Variable Frequency Drives – Planning your system

   

                   Many      irrigation    districts  are          demanding of the motor because of

              installing       Variable      Frequency             the electrical distortions they cause.

              Drives     (VFDs)     to   increase   their          To     determine      if   a    motor     is

              level   of   service   to   their   customers        appropriate     for  a  VFD     retrofit,  a

              and/or     to    reduce    their    energy           Meggar test is used.       A Meggar test

              consumption.                                         measures      the   resistance    between

                   However, not all VFDs are the                   the   motor    windings     and    ground.

              same.     The performance quality of                 This resistance is a measure of the

              a    VFD       will   depend      on    the          condition     of   the   motor    winding

              characteristics      of   the    particular          insulation.    If the resistance is over

              VFD,      as  well   as  how    the   VFD            (about)     300,000     ohms     and    the

              system is set up.      Many times it is              installation criteria described above

              the    seemingly     small    details  that          are    followed,    then   the   motor    is

              will    have     a   large    impact     on          probably   OK   for   the   VFD   retrofit.

              performance in the long run.          This           If    the   resistance     is  less    than

              brochure       is   intended     to    help          300,000      ohms      then   the    motor

              irrigation        districts       properly           should   be   “dipped   and   baked”   to

              evaluate     what   type   of  VFD     they          renew the winding insulation.         One

              need      and    how      to   install    it.        cautionary   note   –   if   the   motor   has

              Attending      to  the  details   outlined           been idle for a long time – several

              here    will   increase   the   efficiency           weeks     or  more    –  then  the   motor

              and life of your VFD system.                         should be run long enough to bring

                                                                   it   up   to   operating   temperature   and

              Before You Buy                                       kept    there   for  an   hour   or   more

                                                                   before conducting the Meggar test.

              Retrofitting       VFDs      to  existing            This   is   because   dust   and   moisture

              motors                                               can   collect   in   the   windings   during

                   In   general,   existing   motors   in          idle   periods.     Running     the  motor

              good     condition    can  be   retrofitted          drives off the moisture and “blows”

              with   VFDs.       There   are,   however,           loose     dust   from     the   windings.

              some     limitations.       The     motor            Moisture   and   dust   in   the   windings

              windings       must     be     in    good            will provide false low values when

              condition.         VFDs       are    more            performing a Meggar test.



                 Irrigation Training and Research Center – www.itrc.org


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                                    Variable Frequency Drives – Planning Your System

                     http://www.itrc.org/reports/vfd/technologyspecs.pdf                             ITRC Report No. 02-009



Automating with a VFD                                                 condition   of   maximum   load.      For   a   centrifugal

     For   pipelines,   an   internal   PID   controller   is        pump,   maximum   load   is   usually   the   maximum

often     sufficient.       Canals     are   special    and           flow rate/minimum TDH condition.            For an axial

generally   require   a   separate   PLC   in   an   RTU   to         flow   pump   (propeller   pump),   maximum load   is

apply     more    sophisticated     control    algorithms.           usually the zero flow/maximum head condition.

Modeling of a canal system is required to select                      Consult     the   pump’s     curve    to  determine     the

the proper control algorithm constants.                               actual maximum loading condition.

     The RTU must automate both the VFD and                                Other    factors   also   impact   the   horsepower

the other pumps that operate in parallel with the                    rating     of  the   VFD     compared      to   the   motor

VFD.                                                                 horsepower.       Among   these   factors   are   altitude,

     The   pump   to   automate   with   a   VFD   (in   a            air temperature, and the cleanliness of the motor,

location     with    multiple    pumps     supplying     the          drive    and   appurtenances.        Consult    the   VFD

same   pipeline)   is   the   smallest   which   will   meet         manufacturer        to   determine      how    VFDs      are

both of the following criteria:                                       impacted by these conditions.



¾ (Flow Rate of the VFD pump) + (Sum of the

     flow rates of all the smaller pumps) must be

     greater than or equal to the flow rate of the

     next bigger pump

¾ The flow rate of no single speed pump can

     exceed the combined flow of all pumps that

     are smaller than it – including the pump with

     the VFD at full speed.



Choosing the Right Size of VFD

     VFDs         are     typically       classified      by

horsepower.          However,       the   size   of    VFD

necessary is probably larger than the size of the

motor being controlled by the VFD.             This is for

two   reasons.    First,   a   motor   can   produce   more

than its rated horsepower.        It is not unusual for a

motor to produce up to 15% over the rated shaft

horsepower       by    utilizing   the   motor’s    service

factor.    Second,   the   VFD   is   rated   by   its   output

horsepower,   which   is   the    input   horsepower   to

the motor. The input horsepower is the motor’s                              It is important to know how to specify the

shaft     horsepower        divided     by     the    motor               performance of what is inside the VFD panel.

efficiency.                                                                                 Patterson ID.

