motor theory

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AC Motor TheoryAC Motor Theory

Over ViewOver View– HistoryHistory– AC Motors Convert Electrical Energy to Mechanical AC Motors Convert Electrical Energy to Mechanical

EnergyEnergy» InductionInduction

» Motor ComponentsMotor Components

» The Magnetic FieldThe Magnetic Field

» SpeedSpeed

» SlipSlip

» Slip ControlSlip Control

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Overview (cont.)Overview (cont.)– AC Motors Convert Electrical Energy to AC Motors Convert Electrical Energy to

Mechanical EnergyMechanical Energy» TorqueTorque

– Speed ControlSpeed Control– Nameplate InformationNameplate Information– Motor Winding ConnectionsMotor Winding Connections– Types of Induction MotorsTypes of Induction Motors

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First Electric Motor Was DC in 1833First Electric Motor Was DC in 1833– Simple to control speedSimple to control speed

First AC Motor in 1899First AC Motor in 1899– Simple and RobustSimple and Robust– Fixed speed and torque characteristicsFixed speed and torque characteristics

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– When a conductor is moving across a magnetic When a conductor is moving across a magnetic

field a voltage is inducedfield a voltage is induced– If the conductor is part of a closed circuit there If the conductor is part of a closed circuit there

will be a current inducedwill be a current induced

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– In a motor, the induction principle is utilized in In a motor, the induction principle is utilized in

reversereverse» A live conductor is placed in a magnetic fieldA live conductor is placed in a magnetic field

» The conductor is influenced by a force which tries The conductor is influenced by a force which tries to move it through the magnetic fieldto move it through the magnetic field

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– The AC motor is made up of two partsThe AC motor is made up of two parts

» The StatorThe Stator The stationary section that contain the windingsThe stationary section that contain the windings

» The RotorThe Rotor The rotating section that contains the conductorsThe rotating section that contains the conductors

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– The Magnetic FieldThe Magnetic Field

» Rotates in the air gap between the stator and the Rotates in the air gap between the stator and the rotorrotor

» Has a fixed location in the stator core but its Has a fixed location in the stator core but its direction variesdirection varies

Speed of direction change is determined by the frequency Speed of direction change is determined by the frequency of the AC lineof the AC line

The field changes 60 times per second with 60 Hz powerThe field changes 60 times per second with 60 Hz power

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AC Motors Convert Electric Energy into Mechanical AC Motors Convert Electric Energy into Mechanical EnergyEnergy– The Magnetic FieldThe Magnetic Field

» When three phases are introduced in the motor three magnetic When three phases are introduced in the motor three magnetic fields are introducedfields are introduced

Make up a symmetrical rotating Make up a symmetrical rotating 120 degrees apart120 degrees apart Poles of opposite polarity are formedPoles of opposite polarity are formed

– Speed = (frequency x 120) / # of polesSpeed = (frequency x 120) / # of poles (60 Hz x 120) / 4 poles = 1800 rpm(60 Hz x 120) / 4 poles = 1800 rpm Theoretical or Synchronous - need to factor Theoretical or Synchronous - need to factor

in “slip” in “slip”

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– SlipSlip

» The rotor is not quite able to keep up with the The rotor is not quite able to keep up with the magnetic field rotation so rotates slightly slowermagnetic field rotation so rotates slightly slower

» Slip is typically 3-8% of synchronous speedSlip is typically 3-8% of synchronous speed

» So, actual motor speed equals:So, actual motor speed equals:

Speed = ((Frequency x 120) / # of poles) - SlipSpeed = ((Frequency x 120) / # of poles) - Slip

((60Hz x 120) / 4 poles) - 50 rpm) = 1750 rpm((60Hz x 120) / 4 poles) - 50 rpm) = 1750 rpm

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– Slip ControlSlip Control

» Slip can be controlled through motor voltageSlip can be controlled through motor voltage» If voltage is reduced then slip will increaseIf voltage is reduced then slip will increase

Soft StartsSoft Starts

» Since AC motors are designed for a certain voltage Since AC motors are designed for a certain voltage and frequency ratio, changing the voltage alone will and frequency ratio, changing the voltage alone will cause improper magnetization and increase losses and cause improper magnetization and increase losses and motor heatmotor heat

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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– Speeds of Typical Motors @ 60 HzSpeeds of Typical Motors @ 60 Hz

2 pole = 3600 rpm - slip2 pole = 3600 rpm - slip




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AC Motors Convert Electric Energy into AC Motors Convert Electric Energy into Mechanical EnergyMechanical Energy– Motor torque is affected by the voltage Motor torque is affected by the voltage – As voltage increases in relation to frequency, As voltage increases in relation to frequency,

torque increasestorque increases

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Torque in AC MotorsTorque in AC Motors– A one hp, four pole motor (1800 rpm sync.) has A one hp, four pole motor (1800 rpm sync.) has

approximately 3 ft-lbs. of torqueapproximately 3 ft-lbs. of torque– If the V/Hz ratio remains constant then the If the V/Hz ratio remains constant then the

torque will remain constant over the speed torque will remain constant over the speed range range

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Pop Quiz!Pop Quiz!

