Adjustable Speed Drive Reference Guide

Adjustable Speed Drive

REFERENCE GUIDE

4th Edition

$15.95 CDN

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First Edition, November 1987Second Edition, March 1991Third Edition, February 1995Fourth Edition, August 1997

Revised by:Richard Okrasa, P.Eng.Ontario Hydro

Neither Ontario Hydro, nor any person acting on its behalf,assumes any liabilities with respect to the use of, or fordamages resulting from the use of, any information,equipment, product, method or process disclosed in this guide.

Printed in CanadaCopyright © 1997 Ontario Hydro

In-House Energy EfficiencyEnergy Savings are Good Business


ADJUSTABLE SPEEDDRIVE

Reference Guide

4th Edition

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i

TA B L E O F C O N T E N T S

INTRODUCTION .............................................................................................. 1Latest Improvements .................................................................................2

CHAPTER 1: CLASSIFICATIONS ......................................................................... 3Classification of Motors .......................................................................... 3Classification of Drives ............................................................................ 3

CHAPTER 2: PHYSICAL APPEARANCE ................................................................. 5

CHAPTER 3: PRINCIPLES OF OPERATION ............................................................ 7Conventional Fixed-speed AC Systems .................................................. 7DC Drives ................................................................................................ 8AC Drives ................................................................................................ 8

Eddy Current Clutches ............................................................................. 8Switched Reluctance Drives ...................................................................... 9Vector Drive .......................................................................................... 10Wound-rotor Motor Controllers ............................................................... 10Variable Voltage Controllers .................................................................... 11

Variable Frequency Drives ..................................................................... 11Components .......................................................................................... 12Types of Inverters .................................................................................. 13Waveforms ............................................................................................ 14Switching Devices (Power Electronics) ........................................................14

Medium Voltage Drives...........................................................................14Recommended Specifications .....................................................................15

CHAPTER 4: COMPARISON OF ASDS ............................................................. 17AC Drives .............................................................................................. 17

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Variable Voltage Inverter (VVI) ............................................................... 17Current Source Inverter (CSI) ................................................................. 18Pulse Width Modulator (PWM) .............................................................. 20Power Factor Comparison ....................................................................... 22

DC Drives .............................................................................................. 23Eddy Current Coupling ......................................................................... 25Cycloconverter .........................................................................................26

CHAPTER 5: STANDARD AND OPTIONAL FEATURES ......................................... 33

CHAPTER 6: ADVANTAGES ............................................................................. 35Speed Control ........................................................................................ 35

Position Control ..................................................................................... 36Torque Control ...................................................................................... 36High Energy Savings Potential ................................................................ 36Soft Start/Regenerative Braking .............................................................. 36Equipment Life Improvement .................................................................. 37Multiple Motor Capability ..................................................................... 37Bypass Capability ................................................................................. 37Safe Operation in Harsh Environments .................................................... 37Temporary or Back-up Operation .............................................................37Reduction in Vibration and Noise Level .................................................... 38Re-acceleration Capability ...................................................................... 38

Tips and Cautions .................................................................................. 38

CHAPTER 7: APPLICATION CONSIDERATIONS .................................................. 39How to Select an ASD ........................................................................... 39Software ...................................................................................................42

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Financial Evaluation ................................................................................42Load Characteristics ............................................................................... 42Application Types by Load ..................................................................... 43

Tips and Cautions .................................................................................. 46Motor/Drive System .............................................................................. 49Thermal Considerations ......................................................................... 54

Other Considerations ............................................................................ 56Efficiency .............................................................................................. 57Reliability of ASDs ................................................................................ 58Applications .......................................................................................... 59Performance Required ............................................................................ 60Starting and Stopping Characteristics ...................................................... 62Torque .................................................................................................. 62Environment .......................................................................................... 63Weight and Space ................................................................................. 63Accessories ............................................................................................ 64Safety .................................................................................................. 65Service and Maintenance ....................................................................... 65


CHAPTER 8: ECONOMICS .............................................................................. 69Economic Factors ................................................................................... 72

Capital Costs ........................................................................................ 72Capital Savings .................................................................................... 73Operating Costs and Savings ................................................................. 73


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CHAPTER 9: HARMONIC DISTORTION ........................................................... 77Harmonics .............................................................................................. 77What Harmonic Distortion Can Do ...................................................... 78Production and Transmission ................................................................ 79

Isolation Transformers ............................................................................ 80Other Guidelines (IEEE 519-1992) ........................................................ 81

APPENDIX A: FORMULAS FOR CALCULATING APPLICATIONS ............................. 83APPENDIX B: CONVERSION FACTORS ............................................................. 93

ABBREVIATIONS ............................................................................................ 95

BIBLIOGRAPHY .............................................................................................. 97

INDEX .......................................................................................................... 99

ASD SUPPLIERS IN ONTARIO ....................................................................... 101

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1. Comparison of Range Process Speed Control ......................................12. Physical Appearance of Variable Frequency

Drive/Motor System ............................................................................ 53. 8/6 Pole Switched Reluctance Motor .................................................. 94. Vector Drive .........................................................................................105. Closed Loop (Feedback) Adjustable Frequency

Inverter System .................................................................................. 126. VVI – Variable Voltage Inverter .......................................................... 177. VVI – Waveforms ............................................................................... 188. CSI – Current Source Inverter ............................................................ 199. CSI – Waveforms ............................................................................... 1910. Block Diagram for a Typical CSI Drive ............................................. 1911. PWM – Pulse Width Modulated Inverter .......................................... 2112. PWM – Waveforms ............................................................................ 2113. Block Diagram for a Typical PWM Drive .......................................... 2114. Power Factor Comparison ................................................................. 2215. DC Drive ............................................................................................ 2316. ECC – Eddy Current Coupling .......................................................... 2617. Cycloconverter Circuit.........................................................................2718. Duty Cycles ....................................................................................... 4319. Variable Torque Load ......................................................................... 4520. Constant Torque Load ....................................................................... 4521. Constant Horsepower Load .............................................................. 4522. Power Required is Proportional to RPM3 Centrifugal

Fan/Blower, Pump .............................................................................. 4623. Power Savings in Fans and Pumps Using ASDs ............................... 48

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L I S T O F F I G U R E S

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24. Motor Derating Curves vs. Speed Range When Applied to Adjustable Frequency AC Drives (6-Step Waveform or PWM) ............................................................. 53

25. Watts Loss (Efficiency) Comparison ................................................ 5726. Typical AC Drive Efficiency ............................................................. 5727. Motor Performance, Typical 60 Hz ................................................. 6328. Ideal Torque-Speed Curves .............................................................. 6429. NEMA Design B Motor Torque-Speed Curve ................................. 6430. Capital Cost Comparison of Motor/Drive

Systems Medium HP, Voltages ........................................................ 7631. Harmonic Distortion ........................................................................ 78

A-1. Calculating Hollow Shafts ............................................................... 88A-2. Calculating the Inertia of Complex,

Concentric Rotating Parts ................................................................ 89

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L I S T O F F I G U R E S

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1. Comparison of Adjustable Speed Drives ............................................. 292. ASD and Electronic Motor Features .................................................... 343. Suitability of Inverters for NEMA Motor Designs ............................... 554. ASD Checklist of Costs/Savings .......................................................... 705. ASD Investment Decision Technique .................................................. 71

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L I S T O F TA B L E S

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An adjustable speed drive (ASD) is a device used to providecontinuous range process speed control (as compared to discretespeed control as in gearboxes or multi-speed motors).

An ASD is capable of adjusting both speed and torque from an induction or synchronous motor.

An electric ASD is an electrical system used to control motorspeed.

ASDs may be referred to by a variety of names, such as variablespeed drives, adjustable frequency drives or variable frequencyinverters. The latter two terms will only be used to refer to certainAC systems, as is often the practice, although some DC drives arealso based on the principle of adjustable frequency.

FIGURE 1. Comparison of Range Process Speed Control

Introduction 1

I N T R O D U C T I O N

Discrete

Spe

ed

Operation

Continuous

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In this guide, “drive” refers to the electric ASD.

Application concerns in connecting electric or mechanical ASDshave similar effects on the driven load, and these are covered in thisguide.

LATEST IMPROVEMENTS

• Microprocessor-based controllers eliminate analogue,potentiometer-based adjustments.

• Digital control capability.

• Built-in Power Factor correction.

• Radio Frequency Interference (RFI) filters.

• Short Circuit Protection (automatic shutdown).

• Advanced circuitry to detect motor rotor position by samplingpower at terminals, ASD and motor circuitry combined to keeppower waveforms sinusoidal, minimizing power losses.

• Motor Control Centers (MCC) coupled with the ASD usingreal-time monitors to trace motor-drive system performance.

• Higher starting torques at low speeds (up to 150% runningtorque) up to 500 MP, in voltage source drives.

• Load-commutated Inverters coupled with synchronous motors.(precise speed control in constant torque applications.

2 Adjustable Speed Drive Reference Guide

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CLASSIFICATION OF MOTORS

• There are two main types of motors, AC (alternating current)and DC (direct current).

• AC motors can be sub-classified as induction (squirrel-cage andwound-rotor) and synchronous.

• Induction motors are often classified as either high efficiency orstandard.

CLASSIFICATION OF DRIVES

• Adjustable speed drives are the most efficient (98% at full load)types of drives. They are used to control the speeds of both ACand DC motors. They include variable frequency/voltage ACmotor controllers for squirrel-cage motors, DC motorcontrollers for DC motors, eddy current clutches for AC motors(less efficient), wound-rotor motor controllers for wound-rotorAC motors (less efficient) and cycloconverters (less efficient).

Chapter 1: Classifications 3

C H A P T E R 1

CLASSIFICATIONS

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• Other types of drives include mechanical and hydrauliccontrollers. Examples of mechanical drives are adjustable beltsand pulleys, gears, throttling valves, fan dampers and magneticclutches. Examples of hydraulic drives are hydraulic clutchesand fluid couplings.

• In this guide, emphasis is on AC variable frequency drives, orinverters, which are used to control industry’s workhorse, thestandard AC induction motor. This is because this motor isreplacing the DC motor for many applications. In addition,some information is provided on the DC motor/drive system,since it remains the most suitable choice for certainapplications.

• Drives may be classified according to size ranges (horsepower,voltage) for which increasing specifications are required indesigning an ASD driven system:

- Less than 500 HP.

- Medium sized (up to 2000 HP).

- Motors rated 4kV and up.

• An output transformer between the drive and motor, commonmode voltage is isolated from the motor and put on the driveside transformer winding.


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• Variable frequency AC drives are comprised of many electricalcircuits and components usually arranged within a cabinet thatprovides heat dissipation and shielding.

FIGURE 2. Physical Appearance of Variable Frequency Drive/Motor System

Chapter 2: Physical Appearance 5

C H A P T E R 2

PHYSICAL APPEARANCE

Can be hundreds of

metres away

Feedback Loop(Optional)

ASD + transformer (if required)

MotorTachometer

LOAD

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• Drives vary greatly in size, depending upon their horsepowerand voltage rating and type.

• Electrical cables connect the motor to the drive, which mightinvolve a considerable distance.

• Small AC drives may be built on to their associated motors.


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• Both AC and DC drives are used to convert AC plant power to an adjustable output for controlling motor operation.

• DC drives control DC motors, and AC drives control ACinduction and synchronous motors.

CONVENTIONAL FIXED-SPEED AC SYSTEMS(AC MOTOR WITHOUT DRIVE)• Standard squirrel-cage induction motors are usually considered

to be constant speed motors.

• These systems require some means of throttling (via valves,dampers, etc.) to meet process changes.

• If a reduction in demand occurs, excess energy is wasted in thecontrol device (dampers, throttling valves, recirculation loops)since the power delivered does not decrease in proportion tothe reduction in demand.

Chapter 3: Principles of Operation 7

C H A P T E R 3

PRINCIPLES OF OPERATION

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DC DRIVES

• The DC motor is the simplest to which electronic speed controlcan be applied because its speed is proportional to the armaturevoltage.

