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

Battery and Motor

  1. Feb 3, 2013 #1
    I saw 15 Amp motor which runs on 120 VDC with 5500 rpm [This motor was attached to Miter Saw tool]. I'm not sure what's HP of that motor, but I read 5.5 HP, however the size of motor is around 6 inch long and 4 inch diameter (Is it possible this small motor have 5.5 Hp? And can we calculate torque of the motor based on above known facts?).

    Now my other question is : Can I run this 120 VDC motor using this 20 Volt DC with 3 Ah rating battery? If so how long I can run this motor using it?

    Thank you.
  2. jcsd
  3. Feb 3, 2013 #2


    User Avatar

    Staff: Mentor

    Well, 15A at 120V and 95% efficiency would be 2.3 horsepower.

    I seem to remember that vacuum cleaner manufacturers play games with horsepower ratings, so it wouldn't surprise me if circular saw manufacturers did too.
  4. Feb 3, 2013 #3
    May be that was mistake of Shop guy who put 5.5 HP sticker on it.

    But other thing is can we run that motor using 20 V DC battery with 3 Ah rating? for 1 or 1 minutes?
  5. Feb 4, 2013 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    A 120V motor will not work on 20V. I doubt whether, on only 17% of its rated voltage, it will even rotate at all. Basically, with 17% Volts, you could expect to get just 2.8% of its power (i.e. 17% of 17%) - even if everything else was perfectly pro-rata - and there is a threshold below which nothing will happen. You might expect about 2.5A to pass.

    BTW, the current that the motor will pass is determined basically by the supply volts - too few supply volts will just not make enough current flow. A low voltage motor is designed with much fewer coils of fatter wire (as in a car starter motor).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook