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$15.95 CDN
Adjustable
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REFERENCE GUIDE
4th Edition
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First Edition, November 1987
Second Edition, March 1991
Third Edition, February 1995
Fourth 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 for
damages resulting from the use of, any information,
equipment, product, method or process disclosed in
this guide.
In-House Energy Efficiency
Energy Savings are Good Business
Printed in Canada
Copyright © 1997 Ontario Hydro
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ADJUSTABLE SPEED
DRIVE
Reference Guide
4th Edition
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TA B L E
Page i
OF
CONTENTS
INTRODUCTION .............................................................................................. 1
Latest Improvements .................................................................................2
CHAPTER 1: CLASSIFICATIONS ......................................................................... 3
Classification of Motors .......................................................................... 3
Classification of Drives ............................................................................ 3
CHAPTER 2: PHYSICAL APPEARANCE ................................................................. 5
CHAPTER 3: PRINCIPLES OF OPERATION ............................................................ 7
Conventional Fixed-speed AC Systems .................................................. 7
DC Drives ................................................................................................ 8
AC Drives ................................................................................................ 8
Eddy Current Clutches ............................................................................. 8
Switched Reluctance Drives ...................................................................... 9
Vector Drive .......................................................................................... 10
Wound-rotor Motor Controllers ............................................................... 10
Variable Voltage Controllers .................................................................... 11
Variable Frequency Drives ..................................................................... 11
Components .......................................................................................... 12
Types of Inverters .................................................................................. 13
Waveforms ............................................................................................ 14
Switching Devices (Power Electronics) ........................................................14
Medium Voltage Drives...........................................................................14
Recommended Specifications .....................................................................15
CHAPTER 4: COMPARISON OF ASDS ............................................................. 17
AC Drives .............................................................................................. 17
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CONTENTS
Variable Voltage Inverter (VVI) ............................................................... 17
Current Source Inverter (CSI) ................................................................. 18
Pulse Width Modulator (PWM) .............................................................. 20
Power Factor Comparison ....................................................................... 22
DC Drives .............................................................................................. 23
Eddy Current Coupling ......................................................................... 25
Cycloconverter.........................................................................................26
CHAPTER 5: STANDARD AND OPTIONAL FEATURES ......................................... 33
CHAPTER 6: ADVANTAGES ............................................................................. 35
Speed Control ........................................................................................ 35
Position Control ..................................................................................... 36
Torque Control ...................................................................................... 36
High Energy Savings Potential ................................................................ 36
Soft Start/Regenerative Braking .............................................................. 36
Equipment Life Improvement .................................................................. 37
Multiple Motor Capability ..................................................................... 37
Bypass Capability ................................................................................. 37
Safe Operation in Harsh Environments .................................................... 37
Temporary or Back-up Operation .............................................................37
Reduction in Vibration and Noise Level .................................................... 38
Re-acceleration Capability ...................................................................... 38
Tips and Cautions .................................................................................. 38
CHAPTER 7: APPLICATION CONSIDERATIONS .................................................. 39
How to Select an ASD ........................................................................... 39
Software ...................................................................................................42
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Financial Evaluation ................................................................................42
Load Characteristics ............................................................................... 42
Application Types by Load ..................................................................... 43
Tips and Cautions .................................................................................. 46
Motor/Drive System .............................................................................. 49
Thermal Considerations ......................................................................... 54
Other Considerations ............................................................................ 56
Efficiency .............................................................................................. 57
Reliability of ASDs ................................................................................ 58
Applications .......................................................................................... 59
Performance Required ............................................................................ 60
Starting and Stopping Characteristics ...................................................... 62
Torque .................................................................................................. 62
Environment .......................................................................................... 63
Weight and Space ................................................................................. 63
Accessories ............................................................................................ 64
Safety .................................................................................................. 65
Service and Maintenance ....................................................................... 65
Tips and Cautions .................................................................................. 67
CHAPTER 8: ECONOMICS .............................................................................. 69
Economic Factors ................................................................................... 72
Capital Costs ........................................................................................ 72
Capital Savings .................................................................................... 73
Operating Costs and Savings ................................................................. 73
Tips and Cautions .................................................................................. 75
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CONTENTS
CHAPTER 9: HARMONIC DISTORTION ........................................................... 77
Harmonics .............................................................................................. 77
What Harmonic Distortion Can Do ...................................................... 78
Production and Transmission ................................................................ 79
Isolation Transformers ............................................................................ 80
Other Guidelines (IEEE 519-1992) ........................................................ 81
APPENDIX A: FORMULAS FOR CALCULATING APPLICATIONS ............................. 83
APPENDIX B: CONVERSION FACTORS ............................................................. 93
ABBREVIATIONS ............................................................................................ 95
BIBLIOGRAPHY .............................................................................................. 97
INDEX .......................................................................................................... 99
ASD SUPPLIERS IN ONTARIO ....................................................................... 101
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LIST
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FIGURES
OF
1. Comparison of Range Process Speed Control ......................................1
2. Physical Appearance of Variable Frequency
Drive/Motor System ............................................................................ 5
3. 8/6 Pole Switched Reluctance Motor .................................................. 9
4. Vector Drive .........................................................................................10
5. Closed Loop (Feedback) Adjustable Frequency
Inverter System .................................................................................. 12
6. VVI – Variable Voltage Inverter .......................................................... 17
7. VVI – Waveforms ............................................................................... 18
8. CSI – Current Source Inverter ............................................................ 19
9. CSI – Waveforms ............................................................................... 19
10. Block Diagram for a Typical CSI Drive ............................................. 19
11. PWM – Pulse Width Modulated Inverter .......................................... 21
12. PWM – Waveforms ............................................................................ 21
13. Block Diagram for a Typical PWM Drive .......................................... 21
14. Power Factor Comparison ................................................................. 22
15. DC Drive ............................................................................................ 23
16. ECC – Eddy Current Coupling .......................................................... 26
17. Cycloconverter Circuit.........................................................................27
18. Duty Cycles ....................................................................................... 43
19. Variable Torque Load ......................................................................... 45
20. Constant Torque Load ....................................................................... 45
21. Constant Horsepower Load .............................................................. 45
22. Power Required is Proportional to RPM3 Centrifugal
Fan/Blower, Pump .............................................................................. 46
23. Power Savings in Fans and Pumps Using ASDs ............................... 48
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LIST
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FIGURES
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 ................................................ 57
26. Typical AC Drive Efficiency ............................................................. 57
27. Motor Performance, Typical 60 Hz ................................................. 63
28. Ideal Torque-Speed Curves .............................................................. 64
29. NEMA Design B Motor Torque-Speed Curve ................................. 64
30. Capital Cost Comparison of Motor/Drive
Systems Medium HP, Voltages ........................................................ 76
31. Harmonic Distortion ........................................................................ 78
A-1. Calculating Hollow Shafts ............................................................... 88
A-2. Calculating the Inertia of Complex,
Concentric Rotating Parts ................................................................ 89
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LIST
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2.
3.
4.
5.
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TA B L E S
Comparison of Adjustable Speed Drives ............................................. 29
ASD and Electronic Motor Features .................................................... 34
Suitability of Inverters for NEMA Motor Designs ............................... 55
ASD Checklist of Costs/Savings .......................................................... 70
ASD Investment Decision Technique .................................................. 71
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INTRODUCTION
An adjustable speed drive (ASD) is a device used to provide
continuous range process speed control (as compared to discrete
speed 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 motor
speed.
ASDs may be referred to by a variety of names, such as variable
speed drives, adjustable frequency drives or variable frequency
inverters. The latter two terms will only be used to refer to certain
AC systems, as is often the practice, although some DC drives are
also based on the principle of adjustable frequency.
Continuous
Speed
Discrete
Operation
FIGURE 1. Comparison of Range Process Speed Control
Introduction
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In this guide, “drive” refers to the electric ASD.
Application concerns in connecting electric or mechanical ASDs
have similar effects on the driven load, and these are covered in this
guide.
L ATEST I MPROVEMENTS
• 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 sampling
power at terminals, ASD and motor circuitry combined to keep
power waveforms sinusoidal, minimizing power losses.
• Motor Control Centers (MCC) coupled with the ASD using
real-time monitors to trace motor-drive system performance.
• Higher starting torques at low speeds (up to 150% running
torque) up to 500 MP, in voltage source drives.
• Load-commutated Inverters coupled with synchronous motors.
(precise speed control in constant torque applications.
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CHAPTER 1
CLASSIFICATIONS
C LASSIFICATION
OF
M OTORS
• There are two main types of motors, AC (alternating current)
and DC (direct current).
• AC motors can be sub-classified as induction (squirrel-cage and
wound-rotor) and synchronous.
• Induction motors are often classified as either high efficiency or
standard.
C LASSIFICATION
OF
D RIVES
• Adjustable speed drives are the most efficient (98% at full load)
types of drives. They are used to control the speeds of both AC
and DC motors. They include variable frequency/voltage AC
motor controllers for squirrel-cage motors, DC motor
controllers for DC motors, eddy current clutches for AC motors
(less efficient), wound-rotor motor controllers for wound-rotor
AC motors (less efficient) and cycloconverters (less efficient).
Chapter 1: Classifications
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• Other types of drives include mechanical and hydraulic
controllers. Examples of mechanical drives are adjustable belts
and pulleys, gears, throttling valves, fan dampers and magnetic
clutches. Examples of hydraulic drives are hydraulic clutches
and fluid couplings.
• In this guide, emphasis is on AC variable frequency drives, or
inverters, which are used to control industry’s workhorse, the
standard AC induction motor. This is because this motor is
replacing 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 certain
applications.
• Drives may be classified according to size ranges (horsepower,
voltage) for which increasing specifications are required in
designing 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, common
mode voltage is isolated from the motor and put on the drive
side transformer winding.
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CHAPTER 2
PHYSICAL APPEARANCE
• Variable frequency AC drives are comprised of many electrical
circuits and components usually arranged within a cabinet that
provides heat dissipation and shielding.
ASD + transformer (if required)
LOAD
Feedback
Loop
(Optional)
Tachometer
Can be
hundreds of
metres away
Motor
FIGURE 2. Physical Appearance of Variable Frequency
Drive/Motor System
Chapter 2: Physical Appearance
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• Drives vary greatly in size, depending upon their horsepower
and voltage rating and type.
• Electrical cables connect the motor to the drive, which might
involve a considerable distance.
• Small AC drives may be built on to their associated motors.
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CHAPTER 3
PRINCIPLES OF OPERATION
• 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 AC
induction and synchronous motors.
C ONVENTIONAL F IXED - SPEED AC S YSTEMS
(AC M OTOR W ITHOUT D RIVE )
• 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 the
control device (dampers, throttling valves, recirculation loops)
since the power delivered does not decrease in proportion to
the reduction in demand.
Chapter 3: Principles of Operation
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DC D RIVES
• The DC motor is the simplest to which electronic speed control
can be applied because its speed is proportional to the armature
voltage.
