# Dual controllers controlling single motor?



## Matej (Dec 4, 2015)

Sorry for cluttering up the forum with my questions recently, but I keep thinking up various obscure setups and then wonder what would happen.
This time I am wondering if it is possible to control a single motor with two controllers, and how such a setup would act.

For example, let us say we have a motor connected to two identical 72V 300Amp controllers fed from a single battery pack and receiving their input from the same source.

1. Does the voltage of the battery pack get split between the controllers, so if the pack is 72V, each controller would see 36V, and the motor would be fed 72V?

2. If the answer to question 1 is yes, would it to be safe to use a 144V battery pack with the two controllers rated for half the voltage, or would they likely get damaged?
I would think they may get fried if the full Voltage of the pack happens to reach one of them a minuscule fraction of a second sooner before the power distribution balances out between them, though I am not sure how harmful or likely that would be.

3. Contradictory to question 1, would each controller see the full Voltage of the battery pack, thus feeding up to 144V into the motor from a 72V battery pack?
I assume no, but I had to ask.

4. Would the Amp ratings add up, so the motor could effectively pull up to 600 Amps with the two 300Amp controllers?
If nothing else, this could be a benefit for a performance application.

5. What would be the main drawbacks of a dual controller setup, or reasons why it would simply not work?


These questions originated from my curiosity of whether two (or more) lesser controllers could function as an alternative for a higher rated performance controller.
Thank you for any answers or input.


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## Coulomb (Apr 22, 2009)

Matej said:


> For example, let us say we have a motor connected to two identical 72V 300Amp controllers fed from a single battery pack and receiving their input from the same source.
> 
> 1. Does the voltage of the battery pack get split between the controllers, so if the pack is 72V, each controller would see 36V, and the motor would be fed 72V?


No, you could never get the voltage to share evenly. Besides, the way that DC controllers work, it just wouldn't work. (The free-wheel diodes could not do their job, for example.)



> 2. If the answer to question 1 is yes,


But it's no.



> 3. Contradictory to question 1, would each controller see the full Voltage of the battery pack, thus feeding up to 144V into the motor from a 72V battery pack?
> I assume no, but I had to ask.


No, the battery is the only source of voltage; the controllers won't provide extra voltage. DC controllers are buck converters; the voltage only does down.



> 4. Would the Amp ratings add up, so the motor could effectively pull up to 600 Amps with the two 300Amp controllers?


This configuration is the one that comes closest to being plausible. You would need both controllers to act in exact synchronism. In fact, the 300 A controller is probably made up of about 20 15 A "controllers" (single MOSFETs) acting in synchronism. That's about all the continuous current you can get out of popular inexpensive TO-220 packaged MOSFETs. But inside the controller, the gate signals are (or should be!) carefully designed so that all the gates get the signal to start (and to stop) conducting at the exact same time. If there is a sifnificant (say one microsecond) delay between the first one starting up and the others, then that first one will attempt to take the whole 300 A for that microsecond, and will likely fry, if not this second then in the next few days. Similarly, if one is the last to turn off, it will have to carry the whole current for a short time, and the 20x overload will wear it out in a short time. Once one lets go, the other 19 have to work even harder, so they pop off even quicker, and so on until they are all dead. With two physically separate controllers, the synchronisation requirement would be even harder to maintain.




> 5. What would be the main drawbacks of a dual controller setup, or reasons why it would simply not work?


As an example, let's say you are at two thirds current, 200 A per controller, 400 A total. Let's say controller A is on for 100 us, off for 50 us (total 150 us, or 6.7 kHz, just for round numbers. If both controllers are on at the same time, then they both see about 200 A each. If one tries to take a bit more, it will get hotter, and will increase in resistance, which will hand current back to the other controller. MOSFETs share current nicely like that. All good so far. But now controller A has to turn off as the current ramps to say 420 A. If controller B turns off at the same instant, then the two free wheel diodes take the current, and it maybe decays to say 380 A till the start of the next cycle.

Here is the first problem: I don't think that the free-wheel diodes will share the current the same as the MOSFETs do. But let's say they somehow do. Then all is well and good.

But suppose that controller B is a bit early with turning off. Then controller A has to carry the 420 A until it turns off; that's a 120 A overload. Suppose controller B was instead a little late in turning off; then when controller A turns off, controller B carries the whole 420 A until its turn to switch off. That might be OK, if it's only for a fraction of a microsecond. But without special provisions, there is nothing to synchronise the two controllers, so even if they happened to start off at exactly the same instant, they would eventually drift out of synchronisation, so that they would be carrying the full load for about a third of a cycle. This would quickly blow them both up.

Suppose you decided to force them to co-operate. Would you use a cable? It might get noise, and there would be delays. Would you use optic fiber? Maybe. But you'd want to have a sort of third controller that controlled the two back ends, so that they'd get the signals at the exact same time. But cheaper optical devices have significant delays that are rather variable. So you'd have to use high speed optics. I just don't see it happening. Just buy a 600 A controller.

