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TUTORIAL ON AC DRIVES AND DRIVE APPLICATIONS Written and Edited by Howard G. Murphy P.E. (C) Allen-Bradley Company 1990 ------- T A B L E O F C O N T E N T S -------- USING AC MOTORS ON ADJUSTABLE FREQUENCY DRIVES 1 HOW FREQUENCY LIMITS SPEED HOW VOLTAGE AFFECTS SPEED AC MOTOR HEATING CONSTANT TORQUE, LOW SPEED OPERATION ALL AC MOTORS ARE NOT THE SAME TYPES OF AC DRIVES, FEATURES AND DIFFERENCES 3 VARIABLE VOLTAGE INVERTER SIX-STEP WAVEFORMS CURRENT SOURCE INVERTER VARIABLE VOLTAGE OR CURRENT CONTROL PULSE WIDTH MODULATED AC DRIVE GENERAL PURPOSE AC DRIVE INDUSTRIAL RATED AC DRIVE INTELLIGENT INTERFACED AC DRIVE APPLYING AC DRIVES FOR VARIABLE SPEED 6 APPLICATIONS RULES TO FOLLOW BASIC RULES THE IMPORTANCE OF WIRING SIZING HOW VOLTAGE DROPS AFFECT TORQUE SELECTING THE REQUIRED WIRE SIZE POWER DISTRIBUTION LINE CONSIDERATIONS 9 LIMITING CURRENT CAUSED BY HIGH VOLTAGE TRANSIENTS SELECTING THE PROPER AC DISTRIBUTION SYSTEM HOW ARE VOLTAGE TRANSIENTS CAUSED? CORRECTING THE DISTRIBUTION SYSTEM APPLICATION AND SELECTION OF TRANSFORMERS 11 ENERGY SAVINGS WITH CENTRIFUGAL FANS AND PUMPS 12 AC Diskware Magazine is a service of the Motion Control Division of Allen-Bradley Company Inc. The information provided in this tutorial is a composite of experience in development, application and field testing with AC drives, ac motors and process systems. The information is intended to be used only as a guide for applying AC drives and ac motors. It is not intended to serve as any specific recommendation for applications and installations where an ac motor is used. The use of or interpretation of any information found in this tutorial is the responsibility of the user. The Allen-Bradley Company assumes no responsibility for the use and/or application of any material presented in this tutorial. USING AC MOTORS ON ADJUSTABLE FREQUENCY DRIVES The ac motor has proven itself to be a reliable power conversion device. It has been designed and built to provide reasonably accurate speed and very efficient operation. Its characteristics can be changed to provide optimum torque without large inrush currents. It has been designed for fixed speed operation using the fixed frequency ac line. In the past few years, energy usage or conservation has become an important concern. With the introduction of the premium efficiency or high efficiency ac motor, the losses of the ac motor have been reduced. However, when used across the line, higher inrush currents can be expected. Although the ac motor is well defined as a fixed speed device, operation as a multiple or variable speed device will require a closer look into how the speed of the motor will be changed and the type of load that the motor is expected to handle. HOW FREQUENCY LIMITS SPEED The speed of an ac motor will be limited by the frequency applied. The following formula shows how the maximum speed of the motor is controlled by its construction (number of poles) and by the frequency applied. 120 x Applied Frequency ACTUAL RPM = ------------------------- - Motor Slip Number of Pole Pairs ----------^--------- -----^----- Synchronous Speed Slip Speed For a 100% loaded, 4 pole motor with 60Hz, rated voltage applied, 3% slip; 1746 RPM = (120 x 60)/4 - 0.03(120 x 60)/4 = 1800 - 54 Synchronous speed is a speed limit set by the applied frequency. Slip speed changes with load. Full load will result in a reduction in the synchronous speed of approximately 50 RPM with a NEMA B design motor. HOW VOLTAGE AFFECTS SPEED Speed regulation is better in a NEMA B, ac motor than in a dc motor. Frequency sets an upper speed limit, however, voltage will control the actual speed. In proper installations, the voltage applied to the motor, at a constant frequency, will affect the actual operating speed of that motor. If the load increases, the reduction in speed will be due to the slip of the motor and, in addition, due to the reduction in the motor terminal voltage due to the drop in the wires providing the power to the motor. A 10% voltage drop in the wire will cause a 18% loss in torque which will cause the motor to slip more than normally expected. If the voltage drop in the wire is kept below 1% at full load amps, excellent speed regulation can be expected. In other NEMA motor design classifications, like A, C, and D, the same slip variations will occur. The actual slip percentage will depend on the design characteristics of a particular motor. Rotor design, air gap and stator design will affect the operating speed torque curve of the motor. -1- AC MOTOR HEATING Since a standard ac motor has been designed to operate at a fixed speed, the application of an ac motor to variable speed applications requires that some consideration be given to the thermal characteristics of the motor. The ac motors losses are due mainly to the copper losses. Since copper losses are a result of the motor current, the watts loss will be proportional to the load. As a motor turns slower, with the same watts loss that occurs at higher speeds, the motor will get hotter. The motor gets hotter because less cooling air is available when the internal fan moves at slower speeds. If the motor is used to