Unit1 _ac Commutation

8/6/2019 Unit1 _ac Commutation

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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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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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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 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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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

Unit1 _ac Commutation

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