R T RESUMESED 013 9413ELECTRIC MOTORS FOR FARM USE.ILLINOIS UNIV. , URBANA, COLL. OF AGRICULTURE

PUB DATEEDRS PRICE MF-1;0.25 HC NOT AVAILABLE FROM ORS. 35P.

VT 0113 0114

DESCRIPTORS- *TEX TECOK S *VOC A T IONAL AGRICULTURE, ..ELECTRICMOTOR S

62

BETWEEN 2 AND 6 HOURS ARE REQUIRED FOR USE OF THISTEXTUAL OR REFERENCE MATERIAL ON ELECTRIC MOTORS. IT WASDEVELOFED EY AN AGRICULTURAL EDUCATION-AGR I CULTURALENGINEERING SPECIALIST CV THE BASIS OF CONFERENCES WITHSUBJECT MATTER SPECIALISTS, TEACHER EDUCATORS, SUPERVISCI:SfAND TEACHERS. THE OBJECTIVES AND SUBJECT MATTER CENTER ARC&JtVTHE FOLLOWING QUESTIONS (1) WHAT ARE THE ADVANTAGES CfELECTRIC W.ITORS, (2) WHAT FACTORS SHOULD I CONSIDER INSELECTING AN ELECTRIC MOTOR, (3) HOW CAN I IDENTIFY ANDSELECT THE PROPER TYPE AND SIZE OF ELECTRIC VOTORS, (4) 1104SHOULD I INSTALL THE MOTOR PROPERLY; (5) WHAT CARE SHOULD IGIVE AN ELECTRIC MOTOR, (6) HOO CAN I DETERMINE WHAT IS WRONGWHEN A MOTOR WILL NOT OPERATE; AND (7) WHAT ARE THE IMPCflANTPRINCIPLES OF ELECTRIC vcaoRs. DDIONSTRATIONS AND SHOPEXERCISES ARE SUGGESTED. ILLUSTRATIONS ARE INCLUDED. DESIGNEDFOR BOTH HIGH SCHOOL AND POST-HIGH SCH(.70L USE, THE MATERIALIS APPROPRIATE FOR THOSE STUDENTS WHO HAVE AVERAGE ABILITY,AGRICULTURAL INTEREST, AND AN OCCUPATIONAL COJECTIVE. THISDOCUMENT IS AVAILABLE FOR 45 CENTS FROM VOCATIONALAGRICULTURE SERVICE, 434 UMFORD HALL, UNIVERSITY CfILLINOIS, URBANA, ILLINOIS 61801. (JM)

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U.S. DEPARTMENT OF HEALTH, EDUCATION & WELFARE

OFFICE OF EDUCATION

THIS DOCUMENT HAS BEEN REPRODUCED EXACTLY AS RECEIVED FROM THE

PERSON OR ORGANIZATION ORIGINATING IT. POINTS OF VIEW OR OPINIONS

STATED DO NOT NECESSARILY REPRESENT OFFICIAL OFFICE OF EDUCATION

POSITION OR POLICY.

MEM

ELECTRIC MOTORS

FOR

FARM USE

Distributed by

VOCATIONAL AGRICULTURE SERVICE

UNIVERSITY OF ILLINOISCOLLEGE OF AGRICULTURE

URBANA, ILLINOIS

ACKNOWLEDGMENT 6,i.This publication was made possible through the cooperative efforts of

several agencies and many different individuals.Special credit is due the members of the Motor and Generator Section

of the National Electrical Manufacturers Association (NEMA). Much ofthe source material and most of the illustrations were provided by NEMA.

Much credit should Also be given to a committee of Illinois vocationalagriculture teachers, composed of Kenneth W. Knell, Mahomet ; William R.Queen, Cuba ; Wendell Schrader, Tuscola ; Frank A. Stansfield, Lawrence-ville; and John W. Matthews, formerly of Shabbona.

Additional contributions were made by members of the Teacher TrainerAdVisory Board of the Illinois Association of Vocational AgricultureTeachers, by state supervisors and teacher trainers of vocational agricul-ture, and by subject-matter specialists at the University of Illinois.

Vocational Agriculture Service

ELECTRIC MOTORS FOR FARM USE

1. What Are the Advantages of Electric Motors?2. What Factors Should I Consider in Selecting an Electric Motor?3. How Can I Identify and Select the Proper Type and Size of Electric Motors?4. How Should I Install the Motor Properly?5. What Care Should I Give an Electric Motor?6. How Can i Determine What Is Wrong When a Motor Will Not Operate?7. What Are the Important Principles of Electric Motors?8. Suggested Demonstrations and Shop Exercises

Rural electrification has made possible greatchanges in farming and in farm living during thepast generation. A type of energy has beenbrought to the farm which can furnish light,heat, and power more effectively and efficientlythan we once dreamed possible.

The number and variety of farm chores whichelectric motors can perform has grown steadily.Motors truly provide the "push" behind "push-button farming". A -ast number of our time- andlabor-saving devices would not be possible withoutthe electric motor, the most efficient source ofmechanical energy on the farm.

Electric motors provide power in one of thenfest forms known. Nevertheless, power in anyform presents the possibility of a hazard, and suit-able precautions must be observed in installationand use. Safety from moving parts will depend onthe guarding described in this publication and suchspecial precautions as may be needed for the par-ticular machine each motor drives. To provideelectrical safety, the National Electrical Codeshould be carefully followed in making each instal-lation, and all parts of the electrical system shouldbe maintained in good condition.

1. WHAT ARE THE ADVANTAGES OF ELECTRIC MOTORS?

Electric motors have many advantages whenwe compare them with any other kind of farmpower. Electric motors are low in first cost, cheapto operate, long in life, highly efficient, simple tooperate, quiet in operation, capable of starting areasonable load, capable of withstanding tempor-ary overloading, capable of being automaticallyand remotely controlled, compact, and safe.

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Low in first cost. The initial cost of an electricmotor is relatively low, considering the power itwill develop. Electric motors are quite simple inconstruction, have few moving parts, and aremanufactured by mass-production methods.

Cheap to operate. Electricity is measured inkilowatt-hours, just as corn is measured in bushels.A kilowatt is equal to 1,000 watts. In terms of

mechanical energy, 746 watts equal one horse-power, although for practical purposes, takingaccount of friction and other losses, it requiresabout 1,000 watts or 1 kilowatt (electrical powerinput) to produce one horsepower (mechanicalpower output.) One kilowatt of power suppliedfor one hour continuously equals the energy unitof one kilowatt-hour. This kilowatt-hour of electri-city, at a cost of 2 or 3 cents, will do as much ormore work than can be performed by a man in aneight-hour day. One kilowatt-hour, if properly used,will, among other chores, milk 20 cows, separate2,000 pounds of milk, ventilate a 25-cow dairybarn for one-half day, pump 400 pails of water,grind 100 to 500 pounds of grain, hoist 2 tons ofhay, shell 20 bushels of corn, shear 60 sheep, mixtwo cubic yards of concrete, or paint 700 squarefeet of surface with a pressure sprayer.

Long in life. Electric motors are noted forlength of life. If given reasonable care, a goodmotor will last 20 to 30 years, or even longer.Many motors have been in operation for this longa time with no expense for repair and no mainte-nance cost other than for occasional lubrication.

Highly efficient. The electric motor is themost efficient source of power available to the

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farmer today. Its efficiency will vary from 65 to85 percent, depending on the type of motor andthe conditions under which it operates.

Simple to operate. Electric motors can bestarted and stopped with the push of a button orthe flick of a switch. They start equally well inhot or cold weather, and the electrical connectionscan generally be made so that the motor will ro-tate in either a clockwise or counter-clockwisedirection.

Quiet in operation. Electric motors operatesmoothly, quietly, and with very little vibration.

Capable of starting a reasonable load. Theability of electric wotors of various types to startunder full load eliminates the necessity for a clutchor low gears for starting loads.

Capable of withstanding temporary overload-ing. The ability of electric motors to carry mo-mentary loads up to one and one-half times theirrated capacity gives them flexibility and adapta-bility. Of course, an electric motor should not becontinuously overloaded.

Capable of being automatically and remotelycontrolled. This is an important advantage whichhas made possible the development of mechanicalrefrigeration and automatic water, heating, andventilating systems on the farm. Remote controlsadd a great deal to the convenience and often tothe safety of electric-motor applications.

Compact. The electric motor has considerablyless weight and bulk than other types of power

2. WHAT FACTORS SHOULD I CONSIDER

When we buy a milk cooler, an air compressor,or any similar electric motor-driven farm ap-pliance, we need only know what type of electricpower is available. Beyond this the equipmentmanufacturer determines what size and type ofmotor to use and equips the appliance accordingly.

However, there are frequent occasions when youmay wish to buy a separate motor for operatinga piece of equipment formerly driven by hand orby some other power unit. There are also timeswhen a motor must be replaced in an emergencyand no exact duplicate is available. In situations

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units. It is often very small compared to themachine it drives.

Safe. Motor designs, in general, prevent con-tact with live wires and electrical parts. They donot operate at high temperatures. Thus electricmotors provide the farmer with one of the safestforms of power available.

IN SELECTING AN ELECTRIC MOTOR?

like these you should be able to intelligently choosethe proper type and size of motor to use.

The selection of motors for farm use shouldtake into consideration the type of electric poweravailable, type and size of load, and conditionsunder which the motor operates.

Type of electric power available. Electricitymay be direct current (d.c.) or alternating (a.c.).It may be one of several voltages. If alternatingcurrent, it may be single phase or three phase,and one of several frequencies. In almost everycase today, however, electric power on the farm is

single phase, 60-cycle-alternating current, avail-able at 120 and 240 volts. The voltage ratings ofsingle-phase motors are 115 and 230 volts and areslightly lower than the line voltages of 120 and240 volts. The reason that the rated voltage of themotor is lower than the line voltage is 1,o allowfor the voltage drop which takes place betweenthe meter and the motor or other electrical appli-cations. The voltage ratings of motors are usedthroughout the remainder of this publication.

