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AC Motor TheoryAC Motor Theory
Over ViewOver View
HistoryHistory
AC Motors Convert Electrical Energy toAC Motors Convert Electrical Energy toMechanical EnergyMechanical Energy
InductionInduction
Motor ComponentsMotor Components
The Magnetic FieldThe Magnetic Field
SpeedSpeed
SlipSlip
Slip ControlSlip Control
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AC Motor TheoryAC Motor Theory
Overview (cont.)Overview (cont.)
AC Motors Convert Electrical Energy toAC 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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AC Motor TheoryAC Motor Theory
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 Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
When a conductor is moving across a magneticWhen a conductor is moving across a magneticfield a voltage is inducedfield a voltage is induced
If the conductor is part of a closed circuit thereIf the conductor is part of a closed circuit there
will be a current inducedwill be a current induced
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
In a motor, the induction principle is utilized inIn a motor, the induction principle is utilized inreversereverse
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 toThe conductor is influenced by a force which tries to
move it through the magnetic fieldmove it through the magnetic field
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC 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 Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
The Magnetic FieldThe Magnetic Field Rotates in the air gap between the stator and theRotates in the air gap between the stator and the
rotorrotor
Has a fixed location in the stator core but itsHas a fixed location in the stator core but its
direction variesdirection varies Speed of direction change is determined by the frequencySpeed 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 Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
The Magnetic FieldThe Magnetic Field When three phases are introduced in the motor threeWhen three phases are introduced in the motor three
magnetic fields are introducedmagnetic fields are introduced
Make up a symmetrical rotatingMake 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 SynchronousTheoretical or Synchronous -- need to factorneed to factorin sli in sli
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
SlipSlip The rotor is not quite able to keep up with theThe 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 3Slip is typically 3--8% of synchronous speed8% of synchronous speed
So, actual motor speed equals:So, actual motor speed equals:
Speed = ((Frequency x 120) / # of poles)Speed = ((Frequency x 120) / # of poles) -- SlipSlip
((60Hz x 120) / 4 poles)((60Hz x 120) / 4 poles) -- 50 rpm) = 1750 rpm50 rpm) = 1750 rpm
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC 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 voltageSince AC motors are designed for a certain voltageand frequency ratio, changing the voltage alone willand frequency ratio, changing the voltage alone will
cause improper magnetization and increase lossescause improper magnetization and increase losses
and motor heatand motor heat
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
Speeds of Typical Motors @ 60 HzSpeeds of Typical Motors @ 60 Hz
2 pole = 3600 rpm2 pole = 3600 rpm -- slipslip
4 pole = 1800 rpm4 pole = 1800 rpm -- slipslip
6 pole = 1200 rpm6 pole = 1200 rpm -- slipslip8 pole = 900 rpm8 pole = 900 rpm -- slipslip
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AC Motor TheoryAC Motor Theory
AC Motors Convert Electric Energy intoAC Motors Convert Electric Energy into
Mechanical EnergyMechanical Energy
Motor torque is affected by the voltageMotor 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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AC Motor TheoryAC Motor Theory
Torque in AC MotorsTorque in AC Motors
A one hp, four pole motor (1800 rpm sync.) hasA one hp, four pole motor (1800 rpm sync.) has
approximately 3 ftapproximately 3 ft--lbs. of torquelbs. of torque If the V/Hz ratio remains constant then theIf the V/Hz ratio remains constant then the
torque will remain constant over the speedtorque will remain constant over the speed
rangerange
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AC Motor TheoryAC Motor Theory
Pop Quiz!Pop Quiz!
Motor A is a Four Pole, 1750 rpm, 100Motor A is a Four Pole, 1750 rpm, 100
hp motorhp motor
Motor B is a Two Pole, 3500 rpm, 200Motor B is a Two Pole, 3500 rpm, 200
hp motorhp motor
Which motor has more torque?Which motor has more torque?
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AC Motor TheoryAC Motor Theory
Answer:Answer: The torque will be the sameThe torque will be the same..
Remember that horsepower is a function ofRemember that horsepower is a function of
speedspeed andand torque. Although motor B hastorque. Although motor B hastwice the horsepower it also has to go twicetwice 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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AC Motor TheoryAC Motor Theory
Answer: 300 ft.Answer: 300 ft.--lbs.lbs.
Since a 1 hp, 1750 RPM motor creates 3 ft.Since a 1 hp, 1750 RPM motor creates 3 ft.--
lbs. then a 100 hp, 1750 rpm motor willlbs. then a 100 hp, 1750 rpm motor will
create 300 ft.create 300 ft.--lbs.lbs.
Remember: Horsepower is not the wholeRemember: Horsepower is not the whole
story!story!
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AC Motor TheoryAC Motor Theory
Speed Control In AC MotorsSpeed Control In AC Motors
Proper Speed VariationProper Speed Variation
Since the rotor follows the rotating magnetic fieldSince the rotor follows the rotating magnetic fieldthen 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 toSince the motor is designed for a certain voltage to
frequency ratio, if we lower voltage in proportion tofrequency 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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AC Motor TheoryAC Motor Theory
Typical Motor Nameplate DataTypical Motor Nameplate Data
Frame sizeFrame size
NEMA standard sizesNEMA standard sizes Lower rpm motors will have larger frame sizes toLower rpm motors will have larger frame sizes to
help cool since the cooling fan is moving less airhelp cool since the cooling fan is moving less air
HorsepowerHorsepower
RPMRPM
VoltageVoltage
Full Load AmpsFull Load Amps
For each voltage (208For each voltage (208--230/460 VAC)230/460 VAC)
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AC Motor TheoryAC Motor Theory
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 ODPODP -- Open, drip proofOpen, drip proof
TEFCTEFC -- Totally enclosed, fan cooledTotally enclosed, fan cooled
TEBCTEBC -- Totally enclosed, blower cooledTotally enclosed, blower cooled
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AC Motor TheoryAC Motor Theory
Motor Winding ConnectionsMotor Winding Connections
Typical motor has nine leadsTypical motor has nine leads
Follow motor instructions for connections forFollow motor instructions for connections foryour nominal voltageyour nominal voltage
230 VAC diagrams will have the windings in230 VAC diagrams will have the windings in
parallelparallel
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 startOn part winding start motors tie both the start
and the run togetherand the run together
Verify that both windings have the sameVerify that both windings have the same
rotationrotation
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AC Motor TheoryAC Motor Theory
Types of AC Induction MotorsTypes of AC Induction Motors
Standard efficientStandard efficient
Energy efficientEnergy efficient Federal government requiring all new motors to beFederal government requiring all new motors to be
energy efficientenergy efficient
Inverter DutyInverter Duty
Higher class insulation, phase paper, mounts forHigher class insulation, phase paper, mounts for
constant velocity fanconstant velocity 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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AC Motor TheoryAC Motor Theory
ConclusionConclusion
AC Motors operate through rotating magneticAC Motors operate through rotating magnetic
fieldsfields The speed of the motor is determined by theThe 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 theThe torque of the motor is determined by the
voltage appliedvoltage applied
AC motors are designed for a fixed voltage toAC motors are designed for a fixed voltage to
frequency ratiofrequency ratio
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