• The DC voltage can be controlled through a phase-controlled
rectifier or by a DC-DC converter if the input power is DC.
This is usually accomplished by a separate motor-generator set
producing a DC output.
• The speed of a DC motor can be adjusted over a very wide
range by control of the armature current and/or field currents
(brushless DC drives, vector controlled DC drives).
AC D RIVES
E DDY C URRENT C LUTCHES
• Eddy current clutches can be used to control standard AC
squirrel-cage induction motors. However, they are low
efficiency 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 poles
and a wound coil that provides the variable flux required for
speed control.
• Efficiency is significantly lower than ASDs.
• A voltage is applied to the coil of wire, which is normally
mounted 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 torque
transmitted, and therefore the speed, can be varied.
S WITCHED R ELUCTANCE D RIVES
• Switched reluctance (SR) drives have a high power to weight
ratio.
• In closed-loop control, they are well suited for speed and
torque control.
FIGURE 3. 8/6 Pole Switched Reluctance Motor
(one phase winding shown)
• The rotor has salient poles with no windings or electric
connections.
• A pair of opposite stator poles magnetically pulls rotor poles
in-line.
• Rotor position sensor controls switch each pole pair in
sequence, giving continuous rotation.
Chapter 3: Principles of Operation
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V ECTOR D RIVE
• Vector drive control of AC motors is similar to DC drive
performance in speed, torque and horsepower.
• It can produce full torque from start to full speed. (The motor
needs to control heat at full torque and low speed.)
• It requires complex electronics (digital signal processors, or
DSPs) to calculate servomotor phase currents.
• Magnitude and direction of armature current together are a
vector 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 by
eliminating magnetizing current and slip values.
Speed
Regulator
2 Phase
to
3 Phase
Encoder
Current
Regulator
Motor
Flux
Command
Controller
Position
Signal
FIGURE 4. Vector Drive
W OUND - ROTOR M OTOR C ONTROLLERS
• Wound-rotor motor controllers are used to control the speed of
wound-rotor induction motors.
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• By changing the amount of external resistance connected to the
rotor circuit through the slip rings, the motor speed can be
varied.
• The slip energy of the motor is either wasted in external
resistance controllers (in the form of heat) or recovered and
converted to useful electrical or mechanical energy. For
conversion to useful electrical energy, the system would be
known as a wound-rotor slip energy recovery drive.
VARIABLE V OLTAGE C ONTROLLERS
• 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 inefficient
at high speed.
• Only applications with narrow speed ranges are suitable.
VARIABLE F REQUENCY D RIVES
• A variable frequency drive controls the speed of an AC motor
by varying the frequency supplied to the motor.
• The drive also regulates the output voltage in proportion to the
output frequency to provide a relatively constant ratio (V/Hz) of
voltage to frequency, as required by the characteristics of the
AC motor to produce adequate torque.
•
In closed-loop control, a change in demand is compensated by a
change in the power and frequency supplied to the motor, and thus
a change in motor speed (within regulation capability).
Chapter 3: Principles of Operation
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Feedback
Signal
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Speed
Reference
from Process
TACHOMETER
REGULATOR
(Controls)
Motor
RECTIFIER
Constant
Frequency
Constant Voltage
AC Power
Supply
LOAD
INVERTER
(Switching
Section)
Fixed or
Variable
DC Voltage
Variable
Frequency
Variable Voltage
AC Power
Output
FIGURE 5. Closed Loop (Feedback)
Adjustable Frequency Inverter System
C OMPONENTS
• A variable frequency drive has two stages of power conversion,
a rectifier and an inverter. (“Inverter” is also used to refer to the
entire 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-down
transformers.)
- The rectifier is a circuit which converts fixed voltage AC
power to either fixed or adjustable voltage DC.
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- The inverter is composed of electronic switches (thyristors or
transistors) that switch the DC power on and off to produce
a controllable AC power output at the desired frequency
and voltage.
- A regulator modifies the inverter switching characteristics so
that the output frequency can be controlled. It may include
sensors to measure the control variables.
T YPES
OF I NVERTERS
• There are three basic types of inverters commonly employed in
adjustable AC drives:
- The variable voltage inverter (VVI), or square-wave six-step
voltage source inverter (VSI), receives DC power from an
adjustable voltage source and adjusts the frequency and
voltage.
- The current source inverter (CSI) receives DC power from an
adjustable current source and adjusts the frequency and
current.
- The pulse width modulated (PWM) inverter is the most
commonly chosen. It receives DC power from a fixed voltage
source and adjusts the frequency and voltage. (PWM types
cause the least harmonic noise.)
• AC/AC adjustable frequency drives are used only for large
horsepower applications (1000 hp and above). They include
cycloconverters (AC/AC) and load-commutated inverters
(LCIs). Both can be used with induction or synchronous
motors. (Since these drives are usually custom-designed for
each application, they will not be fully discussed in this guide.)
Chapter 3: Principles of Operation
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W AVEFORMS
• The voltage and current waveforms produced by inverter
systems approximate, to varying degrees, the pure sine wave.
•
Of the three most common inverter systems, the pulse width
modulated inverter produces output current waveforms that
have the least amount of distortion.
S WITCHING D EVICES
• Advances in Power Electronic technology have greatly
enhanced performance range and reliability of ASDs.
• New switching devices are faster, produce less heat, and less
harmonics into the motor circuit. Some types are:
- SCR (silicon - controlled rectifier).
- Diode.
- GTO (gate turnoff thyristor).
- IGBT (insulated gate bi-thermal thyristor).
M EDIUM V OLTAGE D RIVES
• Voltages above 2300V, and controlling induction motors
between 1,000 HP to 15,000HP are becoming increasingly
available.
- Input line isolation transformer.
- Internal cooling (liquid or air).
- Input circuit breaker, output contactor with isolation
switches.
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- Motor harmonics filter to supply maximum 5% current total
harmanic distortion.
- DC link reactor to prevent saturation at faulted conditions.
R ECOMMENDED S PECIFICATIONS
•
Nominal power at +- 10% voltage, 3 phase, 60 Hz ( +- 2%).
• Capable of operation during temporary voltage drop of 70% to
90% lasting up to 6 voltage wave cycles.
• Bus voltage restored within 5 seconds, drive automatically
restarts, if not, drive automatically trips and shuts down. Manual
reset required to start.
• Uninterruptible Power Source (UPS) recommended to provide
control 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.
Chapter 3: Principles of Operation
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CHAPTER 4
COMPARISON OF ASDS
AC D RIVES
VARIABLE V OLTAGE I NVERTER (VVI)
•
A controlled rectifier transforms supply AC to variable voltage
DC. 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.
AC to DC
Rectifier
DC Link
DC to AC
Inverter
M
Constant
Voltage
Voltage
Smoothing
Variable Voltage/
Frequency Control
FIGURE 6. VVI – Variable Voltage Inverter
•
Output frequency is controlled by switching transistors or
thyristors in six steps.
Chapter 4: Comparison of ASDs
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(Line to
Neutral)
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Current
(Line)
0
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6 Step
Time
FIGURE 7. VVI – Waveforms
•
VVI inverters control voltage in a separate section from the
frequency generation output.
•
Approximate sine current waveform follows voltage.
•
VVI is the simplest adjustable frequency drive and most
economical; however, it has the poorest output waveform.
It requires the most filtering to the inverter.
•
Ranges available are typically up to 500 horsepower but can
be up to 1000 horsepower.
•
Voltage source inverters use a constant DC link voltage.
C URRENT S OURCE I NVERTER (CSI)
18
•
AC current transformers are used to adjust the controlled
rectifier. Input converter is similar to the VVI drive. A current
regulator presets DC bus current.
•
The inverter delivers six step current frequency pulse, which
the voltage waveform follows. Switches in the inverter can be
transistors, SCR thyristors or gate turnoff thyristors (GTOs).
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AC to DC
Rectifier
Page 19
DC to AC
Inverter
DC Link
M
Variable
Voltage
Control
Current
Smoothing
Variable
Frequency
Control
FIGURE 8. CSI – Current Source Inverter
Voltage
(Line to
Neutral)
Current
(Line)
0
0
Time
FIGURE 9. CSI – Waveforms
AC
Line
AC/DC
Converter
Filter
Inverter
Motor
Current
Regulator
Frequency
Control
Speed
Speed or
Voltage
Control
FIGURE 10. Block Diagram for a Typical CSI Drive
Chapter 4: Comparison of ASDs
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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, the
CSI requires a large filter inductor and complex regulator.
•
CSI drives are short circuit proof because of a constant circuit
with 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 control
current and, therefore, control torque. This applies in variable
torque applications.
•
CSI-type drives have a higher horsepower range than VVI and
PWM (typically up to 5000 horsepower).
P ULSE W IDTH M ODULATOR (PWM)
•
Diode rectifiers provide constant DC voltage. Since the inverter
receives a fixed voltage, the amplitude of output waveform is
fixed. The inverter adjusts the width of output voltage pulses as
well as frequency so that voltage is approximately sinusoidal.
•
The better waveforms require less filtering; however, PWM
inverters are the most complex type and switching losses can
be high.
•
The range of PWM inverters is typically up to 3000 horsepower,
but each manufacturer may list larger sizes (usually customengineered).
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AC to DC
Converter
Page 21
DC Link
DC to AC
Inverter
M
Variable
Voltage
Control
Voltage
Smoothing
Variable
Frequency
Control
FIGURE 11. PWM – Pulse Width Modulated
Voltage
(Line to
Neutral)
0
Current
(Line)
0
FIGURE 12. PWM – Waveforms
AC
Line
Diode
Bridge
Rectifier
Filter
Inverter
Motor
Speed
Reference
Voltage &
Frequency
Control
FIGURE 13. Block Diagram for a Typical PWM Drive
Chapter 4: Comparison of ASDs
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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 acceleration
rate matched to total motor load.
•
At low speeds, PWM drives may require a voltage boost to
generate 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 of
PWM drives.
Power Factor
1.0
PWM &
Vector Drive
.75
VVI
.50
CSI
.25
0
450
900
1350
1800
Speed (RPM)
FIGURE 14. Power Factor Comparison
P OWER FACTOR C OMPARISON
• The power factor of VVI and CSI drives declines with speed as
the thyristor firing angle varies in the controlled rectifier.
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• PWM drives have near unity power factor throughout the speed
range, due to the diode rectifier and constant voltage DC bus.
• Note that true Root-Mean-Square (RMS) meters will determine
the real power factor on three-phase systems. It may be less
than the displacement power factor (kW/kVA) which appears
on single-phase meters.
DC D RIVES
• DC drives are a simpler, more mature technology than AC
drives, and they continue to have applications where larger
horsepower 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 rated
motor base speed.
