Download Chapter 9 - Cengage Learning

Chapter 9
Basic Electric Motors
Objectives
• Upon completion of this course, you will
be able to:
– Explain magnetism and the part it plays in
the operation of electric motors
– Explain torque and the purpose of different
types of single-phase motors
– Explain the operation of a basic electric
motor
Objectives (cont’d.)
– Understand how to operate, install, reverse
the rotation (if possible), and diagnose
problems in a shaded-pole motor
– Understand the purpose of capacitors in
the operation of a single-phase motor and
be able to explain the difference between a
starting and running capacitor
Objectives (cont’d.)
– Correctly diagnose the condition of any
capacitor and, using capacitor rules, be able
to substitute a capacitor if a direct
replacement is not available
– Explain how to operate, install, troubleshoot,
and repair (if possible) split-phase and
capacitor-start motors
Objectives (cont’d.)
– Explain how to operate, install, troubleshoot,
and repair (if possible) permanent splitcapacitor motors
– Explain how to operate, install, troubleshoot,
and repair (if possible) capacitor-start–
capacitor-run motors
– Understand how to operate, install, reverse,
and troubleshoot three-phase motors
Objectives (cont’d.)
– Explain how to operate, install, troubleshoot,
and repair (if possible) electronically
commutated motors
– Identify the common, start, and run terminals
of a single-phase compressor motor
Key Terms
• Capacitor
• Capacitor-start
motor
• Capacitor-startcapacitor-run
motor
• Delta winding
• Electromagnet
• Electronically
commutated motor
(ECM)
• Flux
• Hermetic compressor
• Induced magnetism
Key Terms (cont’d.)
•
•
•
•
Magnetic field
Magnetism
Microfarad
Permanent
magnet
• Permanent splitcapacitor motor
•
•
•
•
•
•
Rotor
Running capacitor
Shaded-pole motor
Split-phase motor
Squirrel cage rotor
Star winding
Key Terms (cont’d.)
•
•
•
•
Starting capacitor
Stator
Three-phase motor
Torque
Introduction
• Electric motors
– Change electric energy into mechanical
energy
• Used to drive any device needing energy to
power its movement
– Many types
– Torque: strength the motor produces by
turning
Magnetism
• Physical phenomenon
– Includes attraction of an object for iron
• Exhibited by a permanent magnet or electric
current
• Produced in many different ways
– Example: magnetic fields of the earth
• Compass reaction to earth’s magnetic fields
Magnetism (cont’d.)
• Magnets have two poles: north and
south
– North pole of a bar magnet is brought close
to north pole of another bar magnet
• They will repel
– South pole of a bar magnet is brought
close to the north pole of bar magnet
• They will attract each other
Magnetic Field
• Flux
– Magnetic lines of force of a magnet that
flow between the north and south poles
• Magnetic field
– Area that the magnetic force operates in
– Can flow through material
Induced Magnetism
• Created when a
piece of iron is
placed in a
magnetic field
– The closer an
object is to the
magnet, the
stronger the
magnetic field
Figure 9.5 Magnetic field of an iron core
when a current-carrying conductor
is wound around the core.
(Delmar/Cengage Learning)
Induced Magnetism (cont’d.)
• Permanent magnet
– Magnetic material that has been
magnetized
• Can hold magnetic strength
• Electromagnet
– Magnet produced through electricity
• Electron flow is in a conductor
• Magnetic field is created around the conductor
Basic Electric Motors
• Create a rotating motion and drive
components that need to be turned
– Electric energy is changed to mechanical
energy by magnetism
• Based on induced magnetism
• To make an electric motor rotation continuous
the magnet field must rotate
Types of Electric Motors
• All kinds of AC motors are used to
rotate many different devices
– Different motors are needed
• Not all motors have the same running and
starting characteristics
– Must use the right motor for the right job
Motor Strength
• Generally used to classify motors
– Motors are selected mainly because of
starting torque required
• General types of motors
– Shaded-pole, split-phase, permanent splitcapacitor, capacitor-start-capacitor-run,
capacitor-start, three-phase, and
electronically commutated
Motor Speed
• Formula
– Motor with a load
• Speed = (Flow reversals/second x 120)/
Number of poles
• Common motor speeds
– Two-pole: 3450 rpm
– Four-pole: 1750 rpm
– Six-pole: 1050 rpm
– Eight-pole: 900 rpm
Open and Enclosed Motors
• Open motors
– Have a housing
• Rotate a device not enclosed in housing
• Enclosed motors
– Housed within some type of shell
• Most common enclosure: completely sealed
hermetic compressor
Motor Dimensions
• Established by National Electrical
Manufacturers’ Association (NEMA)
Figure 9.14 Dimension of typical motor frames.
(Delmar/Cengage Learning)
Shaded-Pole Motors
• Used when very small starting and
running torques are required
– Operation: current is induced into shaded
pole from main windings
– Reversing: requires disassembly
– Troubleshooting: easy to identify because
of the copper band around shaded pole
Capacitors
• Consist of two aluminum plates with an
insulator between them
– Two types used in the industry
• Electrolytic or starting capacitor
• Oil-filled or running capacitor
– Troubleshooting: short capacitor life and
malfunctions
• Caused by several different factors
Split-Phase Motors
• Two general classifications:
– Resistance-start-induction-run motor
– Capacitor-start-induction-run motor
• Each has different operating
characteristics
– Similar construction
• Use some method of splitting the phase of
incoming power to produce a second phase
Resistance-Start-InductionRun Motors
• Have starting and running winding
– Most have some method of beginning
rotation
– Operation: phases are split by makeup of
starting windings
– Troubleshooting: probable areas are the
bearings, windings, and centrifugal switch
Capacitor-Start-Induction-Run
Motors
• Produce a high starting torque
– Needed for many applications
– Operate like a split-phase motor
• Except a capacitor is inserted in series with
centrifugal switch and starting windings
• Types
– Open and enclosed
Permanent Split-Capacitor Motors
• Simple design and moderate starting
torque
– Operation: running capacitor is put in series
with starting winding
– Troubleshooting: usually trouble-free for long
periods
• Common failures: bearings, windings, and
capacitor
Capacitor-Start-Capacitor-Run
Motors
• Produce high starting torque and
increase running efficiency
– Operation: begin on a phase displacement
between starting and running windings
– Troubleshooting: sometimes difficult
• Number of added components
Three-Phase Motors
• Rugged, reliable, and more dependable
– Most common: squirrel cage induction type
– Operation: same principles as single-phase
• Except three-phase displacement
– Troubleshooting: ohmmeter is used to
check resistance
Electronically Commutated Motors
• Reverse one-half of each AC cycle to
form a single directional current
– Construction: brushless DC, three-phase
motor with a permanent magnet rotor
• Two part motor: motor and control
– Operation: determined by control inputs
– Troubleshooting: difficult
• Various interfaces, strategies, and modules
Hermetic Compressor Motors
• Induction type motors
– Designed for single- and three-phase current
– Operation: enclosed in a shell with refrigerant
and oil
• Requires special considerations
– Terminal identification: common, start, and
run terminals
– Troubleshooting: often difficult
Service Call Protocol
• Many types of service procedures are
performed each day
– Pre-season startups
– Preventive maintenance calls
– Inoperative system calls
• Technicians
– Must always keep in mind the need for
service calls