control fans in HVAC applications the load normally will decrease as the speed decreases so motor heating is less of a problem. CONSTANT TORQUE, LOW SPEED OPERATION In applications where the motor must develop full torque (100% current) at low speed, oversizing motors or using motors with a higher service factor will be required. In many applications, the running or steady state load is less than the full load amps of the motor. With a careful analysis of the application requirements, it may be determined that the actual load is less than the full nameplate amps of the motor. If that is the case, it may be possible to use a standard 1.15 service factor motor approaching 1/4 of the base speed rating of the motor. To protect motors against damage resulting from higher temperatures, internal motor thermal sensors can provide an effective means of safeguarding the motor. ALL AC MOTORS ARE NOT THE SAME All motors, of the same rating, are not the same. It is very important that the manufacturer of the motor be contacted regarding how the motor will perform above or below the base speed of their motor. It is best to inform the motor manufacturer that the motor will be used with ac drives. The manufacturer can recommend the proper motor for the application. Motor insulation plays an important role in how long the motor will last. In some general purpose motors, the wire insulation or varnish is the other electrical barrier between the wire and the motor frame. It is important that a user ask the motor manufacturer about the insulation and its capability to withstand high voltage transients. A quality motor should be able to withstand, on a continuous basis, twice rated voltage plus 1000 volts at rated frequency, when applied from a winding to the case. It should also be able to withstand a continuous pulsing voltage equal to 2/3 of the rated withstand voltage. A quality motor should contain slot insulation and phase paper between the phases. This extra insulation protection means longer life for the motor. In larger horsepower rated motors, rotor bar designs must be configured for any harmonic currents that circulate in the motor. Too narrow a rotor bar tip can result in higher rotor bar temperatures and shorter motor life. In motor below 20 Hp, the motor frame and rotor construction will normally have sufficient material to prevent abnormal heating at the base speed rating of the motor. For operation above or below base speed, the motor frame must increase if the HP load changes proportionally with speed. Application of an ac motor to variable speed operation requires an operational speed load curve and a motor speed torque curve. These curves can be overlaid to determine if the motor is suitable for the application. The motor manufacturer should be able to provide data on the motor. The speed load curve must be obtained from the application, -2- TYPES OF AC DRIVES, FEATURES AND DIFFERENCES The differences between AC drives can not easily be determined from the data provided by the manufacturers. All AC drives convert the input ac voltage into some form of dc voltage or current and then connect that dc to the leads of the ac motor. There are three basic types of AC drives. They are Variable Voltage, Current Source and Pulse Width Modulated. VARIABLE VOLTAGE INVERTER The oldest type of AC drive is the VVI or Variable Voltage Inverter. The VVI drive changes the input ac voltage to a variable value of dc voltage. This voltage is connected to the motor leads simulating frequency. The dc voltage amplitude is varied in step with the frequency to obtain the required constant volts per hertz relationship. The VVI type of AC drive provides a low quality simulation of a sinewave or ideal waveform for the motor. The motor or output waveform is called a 6-Step waveform. SIX-STEP WAVEFORMS This waveform contains the fundamental frequency or operating frequency which produces the desired operating characteristic in the motor. This waveform also contains other frequencies which do not provide desirable operating characteristics. These other frequencies will cause additional heating and cogging or rough shaft rotation. This type of waveform will limit the speed range for a standard ac motor used in low speed, constant torque applications. CURRENT SOURCE INVERTER The next type of AC drive is the CURRENT SOURCE INVERTER. This type of AC drive controls a level of dc current which is connected to the leads of the ac motor. If the level of current in the windings of the motor is controlled, then the torque that the motor can produce is controlled. The waveform to the ac motor is trapezoidal containing frequencies other than the fundamental operating frequency. The motor characteristics will define the actual shape of the resulting voltage waveform. The CURRENT SOURCE INVERTER (CSI) is dependent on the electrical design of the motor and does not always accept a standard motor when a motor replacement is required. This type of drive is normally designed to operate with a single motor and tach feedback, not with multiple motors. VARIABLE VOLTAGE OR CURRENT CONTROL Most VVI and CSI type AC drives convert the ac input power to a dc supply by using Silicon