% of Rated Power
100
0
0
Armature Voltage
Control
Field Current
Control
Constant Field
Current
Constant Armature
Voltage
Constant
Torque
Constant
Power
100
% of Base Speed
FIGURE 15. DC Drive
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• Field voltage-controlled DC drives provide constant horsepower
and variable torque. A variable voltage field regulator can
provide alternate armature and field voltage control.
• Motor speed is directly proportional to voltage applied to the
armature by the ASD. A phase-controlled bridge rectifier with
logic circuits is used. Tachometer feedback achieves speed
regulation.
• 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 not
possible to bypass the drive to run the motor and maintenance
costs are high due to armature connections through a brush and
commutator ring.
• Regenerative DC drives can invert the DC electrical energy
produced 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’s
flywheel.
• Non-regenerative DC drives are those where the DC motor
rotates in only one direction, supplying torque in high friction
loads such as mixers or extruders. The load exerts a strong
natural brake. If desired, the drive’s deceleration time can affect
speed regulation.
• Flywheel applications such as stamping presses have
overhauling load; hence, braking torque or “dynamic braking” is
applied. All DC motors are DC generators as well.
• Regenerative drives are better speed control devices than nonregenerative but are more expensive and complicated.
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• Armature voltage control DC drives have constant torque
features, capable of rated torque across the motor speed range.
These drives must be oversized to handle constant horsepower
applications.
• Field voltage control of shunt wound DC motors with a voltage
regulator coordinate armature and field voltage for extending
speed range in constant horsepower applications.
• Table 1 compares the electric variable speed drives that may be
used to control the speed of standard squirrel-cage induction
motors. For comparison, information on DC systems is also
provided. Note that this table covers products representative of
the 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 need
for auxiliary equipment, such as step-down transformers, line
filters and chokes.
E DDY C URRENT C OUPLING
• The eddy current coupling (ECC) is similar in principle to a
friction-type clutch. It provides electromechanical coupling
with torque transmitted by eddy currents. The eddy currents
are generated by rotation.
• The ECC has electrically energized magnetic coil windings on
the rotor via slip rings. The magnetic fields in the drum are
caused by eddy currents.
• Horsepower Slip Loss =
motor hp x slip speed RPM
motor RPM
Chapter 4: Comparison of ASDs
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Drum
Motor
Load
SR TR
TD SD
Magnetic Rotor
TD = Drum Torque
SD = Drum Speed
TR = Rotor Torque
SR = Rotor Speed
FIGURE 16. ECC – Eddy Current Coupling
C YCLOCONVERTER
• Mainly used in large synchronous motor drives in low
frequency applications:
- Steel rolling mill end tables.
- Cement mill furnaces.
- Mine hoists.
- Ship propulsion drives.
• Limitation: wave forms become distorted above 40% of input
frequency (i.e., 20Hz from 50Hz supply).
• Advantage: high power factor using synchronous motors.
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A.C. Supply
Bridge
A
Load
Bridge
B
A.C. Supply
FIGURE 17. Cycloconvertor Circuit
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TABLE 1. Comparison of Adjustable Speed Drives
Type of Electric
Drive
Variable Voltage
Inverter (VVI)
Current Source
Inverter (CSI)
Pulse Width Modulated
Inverter (PWM)
MOTOR COMPATIBILITY
• Squirrel-cage induction
or synchronous
• Can handle motors
smaller than inverter
rating
• Squirrel-cage induction
or synchronous
• Can handle motors
smaller than inverter
rating (at reduced rating)
• Squirrel-cage induction
or synchronous
• Can handle motors
smaller than inverter rating
TYPICAL POWER RANGE
(hp)
1 – 1,000
50 – 5,000
SPEED REDUCTION
(typical) =
Maximum Speed
10:1
10:1
CONTROL OPEN LOOP
CAPABILITY
(no feedback)
(Note: Can be improved
with feedback controls)
5%
5%
ADAPTABILITY OF MOTOR
TO HOSTILE
ENVIRONMENTS
Good
Good
EFFICIENCY RANGE
• for system: drive & motor
88 - 93%
88 - 93%
Yes
Yes
Yes
Yes
DC Drive
Wound Rotor with
Slip Energy Recovery
Eddy Current
Coupling (ECC)
Commutated DC
Wound rotor induction
Squirrel-cage induction
0 – 10,000
400 – 20,000
1 – 1,000
20:1 open loop
200:1 with tachometer
5:1
34:1 but may be difficult to
control above 2:1
0.1 - 5%
depending upon feedback
methods
2 - 5%
3 - 5%
Poor due to high
maintenance of motor
Medium
Good
90 - 94%
92 - 96%
0 - 70%
Rotor current
Field winding
5 – 5,000
30:1
Minimum Speed
5%
Good
85 - 95%
TORQUE hp
• Constant
• Variable
• Control Method
Yes
Yes
Yes
Field voltage, armature
voltage or both
600
VOLTAGE RANGE
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TABLE 1. Comparison of Adjustable Speed Drives (cont’d)
Type of Electric
Drive
Variable Voltage
Inverter (VVI)
Current Source
Inverter (CSI)
Pulse Width Modulated
Inverter (PWM)
DC Drive
MULTIPLE MOTOR
CAPABILITY (e.g., two
200 hp motors on a single
400 hp drive)
Yes, unlimited within
inverter rating
No
Yes, unlimited within inverter
rating
SOFT STARTING
Yes
Yes
Power Factor to Motor (PF)
Better than CSI (*2)
Drops with speed
(*2)
Drops with speed
Worst
OUTPUT SYSTEMS
HARMONICS
(dependent on leakage
reactance)
COMPLEXITY OF:
• POWER CIRCUIT
• CONTROL CIRCUIT
PRINCIPLE
Wound Rotor with
Slip Energy Recovery
Eddy Current
Coupling (ECC)
Yes, with manufacturer’s
engineering for load sharing
No
No
Yes
Yes
Yes, if starting resistors
used
Yes
Near unity (excellent)
(*2)
Relatively low (can be
improved with capacitors)
Good
Better than VVI
Least
Yes
Yes
No
Simple
Simple
Simple
Semi-complex
Simple
Complex
Simple
Simple
N/A
Simple
Simple
The inverter receives DC
power from an adjustable
voltage source and adjusts
the frequency.
The inverter receives DC
power from an adjustable
current source and adjusts
the frequency and voltage.
The DC current regulator is
controlled by a closed loop
speed regulator.
The inverter receives DC
power from a fixed voltage
source (diode rectifier) and
controls voltage and
frequency. The RMS
voltage amplitude is fixed,
but the width of voltage
intervals is varied.
Speed is adjusted by
changing field voltage
and/or armature voltage.
Changes current in rotor
circuit by means of a
rectifier and converter
connected to rotor winding.
Energy recovered is usually
fed back into power supply.
30 Adjustable Speed Drive Reference Guide
The output speed is varied
by controlling the magnetic
coupling between two
rotating members. This is
done by means of a field
winding which controls the
clip between them.
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TABLE 1. Comparison of Adjustable Speed Drives (cont’d)
Type of Electric
Drive
Variable Voltage
Inverter (VVI)
Current Source
Inverter (CSI)
Pulse Width Modulated
Inverter (PWM)
DC Drive
Wound Rotor with
Slip Energy Recovery
Eddy Current
Coupling (ECC)
CIRCUIT PROTECTION
• Inverter Open Circuit
Inherent voltage limit
Requires careful design
Inherent voltage limit
Inherent voltage limit
N/A
N/A
• Inverter Short Circuit
Must be carefully designed
to handle DC bus capacitor
discharge
Inherent current limit
Same as for VVI, except
PWM circuit is very fast
acting
Inherent current limit
N/A
N/A
CONTROL VARIABLE
Motor voltage, frequency
Motor voltage, frequency
and current
Motor voltage and frequency
Motor armature voltage,
current and/or field voltage
(not common)
Rotor current
Field between rotating
member
REGENERATIVE BRAKING
Option with added circuitry
Standard
Option
Option
No
No
REVERSE CAPABILITY
Yes
Yes
Yes
Yes
No
Poor
RIDE-THROUGH
CAPABILITY
Difficult
Difficult
Yes, using battery or
capacitive storage
Special applications only
No
No
SIZE & WEIGHT
Intermediate
Large
Small
Intermediate
Small
Small controller; large
rotating element
MAIN ADVANTAGES
• High output frequencies
(higher than 60 Hz if
necessary)
• Can be retrofitted to
existing fixed speed
motor
• Soft start
• Short circuit and overload
protection due to current
control of regulator
• Soft start
• Excellent power factor;
harmonics are minimal
• Can be retrofitted to
existing fixed speed motor
• Soft start
• Simple system
• Wide speed range
• Soft start
• Costs are relatively low
for narrow variable speed
ranges
• Simple circuitry
• Adaptable to existing
wound rotor motors
• Low costs
• Simple compact control
• Wide constant torque
speed range
MAIN DISADVANTAGES
• Harmonics increase
losses in motor
• Standard inverter cannot
operate in a regenerative
mode
• Instability may result
under partial loading
• Harmonics increase
losses in motor
• Difficult to retrofit to
existing fixed speed
motor drive
• Motor is subject to voltage
stresses
• Complex logic circuits
• Brush and commutator
maintenance is high
• Limited to medium and
lower speed applications;
special motor enclosures
may be specified if higher
speed capability is
required (TENV, TEAO)
• Maintenance of brushes
is high
• May pose problems in
hazardous environments
• Relatively low power
factor
• Limited speed range
• Regenerative braking n/a
• Efficiency low at low
speeds
• Lack of reversing
capability
• Limited speed range
• Maintenance of brushed
is required
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TABLE 1. Comparison of Adjustable Speed Drives (cont’d)
Type of Electric
Drive
Variable Voltage
Inverter (VVI)
Current Source
Inverter (CSI)
Pulse Width Modulated
Inverter (PWM)
MAIN DISADVANTAGES
(cont’d)
• Lower horsepower ranges
typically
• Only single motor control
• High initial cost
• Not suitable for
hazardous environments
where explosive gases
may exist
• Expensive, large motor
• Power factor always poor
at low speed
• General purpose lowmedium horsepower
(<500 horsepower),
multiple motor control
• General purpose when
regenerative braking
wanted (hoists)
• Best reliability AC type, at
added cost
• Also suitable for most
applications
•
•
•
•
•
•
•
•
•
•
•
•
•
APPLICATIONS
• General
• Specific
(*1)
(*2)
Conveyors
Machine tools
Pumps
Fans
Pumps
Fans
Compressors
Blowers
Slow speed ranges
Conveyors
Pumps
Fans
Packaging equipment
Wound Rotor with
Slip Energy Recovery
Eddy Current
Coupling (ECC)
• For applications with a
wide range of speed
adjustment and a lowmoderate starting torque
• Used for medium and low
speed applications
• General purpose
• Used if speed range is
narrow (70%-100%) and
reversing not required
• General purpose for
equipment normally
operating at full speed
•
•
•
•
•
•
•
•
•
•
•
• Large pumps & fans with
limited speed range
• Compressors
• Kilns
• Conveyors
• Mixers
•
•
•
•
•
DC Drive
Extruders
Machine tools
Mine hoists
Cranes
Elevators
Rotary kilns
Rubber mills
Printing presses
Shakers (foundry or car)
Winches
Public transportation
A totally enclosed motor is usually required because the ECC is normally used in close proximity to the driven machine (e.g., machine tools).
