Adjustable Speed Drive

Transcription

1 $15.95 CDN Adjustable Speed Drive REFERENCE GUIDE 4th Edition

2 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

3 ADJUSTABLE SPEED DRIVE Reference Guide 4th Edition

4 T ABLE 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 Wound-rotor Motor Controllers Variable Voltage Controllers Variable Frequency Drives Components Types of Inverters Waveforms Switching Devices (Power Electronics)...14 Medium Voltage Drives...14 Recommended Specifications...15 CHAPTER 4: COMPARISON OF ASDS AC Drives i

5 T ABLE OF CONTENTS Variable Voltage Inverter (VVI) Current Source Inverter (CSI) Pulse Width Modulator (PWM) Power Factor Comparison DC Drives Eddy Current Coupling Cycloconverter...26 CHAPTER 5: STANDARD AND OPTIONAL FEATURES CHAPTER 6: ADVANTAGES Speed Control Position Control Torque Control High Energy Savings Potential Soft Start/Regenerative Braking Equipment Life Improvement Multiple Motor Capability Bypass Capability Safe Operation in Harsh Environments Temporary or Back-up Operation...37 Reduction in Vibration and Noise Level Re-acceleration Capability Tips and Cautions CHAPTER 7: APPLICATION CONSIDERATIONS How to Select an ASD Software...42 ii

6 T ABLE OF CONTENTS Financial Evaluation...42 Load Characteristics Application Types by Load Tips and Cautions Motor/Drive System Thermal Considerations Other Considerations Efficiency Reliability of ASDs Applications Performance Required Starting and Stopping Characteristics Torque Environment Weight and Space Accessories Safety Service and Maintenance Tips and Cautions CHAPTER 8: ECONOMICS Economic Factors Capital Costs Capital Savings Operating Costs and Savings Tips and Cautions iii

7 T ABLE OF CONTENTS CHAPTER 9: HARMONIC DISTORTION Harmonics What Harmonic Distortion Can Do Production and Transmission Isolation Transformers Other Guidelines (IEEE ) APPENDIX A: FORMULAS FOR CALCULATING APPLICATIONS APPENDIX B: CONVERSION FACTORS ABBREVIATIONS BIBLIOGRAPHY INDEX ASD SUPPLIERS IN ONTARIO iv

8 L IST OF FIGURES 1. Comparison of Range Process Speed Control Physical Appearance of Variable Frequency Drive/Motor System /6 Pole Switched Reluctance Motor Vector Drive Closed Loop (Feedback) Adjustable Frequency Inverter System VVI Variable Voltage Inverter VVI Waveforms CSI Current Source Inverter CSI Waveforms Block Diagram for a Typical CSI Drive PWM Pulse Width Modulated Inverter PWM Waveforms Block Diagram for a Typical PWM Drive Power Factor Comparison DC Drive ECC Eddy Current Coupling Cycloconverter Circuit Duty Cycles Variable Torque Load Constant Torque Load Constant Horsepower Load Power Required is Proportional to RPM 3 Centrifugal Fan/Blower, Pump Power Savings in Fans and Pumps Using ASDs v

9 L IST OF FIGURES 24. Motor Derating Curves vs. Speed Range When Applied to Adjustable Frequency AC Drives (6-Step Waveform or PWM) Watts Loss (Efficiency) Comparison Typical AC Drive Efficiency Motor Performance, Typical 60 Hz Ideal Torque-Speed Curves NEMA Design B Motor Torque-Speed Curve Capital Cost Comparison of Motor/Drive Systems Medium HP, Voltages Harmonic Distortion A-1. Calculating Hollow Shafts A-2. Calculating the Inertia of Complex, Concentric Rotating Parts vi

10 L IST OF TABLES 1. Comparison of Adjustable Speed Drives ASD and Electronic Motor Features Suitability of Inverters for NEMA Motor Designs ASD Checklist of Costs/Savings ASD Investment Decision Technique vii

11 I NTRODUCTION 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. Discrete Continuous Speed Operation FIGURE 1. Comparison of Range Process Speed Control Introduction 1

12 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. LATEST IMPROVEMENTS Microprocessor-based controllers eliminate analogue, potentiometer-based adjustments. Digital control capability. Built-in Power Factor correction. Radio Frequency Interference (RFI) filters. Short Circuit Protection (automatic shutdown). Advanced circuitry to detect motor rotor position by 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. 2 Adjustable Speed Drive Reference Guide