Type and size of load. Farm-motor loads varywidely, particularly with respect to power neededfor starting. Some are particularly easy for anelectric motor to start, or are of a nature that verylittle load is applied until the machine is up to itsfull running speed. For example, if we are sharp-ening an axe on a power-driven grinding wheel,we start the motor and allow the grinder to reachoperating speed before we start grinding. Thismeans that the motor is only required to start asmall load compared to what it must carry whenwe bear down sharply with the axe. Likewise amachine like a small fan offers very little re-sistance at the start.

Other machines like piston-type water pumps,milking machines, refrigerators, and cream separ-ators require much more effort to start than tokeep them running. It is logical to choose a dif-ferent type of motor for these devices.

There are other differences in load require-ments which have an important bearing on thechoice of motors. Automatic water pumps anu re-frigerators require frequent starting awl stopping.Gasoline tank pumps run only occasionally and forshort periods while milking machines and feedgrinders may run steadily for long periods of time.Elevators and wood saws may be subjected fre-quently to momentary overloads.

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Operating conditions. Farm motors operate inall kinds of surrounding conditions. Dust and dirt,excessive moisture, flammable liquids, explosivegases, exposure to mice and other rodents, andother unfavorable factors often make the choiceof proper type of motor enclosure an importantone for safety and protection of the motor.

3. HOW CAN I IDENTIFY AND SELECT THE PROPER TYPEAND SIZE OF ELECTRIC MOTORS?

Parts of a motor

In learning to identify and select the propertype of motor, you should first become familiarwith the parts of a typical motor. All motors con-sist essentially of a rotating part called the rotorwhich revolves freely within a stationary partcalled the stator (Fig. 1).

Rotor. The rotor consists of a slotted core,made up of thin sections of a special soft steel,

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carefully balanced on a central shaft. This shafthas a ground bearing surface at each end of thecore and extends beyond the bearing surface atone or both ends to provide for pulleys or othermeans of attachment to the device it drives. Rotorsmay be of the squirrel-cage type (Fig. 2a) or of thewound-rotor type (Fig. 2b).

The squirrel-cage rotor got its name from thefact that it resembles the cage sometimes used to

A typical motor consists of a stator (a), and arotor (b). (Fig. 1)

a

b

The rotor may be of the squirrel-cage type (a), orwound-rotor type (b). (Fig. 2)

exercise pet squirrels (Fig. 3). The slots of therotor contain bare copper, brass, or aluminum barswhich are short-circuited together at each end bythe end rings. Most squirrel-cage fotors also havesome type of cooling fan and, in addition, thosefor single-phase motors have a centrifugal devicefor operating the starting-switch mechanism.

The single-phase wound rotor, such as thatfound in a repulsion-start induction motor, hascoils of insulated copper wire wound in the rotorslots. It also has a commutator made up of coppersegments which are insulated from the rotor shaft

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Note the similarity of the actual squirrel cage to thesquirrel-cage rotor. (Fig. 3)

and from each other with mica or a similar sub-stance. The ends of the rotor coils are solderedto individual commutator segments. When themotor brushes contact these segments, they com-plete the circuit, thus permitting currer's; to flowthrough all of the coils in the rotor in a propersequence for starting purposes. These x dors havea cooling fan and a centrifugal device for short-circuiting all the commutator segments when therotor comes up to speed. The wound rotor in Fig.4 also has a brush ring and a device for liftingthe brushes away from the commutator at thesame time the segments are short-circuited.

Some wound rotors are of the brush-lifting type.(Fig. 4)

Stator. The electrical part of the stator con-sists of a slotted core also made of special lamin-ated steel. Insulated copper wire is wound in theslots in such a way as to form one or more pairsof definite magnetic poles (Fig. 5).

For the so-called constant-speed motors, which

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The stator has coils of insulated wire wound to formmagnetic poles. (Fig. 5)

includes most of the types used on the farm, thespeed at which the motor runs is determined bythe frequency of the power supply and the numberof poles. With ordinary 60-cycle current, full-loadrunning speed of a 2-pole motor is about 3450r.p.m.; a 4-pole motor, 1725 r.p.m. ; and a 6-pole

motor, 1140 r.p.m.In addition to the rotor and stator, the motor

has a frame, end shields, and through bolts orcap screws (Fig. 6). The frame supports the entiremotor and provides for mounting. The end shieldshouse the bearings and usually one of the endshields contains the starting switch or the brushesand the terminal box where the attachment ismade to the line. The through bolts or cap screwshold the motor together.

Types of motors

There are three popular types of single-phase,alternating-current motors to choose from for

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farm and home applications. They are the split-phase, capacitor, and repulsion-start induction.Two more types will be discussed briefly. They arethe universal and the three-phase induction type.

The main difference between the first threetypes is in the way they start and come up to run-ning speed. For motors of the same horsepowerrating, there is no practical difference .betweenthese types in the amount of work they will do,nor in the current they will require in doing it,after they have come up to operating speed.

Split-phase. The typical split-phase motorconsists of a squirrel-cage rotor and a stator inwhich are found two different sets of windings(Fig. 7). One is called the main or running wind-ing, and the other the auxiliary or starting wind-ing. In general, the running winding consists ofa greater number of turns of larger diameter wirethan the starting winding and is usually woundin the stator slots first. Tho motor shown in Fig.7 has four distinct poles. The starting and run-ning windings have the same number of poles.The poles of the starting winding are spaced half-way between those of the running winding,

b

The split-phase stator has a running winding (a),and a starting winding (b). (Fig. 7)

At starting, the starting switch in the motoris closed and the current flows through both wind-ings. The rotor commences to turn and when itreaches about three-fourths full speed a centrifu-gal device opens the starting switch. This discon-nects the starting winding from the circuit andthe motor continues to operate on the runningwinding only. When the motor stops, the ttartingswitch again closes so that both windings in thecircuit will be ready for starting. This process ismore completely explained in Section 7.

The split-phase motor is the simplest in con-

struction and, consequently, the least expensiveto buy. However, it has a relatively low startingtorque, or ability to start a load, and requires ahigh starting current. This limits its use to loadsthat are easy to start, and, because of the largestarting current required, split-phase motors arerarely made in sizes larger than 173 horsepower.

Split-phase motors are usually made for onlyone voltage, either 120 or 240, and cannot bereadily changed from one to the other.

Direction of rotation is determined by the di-rection the current flows through the startingwinding with relation to the direction it flowsthrough the running winding. Switching starting-winding leads S, and S., or the running windingleads (Fig. 8) will cause the motor to start andrun in the opposite direction.

The split-phase motor is reversed by switching thestarting winding leads. (Fig. 8)

Direction of rotation of a motor is describedas clockwise (c.w.) or counterclockwise (c.c.w.)when viewing the motor facing the end oppositethe shaft extension. This is also usually the endwhere the motor lead connections are made. Youshould look at this eiid when describing thedirection of rotation for a double-shaft motor.

Capacitor. Two of the widely used types ofcapacitor motors are: capacitor-start ; and capaci-tor-start, capacitor-run (two-value capacitor).

The capacitor-start motor and the split-phase motor are similar in that both have squirrel-cage rotors and two separate windings in thestator, a starting and running winding. The capaci-tor-start motor has in addition a capacitor (con-denser) placed in series with the starting winding.The smaller sizes of capacitor-start motors canusually be identified by the tube-shaped containeron top of the motor which holds the capacitor(Fig. 9). Some motors, however, have the capaci-

The capacitor-start motor has a capacitor or con-denser, often mounted on top the motor. (Fig. 9)

for mounted inside one of the end shields, or inthe motor base.

A capacitor-start motor is, in reality, a greatlyimproved split-phase type. The improvements inthe starting circuit, made possible by the additionof the capacitor, give this type of motor a greaterstarting and accelerating torque for the samestarting current, usually at least twice as greatas a split-phase motor of the same horsepowerrating. Because of greater starting torque, capaci-tor-start motors are widely used for such ap-pliances as water pumps, and air compressorswhich start and stop frequently and have rela-tively high starting torque requirements. Recentdevelopments in capacitors make these motorspractical in sizes up to 5 or 71A horsepower, oreven larger.

Frequently in the larger horsepower ratings acapacitor-start, capacitor-run motor is used (Fig.10). The capacitor-start, capacitor-run motor is

7 1

The capacitor-start, capacitor-run motor is often usedin sizes from 2 to 10 h.p. (Fig. 10)

similar in external appearance to the capacitor-start motor. It has a starting winding and a min-ing winding in the stator with a capacitorconnected in series with the starting winding, anda squirrel cage rotor. However, the capacitor-start,capacitor-run motor differs from the capacitor-start motor in that the motor starting switch doesnot remove the starting winding from the circuitbut serves only to disconnect the starting capaci-tor when the motor comes up to speed.

Capacitor motors smaller than I/2 horsepowerare usually wound for 120 volts while manyrated 1/2 horsepower and larger can be connectedto either 120 or 240 volts t -7 changing the, leadwires. If this is possible, it will be indicated byboth voltages being shown on the nameplate.However, it is to your interest to use the 240-volt conection when 240 volt service is available.The direction of rotation is reversed in the sameway as the split-phase motor.

If in doubt as to whether the motor is a capaci-tor-start or a capacitor-start, capacitor-run motorthe information is generally supplied on the motornameplate or instruction tag.

Repulsion-start induction. The repulsion-startinduction motor is quite different from the twotypes previously described. It has only one wind-ing in the stator which acts as a running wind-ing. It has a wound rotor instead of the squirrel-cage type and therefore has a commutator andbrushes (Fig.11).

There is no direct connection between theline current and the brushes or rotor windings.The brushes merely serve to complete the circuitin certain rotor coils. This creates strong magneticforces within the rotor which react with thoseof the stator causing the motor to start. When itapproaches full running speed, a centrifugal de-vice within the rotor short-circuits all the commu-

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The repulsion-start induction motor has a woundrotor and only a running winding in the stator.

(Fig. 11)

tator bars together so that the rotor operates atfull speed like the squirrel-cage type.