• The DC voltage can be controlled through a phase-controlledrectifier or by a DC-DC converter if the input power is DC.This is usually accomplished by a separate motor-generator setproducing a DC output.

• The speed of a DC motor can be adjusted over a very widerange by control of the armature current and/or field currents(brushless DC drives, vector controlled DC drives).

AC DRIVES

EDDY CURRENT CLUTCHES

• Eddy current clutches can be used to control standard ACsquirrel-cage induction motors. However, they are lowefficiency compared to ASDs and have limited applications.

• An eddy current clutch has essentially three major components:a steel drum directly driven by an AC motor, a rotor with polesand a wound coil that provides the variable flux required forspeed control.

• Efficiency is significantly lower than ASDs.

• A voltage is applied to the coil of wire, which is normallymounted on the rotor of the clutch to establish a flux, and thus relative motion occurs between the drum and its output rotor.


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• By varying the applied voltage, the amount of torquetransmitted, and therefore the speed, can be varied.

SWITCHED RELUCTANCE DRIVES

• Switched reluctance (SR) drives have a high power to weightratio.

• In closed-loop control, they are well suited for speed andtorque control.

FIGURE 3. 8/6 Pole Switched Reluctance Motor(one phase winding shown)

• The rotor has salient poles with no windings or electricconnections.

• A pair of opposite stator poles magnetically pulls rotor poles in-line.

• Rotor position sensor controls switch each pole pair insequence, giving continuous rotation.


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VECTOR DRIVE

• Vector drive control of AC motors is similar to DC driveperformance in speed, torque and horsepower.

• It can produce full torque from start to full speed. (The motorneeds to control heat at full torque and low speed.)

• It requires complex electronics (digital signal processors, orDSPs) to calculate servomotor phase currents.

• Magnitude and direction of armature current together are avector quantity which must be regulated to adjust torque.

• Slip speed and motor speed are tracked by an encoder.

• Synchronous motors can be controlled by vector drives byeliminating magnetizing current and slip values.

FIGURE 4. Vector Drive

WOUND-ROTOR MOTOR CONTROLLERS

• Wound-rotor motor controllers are used to control the speed ofwound-rotor induction motors.


SpeedRegulator

2 Phaseto3 Phase

CurrentRegulator

Flux Command

Controller

Motor

Encoder

PositionSignal

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• By changing the amount of external resistance connected to therotor circuit through the slip rings, the motor speed can bevaried.

• The slip energy of the motor is either wasted in externalresistance controllers (in the form of heat) or recovered andconverted to useful electrical or mechanical energy. Forconversion to useful electrical energy, the system would beknown as a wound-rotor slip energy recovery drive.

VARIABLE VOLTAGE CONTROLLERS

• Variable voltage controllers can be used with induction motors.

• Motor speed is controlled directly by varying the voltage.

• These controllers require high slip motors and so are inefficientat high speed.

• Only applications with narrow speed ranges are suitable.

VARIABLE FREQUENCY DRIVES

• A variable frequency drive controls the speed of an AC motorby varying the frequency supplied to the motor.

• The drive also regulates the output voltage in proportion to theoutput frequency to provide a relatively constant ratio (V/Hz) ofvoltage to frequency, as required by the characteristics of theAC motor to produce adequate torque.

• In closed-loop control, a change in demand is compensated by achange in the power and frequency supplied to the motor, and thusa change in motor speed (within regulation capability).


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FIGURE 5. Closed Loop (Feedback)Adjustable Frequency Inverter System

COMPONENTS

• A variable frequency drive has two stages of power conversion,a rectifier and an inverter. (“Inverter” is also used to refer to theentire drive.)

• The system functions this way:

- 60 Hz power, usually 3-phase, is supplied to the rectifier. The input voltage level is usually standard 208V, 230V, 460V,600V, 4,160V, etc. (Higher than 600V requires step-downtransformers.)

- The rectifier is a circuit which converts fixed voltage ACpower to either fixed or adjustable voltage DC.


INVERTER(SwitchingSection)

REGULATOR(Controls)

TACHOMETER

ConstantFrequencyConstant VoltageAC PowerSupply

SpeedReferencefrom Process

Fixed orVariableDC Voltage

VariableFrequencyVariable VoltageAC PowerOutput

Motor

RECTIFIERLOAD

Signal

Feedback

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- The inverter is composed of electronic switches (thyristors ortransistors) that switch the DC power on and off to producea controllable AC power output at the desired frequencyand voltage.

- A regulator modifies the inverter switching characteristics sothat the output frequency can be controlled. It may includesensors to measure the control variables.

TYPES OF INVERTERS

• There are three basic types of inverters commonly employed inadjustable AC drives:

- The variable voltage inverter (VVI), or square-wave six-stepvoltage source inverter (VSI), receives DC power from anadjustable voltage source and adjusts the frequency andvoltage.

- The current source inverter (CSI) receives DC power from anadjustable current source and adjusts the frequency andcurrent.

- The pulse width modulated (PWM) inverter is the mostcommonly chosen. It receives DC power from a fixed voltagesource and adjusts the frequency and voltage. (PWM typescause the least harmonic noise.)

• AC/AC adjustable frequency drives are used only for largehorsepower applications (1000 hp and above). They includecycloconverters (AC/AC) and load-commutated inverters(LCIs). Both can be used with induction or synchronousmotors. (Since these drives are usually custom-designed foreach application, they will not be fully discussed in this guide.)


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WAVEFORMS

• The voltage and current waveforms produced by invertersystems approximate, to varying degrees, the pure sine wave.

• Of the three most common inverter systems, the pulse widthmodulated inverter produces output current waveforms thathave the least amount of distortion.

SWITCHING DEVICES

• Advances in Power Electronic technology have greatlyenhanced performance range and reliability of ASDs.

• New switching devices are faster, produce less heat, and lessharmonics into the motor circuit. Some types are:

- SCR (silicon - controlled rectifier).

- Diode.

- GTO (gate turnoff thyristor).

- IGBT (insulated gate bi-thermal thyristor).

MEDIUM VOLTAGE DRIVES

• Voltages above 2300V, and controlling induction motorsbetween 1,000 HP to 15,000HP are becoming increasinglyavailable.

- Input line isolation transformer.

- Internal cooling (liquid or air).

- Input circuit breaker, output contactor with isolationswitches.


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- Motor harmonics filter to supply maximum 5% current totalharmanic distortion.

- DC link reactor to prevent saturation at faulted conditions.

RECOMMENDED SPECIFICATIONS

• Nominal power at +- 10% voltage, 3 phase, 60 Hz ( +- 2%).

• Capable of operation during temporary voltage drop of 70% to90% lasting up to 6 voltage wave cycles.

• Bus voltage restored within 5 seconds, drive automaticallyrestarts, if not, drive automatically trips and shuts down. Manualreset required to start.

• Uninterruptible Power Source (UPS) recommended to providecontrol circuit power during supply power disturbances, from 5 seconds up to 15 minutes UPS supply recommended.

- Ambient Indoor Conditions:

- 0°C to 40°C.

- Relative humidity up to 95% non condensing.

- Overload capability: 15% rated current for 60 seconds.

- Class H insulation, class B temperature rise.

- ANSI C57.12.01 construction materials.

- NEMA Std. TR-27 for noise.


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COMPARISON OF ASDS

AC DRIVES

VARIABLE VOLTAGE INVERTER (VVI)

• A controlled rectifier transforms supply AC to variable voltageDC. The converter can be an SCR (silicon-controlled rectifier)bridge or a diode bridge rectifier with a DC chopper. The voltage regulator presets DC bus voltage to motor requirements.

FIGURE 6. VVI – Variable Voltage Inverter

• Output frequency is controlled by switching transistors orthyristors in six steps.

Chapter 4: Comparison of ASDs 17

C H A P T E R 4

AC to DCRectifier

ConstantVoltage

DC Link

VoltageSmoothing

DC to ACInverter

Variable Voltage/Frequency Control

M

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FIGURE 7. VVI – Waveforms

• VVI inverters control voltage in a separate section from thefrequency generation output.

• Approximate sine current waveform follows voltage.

• VVI is the simplest adjustable frequency drive and mosteconomical; however, it has the poorest output waveform. It requires the most filtering to the inverter.

• Ranges available are typically up to 500 horsepower but canbe up to 1000 horsepower.

• Voltage source inverters use a constant DC link voltage.

CURRENT SOURCE INVERTER (CSI)

• AC current transformers are used to adjust the controlledrectifier. Input converter is similar to the VVI drive. A currentregulator presets DC bus current.

• The inverter delivers six step current frequency pulse, whichthe voltage waveform follows. Switches in the inverter can betransistors, SCR thyristors or gate turnoff thyristors (GTOs).

Voltage(Line toNeutral) 0

Current(Line) 0

6 Step

Time

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AC to DCRectifier

VariableVoltageControl

DC Link

CurrentSmoothing

DC to ACInverter

VariableFrequency

Control

M

Voltage(Line toNeutral)

0

Current(Line) 0

Time

FIGURE 9. CSI – Waveforms

FIGURE 10. Block Diagram for a Typical CSI Drive

FIGURE 8. CSI – Current Source Inverter

CurrentRegulator

FrequencyControl

Speed orVoltageControl

FilterAC/DCConverter

Inverter

Motor

ACLine

Speed

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• The capacitor in the inverter is matched to motor size.

• Voltage exhibits commutation spikes when the thyristors fire.

• Because it is difficult to control the motor by current only, theCSI requires a large filter inductor and complex regulator.

• CSI drives are short circuit proof because of a constant circuitwith the motor.

• They are not suitable for parallel motor operation.

• Braking power is returned to the distribution system.

• The CSI drive’s main advantage is in its ability to controlcurrent and, therefore, control torque. This applies in variabletorque applications.

• CSI-type drives have a higher horsepower range than VVI andPWM (typically up to 5000 horsepower).

PULSE WIDTH MODULATOR (PWM)

• Diode rectifiers provide constant DC voltage. Since the inverterreceives a fixed voltage, the amplitude of output waveform isfixed. The inverter adjusts the width of output voltage pulses aswell as frequency so that voltage is approximately sinusoidal.

• The better waveforms require less filtering; however, PWMinverters are the most complex type and switching losses canbe high.

• The range of PWM inverters is typically up to 3000 horsepower,but each manufacturer may list larger sizes (usually custom-engineered).


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AC to DCConverter

VariableVoltageControl

DC Link

VoltageSmoothing

DC to ACInverter

VariableFrequency

Control

M

Voltage(Line toNeutral)

0

Current(Line)

0

FIGURE 12. PWM – Waveforms

FIGURE 13. Block Diagram for a Typical PWM Drive

FIGURE 11. PWM – Pulse Width Modulated

Voltage &Frequency

Control

FilterDiodeBridge

RectifierInverter

Motor

ACLine

SpeedReference

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• Motors run smoothly at high and low speed (no cogging);however, they are current limited.

• PWM drives can run multiple parallel motors with accelerationrate matched to total motor load.

• At low speeds, PWM drives may require a voltage boost togenerate required torque.

• A vector drive can control similar to a DC drive.

• PWM is the most costly of the three main AC ASD types.

• Pulse amplitude modulation (PAM) drives are a variation ofPWM drives.

FIGURE 14. Power Factor Comparison

POWER FACTOR COMPARISON

• The power factor of VVI and CSI drives declines with speed asthe thyristor firing angle varies in the controlled rectifier.


1.0

.75

.50

.25

0

PWM & Vector Drive

VVI

CSI

Pow

er F

acto

r

450 900

Speed (RPM)

1350 1800

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• PWM drives have near unity power factor throughout the speedrange, due to the diode rectifier and constant voltage DC bus.

• Note that true Root-Mean-Square (RMS) meters will determinethe real power factor on three-phase systems. It may be lessthan the displacement power factor (kW/kVA) which appearson single-phase meters.

