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INTRODUCTION TO MOTOR CONTROL OPERATIONS

AND PROTECTION

An Industrial Text and Video Co. Publication

Electrical and Motor ControlsTraining Series

Interactive CD-ROM HandbookCourse #733

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Editor Lucile B. ThompsonIllustrations and Art Direction by Gina KoryLayout Coordination Jane HarawayTechnical Writing and Editing by L.A. Bryan and E.A. Bryan

Copyright All rights reserved. First editionPrinted and bound in the United States of America

Reproduction or translation of any part of this workbeyond that permitted by Sections 107 and 108 of the1976 United States Copyright Act is unlawful.

Due to the nature of this publication and because of the different applications of motors and electrical control circuits, the readers or users andthose responsible for applying the information herein contained must satisfy themselves as to the acceptability of each application and the useof equipment therein mentioned. In no event shall the publisher and others involved in this publication be liable for direct, indirect or conse-quential damages resulting from the use of any technique or equipment herein mentioned.

The illustrations, charts, and examples in this book are intended solely to illustrate the methods used in each application example. Thepublisher and others involved in this publication cannot assume responsibility or liability for actual use based on the illustrative uses andapplications.

No patent liability is assumed with respect to use of information, circuits, illustrations, equipment, or software described in this text.

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Before Watching the Program:

To properly and effectively use this program, you should read the first three sections in this guidebefore watching the video. These sections are:

1—Introduction2—Learning Objectives3—Guide and Reference

The introduction section will give you a general idea of what is to be covered in the program. Theguide and reference will provide you with the information that will actually be presented in theprogram, and it is suggested that you read the handbook before watching the program.

During the Program:

After you read the first three sections, you are ready to go over the program. To make it easier tofollow the program, we have included in the guide section the text of the material to be covered,along with most of the graphics that are in the CD-ROM interactive. Where possible, we have alsoincluded space for writing comments or notes. This guide section will help you to better follow thevideo program and better understand the topics being explained.

During the program, you will also go over several Review Questions which will help you reinforcethe lessons that are being presented.

After Watching the Program:

After you watch the training program, you should read the guide and reference section again toreinforce what you have just learned in the video segments. Section four of this guide provides asummary of the information covered in the program. The last section provides review questionsshown in the program followed by the answers.

Final Exam and Certification:

The CD-ROM interactive also includes a Final Exam. There are a total of 20 interactive questionsthat are presented. These questions are selected ramdomly from a pool of test questions. Aftersatisfactorily passing the test, you will be able to issue a Certificaate of Achevement as instructedby the program.

Further Reference:

This book was prepared to be used with the program, as well as to be used as a stand-alonereference once you have viewed the program.

HOW TO USE THIS PROGRAM HANDBOOK

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Table of Contents

1—INTRODUCTION .................................................................................................................. 52—LEARNING OBJECTIVES .................................................................................................... 53—GUIDE AND REFERENCE .................................................................................................. 6

DISK 1ELECTROMAGNETIC DEVICES .............................................................................. 6MAGNETIC CONTACTORS....................................................................................... 6

Contactor Connection ........................................................................................... 7Differences Between AC and DC Contactors ....................................................... 8ARC Suppression and Protection ......................................................................... 9

MOTOR STARTERS ................................................................................................ 11Types of Magnetic Motor Starters ....................................................................... 13Installation and Wiring ........................................................................................ 15

PLUGGING SPEED SWITCHES ............................................................................. 16DISK 2

BASICS OF FUSES AND CIRCUIT BREAKERS .................................................... 18Fuses .................................................................................................................. 19Fuse Curves ....................................................................................................... 22Other Advantages of Dual Elements in Motor Control Circuits ........................... 23Fuse Types ......................................................................................................... 24Circuit Breakers .................................................................................................. 24

OVERLOADS ........................................................................................................... 26Overload Components and Operation ................................................................ 27Sizing Proper Heater Coils ................................................................................. 29Effects of Ambient Temperature.......................................................................... 30

CONTROLLING MOTOR OPERATIONS ................................................................. 31Safer Control Circuits—Control Power ............................................................... 32Wiring Diagrams ................................................................................................. 33Magnetic Starter Wiring Diagram ....................................................................... 34Wiring Diagram and Ladder Diagram Example .................................................. 36

TWO-WIRE AND THREE-WIRE CONTROL ........................................................... 38Two-Wire Control ................................................................................................ 38Three-Wire Control ............................................................................................. 40

CAUSES OF MOTOR FAILURE .............................................................................. 41Symptoms........................................................................................................... 42Why Motors Fail .................................................................................................. 42

4—SUMMARY ......................................................................................................................... 475—REVIEW QUESTIONS ....................................................................................................... 526—ANSWERS ......................................................................................................................... 67

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INTRODUCTION TO MOTOR CONTROL OPERATIONS AND PROTECTION

5

1—INTRODUCTION

In this video program, you will learn about theoperation of contactors and motor starters andhow these devices are used to control motor op-erations. You will also learn about plugging (zero-speed) switches and how these devices are usedin circuits to stop motors.

In addition, this program covers all protectiveswitches including fuses, circuit breakers, andmotor overload heaters, as well as their respec-tive overload contacts. Methods of troubleshoot-ing these components are also presented.

You will also learn about how two-wire and thee-wire control circuits operate, review what makesmotors fail, and learn how to prevent potentialmotor failure.

2—LEARNING OBJECTIVES

After this two-part video program, you will beable to:

• define electromechanical switches, their useand application

• understand how contactors and magneticstarters are used and the principal compo-nents that form them

• know the difference between contactors andstarters

• understand and apply plugging switches inmotor control circuits

• know the proper use and application of pro-tection fuses, circuit breakers, and motoroverloads

• troubleshoot all these control components

• understand two-wire and three-wire control

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3—GUIDE AND REFERENCE

CD-ROM DISK 1

This video program covers electromagneticswitches as well as their uses and operationsin motor control circuits. You will also cover pro-tective type switches that are used to protectelectrical circuits and the machinery controlledby these circuits. Let’s start by defining thesecontrol elements.

ELECTROMAGNETIC DEVICES

Electromagnetic devices are used in electricalcontrol systems to switch, either directly or in-directly, the on/off operation of lighting andpower loads such as motors (see Figure 1A).These devices are called electromagnetic be-cause they use a magnet and electrical powerto actuate the switching mechanism.

The electromagnetic control devices that arediscussed in this program are used in the con-trol of motor operations, including contactorsand magnetic motor starters. The devices wewill cover, which protect the motor and the mo-tor circuit, include fuses, breakers, and over-loads. Let’s start by looking at contactors.

MAGNETIC CONTACTORS

Magnetic contactors also actuate their contactsby energizing a magnetic coil. Contactors op-erate similarly to control relays. The differencebetween magnetic contactors and magneticcontrol relays is that relays are used for gen-eral purpose control switching of light duty loadswhile contactors are used for switching lightingsystems, heaters, transformer loads, and othercapacitive loads (see Figure 1B).

Figure 1B.

Figure 1A.

ELECTROMAGNETICDEVICES

E/M LOADS

NOTES

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7

123456789

2550

100150300600900

13502500

35

10204075

110175300

510254075

150225350600

NEMA RATINGS (DC)

SIZE120V

POWER (HP)

240VAMP.

Although there are several types of contactor

assembly designs, all operate very similarly

(see Figure 2). When voltage is applied to a

magnetic coil, the armature is attracted and

forces the movable contacts to make a con-

nection with the stationary contacts.

Magnetic contactors are available for single-

phase AC voltages, three-phase AC voltages,

and DC voltages (see Figure 3). Contactors for

each of these three categories are rated ac-

cording to NEMA. These ratings specify the size

of the load that can be handled safely by each

contact in the contactor. The sizes range from

00 to 9 according to the horsepower of the load

and the voltage being switched (see Figure 4).

For example, a NEMA 2 contactor is used when

you need up to 45 amps of continuous current

(see Figure 5). This type of contactor can be

used in a 15- or 25-horsepower load applica-

tion of a 208/240 or 480/600 voltage system,

respectively.

The single-phase NEMA 00, 0, 1, and 2 rat-

ings are only available to 120- and 240-volt sys-

tems from 1/3 to 7-1/2 horsepower. The stan-

dard rating available for DC contactors is from

size 1 to 9 with current capacities ranging from

25 amps to 2500 amps (see Figure 6).

Contactor Connection

The coil in a contactor indirectly controls the

operation of the contactor's load. In other words,

when the coil is turned on, its high-power con-

tacts close, providing power to the load. For

example, a three phase–three pole contactor

Figure 4.

Figure 2.

Figure 5.

000123

918274590

3/423--

11/23

71/21530

1/3123-

25

102550

123

71/2-

NEMA RATINGS (AC)

SIZE

120V 120V 240208/240

3 PHASE

480/600

CONT.AMP.

RATING

SINGLEPHASE

POWER (HP)

Figure 6.

AC/DC MAGCONTACTORS

SINGLEPHASE AC

THREEPHASE AC

DCVOLTAGE

NEMA SIZES

LOAD RATING

CONTACTORNEMA SIZES

0001234

56789

Figure 3.

MOVABLECONTACTS

COIL

ARMATURE

MAGNET

STATIONARYCONTACTS

CONTROLVOLTAGE

INPUT

L1

L2

TOLOAD

SUPPLY

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Figure 7.

L1 L2 L3 L2L1

H

H H

AUX

CON

Figure 12.

L1 L2 L3

TO LOAD

AUX C

Figure 11.

WIRINGDIAGRAM

L1 L2 L3

TO LOAD

POWERCONTACTS

Figure 10.

L1 L2

CON

CON

Figure 9.

Figure 8.CON

CONTACTORREPRESENTATION

CONTROL120 V

L3L2L1

H

H H480VOLTS

(see Figure 7) connected to a three-phase

heater will give power to the heating elements

once there is control power to the coil. This con-

trol power can be 120 volts AC while the power

provided to the heaters through the power con-

tacts can be 480 volts.

Contactors are represented in ladder diagrams

(see Figure 8) similar to the way relays are rep-

resented. Its power contacts, however, are not

represented in the ladder diagram (see Figure

9), but in another electrical diagram called the

wiring diagram (see Figure 10). These

contactors also have what’s known as auxiliary

contacts that react in the same manner as con-

trol relay contacts and are not part of the power

contacts circuit (see Figure 11).

