Chapter 13
AC Motors
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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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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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Induction Motors
• Three-phase motor– Rotation of magnetic field inside a stator– Rotor inside stator housing– Squirrel cage rotor
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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
poles of pairs ofNumber
seconds 60 hertz ofNumber minute)per srevolution(in speed sSynchronou
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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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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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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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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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Dual-Voltage Motors
• Stator coils arranged in pairs
• Three-phase motors• Economical motors
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Single-Phase Induction Motors
• Splitting phases• Used in home
appliances
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Inductive Reactance Phase Splitting
• Starting windings• Running windings
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Capacitor Phase Splitting
• Larger phase displacement
• High starting torque
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Capacitor Start, Capacitor Run Motors
• Heavy mechanical load conditions
• Similar to capacitor start motors
• Single-phase air-conditioning equipment
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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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Shaded-Pole Motors
• Shading the pole• Shading coils• Induced current
opposes change in magnetic field of field coil
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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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Selsyn Units
• Synchros• Two motor units• Energized by ac
power supply• Transmitter and
receiver
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Review
What is the stator in a motor?
Stationary windings in a circular pattern inside a housing
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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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Review
What is synchronous speed?
The speed of the stator’s rotating magnetic field
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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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Review
Which kind of motor has stator coils arranged in pairs?
The dual-voltage motor
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Review
What is the single turn of wire in a shaded-pole motor called?
Shading coil
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Motor Protection
• Dedicated line• Branch circuit protecti
on
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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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Motor Failure
• Excessive bearing wear
• Mechanical overload
• Single-phasing condition
• Locked rotor condition
• Low voltage
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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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Mechanical Overload
• Affects small ac motors
• Overheating can cause irreversible damage
• Can produce short between windings
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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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Locked Rotor Condition
• Causes severe damage
• Caused by mechanical failure in the system
• Excessive current in windings
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Low Voltage
• Excessive motor current in windings
• Causes the rotor to lock
• Motor should be under mechanical load
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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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Review
What is a dedicated line?
A circuit used for only one purpose
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Review
What is the ability of a motor to withstand or avoid damage from an overload condition?
Service factor
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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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Review
What is the most common cause of motor failure?
Excessive bearing wear
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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.