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Electric Motor and Gas Engine Horsepower Equivalency

  1. Jul 18, 2010 #1
    Is there a way to figure out the horsepower of an electric motor if you know the horsepower of a gas engine.

    For example:
    If the torque-to-horsepower equation goes as follows: [(Torque x Engine speed) / 5,252 = Horsepower] and I know that a 161cc gas engine has a Gross Torque of 7.00 ft-lbs, then how can I figure out the equivalent horsepower, while maintaining a constant amount of torque, of an electric motor?
  2. jcsd
  3. Jul 18, 2010 #2
    That was worded in a very confusing way, so i'm not quite sure what the question is.

    You calculate power output (at the shaft) in exactly the same way. torque * angular velocity. Electric motor power input is volts * amps.

    EDIT: I think I understand where you are getting confused.

    That horsepower equation is just the standard torque*angular velocity adapted to use non standrd units (hp and ft.lb). So if you are using the same units for the electric motor, you can use the same eq.
    Last edited: Jul 18, 2010
  4. Jul 18, 2010 #3
    I'm talking like in a mower, for example. it has say 5hp and __ torque. Is the exchange between a gas engine (like above) and an electric motor the same, or would i need a more powerful electric motor to get the same amount of torque?
  5. Jul 18, 2010 #4
    You'd just need the same power (so long as it's delivered at the same rpm), but this depends beause electric motors deliver their torque differently (constant torque).

    If you have the same rated power (so a 5hp electric motor) running at the same rpm (say 2000rpm) as a petrol motor then the torque output will be the same for both the electric motor and the petrol motor.

    EDIT: Sorry thats probably quite confusing to read through, it's pretty rambling.
  6. Jul 18, 2010 #5
    Ok, so if i were to rig a gas mower with an electric motor, and the blade needs X-amount of torque for optimum operation, then I would still use the same 5hp?

    Or since the torque is in constant application, would it be more suitable to use a slightly less powerful electric motor - i.e. less hp?
  7. Jul 18, 2010 #6
    I would tend to prefer swapping the petrol motor (5hp @ xxx rpm) for an identically specced electric motor (5hp @ xxxrpm).

    You could cartainly swap it out for a different lowered powered motor, there is nothing stopping it, the mower could certainly still work, but altering the speed of the blades or the motor output torque may cause a bad cut or some other bit of the mower to fail.
  8. Jul 18, 2010 #7
    Here's another question.... Does the torque change, even though the horsepower stays the same, since it's applied differently through the various sources (gas versus electric)?
  9. Jul 18, 2010 #8
    The relationship between power and torque is very simple:

    Power = Torque * Angular velocity.

    If something is prducing 'X' power at Y RPM then torque will be Z. For the same power output X at 0.5Y RPM then torque must be 2Z.

    So for a given power output, for the torque to differ, the RPM at which the power is delivered will be different. so same power but double the RPM = half the torque. or same power at half the rpm = double the torque.

    In this case:
    If the electric motor is still prducing 5hp, but is doing so with more torque, it will be rotating less quickly.
  10. Jul 18, 2010 #9


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    Electrical motors tend to have torque drop off almost linearly as rpms increase, while gas engines tend to have a torque peak somewhere between 1/2 to 3/4 of their peak rpm. Given this, then a lower maximum power electrical motor might make the same power as a gas engine at lower rpms, if the gas engines peak torque is near 3/4 peak rpm.
  11. Jul 19, 2010 #10
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