We could graphically represent the contactor

circuit of Figure 7 in Figure 12, where the three-

phase power lines are being switched to the

heater. When the switch closes, the contactor’s

coil is energized, thus closing its power con-

tacts and turning on the heater. The auxiliary

contacts of the coil will also close which, if con-

nected to a light, will make it light up.

Differences Between AC and DC Contactors

Even though AC and DC contactors operate in

a similar manner, there are some differences

between the two.

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In DC contactor applications, you can switchonly one of the power lines, which requires onlyone set of contacts, or a contactor having onlyone pole (see Figure 13). In three-phase cir-cuits it is necessary to switch all three lines,

thus requiring three poles (see Figure 14).

The magnetic, or coil, assembly of a DCcontactor is made of solid steel instead of lami-nated steel used in AC contactors. DCcontactors use solid steel because DC currentonly flows in one continuous direction anddoesn’t create eddy current problems. AC andDC contactors also differ in the electrical andmechanical requirements used to suppress thearcing created during the opening and closingof the contacts.

ARC Suppression and Protection

Although the contacts in contactors are con-structed of strong cadmium oxide and silvermaterials to prevent corrosion and wear, theyare still subject to arcing that is present duringthe opening and closing of the contacts (seeFigure 15). This electric arc is created becauseduring the closing or opening of the contacts,there is a short time in which the contacts areneither closed nor open. This break in the cur-rent causes heat in the contacts, creatingenough “metal vapors” between the gaps of thecontacts that are still trying to conduct current(see Figure 16), thus creating the arc. This arccan continuously create heat eventually dam-aging the contact surface.

Because prolonged arcing can damage con-tacts, the sooner this arcing is extinguished,

Figure 16.

ONE POLE SWITCH

C

+ -

CON

Figure 13.

L1 L2 L3

THREE POLE SWITCH

CCON

Figure 14.

Figure 15.

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Figure 17.the longer the contactor will last. There are sev-eral methods used to suppress the arcing.

It's more difficult to suppress DC arcing thanAC arcing (see Figure 17) because DC cur-rent provides a continuous flow of current inone direction instead of fluctuating betweenpositive and negative current like AC currentdoes (see Figure 18). The DC arc lasts untilthe two contacts are so far apart that the arcextinguishes itself (see Figure 19). In contrast,AC arcs are self-extinguishing, thus having asmaller space between contacts (see Figure20). Because of the differences in arcing, ACcontactors tend to run with less heat than DCcontactors and have an increased contact life.

Arcing is much more pronounced during theopening of contacts, although it can also occurduring the closing of contacts, because currentis already flowing through the contacts.

There are three basic suppression methodsused to extinguish and minimize arcing.Contactors provide different types of arc pro-tection such as arc chutes and arc traps some-times incorporated in the cover of the contactor.

Chutes and traps are used to confine, divide,and limit the arc away from the contact surfaceas much as possible. This provides an area inthe contactor where the arc can be extinguishedwithout damage to the contactor. Contactorscan also provide a DC magnetic blowout coilfor arc extinguishing (see Figure 21).

Blowout coils produce a magnetic field acrossthe pole as current flows through the coil. This

SUPPRESSION OF DCARCING IS HARDER

CONSTANTCURRENT

FLOW

Figure 19. Figure 20.

LESS SPACEBETWEEN CONTACTS

Figure 18.

SUPPRESSION OF DCARCING IS HARDER

AC CURRENTFLUCTUATES

+ AND –

+-

BLOW-OUTCOIL

MAGNETICFIELD

CONTACTS

ARC -

+

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11

magnetic field interacts with the arc pushing upor blowing out into the arc chute where it is en-larged and extinguished (see Figure 22). Thepower contacts of a contactor with blowout coilsare shown in Figure 23.

Contactors should not be operated without arcsuppression elements. Without arc suppressionelements, damage can occur to the contactor,the machinery, or the loads the contactor is con-nected to (see Figure 24). During preventivemaintenance, make sure that the arc suppres-sion features of the contactor are installed andworking properly (see Figure 25).

Contactor troubleshooting is done similarly torelay troubleshooting (see Figure 26). Makesure that the coil is being energized and thatthe auxiliary contacts and the power contactsare responding to the energized coil.

MOTOR STARTERS

Motor starters are control devices used to con-trol the on/off state of motors in a manner simi-lar to the operation of contactors. The main dif-ference between motor starters and contactorsis the use of additional motor protection devicesused in motor starters. These protection devicesprevent overcurrents that could cause damageto the motor during overload conditions (seeFigure 27).

There are two types of motor starters—amanual motor starter and a magnetic motorstarter. A manual motor starter functions like aswitch. By activating the switch that closes thecontact, the power lines will connect to the

Figure 22.

L1 L2 L3

AUX C

CONTACTOR

OVERCURRENTPROTECTION

Figure 27.

MOVEMENT DUE TOMAGNETIC FIELD

Figure 23. BLOW-OUT COILREPRESENTATION

• CONTACTORS SHOULD NOT BE OPERATED WITHOUT THEIR ARC SUPPRESSION

• DAMAGE CAN OCCUR

Figure 24.

• CHECK SUPPRESSION FEATURES DURING PREVENTIVE MAINTENANCE

Figure 25.

Figure 26.L1 L2 L3

L1

L2

TO LOAD

AUX C120VSampl

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12

motor. A magnetic starter functions like a mag-netic contactor, using a coil to activate the con-tacts. Both motor starters have overload pro-tection for the motor.

The symbol for a motor starter in a ladder dia-gram is shown in Figure 28. A motor starter'spower contacts are represented in Figure 29.Motor starters also have auxiliary contacts usedfor interlocking connections with start/stop cir-cuits and other components in a ladder diagram(see Figure 30). Power contacts are usually rep-resented in a wiring diagram where they con-nect to the motor that is being controlled.

Starters provide protection to the motor withoverload relays, represented by the symbolshown in Figure 31. These overloads are foundbelow the contactor section of the starter. Over-load relays control a normally closed contactcalled a motor overload contact. A magneticstarter and its overloads are illustrated in Fig-ure 32, with the power contacts at the top andthe overloads just below.

The L1, L2, and L3 lines in a 3-phase voltagesystem are connected to the top of the starterand the bottom of the starter connects to themotor terminals that are marked T1, T2, andT3 (see Figure 32). The coil of the starter, whichenergizes the power contacts and the auxiliarycontacts, is also shown in Figure 32. The nor-mally closed overload contacts are representedin close proximity to the overload coils.

Figure 33 shows the actual location of the com-ponents that form the motor starter. At the topof the illustration, the line power terminals are

Figure 28.

L1 L2 L3

OVERCURRENTPROTECTION

AUX M

Figure 29.

MOTOR STARTER

M

Figure 31.

L1 L2

M

M

Figure 30.

OL

Figure 32.

L1 L2 L3

OL

AUX

M

T1 T2 T3

Figure 33.

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13

located below the coil and the auxiliary con-tacts are located between the coil. Power con-tacts are located behind the coil while the over-load relays and the normally closed overloadcontacts are right below.

Types of Magnetic Motor Starters

In the overload protection ladder diagramshown in Figure 34, the normally closed over-load contacts are located right after the startercoil. These contacts will open if the overloadrelay detects an overload situation, causing themotor to pull more current. An overload situa-tion can occur if the motor is experiencing alocked rotor situation. In a locked rotor situa-tion, the motor needs to be disconnected fromthe circuit or the circuit could burn out.

In Figure 35, the motor starter will be ON if thelimit switch closes. The normally open auxil-iary contacts will close and turn the pilot lighton. The power contacts will also close, provid-ing power to the motor as soon as the startercoil is energized. If the motor pulls too muchcurrent, the overload relays will trip and openthe normally closed overload contacts whichwill disconnect the motor from the circuit.

The types of magnetic motor starters include:

• full-voltage• combination starter• reversing starter• two-speed starter• reduced-voltage starter

L1 L2

M OLLS

OVERLOADPROTECTION

Figure 34.

Figure 35.

L1 L2

M

M

LS OL

PL

NOTES

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14

Full-Voltage Starter

A full-voltage starter is one of the most com-mon starters. This motor starter is also calledan across-the-line starter because it switchesthe full voltage to the motor when the startercoil is energized (see Figure 36).

Combination Starter

In the same cabinet or enclosure, combinationstarters combine a magnetic starter with a dis-connect switch that can be fused or non-fused.This combination, produced by manufacturers,allows for the quick and convenient installationof a motor control in a single enclosure. Start/stop push buttons are also available for the frontof the cabinet.

Reversing Starter

Reversing starters are available for applicationsthat require the reversal of motor rotation (seeFigure 37). Reversing starters can be used withplugging switches in a circuit to reverse the di-rection of the motor. For instance, the directionof a 3-phase motor is reversed by changing anytwo of the three phases. A reversing starter pro-vides the hardware to implement this reversal.

A reversing starter is composed of twocontactor sections with respective auxiliary con-tacts. Contactor sections are used to switch theappropriate phases to the motor. A reversingstarter also provides overload protection for themotor.

NOTES

Figure 36.L1 L2 L3

M

480VSYSTEM

Figure 37.

L1 L2 L3

OL

M

FWD REV

F R

1 2 3 1 2 3

REVERSINGSTARTER

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Two-Speed Starter

A two-speed motor starter provides two com-plete starters each with contactor sides andoverload relay assemblies that control two-speed motors (see Figure 38). The speeds of atwo-speed motor are low and high.

Reduced-Voltage Starter

Reduced-voltage starters are available to con-trol and allow a motor to start at a lower speedand then switch to full speed. There are fourtypes of reduced-voltage starters: auto trans-former, primary resistor, wye-delta, and partwinding.

Installation and Wiring

Wiring connections made during the installa-tion of a motor starter must follow the manu-facturers wiring diagram specifications. Ladderdiagrams serve little purpose during the wiringand installation of a motor starter. Frequently,especially for applications that require multiplemotors to control a line, all starters are housedin motor control centers that are easily acces-sible for troubleshooting.

During installation, it is important to make surethat the proper voltage phases are connectedto the proper terminals. The reversal of phasescan cause a motor to change the direction ofrotation during start-up causing equipmentdamage or personal injury (see Figure 39).

NOTES

Figure 39.

