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Discussion Starter · #1 · (Edited)
I'm trying to find out what performance I can realistically expect for my EV project and am trying to fine tune my spreadsheet. I need to approximate a DC motor torque curve and have attached some figures to see if I have understood what I have read.

Chart 1 is just the torque curve from Netgain for a 9" motor formatted how I'm used to looking at torque curves. It seems that torque is very proportional to motor amps, and I just assumed that the 334 amps at maximum torque is the controller rating and the Red torque curve is just flat from 0 rpm to 2158 rpm.

Chart 2 is just the torque curves.
Red is at the rated 72v and 334amps.
Yellow is at 115v and 334 amps and all points are shifted right by the proportion of 115/72.
Gray is at 115v but with a controller that has the motor amps and battery amps limited to 500amps, so all points from the yellow line are shifted up by the proportion of 500/334.
Blue is at 115v but with a controller that has the motor amps limited to 1000 amps, and the battery amps limited to 500amps. I just figured if the maximum power out of the batteries was 115v*500amps at the knee of the curve, then the curve would start declining at 500/1000 * rpm of max power. That's assuming electrical power would be similar to mechanical power, but I haven't a clue to the efficiencies in this area, so I don't know how close I am.

Does this look right, or is there any data that would shed some clarity on this? And is this how a controller works if the controller is rated at 1000amps with the batteries limited to 500amps? And which controllers can do this?

I'll post my spreadsheet, once I've tuned it up a bit. You should be able to plug in various bit about your car and find your ET's from 0 to 85mph. You can also see where to shift for maximum performance. I plugged in tomofreno's SwiftE using the AC50 motor and he said the numbers look close.

Edit:
I'll post updated spreadsheets (Excel format, zipped) here so you don't have to search the thread for the latest. Also, the charts below have been corrected in the next few posts.
 

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And is this how a controller works if the controller is rated at 1000amps with the batteries limited to 500amps? And which controllers can do this?
Hi max,

When you have a battery current limit, that region is constant power. So I think that portion of the curve is curved, not linear. Hyperbola??

I believe Qer has done software for this and maybe the Zilla has it.

I don't know why you guys take a perfectly nice motor speed torque curve and turn it sideways :)

Regards,

major
 

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Discussion Starter · #3 ·
Thanks for responding major.
When you have a battery current limit, that region is constant power. So I think that portion of the curve is curved, not linear. Hyperbola??
MAN your good! Yes, hyperbolic, that make more sense, torque inversely proportional to rpm for constant power, don't know what I was thinking. The AC50 motor efficiency behind max power falls off as it approaches 0, would any curve fit that? I guess those 2 curves would need to be multiplied for mechanical power out.

And do the other curves look right, being proportional to volts and motor amps?

And I know the torque falls off after peak power due to back emf. I was going to use a couple of linear lines to approximate those, or is there a curve that would make sense?

I don't know why you guys take a perfectly nice motor speed torque curve and turn it sideways :)
I was thinking the same of you DC motor guys :). All in what you're used to I guess. It would make it easier for us mechanical guys if everything were based on rpm, not just the torque. Efficiency based on torque? I still don't get that one :).
 

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Yes, hyperbolic, that make more sense, torque inversely proportional to rpm for constant power,
Hi Max,

The battery current limit makes the motor input a constant power. You're plotting motor output, so it ain't exactly right. But pretty close because motor efficiency is fairly constant over that range.

The AC50 motor efficiency behind max power falls off as it approaches 0, would any curve fit that?
I'm sure some curve (mathematical expression) would fit it, but no simple curve of which I am aware.

And do the other curves look right,
Look pretty good to me, except sideways :)

being proportional to volts and motor amps?
Over the midrange, it is a close approximation to consider RPM proportional to motor voltage. And current and torque are a unique relationship, but not completely linear. For series motors, prior to saturation, torque is proportional to the square of current, after saturation, approximately proportional.

And I know the torque falls off after peak power due to back emf. I was going to use a couple of linear lines to approximate those, or is there a curve that would make sense?
You lost me here. You plotted a series motor on a PWM having a current limit. Unless you have a real high current limit, peak power will be at the power where the PWM goes full on. After that power is linear, for approximation. Prior to that, the power curve is the motor power curve from the motor speed torque curve which kinda resembles a parabola when plotted against torque. Turned sideways, don't know what it looks like.

Efficiency based on torque? I still don't get that one :)
Torque is the load. It makes perfect sense to use it as the independent variable (x axis). The motor speed, current, power output and efficiency all depend on the motor load, when the motor is supplied a particular voltage.

