# Two Alltrax controllers with single throttle



## MalcolmB (Jun 10, 2008)

Hi guys. I'm looking for a little advice again. My Mini currently uses two series-wound motors wired in parallel to a single Alltrax AXE 7245. The motors drive the front wheels independently. Starting torque is naturally underwhelming with a single Alltrax, and since I don't have much spare cash right now I'm thinking about adding a second Alltrax, so each motor will have its own controller.

Alltrax provides a diagram for connecting two controllers to a single throttle. A note on this drawing says that "Each controller has its own battery wire". I'd like to know why this is necessary and roughly how long the separate supply cables should be? I've already emailed Alltrax technical support, but got no reply.


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## major (Apr 4, 2008)

MalcolmB said:


> Hi guys. I'm looking for a little advice again. My Mini currently uses two series-wound motors wired in parallel to a single Alltrax AXE 7245. The motors drive the front wheels independently. Starting torque is naturally underwhelming with a single Alltrax, and since I don't have much spare cash right now I'm thinking about adding a second Alltrax, so each motor will have its own controller.
> 
> Alltrax provides a diagram for connecting two controllers to a single throttle. A note on this drawing says that "Each controller has its own battery wire". I'd like to know why this is necessary and roughly how long the separate supply cables should be? I've already emailed Alltrax technical support, but got no reply.


Hi Malc,

My guess is that they attempt to make the user provide sufficiently large enough conductors by using 2 from the battery opposed to daisy-chain style. 

major


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## MalcolmB (Jun 10, 2008)

Thanks Major, I hadn't thought of that. It would make wiring a lot neater just to daisy-chain the controllers, as they'll be installed right next to each other.


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## MalcolmB (Jun 10, 2008)

Finally got round to testing this out. I've hooked up two Alltrax AXE 7245 controllers with a single throttle input, wired exactly as shown in the diagram above. Each motor is connected to its own controller (via individual reversing contactors). The controllers have been programmed with exactly the same throttle curves and settings (300A current limit for testing).

I've checked the wiring and controller setup several times as I'm getting some weird behaviour:

Moderate acceleration is causing sudden torque steer, so the steering wheel twitches right or left as I accelerate. It's not uncontrollable but definitely not desirable. I had no such problem when I had both motors connected in parallel to a single Alltrax. Straight line stability and low speed manouvering were both fine. I've not changed the motor wiring at all, simply disconnected one motor from the original controller and reconnected it to the new controller.


Any ideas what could be wrong?


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## Qer (May 7, 2008)

MalcolmB said:


> Any ideas what could be wrong?


Is that heat sink supposed to cool both controllers?

I bet things will overheat fast, probably fatally.


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## toddshotrods (Feb 10, 2009)

MalcolmB said:


> ...Moderate acceleration is causing sudden torque steer, so the steering wheel twitches right or left as I accelerate. It's not uncontrollable but definitely not desirable. I had no such problem when I had both motors connected in parallel to a single Alltrax. Straight line stability and low speed manouvering were both fine...


It sounds like you've went from an electronic limited slip diff, to an electronic spool (locked diff), at initial acceleration rates. Both controller/motor combos seem to be pulling at the requested (throttle) rate, regardless of road condition or traction. If one wheel really went into a notable spin that controller should start to reduce the current to the respective motor, but initially there's not enough of a reduction for you to not feel one wheel pulling harder than the other, and vice versa. That's what _it's controllable but not desirable_ sounds like. Like a FWD car on snow and ice. If it was RWD, you probably wouldn't notice it at all.

Is there any way to increase the speed or ate at which the controller begins to reduce current?


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## MalcolmB (Jun 10, 2008)

Qer said:


> Is that heat sink supposed to cool both controllers?
> 
> I bet things will overheat fast, probably fatally.


Hi Qer
Nice to hear from you. No, I've just lashed things up temporarily for testing. I don't plan to go far or push these controllers during initial testing. A full-size heat sink will be first priority if/when I get this to work satisfactorily. I also plan to make up some new cables and fit terminal boots and flexible conduit to all cables.



Todd said:


> If it was RWD, you probably wouldn't notice it at all..


That crossed my mind as well Todd. But if it's twitching like this at the very modest power level I've tried so far I would expect it to be more severe at higher power. Even if it was unnoticeable in RWD it would still be putting strain on the drive system and wasting energy. The twitching is most noticeable in a straight line, probably because that's when I accelerate most.


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## MalcolmB (Jun 10, 2008)

Any more ideas? I realise this is an unusual setup and it's going to accentuate any differences in controller output. I'm willing to accept it's not going to work and move on, but I would like to understand why before I do so. Is the instability just due to slight mismatches in the throttle response of the controllers, or is it a more fundamental issue? Would it make any difference if I ran the controllers independently using two identical throttle pots on the same shaft, or is that just likely to aggravate the instability? I'd really appreciate any insights.


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## major (Apr 4, 2008)

MalcolmB said:


> Any more ideas? I realise this is an unusual setup and it's going to accentuate any differences in controller output. I'm willing to accept it's not going to work and move on, but I would like to understand why before I do so. Is the instability just due to slight mismatches in the throttle response of the controllers, or is it a more fundamental issue? Would it make any difference if I ran the controllers independently using two identical throttle pots on the same shaft, or is that just likely to aggravate the instability? I'd really appreciate any insights.


Hi Malc,

Do those controllers have the selection for speed vs torque control? Where are they set? And one wouldn't think a common throttle pot would cause it, but maybe. Is it too difficult to try separate pots? 

major


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## piotrsko (Dec 9, 2007)

MY $.02 (usd) Until you instrument both motor/controller combos for volt and current differences, you will only be guessing as to the causes of the torque steer if that is what it really is. rotation sensors on either motor shafts or wheels wouldn't hurt either. I'm guessing non computer mini. Tesseract did a explanation regarding his thoughts on dual non-coupled motors perhaps 18 months ago. It would be worth searching for.


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## MalcolmB (Jun 10, 2008)

major said:


> Hi Malc,
> Do those controllers have the selection for speed vs torque control?
> major


Hi Major,
No, they're speed control only, which I guess is not ideal. It wouldn't be all that difficult to arrange a dual pot setup. I might try that just out of curiosity.



piotrsko said:


> MY $.02 (usd) Until you instrument both motor/controller combos for volt and current differences, you will only be guessing as to the causes of the torque steer


Thanks piotrsko, you're right of course. I already have a pair of mV meters in the car. I can use identical lengths of motor cable on each motor as shunts to compare the motor currents. I'll also have a root around for Tesseract's explanation.[/QUOTE]


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## MalcolmB (Jun 10, 2008)

OK, I found an earlier relevant thread here http://www.diyelectriccar.com/forum...differentiali-68527.html?highlight=dual+motor (where I also posted a couple of questions). Tesseract states that the only good way to use twin motors as a differential is to drive each motor with a separate controller, although this was in the context of Soliton controllers, which use torque control. He also added a rider that he thought it might be twitchy.

As piotskro suggested, I need to monitor motor currents and voltages, ideally in both modes, with one controller and with two, to find out what's going on.


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