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Testing with VFD

Discussion in 'Electronic Design' started by EnigmaPaul, Jul 28, 2013.

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  1. EnigmaPaul

    EnigmaPaul Guest

    Hi Folks -

    I need to setup a VFD in a workshop to test the accuracy of my AC voltage and current metering on the line side. VFDs often disrupt the measurements of RMS due to harmonics, conducted, and radiated noise.

    I'm going to get a low cost PWM VFD and need to apply a load to it.

    I'd like to use a motor to most accurately simulate an inductive load but I was wondering:

    1) Can I connect a resistive load bank instead so I can control the current? What can I use for an inexpensive 3-phase load bank?

    2) If I use a motor, is it possible to just use a simple friction brake to load down the motor? Where would I get a suitable brake without having to make one?

  2. Fred Abse

    Fred Abse Guest

    A motor is nowhere near an inductive load.
    Depends on the design of the VFD, as to whether you can use a purely
    resistive load. Modern VFDs monitor the current in the phases (usually
    two, and derive the third), then do some quite sophisticated DSP math, to
    arrive at motor excitation. They expect to see a real motor.
    Any brake (dynamometer) has the problem of dissipating the energy it
    absorbs, usually as heat. A frictional brake will get very hot.
  3. EnigmaPaul

    EnigmaPaul Guest

    Yes, but the testing would only be for short periods of time.
  4. RobertMacy

    RobertMacy Guest

    We used to load a motor by driving a generator and then power a radiant
    room heater with that, or that type of load, you know the giant ceramic
    resistors that can take 100W each.

    By loading the generator it was easy to vary the load on the motor.
  5. Neon John

    Neon John Guest

    No. Three phase VFDs look at the back-EMF as part of their
    determination as to when to step to the next phase rotation.
    I did that for a Science Fair project many moons ago. The research
    was to determine the effects of various porting techniques (2-stroke)
    on power output. The absorber was a disk brake from an Opel with a
    hole bored in the back on axis with the spindle and a 1" shaft welded
    in. This assembly was mounted on pillow blocks and equipped with a 1
    ft long arm that pressed on a scale.

    That is NOT the way to do things today. The cheapest absorber that I
    know of that you can make without a machine shop is an ordinary
    induction motor (squirrel cage) with DC applied to its input

    This causes the motor to become a generator with the electricity
    induced into the rotor's shorted bars. Almost all the absorbed energy
    is dissipated in the rotor so some good forced cooling is necessary.

    You can easily demonstrate this effect. Pick up any old AC motor
    (even a house fan) and put a 9 volt battery across the AC in. Observe
    how difficult the motor is to turn.

    Not too many years ago I build a dyno using this method that used a
    5hp single phase motor as the absorber. I unbolted the foot and
    mounted the motor body between 6 ball bearings, 3 at each end. A 1 ft
    long (from the shaft centerline) pressed on a load cell that fed my
    data acquisition system. A simple Variac and rectifier varied the DC
    voltage. An optical pickup registered the shaft speed. Once you have
    RPM and torque in ft-lbs, hp = (torque * RPM)/ 5252.

    With a high capacity blower blowing air into one end bell and through
    the rotor, this absorber could handle 50 HP for several minutes. I
    used it to test small EV motors.

    Check with your local electric motor shop. It's not that uncommon to
    have a phase open up on a 3 phase motor and burn out 1 or more of the
    windings. Chinese made motors are too cheap to rewind so they get
    discarded for their scrap value. You can probably pick up such a
    motor for the scrap value. One winding should be enough, though 2
    would be better.


    John DeArmond
    Tellico Plains, Occupied TN
    See website for email address
  6. josephkk

    josephkk Guest

    Each to their own. Just the same i would buy, assemble, or rent a
    motor / brake assembly. That way you could test its performance over the
    range of loads from no load to locked rotor.

  7. John S

    John S Guest

    About 20 years ago I worked at a company who produced AC and DC motor
    speed controllers ( Some of our DC drives were
    regenerative. To test VFDs, we had DC motors mechanically coupled to the
    AC motors and used the DC regenerative drives to return the energy from
    the DC motor to the power line. It was possible to vary the amount of
    regeneration so as to control the load on the AC motor. BTW, just
    because you return the energy to the line doesn't mean that the power
    factor is good.