DC DRIVES

• DC drives are a simpler, more mature technology than ACdrives, and they continue to have applications where largerhorsepower is required due to high voltage capacity.

• Armature voltage-controlled DC drives are constant torque drives capable of rated motor torque at any speed up to ratedmotor base speed.

FIGURE 15. DC Drive


100

01000

% of Base Speed

% o

f Rat

ed P

ower

Field CurrentControl

Armature VoltageControl

Constant ArmatureVoltage

Constant FieldCurrent

ConstantTorque

ConstantPower

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• Field voltage-controlled DC drives provide constant horsepowerand variable torque. A variable voltage field regulator canprovide alternate armature and field voltage control.

• Motor speed is directly proportional to voltage applied to thearmature by the ASD. A phase-controlled bridge rectifier withlogic circuits is used. Tachometer feedback achieves speedregulation.

• DC drives have good efficiency throughout the speed range and are larger than AC for the same horsepower. However,with DC drives, the power factor decreases with speed, it is notpossible to bypass the drive to run the motor and maintenancecosts are high due to armature connections through a brush andcommutator ring.

• Regenerative DC drives can invert the DC electrical energyproduced by the generator/motor rotational mechanical energy.

• Cranes and hoists use DC regenerative drives to hold back“overhauling loads,” such as a raised weight or a machine’sflywheel.

• Non-regenerative DC drives are those where the DC motorrotates in only one direction, supplying torque in high frictionloads such as mixers or extruders. The load exerts a strongnatural brake. If desired, the drive’s deceleration time can affectspeed regulation.

• Flywheel applications such as stamping presses haveoverhauling load; hence, braking torque or “dynamic braking” isapplied. All DC motors are DC generators as well.

• Regenerative drives are better speed control devices than non-regenerative but are more expensive and complicated.


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• Armature voltage control DC drives have constant torquefeatures, capable of rated torque across the motor speed range.These drives must be oversized to handle constant horsepowerapplications.

• Field voltage control of shunt wound DC motors with a voltageregulator coordinate armature and field voltage for extendingspeed range in constant horsepower applications.

• Table 1 compares the electric variable speed drives that may beused to control the speed of standard squirrel-cage inductionmotors. For comparison, information on DC systems is alsoprovided. Note that this table covers products representative ofthe types available. Actual product lines may differ. In addition,special order equipment may not conform to these guidelines.Voltage ranges depend on the manufacturer as well as the needfor auxiliary equipment, such as step-down transformers, linefilters and chokes.

EDDY CURRENT COUPLING

• The eddy current coupling (ECC) is similar in principle to afriction-type clutch. It provides electromechanical coupling with torque transmitted by eddy currents. The eddy currentsare generated by rotation.

• The ECC has electrically energized magnetic coil windings onthe rotor via slip rings. The magnetic fields in the drum arecaused by eddy currents.

• Horsepower Slip Loss = motor hp x slip speed RPMmotor RPM


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FIGURE 16. ECC – Eddy Current Coupling

CYCLOCONVERTER

• Mainly used in large synchronous motor drives in lowfrequency applications:

- Steel rolling mill end tables.

- Cement mill furnaces.

- Mine hoists.

- Ship propulsion drives.

• Limitation: wave forms become distorted above 40% of inputfrequency (i.e., 20Hz from 50Hz supply).

• Advantage: high power factor using synchronous motors.


Motor

Drum

TD SD SR TR

Magnetic Rotor

Load

TD = Drum Torque

SD = Drum Speed

TR = Rotor Torque

SR = Rotor Speed

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FIGURE 17. Cycloconvertor Circuit


Load

BridgeA

A.C. Supply

A.C. Supply

BridgeB

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TABLE 1. Comparison of Adjustable Speed Drives

Eddy CurrentCoupling (ECC)

Squirrel-cage induction

1 – 1,000

34:1 but may be difficult tocontrol above 2:1

3 - 5%

Good

0 - 70%

Field winding

Type of ElectricDrive

MOTOR COMPATIBILITY

TYPICAL POWER RANGE(hp)

SPEED REDUCTION(typical) =

Maximum Speed

Minimum Speed

CONTROL OPEN LOOPCAPABILITY(no feedback)(Note: Can be improvedwith feedback controls)

ADAPTABILITY OF MOTORTO HOSTILEENVIRONMENTS

EFFICIENCY RANGE• for system: drive & motor

TORQUE hp• Constant• Variable• Control Method

VOLTAGE RANGE

Variable VoltageInverter (VVI)

• Squirrel-cage induction or synchronous

• Can handle motorssmaller than inverterrating

1 – 1,000

10:1

5%

Good

88 - 93%

YesYes

Pulse Width ModulatedInverter (PWM)


• Can handle motorssmaller than inverter rating

5 – 5,000

30:1

5%

Good

85 - 95%

YesYes

600

Current SourceInverter (CSI)


• Can handle motorssmaller than inverterrating (at reduced rating)

50 – 5,000

10:1

5%

Good

88 - 93%

YesYes

DC Drive

Commutated DC

0 – 10,000

20:1 open loop200:1 with tachometer

0.1 - 5%depending upon feedbackmethods

Poor due to highmaintenance of motor

90 - 94%

Yes

Field voltage, armaturevoltage or both

Wound Rotor withSlip Energy Recovery

Wound rotor induction

400 – 20,000

5:1

2 - 5%

Medium

92 - 96%

Rotor current


ASD tables 2/12/01 10:26 AM Page 25

TABLE 1. Comparison of Adjustable Speed Drives (cont’d)


No

Yes

Good

No

Simple

The output speed is variedby controlling the magneticcoupling between tworotating members. This isdone by means of a fieldwinding which controls theclip between them.


MULTIPLE MOTORCAPABILITY (e.g., two 200 hp motors on a single400 hp drive)

SOFT STARTING

Power Factor to Motor (PF)

OUTPUT SYSTEMSHARMONICS(dependent on leakagereactance)

COMPLEXITY OF:• POWER CIRCUIT• CONTROL CIRCUIT

PRINCIPLE


Yes, unlimited withininverter rating

Yes

Better than CSI (*2)

Drops with speed

Worst

SimpleSimple

The inverter receives DCpower from an adjustablevoltage source and adjuststhe frequency.


Yes, unlimited within inverterrating

Yes

Near unity (excellent)

Least

SimpleComplex

The inverter receives DCpower from a fixed voltagesource (diode rectifier) andcontrols voltage andfrequency. The RMSvoltage amplitude is fixed,but the width of voltageintervals is varied.


No

Yes

(*2)

Drops with speed

Better than VVI

SimpleSemi-complex

The inverter receives DCpower from an adjustablecurrent source and adjuststhe frequency and voltage.The DC current regulator iscontrolled by a closed loopspeed regulator.

DC Drive

Yes, with manufacturer’sengineering for load sharing

Yes

(*2)

Yes

SimpleSimple

Speed is adjusted bychanging field voltageand/or armature voltage.


No

Yes, if starting resistorsused

Relatively low (can beimproved with capacitors)

Yes

N/ASimple

Changes current in rotorcircuit by means of arectifier and converterconnected to rotor winding.Energy recovered is usuallyfed back into power supply.





N/A

N/A

Field between rotatingmember

No

Poor

No

Small controller; largerotating element

• Low costs• Simple compact control• Wide constant torque

speed range

• Efficiency low at lowspeeds

• Lack of reversingcapability

• Limited speed range• Maintenance of brushed

is required


CIRCUIT PROTECTION• Inverter Open Circuit

• Inverter Short Circuit

CONTROL VARIABLE

REGENERATIVE BRAKING

REVERSE CAPABILITY

RIDE-THROUGHCAPABILITY

SIZE & WEIGHT

MAIN ADVANTAGES

MAIN DISADVANTAGES


Inherent voltage limit

Must be carefully designedto handle DC bus capacitordischarge

Motor voltage, frequency

Option with added circuitry

Yes

Difficult

Intermediate

• High output frequencies(higher than 60 Hz ifnecessary)

• Can be retrofitted toexisting fixed speedmotor

• Soft start

• Harmonics increaselosses in motor

• Standard inverter cannotoperate in a regenerativemode



Same as for VVI, exceptPWM circuit is very fastacting

Motor voltage and frequency

Option

Yes

Yes, using battery orcapacitive storage

Small

• Excellent power factor;harmonics are minimal

• Can be retrofitted toexisting fixed speed motor

• Soft start

• Motor is subject to voltagestresses

• Complex logic circuits


Requires careful design

Inherent current limit

Motor voltage, frequencyand current

Standard

Yes

Difficult

Large

• Short circuit and overloadprotection due to currentcontrol of regulator

• Soft start

• Instability may resultunder partial loading

• Harmonics increaselosses in motor

• Difficult to retrofit toexisting fixed speedmotor drive

DC Drive


Inherent current limit

Motor armature voltage,current and/or field voltage(not common)

Option

Yes

Special applications only

Intermediate

• Simple system• Wide speed range• Soft start

• Brush and commutatormaintenance is high

• Limited to medium andlower speed applications;special motor enclosuresmay be specified if higherspeed capability isrequired (TENV, TEAO)


N/A

N/A

Rotor current

No

No

No

Small

• Costs are relatively lowfor narrow variable speedranges

• Simple circuitry• Adaptable to existing

wound rotor motors

• Maintenance of brushesis high

• May pose problems inhazardous environments

• Relatively low powerfactor

• Limited speed range• Regenerative braking n/a





• General purpose forequipment normallyoperating at full speed

• Fans• Pumps• Blowers• Fluid propulsion systems• Driving extruders


MAIN DISADVANTAGES(cont’d)

APPLICATIONS• General

• Specific


• Lower horsepower rangestypically

• General purpose low-medium horsepower(<500 horsepower),multiple motor control

• Conveyors• Machine tools• Pumps• Fans


• High initial cost

• Best reliability AC type, atadded cost

• Also suitable for mostapplications

• Slow speed ranges• Conveyors• Pumps• Fans• Packaging equipment


• Only single motor control

• General purpose whenregenerative brakingwanted (hoists)

• Pumps• Fans• Compressors• Blowers

DC Drive

• Not suitable forhazardous environmentswhere explosive gasesmay exist

• Expensive, large motor• Power factor always poor

at low speed

• For applications with awide range of speedadjustment and a low-moderate starting torque

• Used for medium and lowspeed applications

• General purpose

• Extruders• Machine tools• Mine hoists• Cranes• Elevators• Rotary kilns• Rubber mills• Printing presses• Shakers (foundry or car)• Winches• Public transportation


• Used if speed range isnarrow (70%-100%) andreversing not required

• Large pumps & fans withlimited speed range

• Compressors• Kilns• Conveyors• Mixers

(*1) A totally enclosed motor is usually required because the ECC is normally used in close proximity to the driven machine (e.g., machine tools).(*2) The VVI, CSI and DC drives have power factors that decrease with speed. For the AC inverters, this can be corrected by implementing a diode and chopper control.

This will slightly increase acoustical noise and slightly reduce efficiency.

N/A Not Applicable



• See Table 2 on the following page for a general guideline list ofstandard and optional features for AC variable frequency drivesand new power electronic devices. Note, however, thatmanufacturers may differ on some factors.

