# 20kw?



## QuodMagusZerum (Dec 23, 2018)

I'm thinking of building a 2,000 lbs mini car with a motor for each wheel. In reading various threads around here, I'm getting the impressing I could use four 5kw motors with appropriate gearing to power this thing. Am I on the right track?


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## brian_ (Feb 7, 2017)

QuodMagusZerum said:


> I'm thinking of building a 2,000 lbs mini car with a motor for each wheel. In reading various threads around here, I'm getting the impressing I could use four 5kw motors with appropriate gearing to power this thing. Am I on the right track?


I'm thinking that's not enough for satisfactory performance. See your other thread for a similar discussion from a torque and force viewpoint; however, you can look at this in terms of power as well.

For comparison, a typical compact EV weighs twice as much as your planned car, but has four times the total power available.

There are various calculation tools available to predict performance. From the most basic viewpoint, the greatest possible acceleration would occur in the (highly unrealistic) situation that you have no rolling drag and no air drag; in that case...

to accelerate to 60 mph would require 327 kJ (energy = 1/2*mass*velocity squared),
and 20 kW for about 16 seconds would deliver 327 kJ,
so it would take 16 seconds to get from zero to 60.
In fact it would take much longer, since much more energy would be required to overcome drag. Is that quick enough?

Of course, if you mean that you would use motors rated for 5 kW each, but drive them harder and get more power out of them (keeping bursts of higher power short enough to avoid overheating them), you would get better performance.


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## QuodMagusZerum (Dec 23, 2018)

Excellent! That is the math I was looking for. It would seem I need 50-100 kw per ton of automobile. That's a ratio I can plug into my spreadsheet to make the rest of my calculations.


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## Emyr (Oct 27, 2016)

brian_ said:


> I'm thinking that's not enough for satisfactory performance. See
> 
> to accelerate to 60 mph would require 327 kJ (energy = 1/2*mass*velocity squared),
> and 20 kW for about 16 seconds would deliver 327 kJ,
> so it would take 16 seconds to get from zero to 60.




No, E=0.5MV^2 gives you the kinetic energy, not the energy required to accelerate. The sum energy to accelerate involves integrating the equations for aerodynamic drag, mechanical drag, fed into Force = Mass x Acceleration, where Force is net of the drag components.


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## brian_ (Feb 7, 2017)

Emyr said:


> No, E=0.5MV^2 gives you the kinetic energy, not the energy required to accelerate. The sum energy to accelerate involves integrating the equations for aerodynamic drag, mechanical drag, fed into Force = Mass x Acceleration, where Force is net of the drag components.


The time to deliver the required kinetic energy provides a first approximation of the minimum value of the acceleration time. I don't think that QuodMagusZerum was looking for a more sophisticated model, and this first approximation served the intended purpose.

Of course, drag must be considered for a more accurate estimate, which is why I included this:


brian_ said:


> In fact it would take much longer, since much more energy would be required to overcome drag.


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## QuodMagusZerum (Dec 23, 2018)

It should be accurate enough. 100KW per ton should get me in a good zone.


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## galderdi (Nov 17, 2015)

Why did you choose 2000lbs? That doesn't seem like a mini car to me. Is there any reason you can't target a weight more like 1200lbs. This would significantly reduce the batteries, motor/s and controllers you would require.
My first EV was 1160lbs and was a 2 seater with great performance from a 1000amps and 172v. Where do you see the extra 800lbs?


Also I would steer clear of motors on each wheel because unsprung weight has a big impact on handling. Unless you are driving each wheel from an inboard motor via a drive shaft?


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