Adjustable
Speed Drive
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.
Printed in Canada
Copyright © 1997 Ontario Hydro
In-House Energy Efficiency
Energy Savings are Good BusinessA
DJUSTABLE
S
PEED
D
RIVE
Reference Guide
TA B L E O F CO N T E N T S INTRODUCTION... 1 Latest Improvements ...2 CHAPTER1: CLASSIFICATIONS ... 3 Classification of Motors ... 3 Classification of Drives ... 3
CHAPTER2: PHYSICALAPPEARANCE... 5
CHAPTER3: PRINCIPLES OFOPERATION... 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
CHAPTER4: COMPARISON OFASDS ... 17
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
CHAPTER5: STANDARD ANDOPTIONALFEATURES ... 33
CHAPTER6: 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
CHAPTER7: APPLICATIONCONSIDERATIONS ... 39
How to Select an ASD ... 39
Software ...42 TA B L E O F CO N T E N T S
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
CHAPTER8: ECONOMICS ... 69
Economic Factors ... 72
Capital Costs ... 72
Capital Savings ... 73
Operating Costs and Savings ... 73
Tips and Cautions ... 75 TA B L E O F CO N T E N T S
CHAPTER9: HARMONICDISTORTION ... 77
Harmonics ... 77
What Harmonic Distortion Can Do ... 78
Production and Transmission ... 79
Isolation Transformers ... 80
Other Guidelines (IEEE 519-1992) ... 81
APPENDIXA: FORMULAS FORCALCULATINGAPPLICATIONS... 83
APPENDIXB: CONVERSIONFACTORS ... 93
ABBREVIATIONS ... 95
BIBLIOGRAPHY... 97
INDEX ... 99
ASD SUPPLIERS INONTARIO ... 101 TA B L E O F CO N T E N T S
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 LI S T O F FI G U R E S
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
1. Comparison of Adjustable Speed Drives ... 29
2. ASD and Electronic Motor Features ... 34
3. Suitability of Inverters for NEMA Motor Designs ... 55
4. ASD Checklist of Costs/Savings ... 70
5. ASD Investment Decision Technique ... 71 LI S T O F TA B L E S
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.
FIGURE 1. Comparison of Range Process Speed Control IN T R O D U C T I O N
Discrete
Speed
Operation
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.
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).
CH A P T E R 1
• 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.
• Variable frequency AC drives are comprised of many electrical circuits and components usually arranged within a cabinet that provides heat dissipation and shielding.
FIGURE 2. Physical Appearance of Variable Frequency Drive/Motor System CH A P T E R 2
P
HYSICAL
A
PPEARANCE
Can be hundreds of metres away Feedback Loop (Optional)ASD + transformer (if required)
Motor Tachometer
• 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.
• 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.
CH A P T E R 3
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
EDDY CURRENT CLUTCHES
• 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.
• By varying the applied voltage, the amount of torque transmitted, and therefore the speed, can be varied.
SWITCHED RELUCTANCE DRIVES
• 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.
VECTOR DRIVE
• 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.
FIGURE 4. Vector Drive
WOUND-ROTOR MOTOR CONTROLLERS
• Wound-rotor motor controllers are used to control the speed of wound-rotor induction motors.
Speed Regulator 2 Phase to 3 Phase Current Regulator Flux Command Controller Motor Encoder Position Signal
• 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 VOLTAGE CONTROLLERS
• Variable voltage controllers can be used with induction motors. • Motor speed is controlled directly by varying the voltage. • These controllers require high slip motors and so are inefficient
at high speed.
• Only applications with narrow speed ranges are suitable.
V
ARIABLE
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).
FIGURE 5. Closed Loop (Feedback) Adjustable Frequency Inverter System
COMPONENTS
• A variable frequency drive has two stages of power conversion, a rectifier and an inverter. (“Inverter” is also used to refer to 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.
INVERTER (Switching Section) REGULATOR (Controls) TACHOMETER Constant Frequency Constant Voltage AC Power Supply Speed Reference from Process Fixed or Variable DC Voltage Variable Frequency Variable Voltage AC Power Output Motor RECTIFIER LOAD Signal Feedback
- 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.
