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  1. #1

    Default AC motors

    I know we've touched on this before but I can't find the thread so I'll apologize in advance for the redundancy of this post. I'm trying to determine the cost of running an AC motor. The motor is rated 3.5 amps @ 115V. From the formula P=VI, I get 402.5 Watts or 0.402 kw. Does this value represent the power consumption of this motor per hour?? Is it an industry standard to rate motors on a per hour power consumption basis??

    Thanks in advance for all the help.


    Russ



  2. #2
    Veteran Member Carl_NH's Avatar
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    Default Re: AC motors

    Russ,

    1KWH = 1000 watts/hr so you will use 1 KWH for every 2.5 hours of operation at .400KW. If starting and stoppng or the voltage drops this will go up a since initial starting current draw could be 2x or more the running load amperage.

    Carl

  3. #3
    Veteran Member chim's Avatar
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    Default Re: AC motors

    Rus, it will get you in the ballpark. To accurately figure the actual power, you need to know how much work the motor is really doing. The nameplate amperage is the maximum load that should be imposed on it. For example, a bench grinder could have a nameplate amperage of 6A, draw 3A just to spin the wheel, and 12A if you get too aggressive and try to burn it up by pushing the bush hog blade into it. 6A is the most you SHOULD make it draw. If the grinder is sitting there just spinning the wheel, it will be cheaper than when you are grinding away.

    With inductive (motor and other coil loads) or capacitive circuits, there's another component - power factor. Not all the current is in phase with the voltage. The real formula for power in a single phase AC circuit with inductive or capacitive loads is Volts X Amps X Power Factor.

    The power company bills you for KWH (kilowatt hours) not KVA (kilovolt amperes). Generator manufacturers monkey with the nameplate ratings of their units, partly to cover this difference and usually rate the gensets in KW.

    Transformer manufacturers on the other hand usually rate their equipment in KVA. If you have low power factor loads, you will hit your current limit before you get to the maximum power capability of the circuit.

    Awww, too much coffee today, sorry I got carried away and am only scratching the surface anyway. Short answer is it will get you in the ballpark......................chim

  4. #4

    Default Re: AC motors

    Carl and Chim, thanks for the info. It helps me put some numbers on the board to give me an idea of electricity usage.

    Russ

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