13 C HAPTER 1 CLASSIFICATIONS CLASSIFICATION OF MOTORS There are two main types of motors, AC (alternating current) and DC (direct current). AC motors can be sub-classified as induction (squirrel-cage and wound-rotor) and synchronous. Induction motors are often classified as either high efficiency or standard. CLASSIFICATION OF DRIVES Adjustable speed drives are the most efficient (98% at full load) types of drives. They are used to control the speeds of both 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 3

14 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. 4 Adjustable Speed Drive Reference Guide

15 C HAPTER 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. LOAD ASD + transformer (if required) Feedback Loop (Optional) Can be hundreds of metres away Tachometer Motor FIGURE 2. Physical Appearance of Variable Frequency Drive/Motor System Chapter 2: Physical Appearance 5

16 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. 6 Adjustable Speed Drive Reference Guide

17 C HAPTER 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. CONVENTIONAL FIXED-SPEED AC SYSTEMS (AC MOTOR WITHOUT DRIVE) Standard squirrel-cage induction motors are usually considered to be constant speed motors. These systems require some means of throttling (via valves, dampers, etc.) to meet process changes. If a reduction in demand occurs, excess energy is wasted in 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 7

18 DC DRIVES 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 DRIVES 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. 8 Adjustable Speed Drive Reference Guide

19 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. Chapter 3: Principles of Operation 9

20 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. Speed Regulator Flux Command 2 Phase to 3 Phase Current Regulator Motor Encoder Controller Position Signal FIGURE 4. Vector Drive WOUND-ROTOR MOTOR CONTROLLERS Wound-rotor motor controllers are used to control the speed of wound-rotor induction motors. 10 Adjustable Speed Drive Reference Guide

21 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. VARIABLE FREQUENCY DRIVES 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 11

22 Feedback Signal Speed Reference from Process TACHOMETER REGULATOR (Controls) Constant Frequency Constant Voltage AC Power Supply RECTIFIER INVERTER (Switching Section) Fixed or Variable DC Voltage Motor Variable Frequency Variable Voltage AC Power Output LOAD 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. 12 Adjustable Speed Drive Reference Guide

23 - 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.) Chapter 3: Principles of Operation 13

24 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). MEDIUM VOLTAGE DRIVES 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. 14 Adjustable Speed Drive Reference Guide

25 - 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 % 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 C construction materials. - NEMA Std. TR-27 for noise. Chapter 3: Principles of Operation 15

26 C HAPTER 4 COMPARISON OF ASDS AC DRIVES 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. 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 17

27 Voltage (Line to Neutral) 0 6 Step Current (Line) 0 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. CURRENT SOURCE INVERTER (CSI) Time 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). 18 Adjustable Speed Drive Reference Guide

28 AC to DC Rectifier DC Link DC to AC Inverter M Variable Voltage Control Current Smoothing Variable Frequency Control FIGURE 8. CSI Current Source Inverter Voltage (Line to Neutral) 0 Current (Line) 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 19

29 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 customengineered). 20 Adjustable Speed Drive Reference Guide

30 AC to DC Converter 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 Speed Reference Diode Bridge Rectifier Filter Inverter Motor Voltage & Frequency Control FIGURE 13. Block Diagram for a Typical PWM Drive Chapter 4: Comparison of ASDs 21

31 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 PWM & Vector Drive VVI CSI POWER FACTOR COMPARISON Speed (RPM) FIGURE 14. Power Factor Comparison The power factor of VVI and CSI drives declines with speed as the thyristor firing angle varies in the controlled rectifier. 22 Adjustable Speed Drive Reference Guide

32 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 DRIVES 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. 100 Armature Voltage Control Constant Field Current Field Current Control Constant Armature Voltage % of Rated Power Constant Torque Constant Power % of Base Speed FIGURE 15. DC Drive Chapter 4: Comparison of ASDs 23

33 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. 24 Adjustable Speed Drive Reference Guide

34 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 Chapter 4: Comparison of ASDs 25

35 Drum Motor Load T D S D S R T R Magnetic Rotor T D = Drum Torque S D = Drum Speed T R = Rotor Torque S R = Rotor Speed 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. 26 Adjustable Speed Drive Reference Guide

36 A.C. Supply Bridge A Load Bridge B A.C. Supply FIGURE 17. Cycloconvertor Circuit Chapter 4: Comparison of ASDs 27

37 TABLE 1. Comparison of Adjustable Speed Drives 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) 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 Squirrel-cage induction or synchronous Can handle motors smaller than inverter rating 1 1,000 10:1 5% Good 88-93% Squirrel-cage induction or synchronous Can handle motors smaller than inverter rating (at reduced rating) 50 5,000 10:1 5% Good 88-93% Squirrel-cage induction or synchronous Can handle motors smaller than inverter rating 5 5,000 30:1 5% Good 85-95% 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% Wound rotor induction ,000 5:1 2-5% Medium 92-96% Squirrel-cage induction 1 1,000 34:1 but may be difficult to control above 2:1 3-5% Good 0-70% TORQUE hp Constant Variable Control Method VOLTAGE RANGE Yes Yes Yes Yes Yes Yes 600 Yes Field voltage, armature voltage or both Rotor current Field winding Chapter 4: Comparison of ASDs 29