TYPES OF INVERTERS
• 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.)
WAVEFORMS
• 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.
SWITCHING DEVICES
• 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.
- Motor harmonics filter to supply maximum 5% current total harmanic distortion.
- DC link reactor to prevent saturation at faulted conditions.
RECOMMENDED SPECIFICATIONS
• Nominal power at +
- 10% voltage, 3 phase, 60 Hz ( +- 2%). • Capable of operation during temporary voltage drop of 70% 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.
C
OMPARISON OF
ASD
S
AC D
RIVES
VARIABLE VOLTAGE INVERTER (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.
FIGURE 6. VVI – Variable Voltage Inverter • Output frequency is controlled by switching transistors or
thyristors in six steps.
CH A P T E R 4 AC to DC Rectifier Constant Voltage DC Link Voltage Smoothing DC to AC Inverter Variable Voltage/ Frequency Control M
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.
CURRENTSOURCE INVERTER (CSI)
• 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).
Voltage (Line to Neutral) 0 Current (Line) 0 6 Step Time
AC to DC Rectifier Variable Voltage Control DC Link Current Smoothing DC to AC Inverter Variable Frequency Control M Voltage (Line to Neutral) 0 Current (Line) 0 Time
FIGURE 9. CSI – Waveforms
FIGURE 10. Block Diagram for a Typical CSI Drive FIGURE 8. CSI – Current Source Inverter
Current Regulator Frequency Control Speed or Voltage Control Filter AC/DC Converter Inverter Motor AC Line Speed
• 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).
PULSE WIDTH MODULATOR (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 custom-engineered).
AC to DC Converter Variable Voltage Control DC Link Voltage Smoothing DC to AC Inverter Variable Frequency Control M Voltage (Line to Neutral) 0 Current (Line) 0 FIGURE 12. PWM – Waveforms
FIGURE 13. Block Diagram for a Typical PWM Drive FIGURE 11. PWM – Pulse Width Modulated
Voltage & Frequency Control Filter Diode Bridge Rectifier Inverter Motor AC Line Speed Reference
• 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.
FIGURE 14. Power Factor Comparison
POWER FACTOR COMPARISON
• The power factor of VVI and CSI drives declines with speed as the thyristor firing angle varies in the controlled rectifier.
1.0 .75 .50 .25 0 PWM & Vector Drive VVI CSI Power Factor 450 900 Speed (RPM) 1350 1800
• 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.
FIGURE 15. DC Drive 100 0 100 0 % of Base Speed % of Rated Power Field Current Control Armature Voltage Control Constant Armature Voltage Constant Field Current Constant Torque Constant Power
• 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 non-regenerative but are more expensive and complicated.
• 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.
EDDY CURRENT COUPLING
• 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
FIGURE 16. ECC – Eddy Current Coupling
CYCLOCONVERTER
• 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.
Motor Drum TD SD SR TR Magnetic Rotor Load TD = Drum Torque SD = Drum Speed TR = Rotor Torque SR = Rotor Speed
FIGURE 17. Cycloconvertor Circuit Load Bridge A A.C. Supply A.C. Supply Bridge B
TABLE 1. Comparison of Adjustable Speed Drives
Eddy Current Coupling (ECC)
Squirrel-cage induction
1 – 1,000
34:1 but may be difficult to control above 2:1 3 - 5% Good 0 - 70% Field winding Type of Electric Drive MOTOR COMPATIBILITY
TYPICAL POWER RANGE (hp)
SPEED REDUCTION (typical) =
Maximum Speed Minimum Speed CONTROL OPEN LOOP CAPABILITY
(no feedback) (Note: Can be improved with feedback controls) ADAPTABILITY OF MOTOR TO HOSTILE
ENVIRONMENTS EFFICIENCY RANGE • for system: drive & motor TORQUE hp • Constant • Variable • Control Method VOLTAGE RANGE Variable Voltage Inverter (VVI) • Squirrel-cage induction or synchronous • Can handle motors
smaller than inverter rating 1 – 1,000 10:1 5% Good 88 - 93% Yes Yes
Pulse Width Modulated Inverter (PWM)
• Squirrel-cage induction or synchronous • Can handle motors
smaller than inverter rating 5 – 5,000 30:1 5% Good 85 - 95% Yes Yes 600 Current Source Inverter (CSI) • Squirrel-cage induction or synchronous • Can handle motors
smaller than inverter rating (at reduced rating) 50 – 5,000 10:1 5% Good 88 - 93% Yes Yes DC Drive Commutated DC 0 – 10,000 20:1 open loop 200:1 with tachometer 0.1 - 5%
depending upon feedback methods
Poor due to high maintenance of motor
90 - 94%
Yes
Field voltage, armature voltage or both
Wound Rotor with Slip Energy Recovery
Wound rotor induction
400 – 20,000 5:1 2 - 5% Medium 92 - 96% Rotor current
TABLE 1. Comparison of Adjustable Speed Drives (cont’d) Eddy Current Coupling (ECC) No Yes Good No Simple
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.