Both capacitor and repulsion-start inductionmotors are designed to have the same high-start-ing torque. For this reason they can be usedinterchangeably in farm applications under nor-mal voltage conditions. Because of having greaterstarting torque per ampere of current, the repul-sion-start induction motor is less likely to ag-gravate a low-voltage condition or be troubled byvoltage drop. Both types of motors are ruggedlybuilt and give good service on the farm for steadyor intermittent use.

Most repulsion-start induction motors, even inthe fractional horsepower sizes, can be operatedon either 120- or 240-volt current. The stator wind-ing is usually divided into halves and four leadsare brought into the terminal box. These two hal-ves are connected in parallel for 120-volt and, inseries, for 240-volt operation (Fig. 12).

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Directions for connecting to low or high voltage areusually found on the motor. (Fig. 12)

Direction of rotation is determined by the posi-tion of the brushes with respect to the centers ofthe stator coils. Therefore, reversing is accom-plished by shifting the brushes to a different posi-tion. Some motors have a brush-shifting leverwhich extends outside the motor (Fig. 13a) . Withothers it may be necessary to remove a plate onthe end shield, and move an internal brush shiftingdevice (Fig. 13b and c).

Universal motors. The universal motor getsits name from the fact that it will operate on eitherdirect or alternating current of the correct volt-age. .It has a wound rotor and brushes somewhat

a b

To reverse a repulsior-start induction motor shift the brushposition by moving an external lever (a) or by removing a coverand operating an internal shifting device (b or c). (Fig. 13)

like the repulsion-start induction motor (Fig. 14).It is different, however, in several important re-spects. The universal motor is not an inductionmotor. Line current flows through the brushesto the rotor as well as to the stator in such a waythat the two windings are in series with eachother. Both are in the circuit constantly duringoperation.

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The universal motor also has a wound rotor andbrushes. (Fig. 14)

Universal motors do not operate at constantspeed like induction motors. They operate like agas engine with the throttle wide open; that is,they run as fast as their load will permit. Forthis reason, universal motors are usually perman-ently and directly connected to some device orappliance.

Universal motors have high starting torquesand high starting currents. They are widely usedon portable electric tools and on household ap-pliances such as vacuum sweepers, food mixers,and sewing machines.

Three-phase motors. Up to the present time,the availability of three-phase power on the farmhas been limited. This is because the costs in-volved in supplying such service have not beenjustified by the amount of power the averagefarmer was prepared to use. Three-phase electri-

city consists of three distinct currents which re-quire three or more primary (high-line) wires,two or three transformers at the farm, and threeor more secondary wires to the motor. The utiliza-tion voltage may be either 240 or 208 volts.

Three-phase motors are very simple in con-struction and hence relatively low in first cost.They have three phase windings in the stator andusually have a squirrel-cage type rotor (Fig. 15).

The three-phase current produces whatamounts to a rotating magnetic field in the statorin which the rotor will start and run without anyspecial starting device. Squirrel-cage three-phasemotors are notably free from trouble, having nobrushes, starting switch, or short-circuiting de-vice.

Direction of rotation of a three-phase motor isdetermined by the way the three line wires areconnected to the motor. Interchanging the con-nections of any two line leads will cause the motorto rotate in the opposite direction, so reversing is avery simple process. Three-phase motors cannot beused on a single phase line.

191

A three-phase motor has a winding made up ofthree parts in the stator, a squirrel cage rotor, andneeds no starting mechanism. (Fig. 15)

Types of enclosures (motor cases)

Up to this point we have considered selectingthe motor only with regard to basic type. Motorsshould also be chosen according to type of frameprotection or enclosure. The four main types ofenclosures are : open, splash-proof, totally-en-closed, and explosion-proof.

Open. Open motors are designed for use in-doors where the motor is kept dry and theatmosphere is normally clean. Openings for venti-lation are usually of drip-proof design to pre-vent objects or liquids from falling into the vitalparts of the motor (Fig. 16).

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Open ;motors have openings in the end shields topermit ventilaiton. (Fig. 16)

Splash-proof. Splash-proof motors may be

used indoors or sometimes outdoors in mild cli-

mates. This construction will protect the vitalparts of the motor even where it is necessary towash down the equipment with a hose. Splash-proof construction (Fig. 17) is seldom used formotors 3/4, horsepower and smaller.

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Splash-proof motors may be used around splashingliquids. (Fig. 17)

Totally-enclosed. this construction is de-

signed to protect the motor from dirt and grit inthe atmosphere as well as from moisture. Totally-

enclosed motors are recommended for farm use inextremely dirty conditions (Fig. 18). Since thesemotors do not have ventilating openings, an in-ternal and an external frame is sometimes pro-vided with a fan to carry the heat away from thesurface of the frame.

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Totally-enclosed motors have no ventilating openingsto allow dust or dirt to enter. (Fig. 18)

Explosion-proof. Essentially, there are twotypes of explosion-proof motors. Both are totallyenclosed. One is designed to withstand an explosionof gas or vapor inside it without igniting the gasor vapor surrounding the motor. This type iswidely used around gasoline and similar vapors(Fig 19). The other is a dust explosion-proofmotor which is designed and built so as not tocause ignition or explosion of a hazardous dustconcentration on or around the motor. This typeis used in such places as flour mills, feed mills, orgrain elevators or where grain, flour, or starchdusts may be present in hazardous quantities.

Explosion-proof motors are used where flammablegases or combustible dusts are present. (Fig. 19)

Types of bearings

Type of bearings may be another factor to con-sider in selecting a motor. In some sizes and typesof motors, there may be a choice between sleevebearings and ball bearings.

Sleeve bearings. Sleeve bearings are usuallysteel-backed, and babbitt-lined, although some aremade of bronze or a similar alloy. Sleeve-bearingmotors are usually oil-lubricated and are generallydesigned for operation only in a horizontal posi-tion.

Ball bearings. Ball bearings are widely usedon motors. Motors equipped with ball bearings maybe operated in a vertical position. Some ball-bearing motors are designed for relubrication atinfrequent intervals, while others are providedwith prelubricated sealed bearings without pro-vision for relubrication.

Determining the size motor to use

Determining the size of motor to use for agiven load is a frequent problem. The ability ofthe electric motor to withstand momentary over-loads was mentioned under the advantages ofmotors. It is a serious mistake, however, to sub-ject any motor to long and continuous overloadingfor its useful life will be shortened. It is alsoprobably true that many farm motors are toolarge for their loads, resulting in low efficiencyand increased operating cost. The equipment manu-facturer determines the proper size motor to usewith a given device by application of engineeringprinciples and extensive testing. The farmer cansecure valuable advice and help from his powersupplier or his motor dealer.

In some cases it is fairly simple to calculate thetheoretical horsepower required, as, for example,when the job to be done consists of lifting some-thing like hay, grain, or water. One horsepower isequal to 33,000 foot-pounds of work done in oneminute, 550 foot-pounds done in one second, orthe equivalent. A foot-pound of work is done whena weight of one pound is lifted one foot, two poundsone-half foot, or one-half pound two feet.

Suppose, for example, it is desired to hoist aquantity of baled hay weighing about 600 poundsat a rate of two feet per second. How many horse-power would be required ?

600 lb. x 2 ft. per sec. = 1,200 ft.lb. per sec.1,200

= 2.18 h.p.550

This calculation does not, however, take account offriction. To allow for friction, the extra force re-quired to pull the bales away from the load, and areasonable margin of reserve power, the next com-mon larger size, or in this case a 3 horsepowermotor should probably be used.

What size motor should be used on a pumpthat is to raise water 50 feet and deliver it at arate of 10 gallons per minute ?

Wt. of water = 8 lb. per gal.10 gal. = 80 lb.

80 lb. x 50 ft. = 4,000 ft. lb.4,000

== .12 or about 1/8 h.p.33,000

From these calculations we would probablyuse a 1/4 horsepower motor. This would take careof pumping into an open tank, but suppose we wishto pump the water into a pressure tank with thesame lift and at the same rate. What size motorwould be required ?

At sea level, 15 pounds of water pressure persquare inch is equal to about 34 feet of lift. If weassume the water is to be pumped to a pressureof 45 pounds per square inch, it would be equalto lifting water:

45x 34 = 102 ft.

15Total lift = 102 + 50 or 152 ft.

80 lb. x 152 ft. = 12,160 ft. lb.12,160

.37 or about JAI h.p.33,000

This again is the theoretical requirement and itwould be well to use at least a 1/2 horsepowermotor.

One way to determine whether a motor isbeing overloaded is to measure the current it con-sumes in doing its job. This can be done by placingan a.c. ammeter of proper capacity in series withthe circuit as in Fig. 20 when the motor is operat-ing the load in question and comparing the am-meter reading with the nameplate rating. Linevoltage should also be checked to see that it doesnot vary more than about 10 percent from therated voltage of the motor.

a b

An overloaded motor may be detected by measuring the current it uses with a regular a.c. ammeter (a), or a special"clip on" ammeter (b). (Fig. 20)

4. HOW SHOULD I INSTALL THE MOTOR PROPERLY?

Installing a motor properly involves problemsof connecting to the load, mounting the motor,determining proper wire size, protecting and con-trolling the motor, and providing for safety. Theinstallation should conform to provisions set forthin the National Electrical Code.

Connecting to the load

Belts and pulleys. In years past flat beltswere widely used for connecting engines andmotors to their driven loads. Today, V-belts havelargely replaced flat belts for such uses. Someadvantages of the V-belt drive over the flat beltare:

a. V-belts permit a lighter, more compact as-sembly. V-pulleys are narrower and will operatesatisfactorily at close centers without using idlersor belt tighteners.

b. The wedging action of V-belts provides agood grip between belt and pulley so that lessbelt tension is necessary to prevent slippage. Thisreduces bearing wear and increases belt life.

c. V-belts are easier to install and stay onbetter. Precise alignment needed by other drivesand provision for centering the belt on the pulleyare not required.

Standard V-belts are available in a variety oflengths and cross-sectional sizes. The cross-sectionsizes are designated by the letters A, B, C, D, andE. Types A and B will cover most of the applica-tions found on the farm for electric-motor drivesfrom the fractional sizes up to and including 71/2horsepower. Type B belts should not be used forpulleys smaller than 51/2 inches in diameter. Fig.21 gives the cross-section dimensions of Types Aand B belts.