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
32 Adjustable Speed Drive Reference Guide
Fans
Pumps
Blowers
Fluid propulsion systems
Driving extruders
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CHAPTER 5
STANDARD AND OPTIONAL
FEATURES
• See Table 2 on the following page for a general guideline list of
standard and optional features for AC variable frequency drives
and new power electronic devices. Note, however, that
manufacturers may differ on some factors.
Chapter 5: Standard and Optional Features
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TABLE 2. ASD and Electronic Motor Features
ASD Standard
Protection Features
ASD Optional
Features
Overvoltage
Soft start
Undervoltage
Overload protection
Overcurrent
Torque limit
Loss of control power
Power outage
ride-through
Across-the-line start
Line-to-line shorts
on output
Line-to-ground shorts
on output
Brake stop
Coast stop
Bypass
Continuous overload
Motor slip
compensation
Locked rotor
Electronic reversing
Motor single phasing
Voltage boost (at start)
Accel/decel
Regenerative power
protection
Low speed jog
IR compensation
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New Power
Electronic Devices
Metal oxide semiconductor (MOS)
controlled thyristors
(inverter switches)
Insulated-gate bi
thyristors (IGBT are
more capable of rapid
energizing)
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CHAPTER 6
ADVANTAGES
• Electronic AC or DC adjustable speed drives have a number of
advantages over mechanical, hydraulic and fixed speed drives.
They include a continuous speed range from 0 to full speed,
improved process control, improved efficiency and potential
energy savings, enhanced product quality and uniformity, soft
starting/regenerative braking, wider speed, torque and power
ranges, short response time, equipment life improvement,
multiple motor capability (except CSI), easy to retrofit (except
CSI), bypass capability, increased productivity, safe operation in
hazardous environments, reduction in vibration and noise level,
re-acceleration capability, reduced maintenance and downtime
and operation above full load speeds.
• Motor diagnostics are available in feedback controls.
S PEED C ONTROL
• ASDs are used to control production speed in conveyor
systems in the food, paper, automotive, and consumer goods
industries. In mining, ASDs are used in crushers, grinding mills,
rotary kilns, presses, rolling mills, and textile machinery.
Chapter 6: Advantages
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P OSITION C ONTROL
• ASDs are used for machine tools.
T ORQUE C ONTROL
• ASDs are used for tensioning (winders).
H IGH E NERGY S AVINGS P OTENTIAL
• Applications with highest energy savings potential are
centrifugal pumps and fans (power is proportional to speed
cubed), pumping applications (municipal water systems,
centrifugal chillers, chemical/petrochemical industries, pulp and
paper plants and food industries) and replacing damper controls
in air handling and ventilation applications.
S OFT S TARTING /R EGENERATIVE B RAKING
• When a constant speed drive starts up, the surge of inrush
current that moves the motor out of its stationary position is
about six times the ordinary current, thus producing much
stress on the equipment, especially the windings.
• With adjustable frequency drives, acceleration times can be
adjusted from instantaneous up to several minutes, thus
providing soft starting capabilities.
• Regenerative braking is used when the rapid reduction of motor
speed in a controlled manner is needed for production or safety
reasons. It is a form of dynamic braking in which the kinetic
energy of the motor and driven machinery is returned to the
power supply system. The motor becomes a generator when
the driven load is applying torque in the reverse direction.
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E QUIPMENT L IFE I MPROVEMENT
• The soft starting feature reduces water hammer and cavitation
situations 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 during
starting and therefore also reduces stresses on windings,
starting currents and heating.
M ULTIPLE M OTOR C APABILITY
• One multiple motor ASD (except CSI) can control a number of
synchronized motors at the same speed (e.g., in the textile
industry).
B YPASS C APABILITY
• The adjustable frequency drive can be for service, without need
to shut down the driven equipment (with additional circuitry
optional).
S AFE O PERATION
IN
H ARSH E NVIRONMENTS
• Adjustable frequency drives offer safe operation in harsh
environments since the drive can be housed in a remote
location.
T EMPORARY
OR
B ACK - UP O PERATION
• Instead of operating a second pump or fan for temporary
service when extra pressure or flow is required, use a larger
capacity single pump or fan under ASD control to meet the
EXACT requirements at ALL times.
Chapter 6: Advantages
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N OISE L EVEL
• Vibration and noise level are reduced when the operating speed
of the equipment is lowered and because valves or vanes are
eliminated.
R E - ACCELERATION C APABILITY
• Some adjustable frequency drives continue to have power
supply during power losses of short duration, whereas fixed
speed devices would trip out.
T IPS
AND
C AUTIONS
• If using multiple motors, each one must be protected by its
own overload relay. The total current drawn by all the attached
motors must be equal to or less than the current rating of the
controller.
• Equipment life will be prolonged only if the proper precautions
are taken for power conditioning. Poor quality power can cause
overheating, insulation damage and even equipment destruction.
• Consider torsional harmonics. Avoid operating at speeds
coincident with rotating equipment natural frequencies
(resonance).
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CHAPTER 7
APPLICATION
CONSIDERATIONS
H OW
TO
S ELECT
AN
ASD
• Use this section as a general guide. The information provided
does not address differences in types of driven equipment.
• Essentially, selecting an ASD involves matching the
performance 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 simply
be oversized for the needs of the process, if present throttling
devices are frequently on.
- Describe the range of speed control. An ASD offers a
continuous range from 0 to full speed. If only a few select
operating points are required, a multi-speed motor may be a
better choice.
- Estimate the process duty cycle (see Figure 18). Duty cycle is
a listing of the process operating points (for example, fan
pressure and flow) and the duration each point occurs. This is
perhaps the most important part of assessing the need for an
ASD in a particular application. The duty cycle characterizes
the process being served by the motor.
Chapter 7: Application Considerations
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- Gather equipment performance data. Performance curves
supplied by the equipment manufacturer describe the power
requirements of the driven equipment at selected operating
points. 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 note
that performance ratings and field ratings may differ. Consider
getting the help of a qualified installation and set-up contractor to
verify field performance.
- Operating points are the intersection of the particular process
system curve and the equipment’s characteristic
performance curve.
- System curve is the set of points that describes the volume of
flow and resistance to flow as defined by the application.
- Throttling, or dampers, change the system curve by
increasing the resistance to flow.
- Performance curve is the set of points of flow vs. pressure
that the particular fan, pump or blower must follow at a
particular speed and fluid density. Manufacturers usually
supply performance curves that give the selected design
point.
- Brake horsepower and efficiency vs. flow are also supplied by
the manufacturer. They determine the motor and any
gearbox or belt sheave reduction necessary to achieve the
correct speed.
- Calculate constant and ASD power requirements. Using the
formulas in Appendix A, calculate the power required for
each 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 required
at each operating point by the annual hours at the point from
the duty cycle, then sum the total for constant and adjustable
speed.
- Select a drive type and features and estimate costs. Based on
the load type (constant vs. adjustable torque, horsepower,
starting time, speed regulation, speed, torque range,
regeneration, shielding, transformers, installation, control
logic and other specific features listed in this guide), select the
type of drive for the application. Obtain manufacturers’
quotes. Prices will depend greatly on whether you need a
custom-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 annual
energy savings (adjustable vs. constant speed) by the utility
energy rate charge. Divide the total installed cost by annual
energy savings. The result is simple payback in years.
- Consider other ASD savings, such as reduced wear due to
soft start, lower maintenance costs and less material wastage
resulting from more accurate speed adjustment. These
savings are difficult to estimate and can usually be
determined 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.)
Chapter 7: Application Considerations
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S OFTWARE
F INANCIAL E VALUATION
• Software is available from several ASD suppliers, including
some utilities. Be careful to include lower part-load efficiencies
when inputting performance data.
L OAD C HARACTERISTICS
Varying Duty Cycle
• The load profile or duty cycle will also indicate the potential
suitability of an ASD for an application. The duty cycle shows
the typical speeds and corresponding time intervals for which a
motor operates annually. From an energy standpoint, the
ingredients of a good ASD application are high percent throttling
(changing load) and high annual operating hours.
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% Flow
100
Good
Application
0
Time
% Flow
100
Poor
Application
0
Time
FIGURE 18. Duty Cycles
A PPLICATION T YPES
BY
L OAD
• There are three main types of adjustable speed loads: variable
torque/variable horsepower (hp = torque x RPM) (centrifugal
pumps, fans), constant torque and constant horsepower,
(constant tension winders, machine tools).
Chapter 7: Application Considerations
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• The behaviour of the horsepower and torque as a function of
percent speed partially determines the requirements of the
motor/controller system.
• For an induction motor, the speed-torque relationship depends
on the voltage and frequency of the supplied voltage as well as
the characteristics of the rotor conductors.
• Constant torque drives are often supplied as “standard” drives.
To make a variable torque drive, the manufacturer usually adds
a jumper and chopper to the standard model.
• Examples of variable torque loads are centrifugal loads, where
torque is proportional to RPM2, where horsepower is
proportional to RPM3 such as fans, pumps and blowers
(dynamic).
• Examples of constant torque loads are agitators, positive
displacement 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 that
some may not be constant torque loads but require constant
torque drives due to shock overloading, overload or high inertia
load conditions.
• Examples of constant horsepower loads are drilling machines,
lathes, machine tools, milling machines and centre-driven
winders. Note that torque is inversely proportional to speed.
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Percent hp and Torque
100
80
60
40
Torque
hp
20
0
50
Percent Speed
100
FIGURE 19. Variable Torque Load
Percent hp and Torque
Torque
100
80
60
hp
40
20
0
50
Percent Speed
100
FIGURE 20. Constant Torque Load
Percent hp and Torque
hp
100
80
60
Torque
40
20
0
50
Percent Speed
100
FIGURE 21. Constant Horsepower Load
Chapter 7: Application Considerations
45
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C AUTIONS
• The variable torque controller is designed to provide 100%
rated torque continuously with no overload capability. This
should be used only for applications where the load torque
varies proportionally with speed, such as fans and centrifugal
pumps. The current rating of the motor must be checked with
Fan/Blower (incompressible flow)
Outlet Damper Control
ASD Control
Pressure
Unstable
Area
Performance
System
Inlet Guide
Vane Control
Flow
Flow
Pump
Valve Control
ASD Control
System
Perf
nce
Performance
System
Static
Pressure
Dynamic
orma
Flow
Flow
FIGURE 22: Power Required is Proportional to RPM3
Centrifugal Fan/Blower, Pump
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the current rating of the controller to ensure that the controller
can provide the full horsepower capability of the motor.
