1. Chapter 19
AC Motors

2. Objectives After studying this chapter, you will be able to:
Describe the construction of various types of AC motors
Explain the principle of operation of various types of three-phase motors and single-phase motors

3. Objectives (cont’d.)
Discuss the reasons for the difference in the values of starting and running currents in AC motors

4. AC Motor Construction Induction motor is most common AC motor
Two main components: stator and rotor
Stator has electromagnets secured to frame
Rotor made of steel lamination in shape of cylinder
Windings placed in slots on rotor surface
Squirrel-cage rotor winding
Heavy copper bars welded to end rings

5. AC Motor Construction (cont’d.)
Wound-rotor motor
Copper wire wound into rotor slots
Connected in wye
Ends connected to slip rings on shaft

6. Three-Phase Motor Theory
Two-pole, three phase stator winding
When energized from three-phase source, three-phase currents are 120 degrees apart and continuously changing in value and direction
The effect of this variation in strength and direction produces a rotating field
See Figure 19-4C

8. Three-Phase Motor Theory (cont’d.)
Speed of the rotating magnetic field
Synchronous speed of two-pole induction motor is 60 revolutions per second when supplied with 60 Hz.
Lower speeds obtained with more poles

9. Three-Phase Motor Theory (cont’d.)
Rotor speed
Depends on synchronous speed and load
Rotor speed lags behind synchronous speed one percent (no load) to five percent (full load)
Direction of rotor rotation
Depends on the phase sequence
Revolves in same direction as stator field

10. Three-Phase Motor Theory (cont’d.)
Torque
Varies with the strength of the stator and rotor fields
Also affected by phase relationship between the two fields

11. Three-Phase Motor Theory (cont’d.)
Slip
Difference between synchronous speed and rotor speed
May be stated in rpm or a percentage

12. Three-Phase Motor Starting and Running Current
Starting current is limited to four to six times the full load current
Loading a squirrel-cage motor
For large motors, reduced voltage starters are used to limit the starting current to a lower value that will not cause line drops

13. Three-Phase Motor Starting and Running Current (cont’d.)
Double squirrel-cage rotor
Has two squirrel-cage windings
Inner winding has low resistance and high inductance
Outer winding has high resistance and low inductance
Results in large current and high power factor in outer winding; high starting torque

14. Types of Three-Phase Motors
Multispeed squirrel-cage motors
Number of poles may be varied by changing external connections
Wound-rotor induction motors
Provides high starting torque at low current value
Accelerates smoothly under heavy loads
Poor speed regulation when operating with resistance in the rotor circuit

15. Types of Three-Phase Motors (cont’d.)
Adjustable-speed induction motor (brush-shifting motor)
Provides a wide range of speeds, depending on position of the brushes
High-frequency motors
Operate at speeds greater than can be obtained from a 60Hz supply

16. Types of Three-Phase Motors (cont’d.)
Synchronous motors
Operates at constant speed from no load to full load
Power factor can be controlled by varying amount of current in the exciter winding
Generally used for driving loads requiring constant speeds and infrequent starting and stopping

17. Single-Phase Motors Disadvantages when compared to polyphase motors
Torque developed is not as smooth
Larger in physical size and less efficient

18. Starting Single-Phase Motors
Phase-splitting principle: one method of making a single-phase motor self-starting
Stator has two windings: main and auxiliary
Two currents out of phase by less than 90 degrees
Resistance split-phase motor
Resistance connected in series with auxiliary winding, resulting in better starting torque

19. Starting Single-Phase Motors (cont’d.)
Capacitor split-phase motors
Three classes: 1) low starting torque, 2) capacitor start, induction run, 3) capacitor start, capacitor run
Reversing split-phase motors
Reverses direction of rotor rotation by reversing the polarity of one of the fields

20. Starting Single-Phase Motors (cont’d.)
Shaded-pole motors
Started by a low-resistance, short-circuited coil placed around one tip of each pole
Have poor starting torque
Repulsion motors
Starts based on the principle that like poles repel

21. Starting Single-Phase Motors (cont’d.)
Series AC motors
More complex structure and heavier per horsepower
More expensive than DC motors of the same rating
Operating characteristics very similar to DC motors

22. Universal Motors Series motor that will operate on both AC and DC
Manufactured in small sizes with fractional horsepower ratings
Used for vacuum cleaners, portable electrical tools, and small household appliances

23. Selection of Single-Phase Motors
Shaded-pole motors adequate for small fans, measuring instruments and other applications requiring little starting torque
Split-phase motors can be used for large machines that start without a load (small lathes, drills, grinders)
Capacitor and repulsion motors for machines that start under load

24. Torque Motors Have greatest torque output when stalled
Applications: opening and closing valves, dampers, doors, gates, windows, etc.
Large torque motors operate on three-phase
Small ones versions of universal motor
Not intended for continuous duty

25. Dual-Voltage Windings
Motors that can operate on either of two voltages
Accomplished by dividing each phase into two sections (either delta or wye)

26. Multispeed Induction Motors
Synchronous speed of induction motor depends on supply frequency and number of poles
Changing speed by varying frequency requires use of variable frequency drive and means of adjusting motor current
Solid-state controller can be used to accomplish this

27. Summary Induction motor made up of stator and rotor
Types of three-phase motors include multispeed squirrel-cage, wound-rotor induction, adjustable speed induction, high-frequency and synchronous motors
Single-phase motors are larger in size and less efficient than polyphase motors

28. Summary (cont’d.) Universal motors can operate on either AC or DC
Dual voltage windings can operate on either of two voltages
Multispeed induction motors offer varying speeds by changing frequency or number of poles