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Can power factor be greater than 1?

  1. Mar 31, 2009 #1
    Can the power factor of an electrical system ever be greater than 1? if yes.. then when and how?
     
  2. jcsd
  3. Mar 31, 2009 #2
    No. Cosx<=1. Mathematically impossible.
     
  4. May 10, 2010 #3
    Ok, well, This question is on the TCEQ A-Operator exam:

    5. You have a newly installed deep well vertical turbine pump with centrifugal motor driver with a guaranteed overall efficiency of at least 74%. The pump is pumping 500 gpm from a well with a 60-foot static level and a specific capacity of 12.5 into a 180-foot elevated tank. The pump driver is a three-phase motor with a power factor of 1.05. It is drawing 43 amps on a 480 volt service. A gauge 5 feet above ground level at the tank site is reading 69 psi. The total head loss is 8%. What is the actual overall efficiency?
    A. 95.2%
    B. 84.7%
    C. 74.1%
    D. 71.7%
    E. 60.3%

    It states that the power factor is 1.05! Can you explain that?
     
  5. May 11, 2010 #4

    mheslep

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    Gold Member

    Good question. Then that's either a typo, or the test question is abusing the electric term 'power factor' in some way. A ~30HP motor like this one might (?) have a power factor of 0.95. As indicated above, PF can not exceed 1.0.

    The given information is typical for motor ratings (except for the 1.05?), so the question is phrased to allow you to calculate the actual power used by the motor (PF x volts x amps). The rate that work is done on the water delivered to the elevated tank divided by that motor power gives you the overall system efficiency.
     
  6. May 12, 2010 #5
    Yes it is always greater than 1, indeed he says about power factor margin which means the relation of rated power of electrical driver and its related mechanical load.

    PF = Rated power of electrical motor / Required maximum mechanical power

    Of course when considering centrifugal machines it is important to base the motor rating on the ‘end of curve’ condition of the driven machine, because in practice the machine may need to run at this extreme condition for a reasonably long period of time. This condition is generally defined as 125% of the capacity of the machine at the maximum working efficiency point on the ‘head-flow’ curve for the designed shaft speed.


    --------------------------------
    Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electrical-riddles.com
     
    Last edited: May 12, 2010
  7. May 12, 2010 #6

    mheslep

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    Gold Member

    Ug, looks like some cross discipline language mangling in play here. http://en.wikipedia.org/wiki/Power_factor" [Broken] is always one or less.
     
    Last edited by a moderator: May 4, 2017
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