Type of Electric Drive
MULTIPLE MOTOR CAPABILITY (e.g., two 200 hp motors on a single 400 hp drive)
SOFT STARTING
Power Factor to Motor (PF)
OUTPUT SYSTEMS HARMONICS (dependent on leakage reactance) COMPLEXITY OF: • POWER CIRCUIT • CONTROL CIRCUIT PRINCIPLE Variable Voltage Inverter (VVI)
Yes, unlimited within inverter rating
Yes
Better than CSI(*2)
Drops with speed Worst
Simple Simple
The inverter receives DC power from an adjustable voltage source and adjusts the frequency.
Pulse Width Modulated Inverter (PWM)
Yes, unlimited within inverter rating
Yes
Near unity (excellent)
Least
Simple Complex
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. Current Source Inverter (CSI) No Yes (*2)
Drops with speed Better than VVI
Simple Semi-complex 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.
DC Drive
Yes, with manufacturer’s engineering for load sharing
Yes (*2) Yes Simple Simple Speed is adjusted by changing field voltage and/or armature voltage.
Wound Rotor with Slip Energy Recovery
No
Yes, if starting resistors used
Relatively low (can be improved with capacitors) Yes
N/A Simple
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.
TABLE 1. Comparison of Adjustable Speed Drives (cont’d)
Eddy Current Coupling (ECC)
N/A N/A
Field between rotating member
No Poor No
Small controller; large rotating element • Low costs
• Simple compact control • Wide constant torque
speed range
• Efficiency low at low speeds
• Lack of reversing capability • Limited speed range • Maintenance of brushed
is required
Type of Electric Drive
CIRCUIT PROTECTION • Inverter Open Circuit • Inverter Short Circuit
CONTROL VARIABLE
REGENERATIVE BRAKING REVERSE CAPABILITY RIDE-THROUGH CAPABILITY SIZE & WEIGHT
MAIN ADVANTAGES
MAIN DISADVANTAGES
Variable Voltage Inverter (VVI)
Inherent voltage limit Must be carefully designed to handle DC bus capacitor discharge
Motor voltage, frequency
Option with added circuitry Yes
Difficult
Intermediate
• High output frequencies (higher than 60 Hz if necessary) • Can be retrofitted to
existing fixed speed motor
• Soft start • Harmonics increase
losses in motor • Standard inverter cannot
operate in a regenerative mode
Pulse Width Modulated Inverter (PWM)
Inherent voltage limit Same as for VVI, except PWM circuit is very fast acting
Motor voltage and frequency
Option Yes
Yes, using battery or capacitive storage Small
• Excellent power factor; harmonics are minimal • Can be retrofitted to
existing fixed speed motor • Soft start
• Motor is subject to voltage stresses
• Complex logic circuits
Current Source Inverter (CSI)
Requires careful design Inherent current limit
Motor voltage, frequency and current
Standard Yes Difficult
Large
• Short circuit and overload protection due to current control of regulator • Soft start
• Instability may result under partial loading • Harmonics increase
losses in motor • Difficult to retrofit to
existing fixed speed motor drive
DC Drive
Inherent voltage limit Inherent current limit
Motor armature voltage, current and/or field voltage (not common)
Option Yes
Special applications only
Intermediate
• Simple system • Wide speed range • Soft start
• Brush and 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)
Wound Rotor with Slip Energy Recovery
N/A N/A Rotor current No No No Small
• Costs are relatively low for narrow variable speed ranges
• Simple circuitry • Adaptable to existing
wound rotor motors
• Maintenance of brushes is high
• May pose problems in hazardous environments • Relatively low power
factor
• Limited speed range • Regenerative braking n/a
