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Max speed of an ev at give HP

  1. Aug 20, 2007 #1
    Problem is to solve for max speed of an electric vehicle, given is the HP of motor (3 total), and the wind+rolling resistance of the vehicle at 60mph. Wind resistance should be regarded as a constant for this problem. Also given is EV weight with cargo and rider

    Motor HP = 5, 10 , 20
    Resistance is constant @ 165lbs.
    EV weight = 2250lbs.

    Solve for the given 3 hp varables.
  2. jcsd
  3. Aug 20, 2007 #2


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    Homework Helper

    You really have to at least try and solve the question - see the sticky at the top of the thread,

    To get you started.
    Energy is force * distance
    Power is energy / time
    Speed is distance / time
    At constant speed you only have to overcome resistance
  4. Aug 20, 2007 #3
    I can solve for horsepower needed at a given speed. For example: with the above constants it is 22.3hp to maintain 60mph. However I can not take a power amount and result with the max speed. I could use trial and error perhaps, until I get results with the required hp ratings but this is a very poor way to answer the question.
  5. Aug 20, 2007 #4
    PS this is not homework I am building this EV, I need to know what size engine.
  6. Aug 20, 2007 #5


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    Ok then, 5hp is 3728W 165lbs force = 734 Newtons.
    Power = force * distance in 1 sec
    speed = distance/s = 3728/734 = 5 m/s = 11.2 MPH
    Because you are going at a constant speed on the flat the mass is ignored.

    ps above about 10mph air resistance is proportional to velocity squared, so you constant resistance isn't a good model.
  7. Aug 21, 2007 #6
    I see, the problem I was having was I neglected to ignore the mass. Thank you.
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