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  1. #1
    Join Date
    Apr 2003
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    ottawa canada
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    Question


    Hi guys i figured someone here could answer these questions once and for all!

    Four questions:
    #1 on a loss of load on a three phase motor, does the CURRENT DRAW increase or decrease?
    #2 on a loss of load on a three phase motor, does the POWER FACTOR increase or decrease?
    #3 on a loss of load on a single phase motor, does the CURRENT DRAW increase or decrease?
    #4 on a loss of load on a single phase motor, does the POWER FACTOR increase or decrease?

    Simple question it seems but, I have asked many people and everyone has a different answer. please if you cant back up your answer with therory dont reply. thanks.

  2. #2
    Join Date
    Jul 2000
    Location
    Dallas,Texas
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    4,956
    Power factor in an AC circuit is a function of the effective voltage and effective current.
    In an AC motor circuit current lags voltage because it is an inductive circuit.

    For the purposes of your question, if we assume that the loss of load means; The motor is no longer connected to the electrical supply.....

    #1 Current draw ceases.
    #2 Power factor is no longer affected by this motor because there is no current draw. If the motor is large enough the power factor to the service could conceivably increase with the absence of this load.

    #3 Same as #1

    #4 Same as #2

  3. #3
    Join Date
    May 2000
    Location
    Urbandale IA. USA
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    4,934
    Originally posted by rustico79

    Hi guys i figured someone here could answer these questions once and for all!

    Four questions:
    #1 on a loss of load on a three phase motor, does the CURRENT DRAW increase or decrease?
    #2 on a loss of load on a three phase motor, does the POWER FACTOR increase or decrease?
    #3 on a loss of load on a single phase motor, does the CURRENT DRAW increase or decrease?
    #4 on a loss of load on a single phase motor, does the POWER FACTOR increase or decrease?

    Simple question it seems but, I have asked many people and everyone has a different answer. please if you cant back up your answer with therory dont reply. thanks.
    If the loss of load were a broken belt on a blower...
    #1 I know the CURRENT DRAW would decrease
    #2 I believe the POWER FACTOR would not change as nothing electrical has changed
    #3 I would consider this to be the same as #1
    #4 I think this would be the same as #2

    When voltage remains the same, but amperage changes due to load, has the power factor changed? I don't believe it has, but I have been wrong in a previous life also...

    Those who dance, appear insane to those who do not hear the music.
    Those who believe, appear ignorant to those who do not know God.

  4. #4
    Join Date
    May 2003
    Posts
    77
    1: decrease
    2: decrease
    3: decrease
    4: decrease
    Provided loss of load means a decrease in load, not a total loss of load.

  5. #5
    Join Date
    Apr 2003
    Location
    ottawa canada
    Posts
    10
    thanks guys. confirms what i thaught.

  6. #6
    Join Date
    Aug 2000
    Location
    Austell, Ga.
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    1,296
    Previous answers are correct if motor is not connected to a modern VSD...
    While the Currents will of course decrease, the Power factor will never go below .96lag
    An induction motor running "Bare-Shaft"(No Load) will have a Power factor of .3 to .4Lag...
    However...since type of motor was not specified, I must add that a Syncronous Motor would also decrease in current....but the Power facor would rise...Very possibly beyond "Unity Power Factor"..which is 1.. to an unacceptable Leading Power Factor of up to .3Lead..
    Ain't "None" of us as smart as "All" of us..

  7. #7
    Join Date
    Jul 2000
    Location
    Dallas,Texas
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    I wondered if you would wade into this Richard.

    Do you agree with all the answers from Tarz?
    Specifically, the statements about power factor decreasing.

  8. #8
    Join Date
    Aug 2000
    Location
    Austell, Ga.
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    1,296
    [QUOTE]Originally posted by wolfdog
    [B]I wondered if you would wade into this Richard.

    Do you agree with all the answers from Tarz?
    Specifically, the statements about power factor decreasiNG.

    My assumptions were based on the motor losing it's load, Belts coming off, Compressor unloading etc.
    while still connected to it's power source..

    The same applies to a transformer that is operating at less than its rated KVA rating. The power factor runs about .3 to .4Lag on a transformer with its secondary(Load) disconnected.
    Since their is no such thing as "A" three phase transformer, all four questions are answered by referencing a transformewr in place of a motor...After all....An "Induction" motor is nothing more....nothing less than a transformer with a rotating secondary winding.......
    Ain't "None" of us as smart as "All" of us..

  9. #9
    Join Date
    Jul 2000
    Location
    Dallas,Texas
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    I am not trying to start a fight.
    Either I didn't ask a good question or I don't understand your answer.

    Will the power factor change if the load decreases?

    Or is the power factor a function of the motor windings and stays constant for that particular motor regardless of loading?

  10. #10
    Join Date
    May 2003
    Posts
    77
    If you oversize your motors in a plant,etc., your power factor will decrease. Very important to size motors correctly in areas where utility will penalyze companies for a pf lower than .95 (depending on area of course). Obviously a vfd changes everthing, including power factor. I was speaking of motors not connected to variable frequency drives.

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