Controlled Rectifiers or SCRs. This is the same type of power device used by DC motor Controllers. The SCR type conversion method is well known for its poor input power factor/speed range characteristic. SCR type conversion causes high ac line distortion due to the commutation action which momentarily short circuits the ac line. In general, the VVI ac motor controllers, used in constant torque applications, will require line reactors or input transformers to reduce ac line distortion and high ac line currents at low operating speeds. Some VVI controllers use diodes for converting ac to fixed dc and a chopper to convert the fixed dc to a variable dc. The variable dc is then connected to the motor as a variable frequency supply. -3- PULSE WIDTH MODULATED AC DRIVE Of the three basic types of AC drives, the PWM or Pulse Width Modulated AC drive, offers the most efficient control of an ac motor. All Pulse Width Modulated AC drives are not the same. A PWM drive can be different from different manufacturers and is not necessarily the same type drive from the same manufacturer. The main differences can be characterized in the following manner. Of the PWM drives available in the market, three types of PWM drives can be defined in terms of the features and the type of waveform that it creates. The first type is the GENERAL PURPOSE DRIVE. This drive will provide the means to control the speed of an ac motor, but will not provide the best use of electrical power. This type of PWM AC drive will provide the basic features, but can be very sensitive in some types of installations. Some of the differences exist in the conversion section of the drive. This type of drive tends to be more voltage sensitive. GENERAL PURPOSE AC DRIVE The GENERAL PURPOSE AC DRIVE is designed with a simple dc filter. All PWM drives convert or rectify the input ac voltage to a dc voltage. The dc voltage should be filtered before transferring the power to the motor in the form of an AC voltage. The simple filter is a capacitor. With a simple filter, the input power factor reflected back to the ac line can be much lower than the power factor of the motor it is controlling. The PWM AC drive with a simple filter can cause a higher power factor penalty than would occur when operating the ac motor across the line. Dependent upon the installation or characteristics of the distribution system providing power to the drive, the GENERAL PURPOSE AC DRIVE can have a input power factor as low as 0.60. The GENERAL PURPOSE AC DRIVE package is normally offered in an open panel construction, a box type enclosure or in a NEMA 1 type enclosure. Other types of enclosures would be "custom". The GENERAL PURPOSE AC DRIVE custom drive package is normally a standard drive mounted in a specified enclosure, such as a NEMA 12. This type of construction will be different than a true factory built AC drive. The factory built AC drive is tested as a complete unit, which will meet all the requirements for a NEMA 12 AC drive. The enclosure variations and construction will vary as widely as the number of panel shops that mount the ac controllers inside a "custom" enclosure. INDUSTRIAL RATED AC DRIVE The next class of PWM drive is the INDUSTRIAL RATED AC DRIVE. This drive contains an LC filter. The LC dc filter exists if reactors or inductors are inserted before the filter capacitor. This significantly improves the input power factor. When continuous current exists in the dc link choke or inductor, a high power factor and low harmonic current distortion will be the characteristics of the INDUSTRIAL RATED PWM AC DRIVE. The INDUSTRIAL RATED AC DRIVE has the ability to control motor current and to handle momentary overload conditions. Many ac drives will trip off on a momentary overload rather than "ride-thru" the overload. The INDUSTRIAL RATED AC DRIVE can provide protection for severe overloads, while ignoring peak loads that are greater than the overload rating of the drive. The superiority of the INDUSTRIAL RATED AC DRIVE frequently is overlooked until it replaces the GENERAL PURPOSE AC DRIVE in those more demanding applications or installations. -4- INTELLIGENT INTERFACE AC DRIVE The last class of PWM drives is the INTELLIGENT INTERFACE AC DRIVE. This AC drive contains some form of programming from which operating settings can be selected. This interface does not add to the drive's ability to power the motor, but may simplify the interface between the AC drive and an external master controller. This class of AC drive is digital based, rather than an analog-based drive. Some form of microprocessor or custom digital chip is used for processing external signals and for control of the output power transistors. The INTELLIGENT INTERFACE AC DRIVE may have some form of distributed control such as PID for process control and TREND buffer that is used as an event recorder. This event recorder is used to monitor the process and provide a high level of diagnostics. In all PWM ac drives, the type of output waveform will vary. Most PWM drives use a "unipolar" type switching method. A few drives use "bipolar" type switching methods. The "bipolar" method provides greater control of the dc voltage applied to the