What do you plot efficiency against? Time of day, phase of the moon, or speed or something :)

I end up explaining motor curves to quite a few of you guys. Like this: http://www.diyelectriccar.com/forums/showthread.php/dc-motor-graph-39649.html You want to take that information and transpose it over to some other format, fine, but I think you lose some of the basic understanding of the characteristic behavior of the motor.

Regards,

major
 

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Discussion Starter · #5 ·
The battery current limit makes the motor input a constant power. You're plotting motor output, so it ain't exactly right. But pretty close because motor efficiency is fairly constant over that range.
That's cool, if the efficiency is relatively contstant at constant power, close enough.
You lost me here. You plotted a series motor on a PWM having a current limit. Unless you have a real high current limit, peak power will be at the power where the PWM goes full on. After that power is linear, for approximation. Prior to that, the power curve is the motor power curve from the motor speed torque curve which kinda resembles a parabola when plotted against torque. Turned sideways, don't know what it looks like.
I think I got it, found a curve that looks pretty good.
What do you plot efficiency against? Time of day, phase of the moon, or speed or something :)
Speed, on Thursdays at noon :).
I end up explaining motor curves to quite a few of you guys. Like this: http://www.diyelectriccar.com/forums...aph-39649.html You want to take that information and transpose it over to some other format, fine, but I think you lose some of the basic understanding of the characteristic behavior of the motor.
I appreciate you taking the time to explain it again. I see it might make the motor easier to understand (if that were my goal at this point). I just want as few equations as possible to describe a motor curve, and I think I have it now. Look for a spreadsheet shortly.

I'll use the updated attached chart.
 

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Discussion Starter · #6 ·
Didn't read your linked post, major, before my reply.
I think the thing which confuses people about these motor curves is they compare it to stepping on the throttle in the car. The motor speeds up and produces more power as it speeds up. But remember, as you increase the motor speed in the car, you are increasing the motor voltage with the controller. These graphs show the motor output at a constant voltage. The motor factory makes the motor for the rated voltage and does not know what controller you will put on it. They give you the motor graph to tell you how the motor will perform at that rated voltage.

Hope that explained it for ya,

major
Gonna have to get my head wrapped around that one. So, is the curve I'm making going to represent what torque the motor is producing in the car?
 

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Discussion Starter · #8 ·
Spreadsheet (zipped, Excel format) is attached to Post #1. I'll keep editing Post #1 so you won't have to search the thread for the latest corrected spreadsheet.

Feedback is appreciated. I'll add other motors if desired.
 

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Thank you for putting this together. I find all the formulas a little confusing on how to translate to different amps and voltages.

I put in some data for my setup and it shows the torque falling WAY off after 1000 RPMs. IS that right? Interesting information if it is.

Thaniel

Think I found my error. I was putting in the wrong voltage in "cell Volts Each". Torque is now flat until 4500 RPM.
 

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Discussion Starter · #10 ·
Thanks for the feedback Thaniel. Let me know if there is something I can make more clear. I probably shouldn't have interchanged cell and battery, that might be confusing, maybe I should just call them batteries.

Also, if there is any real world data that might confirm if the spreadsheet is close, that would be appreciated.
 

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Discussion Starter · #14 ·
Just updated the spreadsheet (get from post #1).

Added the WarP 8, ImPulse 9, WarP 11, and the AC50/1238-75 to the WarP 9, just chose the number and all the relevant info will pop in.

Added 1/4 mile (user selectable distance in miles) speeds and times. Max speed is 100 mph, so may not be useful for the super serious drag racers.

Just changed wording a little so hopefully it will be more understandable.

I don't anticipate more changes unless anyone wants me to add another motor or finds errors.

Enjoy.
 

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Have you considered adding the HP line to the RPM torque curve like in Dyno plots for ICE?

 

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Discussion Starter · #18 ·
Have you considered adding the HP line to the RPM torque curve like in Dyno plots for ICE?
No, but it would be very easy to do since it's just a function of torque and rpm. How would you use it?

Have you compared the spreadsheet to your performance, is it close?
 

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Have you considered adding the HP line to the RPM torque curve like in Dyno plots for ICE?

Is this data just an example or are you really pulling enough amps to give you 200ftlbs of torque from a warp9 and 4500+ rpm??? Isn't that like 800-1000A?? If it's real data then thats great to hear.
 

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Hi Max
I have just had a quick look at your spreadsheet
Did you do it all in Imperial units?
If so then I am impressed -
The only way I can do anything serious with Imperial is by converting it to Metric and then converting the answers back

Slugs -- Ughhhh
 
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