• Low speed motor cooling does not limit the speed range with a
variable torque load since the load requires less torque at lower
speeds. For this type of load, it is important to choose a
horsepower rating for the highest speed attained.
• The minimum allowable motor speed for continuous constant
torque or constant horsepower operation is determined by the
motor cooling requirements at low speeds. These methods can
be 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 its
nameplate rating. This provides additional mass for heat
dissipation. However, this may result in oversizing the drive
to compensate for the increased magnetizing current.
- Use a motor with a high service factor. Specify class F or H
insulation.
- Use a high efficiency motor.
• Also, see “Thermal Considerations.”
• Torsional harmonics may occur if resonant frequencies coincide
with reduced speeds. These can be programmed out by
the ASD.
• Low speed operation can cause mechanical instability if it
results 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 of
parallel pumps or fans.
• Sizing the drive means matching torque, speed, voltage, current
and 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 or
recirculation lines are replaced by higher efficiency ASDs.
• ASDs offer the best potential for energy savings when controlling
the speed of centrifugal fans, pumps and blowers. The power
required is proportional to RPM3. Therefore, a 10% drop in speed
results in a 27% drop in power consumption (1.0-0.93).
Power Required
Damper
Control
Saving
ASD
Control
Speed
FIGURE 23. Power Savings in Fans and Pumps Using ASDs
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• Demand savings are not attributable to ASD control, however,
since achieving better speed control does not usually result in
downsizing absolute power requirements. There may be “time
of use” demand savings (taking advantage of reduced speed
operation during utility peak demand periods).
• In-rush current is about 600% rated current when started at full
voltage and frequency. If the motor is started at low voltage and
frequency through an ASD, it will never need more than 150%
of rated current (started at 2 Hz). This soft start reduces stresses
on the motor, extending its life.
M OTOR /D RIVE S YSTEM
• If, after examining the load characteristics and process
requirements of an application, it appears that an ASD may be
an asset, investigate motor/drive compatibility.
• If a drive is to be retrofitted to an existing motor, get this
information from the motor: nameplate voltage and
horsepower, current and torque data, insulation class and
NEMA design characteristic.
• Manufacturers’ curves should be consulted to aid in motor
selection for new systems.
• When considering the information here, also look at Table 1,
because the table lists typical applications for each of the drives
and may help you narrow the choices available for a particular
application. It should be used when conducting the remainder
of the selection process.
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Motor Type
• Your choice of available drives depends to a large extent on the
motor used. Although DC systems were largely used in the
past, AC motors are much more popular now due to their
relatively low cost, low maintenance requirements and better
reliability. 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 to
approximately 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 by
current source inverters or by load-commutated inverters or
cycloconverters. These high-powered systems are very
expensive to purchase for use in the lower end of their operating
ranges. Medium Voltage AC induction motors are now available
under ASD control.
• It is important to determine the maximum horsepower
requirements of the driven load and how the required power
varies 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
50-200
200-1,000
208V to 600V three-phase
460V to 600V three-phase
low voltage (460V, 600V) and medium
voltage (2,300V, 4,160V)
mostly medium voltage
(2,300V, 4,160V)
medium voltage
(4,160V, 6,900V, 13,800V)
13,800V
1,000-2,500
*2,500-10,000
*>10,000
(usually DC, or wound rotor)
• Note that suitably rated transformers may be used to match the
drive voltage rating to that of available power supply voltages.
• The system voltage should be within the deviation permitted
by the specifications for the ASD. This is usually +10% and
-5% per NEMA standards. Specific values can be obtained from
the manufacturer.
Torque and Current
• After checking the horsepower requirements, ensure that the
starting torque and full load torque are within the motor’s
rating.
• Continuous permissible running torque decreases with motor
speed.
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• It is important to ensure that the drive can supply the required
current. Inverters are current-limited and may only allow a
relatively high output current for short time periods. An
estimate of the motor torque to current ratio can be made by
referring to the motor speed, torque and current characteristics.
• The drive must have a maximum continuous current rating that
is 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 standard
efficiency, class F insulated motor is applicable only to a 2:1
constant torque speed ratio. A high efficiency motor can
provide a 3:1 ratio. To obtain wider speed ranges, the motor
can be oversized.
• Below 6 Hz, however, significant motor cogging may occur as
the motor tries to follow the waveshape. A practical speed
range 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 or
synchronous reluctance motor can be used for an AC system.
Otherwise, a DC system could be used.
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Speed Range
Percent (%) of 60 Hz Torque Rating
100
z
0-60 H
2:1 3
z
0-6 0 H
3:1 2 0 Hz
6
5
1
4:1
0 Hz
1 0-6
6:1
0 Hz
6
5
.
7
8:1
0
-6 H z
1 0:1 6
90
80
70
60
Induction Motor:
Constant Torque Load,
USEM 4-P TEFC 460 V
30 60 Hz Motor With
Boost At Low Frequency.
50
2 0 : 1 3-6 0
40
Hz
Source Data:
EIC Program Based on
Constant Temp. Method
30
0
1
20
40 50
75
100
125 150
200
Motor Horsepower – 60 Hz Rated
250
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 a
drive above 60 Hz, provided the V/Hz ratio is maintained. The
motor is rated at V/Hz; as speed increases at constant rated
torque, the horsepower output increases. The drive must be
sized to accommodate the horsepower rating as well as motor
current and voltage.
Speed Regulation
• Mechanical loads cause a drop in motor speed (according to its
speed/torque curve).
• Tachometers can monitor motor shaft speed through a feedback
loop to the drive controller, which sends a compensation speed
increase signal to the ASD.
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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, which
determines ASD speed regulation capability.
Time required to accelerate the load:
T(sec) = WK2 (lb-ft2) x change in RPM
308 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 5250
RPM
T HERMAL C ONSIDERATIONS
• If variable frequency controllers are used, there are a number of
important factors to consider to ensure that the motor/drive
system is compatible from a thermal standpoint.
•
The main concern when retrofitting existing motors with variable
frequency drives is to ensure that the controller can provide the
current required for the load torque to prevent motor overheating.
• Since the cooling systems of most motors are designed for a
fixed speed, the cooling action will be reduced when operating
at reduced speeds (since cooling fan speed decreases with
motor speed). This is especially true for constant torque
applications and applications in which CSI drives are used. For
these situations it is important to provide additional cooling or
overframe or derate the motor. An overframed motor may also
require 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 overheat
because of the harmonics in the AC waveform supplied by the
inverter. This is especially true for standard motors. (See
Chapter 9, “Harmonic Distortion.”)
• Harmonic losses are affected by the design type of NEMA
speed torque characteristics as well as the characteristics of
the motor under consideration. The motor leakage reactance,
which limits harmonics, varies with each NEMA design. The
compatibility of variously rated motors with inverters is useful
to know. See Table 3 for the most suitable motor
design/inverter combination to use.
TABLE 3. Suitability of Inverters for NEMA Motor Designs
Motor NEMA Design
VVI
CSI
PWM
High Efficiency Motor
A
B
C*
D*
X
X
X
X
X
X
* These motors are very undesirable for adjustable frequency control, due to
high harmonic losses.
• NEMA design B squirrel-cage induction motors are commonly
used in industry.
• Energy efficient motors have lower losses than standard motors
and therefore provide wider torque capability when used with
variable frequency drives.
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O THER C ONSIDERATIONS
• The next step in the decision process is to evaluate the relative
importance of each of the remaining factors to be considered.
One of these factors may exclude one drive system. For
example, if the system is to be used in an explosive
environment, commutators and brushes cannot be used
because of the sparks that would be generated.
• These are some other selection considerations: economics,
process requirements and load characteristics, performance
required (speed regulation/control accuracy, efficiency and
reliability), starting and stopping characteristics (load inertia),
torque (breakaway torque, accelerating time and torque and
decelerating 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 at
the beginning of this chapter. Although initially used as
indicators, the importance of these factors should now be
compared with all other considerations.
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E FFICIENCY
• At full speed and full load, VVI, CSI and PWM drives are all
about 95% efficient. Efficiency drops at approximately a square
rate with speed, as commutation losses (thyristor closing) vary
with torque and current.
Driver Losses (kW)
4.0
3.5
PWM
3.0
2.5
2.0
VVI
1.5
CSI
1.0
.5
450
900
1350
1800
Speed (RPM)
FIGURE 25. Watts Loss (Efficiency) Comparison
100%
Percent Efficiency
75% Load
100% Load
94%
90%
50% Load
86%
25% Load
82%
78%
74%
0
20
40
60
80
100
Percent Speed
FIGURE 26. Typical AC Drive Efficiency (PWM)
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• CSI drives tend to be more efficient than VVI and PWM as
speed is reduced.
• Higher horsepower sizes, as well as drives operating close to
their maximum design rating, tend to be at higher efficiency.
• Information about efficiency of drives is generally not easily
obtained from manufacturers since so many factors affect it.
• Motor efficiency at reduced speed needs to be recalculated.
R ELIABILITY
OF
ASD S
• Reliability of ASDs has improved as power electronics
technology has advanced. Thyristors convert to AC to DC
power and GTO designs improved reliability. Metal oxide
semi-conductor controlled thyristors, surface mount technology
and 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 with
centrifugal loads due to the drop in motor current and I2R
losses.
• CSI drives use the motor as part of the circuit, so selecting the
motor and drive together will minimize risk of mismatching.
• Transistors can be made for high current and voltage and faster
response than thyristors.
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• Constant voltage/frequency ratio means the motor will not stall
when overloaded, maintaining constant speed regardless of load.
• The motor may trip out when decelerating rapidly. With large
inertia loads, regeneration of power back through the drive
may trip the voltage protection bus. Elevators and lowering
conveyors are examples. Sizing the protective bus to suit the
application should prevent it, (see recommended technical
specifications for medium voltage drive).
A PPLICATIONS
• Constant torque (hoists, presses and conveyors) operation up to
120 Hz can be provided by applying constant V/Hz to the
motor. This requires an AC drive with twice the voltage output
capability than the supply voltage to the motor (@ 60 Hz).
Since a motor is rated at V/Hz, it can be operated at rated
torque and twice the speed if voltage and frequency are both
doubled. Operation at twice the motor-rated horsepower
requires sizing the AC drive at that horsepower and considering
stresses and balancing on the motor.
• Position control is important in materials handling, machining
and robotics.
• Multiple motor operation in parallel by a single voltage inverter
AC drive can be done by sizing the drive to the sum of the
maximum continuous running currents of each motor. All
motors start and stop together. If motors are coupled together
through the load, load sharing must be considered. High-slip
NEMA design D motors may be required. Also, individual
motor overload protection is necessary.
• Cogging refers to torque pulsation at below 6 Hz frequency. If
smooth 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 Hz
maximum frequency, (eliminated with vector drives).
• IR compensation is a circuit that senses changing motor load
and 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 generator
when driven by the load. The energy is returned to the power
lines through the drive. The drive must be sized to handle the
energy absorbed. Hoists, flywheels and other constant torque
applications make use of regenerative braking. Centrifugal loads,
such as fans, pumps and blowers, do not.