motor. It will result in fewer harmonics and a better RMS to peak current ratio. Pulse Width Modulation is the method used to control the voltage to the motor. The pulse pattern used will vary with drives. Many terms are used to define the switching method used. Sine weighted, Star Modulation, and Sine Modulated are examples of the terms that are used. The true test of switching methods is not in a definition of a term, but in the actual performance of the motor. The ac motor, when operated on a adjustable frequency controller, should exhibit some easily measurable parameters. When compared against an ac motor operated across the line, the ac motor temperature should not, when operated at base speed, achieve a surface temperature that is 3% greater than the line operated motor. The actual measured current may be different due to the non-sinusoidal waveform, but the additional heating should not be significantly different. Rotational performance should not be degraded when applying an adjustable frequency controller. Cogging or pulsating shaft rotation should not be observed at applied frequency greater than 6 Hertz. Observed pulsation are often due to variations in loading due to machine friction variations during rotation or when converting the rotation motion to a linear motion. With PWM drives, audible noise can become a consideration. Some switching methods use a carrier frequency above 12,000 Hertz to place the "noise" above the normal range of hearing. This type of method results in greater heat losses in the drive and the motor. A higher frequency carrier also placing more stress on wire insulation. This can result in shorter motor life. With "standard" ac motor designs, a carrier frequency range from 600 to 3000 Hertz provides a reasonable efficiency. With special motors, a wider range for the carrier frequency could be used. Pulse Width Modulated drives, with an internal LC filter or input line reactors provides the best method for converting electrical energy to mechanical energy in variable speed applications. High input power factor and improved ability to ignore ac line conditions make the PWM drive the most effective power conversion product. As digital based products, the PWM drive can provide troublefree and predictable operation. Reliability, in PWM drives, today far exceeds early type ac drives and can be expected to improve with each new product. The trouble areas becoming more evident are with the ac line and the ac motor. Voltage transients and insulation stress have become the leading problem in variable speed applications. -5- APPLYING AC DRIVES FOR VARIABLE SPEED When compared to alternative methods of controlling speed, the AC drive combined with the standard ac motor is the simplest method for speed control. Replacing an existing motor starter with an AC drive will provide not only the means to control the speed of that motor, but will reduce the mechanical strain on belts, gear boxes and the electrical distribution system. There are some simple rules which will insure a successful installation and long term operation. APPLICATION RULES TO FOLLOW The first rule is to follow proper grounding methods. The second is to follow proper wiring methods. The last rule is to insure that correct components and component ratings have been selected to do the job. BASIC RULES The basic rule in proper grounding methods is that all equipment is tied to earth ground at one location. This means that a 3 phase electrical system will require 4 wires. The 4th wire is the ground conductor. For the input power, the best source of power for electronic power equipment is a WYE configured source. This source could be the plant distribution system or the secondary of an isolation or distribution transformer. With a 4 wire system, all current will be contained within the 4 wires and will reduce the possibilities of creating interference on the input power lines. To reduce interference in the output of power electrical equipment, the 4th wire should be used as a fixed ground connection between the drive enclosure and the motor case. THE IMPORTANCE OF WIRE SIZING The basic rule in proper wiring methods starts with the National Electrical Code and local codes, but continues with wire size selection for minimum voltage drops and metallic conduit to eliminate or reduce magnetic radiation or electrical noise. In adjustable speed applications using electric motors, the motor voltage is proportional to speed. When the speed is near full or base speed, the voltage is near maximum. At full speed, the voltage drop in the wire to the motor will be less critical than when the speeds are lower. At low speeds, the voltage to the motor is low, so that a few volts drop in the wires to the motor will prevent the motor from providing full capacity. HOW VOLTAGE DROPS AFFECT TORQUE Applications that require full torque at low speed will be sensitive to voltage drops in the wire caused by rated motor currents. A normal 5% voltage reduction at full speed would result in a 50% reduction in voltage at 10% speed. The voltage boost available in AC drive is not always sufficient to overcome large voltage drops. For constant torque applications or when starting