# Amp Draw Estimation Question.



## Stanjo58 (Aug 28, 2010)

I have worked out the HP requirements for my vehicle at various speeds. Reading the continuous rate graphs for various AC motors you can know the HP rating at different RPM in 1000 RPM increments then calculate the speed of the vehicle at each RPM. In all cases the available continuous HP from the dyno charts exceeded the HP requirement at that given speed.

My question is .... if required power is X and available power is 2X then can the amp draw stated on the dyno chart simply be divided by 2 to provide a realistic estimate of the actual Amp draw at a given speed.

God, I hope that made sense. Thanks Stan


----------



## major (Apr 4, 2008)

Stanjo58 said:


> ..... Reading the continuous rate graphs for various AC motors you can know the HP rating at different RPM in 1000 RPM increments then calculate the speed of the vehicle at each RPM.....


Hi Stan,

Something doesn't sound right with that statement. Please give an example of the motor graphs to which you refer. While you're at it, please post the load curve for your vehicle and we'll see how it looks and use it for the calculations.

Thanks,

major


----------



## Stanjo58 (Aug 28, 2010)

Well, for example. The AC-35X2 at 4K RPM would produce a speed of 69 MPH. I can calculate the power requirement to maintain that speed on a level road at 23.6 HP. By interpolating the continuous graph I read 45 HP produced or about twice what is required. 

The continuous DC amp draw from the graph is about 260 amps would it not follow that the draw would be roughly half of what was needed to produce 45 HP as only 23.6 HP is required. Less work = less battery draw, just not sure if it's that simple.

Donor vehicle is Triumph Spitfire. 1800 lbs. 0.42 Drag Coefficient Frontal area 19.2 Sq. Ft. 
Direct drive through 3.89 Diff with 22.6" tires.


----------



## major (Apr 4, 2008)

Please post the graph you use or a direct link to it.


----------



## Stanjo58 (Aug 28, 2010)

The data graphs are from HPEVS web site. Wish I could get the data in spread sheet form but I can't find that on the site.

http://www.hpevs.com/power-graphs-ac-35x2.htm

Thanks Stan


----------



## major (Apr 4, 2008)

Stanjo58 said:


> The data graphs are from HPEVS web site. Wish I could get the data in spread sheet form but I can't find that on the site.
> 
> http://www.hpevs.com/power-graphs-ac-35x2.htm
> 
> Thanks Stan


We're getting closer. There are 4 continuous graphs linked from there. Which one are you using?


----------



## Stanjo58 (Aug 28, 2010)

I am simply interpolating HP and DC Current from the continous graph. Then comparing that (assumed produced on Dyno and would be max at given motor temp) to calculated HP requirement of vehicle at a given speed.


----------



## major (Apr 4, 2008)

Stanjo58 said:


> I am simply interpolating HP and DC Current from the continous graph. Then comparing that (assumed produced on Dyno and would be max at given motor temp) to calculated HP requirement of vehicle at a given speed.


I understand that. And there are 4 different continuous graphs. I need to know which graph you are using so I can answer your question. 

When you ask for help, it is courteous to supply the information in enough detail such that the responder is not belabored to search for that and make assumptions such as "which graph was used". 

All you need to do is copy and paste the URL (web address) of the graph into your post then there will be no ambiguity.


----------



## dougingraham (Jul 26, 2011)

The dyno graphs are showing a load curve for a particular motor on a particular controller being fed from a battery of a particular voltage.

You cannot look at 4000 RPM on the dyno plot and compute your speed in gear from that and assume that it will take that many horsepower to maintain the speed.

What you will see in your vehicle at a steady state of 4000 rpm on level ground will be a battery current and voltage that will equal 17605.6 watts (23.6 hp) plus a bit extra for loss of efficiency energy. So if your battery voltage was 96 volts the battery current would be around 183 amps (17606 watts/96 volts). If the battery volts is 144 then the battery current will be 122 amps (17606/144). If the peak power output of the motor/controller combination at 4000 RPM is 45 HP then you are not going to be in a steady state but accelerating.

The dyno plots will tell you max torque at a given RPM and from that you could back compute acceleration if you know enough about your vehicle. Things like mass and gear ratios, tire size, rolling resistance, cd, frontal area, and incline. The dyno plots only apply at maximum load to arrive at a given RPM. And they are calculated from accelerating a fixed mass or load and careful measurement.


----------



## Stanjo58 (Aug 28, 2010)

I sincerely apologize for being ambiguous. I truly appreciate advise and information from those that know a heck of a lot more about EVs than me. I was referring to the AC-35X2 fan cooled continuous rate which can be found here http://www.hpevs.com/Site/images/torque-curves/ac-35x2/144%20volt/imperial/continuous%20graphs/AC-35X2%20144v%20continuous%20FC%20Imperial.pdf. but I didn't make that clear.

I am apparently not explaining myself well at all. By knowing the RPM of the motor in this case available in 1K increments the gearing and tire Dia. it is possible to calculate a speed for that RPM. With speed known, if you know the weight, areo and frontal area you can calculate the HP required to maintain that speed on a level road with no wind.

In my case about 24 HP at 4K RPM at 69 MPH. The graph indicates that there are 43 to 45 HP available at 4K. Yes, more than enough to accelerate the vehicle, but the graph also indicates a 260 amp DC current draw. However, just to maintain that speed you don't 45 HP or 260 amps. You need 53% of the HP. Does that equate to needing only 53% of the amps or about 140 amps?

Edit...just saw last post you have answered my question and I thank you.


----------

