# How much power do i need to run a ac & dc motor



## tom13o (Oct 18, 2009)

I know it depends on the size of the motors but I just just curious if anyone could give me an example of around how much power is needed to run a ac motor and a dc motor also. Thank you


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## Qer (May 7, 2008)

The amount of POWER (ie kW) you need depends on top speed, drag, weight etc. Your range will depend on the total amount of ENERGY (ie kWh). AC versus DC*won't affect that, but an AC system typically needs higher voltage where DC usually needs lower voltage but higher Ah. However power stays approximately the same.


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## WarpedOne (Jun 26, 2009)

> Your range will depend on the total amount of ENERGY (ie kWh). AC versus DC*won't affect that, but an AC system typically needs higher voltage where DC usually needs lower voltage but higher Ah. However power stays approximately the same.


This is true as "first approximation". When you start looking at efficiency/rpm and cable/battery/controller losses things start to change a bit. Higher voltage means lower amps. Halve the amps and you halved the heat losses twice. 

Everyone likes to talk about how that they can start the car in second or third gear and never change the gear. Doing so requires huge motor power at very different rpm. DC motors are very efficient under very narrow rpm band as AC motor are very efficient over their whole rpm band.

So, AC should give you a bit more range everything else being the same/simialr. If you also consider regen braking that is practically impossible to implement reliably with DC, AC is a winner. 

On the other hand cost of AC is still so prohibitively high that DC makes sense.


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## Qer (May 7, 2008)

WarpedOne said:


> This is true as "first approximation". When you start looking at efficiency/rpm and cable/battery/controller losses things start to change a bit. Higher voltage means lower amps. Halve the amps and you halved the heat losses twice.
> 
> Everyone likes to talk about how that they can start the car in second or third gear and never change the gear. Doing so requires huge motor power at very different rpm. DC motors are very efficient under very narrow rpm band as AC motor are very efficient over their whole rpm band.


That horse has been beaten to its death in other threads that it's mostly tragic and even though that your claim has some merit, for a commuting car there will be other factors that will matter more. I bet even the difference between a good and a bad charger will matter more for the daily driving since a badly charged pack (imbalances, not fully charged, damaged by over charging etc) won't perform very well, not to mention what a stuck brake or tires with high friction will do to your performance. Besides, increased losses in cables can easily be adjusted by more cable area.

If you're doing a light weight, extremely aerodynamic car those last few percent will probably matter a lot, for most people that convert old clunkers it's more theory than reality.


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## WarpedOne (Jun 26, 2009)

> for a commuting car there will be other factors that will matter more


Absolutely true.


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