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Electric Motors

UNEP 2006

ical Equipment/ctric Motors


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UNEP 2006

Agenda: Electric Motors

IntroductionTypes of electric motorsAssessment of electric motorsEnergy efficiency opportunities



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UNEP 2006

Introduction

Electromechanical device that converts

electrical energy to mechanical energy Mechanical energy used to e.g.

o Rotate pump impeller, fan, blowero Drive compressorso Lift materials

Motors in industry: 70% of electrical load

What is an Electric Motor?



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Introduction

How Does an Electric Motor Work?


(Nave, 2005)

1

2

3

4


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Introduction

Three types of Motor Load


Motor loads Description Examples

Constanttorque loads

Output power variesbut torque is constant

Conveyors, rotary kilns,constant-displacementpumps

Variabletorque loads

Torque varies withsquare of operationspeed

Centrifugal pumps, fans

Constantpower loads

Torque changesinversely with speed

Machine tools


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Training Agenda: Electric Motors

IntroductionTypes of electric motorsAssessment of electric motorsEnergy efficiency opportunities



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Type of Electric Motors

Classification of Motors


Electric Motors

Alternating Current(AC) Motors

Direct Current (DC)Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparately

Excited

Series ShuntCompound


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Field poleo North pole and south poleo Receive electricity to form

magnetic field Armature

o Cylinder between the poleso Electromagnet when current goes througho Linked to drive shaft to drive the load

Commutatoro Overturns current direction in armature

DC Motors Components


(Direct Industry, 1995)


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Speed control without impact powersupply qualityo Changing armature voltageo Changing field current

Restricted useo Few low/medium speed applicationso

Clean, non-hazardous areas

Expensive compared to AC motors

DC motors



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Relationship between speed, field fluxand armature voltage

DC motors


Back electromagnetic force: E = K NTorque: T = K Ia

E = electromagnetic force developed at armature terminal (volt)= field flux which is directly proportional to field current

N = speed in RPM (revolutions per minute)T = electromagnetic torqueIa = armature currentK = an equation constant


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Separately excited DC motor: field currentsupplied from a separate force

Self-excited DC motor: shunt motor


Field winding parallelwith armaturewinding

Current = fieldcurrent + armaturecurrent

Speed constantindependent of loadup to certain torque

Speed control:insert resistancein armature orfield current

DC motors

(Rodwell Int.

Corporation, 1999)


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UNEP 2006


Self-excited DC motor: series motor


DC motors

(Rodwell Int.

Corporation, 1999)

Field winding inseries with armaturewinding

Field current =armature current

Speed restrictedto 5000 RPM Avoid running

with no load:speeduncontrolled

Suited for high

starting torque:cranes, hoists


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UNEP 2006


DC compound motor


DC motors

Field winding inseries and parallelwith armaturewinding

Good torque and

stable speed

Higher %compound in

series = highstarting torque

Suited for high

starting torque if high% compounding:cranes, hoists


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UNEP 2006


Classification of Motors


Electric Motors

Alternating Current(AC) Motors

Direct Current (DC)Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparatelyExcited

Series ShuntCompound


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UNEP 2006


Electrical current reverses direction Two parts: stator and rotor

o Stator: stationary electrical componento Rotor: rotates the motor shaft

Speed difficult to control Two types

o Synchronous motoro Induction motor

AC Motors


(Integrated Publishing, 2003)


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UNEP 2006


Constant speed fixed by systemfrequency

DC for excitation and low startingtorque: suited for low load applications

Can improve power factor: suited forhigh electricity use systems

Synchronous speed (Ns):


AC Motors Synchronous motor

Ns = 120 f / PF = supply frequencyP = number of poles


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UNEP 2006


Most common motors in industry Advantages:

o Simple designo Inexpensiveo High power to weight ratioo Easy to maintaino Direct connection to AC power source


AC Motors Induction motor


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UNEP 2006



Components Rotor

o Squirrel cage:conducting barsin parallel slots

o Wound rotor: 3-phase, double-layer,distributed winding


Statoro Stampings with slots to carry 3-phase windingso

Wound for definite number of poles

(Automated Buildings)


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UNEP 2006




How induction motors work Electricity supplied to stator Magnetic field generated that moves around rotor Current induced in rotor

Electromagnetics

Stator

Rotor

Rotor produces secondmagnetic field thatopposes stator magneticfield

Rotor begins to rotate (Reliance)


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Single-phase induction motoro One stator windingo Single-phase power supplyo Squirrel cage rotoro Require device to start motoro 3 to 4 HP applicationso Household appliances: fans, washing machines,

dryers


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UNEP 2006




Three-phase induction motoro Three-phase supply produces magnetic fieldo Squirrel cage or wound rotoro Self-startingo High power capabilitieso 1/3 to hundreds HP applications: pumps,

compressors, conveyor belts, grinderso 70% of motors in industry!


