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Thread: Why does higher voltage results in higher current drawn?

  1. #11
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    Thank you everyone for your views. I was hoping to get a curve before I post again but looks like I will not have it so soon. I had also checked the medium used in factory and on site, I was told they are generally the same.

    More information - This is a positive displacement pump and i learnt that the same flow meter was used (despite some months later) for both factory test and on site. Even the manner of connecting the flow meter was the same, they removed the connecting hard pipes and attached the flow meter with a 1 meter hose that goes from supply to return outlet directly. The pressure was kept about the same (~1.5% different from factory) and I supposed the head remains the same? Are there any other mechanical parameters that I have missed out or I should measure?

    Both frequency are the same ~ 60Hz and they measured almost the same rpm. The power factor for the power supply are almost the same too (0.8 - 0.85)

    I think the only main difference is the power being supplied. In factory, they claim they use 460V while on site, the power supply is about 480V before 1-2% voltage drop. Hence, I was puzzled why a slightly higher voltage led to higher current measured.

    It is currently working in the service factor range and we obviously hope it will be within the full load current.

  2. #12
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    If the flow meter is accurate the only thing that can account for an increase in power is an increase in Q
    and for a given pump curve that means lower head
    this imho is not an elec issue but mech/hydraulic



  3. #13
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    Quote Originally Posted by Crowbar View Post
    This is a positive displacement pump ...
    With similar fluids and pressures, flow rate is directly proportional to speed. That the current only went up the same 10% says the pressure is "the same". Remember that pressure times flow times a system of units constant is power ...

    Quote Originally Posted by Crowbar View Post
    Both frequency are the same ~ 60Hz and they measured almost the same rpm.
    10% higher flow rate says the speed ___OF THE PUMP___ is 10% higher.

    Can you give us a pump model?

  4. #14
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    Quote Originally Posted by Ingenieur View Post
    If the flow meter is accurate the only thing that can account for an increase in power is an increase in Q
    and for a given pump curve that means lower head
    this imho is not an elec issue but mech/hydraulic
    Positive displacement pump ... no typical pump curves, Q=k*(rpm)

  5. #15
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    Yes, it being a PD pump makes this more complicated, assuming the given information is accurate. It's true, the only way the current can be that much higher is if the flow rate is that much higher, which in a PD pump must be related to the speed. +5% (or even less) voltage at the motor terminals is basically insignificant.

    I'm now thinking more along the lines of measurement error. Values of current and/or voltage and or frequency in one of the tests was assumed, not measured, or measured incorrectly. How / where are the measurements being taken?
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  6. #16
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    We know this
    v ~ the same
    speed constant
    i + 10%
    Q + 10%

    although the pump 'curve' is linear with a nearly vertical slope the operating point varies based on the system curve
    sith all the various measurements imosafe tosay the only thing that changed is the system curve



  7. #17
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    Based on the last from OP, the test voltage of 460 is assumed, not measured.
    I'm also going with some kind of measurement error.

  8. #18
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    PD pump throws out most of previous discussions prior to that significant bit of information.


    Unless same electrical meters used at both locations and piping nearly the same, agree that measurement error most likely culprit - unless some valve position was different in the 2 tests, eh?

    Hmm - direct drive or belt driven? Anyone replace an old worn slipping belt, etc.. etc...

  9. #19
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    I would think a lot details need to be nearly identical at both locations for a PD pump to have the same load on it at both locations then what has to be identiacl for centrifugal pumps to have same load at both locations.

    Centrifugal pumps are primarily loaded by flow, PD pump loading is effected more by changes in pressure then a centrifugal pump, so if a line is different size, length etc. it can have a bigger impact on loading of a PD then a centrifugal pump.

  10. #20
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    Quote Originally Posted by kwired View Post
    I would think a lot details need to be nearly identical at both locations for a PD pump to have the same load on it at both locations then what has to be identiacl for centrifugal pumps to have same load at both locations.

    Centrifugal pumps are primarily loaded by flow, PD pump loading is effected more by changes in pressure then a centrifugal pump, so if a line is different size, length etc. it can have a bigger impact on loading of a PD then a centrifugal pump.
    Correct about the effects of pressure on loading. But he also reported that the flow increased by 10% post installation. The only thing that can cause a PD pump to do that is to proportionately increase the speed.

    I think we are presuming a standard AC induction motor and constant speed transmission (direct coupled, belted, etc.).
    I still think something is wrong with the data collection.

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