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NEMA MG 2 - 2001

6.3 ProperSelection of

Machines

Machines shouldbe properly

selected withrespect to their

usual or unusualservice conditions,

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both ofwhich

involve theenvironmental and

operatingconditions towhich the

machine issubjected.

Machinesconforming to the Scope of

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this publication

are designed foroperation in

accordance withtheir ratingsunderusual service

conditions. Sometypical

applications formotors of

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standard Design

types are giveninTable 3. Typical

speed torquecharacteristics areshown in Figure 4.

Some machinesmay also be

capableofoperating in

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accordance with

their ratings underone or more

unusual serviceconditions.Definite-purpose

or special-purposemachines may be

required for someunusual

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conditions.Good

energymanagement is

the successfulapplication of themotor controller,

motor, and thedrivencomponents

that results in theleast consumption

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of energy. Since

all motors do nothave the

sameefficiency,carefulconsideration

must be given totheir selection and

application. Forfurther

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informationand

guidance, see MG10EnergyManagementGuide for

Selection and Useof Fixed

FrequencyMedium AC

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Squirrel-Cage

PolyphaseInduction Motors

.Serviceconditions, otherthan those

specified as usual,may involve some

degree of hazard.Theadditional

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hazard depends

upon the degreeof departure from

usual operatingconditions and theseverityof the

environment towhich the

machine isexposed. The

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additional hazard

results from suchthings

asoverheating,mechanicalfailure, abnormal

deterioration ofthe insulation

system, corrosion,fire,

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orexplosion.Althou

gh pastexperience of the

user may often bethe best guide, themanufacturer of

the drivenordriving

equipment or themachine

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manufacturer, or

both, should beconsulted for

furtherinformationregarding any unusual

service conditionswhich increase

the mechanical orthermal duty of

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the machineand,

as a result,increase the

chances for failureand consequenthazard. This

further informationshouldbe

considered by theuser, his

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consultants, or

others mostfamiliar with the

details of theapplicationinvolved when making

the final decision.6.3.1 Variation

From RatedVoltage and

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Rated

Frequency6.3.1.1Induction

Motors6.3.1.1.1RunningMotors will

operatesuccessfully under

running conditionsat rated load with

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a variation in the

voltage orthefrequency up

to the following:a)Plus or minus 10percent of rated

voltage with ratedfrequency.b) Plus

or minus 5percent of rated

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frequency with

rated voltage.c) Acombined

variation involtage andfrequency of 10

percent (sum ofabsolute values)

of the ratedvalues,provided the

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frequency

variation does notexceed plus or

minus 5 percent ofratedfrequency.Perform

ance within thesevoltage and

frequencyvariations will not

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necessarily be in

accordance withthestandards

established foroperation at ratedvoltage and

frequency.6.3.1.1.2 StartingThe limitingvalues of voltage

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and frequency

under which amotor will

successfully startand acceleratetorunning speed

depend on themargin between

the speed-torquecurve of the motor

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at rated voltage

andfrequency andthe speed-torque

curve of the loadunder startingconditions. Since

the torquedeveloped bythe

motor at anyspeed is

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approximately

proportional to thesquare of the

voltage andinverselyproportional to the square

of the frequency, itis generally

desirable todetermine what

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voltage

andfrequencyvariations will

actually occur ateach installation,taking into

account anyvoltage drop

resultingfrom thestarting current

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drawn by the

motor. Thisinformation and

the torquerequirements ofthe drivenmachine

define the motorspeed torque-

curve, at ratedvoltage and

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frequency, which

is adequate fortheapplication.