Chapter 5: Standard and Optional Features 33

C H A P T E R 5

STANDARD AND OPTIONALFEATURES

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TABLE 2. ASD and Electronic Motor Features

ASD StandardProtection Features

Overvoltage

Undervoltage

Overcurrent

Loss of control power

Across-the-line start

Line-to-line shorts on output

Line-to-ground shortson output

Continuous overload

Locked rotor

Motor single phasing

ASD OptionalFeatures

Soft start

Overload protection

Torque limit

Power outage ride-through

Brake stop

Coast stop

Bypass

Motor slipcompensation

Electronic reversing

Voltage boost (at start)

Accel/decel

Regenerative powerprotection

Low speed jog

IR compensation

New PowerElectronic Devices

Metal oxide semi-conductor (MOS)controlled thyristors(inverter switches)

Insulated-gate bithyristors (IGBT aremore capable of rapidenergizing)

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• Electronic AC or DC adjustable speed drives have a number ofadvantages over mechanical, hydraulic and fixed speed drives.They include a continuous speed range from 0 to full speed,improved process control, improved efficiency and potentialenergy savings, enhanced product quality and uniformity, softstarting/regenerative braking, wider speed, torque and powerranges, short response time, equipment life improvement,multiple motor capability (except CSI), easy to retrofit (exceptCSI), bypass capability, increased productivity, safe operation inhazardous environments, reduction in vibration and noise level,re-acceleration capability, reduced maintenance and downtimeand operation above full load speeds.

• Motor diagnostics are available in feedback controls.

SPEED CONTROL

• ASDs are used to control production speed in conveyorsystems in the food, paper, automotive, and consumer goodsindustries. In mining, ASDs are used in crushers, grinding mills,rotary kilns, presses, rolling mills, and textile machinery.

Chapter 6: Advantages 35

C H A P T E R 6

ADVANTAGES

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POSITION CONTROL

• ASDs are used for machine tools.

TORQUE CONTROL

• ASDs are used for tensioning (winders).

HIGH ENERGY SAVINGS POTENTIAL

• Applications with highest energy savings potential arecentrifugal pumps and fans (power is proportional to speedcubed), pumping applications (municipal water systems,centrifugal chillers, chemical/petrochemical industries, pulp andpaper plants and food industries) and replacing damper controlsin air handling and ventilation applications.

SOFT STARTING/REGENERATIVE BRAKING

• When a constant speed drive starts up, the surge of inrushcurrent that moves the motor out of its stationary position isabout six times the ordinary current, thus producing muchstress on the equipment, especially the windings.

• With adjustable frequency drives, acceleration times can beadjusted from instantaneous up to several minutes, thusproviding soft starting capabilities.

• Regenerative braking is used when the rapid reduction of motorspeed in a controlled manner is needed for production or safetyreasons. It is a form of dynamic braking in which the kineticenergy of the motor and driven machinery is returned to thepower supply system. The motor becomes a generator whenthe driven load is applying torque in the reverse direction.


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EQUIPMENT LIFE IMPROVEMENT

• The soft starting feature reduces water hammer and cavitationsituations for fluid systems to prolong equipment life.

• Operation of motors, transformers, cables, pump seals, pipes,valves and impellers may be prolonged.

• Soft starting reduces inrush current and voltage drop duringstarting and therefore also reduces stresses on windings,starting currents and heating.

MULTIPLE MOTOR CAPABILITY

• One multiple motor ASD (except CSI) can control a number ofsynchronized motors at the same speed (e.g., in the textileindustry).

BYPASS CAPABILITY

• The adjustable frequency drive can be for service, without needto shut down the driven equipment (with additional circuitryoptional).

SAFE OPERATION IN HARSH ENVIRONMENTS

• Adjustable frequency drives offer safe operation in harshenvironments since the drive can be housed in a remote location.

TEMPORARY OR BACK-UP OPERATION

• Instead of operating a second pump or fan for temporaryservice when extra pressure or flow is required, use a largercapacity single pump or fan under ASD control to meet theEXACT requirements at ALL times.

Chapter 6: Advantages 37

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REDUCTION IN VIBRATION AND NOISE LEVEL

• Vibration and noise level are reduced when the operating speedof the equipment is lowered and because valves or vanes areeliminated.

RE-ACCELERATION CAPABILITY

• Some adjustable frequency drives continue to have power supply during power losses of short duration, whereas fixedspeed devices would trip out.

TIPS AND CAUTIONS

• If using multiple motors, each one must be protected by itsown overload relay. The total current drawn by all the attachedmotors must be equal to or less than the current rating of thecontroller.

• Equipment life will be prolonged only if the proper precautionsare taken for power conditioning. Poor quality power can causeoverheating, insulation damage and even equipment destruction.

• Consider torsional harmonics. Avoid operating at speedscoincident with rotating equipment natural frequencies(resonance).


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HOW TO SELECT AN ASD

• Use this section as a general guide. The information provideddoes not address differences in types of driven equipment.

• Essentially, selecting an ASD involves matching theperformance of the ASD to the needs of the motor and load.

- Determine the need for speed or process flow control.Without varying speed requirements, equipment may simplybe oversized for the needs of the process, if present throttlingdevices are frequently on.

- Describe the range of speed control. An ASD offers acontinuous range from 0 to full speed. If only a few selectoperating points are required, a multi-speed motor may be abetter choice.

- Estimate the process duty cycle (see Figure 18). Duty cycle isa listing of the process operating points (for example, fanpressure and flow) and the duration each point occurs. This isperhaps the most important part of assessing the need for anASD in a particular application. The duty cycle characterizesthe process being served by the motor.

Chapter 7: Application Considerations 39

C H A P T E R 7

APPLICATIONCONSIDERATIONS

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- Gather equipment performance data. Performance curvessupplied by the equipment manufacturer describe the powerrequirements of the driven equipment at selected operatingpoints. It is necessary, however, to check that the “as installed”performance matches that of the performance curves. Otherwise,improper performance selection of the ASD may result. Also notethat performance ratings and field ratings may differ. Considergetting the help of a qualified installation and set-up contractor toverify field performance.

- Operating points are the intersection of the particular processsystem curve and the equipment’s characteristicperformance curve.

- System curve is the set of points that describes the volume offlow and resistance to flow as defined by the application.

- Throttling, or dampers, change the system curve byincreasing the resistance to flow.

- Performance curve is the set of points of flow vs. pressurethat the particular fan, pump or blower must follow at aparticular speed and fluid density. Manufacturers usuallysupply performance curves that give the selected designpoint.

- Brake horsepower and efficiency vs. flow are also supplied bythe manufacturer. They determine the motor and anygearbox or belt sheave reduction necessary to achieve thecorrect speed.

- Calculate constant and ASD power requirements. Using theformulas in Appendix A, calculate the power required foreach operating point in the duty cycle for constant speed(throttling flow control) and adjustable speed cases.


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- Calculate energy consumption. Multiply the power requiredat each operating point by the annual hours at the point fromthe duty cycle, then sum the total for constant and adjustablespeed.

- Select a drive type and features and estimate costs. Based onthe load type (constant vs. adjustable torque, horsepower,starting time, speed regulation, speed, torque range,regeneration, shielding, transformers, installation, controllogic and other specific features listed in this guide), select thetype of drive for the application. Obtain manufacturers’quotes. Prices will depend greatly on whether you need acustom-designed ASD or an off-the-shelf model.

- Calculate simple payback (based on energy savings alone).Total the cost to install a drive. Multiply the estimated annualenergy savings (adjustable vs. constant speed) by the utilityenergy rate charge. Divide the total installed cost by annualenergy savings. The result is simple payback in years.

- Consider other ASD savings, such as reduced wear due tosoft start, lower maintenance costs and less material wastageresulting from more accurate speed adjustment. Thesesavings are difficult to estimate and can usually bedetermined only through ASD operating experience.

- Note:Measure power in kW, not kVA.Use power meters, not ammeters.Power factor must be measured. kW = kVA x p.f.Check that phases are balanced in a three-phase system.(Do not assume three phase = 1.73 x single phase.)


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SOFTWARE

FINANCIAL EVALUATION

• Software is available from several ASD suppliers, includingsome utilities. Be careful to include lower part-load efficiencieswhen inputting performance data.

LOAD CHARACTERISTICS

Varying Duty Cycle

• The load profile or duty cycle will also indicate the potentialsuitability of an ASD for an application. The duty cycle showsthe typical speeds and corresponding time intervals for which amotor operates annually. From an energy standpoint, theingredients of a good ASD application are high percent throttling(changing load) and high annual operating hours.


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FIGURE 18. Duty Cycles

APPLICATION TYPES BY LOAD

• There are three main types of adjustable speed loads: variabletorque/variable horsepower (hp = torque x RPM) (centrifugalpumps, fans), constant torque and constant horsepower,(constant tension winders, machine tools).


100

0Time

% F

low

100

0Time

% F

low

GoodApplication

PoorApplication

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• The behaviour of the horsepower and torque as a function ofpercent speed partially determines the requirements of themotor/controller system.

• For an induction motor, the speed-torque relationship dependson the voltage and frequency of the supplied voltage as well asthe characteristics of the rotor conductors.

• Constant torque drives are often supplied as “standard” drives.To make a variable torque drive, the manufacturer usually addsa jumper and chopper to the standard model.

• Examples of variable torque loads are centrifugal loads, wheretorque is proportional to RPM2, where horsepower isproportional to RPM3 such as fans, pumps and blowers(dynamic).

• Examples of constant torque loads are agitators, positivedisplacement compressors, conveyors (belt, batching, chain,screw), crushers, drill presses, extruders, hoists, kilns, mixers,packaging machines, positive displacement pumps,screwfeeders, roll out tables and winders-surface. Note thatsome may not be constant torque loads but require constanttorque drives due to shock overloading, overload or high inertiaload conditions.

• Examples of constant horsepower loads are drilling machines,lathes, machine tools, milling machines and centre-drivenwinders. Note that torque is inversely proportional to speed.


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100

80

60

40

20

Percent hp and Torque

10050Percent Speed

0

Torque hp

Percent Speed

100

80

60

40

20


100500

Torque

hp

FIGURE 20. Constant Torque Load

FIGURE 21. Constant Horsepower Load

FIGURE 19. Variable Torque Load

Percent Speed

100

80

60

40

20


100500

Torque

hp

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TIPS AND CAUTIONS

• The variable torque controller is designed to provide 100%rated torque continuously with no overload capability. Thisshould be used only for applications where the load torquevaries proportionally with speed, such as fans and centrifugalpumps. The current rating of the motor must be checked with


Flow

Pre

ssur

e

Outlet Damper Control

System

Performance

Flow

ASD Control

UnstableArea

Inlet GuideVane Control

Flow

Pre

ssur

e

Valve Control

System

Performance

Performance

Sta

ticD

ynam

ic

Flow

ASD Control

System

FIGURE 22: Power Required is Proportional to RPM3

Centrifugal Fan/Blower, Pump

Pump

Fan/Blower (incompressible flow)

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the current rating of the controller to ensure that the controllercan provide the full horsepower capability of the motor.

• Low speed motor cooling does not limit the speed range with avariable torque load since the load requires less torque at lowerspeeds. For this type of load, it is important to choose ahorsepower rating for the highest speed attained.

• The minimum allowable motor speed for continuous constanttorque or constant horsepower operation is determined by themotor cooling requirements at low speeds. These methods canbe used to increase the motor’s constant torque speed range:

- Use a separate blower for motor cooling.

- Use an oversized motor, and operate it at less than itsnameplate rating. This provides additional mass for heatdissipation. However, this may result in oversizing the driveto compensate for the increased magnetizing current.

- Use a motor with a high service factor. Specify class F or Hinsulation.

- Use a high efficiency motor.

• Also, see “Thermal Considerations.”

• Torsional harmonics may occur if resonant frequencies coincidewith reduced speeds. These can be programmed out bythe ASD.

• Low speed operation can cause mechanical instability if itresults in operating too far up the fan/pump performance curve(the unstable region before peak pressure).


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• Multiple fan/pump systems will run at the same pressure if in parallel operation. So, do not put an ASD on only one ofparallel pumps or fans.

• Sizing the drive means matching torque, speed, voltage, currentand horsepower to the load and motor requirements.

• The cost for custom-engineered applications (mostly DC,synchronous or wound-rotor motors with slip energy recovery,load-commutated inverters) will be higher.

• ASDs are generally selected for their speed control capability,not specifically for energy savings. Energy savings are achieved,however, when process control dampers or throttling valves orrecirculation lines are replaced by higher efficiency ASDs.

