Chapter 13

2. Chapter 13 AC Motors

3. Objectives • Discuss the operation of an induction motor. • Identify and explain the operation of various three-phase motors. • Explain how a split-phase condition is created. Permission granted to reproduce for educational use only.

4. Objectives • Discuss the purpose of ac motor protection circuits. • List the common causes of motor failure. • Explain basic troubleshooting techniques for ac motors. Permission granted to reproduce for educational use only.

5. Induction Motors • Three-phase motor • Rotation of magnetic field inside a stator • Rotor inside stator housing • Squirrel cage rotor Permission granted to reproduce for educational use only.

6. Synchronous Speed • Dependent on frequency and number of poles • As the number of coils or poles per phase increases, the rpm of the motor decreases Permission granted to reproduce for educational use only.

7. Slip • Rotor travels slower than synchronous speed of magnetic field • 4%–5% of motor’s rpm • Rotor’s rpm depends on number and size of rotor bars • As slip increases, greater current circulates, leading to greater torque Permission granted to reproduce for educational use only.

8. Types of Induction Motors • Three-phase synchronous motor • Wound rotor motor • Dual-voltage motor • Single-phase induction motor • Capacitor start, capacitor run motor • Repulsion-induction motor • Shaded-pole motor • Stepping motor Permission granted to reproduce for educational use only.

9. Three-Phase Synchronous Motors • Motor and magnetic field rotate at same speed • Rotor windings are energized, so rotor is an electromagnet • No slip to maintain • Used where constant speeds exist under changing mechanical loads Permission granted to reproduce for educational use only.

10. Wound Rotor Motors • Use three sets of windings wrapped around soft iron laminations • Can be brought to near synchronous speed • Varying induced current in rotor windings • Rotor speed can be adjusted Permission granted to reproduce for educational use only.

11. Dual-Voltage Motors • Stator coils arranged in pairs • Three-phase motors • Economical motors Permission granted to reproduce for educational use only.

12. Single-Phase Induction Motors • Splitting phases • Used in home appliances Permission granted to reproduce for educational use only.

13. Inductive Reactance Phase Splitting • Starting windings • Running windings Permission granted to reproduce for educational use only.

14. Capacitor Phase Splitting • Larger phase displacement • High starting torque Permission granted to reproduce for educational use only.

15. Capacitor Start, Capacitor Run Motors • Heavy mechanical load conditions • Similar to capacitor start motors • Single-phase air-conditioning equipment Permission granted to reproduce for educational use only.

16. Repulsion-Induction Motors • Resemble dc motors • Brushes connected to each other, not power source • Rotation started by repulsion • Currents displaced by brushes • New poles opposing stator poles Permission granted to reproduce for educational use only.

17. Shaded-Pole Motors • Shading the pole • Shading coils • Induced current opposes change in magnetic field of field coil Permission granted to reproduce for educational use only.

18. Stepping Motors • Rotate in steps • Gear teeth inside motor • Pulses of power, not continuous • Speed of power pulses determines rotor speed • Used in robots and disk drive systems Permission granted to reproduce for educational use only.

19. Selsyn Units • Synchros • Two motor units • Energized by ac power supply • Transmitter and receiver Permission granted to reproduce for educational use only.

20. Review What is the stator in a motor? Stationary windings in a circular pattern inside a housing Permission granted to reproduce for educational use only.

21. Review What differentiates a squirrel cage from other rotors? It is common in the three-phase induction motor, used to produce rotation instead of just vibration Permission granted to reproduce for educational use only.

22. Review What is synchronous speed? The speed of the stator’s rotating magnetic field Permission granted to reproduce for educational use only.

23. Review What happens to the rpm of a motor as the number of coils or poles per phase increases? The rpm decreases Permission granted to reproduce for educational use only.

24. Review Which kind of motor has stator coils arranged in pairs? The dual-voltage motor Permission granted to reproduce for educational use only.

25. Review What is the single turn of wire in a shaded-pole motor called? Shading coil Permission granted to reproduce for educational use only.

26. Motor Protection • Dedicated line • Branch circuit protection Permission granted to reproduce for educational use only.

27. Motor Protection (Cont.) • Run protection • Service factor • Based on percentage of normal current under full load condition • (Percentage of current – 1) × 100 Permission granted to reproduce for educational use only.

28. Motor Failure • Excessive bearing wear • Mechanical overload • Single-phasing condition • Locked rotor condition • Low voltage Permission granted to reproduce for educational use only.

29. Excessive Bearing Wear • Most common cause of failure • Some bearings should be lubricated • Dry bearings cause friction • Causes motor to go into overload condition Permission granted to reproduce for educational use only.

30. Mechanical Overload • Affects small ac motors • Overheating can cause irreversible damage • Can produce short between windings Permission granted to reproduce for educational use only.

31. Single-Phasing Condition • Affects three-phase motors • Power supply to one line is lost to open or blown fuse • Increased current will damage motor • Thermo protection prevents overheating Permission granted to reproduce for educational use only.

32. Locked Rotor Condition • Causes severe damage • Caused by mechanical failure in the system • Excessive current in windings Permission granted to reproduce for educational use only.

33. Low Voltage • Excessive motor current in windings • Causes the rotor to lock • Motor should be under mechanical load Permission granted to reproduce for educational use only.

34. Troubleshooting Motors • Clamp-on ammeters • Used only for ac circuits • Circuit does not have to be opened for ammeter to be connected • Current should be below motor’s rating • Compare all three lines of a three-phase motor • Megohmmeters Permission granted to reproduce for educational use only.

35. Review What is a dedicated line? A circuit used for only one purpose Permission granted to reproduce for educational use only.

36. Review What is the ability of a motor to withstand or avoid damage from an overload condition? Service factor Permission granted to reproduce for educational use only.

37. Review In a case where the motor is energized, but the rotor is not turning, what component can be most severely damaged? The motor windings Permission granted to reproduce for educational use only.

38. Review What is the most common cause of motor failure? Excessive bearing wear Permission granted to reproduce for educational use only.

39. For Discussion • Why does an overloaded motor heat up? • Why are motor starters necessary on heavy-duty motors? • Compare an induction motor to a transformer. Permission granted to reproduce for educational use only.