1. AC Machines
6077SA
NUE 046

2. 3 Phase Induction Motor Construction
At the end of this section you will be able to:
Identify the basic components of a 3Ø induction motor
List the types of rotors used
Test motor windings for suitability to connect to the supply
Connect in both star and delta
List the steps for dismantling it

3. 3Ø Induction Motors Why are they so common in industry?
Simple in construction
Only one moving part
Don’t have brush gear and commutators
Now with VSD technology, Variable speed
Speed of machine fixed to supply frequency

4. Rotor (The bit that Rotates)
Construction
Fixed Stator (stationary)
Rotor (The bit that Rotates)

5. Construction Frame can be made from cast
Iron
Steel
Aluminium
End caps/shields bolted to each end of frame

6. Drip Proof Designed for air flow into motor Not waterproof
Not suitable for flame proof atmospheres

7. Totally Enclosed
No airflow between motor inside and outside atmosphere
Waterproof
Easier to adapt to for flame proof atmospheres

8. Open Designed for air flow into motor Not waterproof
Very rare due to OH&S considerations

9. Core Laminations Carries the stator windings
Made of silicone steel 0.3 to 0.5mm thick
Insulated from one another to reduce eddy current loss

10. Stator windings
3 identical phase windings that are displaced by 120° electrical around the stator
Each winding consists of series, parallel connected coils to achieve the number of poles required.
2, 4, 6, & 8 poles per phase are common
Windings are generally connected in delta but can be connected in star

11. Rotors Two Main Types Wound rotor (not common) Squirrel cage
(most common)

12. Squirrel Cage Rotor

13. Squirrel Cage Rotor Aluminium Copper Copper alloy Can be Cast
Copper/Copper alloy bars inserted into the laminations and welded/brazed to shorting ring

14. Wound Rotor Slip ring Same number of poles as stator.
One end of coil is connected to a star point
Other end is terminated on a slip ring
Resistance is added to change the mechanical and electrical characteristics of motor.

15. Motor Terminals

16. Motor Terminals

17. Motor Terminals
U V W1
V W U2

18. Motor Terminals
U V W1
V W U2

19. To Make Star Connection
Motor Terminals
U V W1
V W U2
To Make Star Connection

20. To Make Delta Connection
Motor Terminals
U V W1
V W U2
To Make Delta Connection

21. Electrical Testing Each winding should have the same resistance
ie 1-10
Voltage between windings =400V
Insulation test level =1000V
IR reading no less than 1M