# Electronic differential



## aquilante (Feb 12, 2011)

If you have two independent motors connected directly to the wheels, how do you calculate and control the speed of each wheel as a differential mechanically do? 

Is there an already open discussion on this issue?


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## alexcrouse (Mar 16, 2009)

aquilante said:


> If you have two independent motors connected directly to the wheels, how do you calculate and control the speed of each wheel as a differential mechanically do?
> 
> Is there an already open discussion on this issue?



If they are both Series DC, they will automatically allow different speeds because they will apply a set amount of torque per amp. AC motors care a little more about speed, and require a more complex system per my understanding. If they are DC, from what i can tell in doing years of robotics, don't worry about it. They will just work! Old guys: Please let me know if i'm wrong!


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## major (Apr 4, 2008)

aquilante said:


> If you have two independent motors connected directly to the wheels, how do you calculate and control the speed of each wheel as a differential mechanically do?
> 
> Is there an already open discussion on this issue?


Sort of  http://www.diyelectriccar.com/forums/showthread.php?t=55656 

And like I say there, control the torque and it works fine. There have also been previous discussions on the subject. But a note, you should not drive the wheels directly, use a speed reduction of some sort. Otherwise the motor runs too slow and becomes oversize for the required power.


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## aquilante (Feb 12, 2011)

Hi Alexcrouse, it seems easy... since the torque is the same one controller is enough or I need two?

thanks Major for the link!


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## madderscience (Jun 28, 2008)

If I'm not mistaken, if you run two series wound DC motors in parallel, you will basically get the equivalent of a limited slip differential.

Reason being, a series wound motor produces back EMF the faster it is turning (basically, the faster it is going the more voltage must go across it to cause current to flow). because of this, when two such motors are in parallel, a stalled or slowly turning motor will draw all the power while a freely spinning one will not. Since presumably the stalled motor is the one with more traction, the power will magically go where it is needed.

I know it could be slightly more complicated than that and I haven't tried it myself but based on my noodling on it that is what I have concluded.

Note that high performance, dual motor EVs run their series wound motors in series at low speeds and parallel at high speeds. However since the motors are mechanically connected there should never be a case where they aren't turning at the same speed.


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## alexcrouse (Mar 16, 2009)

madderscience said:


> If I'm not mistaken, if you run two series wound DC motors in parallel, you will basically get the equivalent of a limited slip differential.


EXACTLY my understanding.


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## aquilante (Feb 12, 2011)

great!
thanks a lot!


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## Nunkmolian (Oct 31, 2020)

This article is very helpful


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