Efficiency Opportunities with Adjustable Speed Drives

Transcription

1 APPLICATION NOTE An In-Depth Examination of an Energy Efficiency Technology Efficiency Opportunities with Adjustable Speed Drives Summary...1 How This Technology Saves Energy...2 Types of Adjustable Speed Drives... 3 Applicability...5 Field Observations to Assess Feasibility...6 Estimation of Energy Savings...8 Cost and Service Life...10 Definitions of Key Terms...11 References to More Information...11 Major Manufacturers...12 Summary The primary purpose of an adjustable speed drive is to let an AC-motor run slower. These drives allow the motordriven machines to produce the same output as standard counterparts but with less electrical input, often improving manufacturing quality as well. Such drives can also enhance motor and machine life and reduce maintenance by allowing operation at lower temperatures and smoothing machine startup and shutdown. The most common type is the pulse-width modulation drive, which provides good power factor through a wide range of speeds, can control several motors from a single drive, has very good speed control response and is applicable to all AC motor sizes. Costs vary greatly depending on features and ruggedness. Per-horsepower costs decrease significantly with size, from an average of about $640 per horsepower for a 20-hp application to about $1 per horsepower for a 20,000-hp application. The drives last on average about 16 years, but lifetime varies significantly based on design. Overall system efficiency improvements vary significantly depending on the application. The best opportunities come with machines that operate for significant periods only partially loaded. It is usually appropriate to consider adjustable speed drives when (1) designing new facilities, (2) modifying existing installations or processes, (3) procuring pre-packaged equipment, (4) replacing oversized or underloaded motors, (5) setting up an energy management or preventive maintenance program. Copyright May 1997, Pacific Gas and Electric Company, all rights reserved. Revised 4/25/97

2 How This Technology Saves Energy Most commercial and industrial electric motors are three-phase, alternating current, induction motors. They are reliable, moderately priced, and reasonably efficient, but operate only at fixed speeds. Many operations regulating the flow of liquid or gas from pumps or compressors, for example require varying speed or torque 1. The conventional way Feedback Signal from Process Regulator (Controls) Rectifier (ac to dc) dc Link Inverter (dc to ac) 60-Hz Power Input Adjustable Frequency and Voltage Output Motor Load Figure 1: Typical ASD System to meet this need is with devices that throttle machine output, (such as throttling valves on pumps), inlet vanes and dampers (on fans), or devices that adjust machine drive speed, such as eddy current clutches or adjustable belt pulley drives. 1 Bold italicized words are defined in the section titled Definition of Key Terms Adjustable Speed Drives (ASDs) do this job more efficiently, significantly reducing energy consumption. In addition, ASDs offer more precise process control and greater equipment reliability than conventional controls. Induction motors with conventional full voltage starters draw high inrush current upon starting with very low power factor. The resulting voltage sag may affect other loads on the system. Motors using ASDs ramp up to the operating speed setpoint smoothly, with a gradual increase in torque, eliminating problems associated with high inrush currents and prolonging the life of the equipment. Figure 1 shows a typical ASD system, which has four basic components: rectifier, DC link, inverter, and regulator. The rectifier converts line frequency alternating current (AC) to direct current (DC). The DC link smoothes the output of the rectifier. And, the inverter generates an adjustable frequency and volt- PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 2