• MAKE SURE THAT THE

PROPER VOLTAGE PHASES

ARE CONNECTED

• AN UNPREDICTED REVERSAL

OF PHASE CAN CAUSE

SERIOUS DAMAGE AND/OR

INJURY

L1 L2 L3

1 2 3 1 2 3

TO MOTORSHIGH SPEEDTERMINALS

TO MOTORSLOW SPEEDTERMINALS

HIGH

LOW

Figure 38.

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Most motor starters are supplied from manu-facturers with some prewiring (see Figure 40).This prewiring includes the wire from one sideof the starter coil to the normally closed over-load contacts, and from the other side of theoverload contact to the return, or L2 terminalconnection, on top of the starter. Anotherprewired connection is from one side of theauxiliary contacts, labeled terminal 3, to the coil.This side of the coil is where the control poweris connected that turns on the starter coil.Prewired connections save time during the in-stallation of three-wire motor control circuits.

PLUGGING SPEED SWITCHES

Plugging switches are used to quickly stopmotor rotation. A plugging switch is also calleda zero-speed switch because it detects the ro-tation of the motor at zero speed or when it isclose to stopping.

A plugging switch is attached to the shaft of themotor pulley, which is turned by the rotatingequipment, and its shaft rotates with the motor.Figure 41 shows several types of pluggingswitches. Inside a plugging switch, there are twosets of double pole–double throw contacts thatopen and close according to the direction of theshaft rotation. The mechanism that actuatesthese contacts is called a centrifugal switch.

A plugging zero-speed switch is representedby the circular arrow symbol shown in Figure42, indicating a rotating type of activation switch.

L1 L2 L3

OL

AUX

3

2

Figure 40.

Figure 41.

Interchangeable Mounting Brackets

Courtesy of Allen-Bradley

Base 3 Point Flange 4 Point Flange

Figure 42.

REV

FWD

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NOTES

Figure 43 illustrates how a motor can go to themaximum forward or reverse speed accordingto a motor control circuit. If a plugging switch isattached to the shaft of a motor, the centrifugalforce created by the rotation of the shaft willcause the forward set of contacts to close whenthe motor starts to go forward. The speed orRPMs at which the switch will close is adjust-able.

To illustrate motor speed, let's assume, for ex-ample, that the contacts close around the dot-ted line representation for the forward and re-verse motions as shown in Figure 43. Once themotor starts and passes the forward speedsetting, the forward contacts will close. If themotor is stopped, the switch contacts will beOFF after the motor is slowed down at this trig-ger speed.

The simple operation of a plugging switch isused in interlocking circuits when it is neces-sary to stop a motor very rapidly by reversingthe direction of the motor. When the directionof a motor is reversed, the motor will stop fasterbecause it is going backwards. The sectionbetween zero speed and the speed at whichthe switch closes during forward will allow thecontrol circuit to detect when the motor is al-most at rest, disconnecting the circuit thatcaused the motor to go into reverse.

A plugging switch has many applications in con-veyor systems, where it is better for a motorthat controls the carrying of parts to stop sud-denly instead of coasting to a stop. This alsoapplies in emergency stop circuits in which amotor would be brought to a fast stop whennecessary.

Figure 43.

L1 L2

FULL REVSPEED

ZEROSPEED

FULL FWDSPEED

REVCONTACTS

CLOSE

FWDCONTACTS

CLOSE

REVREV

FWD

FWD

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CD-ROM DISK 2

The distribution of electrical power in a plantcan be very complicated. In addition, the cir-cuits that are receiving power are subject to de-structive overcurrents and short circuits. To pro-tect these circuits and power systems fromblackouts, prolonged downtime, and fire haz-ards, fuses and circuit breakers are widely used.Let's first define what an overcurrent is.

An overcurrent is the result of an overload cur-rent, a short-circuit, or a ground-fault current.An overload current is an excessive current,relative to the normal operating current, that isconfined to the normal conductive path providedby conductors, other components, and loads,such as motors, that form part of the power dis-tribution system (see Figure 44). A short-cir-cuit or a ground-fault current is an excessivecurrent that flows outside the normal conduct-ing path (see Figure 45).

While overloads are most often at no more than6 to 10 times the normal current levels, short-circuit currents can be hundreds of times thatof the normal operating current. This high cur-rent, if not stopped in a few thousands of a sec-ond, can cause severe insulation damage. Inaddition, overloads can cause melting of metaland conductors, vaporization of metal, ioniza-tion of gases, and arcing, all of which can re-sult in a fire.

OVERLOAD

Figure 44.

Figure 45.

NOTES

BASICS OF FUSES AND CIRCUIT BREAKERS

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Figure 47.

Fuses

Fuses are used to protect a circuit in which ashort-circuit fault can occur. This protection issupplied by the design built into the fuse, whichquickly disconnects the power provided to asystem. Fuses are represented by the symbolshown in Figure 46.

Inductive loads, such as squirrel-cage motors,will pull between 6 to 10 times the amount offull-load, normal current when they are firststarted. For example, a 200-volt, 10-horse-power motor will pull 193.2 amps for a shortduration before it reaches its full-load amper-age current (see Figure 47). In this case, it pulls6 times its full-load current. When this circuit isprotected with fuses against short circuits andground faults (see Figure 48), the fuse mustbe able to allow this overcurrent situation tooccur without breaking or disconnecting powerto the motor circuit.

Single element and dual element fuses areused in different applications. A single elementfuse, also called a non time-delay fuse, is madeof one conducting element that has one or morelinks enclosed in a tube, or cartridge, sur-rounded by arc-quenching filler material.

Under normal operation, the fuse conductscurrent simply by acting as a conductor (seeFigure 49). If an overload occurs and persistsfor a short interval, the temperature createdby the overcurrent will reach a level that meltsthe link forming a gap and breaking the circuit.

SWITCH

L1 L2 L3

3O200V

10 HP

32.2

CURRENT193.2A

MOTORSTARTER

Figure 48.

SWITCH

L1 L2 L3

3O200V

10 HP

32.2

CURRENT193.2A

MOTORSTARTER

FUSE SYMBOL

F

Figure 46.

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While overload currents are normally 6 to 10 timesthe normal current, short-circuit currents are muchhigher than the normal current (see Figure 50).Single-element fuses can be subjected to short-circuit currents of 50,000 amps or higher. Theresponse to these currents must be extremelyfast to avoid serious damage and run the riskof fire. In fact, if a large ground fault occurs, theresponse of a fuse must be within a matter of afew milliseconds.

A single element fuse, when exposed to shortcircuit conditions, will melt its links simulta-neously. A short-circuit current is cut off in lessthan 1/2 cycle, or about 8.3 thousands of a sec-ond. This is long before the short-circuit cur-rent can reach its full value.

Single element fuses are an excellent sourceof short-circuit protection (see Figure 51). How-ever, temporary and harmless overloads likethose created by motors can cause nuisanceopenings unless the fuses are oversized. Theseoverloads will occur when a fuse is sized toprotect a motor circuit and it trips during theinrush current generated when the motor isstarted (see Figure 51).

However, if a fuse is allowed to withstand theinrush current, it will not provide adequate pro-tection when a motor is running at full loadcurrent. For example, if a 10-horsepower mo-tor circuit is fused to the maximum inrush cur-rent of 193.2 amps, the motor runs the risk ofbeing burned out if an overcurrent exists at, forinstance, double its full load current for a pe-riod of time (see curve in Figure 51). The singleelement fuse must be sized higher than the fullload current of a motor in a circuit to allow the

STARTER

M

SWITCH

L1 L2 L3

MOTORSTARTER

AMMETER

10,000

SHORT

Figure 49.

Figure 50.

Figure 51.

32.2

193.2A

64.2

OVERLOAD

SINGLEELEMENT

RATED40A 10 HP

MSTARTERSampl

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motor to start. The proper sizing of non time-delay fuse is specified according to the NationalElectric Code.Dual-element time delay fuses are used to pro-tect conductors and circuits from both short cir-cuits and ground faults. Dual-element fuses notonly protect against short circuits, but also pro-tect motors from overcurrents caused by stall-ing, overloads, worn bearings, improper volt-age, single phasing, and other possible causes.

The dual-element time-delay fuse has two over-load and short-circuit elements built in. If a fuseexperiences a short-circuit overcurrent, the fusewill separate the links the same way a singleelement fuse does. If the fuse experiences anoverload condition for 10 seconds, at about 5times the value of the rated fuse, the overloadelement will snap out of the connector, disen-gaging the fuse from the circuit.

This fuse allows motor protection from overload-ing. For example, if a dual element fuse ratedat 40 amps is protecting a circuit of a 10-horse-power motor, we could sustain the overload in-rush current of 193 amps for a little over 10seconds before the fuse would open (see Fig-ure 52).

In normal motor control circuit applications, adual-element time-delay fuse serves as an in-valuable motor-running backup protection to themotor starter overloads (see Figure 53). A dual-element time-delay fuse can be particularlyhelpful if a phase is lost. In addition, if the mag-netic starter’s overload contacts fail to open andthe power contacts remain closed because a

NOTES

Figure 52.

SWITCH

L1 L2 L3

3O200V

10 HP

32.2

CURRENT193.2A

MOTORSTARTER

STARTER

M

STARTERPROTECTION

BACK- UPPROTECTION

DUALELEMENT

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severe overload has melted the contacts, thebackup protection provided by a dual-elementfuse will save the motor and circuit.During troubleshooting, if a fuse is found to beopen, it is a good idea to cut it in half and deter-mine if the cause of the opening was a shortcircuit or an overload. This will help determinethe source of a problem.

Fuse Curves

In the National Electric Code, non time-delayand dual-element time-delay fuses are ratedunder normal conditions in the range of 300and 175 percent of the full load current ofmotors. This gives, for example, a non time-delay fuse only three times its rating to allowfor the surge of current created by starting amotor (see Figure 54). A motor may pull at leastsix times the full load current when it is firststarted. Fuses, however, do not open instantlywhen current is just above the fuse rating.

The graph in Figure 55 represents single-ele-ment and dual-element time-delay fuses, ratedat 100 amps. It takes 10 seconds (see Figure56) for the time-delay fuse to open at a currentof 500 amps. However, it only takes 2 tenths ofa second for the non time-delay fuse to open ata current of 500 amps (see Figure 57).