P ERFORMANCE R EQUIRED
Speed Regulation/Control Accuracy
• The importance of the drive’s sensitivity to changes in load,
temperature, humidity, drift and line voltage fluctuations should
be determined.
• If there is to be no speed deviation, a synchronous motor
is used.
• Vector drives can smoothly hold position and speed and torque
over a full range from 0% to 100% of scale.
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Efficiency
• System efficiency = mechanical power output from motor shaft
electrical power input to drive
• The efficiency of a motor/drive system depends on
characteristics of the connected motor (power factor, efficiency),
speed range and duty cycle, load, measurement method and
instrument accuracy, inverter size and horsepower rating, input
power 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 largest
contributors to efficiency differences.
• High efficiency motors are more susceptible to tripping due to
heat, voltage or current drops.
• Multi-speed motors (i.e., pole changing motors) offer fixed
speed combinations (two to four is typical) that are a much
cheaper alternative to ASDs if continuous speed adjustment is
not 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 other
inverters as operating speed is reduced.
S TARTING
AND
S TOPPING C HARACTERISTICS
• Are soft starts or controlled acceleration needed for the driven
machine?
• Does the power supply system need reduced voltage starting or
controlled acceleration?
• Does the driven machine require accurate positioning,
controlled deceleration or regenerative braking?
T ORQUE
• The ability of the drive to reach the torque required at various
points in the process cycle should be considered.
• Breakaway or locked rotor torque is needed to start the load
from rest to overcome static friction. An ASD can provide a
voltage boost that will permit this torque to be higher than
normal. The inverter components may have to be sized larger
and the current limit set higher if this is the case.
• Accelerating time and torque is needed to increase the speed of
the 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 can
provide 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.
E NVIRONMENT
• Abrasive, moisture laden, explosive, dusty or otherwise difficult
environments may affect the ability of the drive/motor system
to function and the ability to provide adequate maintenance.
The effects can be eliminated by careful design or locating the
drive in a clean, cool room. Providing an adequate cooling air
supply for air-cooled converters is another important
consideration.
W EIGHT
AND
S PACE
Motor Torque 7 hp (Continuous)
% Motor Full Load Rating
• There may be space and economic considerations involved in
decisions concerning large drives due to their size.
Standard Operating
Speed Range
Extended
Speed Range
150
High Eff. Motors
100
80
Torque
hp
hp
1.15 SF Motors
50
Torque
hp
0
6 15
30
60
Frequency (Hertz)
90
120
FIGURE 27. Motor Performance, Typical 60 Hz
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Percent Torque
300
200
100
2
10
20
30
40
Frequency (Hertz)
50
60
FIGURE 28. Ideal Torque-Speed Curves
Pull Out or
Breakdown Torque
200
Locked
Rotor
or Stall
Torque
100
NEMA Design B Motor
Torque vs. Speed
Operating
Point
Load Torque
Slip
Percent
Torque 0
10 20 30 40 50 60 70 80 90 100
Percent Speed
Synchronous
Speed
Operating Speed
FIGURE 29. NEMA Design B Motor Torque-Speed Curve
A CCESSORIES
• Accessories include auto transformers (for voltage overload
protection), regenerative braking circuits (overhauling loads in
constant torque such as cranes), bypass loop (for operating the
motor directly bypassing the drive), filters and the line chokes
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(to reduce electric harmonics), cooling fans and programmable
logic controllers (PLC) for speed control through feedback
process monitoring.
S AFETY
• Follow NEMA recommended enclosure design and installation
specifications. High voltage and current are present. DC power
is more dangerous than AC; DC is found in both AC and DC
drives. The DC bus can be more than 600V in AC drives if input
power is not checked in harmonic spikes. All circuit boards
should be covered in metal for shielding and cooling. Access
interlocks should shut down and disconnect the drive input
power before the cabinet can be opened. Manual control panels
are restricted to 120V.
• It is important to separate control wiring from power wiring.
Use separate metal conduits to reduce electronic “noise” from
power to control circuits (see Chapter 9).
S ERVICE
AND
M AINTENANCE
• AC and DC drives can be located several hundred feet away from
the 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 electrical
connection to the motor and power lines.
• Solid-state electronics are relatively maintenance free. Most drive
manufacturers supply built-in diagnostics as well as protection
relays and fuses. As proper drive performance depends on
matching motor and load requirements, expert trouble-shooting
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may be necessary. For this reason, consider purchasing a service
contract until you have experience with the particular drive.
• The frequency and degree of complexity of maintenance
requirements for a particular system can be a significant factor.
Are company personnel restricted to the type of equipment they
service? Are self-diagnostics supplied with the control module? Is
the supplier willing to service the drive after purchase? Does the
supplier have representatives located close to you? Purchasing
equipment from a supplier who has no representatives close to
the installation may result in the supplier losing interest in
installation and maintenance after the purchase is made.
• Mechanical devices that provide adjustable speed need more
maintenance than their electrical counterparts.
• Servicing a PWM inverter requires a complex diagnostic aid
equivalent to a logic analyzer. CSIs and VVIs, on the other
hand, are easy to service since each part of the system can be
operated independently to isolate problems.
Programmability Needed
• Will it be necessary to frequently change the operating characteristics
of the drive, as offered by a PC or a drive equipped with a
microprocessor?
Lead Time for Delivery
• Lead time for delivery depends on whether the application requires a
small horsepower drive (the equipment may be purchased off the
shelf) or if increased horsepower or custom-engineered features, such
as line filters, chokes and autotransformers, are needed. Drive
sophistication or special options, such as cooling requirements,
unique process control interface or packaging, tend to lengthen lead
time. 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 imposed
for an installation, this should be considered. Normally, the
power factor is satisfactory.
Mechanical Considerations
• If adjustable speed is being considered, the natural torsional
frequencies of the connected mechanical loads should be
checked to ensure that they do not correspond to the frequencies
produced at lower operating speeds. This may still be a concern
even if the motor is mounted to a massive support pad.
T IPS
AND
C AUTIONS
• When a few discrete speeds are needed, a multiple speed
motor may be satisfactory. It would be significantly cheaper
than purchasing a variable frequency drive. This can be
accomplished by using a special stator winding arrangement.
This method is often used for applications involving pumps,
fans, blowers, conveyors and printing presses. For example, a
2:1 speed ratio is easily obtained from a single stator winding
by reconnection.
• On new installations, an ASD can replace the standard motor
starter. All that is needed is a feeder breaker to protect the
cables to the controller.
• If retrofitted, the existing motor is usually retained, but it must
be derated.
• It is important to ensure that the electrical supply line uses the
correct voltage. This is particularly important in view of the
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large number of electrical drives that are manufactured outside
Canada and where different standards may be used.
• Circuit breakers, transformers, fuses and disconnect switches
may or may not be included in an ASD system. If this
apparatus is to be mounted separately, its current rating should
be based on the input current of the ASD, not the motor full
load current. This is due to harmonic effects which cause the
ASD input current to be greater than the motor full load current
for a given power level.
• If a motor drives a load through a gearbox at reduced speed
under ASD control, it may not deliver enough running torque.
Check minimum torque times speed requirements.
• Induction motors cause supply current to lag behind supply
voltage. The ratio of kW to kVA (true power to apparent power)
is the “displacement power factor.” This is the cosine of the
phase angle between current and voltage, when current and
voltage are assumed to be clean sine waveforms.
• Harmonic currents from inverter switching may increase
apparent power and decrease power factor. This “true power
factor” will be less than displacement power factor cosine of
current 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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CHAPTER 8
ECONOMICS
• Economics is usually one of the most important factors
involved in selecting industrial equipment, but it is not
straightforward. Many economic aspects are often ignored in
ASD evaluations.
• Use the Table 4 ASD checklist of costs and savings to avoid
overlooking certain economic aspects.
• The simple payback method is frequently used to determine how
long it would take for a piece of equipment to “pay for itself” in
terms 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 underestimate
the true period until breakeven.
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• The net present value method is a good technique that can be
used to appraise the profitability of an investment. By using
the discounted cash flow technique, it takes into account the
time value of money. A summary of this approach appears in
Table 5.
TABLE 4. ASD Checklist of Costs/Savings
Capital Costs
Capital
Savings
Drive
Control valves
Motor
Gear box
Power
conditioning
equipment
Fluid coupling/
mechanical
speed
changing
equipment
Installation
Electrical
system
upgrade
Reducedvoltage starters
Torsional
analysis
Space
requirements
Cooling
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Operational
Costs and
Savings
Energy (total
energy
consumed,
peak demand
charge)
Maintenance/
useful life/
downtime
Overspeed
capability
Other
Salvage value
Tax
implications
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TABLE 5. ASD Investment Decision Technique
For detailed examples of this procedure and relevant software, contact your local
utility.
1.
Evaluate the cost/savings of the factors in Table 4 for each option you are
considering (for example, purchasing an ASD, purchasing a mechanical
drive system, not purchasing a variable speed drive). Capital costs will be
expressed in total dollars; operating expenses will be expressed in terms of
time.
2.
Determine the real discount rate that should be used for each timedependent and future-valued factor. For example, for energy savings
calculations:
x% per annum = nominal discount rate
y% 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 be
discounted using the rate at which the interest rate is expected to rise
between 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 timedependent factors should be chosen as a reasonable payback period.
Present value tables and annuity tables are useful for the discounting
process.
4.
The net present value (NPV) of each option is found by summing the costs
and savings that have been calculated in present value terms for each
factor.
5.
For any option, if
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 relative
difference between the option for each factor and finding one NPV.
NPV >0,
NPV <0,
NPV = 0,
there is a net gain
there is a net loss
breakeven occurs at the time
under consideration.
Chapter 8: Economics
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E CONOMIC FACTORS
C APITAL C OSTS
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, a
battery back-up for the controls may be provided for auto restart 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 as
harmonic filters, should be included. This includes filters for
incoming power to the motor as well as power conditioners
for harmonic voltages and currents sent back to the power
supply from the drive.
Installation
• Installation, labour and commissioning charges for the drive and
motor and power conditioning apparatus should be determined.
Electrical System Upgrade
• Upgrading of the electrical system may be necessary if higher
reliability is required than the present system can offer. Potential
upgrades include relay protective systems, supply transformer
redundancy, transfer switching/alternate feeders, maintenance and
emergency staff training and preventive maintenance programs.
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Torsional Analysis
• A torsional analysis will define the vibration effects of inverter
harmonics in the drive train. It should be conducted for large
drive 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. For
large applications, although HVAC equipment is often used,
water cooling may be a much more economical alternative.
C APITAL S AVINGS
• Use of an ASD may avoid certain capital investments. Examples
are gear boxes, control valves, fluid coupling/mechanical speed
changing equipment and reduced voltage starters.