3 age AC output to the motor. Some applications may require regulators, which help control system output to optimize the process requirements. Types of Adjustable Speed Drives ASDs are also commonly referred to as variable speed drives (VSDs), variable frequency drives (VFDs), and adjustable frequency drives (AFDs). Several types of ASDs are available; each with its own practical applications. All are similar in controlling motor speed by varying electrical voltage and frequency. They differ in how they accomplish AC-to-DC conversion and DC-to-AC inversion. The three most common rectifier/inverter types are discussed below; many hybrid forms combine characteristics of these basic types. Current Source Inverter (CSI) A CSI actually uses the inductive characteristic of the motor to stabilize DC as it reaches the inverter. Because the inductor has to be fairly large to keep the current source stable, this type of electronic ASD is generally used with - hp and larger drives. It is harder to tune in the field because it cannot operate without connection to the motor. Advantages of CSIs include regenerative braking capability (acting as a generator while motor is slowing down), good short-circuit protection, high efficiency at a wide range of speeds above percent, quiet operation, relative ruggedness, and simplicity. Drawbacks include poor power factor at low speed/load, potential for oscillating (reversing) torque at speeds less than 10 percent, and inability to test the drive without attaching the motor (which must be matched to the inverter). This type of drive is usually used to drive only one motor at a time, although more than one is physically possible. Voltage Source Inverter (VSI) The VSI is sometimes called a variable voltage inverter (VVI) or six-step inverter. It uses a capacitor before the inverter to store energy from the rectifier and stabilize the voltage entering the inverter. It is used in low to medium power applications, generally up to a few hundred horsepower. Advantages include simple circuit configuration, high efficiency over a wide speed range (10 to 200 percent) and ability to control several motors at once. A disadvantage is that in most arrangements it has no regenerative braking capability. Nor is it well suited to operation at less than 10 percent of rated speed, where power factor is very poor and harmful cogging can occur. Pulse-Width Modulated Inverter (PWM) The PWM inverter develops the voltage output by chopping pulses of varying widths to synthesize the desired waveform. PWM drives use complex software algorithms to determine timing, duration, and frequency of the voltage pulses delivered to the motor. Each manufacturer employs its own algorithms, causing significant differences in motor performance and life. This is presently the most common ASD type for up to 400-hp motor sizes. These drives are well suited for motors up to,000 hp, but PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 3

4 above 1,000 hp, electronic components and inverter controls are more complex. Advantages include good power factor through a speed range from 0 to percent and low harmonic distortion generation on power systems. It can control several motors from a single drive, can be tested without being connected to a motor and has fast response and close control characteristics. Disadvantages include no regenerative braking capability, need for greater skill in servicing, greater motor heating due to motor harmonic distortion generation and limited cable distance between drive and motor (- feet). Table 1 summarizes characteristics of the three basic types of ASD. ASD Costs Hardware and installation costs vary widely depending on features and ruggedness, but per-horsepower costs decrease with size a 20-hp application can range from $385 to $900 per horsepower while a -hp application can range from $230 to $515. VSI CSI PWM Ease in Retrofitting Yes No Yes Soft Start Yes Yes Yes Regeneration Option* Inherent Option* Motor Heating Low Low High Motor Noise Low Low High Partial Loading Yes Unstable Yes Low Speed Operation No No Smooth Low Speed Torque Pulsation Yes Yes No Frequency Above 60 Hz Yes No Yes Open Circuit Protection Inherent Required** Inherent Short Circuit Protection Required** Inherent Required** Overload Protection Required** Inherent Required** Multi-Motor Drive Yes Option* Yes Controller and Logics Simple Semi-complex Complex 60 Hz Power Factor Poor Poor High 60 Hz Harmonics High High Low Motor Harmonics Moderate Moderate High Voltage Stresses on Motor No Yes Yes DC Filter Size Large Large Small Inverter Noise Medium Medium High Transistor/GTO Technology Yes No Yes Inverter Switches Low frequency Low frequency High frequency Size and Weight Medium Large Small * Feature is available at extra cost. ** Feature must be provided by the system design. Table 1: Characteristics of ASD Types (Source: BPA) PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 4

5 Case Study: Water Distribution Pumping System Saves Energy at University Complex A university laboratory complex had a hot water circulation system with both a 5-hp and a 30-hp pump. Pressure and flow were controlled by pressurereducing valves at the discharges of the pumps. After a feasibility study, an ASD was applied to the 30-hp pump to eliminate the use of valves. Factors making this an attractive ASD installation included: high number of hours of operation; wide variation between maximum and minimum demands; and the use of throttling valves. Payback period, without utility incentives, was about four years. Applicability Tables 2a and 2b list a variety of commercial and industrial ASD applications, generic examples of potential applications, and important characteristics of the various drive types. Table 2a lists the process requirements and Table 2b shows the drive system types and some of the key technical factors that apply to various machine types. The table presents data for a range of typical drive systems. Some key technical factors are determined by simple inspection; others may require basic analysis. For example, reversing requirements and multiple drive arrangements, (running more than one motor from one drive), are determined by inspection, but torque requirements may require taking motor current readings, investigating the design of the equipment, or contacting the Machine Types Blenders Crushers & Mills Conveyors Fans Cranes Sanders Drum Barkers Lathes Milling Machines Planers Ring Barkers Rewinders/Slitters Stock Cleaners Stock Washers Mixers Filter Press Compressors Trays Sorters Dryers Glue Spreaders Feeders Pumps Forklift Grinder Extruder Process Equipment Production Control Mass Flow Rates Precision/Finish Extrusion/Molding Machining/Fabrication Chemical Processes Material Handling Gas and Liquid Flows Winding/Unwinding Table 2a: Typical Process Requirement for ASD Applications (Source: BPA) manufacturer or designer of the drive. Regeneration ability or braking requirements may be apparent by simple inspection. A drive able to absorb inertial energy when the system is slowed down or stopped can reduce energy use con- PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 5