[ 12 I

Type A

II/32"

11/16"II-'.:00110011111111111011111111;:1111111111111111: :: 7,I1

Most V-belts used with electric motors on the farmare Type A or Type B. (Fig. 21)

The number and type of V-belts to use for agiven drive depends chiefly on the size of themotor pulley and the speed and horsepower of themotor. Table 1 can be followed as a guide.

TABLE 1.NUMBER AND TYPE OF 'V -BELTSRECOMMENDED FOR 1750 R.P.M. MOTORS

Diameter ofmotor pulley Size of motor, horsepower

in. zA 4/4 1 11/2 2 3 5 7 542 1-A 2A ..21/2 1-A 1-A3 A 1-A 1-A 2-A 2-A 3-A 5-A 8A31/2 1-A 1-A 1A 2-A 2-A 3-A 4-A 7-A4 1-A 1 -A 1A 1-A 2-A 2-A 3A 5A41/2 1-A 1-A 1-A 1-A 1-A 2-A 3-A 5A5 1-A 1-A 1-A 1-A 1-A 2A 3A 4A51/2 1-A 1-A 1-A 1-A 1A 1 -B 2B 3B6 1A 1-A 1-A 1-A 1-A 1-B 215 2-B7 1A 1-A 1-A 1-A 1-A 1-B 2B 2-38 1-A 1A 1-A 1-A 1A 116 121 2-B

Pulleys less than 3 inches in diameter ehould not be used for motors1 h.p. and larger.

',Type A could be used instead of Type B.

Belt length can be determined with sufficientaccuracy for most installations by measuringaround the pulleys with a tape or string whilethe motor is mounted in place. Measurementshould be taken at the point of pitch diameter onthe pulleys. Pitch diameter is 8/8 inch less thanthe outside diameter if the pulley is made for andused with a type A belt, and 1/2 inch less if type

B. In planning the desirable distance to place themotor from the driven machine, use the followingrule. Distance between shaft centers should notbe more than three times the sum of the pulleydiameters, nor less than the diameter of thelarger pulley.

Most V-belt drives are of the type known asV-V drive. This type uses a grooved pulley onboth the motor and the driven machine (Fig. 22a).

Occasionally a drive known as V-flat is usedin which the motor has a V-pulley but the drivenmachine has a flat pulley, although a V-belt isused (Fig. 22b). This type of drive is quite satis-factory when the flat pulley is large so that thearea of contact supplies enough friction to preventslippage.

b

V-belt drives may be the V-V type (a), or V-flattype (b). (Fig. 22)

Sometimes the driving and driven pulleys can-not be arranged in parallel and must be placedat right angles to each other. This type of driveis known as a quarter-turn drive and is satis-factory if the speed ratio between pulleys is notgreater than 21/2 to 1 and the distance betweencenters is about 6 to 61/2 times the diameter of thelarger pulley.

On some machines a variable-speed drive isdesirable. This may be accomplished in two ways.Multiple-step pulleys may be used on the motor,on the driven machine, or both (Fig. 23a). If usedon both, the same belt can be used for all speedswithout changing the motor position. Pulleys canalso be used that have the space between thegroove walls adjustable. This allows the V-beltto ride higher or lower in the groove which ineffect changes the pitch diameter of the pulley(Fig. 23b).

Pulley combination. In selecting the properpulley combination, it is necessary to know thespeed (r.p.m.) of the motor and the speed (r.p.m.)required for the driven device. The speed of themotor may be obtained from the nameplate. Indetermining proper pulley sizes, the following

Different speeds may be obtained by using multiple-step pulleys (a), or adjustable-groove pulleys (b).

(Fig. 23)

equation can be used :Diameter of motor pulley x r.p.m. of motorDiameter of driven pulley x r.p.m. of drivenmachineFor example, what size pulley should be used

on a corn sheller to drive it at 210 r.p.m. usinga motor operating at 1750 r.p.m. with a 3-inchdiameter pulley ?

3 x 1750 = P x 2105250 = 210 P

25 = P, or a 25 inch pulley is requiredFor quick calculations, the outside diameter

may be used to figure pulley sizes. Where accuratedeterminations are required, the pitch diameterof the V-pulleys should be used as described pre-viously.

Direct drive. When both the driven machineand the motor have similar operating speeds andmounting conditions are satisfactory, it may bedesirable to make a direct, end-to-end shaft coupling between the motor and the load (Fig. 24),Direct connections are often used in driving rotarypumps, blowers, fans, and numerous other ma-chines. Direct drive requires very careful align-ment and the use of a coupling device that is atleast partly flexible. If properly mounted, direct-

[ 131

Many devices are connected to motors by directdrive. (Fig. 24)

jOn floor]

driven machines result in a minimum of wear onmotor and shaft bearings.Mounting the motor

Sleeve-bearing motors, unless specially de-signed, should never be used in any position other

mwolormr..;

MU

On wall

From ceilinI I I

Motors may be mounted on the floor, wall, or ceilingby rotating the end shields to keep oil holes upright.

(Fig. 25)

[ 14 ]

than horizontal, that is, with the shaft level.However, most motors can be mounted on thefloor, on a side wall, or on the ceiling, by rotatingthe end shields to keep the oil holes and reser-voirs in an upright position (Fig. 25). Ball-bearingmotors can usually be mounted in any position,including vertical.

Motors should be mounted with some provisionfor tightening and loosening the belt. Only areasonable tension is necessary with a V-belt drive.When in operation, the tight side of the beltshould form a straight line from pulley to pulleywhile there should be a slight sag in the slackside. Running belts tighter than necessary to pre-vent slippage causes extra wear on the belts andmotor bearings. Belts should never be forced overpulleys. More belts are broken from this causethan from actual failure in service. It pays toloosen the motor mounting so that belts can beslipped on easily. Care should be taken not to drawa motor down tight on an irregular surface whichwill tend to place a strain on it and throw thebearings out of line.

In place of a fixed, permanent mounting, somefarm-motor jobs are of a nature that makes theuse of portable motors practical. Machines thatare used only seasonally or at infrequent intervalscan be operated with a portable motor, resultingin a considerable saving in investment.

A portable 1/4, to 1/2 horsepower motor is rela-tively inexpensive and can be used almost any-where without special wiring as it can be pluggedinto a regular120 -volt outlet. Equipment neededto make a small motor portable is shown in Fig.26. No. 10 insulated wire can be twisted togetherto make a carrying handle. Short pieces of pipemay be used to make the motor rails and themotor is equipped with a hard service cord ofample size and a multiple-step V-pulley.

C

d

b

Materials needed to make a small motor portable are:(a) No. 10 insulated wire for carrying handles.(b) Pieces of 1/2-inch pipe and bolts for motor rails.(c) Hard service cord.(d) Multiple-step V-pulley (Fig. 26)

Fig. 27 shows how a small portable motor maybe attached to its load, using the weight of themotor to provide proper belt tension.

Tie portable motor may be held in position withpipe straps (a), (Fig. 27)

Determining proper wire size

Using a circuit conductor (wire) that is toosmall is a serious mistake often made in connectingfarm motors. When the circuit conductors (wires)are too small, the voltage at the motor terminalsis lower than that for which the motor was de-signed. Since the power produced by a motor is aresult of both voltage and current, a voltage dropcauses an increase in the current required. Thishigher current causes an increase in the heatingeffect of the motor. Since the increase in heatingeffect is not in direct proportion to the increase incurrent, but according to its square, doubling thecurrent increases the heating effect four times.This explains why low voltage causes a motor tooverheat and often burn out.

Table 2 shows the minimum size of wire touse in connecting motors according to size and

distance from the center of distribution on thefarm.

Protecting and controlling the motor

A proper electric-motor installation includesmeans for controlling (starting and stopping) itas well as provision for protecting the motor, thewiring, and other equipment from damage due tooverloads or shut circuits. Rules for the safe useof control and protective devices are contained inthe National Electrical Code, which is the safetystandard commonly followed throughout theUnited States. The following diagram (Fig. 28)is a simplified version of one appearing in theCode and applies to circuits for farm motors notlarger than 71/2 horsepower.

The four units, designated a, b, c, and d arenecessary in all properly installed motor circuits.Each will be discussed separately, although inactual practice, two or more are often combined.

Motor branch-circuit overcurrentprotection (Fig. 28a)

Fuses or circuit breakers are installed to pro-tect the entire motor branch-circuit against exces-sive current due to short-circuits or grounds. Thisbranch circuit includes the motor, the controlapparatus, and the wires supplying power to themotor. The maximum limits for these fuses orcircuit breakers as prescribed in the Code areabout 250 to 300 percent of the full-load currentof the motor. This high a rating is permitted be-cause motors draw considerably more than theirrated current during the starting period. Theamount of current required and the length of timeneeded for the motor to reach full speed dependon the type of motor and the character of theparticular load to which it is connected.

TABLE 2.WIRE SIZES FOR INDIVIDUAL SINGLE-PHASE MOTORSBased on 2 Percent Voltage Drop on Full-Load Current

MotorHorsepower

..........

1

35 ....10

VoltsApproximate

Full-LoadCurrant,Amperes

Length of Run in Feet (One Way)

50 75 100 150 200 250 300 350 400 500 600 700

115 4.4 14 14 12 12 10 8 8 8 6 6 6 4

115 5.8 14 12 12 10 8 8 6 0 6 4 4 4

115 7.2 14 12 10 10 8 6 6 6 4 4 4 2

115 9.8 12 10 10 8 0 6 4 4 4 2 2 2

115 13.8 10 10 8 6 4 4 4 2 2 2 1 0

115 16.0 10 8 8 6 4 4 2 2 2 1 0 00

230 2.9 14 14 14 14 14 14 12 12 12 10 10 10

230 3.6 14 14 14 14 14 12 12 12 10 10 10 8

230 4.9 14 14 14 14 12 12 10 10 10 8 8 8

230 6.9 14 14 14 12 10 10 10 8 8 8 6 0

230 8.0 14 14 14 12 10 10 8 s s 0 0 4

230 10.0 14 14 12 10 10 8 8 6 6 6 4 4

230230

230

12.017.028.0

141010'

121010'

12108

1080

884

804

664

642

042

441

421

420

230 40.0 10" 8' 0 4 4 2 2 1 1 0 00 000

230 50.0 6' 6' CV 4 2 2 1 1 0 00 000 000

For wires in cable or conduit, use next size larger." For wires in cable or conduit, use two sizes larger.NOTE: For exterior wiring, overhead conductors shall not be smaller than No. 10 for spans up to 50 feet in length, and not smaller than No. 8 for longer spans.