     For example, a motor with a 100 horsepower

output rating and a motor efficiency of 93% that                      What to Expect from the

is being used at the limit of a 1.15 service factor                   Manufacturer

would     require    a  VFD    with   an   output   of   124

horsepower.         This   is  24%    over   the   nominal            Technical Specifications

motor     horsepower      rating.    A    125    HP    VFD                 The     Variable     Frequency       Drive     (VFD)

would be marginal in this case, for the 100 HP                        should    be   microprocessor-based        inverter   logic

motor.                                                                isolated    from   power    circuits,  with   a  Buffered

     For    VFD     retrofits  to  existing   motors,    the          Pulse     Width      Modulated        (BPWM)        output

best    way   to  determine     the  required    size  of  a         waveform       using   4th  generation    Insulated    Gate



VFD      is  to  measure    the  electrical   current   and           Bipolar    Transistors     (IGBT)    technology.      The

voltage     supplied    to  the  existing    motor    under          VFD   package   should   be   completely   assembled

actual     working      conditions     that   include    the          and    tested   by   the   manufacturer.      The     VFD



                                     Irrigation Training and Research Center – www.itrc.org


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                                    Variable Frequency Drives – Planning Your System

                     http://www.itrc.org/reports/vfd/technologyspecs.pdf                            ITRC Report No. 02-009



should     employ    a  full  wave    rectifier,  DC    link                    ¾ Output Frequency

reactor,   capacitors,   and   fourth generation IGBT                           ¾ Motor Speed (RPM, % or

as    the  output    switching     device.      The   drive                         Engineering units)


efficiency   must   be   97%   or   better   at   full   speed                  ¾ Motor Current


and full load.      Displacement power factor must                              ¾ Output Voltage


be between 100% and 95% lagging at all speeds                                   ¾ Elapsed time meter


and loads.                                                            11. Speed       command        input    made      by    the

     The      VFD      should     have    the    following                following:


capabilities:                                                                   ¾ Keypad


                                                                                ¾ Local Manual Potentiometer


1.   Automatic      restart    after   an    overcurrent,

                                                                                ¾ 4-20 madc input


     overvoltage,   undervoltage,   or   loss   of   input

                                                                      12. I/O       Communications            for      network

     signal    protective     trip.  –  The    number     of

                                                                          communication   with   the   PLC.   –   The   PLC

     restart attempts, trial time, and time between

                                                                          protocol will be Modbus.

     reset attempts should be programmable.

2.   An     anti-back    spin   device    to  prevent    the

     pump from starting while it is rotating in the                   Operation and Maintenance Manuals

     reverse direction.                                                   Operation      and   Maintenance      manuals     must

3.   Control   logic   ride-through   in   the   event   of         be    provided     with   each    VFD.       They    should

     power outages up to 2 seconds in duration.                      include the following:



4.   A   3-position   Hand-Off-Auto   (HOA)   switch

     and speed potentiometer. – When in “Hand”,                       ¾ Spare parts listing; source and current prices

     the   VFD   will   be   manually   started,   and   the              of replacement parts and supplies, including

     speed     will  be   controlled    from    the   speed               recommended          spare     parts    to   be    pre-

     potentiometer.      When   in   “OFF”,   the   VFD                   purchased   by   the   district   after   the   warranty

                                                                          ends

     will be stopped.       When in “Auto”, the VFD

     will start via an external signal from a PLC,                    ¾ Recommended              maintenance       and     repair

     and    its  speed   will  be   controlled    via  PLC                procedures         and     intervals.          Include

     communications.           For   units   with   bypass                dimensioned as-built drawings

     capability,     a  3-position     Drive-Off-Bypass               ¾ Test and Calibration procedures

     switch is required.                                              ¾ Recommended cleaning methods

5.   The   rating   of  the   drive   should    allow   for          ¾ Instructions for troubleshooting diagnostics

     100%      continuous     operation,   and   operation            ¾ Wiring diagrams

     with   an   overload   current   of   120%   for   one           ¾ Full        compliment        of    user     instruction

                                                                          materials

     minute.

6.   Input    line   fuses    standard    in   the   drive

     enclosure.                                                       Warranty

7.   Optimized      for  a  2kHz    carrier   (switching)                Warranty should be 24 months from the date

     frequency.   –   The   carrier   frequency   should             of    first  successful   start-up,   not   to  exceed    30

     be   adjustable   to   a   minimum   of   8kHz.   The           months from the date of shipment.           The warranty

     carrier   frequency   should   be   adjusted   to   the         must     include   all  parts,  labor,  travel   time,  and

     minimum   frequency   that   eliminates   audible               expenses.

     “hums” in the motor and drive.

8.  At least 12 Pulse operation. – The number of                   Installing the VFD

     pulses should be selectable.

                                                                      The Contractor should Specify in the Bid

9.   Both     volts   per    Hertz    and    “Sensorless

     Vector”     operation.    –  The    operating    mode            • The maximum overvoltage and undervoltage

     should be “Sensorless Vector” operation                              prior to trip

10. Standard operating information on the VFD                         • Maximum overcurrent capacity prior to trip

     digital display:                                                 • Maximum transient protection



                                     Irrigation Training and Research Center – www.itrc.org


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                                   Variable Frequency Drives – Planning Your System

                     http://www.itrc.org/reports/vfd/technologyspecs.pdf                           ITRC Report No. 02-009



Installation and Operation                                           ƒ All wires must be individually numbered or

     To minimize the negative impacts of VFDs,                            labeled at both ends.

several steps should be followed.                                    ƒ The disconnect handle should be a thru-the­

                                                                          door    type,   and   be   lockable    in  the  “Off”

ƒ Keep the cables connecting the VFD to the

                                                                          position using a padlock

     motor as short as practical.