torque requirements are high, a good rule of thumb is to size the wire so that no more than 1/2 volt is dropped in a single wire when carrying the full load amps of the motor. By using this rule of thumb, the maximum resistance of the wire is defined and the wire size can be selected based on the total length of wire between the drive and the motor. -6- All AC drives control the volts per hertz ratio. This ratio insures that the air gap flux in the motor is maintained at the selected value. When the voltage at the terminals of the motor vary, the air gap flux is the motor will vary. Controlling the air gap flux in the ac motor controls the performance of the motor. In some applications, any variation in the air gap flux will result in rotational variations or varying torque capability as the shaft of the motor rotates. A technology termed "vector control" can be employed which will reduce the amount of rotational variation. To accomplish "vector control", some form of rotor position feedback is used. By knowing the timing relationship between the stator voltage and rotor position, the stator voltage can be adjusted to keep the relationship between the rotor and stator (slip) defined and stable. All PWM drives transfer the power on the ac line to the motor. In most drives, that output voltage will vary if the input line voltage varies. Any variation in the output voltage will result in a variation in speed. At lower operating speeds, a significant speed variation can occur due to any changes in the output voltage. Since percent slip is constant, when a constant air gap flux is maintained in an ac motor, the actual RPM change will be greater as the speed of the motor is reduced. It is very important that the terminal voltage be maintained within +/- 2% to obtain the best motor performance. A few PWM drives regulate the output voltage by correcting the modulation to compensate for input voltage variations. By correcting for incoming power variations, performance variations in the motor are reduced. With input voltage compensation, speed variations, current variations and any operating temperature variations can be reduced. SELECTING THE REQUIRED WIRE SIZE Since the performance of all ac motors will be affected by any voltage variations, the selection of the proper wire size will be important to the application. The National Electric Code provides some guidelines for the selection of wire sizes for current carrying capacity. These types of guides generally assume fixed voltage supplies. When adjustable frequency controllers are used, the output voltage will change with frequency. At low speeds or lower frequencies, the corresponding reduced voltage will intensify the affect of any voltage loss in the wires between the drive and the motor. To reduce the impact of the voltage loss, the resistance of the wire should be kept as low a value as practical. The maximum resistance in ohms, will be defined by the length and AWG size of the wire. The value of that maximum resistance is equal to 1/2 volts divided by the nameplate amps of the motor. The longer the wire length, the larger the cross-sectional area of the wire(smaller American Wire Gauge Number). The low voltage drop in the wire will insure that the motor receives as much of the voltage present on the output terminals of the AC motor controller. In some AC drives, the output current can contain many harmonic currents. Harmonic current consist of high frequency currents. These currents will tend to compress to the outside surface of the wire. This phenomenon is called "skin effect". It is important that the wire size be selected to insure that any additional heating that occurs due to the "skin effect" is considered. -7- POWER DISTRIBUTION LINE CONSIDERATIONS The electrical power line from which the AC drive takes its power has a more important role that merely providing RMS power. The input voltage to any solid state equipment must always provide a voltage waveform which stays within an acceptable RMS value and also provide a voltage waveform that stays within the acceptable instantaneous voltage value. The RMS value does not accurately define an acceptable waveform to solid state equipment. It never defines the shape of the waveform, only the probable heating that might occur in the equipment due to the current. With solid state equipment the term RMS could be defined as "Roughly Means Something". In most solid state equipment, the ac line voltage is changed to a dc voltage through a rectification process. The only thing a rectification process is concerned with is the instantaneous value of the voltage. In the filtering function of the process, the instantaneous value of voltage can cause the filter capacitor voltage value to change to the instantaneous value of the ac line. As long as the instantaneous value of the input voltage is greater than the voltage on the capacitor (dc bus), current will flow into the capacitor. All solid state equipment has limits to the voltage that it can sustain. When the voltage level reaches that value, the equipment will attempt to protect components used within the equipment. To eliminate overvoltage trips caused by high voltage transients, the