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UNEP 2006




Speed and slip Motor never runs at synchronous

speed but lower base speed Difference is slip Install slip ring to avoid this Calculate % slip:

% Slip = Ns Nb x 100Ns

Ns = synchronous speed in RPMNb = base speed in RPM


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Relationship load, speed and torque

At start: highcurrent and lowpull-up torque

At start: highcurrent and lowpull-up torque

At 80% of full

speed: highestpull-outtorque andcurrent drops

At full speed:torque andstator currentare zero


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IntroductionTypes of electric motorsAssessment of electric motorsEnergy efficiency opportunitiesical Equipment/

ctric Motors


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UNEP 2006

Assessment of Electric Motors

Motors loose energy when serving a load

Fixed loss

Rotor loss Stator loss Friction and rewinding

Stray load loss

Efficiency of Electric Motors


(US DOE)


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UNEP 2006

Factors that influence efficiency

Age

Capacity Speed Type

Temperature

Rewinding Load





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UNEP 2006

Motor part load efficiency Designed for 50-100% load

Most efficient at 75% load

Rapid drop below 50% load



(US DOE)



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UNEP 2006

Motor load is indicator of efficiency Equation to determine load:

Motor Load

ical Equipment/ctric Motors Load = Pi x HP x 0.7457

= Motor operating efficiency in %HP = Nameplate rated horse powerLoad = Output power as a % of rated powerPi = Three phase power in kW



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UNEP 2006

Three methods for individual motors Input power measurement

o Ratio input power and rate power at 100%loading Line current measurement

o Compare measured amperage with ratedamperage

Slip methodo Compare slip at operation with slip at full load

Motor Load




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UNEP 2006

Input power measurement Three steps for three-phase motors

Step 1. Determine the input power:

Motor Load


Pi = Three Phase power in kWV = RMS Voltage, mean line toline of 3 PhasesI = RMS Current, mean of 3phasesPF = Power factor as Decimal



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Input power measurementStep 2. Determine the rated power:

Step 3. Determine the percentage load:

Motor Load


Load = Output Power as a % of RatedPowerPi = Measured Three Phase power inkWPr = Input Power at Full Rated load inkW

Pr = Input Power at Full Rated load inkWhp = Name plate Rated Horse Power

r = Efficiency at Full Rated Load



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UNEP 2006

Result1. Significantly

oversized andunderloaded

2. Moderatelyoversized andunderloaded

3. Properly sized butstandard efficiency

Motor Load


Action Replace with more efficient,

properly sized models Replace with more efficient,

properly sized models whenthey fail

Replace most of these with

energy-efficient models whenthey fail



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IntroductionTypes of electric motorsAssessment of electric motorsEnergy efficiency opportunitiesical Equipment/

ctric Motors


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UNEP 2006

1.Use energy efficient motors2.Reduce under-loading (and avoid over-

sized motors)3.Size to variable load4.Improve power quality5.Rewinding6.Power factor correction by capacitors7.Improve maintenance8.Speed control of induction motor


Energy Efficiency Opportunities

O


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UNEP 2006

Reduce intrinsic motor losses Efficiency 3-7% higher Wide range of ratings More expensive but

rapid payback Best to replace when

existing motors fail

Use Energy Efficient Motors


(Bureau of Indian Standards)


E Effi i O i i


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UNEP 2006


Power Loss Area Efficiency Improvement

1. Fixed loss (iron) Use of thinner gauge, lower loss core steel reduces eddycurrent losses. Longer core adds more steel to the design,

which reduces losses due to lower operating flux densities.