• ASDs offer the best potential for energy savings when controllingthe speed of centrifugal fans, pumps and blowers. The powerrequired is proportional to RPM3. Therefore, a 10% drop in speedresults in a 27% drop in power consumption (1.0-0.93).

FIGURE 23. Power Savings in Fans and Pumps Using ASDs


Pow

er R

equi

red

DamperControl

Speed

ASDControl

Saving

ASD body (0-82) 2/12/01 9:52 AM Page 48

• Demand savings are not attributable to ASD control, however,since achieving better speed control does not usually result indownsizing absolute power requirements. There may be “timeof use” demand savings (taking advantage of reduced speedoperation during utility peak demand periods).

• In-rush current is about 600% rated current when started at fullvoltage and frequency. If the motor is started at low voltage andfrequency through an ASD, it will never need more than 150%of rated current (started at 2 Hz). This soft start reduces stresseson the motor, extending its life.

MOTOR/DRIVE SYSTEM

• If, after examining the load characteristics and processrequirements of an application, it appears that an ASD may bean asset, investigate motor/drive compatibility.

• If a drive is to be retrofitted to an existing motor, get thisinformation from the motor: nameplate voltage and horsepower, current and torque data, insulation class andNEMA design characteristic.

• Manufacturers’ curves should be consulted to aid in motorselection for new systems.

• When considering the information here, also look at Table 1,because the table lists typical applications for each of the drivesand may help you narrow the choices available for a particularapplication. It should be used when conducting the remainderof the selection process.


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Motor Type

• Your choice of available drives depends to a large extent on themotor used. Although DC systems were largely used in thepast, AC motors are much more popular now due to theirrelatively low cost, low maintenance requirements and betterreliability. For most low- and medium-speed applications,squirrel-cage AC induction motors are now used.

• Variable Speed Brushless DC “Electronically Commutated”motors are available in ≤600 horsepower sizes.

Horsepower Rating

• Induction motors are best suited for power levels up toapproximately 500 horsepower (325 kW), although they can be used for higher power levels. Above 1,000 horsepower,synchronous motors are often used and are usually driven bycurrent source inverters or by load-commutated inverters orcycloconverters. These high-powered systems are veryexpensive to purchase for use in the lower end of their operatingranges. Medium Voltage AC induction motors are now availableunder ASD control.

• It is important to determine the maximum horsepowerrequirements of the driven load and how the required powervaries with speed.


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Voltage Requirements

• These are the size ranges usually available for AC variable frequency drives:

Horsepower Range Voltages Available

<50 208V to 600V three-phase

50-200 460V to 600V three-phase

200-1,000 low voltage (460V, 600V) and medium

voltage (2,300V, 4,160V)

1,000-2,500 mostly medium voltage

(2,300V, 4,160V)

*2,500-10,000 medium voltage

(4,160V, 6,900V, 13,800V)

*>10,000 13,800V

(usually DC, or wound rotor)

• Note that suitably rated transformers may be used to match thedrive voltage rating to that of available power supply voltages.

• The system voltage should be within the deviation permittedby the specifications for the ASD. This is usually +10% and-5% per NEMA standards. Specific values can be obtained fromthe manufacturer.

Torque and Current

• After checking the horsepower requirements, ensure that thestarting torque and full load torque are within the motor’s rating.

• Continuous permissible running torque decreases with motorspeed.


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• It is important to ensure that the drive can supply the requiredcurrent. Inverters are current-limited and may only allow arelatively high output current for short time periods. Anestimate of the motor torque to current ratio can be made byreferring to the motor speed, torque and current characteristics.

• The drive must have a maximum continuous current rating thatis greater than or equal to the motor’s full-load current rating.

Speed and Speed Range

• Consider the minimum and maximum speed requirements.

• The speed range depends on the motor used. A standardefficiency, class F insulated motor is applicable only to a 2:1constant torque speed ratio. A high efficiency motor canprovide a 3:1 ratio. To obtain wider speed ranges, the motor can be oversized.

• Below 6 Hz, however, significant motor cogging may occur asthe motor tries to follow the waveshape. A practical speedrange of 10:1 below 60 Hz is suggested for VVIs and CCIs.(This is not a concern for PWMs.)

• If precise speed control is needed, a synchronous orsynchronous reluctance motor can be used for an AC system.Otherwise, a DC system could be used.


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FIGURE 24: Motor Derating Curves vs. Speed Range When Applied to Adjustable Frequency AC Drives

(6-Step Waveform or PWM)

• The speed range of an AC motor can be extended in using adrive above 60 Hz, provided the V/Hz ratio is maintained. Themotor is rated at V/Hz; as speed increases at constant ratedtorque, the horsepower output increases. The drive must besized to accommodate the horsepower rating as well as motorcurrent and voltage.

Speed Regulation

• Mechanical loads cause a drop in motor speed (according to itsspeed/torque curve).

• Tachometers can monitor motor shaft speed through a feedbackloop to the drive controller, which sends a compensation speedincrease signal to the ASD.


2:1 30-60 Hz

20:1 3-60 Hz

10:1 6-60 Hz8:1 7.5-60 Hz6:1 10-60 Hz4:1 15-60 Hz3:1 20-60 Hz

1 20 40 50 75 100 125 150 200 250

Motor Horsepower – 60 Hz Rated

Speed RangeP

erce

nt (

%)

of 6

0 H

z T

orqu

e R

atin

g

30

40

50

60

70

80

90

100

0

Induction Motor:Constant Torque Load,USEM 4-P TEFC 460 V30 60 Hz Motor WithBoost At Low Frequency.

Source Data:EIC Program Based onConstant Temp. Method

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• NEMA design B is the most common standard duty AC motor.Speed can be held within 3% of setpoint (which is motor slip).

• Thyristors are limited in their switching speed, whichdetermines ASD speed regulation capability.

Time required to accelerate the load:

T(sec) = WK2 (lb-ft2) x change in RPM308 x torque (lb-ft)

(load inertia: WK2 total = sum of WK2 components, W is weight, K is radium of gyration.)

Torque (lb-ft) = HP x 5250RPM

THERMAL CONSIDERATIONS

• If variable frequency controllers are used, there are a number ofimportant factors to consider to ensure that the motor/drivesystem is compatible from a thermal standpoint.

• The main concern when retrofitting existing motors with variablefrequency drives is to ensure that the controller can provide thecurrent required for the load torque to prevent motor overheating.

• Since the cooling systems of most motors are designed for afixed speed, the cooling action will be reduced when operatingat reduced speeds (since cooling fan speed decreases withmotor speed). This is especially true for constant torqueapplications and applications in which CSI drives are used. Forthese situations it is important to provide additional cooling oroverframe or derate the motor. An overframed motor may alsorequire a larger controller. See “Tips and Cautions.”


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• NEMA type 1 vented enclosure to dissipate ASD heat within.Ambient limits as specified.

• It is also important to ensure that the motor will not overheatbecause of the harmonics in the AC waveform supplied by theinverter. This is especially true for standard motors. (SeeChapter 9, “Harmonic Distortion.”)

• Harmonic losses are affected by the design type of NEMAspeed torque characteristics as well as the characteristics of the motor under consideration. The motor leakage reactance,which limits harmonics, varies with each NEMA design. Thecompatibility of variously rated motors with inverters is usefulto know. See Table 3 for the most suitable motordesign/inverter combination to use.

TABLE 3. Suitability of Inverters for NEMA Motor Designs

Motor NEMA Design VVI CSI PWM

High Efficiency Motor

A X

B X X X

C* X X

D*

* These motors are very undesirable for adjustable frequency control, due tohigh harmonic losses.

• NEMA design B squirrel-cage induction motors are commonlyused in industry.

• Energy efficient motors have lower losses than standard motorsand therefore provide wider torque capability when used withvariable frequency drives.


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OTHER CONSIDERATIONS

• The next step in the decision process is to evaluate the relativeimportance of each of the remaining factors to be considered.One of these factors may exclude one drive system. Forexample, if the system is to be used in an explosiveenvironment, commutators and brushes cannot be usedbecause of the sparks that would be generated.

• These are some other selection considerations: economics,process requirements and load characteristics, performancerequired (speed regulation/control accuracy, efficiency andreliability), starting and stopping characteristics (load inertia),torque (breakaway torque, accelerating time and torque anddecelerating time and torque), environment, weight and space,maintenance, programmability needed, lead time for delivery,line power factor and mechanical considerations.

• Process requirements and load characteristics were discussed atthe beginning of this chapter. Although initially used asindicators, the importance of these factors should now becompared with all other considerations.


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EFFICIENCY

• At full speed and full load, VVI, CSI and PWM drives are allabout 95% efficient. Efficiency drops at approximately a squarerate with speed, as commutation losses (thyristor closing) varywith torque and current.


.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

450 900 1350 1800

Driv

er L

osse

s (k

W)

Speed (RPM)

PWM

VVI

CSI

74%

78%

82%

86%

90%

94%

100%

0 20 40 60 80 100

Per

cent

Effi

cien

cy

Percent Speed

75% Load 100% Load

50% Load

25% Load

FIGURE 25. Watts Loss (Efficiency) Comparison

FIGURE 26. Typical AC Drive Efficiency (PWM)

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• CSI drives tend to be more efficient than VVI and PWM asspeed is reduced.

• Higher horsepower sizes, as well as drives operating close totheir maximum design rating, tend to be at higher efficiency.

• Information about efficiency of drives is generally not easilyobtained from manufacturers since so many factors affect it.

• Motor efficiency at reduced speed needs to be recalculated.

RELIABILITY OF ASDS

• Reliability of ASDs has improved as power electronicstechnology has advanced. Thyristors convert to AC to DCpower and GTO designs improved reliability. Metal oxide semi-conductor controlled thyristors, surface mount technologyand specific integrated circuits are reducing drive sizes.

• Voltage drop temporary “ride-through” (see Harmonics section).

• Current rise or drop limits are features specified.

• Sizing the controller to handle required load currents is important.

• Motor heating at low speeds will not be a problem withcentrifugal loads due to the drop in motor current and I2Rlosses.

• CSI drives use the motor as part of the circuit, so selecting themotor and drive together will minimize risk of mismatching.

• Transistors can be made for high current and voltage and fasterresponse than thyristors.


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• Constant voltage/frequency ratio means the motor will not stallwhen overloaded, maintaining constant speed regardless of load.

• The motor may trip out when decelerating rapidly. With largeinertia loads, regeneration of power back through the drive may trip the voltage protection bus. Elevators and loweringconveyors are examples. Sizing the protective bus to suit theapplication should prevent it, (see recommended technicalspecifications for medium voltage drive).

APPLICATIONS

• Constant torque (hoists, presses and conveyors) operation up to120 Hz can be provided by applying constant V/Hz to themotor. This requires an AC drive with twice the voltage outputcapability than the supply voltage to the motor (@ 60 Hz).Since a motor is rated at V/Hz, it can be operated at ratedtorque and twice the speed if voltage and frequency are bothdoubled. Operation at twice the motor-rated horsepowerrequires sizing the AC drive at that horsepower and consideringstresses and balancing on the motor.

• Position control is important in materials handling, machiningand robotics.

• Multiple motor operation in parallel by a single voltage inverterAC drive can be done by sizing the drive to the sum of themaximum continuous running currents of each motor. Allmotors start and stop together. If motors are coupled togetherthrough the load, load sharing must be considered. High-slipNEMA design D motors may be required. Also, individualmotor overload protection is necessary.

• Cogging refers to torque pulsation at below 6 Hz frequency. Ifsmooth operation is needed at low speed, it may be necessary


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to use a six- or eight-pole motor with a 90 Hz or 120 Hzmaximum frequency, (eliminated with vector drives).

• IR compensation is a circuit that senses changing motor loadand reduces voltage boost when the motor is lightly loaded.This improves starting torque and low speed overload capability.