(National Electrical Code, Art. 730-6)15

Feederfrom supply

a. Motor Branch-CircuitOvercurrent Protection

Motor b. Disconnecting MeansBranch-CircuitConductors

c. Motor Controller

d. Motor-running Over-current Device

Motor

A properly installed motor requires the above unitsto control and protect it. (Fig. 28)

Disconnecting means (Fig. 28b)

To permit motors to be serviced safely, theremust be provision for disconnecting the motor andcontroller from the power supply. According to theCode, this disconnecting means must open all un-grounded wires, be readily accessible, and plainlyindicate whether it is "on" or "off." Unless insight of and within 50 feet of the controller, thedisconnecting means must be made so it can belocked in the "off" position.

The Code requires motors rated over two horse-power to have, for disconnecting purposes, eithera switch of correct horsepower rating made speci-fically for motor-circuit use or a circuit breaker ofproper rating. For two-horsepower or smallermotors, the switch need not be rated in horse-power. providing its ampere rating is at leasttwice the full-load current of the motor. For port-able motors the attachment plug may serve as thedisconnecting means. For stationary motors I/8horsepower or less, the branch-circuit protectivedevice may serve as means for disconnecting.Motor controller (Fig. 28c)

To make the motor perform the desired taskat the right time, some form of controller mustbe provided. This means a device which will startand stop it, and perhaps control its speed and di-rection of rotation.

16

Common forms of controllers are manual andmagnetic. Manual controllers have a handle forhand operation. The magnetic type operates by anelectromagnet which is generally controlled bya push button at the controller or at some conven-ient remote location. For example, in a corn cribit is often desirable to mount the motor at thetop of the elevator and place the remote-controlbutton down in the driveway where grain is un-loaded. Frequently it is desirable when the buildingis wired, to make the entrance- into the cupola. Thisrequires only a few feet of heavy wiring insidethe building since the wire down to the controlbutton does not carry the motor load and cannormally be as small as No. 14.

The push button may be replaced by an auto-matic device such as a thermostat, a pressure-actu-ated switch, or a tank-float switch. Some of theseautomatic devices are large enough to operate asmall motor directly and when so used are in them-selves "controllers."

The Code requires controllers to be markedwith the voltage and current or horsepower ratingso that they can be selected accordingly. An ap-proved controller of proper horsepower ratingshall be used except as permitted in the followingcases : (1) for a stationary motor rated two horse-power or less, a general-use switch having an am-pere rating of at least twice the full-load ratingof the motor may be used; (2) for a portablemotor of lAt horsepower or less, the controller maybe an attachment plug; or (3) a circuit breaker,rated in amperes only may be used in the branchcircuit as a controller.

It is required for safety that the controlleror a manually-operable switch which will preventthe starting of the motor be in sight of and within50 feet of the motor unless the controller or dis-connecting means can be locked in the "off" posi-tion.

Motor-running overcurrent device (Fig. 28d)

Since a motor draws less current running thanstarting, a "motor-running overcurrent device,"sensitive to currents in excess of the normal run-ning current of the motor, is necessary to preventdamage to the motor and the motor circuit, if themotor becomes overloaded while running. Motoroverloads may be caused by using a motor thatis too small for the machine it drives, lack of lub-rication, too tight a belt, or a number of otherabnormal conditions.

There are several different types of motor-running overcurrent devices which are recognizedby the Code. Continuous-duty motors* of morethan one horsepower may be controlled by a

separate or integral (built-in) overcurrent device.A separate device shall be rated or set at not morethan 125 percent of the motor full-load currentrating for a motor marled to have a temperaturerise not over 40°C, and at not more than 115 per-cent for all other types of motors. If this limitdoes not correspond with a standard rating, thenext larger standard size may generally be used.An integral protective device shall be of the ratingapproved for a specific motor, which will be markedto indicate this fact.

Motors of one horsepower or less may be pro-tected the same as larger motors: However, if themotor is manually operated and within sight ofthe operator, it may be considered protected bythe motor branch-circuit overcurrent protection(Fig. 28a) under most conditions.

An automatically-started motor, rated onehorsepower or less, may be installed without speci-fic running overcurrent protection if it is part ofan approved assembly equipped with other safetycontrols, such as the safety combustion controls ofa domestic oil burner. If the assembly containssuch protective equipment, it is indicated on thenameplate.

Motor-running overcurrent devices other thanfuses are required to have a rating of at least115 percent of the full-load current rating of themotor.

Combination devices

Under the preceding four headings, a briefdescription of the application of the four elements(shown as a, b, c, and d in the diagram) has beengiven. They are always required for the control

* Any motor is considered to be for continuous duty unlessthe nature of the apparatus it drives is such that the motorcannot operate continuously with load under any conditionof use.

and protection of motors. As has already beenmentioned, however, some or all of these functionsare frequently combined.

One such combination is for the disconnectingmeans and the controller to be in the same en-closure. The motor branch-circuit protectiN e deviceand the motor running overcurrent protective de-vice may also be contained in the same enclosure.To go a step farther, a switch or circuit breakermay serve as both controller and disconnectingmeans if it meets the requirements for both typesof use. Similarly, the motor branch-circuit over-current protection and motor running overcurrentprotection may be combined in a single overcurrentdevice if its rating or setting provides the specifiedrunning overcurrent protection. A portable motorof 1/3 horsepower or less may have a single attach-ment plug and receptacle to serve as both a discon-necting means and controller.

Providing for safety

Motor installations should be made as safefrom mechanical and electrical hazards as possible.Guards should cover moving parts such as pulleysand belts, as is recommended with any machine.Mention has been made of the importance of thetype of motor enclosure used around flammableor explosive materials.

Motors are electrically safe when they are in-stalled in accordance with the Code. Particularattention should be given to the Code provisionson grounding. Many farm motors operate in wetlocations that would be rated as hazardous fromthe standpoint of electric shock. Proper groundingnot only removes the danger of fatal shock,should motor insulation fail and the frame be-come charged, but prevents a more common ac-cidentthat of a person jumping or falling intoa moving machine as a result of a light shock.

5. WHAT CARE SHOULD I GIVE AN ELECTRIC MOTOR?

The electric motor will give years of trouble-free service with a very minimum of care andmaintenance. Normal care consists of cleaning,lubricating, storing, and caring for brushes andcommutator (of motors so equipped).Cleaning

Cleanliness is an important factor in the lifeand operation of an electric motor. A majority ofgeneral-purpose motors have openings in themfor ventilation and these allow dirt and foreignmatter to enter the motor. Under most conditionsa motor will operate for a long period of time

[17)

without requiring a thorough cleaning. However,in some places dirt may accumulate in the motorto such an extent that difficulties will occur. Inmost cases, general-purpose single-phase motorshave either a starting switch or brushes whichoperate only while the motor is starting. Dirt orcorrosion on these parts may make the motor failto start, or may cause overheating. Even if themotor does operate, excessive dirt will cause mov-ing parts to wear rapidly. A periodic inspectionshould be made to determine if the motor requirescleaning.

If it becomes necessary to disassemble andthoroughly clean an electric motor, the outsideshould be wiped off first to remove all dirt andgrease. Considerable damage may be done byimproper disassembling of the motor. Extremecare should be taken if thir, is required. Beforetaking the motor apart liiark the exact positionof the end shields on the motor frame with a sharpcenter punch or file (Fig. 29) . This will permitreassembling the motor just as it was for truebearing alignment.

Mark position of end shields with a sharp centerpr nch or file before disassembling. (Fig. 29)

Next, remove the nuts and through bolts orcap screws which hold the end shields in placeand carefully remove the rotor with its end shield.If the motor has brushes, it is often advisable toremove them first to avoid breaking them whenremoving the rotor. The end shield opposite theshaft extension usually has the motor lead wiresattached to it and one mIlst be careful to avoidtearing them loose from the motor windings.Special pains must be taken if the motor has ballbearings as bearings and races are often difficultto remove.

If available, use compressed air at low pressureor a vacuum cleaner to remove dust and loosedirt from inside the motor. A soft brush may also'De used to clean out loose dirt. To remove greaseand oil, apply safe cleaning solvent with a smallpaint brush and wipe clean with a cloth. Avoidusing excessive amounts of cleaning fluid directlyon the windings as the insulation may be damaged.

If the motor has sleeve bearings, be sure toremove the yarn or oil wick and wash out the oilwell. It is advisable to replace the yarn or oil wickif new is available. Some types of ball-bearingmotors are so constructed that the bearings can becleaned and relubricated while the motor is apart.If the motor has sealed ball bearings, do not allowany of the cleaning fluid to enter the bearings.

[ 18

After all parts of the motor have been thor-oughly cleaned, place them on a clean surfaceand wipe dry with a clean cloth. If much cleaningfluid has been used, it is well to use an electricheater, a heat lamp, or a large light bulb tofurther dry out the windings.

When clean and dry, reassemble the motorcarefully. Be sure that the motor leads are pulledout of the way of the rotor fan or other movingparts which might catch and tear them loose whenthe motor starts. Tighten the through bolts or capscrews gradually and evenly, being sure that theend shields fit tightly all the way around and thatthe motor shaft finally turns freely.

Lubricating

Proper lubrication is a very important step inelectric-motor maintenance. It means the use ofthe right lubricant, in the right amount, and atthe right time intervals. Overlubrication is justas serious as underlubrication. The correct amountof lubricant will remain in the bearings to reducefriction heat awl wear. Excess oil or grease willspread to other parts of the motor, cause the motorto plug with dirt, and eventually cause the insula-tion to break down. Manufacturers' directionsshould be followed closely in lubricating motors.