6 Machine Types Blenders Crushers & Mills Conveyors Fans Cranes Sanders Drum Barkers Lathes Milling Machines Planers Ring Barkers Load Characteristics Horsepower: < - 2 > 2 Motor Types: Induction Synchronous Torque Requirements: High Low Duty Cycle: High Low Reversing Regenerative Control Strategy: Flow Speed Pressure Multiple Drive low speeds, starting time gives an indication of system torque requirements. As a general rule, if the motor can come up to speed in less than one second the torque requirement is relatively low; if the time exceeds three seconds, the torque requirement is considered high. Identifying high-torque drives is particularly important where frequent starts and stops or accelerations and decelerations are required. Control strategies are important initial considerations. Speed controlled drives typically use a simple tachometer on the motor to provide the feedback signal. Rewinders/Slitters Stock Cleaners Stock Washers Mixers Filter Press Compressors Trays Sorters Dryers Glue Spreaders For retrofits where a gearbox and motor exist, the ASD may be inserted between the starter and the induction motor. As long as the minimum speed of the system is over 10 percent of top speed, there should be no concern about cogging. Feeders Pumps Forklift Grinder Extruder Field Observations to Assess Feasibility Table 2b: Typical Drive Characteristics for ASD Applications (Source: BPA) siderably, such as a crane hoist where energy can be regenerated while lowering a load. In systems, such as fans, which conventionally operate at a fixed speed with no regenerative braking, regeneration can be incorporated when they are converted to ASD systems. This may become necessary to get the fan to respond to slow-down commands in reasonable times. Rapid acceleration and/or deceleration may be required in many applications. This section discusses field observations and questions that help identify appropriate situations for adjustable speed drives. Related to Applicability Opportunities may exist if the customer is: Designing new facilities. Modifying existing installations or processes. Unless the load is high at standstill and PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 6

7 Procuring pre-packaged equipment or systems with electric motor components. Replacing oversized or underloaded motors. Setting up an energy management or preventive maintenance program. Related to Energy Savings To establish whether opportunities for energy savings exist, answer these questions: Are there motors that drive fans, pumps, or compressors which are modulated by a damper or valve? If so, energy savings can be expected from installing an ASD. However, savings will depend on the load type of the machine. Obvious cases will be where the modulating damper or valve actively controls output to a variable load; less obvious cases may be fixed throttling valves or dampers used to limit flow or pressure from an oversized fan, pump or compressor. Are there motors using speed control devices, such as adjustable belt pulley drives, fluid drives or eddy current clutches? Can the present device be removed or disabled? Is there machinery that can be operated at other than its current speed? Effects on product quality and production rate will be key factors in determining the feasibility of any such changes. For the motors identified by the above three questions, what is the duty cycle and load profile? The higher the duty cycle and lower the load profile, the larger the potential savings. Any motor that operates at 25 percent or more duty cycle (around 2,000 hours per year) should have a welldocumented load profile. Methods of determining motor duty include monitoring hours of operation per month (if the motor controller has a run hour meter) or attaching a recording current meter. There are several ways to estimate motor loading, each with its own sources of inaccuracy. In general, however, the preferred method is to measure total motor input power with a wattmeter. Multiplying this figure by the approximate motor efficiency (from the nameplate or manufacturer s data) yields the motor load. Reference 3 contains additional information on estimating motor loading. Typically it is helpful to estimate what portion of the equipment's running time is spent at, 75,, and 25 percent of the maximum process requirement. Related to Implementation Costs To establish if installing ASDs will be cost effective, answer these questions: How will the ASD improve quality (better speed control, elimination of waste, product reversion, etc.) and/or ultimately result in lower product costs? What costs associated with existing motor drive inefficiencies (friction heat, cooling water, etc.) can be reduced by an energy-efficient ASD system? PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 7

