# Max amps for warp11 at 5000rpm



## kennybobby (Aug 10, 2012)

*It depends, how much torque do you need at 5000?*

And for how long a time period?
i didn't see your other thread but i'm guessing it must be some sort of drag car.
Asking for 185 hp from a 30 hp device...

The motor current draw depends upon the motor torque load, and the resulting motor speed at that load will be determined by the available voltage minus the backemf.
The no-load speed at 172 volts will be close to 10,000 rpm.
A warp 9 makes about 200 ft-lbs at 800 amps, i would figure a warp 11 to make about 230 ft-lbs.
http://www.diyelectriccar.com/forums/showpost.php?p=315821&postcount=57


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## appleseed (Sep 12, 2010)

Thank you but i am assuming that 10000rpm is a guess?
I was hoping for real measured numbers of the wires resistance (probably in the milliohm range) and RPM/volt to estimate back emf at certain speeds.

Kinda odd that netgain doesnt post such simple numbers. Almost makes me want to buy from kostov


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## kennybobby (Aug 10, 2012)

*No guess, using your numbers from netgain.*



appleseed said:


> Thank you but i am assuming that 10000rpm is a guess?
> I was hoping for real measured numbers of the wires resistance (probably in the milliohm range) and RPM/volt to estimate back emf at certain speeds.
> 
> Kinda odd that netgain doesnt post such simple numbers. Almost makes me want to buy from kostov


No it's calculated from your numbers. On paper it would do it--but i'd guess that the brushes would start arcing before then. It takes about 18.5 volts per 1krpm based upon the 72 volt dyno data. Double the voltage and you double the speed (if it could survive at that speed).

He only tested to 72 volts because it is quite difficult to make a power supply that won't sag under high currents (~500 amps)--and not many folks have a dyno that can dissipate the power and test motors at that sort of torque and speed. My largest dyno can only test up to 40 ft-lbs at 5000 rpm for a short time. A 15kW power supply that won't sag is very big, heavy and expensive, much more for a 50kW supply and a dyno that can handle that too.


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## appleseed (Sep 12, 2010)

Ok, then is my .1 ohm calculation for resistance of the wiring good? I know i am missing the effects of eddy currents and hysteresis losses but Im not sure how large of a part they play.


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## major (Apr 4, 2008)

The problem is that you can't use a RPM/volt constant for the series wound motor because the flux isn't constant like you have with a PM machine.


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## appleseed (Sep 12, 2010)

Thank you major.

Is there a way to determine the needed voltage to achieve a given current at a given rpm?

EX: 800 amps at constant 4500 RPM for the warp 11?


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## major (Apr 4, 2008)

appleseed said:


> Thank you major.
> 
> Is there a way to determine the needed voltage to achieve a given current at a given rpm?
> 
> EX: 800 amps at constant 4500 RPM for the warp 11?


Use the motor performance characteristic graphs. You'll likely have to extend the curves out to the load of interest and then proportion the applied voltage to increase the RPM to the desired level or vice versa. It is estimation but works fairly well. The big caution is that the published motor curves are notorious for inaccuracy 

With modern day high power DC motor controllers it is best to make your estimation and then choose a battery voltage well in access of that. This will also cover your azz when battery sag turns out greater than you expect. The controller should allow you to limit the motor voltage as needed as well as limit the battery current.


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## appleseed (Sep 12, 2010)

the very inaccuracies of the chart was what i was hoping to avoid with a measured RPM/volt reading. Oh well. When i do the calculations i will add in this margin of error. 

What do you think of my calculation for winding resistance? is .1 ohm too high? Based on that, you will need 80 volts at 0 rpm to get 800 amps. Does that sound right according to real life uses of the motor?


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## major (Apr 4, 2008)

appleseed said:


> What do you think of my calculation for winding resistance? is .1 ohm too high? Based on that, you will need 80 volts at 0 rpm to get 800 amps. Does that sound right according to real life uses of the motor?


I have not used this particular motor but ones similar. I'd be very, very surprised if the actual motor armature and field resistance was greater than 0.01 Ω. There will be a voltage drop due to the brushes. But I would expect the stall current at 80 volts applied to the motor terminals to be several thousand amps.


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## kennybobby (Aug 10, 2012)

*800 amps at 80 volts at 0 rpm?*



appleseed said:


> the very inaccuracies of the chart was what i was hoping to avoid with a measured RPM/volt reading. Oh well. When i do the calculations i will add in this margin of error.
> 
> What do you think of my calculation for winding resistance? is .1 ohm too high? Based on that, you will need 80 volts at 0 rpm to get 800 amps. Does that sound right according to real life uses of the motor?


More like 3400 amps!

Here's some resistance data for a warp 9 measured and reported in another thread here:
WarP-9 Field resistance: 3.8 mΩ
WarP-9 Armature resistance: 8.6 mΩ

Measured via 4-terminal (Kelvin) method with the shaft stalled at 40A and 100A to verify correlation. Resistance was the same at both currents. So, total resistance of the motor (minus the field to armature jumper) is 12.4 mΩ. 

I previously calculated a total motor loop resistance (ie - including the cables between controller and motor on our dyno setup) of 23 mΩ which despite major's protests strikes me as perfectly reasonable..."

lifted from Tesseract's post here: http://www.diyelectriccar.com/forums/showpost.php?p=236548&postcount=45


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## appleseed (Sep 12, 2010)

Since the warp 11 is larger and can handle more amps, most likely it would have a lower resistance than the warp 9, right?

Well it seems that it wont take much if hardly any voltage to get alot of amps at stall. Thanks everyone. I've been trying to figure out the motor size needed for my plans, which i will post about soon enough.


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