O PERATING C OSTS
AND
S AVINGS
Energy
• There may be savings in terms of both energy consumed and
peak demand charge. The extent of these savings depends on
the local utility’s rate schedule. If an ASD is installed, the total
energy 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 peak
current it must deliver during the month. The most significant
factor affecting KVA demand is the power required by the load,
Chapter 8: Economics
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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 energy
saved are the last ones that would have otherwise been
purchased. The use of average energy cost can be very
misleading.
Maintenance/Useful Life/Downtime
• The reduction of maintenance and downtime may be quite
substantial if an AC variable frequency drive is employed.
Contributing factors are elimination of control valves, current-limit
feature (prevents motor burnouts caused by multiple restarts) and
protection of the motor insulation (so it is shielded from voltage
problems).
• Useful life of equipment, such as bearings, can be extended by
operating at reduced speeds. Stresses and metal fatigue in the
drive train shafts will be lowered.
• Repairs to variable frequency drive systems do not usually take
much time.
Overspeed Capability
• The overspeed capability of adjustable frequency drives can
save considerable operating costs, as well as investment, if
increases in production levels occur. For example, the airflow
through an existing fan can be increased by retrofitting a
variable speed drive to its motor so that the motor is supplied
with a frequency higher than an existing 60 Hz rating.
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C AUTIONS
• An adjustable frequency drive is the most cost effective choice
if the duty cycle is more evenly distributed over the entire range
of flow rates.
• Relative energy savings improve if the performance and system
resistance curves are steep.
• Many potentially good ASD applications are passed up because
benefits other than energy savings are overlooked. Frequently,
however, process control and reliability far outweigh efficiencyrelated benefits to the user.
• By using the average cost of energy in savings analyses, the
savings can be significantly overstated for variable frequency
applications. Instead, both the energy and demand charges of
the local utility’s rate schedule should be used.
• For variable torque loads, the variable frequency drive is very
energy conscious, since the horsepower varies proportionally to
the cube of the speed.
• Coupling systems (eddy current and hydraulic) have the
quickest economic payback. Electronic drive systems have the
highest dollar return.
• Cost savings through reduced energy consumption often result
in ASD payback periods of five to six years, rather than the two
to three years normally required by industry.
• Induction motors tend to be cheaper than DC motors for
similar horsepower ratings.
Chapter 8: Economics
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• See Figure 27 for a relative purchase price comparison for
various motor/drive systems. For explosion-proof
environments, the relative cost of the DC motor would
increase substantially.
• For horsepower applications above 50, installation costs are
usually comparable to the total capital cost for the drive. Below
this power rating, installation costs may be as much as double
the drive cost.
• Software packages which evaluate the economic aspects of
adjustable speed drives are available. It is important to keep in
mind that these programs require part-load efficiency within
their analyses.
Controller
Valve
Eddy
Current
Coupling
Motor
Motor
Valve
Control
Slip
Control
Motor
Controller
Motor
DC
AC
Solid-state
Control
FIGURE 30. Capital Cost Comparison of Motor/Drive Systems
Medium HP, Voltages
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CHAPTER 9
HARMONIC DISTORTION
• Adjustable Speed Drives have defined capability to withstand
voltage and current waveform distortion.
• The latest IEEE standard 519 defines harmonic distortion limits
acceptable on the input side to the AC power system.
• For a full discussion of harmonic distortion and mitigation
techniques, see Ontario Hydro’s Power Quality Reference Guide
and Power Quality Mitigation Reference Guide.
H ARMONICS
• There are two types or harmonics: electrical and mechanical.
• The inverter (switching) section of an ASD generates
harmonics.
• Electrical harmonics cause waveform distortion. They are
currents or voltages that oscillate at integer multiples of the
fundamental 60 Hz frequency, which is the main power
frequency. For example, a frequency five times the fundamental
frequency is called the fifth harmonic.
Chapter 9: Harmonic Distortion
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Bus Voltage
Line Current
FIGURE 31. Harmonic Distortion
• If large, electrical harmonics may cause power system
waveforms to deviate from perfect sinusoids, eg.: capacitor
switching, large induction, motors start-up.
• Any static power converter that converts AC to DC or DC to
AC, or any solid-state switch, generates harmonics (e.g.,
thyristors or SCRs).
• All adjustable frequency drives with power switching devices
generate harmonics.
• The odd harmonic amplitudes usually decrease with
increasing frequency, so the lowest order harmonics are the
most significant. Even harmonics are normally not generated
by three-phase converters.
W HAT H ARMONIC D ISTORTION C AN D O
• As with many other forms of pollution, harmonic distortion
affects the whole electrical environment. It propagates through
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the power system and may even show up at distant points
outside the plant, thus causing problems for other equipment
connected to the power supply.
• Typical effects of harmonics on the motor/drive system are
reduced motor efficiency due to increased losses, increased
heating of motors, cables and transformers, excessive voltage
stress on insulation of motor windings and torque pulsations,
(“torsional harmonics”).
• General problems caused by harmonics are general degradation of
power quality, voltage dips or voltage ripple, premature
equipment failure, improper operation of important control and
protection equipment, interference with telecommunication or
computer systems, amplification of harmonic levels resulting
from resonance, incorrect readings on mechanical timing relays
and watt-hour meters and blown fuses.
• All capacitors, including those used for power factor correction,
tend to be very susceptible to harmonic damage. Disastrous
consequences can occur if capacitors are exposed to excessive
harmonic voltages or currents.
• The harmonics produced by a converter may increase motor
losses by 5% to 10%.
P RODUCTION
AND
T RANSMISSION
• Harmonics are produced in utility or industrial electrical
systems by equipment that switches repetitively in less than a
cycle, such as variable frequency AC drives, cycloconverters,
DC drives, rectifiers, UPS systems, arc furnaces and static VAR
generators. Fluorescent and gaseous discharge lamps can also
produce harmonics.
Chapter 9: Harmonic Distortion
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• Harmonics occur as long as the harmonic generating equipment
is in operation. They tend to be of a steady magnitude.
• Harmonic currents flow through the impedance of the
transmission and distribution network and generate harmonic
voltages, 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 drive
depend on the drive configuration and the system impedance.
• Harmonics may be greatly magnified by power factor
correction capacitors. Supply system inductance can resonate
with the capacitors at some harmonic frequency, causing large
currents and voltages to develop. This may damage equipment.
In addition, since the impedance of a capacitor decreases with
increasing frequency, capacitors tend to act as sinks for higherorder harmonics.
I SOLATION T RANSFORMERS
• Isolation transformers are frequently used to protect the drive as
well as the AC line from distortion. They may also decrease the
available short circuit current in a fault situation and prevent
drive shutdown and possible damage in the event of a motor
line 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 ordering
equipment should include fault level at the service entrance,
rating and impedance of transformers between the service
entrance and the input to the power conditioning equipment
and details of all capacitor banks on the same utility substation.
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O THER G UIDELINES
• There are no current CSA standards specifically relating to
ASDs. CSA approval may be granted to many different drive
designs, many of which are imported.
• Minimum guaranteed full load, full speed ASD efficiency of
95%, (including any supplied equipment: isolation transformers
and filters).
• Harmonic Distortion: Latest recommended specification: IEEE
519-1992: total voltage harmonic distortion shall not exceed
5% at common coupling ASD to motor.
Chapter 9: Harmonic Distortion
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APPENDIX A
APPENDIX A
Formulas for Calculating Applications
C ALCULATING H ORSEPOWER
Once the machine BHP (speed x torque) requirement is
determined, horsepower can be calculated using the formula:
rated motor hp = motor efficiency (%)
100
= available hp
BHP =
TxN
5,250
(required hp)
Where,
hp = horsepower, supplied by the motor
T = torque (lb-ft), force x radius
N = base speed of motor (rpm)
If the calculated horsepower falls between standard available motor
ratings, select the higher available horsepower rating. It is good
practice to allow some margin when selecting the motor
horsepower.
Appendix A
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For many applications, it is possible to calculate the horsepower
required without actually measuring the torque required. The
following will help:
BHP = brakehorsepower, the mechanical load required by the
driven equipment
F OR C ONVEYORS
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
F OR W EB T RANSPORT S YSTEMS
AND
S URFACE W INDERS
Note that the tension value used in this calculation is the actual
web tension for surface winder applications, but it is the tension
differential (downstream tension – upstream tension) for
sectional drives.
C ENTRE W INDERS (A RMATURE C ONTROL O NLY )
hp =
tension (lb) x line speed (FPM) x buildup
33,000 x taper
C ENTRE W INDERS (F IELD C ONTROL )
If taper x field range ³ buildup, then
hp =
84
tension (lb) x line speed (FPM)
33,000
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If taper x field range ² buildup, then
tension (lb) x line speed (FPM) x buildup
hp =
33,000 x taper x field range
Note that these formulas for calculating horsepower do not include
any allowance for machine function windage or other factors.
These factors must be considered when selecting a drive for a
machine application.
F OR FANS
AND
B LOWERS
Effect of speed on horsepower
hp = k1 (RPM)3 – horsepower varies as the 3rd power of speed
T
= k1 (RPM)2 – torque varies as the 2nd power speed
Flow = k3 (RPM) – flow varies directly as the speed
hp
=
hp
=
CFM x pressure (lb/in2)
229 x (eff. of fan)
CFM x (inches of water gauge total pressure)
6,362 x (eff. of fan)
Total pressure = static pressure + velocity pressure
Velocity pressure =
*(velocity in fpm)
2
x air density
(velocity*
1,096 )
Appendix A
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F OR P UMPS
hp =
GPM x head (ft) x (specific gravity)
3,960 x (% eff. of pump)
Specific gravity of water = 1.0
1 ft3 per sec = 448 GPM
1 PSI = A head of 2.309 ft for water weighing
62.36 lb/ft3 at 62°F
C ONSTANT D ISPLACEMENT P UMPS
Effect of speed on horsepower
hp = k(RPM) – horsepower and capacity vary directly
as the speed.
Displacement pumps under constant heat require approximately
constant torque at all speeds.
C ENTRIFUGAL P UMPS
Effect of speed on horsepower
= k1 (RPM)3 – horsepower varies as the 3rd power
of speed
T
= k1 (RPM)2 – torque varies as the 2nd power of speed
Flow = k3 (RPM) – flow varies directly as the speed
hp
P UMP E FFICIENCY ( 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.
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H ORSEPOWER R EQUIRED
hp = torque (lb-ft) x speed (RPM)
5,250
hp = torque (lb-in) x speed (RPM)
63,000
Torque (lb-ft) =
hp x 5,250
speed (RPM)
Accelerating torque (lb-ft) =
WK2 (lb-ft2) x RPM
308 x t (sec)
Where,
WK2 = inertia (lb-ft2) reflected to motor shaft
ÆRPM = change in speed
t = 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
2
Load RPM
( Motor
RPM )
I NERTIA (WK 2)
The factor WK2 is the weight (lb) of an object multiplied by the
square of the radius of gyration (k). The unit measurement of the
radius of gyration is expressed in feet.