TABLE 1. Comparison of Adjustable Speed Drives (cont’d)
Eddy Current Coupling (ECC)
• General purpose for equipment normally operating at full speed
• Fans • Pumps • Blowers
• Fluid propulsion systems • Driving extruders Type of Electric Drive MAIN DISADVANTAGES (cont’d) APPLICATIONS • General • Specific Variable Voltage Inverter (VVI)
• Lower horsepower ranges typically
• General purpose low-medium horsepower (<500 horsepower), multiple motor control
• Conveyors • Machine tools • Pumps • Fans
Pulse Width Modulated Inverter (PWM)
• High initial cost
• Best reliability AC type, at added cost
• Also suitable for most applications
• Slow speed ranges • Conveyors • Pumps • Fans • Packaging equipment Current Source Inverter (CSI)
• Only single motor control
• General purpose when regenerative braking wanted (hoists) • Pumps • Fans • Compressors • Blowers DC Drive
• Not suitable for hazardous environments where explosive gases may exist
• Expensive, large motor • Power factor always poor
at low speed
• For applications with a wide range of speed adjustment and a low-moderate starting torque • Used for medium and low
speed applications • General purpose • Extruders • Machine tools • Mine hoists • Cranes • Elevators • Rotary kilns • Rubber mills • Printing presses • Shakers (foundry or car) • Winches
• Public transportation
Wound Rotor with Slip Energy Recovery
• Used if speed range is narrow (70%-100%) and reversing not required
• Large pumps & fans with limited speed range • Compressors • Kilns • Conveyors • Mixers
(*1) A totally enclosed motor is usually required because the ECC is normally used in close proximity to the driven machine (e.g., machine tools).
(*2) The VVI, CSI and DC drives have power factors that decrease with speed. For the AC inverters, this can be corrected by implementing a diode and chopper control.
This will slightly increase acoustical noise and slightly reduce efficiency. N/A Not Applicable
• See Table 2 on the following page for a general guideline list of standard and optional features for AC variable frequency drives and new power electronic devices. Note, however, that
manufacturers may differ on some factors. CH A P T E R 5
S
TANDARD AND
O
PTIONAL
TABLE 2. ASD and Electronic Motor Features ASD Standard Protection Features Overvoltage Undervoltage Overcurrent
Loss of control power Across-the-line start Line-to-line shorts on output Line-to-ground shorts on output Continuous overload Locked rotor Motor single phasing
ASD Optional Features Soft start Overload protection Torque limit Power outage ride-through Brake stop Coast stop Bypass Motor slip compensation Electronic reversing Voltage boost (at start) Accel/decel
Regenerative power protection
Low speed jog IR compensation
New Power Electronic Devices
Metal oxide semi-conductor (MOS) controlled thyristors (inverter switches) Insulated-gate bi thyristors (IGBT are more capable of rapid energizing)
• 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.
CH A P T E R 6
POSITION CONTROL
• ASDs are used for machine tools.
TORQUE CONTROL
• ASDs are used for tensioning (winders).
HIGH ENERGY SAVINGS POTENTIAL
• Applications with highest energy savings potential 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.
SOFTSTARTING/REGENERATIVE BRAKING
• 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.
EQUIPMENT LIFEIMPROVEMENT
• 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.
MULTIPLE MOTOR CAPABILITY
• One multiple motor ASD (except CSI) can control a number of synchronized motors at the same speed (e.g., in the textile industry).