current into the capacitor must be limited. LIMITING CURRENT CAUSED BY HIGH VOLTAGE TRANSIENTS The easiest way to limit current is select an impedance which delays how much input current is permitted while the voltage transient exists. All INDUSTRIAL RATED DRIVES add an inductor to the filtering circuit. This inductor prevents current from increasing rapidly, delaying an increase in the capacitor voltage. The inductor does not prevent the voltage from changing but merely extends the time for the change to take place. It is important to remember that power must be taken from the dc capacitor to prevent an overvoltage condition from occurring. In fan applications, low speed operation requires less power. An over voltage condition can occur if more energy goes into the capacitor than is removed. This is what occurs when a fan is being operated at less than base speed and a high voltage transient occurs on the ac line. The energy within the voltage transient causes current to flow into the dc filter circuit of the drive. If the current causes the filter circuit to charge to the trip level of the drive, the drive will shutdown. In the same application, when the fan is operating at full speed, more energy goes to the motor and helps to keep the filter circuit from charging to the over voltage trip level. In most cases, the voltage transients found on the ac line will not be great enough to cause the drive to trip. In severe cases, the affect of high voltage transients can be reduced by adding input line reactors or shielded, isolation transformers. Power line filters are commercially available to clamp the voltage transients to within a few percent of the nominal ac line. Power factor switching capacitors tend to create the greatest occurrence of high voltage transients. To avoid nuisance tripping of solid state equipment, the peak voltage transient should not be more than 125% of the nominal peak ac line with a time duration not exceeding 1/10th of the period of the applied frequency. -8- SELECTING THE PROPER AC DISTRIBUTION SYSTEM There are many variations in ac power distribution systems. Delta or Wye systems which could be grounded or ungrounded are typically used. With AC drives which rectify the ac line and store power in a dc bus, the ac line current waveform is pulse shaped and consists of the fundamental current (50/60Hz) and many harmonic currents. To minimize the harmonic currents, a Wye configuration is used to eliminate any harmonic current whose frequency is divisible by three. By using a 4th wire for neutral or ground in a Wye system all current paths will be defined, minimizing voltage unbalances that occur when currents are conducted though an earth ground "conductor". The harmonics that are caused in a distribution system are more often due to any unbalance between the phase voltages than by the power equipment taking power from that distribution system. To minimize harmonics, the ac line must have equal voltage waveforms in the positive and negative cycles and must have the same form or shape. Any deviation will create harmonics currents when power is drawn from the distribution system. If a Delta configuration is used and one phase is grounded, the equivalent Wye circuit is no longer balanced. The resulting line currents will not be equal. This can cause harmonic heating, premature line fuse failure and can cause failure in the input rectifiers used in drive equipment. HOW ARE VOLTAGE TRANSIENTS CAUSED? Any deviation for the ideal voltage sinewave can be defined as a transient or voltage spike. The deviation can be greater or less than the ideal voltage value. Voltage transients do not have to be greater than the ideal input voltage in order to cause overvoltage trips. An overvoltage trip is usually caused whenever the dc bus voltage exceeds a specified value. The capacitor current resulting from a difference in ac line phase voltages actually causes the overvoltage condition. Note the word "difference" in ac line phase voltage. If "A" phase voltage is the ideal value and "B" phase voltage is less than the ideal value or phase shifted, the amount of current flowing from phase A to phase B can be greater than expected. When a fixed speed motor is connected across the line, it is possible that a single phase may experience a voltage reduction when compared to the other phases. This "phase voltage droop" can result in higher than normal current flow in other equipment connected to that ac distribution system. When power factor correction capacitors are used in a system, voltage waveform distortion will occur. When those capacitors are switched, voltage droops will occur and dependent on the characteristics of the distribution system, ie. R,L and C, voltage ringing or oscillation can occur. Voltage oscillations will further distort the waveform and cause unwanted "electrical noise". With the introduction of switching mode power supplies in energy saving ballasts for fluorescent lights, for computers and for power supplies in commercial equipment such as television and VCRs, the level of harmonic current distortion has increased on utility distribution systems. With single phase switching mode power supplies, harmonic