Appendix A
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For solid or hollow cylinders, inertia may be calculated by using
the 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 calculated
by breaking the part up into simple rotating cylinders, calculating
their inertias and summing their values, as shown in Figure A-2.
L
Hollow
D1
Solid
D2
D
L
WK2 = .000681 r LD4
WK2 = .000681 r L(D24 – D14)
WK2 = lb.ft.2
D1D2, D1 and L = in.
r = lb.in.3
r (aluminum)
r (bronze)
r (cast iron)
r (steel)
r (paper)
FIGURE A-1. Calculating Hollow Shafts
88
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=
=
=
=
=
.0924
.320
.260
.282
.0289
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+
=
2
+
2
2
WK 2tot = WK 1 = WK 2 = WK 3
FIGURE A-2. Calculating the Inertia of Complex,
Concentric Rotating Parts
WK 2
OF
R OTATING E LEMENTS
In practical mechanical systems, all the rotating parts do not operate at the same speed. The WK2 of all moving parts operating at
each speed must be reduced to an equivalent WK2 at the motor
shaft, so that they can all be added together and treated as a unit,
as follows:
Equivalent WK2 = WK2
( NN )
2
m
Where,
WK2 = inertia of the moving part
N
= speed of the moving part (RPM)
Nm = speed of the driving motor (RPM)
When using speed reducers, and the machine inertia is reflected
back to the motor shaft, the equivalent inertia is equal to the
machine inertia divided by the square of the drive reduction ratio.
Appendix A
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WK 2
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L INEAR M OTION
Not all driven systems involve rotating motion. The equivalent
WK2 of linearly moving parts can also be reduced to the motor
shaft 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 a
continuous fixed relationship to the motor speed, such as a
conveyor.
Synchronous (RPM) motor speed =
% Slip =
synchronous RPM
Amperes =
volts
ohms
volts
amperes
Volts = amperes x ohms
90
no. of poles
synchronous (RPM – full load RPM) x 100
O HMS L AW
Ohms =
Hz x 120
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P OWER
IN
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DC C IRCUITS
volts x amperes
Horsepower
=
Watts
= volts x amperes
Kilowatts
=
Kilowatt-hours
=
P OWER
IN
Page 91
746
volts x amperes
1,000
volts x amperes x hours
1,000
AC C IRCUITS
Kilovolt-amperes (kVA)
kVA (single-phase) =
kVA (three-phase) =
volts x amperes
1,000
volts x amperes x 1.73
1,000
Kilowatts (kW)
kW (single-phase) =
kW (three-phase)
=
Power factor
=
volts x amperes x power factor
1,000
volts x amperes x power factor x 1.73
1,000
kilowatts
kilovolts x amperes
Appendix A
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T HREE - PHASE AC C IRCUITS
HP = E x I x
3 x EFF x PF
746
Motor amps
=
Motor amps
=
Motor amps
Power factor
hp x 746
E x 3 x EFF x PF
kVA x 1,000
3xE
kW x 1,000
=
3 x E x PF
=
Kilowatt-hours =
kW x 1,000
ExIx 3
E x I x hours x
1,000
3 x PF
PF = displacement power factor = cos q =
kW
kVa
Power (watts) = E x 1 x 3 x PF
EFF = mechanical efficiency
E = volts
I = amps
kVA
1 kW =
1 Ton=
1 hp =
=
=
=
92
56.88 BTU/min
200 BTU/min
0.7457 kW
550 lb-ft per sec
33,000 lb-ft per min
2,545 BTU per hour
f
Adjustable Speed Drive Reference Guide
kW
kVARi
kVARc
I
N
D
U
C
T
I
V
E
C
A
P
A
C
I
T
I
V
E
(AC
Added
motors) (to correct
KVARi) to
improve
PF
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APPENDIX B
APPENDIX B
Conversion Factors
Length
Torque
Rotation
Multiply
By
To Obtain
Metres
3.281
Feet
Metres
39.37
Inches
Inches
.0254
Metres
Feet
.3048
Metres
Millimetres
.0394
Inches
Newton-Metres
.7376
lb/ft
lb-ft
1.3558
Newton-Metre
lb-in
.0833
lb-ft
lb-ft
12.00
lb-in
RPM
6.00
Degrees/sec
RPM
.1047
Rad/sec
Degrees/sec
.1667
RPM
Rad/sec
9.549
RPC
Appendix B
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Moment of
Inertia
Power
Temperature
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Multiply
By
To Obtain
Newton-Metres2
2.42
lb-ft2
oz-in2
.000434
lb-ft2
lb-in2
.00694
lb-ft2
Slug-ft2
32.17
lb-ft2
oz-in-sec2
.1675
lb-ft2
lb-in-sec2
2.68
lb-ft2
Watts
.00134
HP
lb-ft/min
.0000303
HP
hp
746.
Watts
hp
33000.
lb-ft/min
Degree C = (Degree F-32) x 5/9
Degree F = (Degree C x 9/5) + 32
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A B B R E V I AT I O N S
ABBREVIATIONS
AC
ANSI
ASD
BHP
CSA
CSI
DC
DSP
ECC
GTO
HDF
IGBT
IEEE
LCI
NEMA
NPV
PAM
PLC
PWM
SCR
SR
V
VSI
VVI
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
alternating current
American National Standards Institute
adjustable speed drive
brakehorsepower
Canadian Standards Association
current source inverter
direct current
digital signal processor
eddy current coupling
gate turnoff (thyristor)
harmonic distortion factor
insulated gate bi-thermal thyristor
Institute of Electrical and Electronics Engineers
load-commutated inverter
National Electrical Manufacturers Association
net present value
pulse amplitude modulation
programmable logic controller
pulse width modulated (inverter)
silicon-controlled rectifier
switched reluctance
voltage
variable source inverter
variable voltage inverter
Abbreviations
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BIBLIOGRAPHY
ANSI/IEEE Standard 446-1987 (February 1987): IEEE
Recommended Practice for Emergency and Standby Power
Systems 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 Drive
Capabilities 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 Speed
Drives in Flow Control,” Pulp and Paper Canada, 88:6 (1987).
Pollack, J.J. “Some Guidelines for the Application of AdjustableSpeed AC Drives,” Adjustable Speed Drive Systems. New York: IEEE
Press, 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
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Reason, J. “Special Report: Powerplant Motors,” Power Magazine,
March 1986.
Stevenson, A.C. “Fundamentals and Applications of Static Power
Conversion,” IEEE 1984 Conference Record of Pulp and Paper
Industry Technical Conference.
Wennerstrom, C.H., et al. “Motor Application Considerations on
Adjustable Frequency Power,” IEEE 1984 Conference Record of
Pulp and Paper Industry Technical Conference.
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INDEX
Adjustable speed drive (ASD)
cost, 70
definition, 1
Advantages
comparison, 31
general, 35
Applications
comparison, 29
energy savings, 36, 43, 73
speed control/process
requirements, 35
Constant horsepower, 44
Constant torque, 44
Control accuracy, 60
Current-source
inverter (CSI), 13, 18, 29
DC drive
general, 23
principle of operation, 8
Delivery time, 66
Economics
cost/saving factors, 69
general, 72
methods, 69
Eddy current clutch
general, 29
principle of operation, 8
Efficiency
comparison, 29
discussion, 57
Environment, 63
Harmonics
comparison, 29
definition, 77
effects, 78
guidelines, 81
losses, 55
production and
transmission, 79
Horsepower rating
comparison, 29
general, 50
Inverter (see also variable
frequency drive), 12
Maintenance, 65
Motors
classification, 3
motor/drive requirements, 49
NEMA motor designs, 55
Net present value, 70
Power conditioning
equipment, 72
Pulse width modulated
inverter (PWM), 13, 20, 25
Index
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Rectifier, 12
Regenerative braking
comparison, 31
description, 36
simple payback, 69
Regulator, 13
Softstarting
comparison, 30
description, 36
Speed
regulation, 53, 60
requirements, 29
Thermal considerations, 54
Torque
considerations, 62
requirements, 51
100
Adjustable Speed Drive Reference Guide
Page 100
Variable frequency drive (see also
adjustable speed drive)
comparison, 29
principle of operation, 11
types, 13
Variable torque, 44
Variable voltage controllers, 11
Variable voltage
inverter (VVI), 13, 17, 29
Voltage requirements, 51
Voltage source inverter (VSI) - (see
variable voltage inverter)
Waveforms, 14
Wound rotor motor controllers
general, 29
principle of operation, 10
ASD appendix (83-102)
2/12/01
10:09 AM
ASD SUPPLIERS
IN
Page 101
O N TA R I O *
ABB
4410 Paletta Court
Burlington, Ontario L7L 5R2
Contact: Steve Seppanen
(905) 577-1986
Fax: (905) 681-2810
ITT Fluid Products Canada
55 Royal Road
Guelph, Ontario N1H 1T1
Contact: Phil Searle
(519) 821-1900
fax: (519) 821-5316
Canadian Drives Inc.
40 Claireville Drive
Etobicoke, Ontario M9W 5T9
Contact: Andrew J. Houston
(416) 213-1022
Fax: (416) 213-0821
Rockwell Automation/Allen-Bradley
135 Dundas Street
Cambridge, Ontario N1R 5X1
Contact: (519) 623-1810
Cegelec Automation
5112 Timberlea Boulevard
Mississauga, Ontario L4W 2S5
Contact: Roger D. Coote
(905) 624-2026
Fax: (905) 629-8203
G.E. Canada Inc.
2300 Meadowvale Boulevard
Mississauga, Ontario L5N 5P9
Contact: Mike Marshall
(905) 858-5128
fax: (905) 858-5132
Siemens Electric Limited
Energy and Automation Division
2185 Derry Road West
Mississauga, Ontario L5N 7A5
Contact:: Drives Sales Representative
(905) 819-5800 ext. 6414
Fax: (905) 819-5802
Toshont-Toshiba
2295 Dunwin Drive
Unit #4
Mississauga, Ontario L5L 3S4
Contact: Tom Johnson
(905) 607-9200
Fax: (905) 607-9203
*at time of printing
Suppliers
101
ASD cover
2/12/01
10:40 AM
Page 4
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 Rouse
Account Executive
Ontario Hydro
700 University Avenue, H10-F18
Toronto, Ontario
M5G 1X6
Telephone (416) 592-8044
Fax
(416) 592-4841
E-Mail
srouse@hydro.on.ca
ASD cover
2/12/01
10:40 AM
Page 1
In-House Energy Efficiency
Energy Savings are Good Business
"The sun represents sustained life
while the lightning bolt depicts energy. The integration
represents the perfect partnership of energy utilization and the
environment that encourages wise use and respect for all natural
resources. The roof represents the in-house aspect of energy
efficiency throughout Hydro."
Marcel Gauthier
Georgian Bay Region - Retail
Printed on
recycled papers
Ontario Hydro