Motor A is a Four Pole, 1750 rpm, 100 Motor A is a Four Pole, 1750 rpm, 100 hp motorhp motor

Motor B is a Two Pole, 3500 rpm, 200 Motor B is a Two Pole, 3500 rpm, 200 hp motor hp motor

Which motor has more torque?Which motor has more torque?

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Answer: Answer: The torque will be the sameThe torque will be the same. . Remember that horsepower is a function of Remember that horsepower is a function of speed speed andand torque. Although motor B has torque. Although motor B has twice the horsepower it also has to go twice twice the horsepower it also has to go twice the speed.the speed.

So, how much torque will they have?So, how much torque will they have?

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Answer: 300 ft.-lbs.Answer: 300 ft.-lbs.

Since a 1 hp, 1750 RPM motor creates 3 ft.-lbs. Since a 1 hp, 1750 RPM motor creates 3 ft.-lbs. then a 100 hp, 1750 rpm motor will create then a 100 hp, 1750 rpm motor will create 300 ft.-lbs.300 ft.-lbs.

Remember: Horsepower is not the whole story! Remember: Horsepower is not the whole story!

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Speed Control In AC MotorsSpeed Control In AC Motors– Proper Speed VariationProper Speed Variation

» Since the rotor follows the rotating magnetic field Since the rotor follows the rotating magnetic field then the rotor will slow with a lower frequencythen the rotor will slow with a lower frequency

» Since the motor is designed for a certain voltage to Since the motor is designed for a certain voltage to frequency ratio, if we lower voltage in proportion to frequency ratio, if we lower voltage in proportion to the frequency the torque will remain constantthe frequency the torque will remain constant

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Typical Motor Nameplate DataTypical Motor Nameplate Data– Frame sizeFrame size

» NEMA standard sizesNEMA standard sizes

» Lower rpm motors will have larger frame sizes to help cool Lower rpm motors will have larger frame sizes to help cool since the cooling fan is moving less airsince the cooling fan is moving less air

– HorsepowerHorsepower– RPMRPM– VoltageVoltage– Full Load Amps Full Load Amps

» For each voltage (208-230/460 VAC)For each voltage (208-230/460 VAC)

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Typical Motor Nameplate DataTypical Motor Nameplate Data– Power Factor (not on all motors)Power Factor (not on all motors)

» Ratio of how much current is active to reactiveRatio of how much current is active to reactive Active current goes to shaft outputActive current goes to shaft output Reactive current builds the magnetic fieldReactive current builds the magnetic field

– Insulation classInsulation class

– EnclosureEnclosure» ODP - Open, drip proofODP - Open, drip proof

» TEFC - Totally enclosed, fan cooledTEFC - Totally enclosed, fan cooled

» TEBC - Totally enclosed, blower cooledTEBC - Totally enclosed, blower cooled

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Motor Winding ConnectionsMotor Winding Connections– Typical motor has nine leadsTypical motor has nine leads– Follow motor instructions for connections for your Follow motor instructions for connections for your

nominal voltagenominal voltage» 230 VAC diagrams will have the windings in parallel230 VAC diagrams will have the windings in parallel

» 460 VAC diagrams will have the windings in series460 VAC diagrams will have the windings in series

– On part winding start motors tie both the start and the On part winding start motors tie both the start and the run togetherrun together

» Verify that both windings have the same Verify that both windings have the same

rotationrotation

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Types of AC Induction MotorsTypes of AC Induction Motors– Standard efficientStandard efficient– Energy efficientEnergy efficient

» Federal government requiring all new motors to be energy Federal government requiring all new motors to be energy efficientefficient

– Inverter DutyInverter Duty» Higher class insulation, phase paper, mounts for constant Higher class insulation, phase paper, mounts for constant

velocity fanvelocity fan

– Vector DutyVector Duty» Same as inverter duty but with a shaft encoderSame as inverter duty but with a shaft encoder

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ConclusionConclusion– AC Motors operate through rotating magnetic AC Motors operate through rotating magnetic

fieldsfields– The speed of the motor is determined by the The speed of the motor is determined by the

frequency of the power supplyfrequency of the power supply– The torque of the motor is determined by the The torque of the motor is determined by the

voltage appliedvoltage applied– AC motors are designed for a fixed voltage to AC motors are designed for a fixed voltage to

frequency ratio frequency ratio

motor theory

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