A non time-delay fuse can be used to protecta circuit if the motor inrush current can be sus-tained by a fuse during the inrush time. If themotor current passes this limit, the fuse willopen. If this is the case, a slightly larger fuse,up to 4 times the full load current according tocode, can be used to protect the circuit. Oth-

Figure 54.

TIME

CU

RR

EN

T

6

3 300%

TIME-DELAY

NON-TIME DELAY

Figure 55.

TIME IN SECONDS

CU

RR

EN

T IN

AM

PE

RE

S

.01 .1.2

1 10 100 1,00010

100

1,000

500

10,000

100,000

3,000

Figure 57.

TIME IN SECONDS

CU

RR

EN

T IN

AM

PE

RE

S

.01 .1.2

1 10 100 1,00010

100

1,000

500

10,000

100,000

3,000

TIME-DELAY

NON-TIME DELAY

Figure 56.

TIME IN SECONDS

CU

RR

EN

T IN

AM

PE

RE

S

.01 .1.2

1 10 100 1,00010

100

1,000

500

10,000

100,000

3,000

TIME-DELAY

NON-TIME DELAY

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erwise, it is best to select a dual-element time-delay fuse that will provide adequate time forthe inrush current.

Other Advantages of Dual Elementsin Motor Control Circuits

To illustrate the advantages of dual elements,let's use as an example with both single-ele-ment and double-element fuses for a 10-horse-power, 200-volt motor (see Figure 58). Thismotor has a full-load amperage rating of 32.2amps. According to rule 430-52 of the NationalElectric Code, we could select a single-element,non time-delay fuse of 90 amps to protect thecircuit.

The fuse curve shown in Figure 59 for this nontime-delay fuse should be enough to let the mo-tor start for a short duration. Remember thatfuses, according to the curves, will let motorsstart if the motor start-current curve does nottrip the fuse. For instance, this 90-amp fusedoes not provide protection for the motor thatis under overload as shown by the dotted lineof the motor curve in Figure 59. Therefore, itwould require overload protection at the starter.

According to article 430-32 of the NEC, if wewant to use a dual-element time-delay fuse toprotect the motor from overloads and the cir-cuit from short circuits, we would select a 40-amp fuse. This fuse will protect the circuitagainst short circuits and overload conditionsand allow the motor to start when it pulls thestarting inrush current. In fact, if motor over-load relays are used in a motor control circuit,the dual-element fuses will serve as motorbackup protection just in case the overload re-

STARTER

M

SWITCH F

SELECTIONSINGLE-ELEMENT(NON-TIME DELAY)

90A(NEC 430-52)

• NO MOTOR O/LPROTECTION

10 Hp, 200V32.2 FLA

DUAL-ELEMENT(TIME DELAY)

40A(NEC 430-32)

• MOTOR O/LPROTECTION

Figure 58.

Figure 59.

TIME

CU

RR

EN

T

FUSE

MOTOR

Figure 60.

STARTER

M

SWITCH F

TIME

CU

RR

EN

T MOTORDAMAGE CURVE

TIMEDELAYCURVEOVERLOAD

HEATER CURVE

100%

600%

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lays are the wrong size or fail to operate. Thegraph in Figure 60 shows the motor damagecurve and the protection curve provided by themotor overload heaters. The dual-elementcurve is shown as backup protection. In addi-tion, the disconnect switch used in the dual-element circuit has a smaller current-carryingcapacity than the dual-element circuit in the 90-amp circuit, therefore reducing space andmoney (see Figure 61). A disconnect switch rat-ing must be equal to or larger than the amper-age rating of the fuse protecting the circuit.

Fuse Types

There are several different types of fuses avail-able, depending on the base the fuse mountsto, the socket or adapter that is used, and theinterrupting current ratings. The two categoriesof fuse sizes include fuses that are 600 volts orless and fuses that are 600 volts or more.

There are also classes of fuses, such as H, K,R, G, J, L, and T, which are rated for specificvoltages and currents, depending on the appli-cation. Classes of fuses are placed in their cor-responding holders. For example, class R re-jection clips, or holders, will accept only classR type fuses.

Circuit Breakers

As defined by NEMA, a circuit breaker is a de-vice that opens and closes a circuit by nonau-tomatic means if used as a disconnect. In otherwords, a circuit breaker opens the circuit auto-matically because of predeterminedovercurrents created by an overload or a shortcircuit. A circuit breaker trips a mechanism in-side which disconnects the circuit from theovercurrent condition (see Figure 62).

Figure 61.

STARTER

M

SWITCH F

STARTER

M

SWITCH F

PROTECTION

PROTECTION

90ANON-TIME

DELAY

40ATIME DELAY

BACK-UPPROTECTION

100A

60A

CB

CONNECTED

DISCONNECTEDCB

Figure 62.

NOTES

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The basic part of a circuit breaker, a circuit in-

terrupter, is shown in Figure 63 for a three-pole

circuit. This section of the breaker, which is

nonautomatic, is followed by the representa-

tion of the automatic breaker section.

Circuit breakers use two types of tripping ele-

ments: a bimetal, or thermal tripping element,

or a magnetic tripping element. Circuit break-

ers occasionally use both types of tripping ele-

ments. These devices can trip instantly in what

is called inverse time (see Figure 64).

Inverse time means that the time it takes the

breaker to trip is inversely proportional to the

amount of current. The higher the amount of

current, the less time needed for the circuit

breaker to trip. For instance, a 20-amp breaker

may take several minutes to trip a current of 25

amps (see Figure 65). However, if the same

20-amp breaker is subjected to a 50-amp cur-

rent, it will trip in a fraction of a second.

A bimetal breaker responds in an inverse time

manner. As the current passes, it bends the

bimetal breaker and makes the circuit trip (see

Figure 66).

A magnetic breaker has an electromagnet ele-

ment which responds to a high current gener-

ated by a short circuit. The activation in a mag-

netic breaker is instantaneous (see Figure 67).

Therefore, the most desirable circuit breaker

has both bimetal and magnetic mechanisms:

an element that responds to time delay and an

Figure 67.

CU

RR

EN

T

.01 1 10 SECONDS

TRIPPOINT

LINETO

LOAD LINETO

LOAD

Figure 66.

LINETO

LOAD

TRIPBAR

TIME

CU

RR

EN

T 500%

135%

30 MIN10 SEC

LINETO

LOAD

TRIPBAR

L1

L2

L3

CURRENT25A

CURRENT50A

20AFigure 65.

L1

L2

L3

DISCONNECTTRIP

ELEMENT

– THE HIGHER THE CURRENTTHE LESS TIME TO TRIP

INVERSE TIME

Figure 64.

Figure 63.

L1

L2

L3

CIRCUITINTERUPTER(NON-AUTO)

CIRCUITBREAKER

(AUTO)

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OVERLOADS

Motors are required to have protection againstoverload situations when the motor draws ex-cessive current for a certain period of time (seeFigure 70). For instance, excessive current oc-curs when a motor is overloaded by a jammingcondition in the line that stops motor rotation.A motor can also overheat if it is started andstopped too frequently.

When overloading occurs in a motor, the motorwill overheat, resulting in a deterioration of itsinsulation and causing damage to the motor.

To meet the protection requirements of a mo-tor, overload relays provide a time delay to al-low for temporary overcurrent during start up,tripping capability of a motor circuit once a dan-gerous level of overcurrent has been detectedfor a period of time, and the means for reset-ting the circuit.

Overload relays are represented by an S andby the symbol shown in Figure 71. The over-loads illustrated in Figure 72 are available forsingle pole–single phase, double pole–singlephase, and three pole–three phase motor cir-cuits. There are two main types of overload re-lays: mechanical state, operated by a heaterelement, and solid-state.

Figure 72.

MMM

THREEPOLE

DOUBLEPOLE

SINGLEPOLE

OVERLOADSYMBOL

OL

Figure 71.

Figure 70.

STARTER

M

SWITCH F

TIME

CU

RR

EN

T MOTORPROTECTION

CURVE

element that responds instantaneously (seeFigure 68).

The symbols shown in Figure 69 represent athermal-magnetic circuit breaker. Notice thatone side represents the bimetal or thermalmechanism and the other side represents themagnetic component.

Figure 68.

CU

RR

EN

T

.01 SECONDS60

LINETO

LOAD

MAGNETIC

THERMAL250%

Figure 69.

L1

L2

L3

THERMAL-MAGNETICCIRCUIT BREAKER

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Figure 76.

RACHETWHEEL

RACHETPIN SOLDER

HEATER

Figure 75.

OPERATINGSPRING

LOAD TERMINALS

HEATER SOLDER

RATCHET PAWL

RESET

RESET

Figure 73. Figure 74.

OLCONTACTS

HEATER

TO MOTOR

LINE

OVERLOAD RELAYHEATER

TO MOTOR

LINE

• MEASURE HEAT GENERATED BY CURRENT

Overload Components and Operation

The assembly of a mechanical overload relayis formed by the heater coil, also called an over-load heater, and the contact mechanism (seeFigure 73). Although there are different typesof heaters made by different manufacturers, allof them work by the same principle of measur-ing the heat generated by flowing current (seeFigure 74). There are two design types that arecommonly used: the eutectic melting alloy andthe bimetal overload relays.

The eutectic melting alloy overload uses aheater element surrounding a solder-likemechanism that holds the pin of a rachet wheelelement (see Figure 75). Eutectic means low-melting temperature solder.

When excessive current passes through aheater, the solder will melt and will transforminto a liquid state very quickly without goingthrough a plastic stage (see Figure 76). Thiswill cause the rachet wheel to release the pawlit is holding.

Once the pawl is released with pressure of theloaded spring, the normally closed contacts willopen (see Figure 77). When the solder-like melt-ing alloy has cooled down to a solid state, theoverload relay contacts can be reset to the nor-mal state. If you try to reset the overload relaycontacts while the alloy is still liquid, the con-tacts will open because the rachet will not beable to hold the pawl mechanism.

