-Phase AC machines
Resource 6
Three-Phase Wound Rotor Induction Motors

2. Three-Phase AC Machines Resource 6
Three-Phase Wound Rotor Induction Motors
Aim
To understand the construction, operation and performance of a Three-Phase Wound Rotor Induction Motor

3. Objectives Three-Phase AC Machines Resource 6
Three-Phase Wound Rotor Induction Motors
Objectives
To be able to describe the construction of a three-phase wound rotor induction motor
To be able to describe the operation of a three-phase wound rotor induction motor
To be able to describe the performance of a three-phase wound rotor induction motor

4. Motor Construction Outside View 1. Terminal Box 2. Outer Casing7
3. Mounting Bolts 2
4. Inspection Cover
5. Air Intake Vents 6
6. End Plate 5 4 3
7. Rotor Shaft

5. Motor Construction Terminal Box 1. Rotor Terminals K, L, M2
Terminal Box
1. Rotor Terminals K, L, M
2. Stator Terminals U1, V1, W1 3
3. Stator Windings
4. Stator Terminals U2, V2, W2
5. Rotor Windings 4 6 5
6. Earth Terminal

6. Motor Construction The Stator 1. Motor Outer Casing 2. Stator Coils6
3. Stator Slots
4. Air Intake Vents
5. Stator Laminations 3 5
6. Slots Insulators 4

7. Motor Construction The Rotor 1. Rotor Coils 2. Slip Rings 3. Brushes
4. Rotor Terminals
5. Rotor Laminations
6. Rotor Slots

8. Motor Construction The Rotor 1. Slip Rings 2. Rotor Coils3 2 1
1. Slip Rings
2. Rotor Coils
3. Cooling Fan
4. Rotor Slots
5. Bearing 4 5

9. Motor Construction The Brush Gear 1. Brass Brush Holders6
The Brush Gear 1
1. Brass Brush Holders
2. Carbon Brushes 5 2
3. Springs
4. Nylon Insulator
5. Rotor Terminals
6. External Connecting Wires 3 4

10. Motor Operation The Rheostat Method of Starting
1. Motor starts with 3-gang Rheostat set to highest resistance giving low starting current and slow speed.
2. The Rheostat is adjusted slowly thus reducing rotor resistance to zero.
3. This increases rotor speed up to its maximum value
4. Starting torque can be easily adjusted by initial resistance setting to suit the load.

11. Motor Operation The Contactor Method of Starting
Stator Connections
The Contactor Method of Starting
1. Motor starts with 3 resistors in each rotor circuit giving low starting current and slow speed.
Earth Terminal
Shorting Relays K1, K2, K3
Rotor Connections
2. K3 is energised shorting out resistors R31, R32, R33 giving increased speed.
Rotor Resistor Bank 1
3. K2 is energised shorting out resistors R21, R22, R23 giving increased speed.
Rotor Resistor Bank 2
4. K1 is energised shorting out resistors R11, R12, R13 giving full speed.
Rotor Resistor Bank 3

12. Motor Performance Torque-Speed Curve for a four pole motor
Increasing Rotor Resistance
1. Does not change synchronous speed
2. Does not change maximum torque
3. Increases Starting Torque
4. Decreases Speed in running region for given load
5. Decreases Speed where maximum torque occurs
6. Decreases Starting Current