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Calculating Power for a car, and relating it to the motor curve

  1. Apr 30, 2018 #1
    Hello all,

    I am trying to build a simulation to understand what power is required by an electric car in order to accelerate from 0-50kmh. As per my understanding the total power required by engine is calculated using Total Power required at given velocity formula as stated below. I am calculating the Total Force from motor and multiplying with the velocity in range from 1m/s to 50m/s in order to plot a Power vs Velocity and Power vs RPM curve. Now after plotting this Power vs RPM I want to relate to Power vs RPM curve in my electric motor datasheet. My problem is relating the two curves. For instance the read value from my graph is 30KW at an RPM of 600 and is then rising linearly. What I find difficult is that if this 30KW is needed to accelerate the car to RPM of 600/50kmh needed for every RPM starting from 1-600 or is it the power required at the point when 600 RPM/50kmh is reached. And how do I counter check with my motor Power vs RPM graph if 30 KW is available for what RPM range?

    Total Force from motor = Drag + Rolling Resistance + Grad + Accelerating (i.e. mxa)
    Total Power required at given velocity V = Total Force from Motor x V
  2. jcsd
  3. May 2, 2018 #2

    jack action

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    Gold Member

    Any bit of power that is left after the resistances have been dealt with, will accelerate your vehicle; maybe very slowly, but it will accelerate your vehicle.

    What relates your vehicle velocity to your motor rpm is the wheel radius and any gearing you could have between the two. The power needed at a given velocity must be produced by motor at the related RPM.

    Usually, you would go the opposite way that you are going right now. Since you know your motor specs, you should find the acceleration you can get over the speed range you want to achieve. So you should prefer using those equations instead:

    Total Force from Motor = Total Power required at given velocity V / V
    Accelerating (i.e. mxa) = Total Force from motor - (Drag + Rolling Resistance + Grad)

    This will tell you what level of acceleration you can have at any given velocity (i.e. Acceleration vs. Velocity graph).

    For more info, check this simulator and the theory behind it.
  4. May 18, 2018 #3
    You have to start with a force vs. velocity curve. The power curve would be the values of the force curve X velocity. To get the available force you multiply total gear reduction (if there are any gears, including the differential) and divide by the rolling radius of the drive(n) wheel(s).
    Factor in drag, rolling resistance and gradient as mentioned above, subtract from the available driving force, and you are there.
  5. Jun 1, 2018 #4
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