OPERATINGSPRING

LOAD TERMINALS

HEATER SOLDER

RESET

RESET

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The bimetal overload relay operates similarlyto the eutectic melting alloy, except that the bi-metal overload relay uses a bimetal strip madeof two bonded metals that expand with heat(see Figure 78). This heat is the result of over-load current created by the heater element andthen transferred to the bimetal element. Whenan overload condition is detected, the bimetalelement will bend and open the normally closedoverload contacts (see Figure 79). After cool-ing off from the heat created by the current, thebimetal may return to its original position auto-matically or manually, depending on the assem-bly. The automatic resetting feature of theseoverloads is restricted according to Article 430-43 of the National Electric Code because ofthe potential for injury and equipment dam-age due to an automatic restart of a motor (seeFigure 80).

The bimetal overload relays will disconnect themotor from the circuit as soon as the heatercoils detect enough current to disengage thenormally closed contacts (see Figure 81).Therefore, it is important for the heater elementsto be properly sized.

The solid-state overload relay in a motor circuitis represented by the symbol shown in Figure82. This overload relay provides the same ac-tion as the heaters. The overload relay detectsovercurrents by sensing the AC current mag-netically from the motor leads passing throughits current loops. This overload assembly pro-vides an automatic reset, producing the addi-tional circuitry necessary to provide for themanual reset. One advantage that this overloadassembly provides is that it is unaffected by

Figure 82.

ELECTRONICOVERLOAD RELAY

M

Figure 80.

HEATERLOAD

TERMINALS

BIMETAL

DEFLECTIONWITH TEMP.INCREASE

OPERATER

CONTROL CIRCUITTERMINALS

Figure 79.

Figure 78.HEATER

LOADTERMINALS

BIMETAL

OPERATER

CONTROL CIRCUITTERMINALS

Figure 81.

TOCONTROLCIRCUIT

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Figure 83.

"A" Underwriter's Requirements"B" Melting Alloy With 'H' Heaters (Standard Trip)"C" Bimetal With 'E' Heaters (Standard Trip)

% o

f Trip

Cur

rent

ambient temperature as heater coils are, thuseliminating the problem of tripping during hotweather.

Sizing Proper Heater Coils

The National Electric Code specifies theamount of protection that must be provided toa motor. For example, a three-phase, AC in-duction motor with a service factor of 1.15 wouldrequire three overloads selected to trip at nomore than 125% of a motor’s full load currentrating.

The selection of the appropriate size of heatercomponents is important. The standard trippingcurves of heater elements are specified at anambient temperature of 40°C to correspond withthe temperature at which most motors arespecified for ambient conditions. At a tempera-ture of 40°C, Underwriters Laboratory requiresthat an overload relay trip in 4 hours of opera-tion at 100% the trip current, in 8 minutes at200%, and in 30 seconds at 600% (see Figure83).

NEMA rates heater elements by the amount oftime it takes to melt the alloy when the motor isdrawing 6 times the full load current.

These heaters are grouped into three catego-ries: class 10, class 20, and class 30. Class 10heaters will melt the alloy in 10 seconds, theclass 20 heaters will melt the alloy in 20 sec-onds, and the class 30 heaters will melt the al-loys in 30 seconds (see Figure 84). Class 10heaters should be used in applications to pro-tect hermetic motors like compressors in air

Figure 84.

CLASS

NOTES

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conditioning systems, submersible pumps, andother motors with short locked-rotor times.Class 20 heaters are the most common sincethey can be used in many types of applications.Class 30 heaters should only be used in motorapplications with high inertia loads, such asdriving grinding wheels and fly wheels.

Manufacturers provide tables to use as a refer-ence in choosing the proper heater for a motorpulling the indicated amount of current. Whenchoosing a heater, it is important to make surethat the one you are referencing in the tablehas the appropriate amount of current. Thecurrent of a heater can be calculated at 125%of the full load current, or without the 125% re-quired by the code.

Effects of Ambient Temperature

Thermal overloads are sensitive to heat, includ-ing ambient temperature. For example, if thetotal heat required by the overload relay to tripthe motor is as shown on the left side of Figure85, and the ambient temperature creates partof the heat, the rest of the heat is left forovercurrents due to an overload. If the motordraws the full load current for a long period oftime, the overload relay will trip. If the ambientheat is as shown in the middle of Figure 85,the overload can be tripped at less overcurrent.Conversely, if the ambient heat is high (seeFigure 85 right side) the amount of heat nec-essary by the heater will be as much as indi-cated at the top right of the table, requiring morecurrent than necessary to protect the motor.Excessive current can result in the motor burn-ing out due to overheating.

Figure 85.

HEATGENERATEDBY MOTORCURRENT

AMBIENTHEAT

HEAT GENERATEDBY MOTOR CURRENT

AMBIENTHEAT

HEATGENERATEDBY MOTORCURRENT

AMBIENTHEAT

TOTAL HEAT REQUIRED TO TRIP OVERLOAD RELAY

A B C

NOTES

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Figure 86.

Compensating Bimetal Strip

HEATERLOAD

TERMINALS

BIMETAL

HEATERLOAD

TERMINALS

BIMETAL

Figure 87.

Compensating Bimetal Strip

Mu

ltip

lier

Fo

r U

ltim

ate

Tri

p C

urr

ent

Ambient Temperatrue Degrees C

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.50 10 20 30 40 50 60 70 80 90

Figure 88.

Bimetal overloads may provide a compensat-

ing bimetal strip that is added to the assembly

(see Figure 86). This compensating strip allows

the relay to adjust for changes in ambient tem-

perature at the location of the overload assem-

bly which is usually close to the motor starter

(see Figure 87).

The curve shown in Figure 88 indicates the re-

lationship between ambient temperature and

the trip current. For example, at 40°C, a trip

current of 10 amps will trip the heater, while at

60°C, the same heater will trip at 8 amps. At

20°C, the heater will trip at about 11.5 amps.

A compensating overload relay will make the

curve essentially flat across the different tem-

peratures. Therefore, if you have an application

in which the motor is in a constant ambient and

the overload is in a varying ambient, you should

use a compensating overload relay.

CONTROLLING MOTOR OPERATIONS

Most often the control of circuits involves the

control of motor operations such as a simple

ON and OFF control, controlling the speed of

the motor, or reversing the motor direction. It is

important to create a safe control circuit, un-

derstand wiring diagrams, and understand the

use of two- and three-wire control circuits.

Ambient Temperature—Degrees Celsius

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H2 H3 H4H1

X1 X2

240V

120V

2:1

H2 H3 H4H1

X1 X2

480V

120V

4:1

Figure 93.

Figure 92.

H2 H3 H4H1

X1 X2

240V / 480V

120V

L1 L2

START STOP

MOL

M1

MOTORSTARTER

CONTROL VOLTAGE120 VAC

L1

L2

L3M

MOTOR

480VSUPPLY

H2 H3 H4H1

X1 X2

Figure 91.

L1

L2

L1

L2

L3M

MOTOR

STARTSTOP OL

M

MOTORSTARTER

240VOR

480VSUPPLY

M120V

SUPPLY

Figure 90.

Figure 89.Safer Control Circuits—Control Power

Generally, control circuits show power betweenL1 and L2 and show the voltage between thetwo lines, such as 120 volts AC (see Figure 89).This voltage is called the control power or con-trol voltage.

In many instances, applications use voltagesupplies to motor circuits with 240 and 480 voltsAC (see Figure 90). For safety reasons, in caseslike this you may want to use lower voltage lev-els, such as 120 volts or lower, for the controlcircuit. If a separate power source is being usedfor the control circuit, this power source mustalso have its own disconnect means.

However, to reduce the load supply voltage foruse in the control circuit, a control transformercan be used (see Figure 91). These transform-ers have dual primary voltages of 240/480 voltswith secondaries of 120 volts (see Figure 92).Depending on how the primary section is con-nected, you can transform 240 volts to 120 volts,a 2 to 1 ratio, or 480 volts to 120 volts, a 4 to 1ratio (see Figure 93).

Control transformers are relatively small andcompact and are connected to the L1 and L2supply lines of the motor power circuit on theprimary side (see Figure 94). The secondaryside is connected to the control circuit, usuallythrough a fuse.

Control transformers are designed to handle thehigh current inrush requirements of control re-lays, contactors, starter coils, solenoids, and

other control devices. However, it is recom-

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Figure 94.

L1

L1

L2

L2

L3M

480V

120V

L1

L2

L3

L2

L1

M480V

120V

M

Figure 95.

L1

L2

L3M480V

120V

M

L1 L2

Figure 96.

mended that when mounted, you leave suffi-

cient air space around the transformer for heat

to dissipate. Also, be aware that the primary

wiring of 480 volts, for example, is not easily

accessible for someone who is troubleshoot-

ing the control circuit.

Control transformers eliminate the need to bring

an additional power source to the control cir-

cuit (see Figure 95). By adding a control trans-

former, you can also eliminate the need of a

separate disconnect means. Notice that in the

diagram shown in Figure 96, the transformer’s

primary connections are made after the dis-

connect means in the power circuit. Disconnect-

ing the motor control circuit will also discon-

nect the motor load.

Wiring Diagrams

Wiring diagrams, unlike ladder diagrams, are

intended to show the actual connection and

location of components, including the power

circuits, as realistically as possible.

Wiring diagrams are useful to electricians and

technicians for troubleshooting or designing a

control system because they provide a road

map of the actual wiring. This is especially true

in motor control circuits that use magnetic start-

ers and other control devices.

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L1 L2 L3

T1 T2 T3

Figure 100.

L1 L2 L3

T1 T2 T3

L1 L2 L3

T1 T2 T3

L1

L2

TO CONTROLCIRCUIT

ORTRANSFORMER

Figure 97.

Figure 98.

Figure 99.

MAGNETICSTARTER

CONTACTORSECTION

OVERLOADSECTION

Magnetic Starter Wiring Diagram

The magnetic starter in Figure 97, for example,can also be represented by the wiring diagramin Figure 98. The wiring diagram shows all ofthe connections to the starter as well as thelocation of the components within the starter.The symbols in the wiring diagram show thelocation of these components just as if you werelooking at the actual starter from top to bottom.

From the top of a starter, there are three termi-nals that connect to L1, L2, and L3 (see Figure98). Immediately below the power terminals,another three terminals are shown in smallercircles. These three terminals represent, andare used for, the wiring connections of controlcircuits or the control transformer that providespower to the circuit (see Figure 99).

The actual contacts are located that turn themotor load on are located right below theseterminals. Below these contacts, you find an-other set of terminals that are used when thereare reversing circuit connections. These ter-minals are also considered power connect-ing terminals, as opposed to control circuitterminals.