2. Stator I2R Use of more copper & larger conductors increases crosssectional area of stator windings. This lower resistance (R)of the windings & reduces losses due to current flow (I)

3 Rotor I2R Use of larger rotor conductor bars increases size of crosssection, lowering conductor resistance (R) & losses due to

current flow (I)

4 Friction & Winding Use of low loss fan design reduces losses due to airmovement

5. Stray Load Loss Use of optimized design & strict quality control proceduresminimizes stray load losses

(BEE India, 2004)

Use Energy Efficient Motors


E Effi i O t iti


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UNEP 2006


Reasons for under-loadingo Large safety factor when selecting motoro Under-utilization of equipmento Maintain outputs at desired level even at low

input voltageso High starting torque is required

Consequences of under-loadingo Increased motor losseso Reduced motor efficiencyo Reduced power factor

2. Reduce Under-loading


E Effi i O t iti


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UNEP 2006


Replace with smaller motoro If motor operates at


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UNEP 2006


Motor selection based ono Highest anticipated load: expensive and risk of

under-loadingo Slightly lower than highest load: occasional

overloading for short periods But avoid risk of overheating due to

o Extreme load changeso

Frequent / long periods of overloading

o Inability of motor to cool down

3. Sizing to Variable Load


X

Motors haveservice factor

of 15% aboverated load



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UNEP 2006


Motor performance affected by Poor power quality: too high fluctuations in

voltage and frequency Voltage unbalance: unequal voltages to threephases of motor

4. Improve Power Quality


Example 1 Example 2 Example 3

Voltage unbalance (%) 0.30 2.30 5.40

Unbalance in current (%) 0.4 17.7 40.0

Temperature increase (oC) 0 30 40



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UNEP 2006


Keep voltage unbalance within 1% Balance single phase loads equally

among three phases Segregate single phase loads and

feed them into separate

line/transformer

4. Improve Power Quality




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UNEP 2006


Rewinding: sometimes 50% of motors Can reduce motor efficiency Maintain efficiency after rewinding byo Using qualified/certified firm

o Maintain original motor designo Replace 40HP, >15 year old motors instead of

rewindingo

Buy new motor if costs are less than 50-65% ofrewinding costs

5. Rewinding




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UNEP 2006


Use capacitors for induction motors Benefits of improved PF

o Reduced kVAo Reduced losseso Improved voltage regulationo Increased efficiency of plant electrical system

Capacitor size not >90% of no-load

kVAR of motor

6. Improve Power Factor (PF)




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UNEP 2006


Checklist to maintain motor efficiency Inspect motors regularly for wear, dirt/dust

Checking motor loads for over/under loading

Lubricate appropriately Check alignment of motor and equipment Ensure supply wiring and terminal box and

properly sized and installed Provide adequate ventilation

7. Maintenance




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UNEP 2006


Multi-speed motorso Limited speed control: 2 4 fixed speeds

Wound rotor motor driveso Specifically constructed motoro Variable resistors to control torque performanceo >300 HP most common

8. Speed Control of Induction Motor




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UNEP 2006


Variable speed drives (VSDs)o Also called inverterso Several kW to 750 kWo Change speed of induction motorso Can be installed in existing systemo Reduce electricity by >50% in fans and pumpso Convert 50Hz incoming power to variable

frequency and voltage: change speedo Three types





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UNEP 2006


Direct Current Drives Oldest form of electrical speed control Consists of

o DC motor: field windings and armatureo Controller: regulates DC voltage to armature

that controls motor speedo Tacho-generator: gives feedback signal to

controlled



Training Session on Energy


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Equipment

Electric MotorsTHANK YOU

FOR YOUR ATTENTION

UNEP 2006


Disclaimer and References


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rical Systems/ctric motors

UNEP 2006

Disclaimer and References

This PowerPoint training session was prepared aspart of the project Greenhouse Gas EmissionReduction from Industry in Asia and the Pacific(GERIAP). While reasonable efforts have been madeto ensure that the contents of this publication are

factually correct and properly referenced, UNEP doesnot accept responsibility for the accuracy orcompleteness of the contents, and shall not be liablefor any loss or damage that may be occasioneddirectly or indirectly through the use of, or reliance

on, the contents of this publication. UNEP, 2006.

The GERIAP project was funded by the SwedishInternational Development Cooperation Agency (Sida)

Full references are included in the textbook chapterthat is available on www.energyefficiencyasia.org

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