


                 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-
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