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three phase motor - question about kW/HP-measurement

Discussion in 'Electronic Basics' started by Alexander Fischer, Jul 9, 2003.

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  1. Hi

    I have a problem of understanding concerning the power a motor should
    deliver. In europe, normally the powere is measured in kW, while at
    the US-Net, it is given in HP. As example, a supplier speaks of 200kW
    at european 460V/50Hz-Net and of 250 HP at the US-Net with 480V/60Hz.
    The conversion of 250 HP would be 184 kW.

    Does the motor delivers only 184 kW in such case, or is there a
    difference between the values of kW in Europe and HP in the US?
    Someone mentioned, that it might be that the european value is
    measured by the power the motor takes from the net, while the us-value
    is measured by the power available at the shaft.

    Could someone clear me up about those things?

  2. Alan McClure

    Alan McClure Guest

    Assuming that the HP doesn't change because of the difference in
    I would say that the motor in question has a specified efficiency of 92%
    at full
    load. Not bad, IMHO.
    HP delivered should equal power consumed times efficiency.

  3. Mark Empson

    Mark Empson Guest

    In both cases, the power rating is the shaft power of the motor, and to get
    HP form KW, divide by 0.746, or to get KW from HP multiply by 0.746. The
    conversion is voltage and frequency independent.
    Note that the KW rating of the motor is not the same as the power drawn by
    the motor, that includes the losses and is the shaft power divided by the

    Best regards,
    Mark Empson
  4. Jon Elson

    Jon Elson Guest

    I think there is, but it is infinitesimal.
    No. The difference is that on 60 Hz power, the motor delivers full
    rated torque at 20% higher speed! Since Hp is torque x speed, it
    therefore produces 20% more Hp (or watts) on US power.

  5. BG

    BG Guest

    Or 20% less torque, at the same HP.
  6. ~^Johnny^~

    ~^Johnny^~ Guest

    You can't get something for nothing.
    More slip = more torque, but at a lower than rated speed.
    Since the motor is rated for a certain slip speed at 50 Hz,
    slip increases at 60 Hz, to provide the same performance, at a
    higher current.

    Increasing current in the windings can drive stator cores into
    saturation, increase I-squared-R losses (exponentially, mind you),
    among other things... and I =love= to wake up to that BBQ smell of
    rosin, phenolic, and enamel all burning at the same time :)))

    I had a buddy, back in the seventies, who called me up with a problem
    with a generator he had purchased (Army surplus). He said it was
    frying things, like his air compressor motor...
    stereo and TV and other electronics seemed to work fine with it, and
    it put out the required 120 volts. I plugged the shop fluorescents
    into it, and they flickered, then smoked (damned mag ballasts of the
    70's). Then I accidentally touched the input RCA jack center
    conductor on the still-playing stereo, and heard that telltale
    400-cycle hum...

    Frequency does matter. :-O

    "The first step in intelligent tinkering is to
    save all the parts." - Aldo Leopold
  7. ~^Johnny^~

    ~^Johnny^~ Guest

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