Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Negative Power Factor on a motor?

  1. Nov 22, 2006 #1
    Negative Power Factor...on a motor?!?!

    Howdy Folks,

    I don't post too much here, but I have run into a bit of a stumper.

    I am in the process of interpretting some data for an energy audit of a commercial building, it is a 15 story office building, heated by heat pumps.

    We hooked up our Fluke Power Quality Analyzer, which records the whole shebang, P, S, Q, PF, DPF, Vrms and I to each of the 11 pumps which supply hot water to the baseboards throughout the building and also for their water supply.

    The motors are 3 phase delta connected so the meter was setup accordingly.

    Upon reviewing the data record weve found that the power factor was negative for 9/11 of the pumps...the other two were fine. This negative power factor means that the motors are a capacitive load and are actually supplying power back to the service. This makes no sense to me whatsoever.

    So my question is this, what else could the observed negative power factor and in turn negative real power, physically mean when measuring the 3 phase power across the lines of a delta connedcted motor.

    Thank you for your time and thoughts in advance.
    -Dave

    PS- All I can think of is that perhaps in the meter there is a setting to switch the meter from capacitive load to inductive load...I will investigate this further, but I am skeptical that this will solve the problem.
     
  2. jcsd
  3. Dec 16, 2007 #2
    I think if your measurement of current in reverse polarity there is a chance for negetive pf instead of positive pf.

     
  4. Dec 17, 2007 #3
    ya thats what i was thinking immediately, maybe you have your polarities switched from what the measurement device reads?
     
  5. Dec 17, 2007 #4

    RonL

    User Avatar
    Gold Member

    You did not mention if all the motors are rated the same, or if they are totally isolated from each other, but it sounds a little like a design of a rotary phase converter where a larger three phase motor supplies power to several smaller units ??

    I know nothing about commercial systems, and only have looked into the basics of single phase conversion to three phase power for my shop.
     
  6. Dec 17, 2007 #5

    stewartcs

    User Avatar
    Science Advisor

    Essentially, the power factor is a comparison of the power used by the load (real power) divided by the power supplied to the load (apparent power). The difference between the real power and the apparent power is the reactive power. That is to say that it is the ratio of the useful work performed by an electrical circuit compared to the maximum useful work that could have been performed at the supplied voltage and amperage.

    The negative sign just means that the current is leading the voltage. The net effect is a predominately capacitive load, i.e. the motors aren't absorbing all of the reactive power that is being supplied by the source.

    Do you have some type of power factor correction device on the line? Typically motors produce magnetic fields, which are absorbers of reactive power, so you would typically have the opposite of what you are describing, unless you have some type of power factor correction (devices that produce electric fields which generate reactive power).

    The whole point is to get the power factor as close to unity as possible by varying the reactive power.
     
  7. Dec 17, 2007 #6

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Yep. I've never seen an inductive motor with a negative pf.

    But maybe a little capacitive reactance will help the overall PF of your building.:biggrin:
     
  8. Dec 21, 2007 #7
    First of all I doubt if it's a measurement issue as if it was then the power analyzer would show either current or voltage as a negative quantity.

    The only place where you can get a leading pf is as you said a capacitive load which can be done using a synchronous motor in the overexcited state? So, Are you using a synchronous motor?
     
  9. Dec 21, 2007 #8
    Negative voltage and current readings in an AC system?
     
  10. Dec 22, 2007 #9

    LURCH

    User Avatar
    Science Advisor

    Is it possible that water being pushed by some of the motors in the system is mechanically forcing other motors to turn so that they input power back into the circuit (essentially turning the motors into generators)? I suppose if that were happening, you'd see a big increase in energy consumption by the other motors at the same time you're getting the surpluss from those in question.
     
  11. Dec 22, 2007 #10
    If you hook up the ammmeter of a power analyzer in the reverse way you'll get a negative current measurement.
     
  12. Dec 22, 2007 #11

    russ_watters

    User Avatar

    Staff: Mentor

    No, you don't. A/C is A/C. Power flows in both directions.

    That said, there is no guarantee that the phases are correctly labeled or hooked up in order on the panel. I use a power meter like this (an Amprobe power meter and, in fact, I used it today) and often get negative reactive power readings on it. I just fix it in Excel.
     
  13. Dec 27, 2007 #12
    Sorry guys, I was having a dumb day. You are right, AC is AC, but what I meant was that if you hook up the voltmeter and ammeter on the power analyser (any one of them) with reverse polarity, you'll end up with -vs pf (NOT -ve V and I readings.)

    Sorry.

    P.S. I think 'speak2ranjith' mentioned this before.
     
  14. Dec 27, 2007 #13
    I can give a better explaniation if you say leading or lagging. Lagging is inductive. Either side of the zero power factor is bad but it is best to keep it one RCH (the finest known measurement in electronics) on the lagging side for strictly monitary purposes.
     
    Last edited: Dec 27, 2007
  15. Jul 21, 2010 #14
    Re: Negative Power Factor...on a motor?!?!

    Flip your current clamp and you get negative readings including PF.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?