• Regenerative braking occurs when the motor acts as a generatorwhen driven by the load. The energy is returned to the powerlines through the drive. The drive must be sized to handle theenergy absorbed. Hoists, flywheels and other constant torqueapplications make use of regenerative braking. Centrifugal loads,such as fans, pumps and blowers, do not.

PERFORMANCE REQUIRED

Speed Regulation/Control Accuracy

• The importance of the drive’s sensitivity to changes in load,temperature, humidity, drift and line voltage fluctuations shouldbe determined.

• If there is to be no speed deviation, a synchronous motoris used.

• Vector drives can smoothly hold position and speed and torqueover a full range from 0% to 100% of scale.


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Efficiency

• System efficiency = mechanical power output from motor shaftelectrical power input to drive

• The efficiency of a motor/drive system depends oncharacteristics of the connected motor (power factor, efficiency),speed range and duty cycle, load, measurement method andinstrument accuracy, inverter size and horsepower rating, inputpower tie voltage variation and manufacturing variations.(Sometimes, it’s better to use a high efficiency motor.)

• The motor design and specific operating points are the largestcontributors to efficiency differences.

• High efficiency motors are more susceptible to tripping due toheat, voltage or current drops.

• Multi-speed motors (i.e., pole changing motors) offer fixedspeed combinations (two to four is typical) that are a muchcheaper alternative to ASDs if continuous speed adjustment isnot needed.

• A more important consideration is:

Energy Lost = Output Power – Input Power

• Higher horsepower drives tend to have higher efficiencies.


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• The CSI controller tends to maintain better efficiency than otherinverters as operating speed is reduced.

STARTING AND STOPPING CHARACTERISTICS

• Are soft starts or controlled acceleration needed for the drivenmachine?

• Does the power supply system need reduced voltage starting orcontrolled acceleration?

• Does the driven machine require accurate positioning,controlled deceleration or regenerative braking?

TORQUE

• The ability of the drive to reach the torque required at variouspoints in the process cycle should be considered.

• Breakaway or locked rotor torque is needed to start the loadfrom rest to overcome static friction. An ASD can provide avoltage boost that will permit this torque to be higher thannormal. The inverter components may have to be sized largerand the current limit set higher if this is the case.

• Accelerating time and torque is needed to increase the speed ofthe machine. An ASD permits short or long accelerating times.For high inertia loads, such as machines with flywheels or large blowers, care must be taken to ensure that the system canprovide enough accelerating torque.


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• Decelerating time and torque is needed for high inertia loads.An ASD permits long or short deceleration times.

ENVIRONMENT

• Abrasive, moisture laden, explosive, dusty or otherwise difficultenvironments may affect the ability of the drive/motor systemto function and the ability to provide adequate maintenance.The effects can be eliminated by careful design or locating thedrive in a clean, cool room. Providing an adequate cooling airsupply for air-cooled converters is another importantconsideration.

WEIGHT AND SPACE

• There may be space and economic considerations involved indecisions concerning large drives due to their size.

FIGURE 27. Motor Performance, Typical 60 Hz


0

50

80

100

150

6 15 30 60 90 120

Mot

or T

orqu

e 7

hp (

Con

tinuo

us)

% M

otor

Ful

l Loa

d R

atin

g

Frequency (Hertz)

ExtendedSpeed Range

Standard OperatingSpeed Range

Torque

High Eff. Motors

1.15 SF Motors

hp

hp

Torque

hp

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FIGURE 28. Ideal Torque-Speed Curves

FIGURE 29. NEMA Design B Motor Torque-Speed Curve

ACCESSORIES

• Accessories include auto transformers (for voltage overloadprotection), regenerative braking circuits (overhauling loads inconstant torque such as cranes), bypass loop (for operating themotor directly bypassing the drive), filters and the line chokes


0

100

200

10 20 30 40 50 60 70 80 90 100Percent Speed

Load Torque

Slip

OperatingPoint

Pull Out orBreakdown Torque

SynchronousSpeedOperating Speed

NEMA Design B MotorTorque vs. SpeedLocked

Rotoror StallTorque

PercentTorque

102 20 30 40 50 60Frequency (Hertz)

Per

cent

Tor

que

100

200

300

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(to reduce electric harmonics), cooling fans and programmablelogic controllers (PLC) for speed control through feedbackprocess monitoring.

SAFETY

• Follow NEMA recommended enclosure design and installationspecifications. High voltage and current are present. DC poweris more dangerous than AC; DC is found in both AC and DCdrives. The DC bus can be more than 600V in AC drives if inputpower is not checked in harmonic spikes. All circuit boardsshould be covered in metal for shielding and cooling. Accessinterlocks should shut down and disconnect the drive inputpower before the cabinet can be opened. Manual control panelsare restricted to 120V.

• It is important to separate control wiring from power wiring.Use separate metal conduits to reduce electronic “noise” frompower to control circuits (see Chapter 9).

SERVICE AND MAINTENANCE

• AC and DC drives can be located several hundred feet away fromthe motor where heat, humidity and contaminants can be controlled.

• DC motors require commutator and brush replacement periodically,(except brushless DC types).

• Installation is usually a simple matter of three-phase electricalconnection to the motor and power lines.

• Solid-state electronics are relatively maintenance free. Most drivemanufacturers supply built-in diagnostics as well as protectionrelays and fuses. As proper drive performance depends onmatching motor and load requirements, expert trouble-shooting


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may be necessary. For this reason, consider purchasing a servicecontract until you have experience with the particular drive.

• The frequency and degree of complexity of maintenancerequirements for a particular system can be a significant factor.Are company personnel restricted to the type of equipment theyservice? Are self-diagnostics supplied with the control module? Isthe supplier willing to service the drive after purchase? Does thesupplier have representatives located close to you? Purchasingequipment from a supplier who has no representatives close tothe installation may result in the supplier losing interest ininstallation and maintenance after the purchase is made.

• Mechanical devices that provide adjustable speed need moremaintenance than their electrical counterparts.

• Servicing a PWM inverter requires a complex diagnostic aidequivalent to a logic analyzer. CSIs and VVIs, on the otherhand, are easy to service since each part of the system can beoperated independently to isolate problems.

Programmability Needed

• Will it be necessary to frequently change the operating characteristicsof the drive, as offered by a PC or a drive equipped with amicroprocessor?

Lead Time for Delivery

• Lead time for delivery depends on whether the application requires asmall horsepower drive (the equipment may be purchased off theshelf) or if increased horsepower or custom-engineered features, suchas line filters, chokes and autotransformers, are needed. Drivesophistication or special options, such as cooling requirements,unique process control interface or packaging, tend to lengthen leadtime. Many applications require a custom-engineered drive system.


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Line Power Factor

• If a specific power factor is specified or limitations are imposedfor an installation, this should be considered. Normally, thepower factor is satisfactory.

Mechanical Considerations

• If adjustable speed is being considered, the natural torsionalfrequencies of the connected mechanical loads should bechecked to ensure that they do not correspond to the frequenciesproduced at lower operating speeds. This may still be a concerneven if the motor is mounted to a massive support pad.

TIPS AND CAUTIONS

• When a few discrete speeds are needed, a multiple speed motor may be satisfactory. It would be significantly cheaperthan purchasing a variable frequency drive. This can beaccomplished by using a special stator winding arrangement.This method is often used for applications involving pumps,fans, blowers, conveyors and printing presses. For example, a2:1 speed ratio is easily obtained from a single stator windingby reconnection.

• On new installations, an ASD can replace the standard motorstarter. All that is needed is a feeder breaker to protect thecables to the controller.

• If retrofitted, the existing motor is usually retained, but it mustbe derated.

• It is important to ensure that the electrical supply line uses thecorrect voltage. This is particularly important in view of the


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large number of electrical drives that are manufactured outsideCanada and where different standards may be used.

• Circuit breakers, transformers, fuses and disconnect switchesmay or may not be included in an ASD system. If thisapparatus is to be mounted separately, its current rating shouldbe based on the input current of the ASD, not the motor fullload current. This is due to harmonic effects which cause theASD input current to be greater than the motor full load currentfor a given power level.

• If a motor drives a load through a gearbox at reduced speedunder ASD control, it may not deliver enough running torque.Check minimum torque times speed requirements.

• Induction motors cause supply current to lag behind supplyvoltage. The ratio of kW to kVA (true power to apparent power)is the “displacement power factor.” This is the cosine of thephase angle between current and voltage, when current andvoltage are assumed to be clean sine waveforms.

• Harmonic currents from inverter switching may increase apparent power and decrease power factor. This “true powerfactor” will be less than displacement power factor cosine ofcurrent to voltage phase angle.

• A true RMS meter is required to measure non-linear loads and filter harmonic currents (usually 5th and 7th are most significant.)


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• Economics is usually one of the most important factorsinvolved in selecting industrial equipment, but it is notstraightforward. Many economic aspects are often ignored inASD evaluations.

• Use the Table 4 ASD checklist of costs and savings to avoidoverlooking certain economic aspects.

• The simple payback method is frequently used to determine howlong it would take for a piece of equipment to “pay for itself” interms of savings:

Number of Years =Total Initial Capital Cost (including any service)Total Annual Savings

• This method should only be used as a risk indicator, however,since it is very inaccurate and neglects the impact of inflation.ASD quotes of two- to three-year paybacks often underestimatethe true period until breakeven.

Chapter 8: Economics 69

C H A P T E R 8

ECONOMICS

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• The net present value method is a good technique that can beused to appraise the profitability of an investment. By usingthe discounted cash flow technique, it takes into account thetime value of money. A summary of this approach appears inTable 5.

TABLE 4. ASD Checklist of Costs/Savings


Capital Costs

Drive

Motor

Powerconditioningequipment

Installation

Electricalsystemupgrade

Torsionalanalysis

Spacerequirements

Cooling

CapitalSavings

Control valves

Gear box

Fluid coupling/mechanicalspeedchangingequipment

Reduced-voltage starters

OperationalCosts andSavings

Energy (totalenergyconsumed,peak demandcharge)

Maintenance/useful life/downtime

Overspeedcapability

Other

Salvage value

Taximplications

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TABLE 5. ASD Investment Decision Technique

For detailed examples of this procedure and relevant software, contact your localutility.

1. Evaluate the cost/savings of the factors in Table 4 for each option you areconsidering (for example, purchasing an ASD, purchasing a mechanicaldrive system, not purchasing a variable speed drive). Capital costs will beexpressed in total dollars; operating expenses will be expressed in terms oftime.

2. Determine the real discount rate that should be used for each time-dependent and future-valued factor. For example, for energy savingscalculations:

x% per annum = nominal discount ratey% per annum = rate at which electricity rates will rise

i% = {x/y – 1}%

As another example, a salvage value n years from the present should bediscounted using the rate at which the interest rate is expected to risebetween now and n years.

3. All factors for each option should be discounted to their present values,using the appropriate discount rate. The number of years used for time-dependent factors should be chosen as a reasonable payback period.Present value tables and annuity tables are useful for the discountingprocess.

4. The net present value (NPV) of each option is found by summing the costsand savings that have been calculated in present value terms for eachfactor.

5. For any option, if NPV >0, there is a net gainNPV <0, there is a net lossNPV = 0, breakeven occurs at the time

under consideration.

6. The option with the greatest positive value of NPV is the most profitable.

7. The procedure could be repeated assuming different total time periods.

8. A comparison between two options could also be made by using the relativedifference between the option for each factor and finding one NPV.


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ECONOMIC FACTORS

CAPITAL COSTS

Drive

• The cost of this major item will vary greatly, depending on the options required. The cost should include speed controls,start/stop controls, engineering, cable, conduit, foundations,spare parts and any related modifications. For example, abattery back-up for the controls may be provided for auto re-start or shut-down sequences.

Motor

• The cost of a motor must be considered for a new system.

Power Conditioning Equipment

• The cost of any power conditioning equipment, such asharmonic filters, should be included. This includes filters forincoming power to the motor as well as power conditioners for harmonic voltages and currents sent back to the powersupply from the drive.