BYPASS CAPABILITY
• The adjustable frequency drive can be for service, without need to shut down the driven equipment (with additional circuitry optional).
SAFEOPERATION INHARSH ENVIRONMENTS
• Adjustable frequency drives offer safe operation in harsh environments since the drive can be housed in a remote location.
TEMPORARY OR BACK-UPOPERATION
• 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.
REDUCTION IN VIBRATION AND NOISE LEVEL
• Vibration and noise level are reduced when the operating speed of the equipment is lowered and because valves or vanes are eliminated.
RE-ACCELERATIONCAPABILITY
• 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).
HOW TO SELECT 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.
CH A P T E R 7
A
PPLICATION
- 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.
- 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.)
S
OFTWARE
FINANCIAL EVALUATION
• Software is available from several ASD suppliers, including some utilities. Be careful to include lower part-load efficiencies when inputting performance data.
LOAD CHARACTERISTICS
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.
FIGURE 18. Duty Cycles
APPLICATION TYPES BY LOAD
• 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).
100 0 Time % Flow 100 0 Time % Flow Good Application Poor Application
• 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 RPM3such 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.
100 80 60 40 20
Percent hp and Torque
100 50 Percent Speed 0 Torque hp Percent Speed 100 80 60 40 20
Percent hp and Torque
100 50
0
Torque
hp
FIGURE 20. Constant Torque Load
FIGURE 21. Constant Horsepower Load FIGURE 19. Variable Torque Load
Percent Speed 100 80 60 40 20
Percent hp and Torque
100 50
0
Torque hp
T
IPS AND
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
Flow
Pressure
Outlet Damper Control
System Performance Flow ASD Control Unstable Area Inlet Guide Vane Control Flow Pressure Valve Control System Performance Performance Static Dynamic Flow ASD Control System
FIGURE 22: Power Required is Proportional to RPM3
Centrifugal Fan/Blower, Pump
Pump
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).
• 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).
FIGURE 23. Power Savings in Fans and Pumps Using ASDs
Power Required Damper Control Speed ASD Control Saving
• 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.
MOTOR/DRIVE SYSTEM
• 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.
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.
Voltage Requirements
• These are the size ranges usually available for AC variable frequency drives:
Horsepower Range Voltages Available
<50 208V to 600V three-phase 50-200 460V to 600V three-phase
200-1,000 low voltage (460V, 600V) and medium voltage (2,300V, 4,160V)
1,000-2,500 mostly medium voltage (2,300V, 4,160V) *2,500-10,000 medium voltage
(4,160V, 6,900V, 13,800V) *>10,000 13,800V
(usually DC, or wound rotor)
• Note that suitably rated transformers may be used to match 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.
• 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.
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.
2:1 30-60 H z 20:1 3-60Hz 10:1 6-60 Hz 8:1 7.5-6 0 Hz 6:1 10-6 0 Hz 4:1 15-6 0 Hz 3:1 20-60 H z 1 20 40 50 75 100 125 150 200 250
Motor Horsepower – 60 Hz Rated Speed Range
Percent (%) of 60 Hz Torque Rating 30 40 50 60 70 80 90 100 0 Induction Motor: Constant Torque Load, USEM 4-P TEFC 460 V 30 60 Hz Motor With Boost At Low Frequency. Source Data:
EIC Program Based on Constant Temp. Method
• 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: WK2total = sum of WK2components, W is weight, K is radium of gyration.)
Torque (lb-ft) = HP x 5250 RPM
THERMAL CONSIDERATIONS
• 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.”
• 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 X
B X X X
C* X X
D*
* 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.
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.
EFFICIENCY
• 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.
.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 450 900 1350 1800 Driver Losses (kW) Speed (RPM) PWM VVI CSI 74% 78% 82% 86% 90% 94% 100% 0 20 40 60 80 100 Percent Efficiency Percent Speed 75% Load 100% Load 50% Load 25% Load
FIGURE 25. Watts Loss (Efficiency) Comparison
• 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.
RELIABILITY OF ASDS
• 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
• 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).
APPLICATIONS
• 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
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.
PERFORMANCE REQUIRED
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.
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
