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 split-capacitor 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-start-capacitor-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 split-capacitor 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 corewhen a current-carrying conductoris 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 split-

capacitor, 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-Induction-Run 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