Installation

• Installation, labour and commissioning charges for the drive andmotor and power conditioning apparatus should be determined.

Electrical System Upgrade

• Upgrading of the electrical system may be necessary if higherreliability is required than the present system can offer. Potentialupgrades include relay protective systems, supply transformerredundancy, transfer switching/alternate feeders, maintenance andemergency staff training and preventive maintenance programs.


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Torsional Analysis

• A torsional analysis will define the vibration effects of inverterharmonics in the drive train. It should be conducted for largedrive applications.

Space Requirements

• This includes the cost of any indoor space requirements for the drive and filters, as well as any outdoor space costs, such as those associated with transformers, filters or reactors.

Cooling

• Additional cooling may be required for drive installation. Forlarge applications, although HVAC equipment is often used,water cooling may be a much more economical alternative.

CAPITAL SAVINGS

• Use of an ASD may avoid certain capital investments. Examplesare gear boxes, control valves, fluid coupling/mechanical speedchanging equipment and reduced voltage starters.

OPERATING COSTS AND SAVINGS

Energy

• There may be savings in terms of both energy consumed andpeak demand charge. The extent of these savings depends onthe local utility’s rate schedule. If an ASD is installed, the totalenergy consumed will likely be reduced.

• The other element of electrical power cost is the demand charge,measured in kVA, which compensates the utility for the peakcurrent it must deliver during the month. The most significantfactor affecting KVA demand is the power required by the load,


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which varies with the cube of the speed. Because of this,adjustable speed drives may provide significant savings.

• It is important to keep in mind that the kilowatt-hours of energysaved are the last ones that would have otherwise beenpurchased. The use of average energy cost can be verymisleading.

Maintenance/Useful Life/Downtime

• The reduction of maintenance and downtime may be quitesubstantial if an AC variable frequency drive is employed.Contributing factors are elimination of control valves, current-limitfeature (prevents motor burnouts caused by multiple restarts) andprotection of the motor insulation (so it is shielded from voltageproblems).

• Useful life of equipment, such as bearings, can be extended byoperating at reduced speeds. Stresses and metal fatigue in thedrive train shafts will be lowered.

• Repairs to variable frequency drive systems do not usually takemuch time.

Overspeed Capability

• The overspeed capability of adjustable frequency drives cansave considerable operating costs, as well as investment, ifincreases in production levels occur. For example, the airflowthrough an existing fan can be increased by retrofitting avariable speed drive to its motor so that the motor is suppliedwith a frequency higher than an existing 60 Hz rating.


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TIPS AND CAUTIONS

• An adjustable frequency drive is the most cost effective choiceif the duty cycle is more evenly distributed over the entire rangeof flow rates.

• Relative energy savings improve if the performance and systemresistance curves are steep.

• Many potentially good ASD applications are passed up becausebenefits other than energy savings are overlooked. Frequently,however, process control and reliability far outweigh efficiency-related benefits to the user.

• By using the average cost of energy in savings analyses, thesavings can be significantly overstated for variable frequencyapplications. Instead, both the energy and demand charges ofthe local utility’s rate schedule should be used.

• For variable torque loads, the variable frequency drive is veryenergy conscious, since the horsepower varies proportionally tothe cube of the speed.

• Coupling systems (eddy current and hydraulic) have thequickest economic payback. Electronic drive systems have thehighest dollar return.

• Cost savings through reduced energy consumption often resultin ASD payback periods of five to six years, rather than the twoto three years normally required by industry.

• Induction motors tend to be cheaper than DC motors forsimilar horsepower ratings.


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• See Figure 27 for a relative purchase price comparison forvarious motor/drive systems. For explosion-proofenvironments, the relative cost of the DC motor would increase substantially.

• For horsepower applications above 50, installation costs areusually comparable to the total capital cost for the drive. Belowthis power rating, installation costs may be as much as doublethe drive cost.

• Software packages which evaluate the economic aspects ofadjustable speed drives are available. It is important to keep inmind that these programs require part-load efficiency withintheir analyses.

FIGURE 30. Capital Cost Comparison of Motor/Drive SystemsMedium HP, Voltages


Valve

Motor

EddyCurrent

Coupling

Motor

Controller

Motor

Controller

Motor

ValveControl

SlipControl Solid-state

Control

ACDC

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• Adjustable Speed Drives have defined capability to withstandvoltage and current waveform distortion.

• The latest IEEE standard 519 defines harmonic distortion limitsacceptable on the input side to the AC power system.

• For a full discussion of harmonic distortion and mitigationtechniques, see Ontario Hydro’s Power Quality Reference Guideand Power Quality Mitigation Reference Guide.

HARMONICS

• There are two types or harmonics: electrical and mechanical.

• The inverter (switching) section of an ASD generatesharmonics.

• Electrical harmonics cause waveform distortion. They arecurrents or voltages that oscillate at integer multiples of thefundamental 60 Hz frequency, which is the main powerfrequency. For example, a frequency five times the fundamentalfrequency is called the fifth harmonic.

Chapter 9: Harmonic Distortion 77

C H A P T E R 9

HARMONIC DISTORTION

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FIGURE 31. Harmonic Distortion

• If large, electrical harmonics may cause power systemwaveforms to deviate from perfect sinusoids, eg.: capacitorswitching, large induction, motors start-up.

• Any static power converter that converts AC to DC or DC toAC, or any solid-state switch, generates harmonics (e.g.,thyristors or SCRs).

• All adjustable frequency drives with power switching devicesgenerate harmonics.

• The odd harmonic amplitudes usually decrease withincreasing frequency, so the lowest order harmonics are themost significant. Even harmonics are normally not generatedby three-phase converters.

WHAT HARMONIC DISTORTION CAN DO

• As with many other forms of pollution, harmonic distortionaffects the whole electrical environment. It propagates through


Bus Voltage

Line Current

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the power system and may even show up at distant pointsoutside the plant, thus causing problems for other equipmentconnected to the power supply.

• Typical effects of harmonics on the motor/drive system arereduced motor efficiency due to increased losses, increasedheating of motors, cables and transformers, excessive voltagestress on insulation of motor windings and torque pulsations,(“torsional harmonics”).

• General problems caused by harmonics are general degradation ofpower quality, voltage dips or voltage ripple, prematureequipment failure, improper operation of important control andprotection equipment, interference with telecommunication orcomputer systems, amplification of harmonic levels resultingfrom resonance, incorrect readings on mechanical timing relaysand watt-hour meters and blown fuses.

• All capacitors, including those used for power factor correction,tend to be very susceptible to harmonic damage. Disastrousconsequences can occur if capacitors are exposed to excessiveharmonic voltages or currents.

• The harmonics produced by a converter may increase motorlosses by 5% to 10%.

PRODUCTION AND TRANSMISSION

• Harmonics are produced in utility or industrial electricalsystems by equipment that switches repetitively in less than acycle, such as variable frequency AC drives, cycloconverters,DC drives, rectifiers, UPS systems, arc furnaces and static VARgenerators. Fluorescent and gaseous discharge lamps can alsoproduce harmonics.


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• Harmonics occur as long as the harmonic generating equipmentis in operation. They tend to be of a steady magnitude.

• Harmonic currents flow through the impedance of thetransmission and distribution network and generate harmonicvoltages, which distort the electrical user’s input voltage.

• Harmonic currents often flow in the neutral line.

• The order and magnitude of the harmonics generated by a drivedepend on the drive configuration and the system impedance.

• Harmonics may be greatly magnified by power factorcorrection capacitors. Supply system inductance can resonatewith the capacitors at some harmonic frequency, causing largecurrents and voltages to develop. This may damage equipment.In addition, since the impedance of a capacitor decreases withincreasing frequency, capacitors tend to act as sinks for higher-order harmonics.

ISOLATION TRANSFORMERS

• Isolation transformers are frequently used to protect the drive aswell as the AC line from distortion. They may also decrease theavailable short circuit current in a fault situation and preventdrive shutdown and possible damage in the event of a motorline ground fault. If their use is not properly planned, however,they may cause electrical difficulties elsewhere in the system.

• The description of the power system used when orderingequipment should include fault level at the service entrance,rating and impedance of transformers between the serviceentrance and the input to the power conditioning equipmentand details of all capacitor banks on the same utility substation.


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OTHER GUIDELINES

• There are no current CSA standards specifically relating toASDs. CSA approval may be granted to many different drivedesigns, many of which are imported.

• Minimum guaranteed full load, full speed ASD efficiency of95%, (including any supplied equipment: isolation transformersand filters).

• Harmonic Distortion: Latest recommended specification: IEEE519-1992: total voltage harmonic distortion shall not exceed5% at common coupling ASD to motor.


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CALCULATING HORSEPOWER

Once the machine BHP (speed x torque) requirement isdetermined, horsepower can be calculated using the formula:

rated motor hp = motor efficiency (%)100

= available hp

BHP = T x N (required hp)5,250

Where,hp = horsepower, supplied by the motorT = torque (lb-ft), force x radiusN = base speed of motor (rpm)

If the calculated horsepower falls between standard available motorratings, select the higher available horsepower rating. It is goodpractice to allow some margin when selecting the motorhorsepower.

APPENDIX AFormulas for Calculating Applications

Appendix A 83

A P P E N D I X A

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For many applications, it is possible to calculate the horsepowerrequired without actually measuring the torque required. Thefollowing will help:

BHP = brakehorsepower, the mechanical load required by the driven equipment

FOR CONVEYORS

hp (vertical) = weight (lb) x velocity (FPM)33,000 x efficiency

hp (horizontal) = weight (lb) x velocity (FPM) x coef. of friction

33,000 x efficiency

FOR WEB TRANSPORT SYSTEMS AND SURFACE WINDERS

Note that the tension value used in this calculation is the actualweb tension for surface winder applications, but it is the tensiondifferential (downstream tension – upstream tension) for sectional drives.

CENTRE WINDERS (ARMATURE CONTROL ONLY)

hp = tension (lb) x line speed (FPM) x buildup

33,000 x taper

CENTRE WINDERS (FIELD CONTROL)

If taper x field range ³ buildup, then

hp = tension (lb) x line speed (FPM)

33,000



If taper x field range ² buildup, then

hp = tension (lb) x line speed (FPM) x buildup

33,000 x taper x field range

Note that these formulas for calculating horsepower do not includeany allowance for machine function windage or other factors.These factors must be considered when selecting a drive for amachine application.

FOR FANS AND BLOWERS

Effect of speed on horsepower

hp = k1 (RPM)3 – horsepower varies as the 3rd power of speedT = k1 (RPM)2 – torque varies as the 2nd power speedFlow = k3 (RPM) – flow varies directly as the speed

hp = CFM x pressure (lb/in2)

229 x (eff. of fan)

hp = CFM x (inches of water gauge total pressure)

6,362 x (eff. of fan)

Total pressure = static pressure + velocity pressure

Velocity pressure = (velocity*)2x air density

1,096*(velocity in fpm)

Appendix A 85


FOR PUMPS

hp = GPM x head (ft) x (specific gravity)

3,960 x (% eff. of pump)

Specific gravity of water = 1.01 ft3 per sec = 448 GPM1 PSI = A head of 2.309 ft for water weighing

62.36 lb/ft3 at 62°F

CONSTANT DISPLACEMENT PUMPS


hp = k(RPM) – horsepower and capacity vary directly as the speed.

Displacement pumps under constant heat require approximatelyconstant torque at all speeds.

CENTRIFUGAL PUMPS


hp = k1 (RPM)3 – horsepower varies as the 3rd power of speed

T = k1 (RPM)2 – torque varies as the 2nd power of speedFlow = k3 (RPM) – flow varies directly as the speed

PUMP EFFICIENCY (TYPICAL)

500 to 1,000 gal/min = 70% to 75%1,000 to 1,500 gal/min = 75% to 80%Larger than 1,500 gal/min = 80% to 85%Displacement pumps may vary between 50% to 80% efficiency,depending on size of pumps.