The starter coil is located under the secondset of power terminals (see Figure 100). Thestarter coil is represented in the diagram in themiddle cross section. Also shown are the twoterminal connections used in the wiring of thecontrol circuit. When the starter coil is ener-gized, these motor load contacts close which,in turn, starts the motor. These terminal con-

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Figure 101.

L1 L2

MOL

L1 L2 L3

T1 T2 T3

3

25

4

Figure 103.

L1 L2 L3

T1 T2 T3

3

25

4

OL

OL1 OL2 OL3

TOCOIL

TO L2

Figure 102.

Figure 104.

nections are also represented in the ladder dia-

gram (see Figure 101).

Auxiliary contacts are located on the left side

of the coil (see Figure 102) and are commonly

used as interlocking or sealing contacts. The

terminals are labeled 3 and 2. Depending on

the starter, there may also be another set of

auxiliary contacts that are normally open or nor-

mally closed as shown in Figure 103, with ter-

minals 5 and 4. On the lower left side of the

diagram, there are also normally closed over-

load contacts.

Depending on the starter, you may have three

sets of overload contacts, one for each heater

coil (see Figure 104). If this is the case, you

would connect or wire them in series if they do

not come internally connected. Although most

overload assemblies provide one set of nor-

mally closed contacts for all three power line

circuits, if there is an overload in any of the three

lines the overload contacts will be tripped.

Overload heaters, which control the overload

contacts, are represented on the right side of

the overload contacts (see Figure 103). Finally,

the terminal connections T1, T2, and T3 that

are connected to the motor load are located

below the heaters. In this case, the terminal

connections will be for a three-phase motor.

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Figure 106.

Figure 108.

L1 L2

STARTSTOP

MOL

M

2 3

L1 L2

STARTSTOP

MOL

M

2 3

Motor

L1 L3L2

T1 T3T2

2Hp, 120 VAC, 3O

Figure 107.

Figure 105.L1 L2 L3

T1 T2 T3

3

2

Most often, starters come with some prewired

control circuit connections that are installed at

the time of manufacturing (see Figure 105).

These connections include one side of the

starter coil attached to terminal 3 of the auxil-

iary contact. This connection is also repre-

sented in the wiring diagram (see Figure 106).

Another connection might be from the back side

of the coil to one side of the overloads (see

Figure 105). The other side of the overload will

be connected to the L2 line in the control circuit,

just as it is represented in the ladder diagram.

Wiring Diagram andLadder Diagram Example

As an example, we will produce a wiring dia-

gram for the 2-horsepower, three-phase, 120-

volt motor shown in Figure 107, whose opera-

tion and control is represented by the ladder

diagram in Figure 108.

In the wiring diagram (see Figure 109), one side

of the stop push button is connected to the L1

connection in the starter. Notice that the con-

nection is made to the small circle terminal to

denote a control circuit connection.

The other side of the stop push button is con-

nected to one side of the start push button,

which is also connected to terminal connec-

tion 2. The terminal connection 2 corresponds

to the auxiliary set of contacts in the starter.

The auxiliary contacts are labeled M in the lad-

der diagram (see Figure 108) in which the

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Figure 110.

L1 L2

STARTSTOP

MOL

M

2 3

PL

Figure 109.

StartStop

Motor

3

2

StartStop

Motor

3

2

Figure 111.

starter connections 2 and 3 are also included.

One side of the start push button is wired to

terminal connection 3 of the auxiliary contacts.

From the ladder diagram, you can see that the

connection from terminal 3 is also wired to the

starter coil. The other side of the starter coil is

connected to one side of the overloads and the

other side of the overloads is connected to L2

to complete the circuit.

It is possible that the two wire connections, from

terminal 3 to the coil and from the coil to L2

through the overloads, could already be wired

in the starter when it is shipped from the manu-

facturer.

If a pilot light is added to the ladder diagram

(see Figure 110) to indicate a “motor on” con-

dition, this condition would be included in the

wiring diagram (see Figure 111) as coming from

either the push button terminal or the terminal

3 connection of the starter. In this example, we

are connecting the pilot light to the push but-

ton terminal because we only have one wire

connected to the push button terminal, and we

have two at terminal 3. The other side of the

pilot light will be wired to L2 in the starter to

complete the circuit, resembling the same con-

nection as in the ladder diagram. Every time

the start push button is pressed, the motor will

start and the pilot light will be lit.

From this example, you can see that it is easierto follow the sequence of operation of a controlcircuit in a ladder diagram, such as Figure 110,rather than in a wiring diagram, such as Figure111. However, if you are troubleshooting a cir-

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Figure 112.

NOTES

StartStop

Motor

X1 X2

H1H2 H3

H4

3

2

T1 T2 T3

L1 L2 L3480V

120V

480 Voltscuit and motor load, wiring diagrams tend to beeasier to use because they give a more pre-cise location of starter components. Both cir-cuit diagrams, however, are useful whentroubleshooting.

If the motor in Figure 112 had been a high-volt-age motor of 480 volts, for instance, you woulduse a control transformer for the power goingto the control circuit. These connections wouldalso be represented in the wiring diagram asshown in Figure 112. Now the power is reducedfrom 480 volts to 120 volts. This transformerwould probably be located in the same enclo-sure as the starter.

Notice that a fuse, overcurrent protection forthe control circuit, has also been added. Thisfuse can be placed in other locations using othermethods, depending on the local, state, or NECrules.

TWO-WIRE AND THREE-WIRE CONTROL

When designing or troubleshooting a controlsystem, you will encounter two types of circuitscommonly known as two-wire and three-wirecontrols. These two circuit names may be ref-erenced by equipment manufacturers or by themaker of some motors and motor starters. Two-wire and three-wire controls have been brieflydiscussed, but we will now go into more depth.

Two-Wire Control

Two-wire control circuits are used in applica-tions in which the system operation is automaticand does not require operator intervention. The

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Figure 113.

L1 L2

MOL

POWER

FLOATSWITCH

Figure 114.

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWER

TWOWIRES

Caution When

Troubleshooting!

Figure 115.

control circuit in Figure 113 is called two-wirebecause only two wires are connected to a coilin the motor starter (see Figure 114). Thismakes the control circuit appear as one circuitin series driving the logic to a coil.

Two-wire control circuits are generally used incircuit applications that control a load device ata remote or distant location. For example, two-wire control circuits are sometimes used in thecontrol of pumping stations, HVAC systems,compressors, and line pumps.

Because of the way a two-wire circuit operates,this circuit provides low-voltage release but nolow-voltage protection. This means that, in theevent of a power loss in the control circuit, thecontactor in the starter will be de-energized. Inother words, the starter will create the low-volt-age release from the control circuit and will re-energize if the control device (in this case, thefloat switch) is still closed when the powercomes back to the control circuit.

There is no low-voltage protection becausethere is no way for an operator to intervene orreset the circuit if a loss of power has occurred.Therefore, be cautious (see Figure 115) whentroubleshooting two-wire circuits. If you discon-nect the control power and the control elementsdriving the two-wire circuit are still closed, thecoil will be picked up as soon as power is backon. There is no reset to provide low-voltage pro-tection in the control circuit.

A lack of low-voltage protection is primarily whythis circuit is used in applications that requireautomatic action for a motor to turn back onafter a power failure. An application that mightuse this type of circuit is an air compressor orfan in an HVAC system.

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Figure 116.

Figure 117.

L1 L2

STARTSTOP

MOL

M

2 3

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWERTHREEWIRES

StopStart

Figure 118.START

STOP

1

3

2

PUSH BUTTONSTATION

In the wiring diagram of the pump circuit (seeFigure 114), there are only two wires connectedor driving the logic of the coil in the starter. Notealso that in a two-wire circuit, terminal 2 of thestarter is not used.

Three-Wire Control

A three-wire control circuit (see Figure 116)uses three wires to connect to the coil of themotor starter. In fact, the extra wire is what con-nects the auxiliary contacts. The auxiliary con-tacts, in turn, seal the start push button as inthe wiring diagram shown in Figure 117.

Figure 117 shows the three wires that go to thestarter. Three-wire control provides both lowvoltage release and low voltage protection. Ifcontrol power is lost, the starter coil will drop,therefore turning off the motor and providingthe low voltage release. If the control powercomes back up, the circuit will not turn the mo-tor on unless the operator presses the startpush button again. Thus, this circuit is provid-ing low voltage protection by not allowing themotor to turn back on automatically without op-erator intervention.

A three-wire circuit can also be represented inthe standard push button station (see Figure118) used for three wire control where thereare three terminals for the three wires. Termi-nal 1 goes to L1 and terminals 2 and 3 go tothe auxiliary set of contacts in the starter. A pushbutton station is represented in a wiring dia-gram as shown in Figure 119. Notice that thetwo push buttons are surrounded by a dottedline to signify a three-wire control push buttonstation.

Figure 119.

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWER

START

STOP

THREEWIRESSam

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Figure 120.

START

RUNJOG

STOP

3

2

1

Figure 121.

L1 L2

STARTSTOP

MOL

M

2 3

JOG RUN

Motor

T1 T2 T3

L1 L2 L3

M

START

STOP

J R

3

2

Figure 122.

There are other three-wire control push buttonstations that may use additional componentssuch as a JOG selector switch to implementthe motor JOG function (see Figure 120).

In the circuit shown in Figure 121, the selectorswitch is used to run the motor when the switchis set to RUN and the start button is pushed.

If the switch is in the JOG position, the motor isonly able to turn on when the push button ispressed. As soon as the push button is re-leased, the motor will be OFF, thus allowing themotor to JOG every time the start push buttonis pressed. The JOG selector eliminates thememory that the sealing auxiliary contacts pro-vide.

A push button station with a JOG is representedin a wiring diagram (see Figure 122), which rep-resents a three-wire control circuit.

CAUSES OF MOTOR FAILURE

Motors are essentially the workhorse of a manu-facturing plant. Motors are designed with theintention of lasting for a long time. However, dueto wear and tear, or age motors eventually fail.When failure occurs, motors need to be re-placed.

Unfortunately, due to conditions in the plant,many motors fail prematurely. These conditionsmust be anticipated and prevented.

NOTES

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NOTES

Figure 123.