                                                                     ƒ Surge        Protection    on   the  incoming     power

     o  Cable length can be as long as 100 feet.

                                                                          lines must be provided

          However, shorter cables mean less EMI

                                                                     ƒ Stainless Steel Door Nameplate

          and RFI and potentially less motor heat.

                                                                     ƒ Space          heater    for    winter     to    prevent

     o  If   a  cable   is  over   40   feet  long   then               condensation

          “inductor      duty”     motors     should     be

                                                                     ƒ Weatherproof               and        dust/insect-proof

          specified     –   particularly   if  the   motor                enclosure

          voltage is 460 Volt or higher.

                                                                     ƒ Fluorescent light (external mounting)

ƒ Provide shielding, like metallic conduit, for

                                                                     ƒ VFD panel cooling

     motor cables.

                                                                          ¾ Water   cooling   using   process   water   and

ƒ Adjust the carrier frequency so that it is high

                                                                               heat   exchanger   with   an   internal   fan   is

     enough to eliminate any objectionable hum.

                                                                               acceptable   only   if   water   temperature   is

     However, keep the carrier frequency on the

                                                                               below 60°F the year-round.          The water-

     low    side.   A   carrier   frequency    between   4

                                                                               cooling     heat  exchanger      for  the  panel

     kHz   and   6   kHz   will   typically   eliminate   the

                                                                               must   be   equipped   with   a   large   capacity

     hum and keep the motor temperature issues

                                                                               water filter having automatic flushing.

     to a minimum.

                                                                          ¾ Outside air cannot be introduced into the

ƒ Measure the EMI and RFI generated by the

                                                                               panel for cooling purposes.

     installation.      If  the   interference     exceeds

                                                                          ¾ Refrigeration   cooling   is   almost   always

     limits defined by IEEE 519-1992 then install

                                                                               required.

     filters.

                                                                          ¾ Calculations are required to demonstrate

                                                                               that     cooling      is    adequate      during

Conditioning of Incoming Power

                                                                               “historically hottest days”

     A   self-contained   control   power   transformer

                                                                     ƒ GFI receptacle (external mounting)

must be supplied to feed the GFI, controls, and

                                                                     ƒ Shading of the panel from direct sunlight

light.      The     RTU     must     have    an   isolated,

conditioned power supply and battery backup.

     Harmonic   filters   must   be   provided for each

leg of incoming power of the VFD.

     If    the   VFD     is  to   be   installed    on   an

ungrounded Delta system, then a 3 phase, Delta

to   Wye     isolation   transformer,    electrostatically

shielded,     should   be   installed   before   the  VFD

with    the   WYE      grounded     with   an   individual

grounding rod.



Lightening Protection

     Recommendations          of  the  NEMA      Standard

No. ICS7 should be followed.



Required Accessories

ƒ All        external     interlocks     and     start/stop

     contacts      must     remain      fully   functional                 A new air conditioned enclosure for a VFD

     whether      the   drive   is  in  Hand,     Auto   or                   application at South San Joaquin ID.

     Bypass.



                                     Irrigation Training and Research Center – www.itrc.org


----------------------- Page 5-----------------------

                                 Variable Frequency Drives – Planning Your System

                   http://www.itrc.org/reports/vfd/technologyspecs.pdf                        ITRC Report No. 02-009



Abbreviations                                                    This information was developed under the PIER

                                                                 End      Use     Agricultural      Sector     Program,

BPWM – Buffered Pulse Width Modulated                            administered       by    the     California     Energy

EEPROM – Electrically Erasable Programmable                      Commission.      It  does  not  necessarily   represent

Read Only Memory                                                 the   views    of   the   Energy    Commission,      its

HOA – Hand-Off-Auto (switch)                                     employees,     or   the  State   of  California.    The

IGBT – Insulated Gate Bipolar Transistors                        Commission,   the   State   of   California,   Cal   Poly,

IHP – Input horsepower, the electric horsepower                  ITRC,      their   employees,      contractors,     and

     delivered   to   the   motor. The   motor’s  shaft          subcontractors     make    no  warranty,    express   or

     horsepower    is   the  input  horsepower    times          implied,   and   assume   no   legal  liability  for   the

     the motor efficiency.                                       information   in   this   report;   nor   does   any   party

madc – milliamps direct current                                  represent that the use of this information will not

MCC – Motor Control Center                                       infringe upon privately owned rights.

OIT – Operator Interface Terminal

PCC – Point of Common Coupling

PID – Proportional/Integral/Differential

     “equation”    used   to  get  “process  value”   to

     converge     on    setpoint   in   the  minimum

     number of tries

PLC – Programmable Logic Controller

PWM – Pulse Width Modulated

RTU – Remote Terminal Unit

TDH – Total Dynamic Head of a Pump

THD – Total Harmonic Distortion

VFD – Variable Frequency Drive