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Thread: Voltage and Amps having negative correltion

  1. #1 Voltage and Amps having negative correltion 
    New Member oversteve's Avatar
    Join Date
    Jul 2007
    If I have say two different .5 horse power electric motors.

    One motor is 110 volts the other 208.

    The 208 motor is rated to draw far less amps that the 110.

    This seems to be the trend with like motors so here is the question.

    If the 208 motor draws less amps and produces the same power then is it not cheaper to run a higher voltage motor or is there some hidden factor I am missing?

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  3. #2  
    Join Date
    Apr 2007
    It is cheaper only because the line losses are less. Neglecting the line loss, the power used is the same and would draw the same current at the primary of the transformer.

    If your 208 volt motor is 3-phase and the 120 volt motor is single phase that could also account for some of the difference.

    To further explain. Let's say you are powering a 120 watt load at 120 volts, 1 amp. You want to change it to 240 volts, 1/2 amp to save current while still supplying 120 watts. You install a step-up transformer with a 2:1 turns ratio. The ideal transformer equation is Vp/Vs=Is/Ip where p is primary and s is secondary. Vp=120, Vs=240, Is=0.5, then Ip=1. So you are still supplying 1 amp at 120 volts at the primary of your transformer.

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