current distortion is significantly greater than with most 3 phase switching power supplies. The problem is that most distribution system are designed or sized to handle linear type loads. Switching mode power supplies are non-linear type loads and will require changing to distribution system equipment. -9- CORRECTING THE DISTRIBUTION SYSTEM Correcting a distribution system assumes that the system or equipment in the system is causing or having problems. The simplest problem to overcome is nuisance tripping due to overvoltage or voltage transient conditions. This problem can usually be overcome by adding inductance in each phase. More difficult problems caused by ac line unbalance will requires re-distribution of power to equipment on that ac line. A problem with most distribution systems is that they are considered low maintenance equipment. In most installations, they are treated as "no" maintenance systems. Maintenance is performed only after a problem has occurred. Terminal connections, wire runs, transformers, circuit breaker, fuses and disconnects are repaired or replaced only after a shutdown has prevented a process operation. There have been, over the last years, a number of problems where aluminum wire or terminals were interfaced to copper conductors. The difference in the material create a long term degradation which can result in a loss of connection or an unintentional switch which creates voltage transients. These voltage transient can cause equipment to shutdown and may cause equipment failure. Correcting a distribution system after problems begin to occur will often be very costly. The first step is correcting a problem is to define the problem. Many problems occur when a new piece of equipment has been added to the system. The assumption is that the new piece of equipment is the cause of the problem. Often the problem exists and is just waiting for the "straw" to bring it to the surface. As new types of technologies are placed on the market, it is important that the user keep a record of the "state of the distribution system". Occasional measurements of the power line will provide information that will indicate that a problem is about to occur. A quarterly distribution system analysis can assist in defining project costs and tasks. The power utility can provide the user with the information and, many times, will provide the service. Often the problems can be solved by re-adjusting equipment that is on the distribution system. Older types of power equipment require more routine maintenance, which is often not provided. Older types of power conversion equipment, using SCRs, can be adjusted to reduce any ac line unbalance caused by the equipment. There is no single solution to a problem that can arise. Distribution system are unique and will require an individual analysis. Frequently, the types of problem are simple in nature and will require only a simple change in the system. As long a no shortcuts were taken in the original distribution system design, additions or changes in the system should be minor to correct problems that may occur when applying new technologies. New drive technologies are being introduced which will minimize problems that can occur due to the temporary loss of the ac line or to "brownout" conditions. Today's AC drives can "ride-through" momentary power losses. The upcoming problems will be associated with transformers and protective devices used with power equipment. Transformers have been pushed to their limit and are generally not rated for the non-linear loads that continue to grow. Existing installations are being taxed as non-linear loads grow. Each installation should be reviewed based on the existing power demand and established a baseline for that distribution system. System should begin a corrective action based on future non-linear loading. Without a distribution system analysis, and the definition that it brings, the distribution system will be regulated by codes and rules which will add costly "canned solutions" to all installations. -10- APPLICATION AND SELECTION OF TRANSFORMERS Transformers have normally been used with the older types of drives. The purpose of the transformer was to buffer the ac line from the affects of the conversion equipment. SCR type controllers would create line notching of the ac line and could affect other equipment on the line. With the introduction of the PWM type AC drive, the need for a transformer to buffer the ac line was reduced. In fact, PWM drives do not require the use of an input isolation transformer to prevent line notching on the ac line. The use of isolation transformers with PWM drives can be restricted to reducing short circuit capacity of the ac line, isolation of power and signal circuits and where isolated equipment is required. Most transformers used today are designed for linear loads. Incandescent lighting, line operated motors and resistive loads are all linear. With the equipment loads today, the characteristic has gone from linear to non-linear. Non-linear loads add an additional demand on transformers. Non-linear loads demand current from the utility which creates higher frequencies. The waveform or shape of the