8 What are the costs of maintaining existing mechanical speed-changing equipment (transmissions, etc.)? Are they obsolete and in need of replacement? What are the costs of maintaining existing electrical speed-changing equipment (wound rotor motor, or reduced voltage starting)? Are they obsolete and in need of replacement? Do other problems of equipment reliability cause production delays and higher product costs? How can they be eliminated by ASDs with selfdiagnostic features? Is there opportunity to create additional space by removing large mechanical equipment (transmissions, etc.) with the installation of an ASD controller? Can plant noise be reduced through lessening mechanical noise by installing an ASD control, or will the noise of the ASD be excessive? What shutdown arrangements are required to provide time to install an ASD? Estimation of Energy Savings Energy savings from an ASD depend highly on the load characteristics and profile of the application. The load characteristics, expressed as torque versus rpm or horsepower versus rpm, can be classified into four basic categories. They are presented in order of increasing potential for energy savings. Constant Torque: Torque is independent of rpm (Figure 2). A typical case is a hoist of a crane. Once at steady state, the magnitude of the weight lifted establishes the force and therefore, the motor torque. Only insignificant deviations occur due to changes in friction at different speeds. In this case, the shaft horsepower, the product of torque and rpm, is linear with rpm. Linear Torque: In some instances, as in certain very low-velocity mixing processes, torque is proportional to rpm (Figure 3). Power, being the product of torque and rpm, will be proportional to the square of the rpm. Therefore, at half speed the torque required is one half of that at full speed, and the power only one quarter. Square Torque: Liquids and gases (including water), when moved, require a pressure proportional to the square of the velocity (i.e., the volume moved). As centrifugal pumps deliver volumes proportional to rpm, pressure will be proportional to the square of rpm. As the motor develops torque to maintain a % Torque (ft-lbs) Power (HP) HP Speed (RPM) Figure 2: Typical Constant Torque Curves (Source: BPA) % PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 8

9 % Torque (ft-lbs) Power (HP) ft-lbs HP Speed (RPM) Figure 3: Typical Square Torque Curves (Source: BPA) pressure inside the pump, the torque will be also proportional to the square of the rpm (Figure 4). Consequently, the power will be proportional to the cube of the rpm. In this case, reducing the flow to one half requires only 12.5 percent of full-flow horsepower. This is often referred to as the affinity laws associated with centrifugal pumps and fans. Variable Torque: Some highly viscous substances (e.g., bread dough) become less viscous as mixing speed increases; such loads have the characteristics shown in Figure 5. A similar property is displayed by some highspeed machine tools. In some instances the torque at start (rpm=0) can be considerably higher; if, for example, the substance becomes very viscous after standing. The relationship between torque (footpounds) and horsepower curves is defined as: Horsepower = torque rpm % Standard Savings Calculation Calculating energy savings from an ASD requires knowing the type of load characteristic curve for the specific application. The curves discussed above are for the ASD installed case. The load curves for the baseline condition are also required for the determination of energy savings. The baseline load curve may be obtained from equipment manufacturers or design documents, or by installing monitoring equipment. The other major requirement for calculating energy savings is the load profile or the amount of time the machine operates at various points along the curve. The kwh consumption for any point along the characteristic curve can be calculated as: kwh = Hp Eff Ophrs where: Hp = Horsepower requirement at full load Eff = Motor efficiency (include the ASD efficiency for the ASD curve) Ophrs = Annual hours of operation at that percent of full load % 25 Torque (ft-lbs) Power (HP) ft-lbs HP Speed (RPM) % Figure 4: Typical Linear Torque Curves (Source: BPA) PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 9