HOT

Symptoms

Most often, motors give symptoms of abnor-mal operation, such as running very hot, run-ning noisily, running at a slower speed thannormal, or even failing to start. Each of thesesymptoms are indications that the motor is notoperating properly and should be fixed.

To detect some of the problems with motors, itis important to be aware of strange noises andsmells (see Figure 123).

The reasons for motor failure are important be-cause, if the motor needs to be replaced, wemust ensure that the problem does not occuragain. It is important that the reason for motorfailure be investigated and documented in themaintenance replacement report.

Why Motors Fail

We know that regardless of how well a motor isbuilt, it will fail no matter how well it is protected.However, the major cause of motor failure isusually related to the deterioration of its insula-tion.

Although most motor manufacturers have at-tempted to solve the problem of insulation de-terioration, it is still the weakest link in any mo-tor. The insulation deterioration is usuallycaused by excessive temperatures experiencedby the motor and its windings.

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Figure 124.

EXCESSIVE LOAD

M

WORK CURRENT =TEMP

EXCESSIVE LOAD

M

OVERLOAD PROTECTION SHOULD BETHE EXCEPTION, NOT THE RULE

Figure 125.

The main causes for heat problems in motorsare:

• excessive loads• excessive duty cycle• high or low line voltages• imbalanced voltages• single phasing• high ambient temperature• lack of proper ventilation

Excessive Loads

If a motor is forced to do more work than it isdesigned for, the motor will attempt to do thework by pulling more current, thus resulting ina temperature increase (see Figure 124). If anoverload condition occurs, the motor’s overloadwill disconnect it from the line. However, a dis-connect should only happen occasionally, notfrequently or continuously (see Figure 125). Infact, if a motor has an automatic overload re-set, the motor can continue cycling until enoughheat has been built up to deteriorate the motorwinding.

Once the burned out motor has been replaced,it is important to measure the current beingdrawn by the new motor with a clamp-on meter(see Figure 126). The closer the reading is tothe full current rating of the motor, the harderthe motor is working with that load. If the cur-rent being drawn is very close to the maximumcurrent in the nameplate, it may be necessaryto install a larger motor. A larger motor will beable to handle normal load increases due tochanges in pulleys, gear sizes, or more loadresistance caused by wear on the equipmentbeing driven.

Figure 126.

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Figure 127.

NOTES

ON

OFF

MM

Excessive Duty Cycle

Excessively turning a motor on and off, referredto as jogging, as well as reversing direction,can be harmful to a motor because these ac-tions draw more current each time the motor isstarted. Excessive current can create a buildup of heat because the continuous self-cool-ing effect of the motor rotation is lost (see Fig-ure 127). If it is necessary to have a heavy dutycycle, a motor with a higher horsepower thanrequired by the load should be chosen.

High or Low Line Voltage

Motors are very sensitive to their rated voltage.Motors are designed to operate within 10% ofthe voltage for which they are rated. A lowerincoming voltage lowers the motor’s startingtorque and increases the temperature.

A higher incoming voltage produces a higherstarting torque and current. Higher currentcauses saturation of the iron in the motor anda rise in temperature. A motor that is runningmore than 10% above or below its rated volt-age can be seriously damaged. When replac-ing or installing a new motor, compare themotor's incoming voltage to the voltage on thenameplate and document the results.

Imbalanced Voltage

An imbalanced voltage is a source of heat cre-ated by a current imbalance in the windings ofa motor. Imbalanced voltage can also cause adecrease in torque, additional noise, and moremotor vibration. To prevent these motor prob-lems, document the occurrence of phase volt-ages that are close to one another. Measuring

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Figure 128.

M

MEASURELINE

VOLTAGES

each line voltage to a good ground will deter-mine whether a phase voltage is too close toanother phase voltage (see Figure 128).

Single Phasing

Single phasing occurs when one of the phasesis damaged due to a blown fuse, a trippedbreaker, or an open switch or contact. If a single-phasing problem occurs while the motor is run-ning, an increase in slip will occur. In otherwords, there will be a decrease in RPMs at fullload. The motor will continue to run as long asthe torque requirements of the load are notgreater than the motor torque. Single phasingincreases heat and vibration which can causesevere damage to the motor if not disconnectedpromptly.

If single phasing occurs when the motor is atrest, the motor will probably not start with a loador will make a humming sound if started. If atechnician assumes that the motor is bad andreplaces it, single phasing will continue to oc-cur in the new motor. Before replacing a motor,check the incoming voltages for a missingphase (see Figure 129). A single-phasing con-dition in a motor at rest could also cause a 3-phase motor with a small load to start in eitherdirection, causing damage to other equipment.

High Ambient Temperature

Just like with high current, a high ambient tem-perature can create unwanted motor problems.These unwanted problems can occur becausea motor is operating in a tight location or is sub-jected to high external or ambient heat.

Figure 129.

NOTES

M

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Most motor overloads do not detect ambienttemperature problems. Overloads monitor thecurrent for heat creation, not the ambient tem-perature. If an ambient temperature problempersists, a motor can burn up before the prob-lem is detected. Make sure that you are awareof what ambient temperature the motor will beoperating in so that the motor does not exceedits rating.

Lack of Proper Ventilation

Insufficient ventilation can happen when the airpassages within a motor are clogged with dirtor other material that prevents or blocks thenatural cooling effect of the motor. Under theseconditions, a motor can reach high tempera-tures without increases in current. A motor canburn up because a lack of ventilation has goneundetected. Proper preventive maintenanceshould prevent this problem from occurring.

In this program, we have covered important in-formation about the operation of contactors,motor starters, and how contactors and motorstarters are used in the control of motor opera-tions. We also covered the importance of us-ing fuses in the circuits and overload heatersin the starter for motor protection. We also pre-sented the important topic of two- and three-wire control and the different causes of motorfailure.

NOTES

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4—SUMMARY

• Electromagnetic devices use a magnet and electric power to actuate a switching

mechanism.

• Magnetic contactors are available for single-phase AC voltages, three-phase AC

voltagesand DC voltages.

• In DC contactor applications, you can switch only one of the power lines, which

requires onlyone set of contacts, or a contactor having onlyone pole. In three-

phase circuits it is necessary to switch all three lines, thus requiring three poles.

• Arcing is much more pronounced during the opening of the contacts because

current is already flowing through the contacts.

• Contactors should not be operated without arcsuppression elements.

• The two types of motor starters are manual and magnetic.

• Starters use overload relays to provide protection to a motor.

• The types of magnetic motor starters used in motor control operations include:

- full-voltage

- combination starter

- reversing starter

- two-speed starter

- reduced-voltage starter

• A full-voltage starter is also called an across-the-line starter.

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• Most motor starters are supplied from manufacturers with some prewiring asshown in the diagram below.

• A plugging switch, also called a zero-speed switch, is used to stop motor rota-tion quickly.

• A plugging zero-speed switch is represented by the circular arrow symbolshown indicating a rotating type of activation switch.

• The mechanism that actuates the contacts inside the plugging switch is called acentrifugal switch.

L1 L2 L3

T1 T2 T3

3

2

Interchangeable Mounting Brackets

Base 3 Point Flange 4 Point Flange

REV

FWD

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• An overcurrent is the result of an overload current, a short-circuit, or a ground-faultcurrent.

• Fuses are used to protect a circuit in which a short-circuit fault can occur and are repre-sented by this symbol:

• A single-element fuse is also known as a non time-delay fuse.

• Dual-element time-delay fuses can also protect conductors and circuits from short-circuit and ground faults.

• A motor may pull at least six times the full load current when it is first started.

• When used as a disconnect device, a circuit breaker can open and close a circuitnonautomatically.

• Circuit breakers use two types of tripping elementsóbimetal and magnetic.

• The activation of a magnetic circuit breaker is instantaneous.

• Overload relays are used to protect a motor.

• Overload relays are represented by the symbol:

• The eutectic melting alloy overload uses a heater element surrounding a solder-likemechanism that holds the pin of a ratchet wheel element.

FUSE SYMBOL

F

OVERLOADSYMBOL

OL

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• A solid-state overload relay is unaffected by ambient temperature.

• Heater elements are rated by NEMA according to the amount of time it will take to meltthe alloy when the motor is drawing six times its full load current.

• Heaters are grouped into three categoriesóclass 10, 20, and 30.

• A compensating bimetal strip should be used in the relay when the ambient tempera-tures of the motor and the overload are different.

• Control transformers eliminate the need to bring an additional power source to the controlcircuit and can also eliminate the need of a separate disconnect means.

• Wiring diagrams are used to show the actual connections and placement of compo-nents.

• Most often, starters come with some prewired control circuit connections that are in-stalled at the time of manufacturing.

MAGNETICSTARTER

CONTACTORSECTION

OVERLOADSECTION

L1 L2 L3

T1 T2 T3

3

25

4

L1 L2 L3

T1 T2 T3

3

2

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• Two-wire circuits provide low-voltage release but no low-voltage protection.

• Two-wire control circuits are generally used in circuit applications that control a loaddevice at a remote or distant location.

• Three-wire circuits provide both low-voltage release and low-voltage protection.

• A three-wire control circuit uses three wires to connect to the coil of the motor starter.

• A three-wire circuit can also be represented in the standard push button station used forthree wire control where there are three terminals for the three wires.

• The major cause of motor failure is due to the deterioration of its insulation caused byexcessive temperatures.

• Heat problems in motors are caused by situations such as, excessive loads, excessiveduty cycles, high or low line voltages, imbalanced voltages, single phasing, high ambienttemperatures, and lack of proper ventilation.

• The reasons for motor failure are important because, if the motor needs to be replaced,we must ensure that the problem does not occur again.

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWERTHREEWIRES

StopStart

START

STOP

1

3

2

PUSH BUTTONSTATIONSam

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5—REVIEW QUESTIONS

1. Devices that use a magnet and electrical power to actuate a switch are called __________devices.

a. electromagnetic

b. manual

c. plugging

d. multichannel

2. Magnetic contactors are used to switch loads that require large current handling, suchas__________ and ____________ (select two).

a. motor starters

b. heaters

c. small loads

d. variable frequency drives

e. large lighting systems

3. In this diagram, the first step that must take place in order to turn on the pilot light is to:

a. attract the armature

b. change the magnet

c. move the stationary contacts

d. apply control voltage to the magnetic coil

MOVABLECONTACTS

COIL

ARMATURE

MAGNET

STATIONARYCONTACTS

CONTROLVOLTAGE

INPUT

L1

L2

TOLOAD

SUPPLY

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4. True/False. In the diagram shown, the120 volts coil indirectly controls the actions of the threecontacts switching the heater. However, the three-phase power cannot be more than 120volts.