    It is also possible to do the same with two coupled AC induction motors,
    but, unless times have changed quite a bit, you will probably not find
    many regenerative AC drives that return the energy to the line. Most of
    them burn up the energy with some sort of load on the bus (called
    dynamic braking, back then). Ours had a regenerative bridge.

    For our customer, the VFD had to drive an induction motor which was
    coupled to a hydraulic motor. Then, when the hydraulic motor drove the
    induction motor, it had to load the hydraulic motor. The amount of
    energy meant that it was prohibitive to just dissipate (I think it was
    about 100Hp or so). That's the reason we put it back into the line even
    if the power factor was not good; it gave it a place to go.

    Just thought for you. Good luck.
  8. Fred Abse

    Fred Abse Guest

    That's basically what I'd do, except I'd use a shunt wound DC motor as a
    generator. Generated voltage into a fixed resistive load, control by
    varying DC field current. The hot load can go somewhere outside, where it
    won't cook the operator. There must be lots of redundant DC shunt motors
    lying around, off retrofitted machine tools, cranes, etc.
  9. Neon John

    Neon John Guest

    The problems with that method are a) you have to come up with some
    method of converting the electricity to heat and disposing of it.
    That's a non-trivial or at least non-inexpensive proposition for
    higher powered motors.

    b) The armature, being solid, has no real way to be cooled and so it
    can't handle current much over its rating for very long. The rotor on
    an AC motor, being perforated with cooling holes, CAN handle the
    overload. All that is required is more cooling air.

    The beauty of the induction motor-as-a-load is that all it requires is
    a relatively small Variac, a bridge rectifier and a blower for the


    John DeArmond
    Tellico Plains, Occupied TN
    See website for email address
  10. Fred Abse

    Fred Abse Guest

    The energy, or most of it, goes into the *load*, which can be fan- or
    water-cooled. A resistor bank, external to the generator, and even the
    building, if you want.
    The armature will generate no more internal heat than it did running as a
    motor at full load. The current is the same, motoring, or generating. The
    armature resistance is the same. If it didn't need cooling as a motor, it
    won't as a generator. Those really big ones that did had a snail fan built

    The rotor of an induction motor is solid, too. Sometimes having cast-in
    aluminum bars.
    *All* the absorbed energy has to be dissipated in the rotor bars.
  11. I guess if had to do something like that all day you could get a grid tied inverter and stuff it back into the grid

    I know a guy that has a "rolling road" in the garage for tuning cars, most of the load is inertia from the big rotating drum but it also has a brake
    afaict it is a rotating electro magnet between two water cooled iron discs

    The cooling water is actually plumbed in to the house water heating

  12. Phil Allison

    Phil Allison Guest

    "Enigma Paul = PITA "
    ** Is that a VFD that operates from single phase power?

    ** Only if there is no filter electro after the rectifier - only possible
    with VFDs that take in 3 phase power.

    ** So it IS a *single phase input* VFD with 3 phase output.

    The input current wave will be just like with any rectifier & capacitor
    oad - ie 120Hz half sine shaped pulses.

    The problems you anticipate only happen when monitoring on the MOTOR side.

    .... Phil
  13. Fred Abse

    Fred Abse Guest

    They used to use salt water rheostats to control the starting rotor
    current of big 3-phase slip-ring induction motors. Steaming by the time
    the motor was up to speed and the rheostats were shorted out with the
    blades at the bottom of the tank.
  14. josephkk

    josephkk Guest

    Um. that is not the case when being forcibly turning way slower than
    rated speed. Especially not during "locked rotor" testing.
    Well at least somewhere in the motor. Much can be dissipated in the
    stator windings as well.

  15. Fred Abse

    Fred Abse Guest

    What I was describing is a DC motor, used as a generator, into a fixed
    resistive load, and it's torque reaction controlled by controlling the
    field excitation. With the right choice of load resistance, and a limit on
    the maximum field current, it should be impossible to exceed either
    armature rated current or voltage.

    If you want to try to lock it with a dead short, you're on your own.
  16. josephkk

    josephkk Guest

    Not so much the DC motor but the induction motor fed by the VFD (which
    must be able to cope for at least a little while). Meanwhile it is
    feeding tons of hash back into the mains which need to be measured.
    Lots of higher than mains frequency stuff there. Measuring all useful
    values while this is occuring is part of OPs question the way i see it.

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