10 % Torque (ft-lbs) 200 Power (HP) ft-lbs Cost and Service Life Factors That Influence First Cost Many factors influence the cost of installing an ASD degree of control, effects on the motor and related systems and other characteristics related to the application. Consideration should be given to: HP Rectifier/inverter Type: CSI, VSI, PWM, etc. Speed (RPM) Figure 5: Typical Variable Torque Curves (Source: BPA) Annual kwh savings are the difference between the arithmetic sum of the baseline kwh consumption and the arithmetic sum of the ASD kwh consumption. The energy savings equations can be applied to various points of the curve to represent the entire operating time of the machine. Reasonable estimates can be made if grouped into four points (25,, 75, and percent). Default curves for a variety of variable flow fan configurations are in Reference 4. They include baseline curves for outlet damper, inlet vane, and variable pitch fan controls in addition to the ASD curve. % Harmonic Effects: harmonic distortion caused by the ASD system, need for filtering equipment on either the ASD or other sensitive electronic equipment, utility system requirements, mechanical torque pulsations, heating by harmonics, derating of existing motor, etc. Isolation Requirements: for harmonic distortion effect reduction and/or for limiting ground currents, etc. Control Specifications: sensor selection, signal interference, system response and accuracy, limitations of speed or torque, etc. Special Requirements: multiple drives, process control integration, manufacturer s tests, startup services, etc. Training Requirements: instruction manuals, electrical drawings, shop floor or classroom sessions, specialized test equipment needed, etc. PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 10

11 Typical Service Life The PG&E CEE program assumption for service life is 16 years. Operation and Maintenance Requirements Proper maintenance can prolong the life of ASD equipment. Some key aspects of a solid maintenance program include: Correct installation, with proper sensor installation, electrical isolation, limits on speed or torque and diagnostic software. Regular cleaning of air filters and checks to ensure cooling fans are operational. Protection from hostile operating conditions. Regular checks for reasonable motor speed operation and sensor calibration. Definitions of Key Terms Cogging: A condition in which a motor does not rotate smoothly but steps or jerks from one position to another as the shaft rotates. Duty Cycle: The amount of time a motor system is operating, usually measured in percent per day or year, shifts per day, or hours per year. Eddy Current Clutch: Couples a motor to a machine and allows the machine to rotate at a slower speed than the motor. Two concentric metal elements behave much like an electric motor; torque is created magnetically and varies with the strength of a controlled DC magnetic field. Harmonic: A multiple of a fundamental frequency. The distortion of a periodic wave can be described in terms of the fundamental frequency and a set of harmonics with varying amplitudes. Inrush Current: The electrical current required to initiate motor rotation. Significantly greater than current at normal operating conditions, and lasts for less than one voltage cycle (about seconds for 60 cycle power). Load Profile: Time distribution of a motor s load (measured in percent of full load), usually expressed on an hourly basis over a day, but may also be expressed on a seasonal basis over a year. Modulated: A control point to vary the output. A control valve opens and closes to varying degrees and modulates the fluid flow through it. Torque: The twisting force exerted by the motor shaft on the load, measured in units of force times length (pound feet). For smaller motors, torque can be measured in inch-pounds or ounce-pounds. References to More Information 1. Bonneville Power Administration, Adjustable Speed Drive Application Guidebook, PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 11

12 2. Electric Power Research Institute, Inc., Adjustable Speed Drives Application Guide, E-Source, Drivepower, Technology Atlas Series, Volume IV, PG&E, PG&E Custom Financial Incentives Programs Resource Binder, PG&E, The Adjustable Speed Drive: What s In It For You., U.S. Department of Energy, Motor Challenge Sourcebook, Major Manufacturers ABB Industrial Systems, Inc. 162 W. Glendale Dr. New Berlin, WI Tel (414) Fax (414) Allen-Bradley Co., Inc. Box 760 Mequon, WI Tel (414) Fax (414) MagneTek Drives & Systems W. Ryerson Rd. New Berlin, WI Tel (414) Fax (414) Additional lists of motor manufacturers can be found in References 3 and 6. Information on adjustable speed drives can also be found by contacting trade organizations such as the National Electrical Manufacturer s Association. PG&E Energy Efficiency Information Adjustable Speed Drive Opportunities Page 12