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.
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4. Objectives Discuss the purpose of ac motor protection circuits.
List the common causes of motor failure.
Explain basic troubleshooting techniques for ac motors.
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5. Induction Motors Three-phase motor
Rotation of magnetic field inside a stator
Rotor inside stator housing
Squirrel cage rotor
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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
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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
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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
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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
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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
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11. Dual-Voltage Motors Stator coils arranged in pairs Three-phase motors
Economical motors
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12. Single-Phase Induction Motors
Splitting phases
Used in home appliances
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13. Inductive Reactance Phase Splitting
Starting windings
Running windings
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14. Capacitor Phase Splitting
Larger phase displacement
High starting torque
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15. Capacitor Start, Capacitor Run Motors
Heavy mechanical load conditions
Similar to capacitor start motors
Single-phase air-conditioning equipment
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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
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17. Shaded-Pole Motors Shading the pole Shading coils
Induced current opposes change in magnetic field of field coil
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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
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19. Selsyn Units Synchros Two motor units Energized by ac power supply
Transmitter and receiver
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20. Review What is the stator in a motor?
Stationary windings in a circular pattern inside a housing
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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
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22. Review What is synchronous speed?
The speed of the stator’s rotating magnetic field
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23. Review
What happens to the rpm of a motor as the number of coils or poles per phase increases?
The rpm decreases
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24. Review Which kind of motor has stator coils arranged in pairs?
The dual-voltage motor
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25. Review What is the single turn of wire in a shaded-pole motor called?
Shading coil
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26. Motor Protection Dedicated line Branch circuit protection
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27. Motor Protection (Cont.)
Run protection
Service factor
Based on percentage of normal current under full load condition
(Percentage of current – 1) × 100
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28. Motor Failure Excessive bearing wear Mechanical overload
Single-phasing condition
Locked rotor condition
Low voltage
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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
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30. Mechanical Overload Affects small ac motors
Overheating can cause irreversible damage
Can produce short between windings
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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
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32. Locked Rotor Condition
Causes severe damage
Caused by mechanical failure in the system
Excessive current in windings
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33. Low Voltage Excessive motor current in windings
Causes the rotor to lock
Motor should be under mechanical load
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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
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35. Review What is a dedicated line? A circuit used for only one purpose
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36. Review
What is the ability of a motor to withstand or avoid damage from an overload condition?
Service factor
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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
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38. Review What is the most common cause of motor failure?
Excessive bearing wear
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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.
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40. Glossary Branch circuit protection Dedicated line
Installation of a fuse or breaker to protect the individual circuit feeding a motor.
Dedicated line
Refers to a circuit that is used for only one purpose.
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41. Glossary Locked rotor condition Rotor
A rotor that is not turning while power is being applied to the motor because of a physical impedance.
Rotor
Rotating part of an ac generator.
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42. Glossary Run protection Running winding
Protects a motor from excessive heat damage or fire while in a normal mode of operation.
Running winding
Turn of heavy wire placed in a slot around the inside of a stator.
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43. Glossary Selsyn unit Service factor
A transformer application designed to relay information through motor units. Also called a synchro.
Service factor
The ability of a motor to withstand or avoid damage from an overload condition.
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44. Glossary Shading coil Shading the pole
A single turn of heavy wire placed in a slot that is cut in the face of the poles of a shaded-pole motor.
Shading the pole
Placing a winding wire in the slot of the starter of a shaded-pole motor.
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45. Glossary Single-phasing condition Slip
Occurs when the power supply to a three-phase motor loses one of its three lines due to an open or blown fuse.
Slip
The difference in the rotor speed and the magnetic field of the stator in ac motors, usually expressed as a percentage.
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46. Glossary Splitting phases Squirrel cage
Dividing a single-phase current into a polyphase current.
Squirrel cage
Made of bars placed in slots of the rotor core and joined at the ends. Used in induction motors.
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47. Glossary Starting windings Stator
Many turns of relatively fine wire in an induction motor.
Stator
Stationary coils of an ac generator.
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48. Glossary Synchro Synchronous speed
Electromechanical device used to transmit the angular position of a shaft from one position to another without mechanical linkage.
Synchronous speed
The speed at which the rotor speed equals the speed of the rotating magnetic field.
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