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Thread: Burning pool motor connection

  1. #11
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    Quote Originally Posted by jumper View Post
    Check your math.
    I thought I did.

    240v(rms 60Hz) 3ph, 20A per leg (measured)
    240 x 20 x 3 = 14.4kW

    even under 240 x 20 x 0.9PF x sqrt3, = 7.7kW, more than the motor rating?
    Last edited by FionaZuppa; 08-10-18 at 10:43 AM.

  2. #12
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    Quote Originally Posted by FionaZuppa View Post
    I thought I did.

    240v(rms 60Hz) 3ph, 20A per leg (measured)
    240 x 20 x 3 = 14.4kW
    7.5HP x 746= 5.5 kW

    Motor chart FLC for a 7.5 240V 3P motor per NEC 430.250=22A

    Seems normal.
    "Electricity is really just organized lightning." George Carlin


    Derek

  3. #13
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    Quote Originally Posted by jumper View Post
    7.5HP x 746= 5.5 kW

    Motor chart FLC for a 7.5 240V 3P motor per NEC 430.250=22A

    Seems normal.
    yes, but the RMS measurements were 20A per leg. 240v per leg, so its a 4 pole delta?

  4. #14
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    Quote Originally Posted by FionaZuppa View Post
    yes, but the RMS measurements were 20A per leg. 240v per leg, so its a 4 pole delta?
    You are going way too given the OPs info.

    We know only what he posted, we cannot hypothesize beyond that with any accuracy. No?
    "Electricity is really just organized lightning." George Carlin


    Derek

  5. #15
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    I thought I did.

    240v(rms 60Hz) 3ph, 20A per leg (measured)
    240 x 20 x 3 = 14.4kW
    No, use square root of three.

    Then there is efficiency, so input KW is greater than output KW always! Or you invented a perpetual motion machine....

  6. #16
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    Quote Originally Posted by FionaZuppa View Post
    I thought I did.

    240v(rms 60Hz) 3ph, 20A per leg (measured)
    240 x 20 x 3 = 14.4kW

    even under 240 x 20 x 0.9PF x sqrt3, = 7.7kW, more than the motor rating?
    We are assuming that this is a 240V delta, so the voltage to neutral is 139V. And 139V x 20A x 3 = 240 x 20A x sqrt(3).

    That is another simple way of deriving the sqrt(3) term in the formula.

    You cannot mix line current and phase voltage in calculating watts (or VA).

  7. #17
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    Quote Originally Posted by GoldDigger View Post
    We are assuming that this is a 240V delta, so the voltage to neutral is 139V. And 139V x 20A x 3 = 240 x 20A x sqrt(3).

    That is another simple way of deriving the sqrt(3) term in the formula.

    You cannot mix line current and phase voltage in calculating watts (or VA).
    ??
    240 3ph wye (3P4W), where does 139v come from? Ph-N is 240v
    sorry, i meant Y. but even with "3P4W 240v Open Delta" or the "3P3W 240 Delta", the ph-ph is 240v, and wild leg to N (B-N) is 208 while AC to N (A-N and C-N) is 120v.
    Last edited by FionaZuppa; 08-10-18 at 05:50 PM.

  8. #18
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    but even in 240 delta, 20A per leg is still ~2.5kW over rated HP, if PF=0.9 under load assumably.

  9. #19
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    Quote Originally Posted by FionaZuppa View Post
    ??
    240 3ph wye (3P4W), where does 139v come from? Ph-N is 240v
    sorry, i meant Y. but even with "3P4W 240v Open Delta" or the "3P3W 240 Delta", the ph-ph is 240v, and wild leg to N (B-N) is 208 while AC to N (A-N and C-N) is 120v.
    IF the supply is 240 wye you need to specify that clearly, as it is relatively uncommon. For 240 wye you would NOT generally use a motor nominally rated at 240V since the motor rating almost always specifies the line to line (across winding) voltage. But, yes there can be motors wound for 240 wye.

    You are correct that when 240 is the wye voltage the power calculation uses a full factor of 3.

    If you are using the line to line voltage, you need to match it with the line to line current, which is NOT equal to the individual line current. That is where the factor of sqrt(3) comes in then.

  10. #20
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    Fair enough. Can we see a pic of the motor spec plate.

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