5. True/False. Contactors are represented in ladder diagrams similar to the way relays are repre-sented, as well as its power contacts.

6. True/False. In DC contactor applications, you must switch both of the power lines, the positiveand negative.

7. True/False. Arcing is more pronounced during the opening of the contacts because current isalready flowing through the contacts.

8. True/False DC arcing is more difficult to suppress that AC arcing.

9. A contactor may provide a DC magnetic __________ to extinguish an arc.

a. blowout coil

b. pole switch

c. fuse

d. overload

L1 L2

M OLLS

OVERLOADPROTECTION

CONTROL120 V

L3L2L1

H

H H480VOLTS

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10. The two types of motor starters are: (select two)

a. magnetic

b. automatic

c. manual

d. fixed

e. suppressed

11. True/False. In this diagram, the overload contact will open if an overload situation is detected.

12. Starters provide protection to the motor with overload relays, represented by the symbol____

13. Two types of magnetic motor starter are _________and ___________ (select two).

a. full-load

b. forward

c. full-voltage

d. reversing

e. auxiliary

L1 L2

M OLLS

OVERLOADPROTECTION

OL

a b

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14. A __________ starter is also called an across-the-line starter.

a. reversing

b. combination

c. full-voltage

d. two-speed

15. True/False. A reversing starter is composed of two contactor sections with respective auxil-iary contacts.

16. True/False. An across-the-line starter, like the one shown in this diagram, switches full voltageto the motor once the starter coil is energized.

17. True/False. Ladder diagrams are very important during the wiring and installation of a motorstarter.

18. True/False. Very seldom do we find motor starters that come with some sort of prewiring.

19.True/False. During installation of a starter, it is very important to make sure that the propervoltage phases are connected to the proper terminals.

L1 L2 L3

M

480VSYSTEM

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20. A plugging switch, like the one shown in this diagram, is used to __________ motor rotation.

a. start

b. maintain

c. quickly stop

d. none of the above

21. Inside a plugging speed switch there are two sets of DPDT contacts that act upon the direc-tion of the motor shaft rotation, the mechanism that actuates these contacts is called_______________.

a. motion detection switch

b. circular switch

c. centrifugal switch

d. centripetal switch

22. In the diagram shown, indicate which side of the plugging switch is wired to in a typical ladderdiagram.

23. True/False. Fuses and breakers can be used to protect motor circuits.

24. True/False. A fuse protects a circuit.

Motor start logic

Reverse circuit logic

Sequential stop logic

Forward circuit logic

connect to

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25. An overcurrent is the result of _____________________

a. an overload current

b. a short-circuit

c. a ground-fault current

d. all of the above

26. What is the function of the fuse (F) in this motor control circuit?

a. to protect the main switchboard

b. to protect the entire branch circuit

c. to protect the motor from overloads

d. to protect the circuit from short-circuit faults

27. A __________ fuse is also known as a non time-delay fuse.

a. dual element

b. two-way

c. full-load amperage

d. single-element

28. True/False. Single element and dual element fuses are used interchangeably in the samemotor applications.

29. True/False. Dual-element fuses not only protect against short circuits, but also protect motorsfrom overcurrents caused by stalling, overloads, worn bearings, improper volt-age, singlephasing, and other possible causes.

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30. True/False. Using this chart, if an overcurrent of 500 amps occurs a 100-amp single elementfuse will open in approximately 0.2 seconds, while a 100-amp double element fuse will openin approximately 10 seconds.

31. Non time-delay and dual-element time-delay fuses are rated under normal conditions in therange of ________ and ______ percent of the full load current of motors.

a. 300

b. 400

c. 275

d. 175

32. A motor may pull at least _______ when it is first started.

a. 50 amps

b. ten times the full load current

c. six time the full load current

d. 15 additional amps to the full load current

TIME-DELAY

NON-TIME DELAY

TIME IN SECONDS

CU

RR

EN

T IN

AM

PE

RE

S

.01 .1.2

1 10 100 1,00010

100

1,000

500

10,000

100,000

3,000

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59

33. True/False. The 90-amp non-time delay fuse provides protection to the motor when it is in anoverload condition as depicted by the dotted lines in the current versus time graph.

34. The diagram shown illustrates the damage curve and the protection curve provided by themotor overload heaters. The dual-element can also be considered ______________ in themotor circuit.

a. non essential protection

b. backup protection

c. under protection


35. True/False. Fuses can be categorized by sizes of 600 volts or less and 600 volts or more.

TIME

CU

RR

EN

TFUSE

MOTOR

STARTER

M

SWITCH F

TIME

CU

RR

EN

T MOTORDAMAGE CURVE

TIMEDELAYCURVEOVERLOAD

HEATER CURVE

100%

600%Sampl

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60

36. A __________ is a device that opens and closes a circuit non-automatically when it is used asa disconnect.

a. capacity switch

b. thermal overload

c. solenoid

d. circuit breaker

37. The two types of tripping elements used in circuit breakers are: (select two)

a. Class R rejection clips

b. static

c. magnetic

d. time-delay

e. bimetal

38. The type of circuit breaker shown in this diagram is a __________ breaker.

a. time-delay

b. static

c. magnetic

d. bimetal

39. Overload relays are used to protect a:

a. branch circuit

b. circuit

c. complete busway

d. motor

40. The symbol in this diagram represents a(n)_______________

a. thermal breaker

b. overload relay

c. circuit breaker

d. fuse

LINETO

LOAD LINETO

LOAD

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61

41. This diagram shows a __________ overload relay.

a. solid-state

b. chattering

c. eutectic melting alloy

d. bimetal

42. The symbols highlighted in this diagram represent a __________ overload relay.

a. eutectic melting alloy

b. bimetal

c. solid-state

d. disconnected

43. True/False. Class 40 is a NEMA-rated category of heaters.

44. Locate the compensating bimetal strip in this diagram (a, b, or c).

a

b

c

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62

45. The curve shown in the diagram shows the relationship between ambient temperature andthe trip current. For example, at 4 degrees CD C, a trip current of 10 amps will trip the heater,while at 60 degrees C, the same heater will trip at ______amps.

a. 10

b. 8

c. 0.8

d. 1.1

Mu

ltip

lier

Fo

r U

ltim

ate

Tri

p C

urr

ent

Ambient Temperatrue Degrees C

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.50 10 20 30 40 50 60 70 80 90

46. Locate the control transformer in this diagram (circle a, b, or c in red).

47. True/False. The secondary side of a control transformer connects directly to the supply linesof a ladder control circuit.

48. True/False. Control transformers eliminate the need to bring an additional power source to thecontrol circuit.

L1

L2

L3

L2

L1

M480V

120V

M

L1

L2

L3M

MOTOR

480VSUPPLY

H2 H3 H4H1

X1 X2

a b

c

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63

49. __________ diagrams show the actual connections and location of components in a system.

a. wiring

b. ladder

c. plant

d. power

50. True/False. In this wiring diagram, the three circles labeled L1, L2, L3 represent the powerterminal inputs to a motor starter.

51. Where in the wiring diagram is the location indicated in the ladder diagram with the arrow.

L1 L2 L3

T1 T2 T3

L1 L2 L3

T1 T2 T3

L1 L2

MOLb

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64

52. In the diagram shown, select where terminal two in the ladder diagram will be connected to inthe wiring diagram.

L1 L2

STARTSTOP

MOL

M

2 3

L1 L2 L3

T1 T2 T3

3

2

b

a

c

53. True/False. It is easier to follow the sequence of operation of a circuit in the wiring diagramrather than the ladder diagram.

54. True/False. If the motor in the diagram is 480 volts, it would not be recommended that acontrol transformer be used to bring the control voltage to 120 volts.

55. True/False. Two-wire circuits, such as the one shown in this diagram, provide low-voltagerelease but no low-voltage protection.

StartStop

Motor

3

2

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWER

TWOWIRES

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65

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWERTHREEWIRES

StopStart

56. The third wire in this three-wire control circuit connects to the _____________.

a. transformer

b. auxiliary contacts

c. power circuits

d. overload contacts

57. True/False. Two-wire control circuits are used in applications that require automatic operationand that do not require operator intervention.

58. Three-wire control shown provides ______________ and _________________ .

a. low-voltage release

b. no low-voltage protection

c. no low-voltage release

d. low-voltage protection

59. True/False. A motor may fail if its insulation has deteriorated.

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

POWER

START

STOP

THREEWIRES

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66

60. In this diagram, if the current reading is very close to the maximum current listed on thenameplate, you should _______________.

a. add another overload relay

b. select a larger motor

c. add more power

d. change the power supply

61. Single phasing occurs when ____________

a. a motorís coil burn out

b. motor power is interrupted

c. one of a motorís phases is lost


M

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67

6—ANSWERS

1. a. electromagnetic

2. b. heaters and e. large lighting systems

3. d. apply control voltage to the magnetic coil

4. False

5. False

6. False

7. True

8. True

9. a. blowout coil

10. a. magnetic and c. manual

11. True

12. b (right symbol)

13. c. full-voltage and d. reversing

14. c. full-voltage

15. True

16. True

17. False

18. False

19. True

20. c. quickly stop

21. c. centrifugal switch

22. Motor start logic

Reverse circuit logic

Sequential stop logic

Forward circuit logic

connect to

connect to

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23. True

24. True

25. d. all of the above

26. d. to protect the circuit from short-circuit faults

27. d. single-element

28. False

29. True

30. True

31. a. 300 and d. 175

32. c. six time the full load current

33. False

34. b. backup protection

35. True

36. d. circuit breaker

37. c. magnetic and e. bimetal

38. c. magnetic

39. d. motor

40. b. overload relay

41. c. eutectic melting alloy

42. c. solid-state

43. False

44. c is the compensating bimetal strip

45. b. 8 amps

46. position c

47. False

48. True

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49. a. wiring

50. True

51. position b

527. position a

53. False

54. False

55. True

56. b. auxiliary contacts

57. True

58. a. low-voltage release and d. low-voltage protection

59. True

60. b. select a larger motor

61. c. one of a motorís phases is lost

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