For sleeve-bearing motors in general, use agood grade of SAE 10 or 20 oil. Lighter or heavieroil may be used if temperatures are extremelylow or high. There is a wide variation in the oilstorage capacity of motors, as found in the threecommon types of oiling systems used with sleeve-bearing motors.

One type uses an oil well below the bearingwith a wick to carry the oil up to the shaft (Fig.30). Twice a year, or so, the oil well should beunscrewed, the old oil cleaned out and the wellrefilled about two-thirds full with new oil.

Some small motors have oilwick lubricated bearings.(Fig. 30)

Another system uses a yarn-packed bearing towhich a few drops of oil should be added every fewmonths. If there is a drain plug at the bottom,accumulated oil can be drained off occasionally(Fig. 31)

yarnBearing

Shaft

01Drain plug

Oil return

Overflow

Many small motors have yarn-packed bearings.(Fig. 31)

A third type has a ring-oiled bearing. Oil iscarried from an oil reservoir below the bearingonto the shaft by a loose ring that turns as themotor runs (Fig. 32). With this type it is neces-sary to keep the oil level up to the filler hole bychecking periodically. Every two or three years itis well to drain off the old oil, flush out the re-servoir, and add new oil.

Bearing

U

I I/4111/ 'MP"

2

\r Shaft

Oil ring

_. -Drain plug

Larger sleeve-bearing motors often have ring-oiledbearings. (Fig. 32)

Lubrication is far less critical with the ball-bearing than the sleeve-bearing motor. Ball bear-ings carry the load by direct contact while sleevebearings carry the load on an oil film. A bail bear-ing could be operated dry if it were not for dirt,corrosion, friction heat, and other adverse factors.However, since such conditions are always present,lubrication is necessary.

The type of ball bearing which is prelubricatedand sealed by the manufacturer should not bedisturbed. The other type of ball bearing can be

relubricated, either by disassembling the motor,or through lubrication openings. Disassembledbearings should be wiped clean of old grease witha soft cloth and repacked half to two-thirds fullof the type of electric-motor ball-bearing greaserecommended by the motor manufacturer.

If the bearing has lubrication openings, removeboth the filler and drain plugs. If the old greaseis hard, run the motor to warm it up and addlight oil until the grease softens and runs out.Stop the motor and add new grease until the re-mainder of the old has been forced out or the newgrease starts to appear at the drain opening (Fig.33) .Then operate the motor again with both holesopen and let the motor force out excess grease.Finally, before the plugs are replaced, remove anadditional quantity of grease with a rod or wireto allow for expansion when the motor is operating.

StoringElectric motors should be stored in a dry place

and kept free from dirt. The following steps arerecommended for preparing a motor for storage :

a. Wipe the outside of the motor with a clothto remove dirt and grease.

191

Some ball-bearing motors are relubricateci with agrease gun containing special ball-bearing grease.

(Fig. 33)

b. Check bearings for lubrication and addfresh oil or grease if required.

c. Cover the shaft extension with a coating ofgrease to prevent rusting.

d. Wrap the motor with heavy paper to keepdust and dirt from accumulating in it.

Caring for commutator and brushes

Proper care of commutator and brushes isimportant to obtain satisfactory service and longlife from commutator-type motors. Sluggish start-ing and excess sparking at the brushes suggeststhat trouble is developing in these parts.

Badly worn brushes should be replaced withnew ones. It is important to secure the properbrushes from the manufacturer or motor-serviceman who can secure them if he has all the infor-mation given on the motor nameplate. Newbrushes should be fitted to the contour of thecommutator, if it is of the axial type. This canbe done by wrapping fine sandpaper around thecommutator and placing the new brush in itsholder. Then by holding the brush against thecommutator with one hand and turning the rotor

back and forth with the other, the brush can beground to the proper contour. Examining thebrushes after the motor has been run for a timewill tell if the fitting has been properly done, asa well-seated brush will appear shiny all over thecontact surface.

Sometimes brushes will fail to make positivecontact with the commutator because of stickingdue to a gummy accumulation of oil and dirt inthe holders, or because of weak or broken springs.The obvious remedy for these conditions is clean-ing or replacing the springs.

A dirty or worn commutator will also causetrouble. With some types of motors, it is possibleto clean the commutator without taking the motorapart, using a clean, lint-free cloth or fine sand-paper. Emery cloth should never be used sinceemery dust is a conductor of electricity and maycause short circuits. If the commutator is rough,pitted, or worn, the motor should be taken to amotor repairman who can resurface the commuta-tor and undercut the mica insulation between thecommutator bars with special equipment for thepurpose.

6. HOW CAN I DETERMINE WHAT IS WRONG WHEN A MOTORWILL NOT OPERATE?

Trouble shooting, or diagnosing motor trouble,is the first step toward getting the machine backinto operation. The most successful trouble shoot-ers are those who look for and eliminate thesimplest difficulties first. Sudden failure of a motorto operate may be due to failure of power supply,excessive load, frozen or worn motor bearings,operation of built-in thermal protection, failure ofmotor-starting mechanism, or failure of motorwindings.

Failure of power supply. This is the firstthing to check. Use a test lamp or voltmeter tomake sure that proper voltage is available rightup to the motor terminals. Be sure to check motorcontrol and protective devices.

Excessive load. Check for excessive load con-ditions in the driven device by removing the belt(if belt driven) and attempting to turn the deviceby hand. If the driven machine is at fault, themotor will run normally with the belt off.

Frozen or worn motor bearings. If the motorwill not run idle, shut off the current and tryturning the shaft by hand. If it does not turnfreely, the trouble may be a dry or worn bearing.Lubrication may remedy a dry bearing but it isoften necessary to take the motor apart to free abearing that has stuck. If the motor shaft has any

[20J

noticeable up and down play, bearings may beworn to the extent that the rotor is dragging, par-ticularly when belt tension is applied. Do not con-fuse this with end play, a slight amount of whichis necessary and desirable. The remedy for wornbearings is replacement, a job for the motor ser-viceman.

Operation of built-in thermal protection. Mo-

tors having this protection usually carry a state-ment so indicating on the nameplate or elsewhereon the motor. If the motor has a reset button,press this to see if it starts. If it is of the auto-matic reset type, wait until the motor cools, andthen turn it on to see if it will run. Like a blownfuse, a tripped thermal protector usually indicatestrouble somewhere.

Failure of motor-starting mechanism. Amotor with this type of trouble will hum when itis turned on and, if the rotor is given a spin byhand, will usually run normally. If the motor is asplit-phase or capacitor-start type, the startingcircuit is usually open at some place. It may be fail-ure of the starting switch to make contact dueto one of a number of causes. The contacts maybe dirty, burned, or pitted. A wire may be burnedin two somewhere. The centrifugal device mayhave failed to close the switch, perhaps due to

too much end play in the rotor shaft. With acapacitor-start motor, the capacitor may be burnedout,

Failure of motor windings. A burned-outwinding, will usually result in a characteristicodor coming from the motor. Charred insulationcan often be seen through the openings in anopen-type motor. An exception to this is when asingle strand of wire quickly burns in two. This

7. WHAT ARE THE IMPORTANT

It may be of interest to take a brief look atsome of the fundamental principles of electricityand magnetism which are involved in the operationof electric motors. We shall consider what makesan induction motor run, and the methods used instarting the split-phase, capacitor, and repulsion-start induction motors.

What makes an induction motor run?

To help understand how an induction motorruns, let us first consider a few of the principlesof electricity and magnetism as they apply tomotors. Permanent magnets made of steel oralloys, such as alnico, will retain their magnetismfor a long period of time. A typical magnet has anorth (N) pole and a south (S) pole, and aninvisible field of magnetic force with lines proceed-ing from the N pole to the S. This magnetic fieldis often demonstrated by placing the magnet undera piece of paper or glass and sprinkling iron filingsover the top. Upon tapping the paper or glass, thefilings are attracted by the lines of magnetic forceand form a definite pattern (Fig. 34).

The magnetic field may be demonstrated with ironfilings sprinkled on a piece of glass over a permanentmagnet. (Fig. 34)

occurs most often in the starting winding, in whichcase the motor shows the characteristics describedunder "failure of motor-starting mechanism."

It is well in discussing motor troubles to pointout the hazards of "tinkering." The operator ofelectric motors should realize that they are deli-cate mechanisms and that he should not go beyondthe limits of his knowledge, skill, and experiencein attempting to service and repair them.

PRINCIPLES OF ELECTRIC MOTORS?

iron nail, the nail assumes all the properties of amagnet. It has a magnetic field of its own and willattract iron filings. It is interesting to note thatthe nail does not have to actually touch the magnetto assume these magnetic qualities. It becomesmagnetized when it is placed within the magneticfield of the permanent magnet. We thus magnetizethe nail by induction rather than by direct contact.

Any wire that has an electric current flowingthrough it is surrounded by a magnetic field,though under ordinary circumstances this field isweak and unimportant. If we wind a coil of insu-later wire around a soft-iron core as in Fig'. 35and pass a current through the coil, we producea magnet called an electromagnet. It may be con-siderably stronger than the permanent magnetdescribed above. The strength of an electromagnetis determined chiefly by the number of turns ofwire in the coil and the amount of current flowingthrough it.

An electromagnet is formed by passing a currentthrough a coil of insulated wire surrounding a soft-iron core. (Fig. 35)

There are two other important ways in whichan electromagnet differs from a permanent mag-net : (1) It is only a temporary magnet, that is,it has magnetism only while current is flowingthrough the coil, and (2) the poles of the electro-magnet change when the direction of current flowin the coil is reversed (Fig. 36 ).

Current flow

N

Current flow

S N

Reversing the direction of current flow in the electro-If we touch one end of the magnet to a soft- magnet coil reverses its magnetic polarity. (Fig, 36)

[ 21 ]

If we experiment with two magnets, we soonobserve another fundamental law of magnetismthat unlike magnetic poles attract and like polesrepel each other. If we suspend a bar magnetwith a string so that it is free to turn and bringanother magnet up to it, we get the reactionshown in Fig. 37.

