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6.0 THEORY

6.1 DESCRIPTION OF AC MOTOR OPERATION

 ThreephaseACmotorsarecomprisedoftwomajorcomponents,thestatorandtherotor.Thestatorisa

setofthreeelectricalwindingsheldstationaryinthemotorhousing.Therotorisametalcylinder,fixedto

the motor drive shaft, which rotates within the stator. The arrangement of the stator coils and the presence

of three phase AC voltage give rise to a rotating magnetic field which drives the rotor. The speed at which

themagneticfieldrotatesisknownasthesynchronousspeedofthemotor.Synchronousspeedisafunction

ofthefrequencyatwhichthevoltageisalternatingandthenumberofpolesinthestatorwindings.

 Thefollowingequationgivestherelationbetweensynchronousspeed,frequency,andthenumberofpoles:

Ss = 120 f/p

 Where:Ss=Synchronousspeed(rpm),f=frequency(Hz),

p = number of poles

In three phase induction motors the actual shaft speed differs from the synchronous speed as load is applied.

Thisdifferenceis knownas “slip”. Slipis commonlyexpressedas apercentageof synchronousspeed. A

typical value is three percent at full load.

The strength of the magnetic field in the gap between the rotor and stator is proportional to the amplitude

ofthevoltageatagivenfrequency.Theoutputtorquecapabilityofthemotoris,therefore,afunctionofthe

appliedvoltageamplitudeatagivenfrequency.Whenoperatedbelowbase(rated)speed,ACmotorsrunin

therangeof“constanttorque”.Constanttorqueoutputisobtainedbymaintainingaconstantratiobetween

voltage amplitude (Volts) and frequency (Hertz). For 60 Hz motors rated at 230, 460, and 575 Vac,

commonvaluesforthisV/Hzratioare3.83,7.66,and9.58respectively.OperatingwiththeseV/Hzratios

generally yields optimum torque capability. Operating at lower ratio values results in lower torque and

power capability. Operating at higher ratio values will cause the motor to overheat. Most standard motors

are capable of providing full torque output from 3 to 60 Hz. However, at lower speeds, where motor

coolingfans becomeless effective,supplemental coolingmaybeneededtooperateatfulltorqueoutput

continuously.

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Leeson Micro Series Compact Inverters Manual de usuario Pagina 19