Chapter 13 AC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Discuss the operation of an induction

Chapter 13

AC Motors

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

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.


Objectives

• Discuss the purpose of ac motor protection circuits.

• List the common causes of motor failure.

• Explain basic troubleshooting techniques for ac motors.


Induction Motors

• Three-phase motor– Rotation of magnetic field inside a stator– Rotor inside stator housing– Squirrel cage rotor


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


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


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


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


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


Dual-Voltage Motors

• Stator coils arranged in pairs

• Three-phase motors• Economical motors


Single-Phase Induction Motors

• Splitting phases• Used in home

appliances


Inductive Reactance Phase Splitting

• Starting windings• Running windings


Capacitor Phase Splitting

• Larger phase displacement

• High starting torque


Capacitor Start, Capacitor Run Motors

• Heavy mechanical load conditions

• Similar to capacitor start motors

• Single-phase air-conditioning equipment


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


Shaded-Pole Motors

• Shading the pole• Shading coils• Induced current

opposes change in magnetic field of field coil


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


Selsyn Units

• Synchros• Two motor units• Energized by ac

power supply• Transmitter and

receiver


Review

What is the stator in a motor?

Stationary windings in a circular pattern inside a housing


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


Review

What is synchronous speed?

The speed of the stator’s rotating magnetic field


Review

What happens to the rpm of a motor as the number of coils or poles per phase increases?

The rpm decreases


Review

Which kind of motor has stator coils arranged in pairs?

The dual-voltage motor


Review

What is the single turn of wire in a shaded-pole motor called?

Shading coil


Motor Protection

• Dedicated line• Branch circuit protecti

on


Motor Protection (Cont.)

• Run protection

• Service factor– Based on percentage of normal current under

full load condition– (Percentage of current – 1) × 100


Motor Failure

• Excessive bearing wear

• Mechanical overload

• Single-phasing condition

• Locked rotor condition

• Low voltage


Excessive Bearing Wear

• Most common cause of failure

• Some bearings should be lubricated

• Dry bearings cause friction

• Causes motor to go into overload condition


Mechanical Overload

• Affects small ac motors

• Overheating can cause irreversible damage

• Can produce short between windings


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


Locked Rotor Condition

• Causes severe damage

• Caused by mechanical failure in the system

• Excessive current in windings


Low Voltage

• Excessive motor current in windings

• Causes the rotor to lock

• Motor should be under mechanical load


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


Review

What is a dedicated line?

A circuit used for only one purpose


Review

What is the ability of a motor to withstand or avoid damage from an overload condition?

Service factor


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


Review

What is the most common cause of motor failure?

Excessive bearing wear


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.

Documents

Chapter 13 AC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Discuss the operation of an induction