Like magnetic poles repel each other; unlike polesattract. (Fig. 37)

If a copper wire, or other conductor forming aclosed circuit is moved through the magnetic fieldof either a permanent magnet or an electromagnetin such a way that the wire cuts across lines ofmagnetic force, an electric current will be causedto flow in the wire. This is a basic principle usedin the magneto and generator. The amount of cur-rent produced in this way is chiefly determined bythe strength of the magnetic field, the number ofturns of wire, and the speed at which the wirecuts the lines of force. This process may also bereversed. Instead of the coil of wire movingthrough the magnetic field, the lines of force maybe caused to move through a stationary coil ofwire, and a flow of electric current will result asbefore.

Let us imagine a coil of wire placed betweenthe poles of a U-shaped magnet as in Fig. 38. Whenthe current commences to flow through the electro-

Currentflow

starts

Currentflowstops

I

MO f #1 kMI

4911/f

,e 4%...;Field building up

tIT #

161..e.e........-4# 4or

.....

N'f

Field collapsing

Lines of magnetic force cut through the coil of wirewhen the current flow starts and stops in the electro-magnet coil. (Fig. 38)

magnet winding, a magnetic field is produced, thatis, lines of magnetic force move into position, cut-ting across the coils of wire. When the current flowstops, the magnetic field disappears or collapsesand the lines of force again cut across the coil. Nocurrent flows in the coil except at the very instantthat lines of force are cutting the coil, or whenthe current starts and stops flowing through theelectromagnet.

If the ordinary 60-cycle, alternating current isconnected to the electromagnet winding, the mag-netic field will build up and collapse once everyalternation or 120 times per second. As a result asimilar 60-cycle, induced alternating current willflow in the coil of wire placed in this magnetic field.We call this process induction because, again,there is no direct contact between the two coils ofwire.

As explained previously, the typical inductionmotor consists of a stationary part called a statorand a rotating part called a rotor (see pages 4-6).The stator has a slotted core made up of thinsections of soft iron or steel. Insulated copper wireis wound in these slots to form an electromagnetwith two or more poles. We will consider, forsimplicity, a stator with two poles as illustrated inFig. 39. If we connect a 60-cycle alternating cur-

Each time the current alternates, the magnetic poles of the suitor change.

[221

(Fig. 39)

A permanent magnet would rotate in this alternating field if started.

rent to the two leads from this winding, the statorbecomes an electromagnet whose magnetic poleswill reverse every time the current alternates, or120 times per second.

If we mount a permanent bar magnet on apivot in the center of this magnetic field (Fig. 40.)and give it a spin as we turn on the 60-cycle alter-nating current to the stator winding, the barmagnet or rotor, will continue to run because of theattraction and repulsion of the alternating polesof the stator.

At the instant that pole A is a north pole andB is a south, the S pole of the rotor will be at-tracted by A and repelled by B. Likewise the Npole will be attracted by B and repelled by A. How-ever, before the rotor can come to rest in linewith poles A and B, the current alternates and thestator poles reverse. Momentum will carry therotor past center and then A will attract N andrepel S while B will attract S and repel N. Thus therotor continues to rotate and would theoreticallyadjust itself to a speed of 60 revolutions per secondor 3600 revolutions per minute. This is knoWn assynchronous speed.

Now we shall substitute a more practical typeof rotor for the bar magnet, using a squirrel-cage

(Fig. 40)

type rotor (Fig. 41) . It has a slotted core made upof thin sections of the same soft iron or steel asfound in the stator core. In its slots are bars ofbare copper or some similar good conductor whichare short-circuited together at each end of therotor.

If we give this type of rotor a spin and turnon the current to the stator coils, currents areinduced into the copper bars of the rotor as theycut the lines of magnetic force. These currentsand the magnetic poles which they create in therotor react with the magnetic field of the statorto make the motor keep on turning.

The actual running speed of this motor wouldbe somewhat less than the synchronous speed of3600 r.p.m. because the current is induced intothe rotor only when its copper bars cut the linesof force in the field of the stator. This occurs onlywhen the rotor turns somewhat slower than thespeed, of the alternations. This difference in speedis known as slip. The greater the slip the morethe lines of force that are cut; and the strongerthe induced current in the rotor becomes. Thiscauses the rotor to pick up speed but the fasterit turns the fewer the lines of force that are cutand the weaker the current and magnetism in the

A squirrel-cage rotor may be substituted for the permanent magnet.

1231

(Fig. 41)

rotor becomes. It then slows down slightly. Thusthe actual running speed becomes a balance be-tween these two tendencies and will usually run4 to 5 percent below synchronous speed, or about3450 r.p.m.

So far we have been giving the motor a spinto start it. This is because we are dealing with asingle current, called a single-phase current, whichdoes not produce a natural starting torque (twist)in an induction motor. The magnetic action in thestator is a back-and-forth attraction and repulsion,or push and pull, which will keep a motor runningbut will not start it. It is like swinging a weightattached to a string around in a circle. After youget it started a simple back-and-forth motion ofthe hand will keep it whirling, but no amount ofback-and-forth motion will start it swinging in acircle. Starting requires a circular moving force.

Split-phase motor

There are several ways to make a single-phasemotor self starting. One is to use the split-phasestarting principle. To do this another winding isadded to the stator. It is made of smaller size wireand fewer turns than the running winding (Fig.42). A and B represent the poles of the mainwinding or running winding and C and D are

A special winding is added to make use of the split-phase principle in starting the motor. (Fig. 42)

the poles of the auxiliary or starting winding.These two windings are connected in parallel tothe line so that the current enters both windingsat the same time. However, because of the differ-ence in size of wire and number of turns, the cur-rent and magnetic effect reaches a peak in thestarting winding slightly before its peak in themain winding. Although both D and A will benorth magnetic poles, the rotor is attracted towardD first, and then, an instant later, toward A, justas though two different currents or phases werepresent in the motor. For starting purposes, thesetwo windings split the single-phase current intotwo phaseshence the name "split-phase motor."

One more feature is needed to improve oursplit-phase motor. Although the starting windingis necessary to make the motor self starting, wehave previously seen that it is not needed to keepthe motor running. In fact, the starting windingwould hinder the running of the motor. Thus, acut-out switch is added in the starting-windingcircuit (Fig. 43). In an actual motor this switchis usually operated by a centrifugal device whichis designed to cut out the starting winding whenthe motor reaches about 75 percent of full run-ning speed. It closes the starting switch againwhen the motor stops to prepare it for the nextstart.

A centrifugal switch cuts out the starting windingwhen the motor reaches the proper speed. (Fig. 43)

Capacitor-start motor

The capacitor-start motor is an improvementover the split-phase motor described above. Thefirst difference noted is that it has a capacitor(condenser) in series with the starting winding.

The common type of motor capacitor consistsof two sheets of aluminum foil separated by alayer of paper or gauze which is impregnated witha liquid such as ethylene glycol. The two sheets ofaluminum foil separated by the insulating layerare rolled up and encased in a metal tube. Eachterminal is connected to one of the sheets ofaluminum foil (Fig. 44).

A capacitor acts as a reservoir in which anelectrical charge can be stored and dischargedback again into the circuit. The capacitor doesseveral things to improve the starting perform-ance of the motor, compared to the simple split-phase type. It has the effect of "splitting" thesingle-phase current wider by creating a greatertime interval between the peak of current andmagnetism in the starting winding as comparedto the running winding. It also permits the use ofmore copper in the starting winding and other im-portant differences in basic design. These thingsresult in a much greater starting torque and a

1 241

A capacitor consists of two conducting surfacesseparated by a nonconductor. (Fig. 44)

lower starting-current requirement. Fig. 45 showshow the capacitor is connected in the circuit of atypical capacitor-start motor.

In a capacitor-start motor the capacitor is connectedin series with the starting winding. (Fig. 45)

Capacitor-start, capacitor-run motorThe capacitor-start, capacitor-run motor has a

running capacitor which remains in series withthe starting winding when the motor is running.The starting winding then becomes an auxiliaryrunning winding. To accomplish this, the centri-fugal switch disconnects only the starting capaci-tor leaving the running capacitor and the startingwinding continuously energized during running.Fig. 46 shows the circuit for this type of motor.In capacitor motors rated at approximately 3horsepower and larger the motor performance atrunning as well as during starting may be im-proved by employing such a circuit.

The capacitor-start, capacitor-run motor has a run-ning capacitor (a) permanently connected in serieswith the starting winding. The starting capacitor (b)is disconnected by the centrifugal switch (c) whenthe motor comes up to speed. (Fig. 46)

Repulsion-start induction motor

The repulsion-start induction motor is consid-erably different in appearance from the split-phaseand capacitor-start types, although the differencesare only employed in starting the motor. It has asingle running winding in the stator which is inthe circuit at all times when the motor is startingand running. It has a wound rotor with commuta-tor and brushes. We shall first consider a two-polerepulsion-start induction motor as shown in Fig.47. If we remove the brushes and introduce analternating current into the stator winding, nocurrent will flow in the rotor because all of therotor coils are open circuits. The ends of thesecoils are attached to the commutator bars but thehare arc insulated from each other. Consequentlyno magnetic forces are created in the rotor and

No magnetism is produced in the rotor of the repul-sion-start induction motor without the brushes.

(Fig. 47)

the motor will neither start nor run.If we add a pair of brushes which are con-

nected together and place them in the properposition, they will complete the circuit in a particu-lar rotor coil. Current will now flow in that coil byinduction and magnetic poles will be formed inthe rotor core (Fig. 48 ). The N pole of the rotorwill be repelled by the N pole of the stator andattracted toward the S pole, causing the rotor toturn in the direction shown by the arrow. As therotor turns, the brushes stay in their relativeposition, completing the circuit in the next rotor

Starting

Brushes complete the circuit in certain rotor coilsto produce magnetic poles. (Fig. 48)

coil, and so on, causing the rotation to continueand the motor to gain speed rapidly.