38 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) 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 Yes, with manufacturer s engineering for load sharing No No SOFT STARTING Yes Yes Yes Yes Yes, if starting resistors used Yes Power Factor to Motor (PF) Better than CSI (*2) Drops with speed (*2) Drops with speed Near unity (excellent) (*2) Relatively low (can be improved with capacitors) Good OUTPUT SYSTEMS HARMONICS (dependent on leakage reactance) Worst Better than VVI Least Yes Yes No COMPLEXITY OF: POWER CIRCUIT CONTROL CIRCUIT Simple Simple Simple Semi-complex Simple Complex Simple Simple N/A Simple Simple PRINCIPLE 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. 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. 30 Adjustable Speed Drive Reference Guide

39 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 Chapter 4: Comparison of ASDs 31

40 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) 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 APPLICATIONS General 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 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 Specific Conveyors Machine tools Pumps Fans Pumps Fans Compressors Blowers Slow speed ranges Conveyors Pumps Fans Packaging equipment Extruders Machine tools Mine hoists Cranes Elevators Rotary kilns Rubber mills Printing presses Shakers (foundry or car) Winches Public transportation Large pumps & fans with limited speed range Compressors Kilns Conveyors Mixers Fans Pumps Blowers Fluid propulsion systems Driving extruders (*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 32 Adjustable Speed Drive Reference Guide

41 C HAPTER 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 33

42 TABLE 2. ASD and Electronic Motor Features ASD Standard Protection Features ASD Optional Features New Power Electronic Devices 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 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 Metal oxide semiconductor (MOS) controlled thyristors (inverter switches) Insulated-gate bi thyristors (IGBT are more capable of rapid energizing) 34 Adjustable Speed Drive Reference Guide

43 C HAPTER 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. SPEED CONTROL 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 35

44 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. SOFT STARTING/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. 36 Adjustable Speed Drive Reference Guide

45 EQUIPMENT LIFE IMPROVEMENT 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). SAFE OPERATION IN HARSH ENVIRONMENTS Adjustable frequency drives offer safe operation in harsh environments since the drive can be housed in a remote location. TEMPORARY OR BACK-UP OPERATION 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 37

46 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-ACCELERATION CAPABILITY Some adjustable frequency drives continue to have power supply during power losses of short duration, whereas fixed speed devices would trip out. TIPS AND CAUTIONS 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). 38 Adjustable Speed Drive Reference Guide

47 C HAPTER 7 APPLICATION CONSIDERATIONS 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. Chapter 7: Application Considerations 39

48 - 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. 40 Adjustable Speed Drive Reference Guide

49 - 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 41

50 SOFTWARE 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. 42 Adjustable Speed Drive Reference Guide

51 100 % Flow Good Application 0 Time 100 % Flow Poor Application 0 Time 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). Chapter 7: Application Considerations 43

52 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 RPM 2, where horsepower is proportional to RPM 3 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. 44 Adjustable Speed Drive Reference Guide

53 Percent hp and Torque Torque hp 0 50 Percent Speed 100 FIGURE 19. Variable Torque Load Percent hp and Torque Torque hp Percent Speed FIGURE 20. Constant Torque Load Percent hp and Torque hp Torque Percent Speed FIGURE 21. Constant Horsepower Load Chapter 7: Application Considerations 45

54 TIPS AND CAUTIONS 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 Unstable Area ASD Control Pressure System Performance Inlet Guide Vane Control Flow Valve Control Pump Flow ASD Control System Pressure Static Dynamic Performance System Flow Performance Flow FIGURE 22: Power Required is Proportional to RPM 3 Centrifugal Fan/Blower, Pump 46 Adjustable Speed Drive Reference Guide

55 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). Chapter 7: Application Considerations 47

56 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 RPM 3. Therefore, a 10% drop in speed results in a 27% drop in power consumption ( ). Damper Control Power Required Saving ASD Control Speed FIGURE 23. Power Savings in Fans and Pumps Using ASDs 48 Adjustable Speed Drive Reference Guide

57 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. Chapter 7: Application Considerations 49