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

Very simple name-plate question

  1. Nov 13, 2015 #1
    If a motor is such-and-such Hp or kW, is that the Output power, or the Input power?
  2. jcsd
  3. Nov 13, 2015 #2


    User Avatar
    Science Advisor
    Gold Member

    from memory its the output power as measured at the driveshaft

  4. Nov 13, 2015 #3
    Huh, ok thats a surprise.

    It's funny that I'm doing a practice question where the input power as calculated from the output power + loss, is different from the rated input current multiplied by rated inut voltage.
    Just thought I'd throw that in as an oddity, I don't understand how.

    anyway, cheers.
  5. Nov 13, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    why do you think it's odd?

    if they wanted to that they could take the BTU rating of the fuel, work out the power generated by the engine, subtract the losses and get an output power
    in the specs listing, its a lot easier just to state it's output

    Ohh and I forgot to add in the first post. the power out will usually be stated for a given rpm
    Last edited: Nov 13, 2015
  6. Nov 13, 2015 #5


    User Avatar
    Science Advisor
    Gold Member

    some reading for you .....


    a way down that page .....

    you can see my driveshaft rating in there amongst other styles .... Brake HP is the other one I'm commonly familiar with

    1HP = 745.7 Watts

  7. Nov 13, 2015 #6
    That is a good point.
    What I think is odd is that I'd have thought that if you multiplied the terminal voltage by the current going into the motor, that, that'd be the total input power. However in my qustion it turned out that the output power + loss' were greater than that, hence the input was greater than VI at the terminal.

  8. Nov 13, 2015 #7


    User Avatar
    Gold Member

    Looking at the following arbitrary motor specs


    the 1 hp motor draws 13.6A at 115V at Full Load, which is 1564 watts. It is then listed on the second page at 68% F.L efficiency and 0.66 F.L. power factor.

    Doesn't the nameplate have the current?
  9. Nov 14, 2015 #8


    User Avatar
    Science Advisor
    Homework Helper

    Can you post the question word for word. In other sections of the forum (eg the homework section) this is a rule because it frequently turns out that the OP has misunderstood or misinterpreted the question.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Very simple name-plate question