[ Edit: the above assumes DC controllers. Suppose you had exactly three single phase AC controllers (it outpurs a single phase). You could use three of those to supply the phases of an AC motor wired in star. But again, you'd have to have a sort of master controller connected to the other controllers via expensive optics to supply the exactly 120 degrees out of phase signals. The other problem is, there isn't really such a thing as a single phase AC motor controller, at least not one designed for automotive use. ]


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## Matej (Dec 4, 2015)

That certainly answered all the questions. Thank you for going in-depth. I learned a lot of new things.
I was considering using two Curtis 1231C controllers to be able to pull up to 1000 Amps and thought they may also operate cooler/safer, since each one would see well under their max voltage rating, assuming I used a 180V pack or so.
I never realized the voltage distribution could be so finicky.

Now, what about if each controller had its own battery pack?
Two 72V 300Amp controllers, each connected to a separate 72V battery pack.
Does the motor see 144V and 600Amps?
Would that be more plausible, or still a terrible idea?

It would still be extremely difficult if not impossible to get the controllers to work precisely in unison, but I am guessing it would not really matter as much in this scenario.


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## PhantomPholly (Aug 20, 2008)

Matej said:


> That certainly answered all the questions. Thank you for going in-depth. I learned a lot of new things.
> I was considering using two Curtis 1231C controllers to be able to pull up to 1000 Amps and thought they may also operate cooler/safer, since each one would see well under their max voltage rating, assuming I used a 180V pack or so.
> I never realized the voltage distribution could be so finicky.
> 
> ...


Complexity is the enemy of success. Just put the whole pack together and buy a bigger controller.

If you were seeking some kind of fail-over, perhaps as in an airplane, there might be value in a standby controller - but they should be wired such that only one can be active at a time. Or, if you have two separate motors you could have separate controllers - particularly if they were not identical motors and you wanted to program different profiles for each. 

But still there would be no real advantage in splitting the pack.


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## Matej (Dec 4, 2015)

The main reasoning was the possibility of an 'economical' alternative for a 144-288V 1000Amp-capable performance controller.
Used 1231C's pop up for sale frequently and one can purchase two or more for less than the price of a Zilla or Soliton. Not to mention if anything happens to one of them, local shops are generally familiar with Curtis products, or one could just buy another used one to replace it with.
I will definitely be a lot more paranoid about something happening to a Soliton (it is a beautiful controller). Though at least now I know for sure that there are no 'cheap' alternatives when it comes to a performance application.


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## AmpEater (Mar 10, 2008)

Matej said:


> The main reasoning was the possibility of an 'economical' alternative for a 144-288V 1000Amp-capable performance controller.
> Used 1231C's pop up for sale frequently and one can purchase two or more for less than the price of a Zilla or Soliton. Not to mention if anything happens to one of them, local shops are generally familiar with Curtis products, or one could just buy another used one to replace it with.
> I will definitely be a lot more paranoid about something happening to a Soliton (it is a beautiful controller). Though at least now I know for sure that there are no 'cheap' alternatives when it comes to a performance application.


Curtis controllers blow all the time. Soliton controllers don't. 

Is it better to have a cheap controller that is a ticking timebomb, or an expensive controller that just works?


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## PhantomPholly (Aug 20, 2008)

Matej said:


> The main reasoning was the possibility of an 'economical' alternative for a 144-288V 1000Amp-capable performance controller.
> Used 1231C's pop up for sale frequently and one can purchase two or more for less than the price of a Zilla or Soliton. Not to mention if anything happens to one of them, local shops are generally familiar with Curtis products, or one could just buy another used one to replace it with.
> I will definitely be a lot more paranoid about something happening to a Soliton (it is a beautiful controller). Though at least now I know for sure that there are no 'cheap' alternatives when it comes to a performance application.


At restaurant's it's called "trying to beat the menu."

Re the above reliability concerns, I can't address. However, if you set your sights on cheap controllers there IS a way to use them together - pair each with a separate motor and then find a way to tie the motors to a common shaft. What I don't know, however, is whether several small motors are cheaper than, or even as cheap as, a single larger motor.

TANSSAAFL.


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## Tesseract (Sep 27, 2008)

It is possible to wire 2 or more DC controllers to 1 motor without any regard to synchronization or matching as long as there is a significant amount of inductance in series with each controller. The inductors perform the same function as the field in the DC motor: integrate the voltage pulses from each controller into a nearly pure DC current, which is what allows the controllers to be paralleled (current sources can be paralleled; voltage sources can't).

Unfortunately, the inductors will be quite large and surprisingly expensive, making this solution more expensive than a single larger controller.


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## Matej (Dec 4, 2015)

Thank you for the input everyone. You helped sell me on the Soliton, once controller purchase time comes around.


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