HORSEPOWER REQUIRED

hp = torque (lb-ft) x speed (RPM)5,250

hp = torque (lb-in) x speed (RPM)63,000

Torque (lb-ft) = hp x 5,250speed (RPM)

Accelerating torque (lb-ft) = WK2 (lb-ft2) x RPM308 x t (sec)

Where,WK2 = inertia (lb-ft2) reflected to motor shaftÆRPM = change in speedt = time (seconds) to accelerate

t = WK2 (lb-ft2) x ÆRPM = time to accelerate (sec)308 x t (lb-ft)

RPM = FPM.262 x diameter (inches)

Inertia reflected to motor = load inertia ( Load RPM )2

Motor RPM

INERTIA (WK2)The factor WK2 is the weight (lb) of an object multiplied by thesquare of the radius of gyration (k). The unit measurement of theradius of gyration is expressed in feet.

Appendix A 87


For solid or hollow cylinders, inertia may be calculated by usingthe equations given here.

To calculate hollow shafts, take the difference between the inertia values for the OD and ID (see Figure A-1).

The inertia of complex, concentric rotating parts may be calculatedby breaking the part up into simple rotating cylinders, calculatingtheir inertias and summing their values, as shown in Figure A-2.

FIGURE A-1. Calculating Hollow Shafts


L

DSolid

HollowD1

D2

L

WK2 = .000681 r L(D24 – D1

4)WK2 = lb.ft.2

D1D2, D1 and L = in. r = lb.in.3

r (aluminum) = .0924r (bronze) = .320

r (cast iron) = .260r (steel) = .282

r (paper) = .0289

WK2 = .000681 r LD4


FIGURE A-2. Calculating the Inertia of Complex, Concentric Rotating Parts

WK2 OF ROTATING ELEMENTS

In practical mechanical systems, all the rotating parts do not oper-ate at the same speed. The WK2 of all moving parts operating ateach speed must be reduced to an equivalent WK2 at the motorshaft, so that they can all be added together and treated as a unit,as follows:

Equivalent WK2 = WK2 ( N )2

Nm

Where,WK2 = inertia of the moving partN = speed of the moving part (RPM)Nm = speed of the driving motor (RPM)

When using speed reducers, and the machine inertia is reflectedback to the motor shaft, the equivalent inertia is equal to themachine inertia divided by the square of the drive reduction ratio.

Appendix A 89

+ +=

tot2

12WK = WK 2

2= WK 32= WK


WK2 OF LINEAR MOTION

Not all driven systems involve rotating motion. The equivalentWK2 of linearly moving parts can also be reduced to the motorshaft speed as follows:

Equivalent WK2 = W (V)2

39.5 (Nm)2

Where,W = weight of load (lb)V = linear velocity of rack and load or conveyor and load

(FPM)

Nm = speed of the driving motor (RPM)This equation can only be used where the linear speed bears acontinuous fixed relationship to the motor speed, such as aconveyor.

Synchronous (RPM) motor speed = Hz x 120

no. of poles

% Slip = synchronous (RPM – full load RPM) x 100

synchronous RPM

OHMS LAW

Amperes = volts

ohms

Ohms = volts

amperes

Volts = amperes x ohms



POWER IN DC CIRCUITS

Horsepower = volts x amperes

746

Watts = volts x amperes

Kilowatts = volts x amperes

1,000

Kilowatt-hours = volts x amperes x hours

1,000

POWER IN AC CIRCUITS

Kilovolt-amperes (kVA)

kVA (single-phase) = volts x amperes

1,000

kVA (three-phase) = volts x amperes x 1.73

1,000

Kilowatts (kW)

kW (single-phase) = volts x amperes x power factor

1,000

kW (three-phase) = volts x amperes x power factor x 1.73

1,000

Power factor = kilowatts

kilovolts x amperes

Appendix A 91



THREE-PHASE AC CIRCUITS

HP = E x I x 3 x EFF x PF746

Motor amps = hp x 746E x 3 x EFF x PF

Motor amps = kVA x 1,0003 x E

Motor amps = kW x 1,0003 x E x PF

Power factor = kW x 1,000E x I x 3

Kilowatt-hours = E x I x hours x 3 x PF1,000

PF = displacement power factor = cos q = kWkVa

Power (watts) = E x 1 x 3 x PFEFF = mechanical efficiencyE = voltsI = amps

kVARi kVARc

(ACmotors)

Added(to correctKVARi) toimprove

PF

kVA

kW

f

CAPACITIVE

INDUCTIVE

1 kW = 56.88 BTU/min1 Ton= 200 BTU/min1 hp = 0.7457 kW

= 550 lb-ft per sec= 33,000 lb-ft per min= 2,545 BTU per hour


Appendix B 93

A P P E N D I X B

APPENDIX BConversion Factors

Length

Torque

Rotation

Multiply

Metres

Metres

Inches

Feet

Millimetres

Newton-Metres

lb-ft

lb-in

lb-ft

RPM

RPM

Degrees/sec

Rad/sec

By

3.281

39.37

.0254

.3048

.0394

.7376

1.3558

.0833

12.00

6.00

.1047

.1667

9.549

To Obtain

Feet

Inches

Metres

Metres

Inches

lb/ft

Newton-Metre

lb-ft

lb-in

Degrees/sec

Rad/sec

RPM

RPC



Moment of

Inertia

Power

Temperature

Multiply

Newton-Metres2

oz-in2

lb-in2

Slug-ft2

oz-in-sec2

lb-in-sec2

Watts

lb-ft/min

hp

hp

By

2.42

.000434

.00694

32.17

.1675

2.68

.00134

.0000303

746.

33000.

To Obtain

lb-ft2

lb-ft2

lb-ft2

lb-ft2

lb-ft2

lb-ft2

HP

HP

Watts

lb-ft/min

Degree C = (Degree F-32) x 5/9

Degree F = (Degree C x 9/5) + 32


AC = alternating currentANSI = American National Standards InstituteASD = adjustable speed driveBHP = brakehorsepowerCSA = Canadian Standards AssociationCSI = current source inverterDC = direct currentDSP = digital signal processorECC = eddy current couplingGTO = gate turnoff (thyristor)HDF = harmonic distortion factorIGBT = insulated gate bi-thermal thyristorIEEE = Institute of Electrical and Electronics EngineersLCI = load-commutated inverterNEMA = National Electrical Manufacturers AssociationNPV = net present valuePAM = pulse amplitude modulationPLC = programmable logic controllerPWM = pulse width modulated (inverter)SCR = silicon-controlled rectifierSR = switched reluctanceV = voltageVSI = variable source inverterVVI = variable voltage inverter

Abbreviations 95

A B B R E V I AT I O N S

ABBREVIATIONS


ANSI/IEEE Standard 446-1987 (February 1987): IEEERecommended Practice for Emergency and Standby PowerSystems for Industrial and Commercial Applications.

Hanna, R, Dr. and Prabhu, S., Study of Medium Voltage Drives.Ontario Hydro, Technology Services Department. 1995.

Jarc, Dennis, and John Robochuck. Reliance Electric: Static Motor DriveCapabilities for Petro. Ind. New York: IEEE Press, 1981.

Mohan, N., et al. Power Electronics: Converters Applications & Design.John Wiley & Sons: 1989.

Persson, E. “Energy Savings and Pay-Back of Adjustable SpeedDrives in Flow Control,” Pulp and Paper Canada, 88:6 (1987).

Pollack, J.J. “Some Guidelines for the Application of Adjustable-Speed AC Drives,” Adjustable Speed Drive Systems. New York: IEEEPress, 1981.

Proceedings of the Symposium on Electric Variable Speed Drives,Ontario Hydro/Ministry of Energy/CCE, 1987.

Radovanovic, V. “Variable Speed Drives,” Electrical Business,June 1987.

Reason, J. “Special Report: AC Motor Control,” Power Magazine,February 1981, Vol. 125, No. 2.

Bibliography 97

B I B L I O G R A P H Y


Reason, J. “Special Report: Powerplant Motors,” Power Magazine,March 1986.

Stevenson, A.C. “Fundamentals and Applications of Static PowerConversion,” IEEE 1984 Conference Record of Pulp and PaperIndustry Technical Conference.

Wennerstrom, C.H., et al. “Motor Application Considerations onAdjustable Frequency Power,” IEEE 1984 Conference Record ofPulp and Paper Industry Technical Conference.



Adjustable speed drive (ASD)cost, 70definition, 1

Advantagescomparison, 31general, 35

Applicationscomparison, 29energy savings, 36, 43, 73 speed control/process

requirements, 35Constant horsepower, 44Constant torque, 44Control accuracy, 60Current-source inverter (CSI), 13, 18, 29

DC drivegeneral, 23principle of operation, 8

Delivery time, 66Economics

cost/saving factors, 69general, 72methods, 69

Eddy current clutchgeneral, 29principle of operation, 8

Efficiencycomparison, 29discussion, 57

Environment, 63Harmonics

comparison, 29definition, 77effects, 78guidelines, 81losses, 55production and transmission, 79

Horsepower ratingcomparison, 29general, 50

Inverter (see also variablefrequency drive), 12

Maintenance, 65Motors

classification, 3motor/drive requirements, 49

NEMA motor designs, 55Net present value, 70Power conditioning

equipment, 72Pulse width modulatedinverter (PWM), 13, 20, 25

Index 99

I N D E X



Rectifier, 12Regenerative braking

comparison, 31description, 36simple payback, 69

Regulator, 13Softstarting

comparison, 30description, 36

Speedregulation, 53, 60requirements, 29

Thermal considerations, 54Torque

considerations, 62requirements, 51

Variable frequency drive (see alsoadjustable speed drive)

comparison, 29principle of operation, 11types, 13

Variable torque, 44Variable voltage controllers, 11Variable voltage

inverter (VVI), 13, 17, 29Voltage requirements, 51Voltage source inverter (VSI) - (see

variable voltage inverter)Waveforms, 14Wound rotor motor controllers

general, 29principle of operation, 10


Suppliers 101

A S D S U P P L I E R S I N O N T A R I O *

ABB4410 Paletta CourtBurlington, Ontario L7L 5R2Contact: Steve Seppanen

(905) 577-1986Fax: (905) 681-2810

Canadian Drives Inc.40 Claireville DriveEtobicoke, Ontario M9W 5T9Contact: Andrew J. Houston

(416) 213-1022Fax: (416) 213-0821

Cegelec Automation5112 Timberlea BoulevardMississauga, Ontario L4W 2S5Contact: Roger D. Coote

(905) 624-2026Fax: (905) 629-8203

G.E. Canada Inc.2300 Meadowvale BoulevardMississauga, Ontario L5N 5P9Contact: Mike Marshall

(905) 858-5128fax: (905) 858-5132

*at time of printing

ITT Fluid Products Canada55 Royal RoadGuelph, Ontario N1H 1T1Contact: Phil Searle

(519) 821-1900fax: (519) 821-5316

Rockwell Automation/Allen-Bradley135 Dundas StreetCambridge, Ontario N1R 5X1Contact: (519) 623-1810

Siemens Electric LimitedEnergy and Automation Division2185 Derry Road WestMississauga, Ontario L5N 7A5Contact:: Drives Sales Representative

(905) 819-5800 ext. 6414Fax: (905) 819-5802

Toshont-Toshiba2295 Dunwin DriveUnit #4Mississauga, Ontario L5L 3S4Contact: Tom Johnson

(905) 607-9200Fax: (905) 607-9203


OTHER IN-HOUSE REFERENCE GUIDES:

• Energy Monitoring & Control Systems• Fans• Lighting• Motors• Power Quality• Power Quality Mitigation• Pumps

COMMENTS:

For any changes, additions and/or comments call or write to:

Scott RouseAccount ExecutiveOntario Hydro700 University Avenue, H10-F18Toronto, OntarioM5G 1X6Telephone (416) 592-8044Fax (416) 592-4841E-Mail [email protected]


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Adjustable Speed Drive Reference Guide