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Converted power range of three phase motor

Discussion in 'Electronic Design' started by nescafe, Oct 5, 2012.

  1. nescafe

    nescafe Guest

    Hi.

    I have 11kw 3 phase motor controlled by variable frequency drive.
    The drive can be powered with one phase and it can give 3 phase output.
    So i was thinking to try this solution. I will connect one phase to the
    drive to see how much power can i get on motor side.

    My question would be, how to calculate maximum horsepower or KW used by
    3 phase motor, if he is be connected to 220 / 50 HZ - 8KW source ?

    Regards !
     
  2. Jamie

    Jamie Guest

    sqrt(3).

    Jamie
     
  3. Tim Williams

    Tim Williams Guest

    Nope. Power factor goes from about 0.9 to 0.5. It's less than half, amp
    for amp.

    Tim
     
  4. Jamie

    Jamie Guest

    Ok, so where's the problem? 0.9/sqrt(3) = 0.52

    If there is going to be a useless debate, I won't be joining in.

    Jamie
     
  5. Tim Williams

    Tim Williams Guest

    No, I mean in addition to the sqrt(3). You get the sqrt(3) from having
    two wires instead of three. 10A at 240V 3ph is 4.16kVA, 10A at 240V 1ph
    is 2.4kVA. The power factor is additionally less, or more like 1.2kW for
    10A 240V.

    I should actually say it's more like a quarter, amp for amp.

    Tim
     
  6. P E Schoen

    P E Schoen Guest

    "nescafe" wrote in message
    I have a 1.5 kW VFD that will work on single phase, but larger drives
    typically need three phase for full power operation. If you can add enough
    capacitance on the DC link to maintain minimum peak voltage during zero
    crossing, you might get full rated power. Drive efficiency is typically 95%,
    so you could get about 7.5 kW from an 8 kW source. However, you need to
    consider power factor of the rectifier/capacitor circuit as well as that of
    the motor. So if your source is actually 8 kVA, and power factor is 60%, you
    might get only about 5 kW for the motor. If you use a PFC DC-DC converter to
    get the DC link voltage, you might get the full 7.5 kW for the motor, but it
    also depends on the PF of the motor and the power rating and losses of the
    VFD.

    If you add capacitors to the DC link, you may need to use larger rectifiers
    than the 3 phase bridge in the drive. For 50 Hz you need a TC of at least 10
    mSec, and the effective load is about 360V/36A = 10 ohms. Thus about 1000 uF
    might be sufficient.

    Paul
     
  7. P E Schoen

    P E Schoen Guest

    "P E Schoen" wrote in message
    ==========================================================
    (nescafe replied by email, 3 times):

    Thank you very much for this answer.
    I got some additional answers on one other newsgroup but the idea is
    almost the same. 2/3 of the power should be reached.

    I use siemens micromaster that is connected to 11kw motor so if i have
    8kw input and if i calculate the 2/3 of the original power then 7,5
    should be reachable. The 8kw is real 8kw input.( 220 - 40A )

    I dont understand the dc-dc part. My micromaster has dc-dc input link.
    Do you think i have to add something to dc link.
    Sorry for this dump question but i didn't work with this kind of
    configuration till now.

    =========================================================
    My reply (which apparently was not received, perhaps because
    <> may be bogus?

    Actually what I meant was an AC-DC power supply with PFC. If you just use AC
    through rectifiers and a capacitor, you will get a very low power factor and
    will draw about twice the current compared to a supply with PFC. And it
    might be difficult to find a DC supply with the proper specifications. (360V
    36A). In fact, if you can get close to unity PF, you only need 22.5 amps for
    8 kW at 360V. So you might be able to use seven 48VDC 25A 1200W power
    supplies to get the DC power you need.

    You might be able to find a 10 kW single phase 240V UPS with PFC. It will
    probably have an internal 300VDC bus you could use for the DC link.

    Paul
     
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