At about 75 percent of full running speed thecommutator bars are all short-circuited togetherby some type of centrifugal short-circuiting device(Fig. 49). It then assumes the properties of asquirrel-cage rotor and rune as an induction motorin a way similar to the split-phase and capacitor-start motors. At running speed, the brushes nolonger function and may or may not continue torest against the commutator.

The direction of rotation is determined by thebrush position when the motor starts. If thebrushes are shifted to position a in Fig. 50 so thatthe magnetic poles of the rotor are directly in linewith the corresponding poles of the stator, themotor will not start at all. A slight shift eitherway from this dead-center position will cause themotor to rotate in that direction (b or C, Fig. 50).However, there is a certain fixed point for thebrushes at which the motor will develop its maxi-mum starting torque in either direction. Thesepoints are usually indicated by a mark on thebrush ring (Fig. 13b).

RunningThe short-circuiting device operates to short all commu-tator bars together when the motor reaches proper speet

(Fig. 49)

When the brushes are in dead-center position a, themotor will not start. When shifted to b, the motorruns counterclockwise; when shifted to c, clockwise.

(Fig. 50)

The speed at which an induction motor runs on60-cycle alternating current is determined by thenumber of poles in the main or running winding.For simplicity we have dealt thus far with two-pole motors which run at about 3450 r.p.m. at fullload. This is one revolution for each cycle of the

current, (3600 r.p.m.) less 4 to 5 percent slip. Afour-pole motor makes one-half a revolution percycle, or about 1725 r.p.m. at full load. A six-polemotor makes one-third revolution per cycle, orabout 1140 r.p.m. at full load, and so on.

8. SUGGESTED DEMONSTRATIONS AND SHOP EXERCISES

To carry on the following demonstrations andshop exercises, certain items of equipment areneeded:

cardboard or glasspermanent magnetsiron filingsbell wireiron rodcarpet tackscompassd.c. laboratory demonstration motor

6-volt test lamp120-volt test lampgrowlerd.c. ammeter test set with self-contained

batterya.c. test set with voltmeter and ammetersplit-phase motor, capacitor-start motor, and

repulsion - start' induction motor of the samehorsepower rating

Torque test stand with pulley, belt, and scale

DEMONSTRATIONS

1. Show the irrignetic field of a magnet

Place a cardboard or glass over a permanentmagnet and sprinkle iron filings over the surface.Gently tap the cardboard or glass and note theattern taken by the filings (see Fig. 36).

2. Show the effect of one magnetic poleupon another

Suspend a bar magnet with a string and holdthe N pole of another magnet near its N pole. Noteresults. Hold the S pole near the N pole of thesuspended magnet and note results (see Fig. 39).

3. Demonstrate how an electromagnet

ironset. S

a.

unctions

rap several turns of the bell wire around therod and connect the ends to the d.c. testhow thatThe electromagnet has the same propertiesas a permanent magnet.t is a temporary magnet. It picks up tacksr bits of iron. Break connection to batterynd show that magnetism is lost.sing a given length of wire, the strength

the electromagnet can be increased bying more turns of wire on the electromag-

b. I0

ac. U

ofus

1 27 1

net. Count the number of tacks it will pickup. Increase the number of turns and counttacks again.

d. The electromagnet can be made to changepolarity. Connect to the test set and testpolarity of the electromagnet with a com-pass. Reverse the battery connections andtest for polarity. Note results.

4. Demonstrate action of d.c. laboratory motor

a. Connect the motor to the d.c. test set andshow how it runs.

b. Show that the commutator changes polarityof the rotor. Remove the permanent magnetsfrom the demonstration motor. Connect themotor to the test set and determine the mag-netic polarity of the rotor by testing withthe compass. Slowly turn the rotor by handand note change in polarity. Show how thecommutator causes this change.

c. Show how the demonstration motor can bereversed. Connect the motor to the test setusing electromagnets as the stator, wiringthe rotor and stator in parallel. To do this,bring both stator and rotor leads to the bat-tery connections. Note direction of rotation.Reverse the stator lead connections and note

results. Reverse the rotor lead connectionsand note results.

5. Show how an alternating current flowsby induction

Attach the growler to a 120 -volt circuit. Con-nect the 6-volt test 'lamp to the ends of the wind-ings of the electromagnet used in the previousdemonstration. Slowly lower the electromagnet,into the magnetic field of the growler (Fig. 51).Note results.

Current flow by induction can be demonstrated witha growler, an electromagnet, and a small bulb..

(Fig. 51)

SHOP EXERCISES

1. Interpret motor nameplate information

Study the information given on the nameplatesof the three motors. Interpret the meaning ofsuch terms as : h.p., r.p.m., cycle, phase, volts,amperes, hours, type, and degrees C. rise.

2. Study variation in starting torque and cur-rent characteristics of split-phase capacitor-start, and repulsion-start induction motors.Set up the equipment for testing as shown in

Fig. 52 and run no-load and locked-rotor tests oneach motor.

aThis equipment is used for making torque and cur-rent tests.

(a) a.c. test set containing voltmeter and ammeter.(b) Torque test. stand and scale. (Fig. 52)

No-load tests should be made before the largewooden pulley is put on the motor shaft. It will benecessary to start the motor several times in orderto get accurate or average readings of voltage andcurrent as the motor starts and when running atfull speed.

For the locked-rotor test, put on the woodenpulley, tighten the set screw, and connect the fab-ric strap to the scale. Turn on the motor for justan instant and take the meter, readings and thescale reading. It may be necessary to do thisseveral times but avoid leaving the current on formore than a moment at a time to avoid blowingfuses or damaging motor windings.

Record the results of your tests as follows:

SPLIT-PHASE

CAPACITOR-START l

REPULSION-START

NO-LOAD TEST

Line volts,starting

Line volts,running

Current (Amps.),starting

Current (Amps.),running

._

LOCKED-ROTOR TEST

Scale reading, lb.

Line volts

Current inamps,

281

3. Wire the terminal board of a split-phasemotor

a. Test and identify starting and running wind-ing leads. Using the d.c. ammeter test set(Fig. 53 ), test and identify the starting andrunning winding leads. Since the starting wind-ing usually has more resistance, it will 'show asthe lower ampere reading.

The d.c. ammeter test set with self-contained batteryis useful in testing and identifying motor leads andcircuits. (Fig. 53)

b. Locate the starting-switch terminals. Deter-mine whether the starting switch is perma-nently attached to two of the bolts on theterminal board. The starting-switch terminalswill show as a high reading on the ammeter, ap-proximately the same as a dead short made bytouching the two test clamps together.c. Wire up the terminal board and run the mo-tor. It will be well to diagram the hook-up fromthe results of your testing before trying themotor. The running winding must be connectedacross the line with starting switch in serieswith starting winding. It is well to make a pre-liminary check with the d.c. test set on theterminals to which you intend connecting the120-volt line. If it shows a low ampere reading,it is safe to go ahead. A high reading shows anerror has been made which would blow a fuseor trip a circuit-breaker in the line. When readyto connect the motor to the line, the "hot" (un-grounded) wire should go to the terminal towhich the running winding and the startingswitch are connected.d. Reverse the direction of rotation. The mo-tor can be reversed by switching either thestarting winding leads or the running windingleads.

4. Wire the terminal board of a capacitor-startmotor

a. Test and identify motor leads. If a smallmotor has six leads, it usually indicates a ther-mal overload protector is built in the motor. Itspair of leads will test on the ammeter as nearlya dead short. The running winding leads willshow a low ampere reading as with the split-phase motor. The starting-winding leads willshow no reading because of the capacitor whichwill not pass a direct current. This means thatthe starting-winding leads can only be found bya process of elmination, or if positive identifica-tion is desired, test first with d.c. and then withthe 120 -volt test lamp in series with the a.c.line. The test lamp will light but no reading willshow on the d.c. ammeter. This is a positiveidentification of the starting-winding circuit.Test the terminal bolts to see if the startingswitch is permanently attached to two of them.b. Wire up the terminal board and run themotor. Diagram your hook-up so that the ther-mal protector is wired in series with the "hot"line leading to both windings and that the start-ing switch and capacitor are in series with thestarting winding.c. Reverse the direction of rotation. This motoris also reversed by switching either the startingor running winding leads.

5. Wire the terminal board of the repulsion-start induction motor and reverse ita. Test and identify the two halves of thestator winding. Use the d.c. test set. The twoleads representing one half of the winding willgive a low reading. The other pair should givethe same reading.b. Wire motor for 120 volts. Connect the twohalves in parallel with the line and try the motoron 120 volts. If it does not run properly, diagramwhat is wrong and correct it.c. Wire motor for 240 volts. Connect the twohalves in series for 240 volts. Make the motorconnection and try it first with a 120-volt line.If your connection is wrong, no damage will bedone to the motor. If your connection is correct,the motor will start but will pick up speedslowly since it is getting only half voltage. Whenit reaches full speed it will run quietly and norm-ally. After you get it to run properly, changethe line connection to 240 volts. Diagram andexplain the possible errors you might make inthe series connection for 240 volts.

d. Reverse the direction of rotation.Repulsion-start induction motors are reversed by shiftingthe brush position on the commutator (see Fig.

13). Locate the mechanism for doing this. It

may be necessary to take off an inspection plate,loosen a set-screw, and shift the brush ring, orthe motor may have an external reversing lever.

QUESTIONS

Some may be interested in determining theanswers to the following questions by experiment-ing with motors:1. Will a split-phase (or capacitor-start) motor

run on the running winding alone, if startedby hand ? In either direction ?

2. Will it run on the starting winding alone ?3. Will it run if the starting switch is wired in

series with the running winding instead of the

30 J

starting winding?4. What will happen if the starting switch is

wired in series with both the starting and run-ning windings ?

5. Will a repulsion-start induction motor run onhalf the stator winding?

6. If the reversing lever of a repulsion-start in-duction motor is shifted while it is runningfull speed, will it immediately reverse itself ?

5M-525M-532M-574M-58

Rev. 5M-623M-65

11206