load current is no longer defined by a single frequency. It is a complex shape which contains many frequencies. These higher frequency currents behave differently than the fundamental or 50/60 hertz currents. High frequency currents will attempt to flow in the surface area on a conductor. When the cross sectional area of the conductor become too restrictive, the conductor will become hot. When the conductor is packaged inside layers of wire as in a transformer, the temperature of the transformer will rise and can create hot spots which will quickly reduce the life of the transformer. When using transformers with non-linear loads, the practice has been to increase the size of the transformer, that is derate a transformer with a higher rating. Using a larger transformer does not always guarantee that it will run at a lower temperature. A larger transformer will use wire with a larger cross-sectional area. Increasing cross sectional area does not provide a proportional increase in surface area. Harmonic currents can cause hot spots in oversized transformers. To correctly select a transformer for non-linear loads, the wire shape must permit a much surface area as possible to offer the least resistance to high frequency currents. Transformers are classified with a K factor. The K factor defines the transformers ability to handle harmonic currents while operating within the thermal capability of the transformer. Linear load transformers are classified with a K factor of 1. Transformers with a K factor of 4 are suitable for moderate levels of harmonic currents. A K factor of 13 is suitable for greater levels of harmonic currents. An INDUSTRIAL RATED AC PWM DRIVE contribute harmonic currents which would be equivalent to a K factor of 2.5. Single phase lighting and computers contribute harmonic currents which could be equivalent to a K factor of 10 or more. When non-linear rated transformers are used, the associated distribution equipment such as circuit breakers can be size to the transformer rating. When oversize linear rated transformers are used, electrical codes will force the selection of larger, more costly circuit breakers. Non-linear equipment requires harmonic currents to operate correctly. Using line filters to reduce the level of harmonic currents transferred back to the distribution system can create voltage problems with non-linear equipment and will add additional losses into the system. -11- ENERGY SAVING PROGRAMS FOR CENTRIFUGAL FANS AND PUMPS The programs contained on this diskette are intended as an aid in determining possible energy savings that might be obtained when using variable speed. The energy savings programs for centrifugal fans and pumps apply equations defining the relationship between flow, pressure and fan or pump speed as defined by the AFFINITY LAWS. 2 3 Q2 N2 P2 [ N2 ] HP2 [ N2 ] FLOW -- = -- PRESSURE -- = [ -- ] HORSE --- = [ -- ] Q1 N1 P1 [ N1 ] POWER HP1 [ N1 ] Where: N=Speed, Q=Flow(CFM), P=Pressure(Static Inches water), HP=Horsepower By applying these laws, it shows that fan or pump characteristics will follow the system curve defined by the demands of the installation. The energy savings program for fans compares the characteristics of outlet dampers and inlet vanes against variable speed. The method of obtaining variable speed has been selected as an adjustable frequency AC drive and ac motor, since both represent the most effective way to convert ac line power to rotating mechanical power. There is one fan program. This program will determine what operating costs can be expected for any single operating flow rate and will also provide for a comparison for a defined operating profile or selection of different operating flow rates. The program can be used to determine what changes to the operating profile might be suitable in order to reduce operating costs. The pump energy saving programs compares characteristics of throttling flow control against flow control with variable speed. One program provides operating costs for a standard pump curve with a static pressure of zero at zero flow. The second program will provide operating cost comparison for custom pump curves with a specified static pressure. To use the energy programs, you will have to define the rating of the ac motor and its operating efficiency. You will have to provide the cost for electrical power, efficiency of the AC drive and the input power factor of the AC drive. Specific operating points for flow and pressure will be asked for to define the pump characteristic more accurately. To determine the correct efficiency for the AC drive, you will have to consult the manufacturers' AC drive product specification sheet for input power factor and efficiency. Since all AC drive are not equal, a small change in efficiency can result in a substantial cost savings. To gain the greatest cost savings, the program can be used to determine the cost benefits that will occur with a slightly slower flow rate for a slightly longer operating time. If the application permits some flexibility by defining a slight change in the operating profile, the program can provide some insight into obtaining the maximum cost savings. **** End of Text ***** -12-