# understanding torque curves.....



## PocoEV (Nov 4, 2014)

Converted an old lead sled..... it came with a 1990s vintage 9" GE 21Hp motor (5BT1346B50 Series) with a Auburn Scientific 700 amp Grizzly controller. Conversion brings in 43 CALB 100AH LiFePO4 watched over by Orion BMS. Vehicle weight is at 2500 pounds. 

Car runs great in the flats but limiting myself to 2C current draw.... I can only get to 25MPH when climbing 500 ft over a steep 1 mile (10% grade). Drawing 200 amps continuous for the 3 minute climb raises average cell temps about 2 degrees C. 

I'm a newbie to DC motor torque curves...see attached. Should I increase current to 3C or 4C for this limited 2-3 minute duration? I have not found any posts talking about current draw when climbing a hill? 

Or is this "old GE motor" tired and needs to be replaced? Would a HPEVS AC-51 provide me better performance in this hill climb challenge?

Thanks for helping in my education....


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## dcb (Dec 5, 2009)

not the expert but 200 amps looks like ~ 20 ft lbs on that graph, which isn't really much at all. What is wrong with 700 amps? You can back it off if the motor gets hot. I would keep an eye on battery amps too, those are only "rated" for 1000 amps for 10 seconds. I mean if all your connections are beefy then you have surplus power exactly for things like getting up to speed or climbing the occasional hill.

force required is: 10% grade=5.24 degrees. sin(5.24)=0.1, stationary force needed = 2500(you included?) * .1 = 250 pounds.

So assuming you went with the miata, ~4.0 rearend, 3.163 1st, lets say 12" radius tires(one foot), is about a 12.5:1 ratio, which is also the ratio between 20 ft lbs and 250 ft lbs.

I don't see evidence that the motor is "tired". batteries might be struggling, or something else. Does the pack voltage sag a lot?

edit, that ratio would put your rpm at 4378, which is a "little" lower than the rpm line at 20ish ft lbs. If the batteries are stiff then Id consider making the pack voltage higher so you can make torque at higher rpm. is 144v an option? get you up to 33 mph up that hill?


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## PocoEV (Nov 4, 2014)

You have piqued my interest with your comment "1000 amps for 10 seconds".... that's 10C. The CALB batteries are rated 2C continuous. Are there "specs" for duration at 3C, 4C, 5C? I want to protect that investment....

Also, I am running 43 cells... so, 138VDC instead of the 108VDC on the curve. Back to understanding torque curves, I assumed higher voltage would allow me to run lower current to get same RPM/torque??? 

Orion BMS software shows pack voltage solid during this 2C draw (these are brand new CALB cells). And yes 2700 pounds with a driver.


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## dcb (Dec 5, 2009)

PocoEV said:


> I am running 43 cells...


oops, missed the voltage bump




PocoEV said:


> Are there "specs" for duration at 3C, 4C, 5C?


I don't know exactly what calb cells you have, this has a couple 100ah listed, 8C and 10C for 10 seconds and they all list 3C continuous. It is probably like a time constant curve, halve that current and quadruple the time or something, till you essentially get to continuous at 2(3?)C

http://store.evtv.me/products.php?cat=10



PocoEV said:


> I assumed higher voltage would allow me to run lower current to get same RPM/torque???


Also note there is a difference between pack current/voltage and motor current/voltage. So with the higher voltage pack, you can convert more power with less battery amps, but motor amps are always higher than battery amps, until the rpm and back-emf pick up. The torque/amp relationship in the graph is based solely on motor amps.

What a higher pack voltage should do is allow you to pump more motor amps in for a given rpm (limited by the controller) and reduce the effects of back-emf limiting rpm (and reduce pack current at lower rpm)



PocoEV said:


> And yes 2700 pounds with a driver.


Ah, multiply my force guesses by 2700/2500 then 

Bump it to 300 amps, watch the temp, see how it does? Try a couple short bursts at 700 amps when you are feeling brave?  THEN look at a whole new motor setup if you still feel the need.


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## major (Apr 4, 2008)

Hi guys,

Don't forget the difference between battery current and motor current. If I'm not mistaken, that controller limits motor current to 700A. I don't know how you limit battery current. Perhaps with the BMS to the controller? Anyway, if the motor and controller are making the grade now, just too slowly, increase battey current limit. New Calbs should handle it especially if they hold voltage at 200A. 

major


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## Sunking (Aug 10, 2009)

Perhaps this might help. DC motors maximum torque is at 0 RPM and declines to 0 at the motor maximum RPM.

So what is happening is when you are going say 50 MPH, and then climb, the motor RPM has to go down to increase the torque needed to climb.


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## PocoEV (Nov 4, 2014)

Thanks gang.... Weather is a challenge right now but will plan testing. Perhaps, I can get out this weekend?

Eval 2C/3C/4C discharge recording MPH and deltaT on motor and batteries while watching CALB battery voltages.

Also, eval "running start" at 50MPH going up a similar hill. 

Great ideas.


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