2 controllers in paralell for one hub motor, is it possible?

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2 controllers in paralell for one hub motor, is it possible?

Post by endonuclease » Nov 13, 2017 6:02 am

Hi, I recently got me a Crystalyte H4080 motor. I run this on a Crystalyte 72 volt 45 A sensor/sensor-less controller (I run it in sensor-less mode and works well). But the case is that I now have two such identical regulators, after I removed two blown MOSFETS (an identical regulator from my previous ebike) and soldered back two new MOSFETs of the same type. The question is perhaps hard to answer, cause I have not found anything on the web or youtube about this scenario.

But here's the question: Is it possible to connect these two identical regulators in parallell to one hub motor ?, to double the current from max 45 A to max 90 A ? They will of course self-synchronize (in sensor-less mode)....... But if it is possible; should I run both controllers in a sensor-less mode, or in sensor-mode ? I can't see any trouble about anything "backfiring" from one controller into the other because they are connected in parallell and the current (let's say at 60A will divide in a Y-connection).

I must emphasize that I'm NOT going to use two hubs, only ONE motor. And ONE speed regulator, because the voltage from the regulator will be the same into each controller through a Y-splitter into both controllers, but the CURRENT out/in to the controllers will get "splitted" on the output side.

So if you can imagine a circuit as a water pipe mesh; the voltage is the pressure in the "pipe-mesh" and the current is the "water-flow". I know mediokre electronics and can't see any dangers, but perhaps there is someone with deeper insight that will see a problem. I've also thought about using diodes, but then the controllers won't get feedback from the hub to synchronize...

Thanks,
Endunuclease

Clyte H4080, 72v nominal 20Ah, 45A (hopefully 65A with two parallell controllers (shared output = 32,5 A per controller and very little heat) if this works, and will make it possible to connect an extra battery in parallell later).

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Re: 2 controllers in paralell for one hub motor, is it possi

Post by amberwolf » Nov 14, 2017 2:43 am

If it was a 6-phase motor, you could do two controllers like that. There's an ongoing recent thread about such a motor / controller setup under that name, and another older thread about it (by 12p3phPMDC

But a regular 3phase motor won't work that way, because the controllers won't operate with identical timing, and they won't synchronize, so at some point in the controllers' commutations of the motor, you'll be shorting battery to ground between the FETs of the different controllers, and parts will probably fail (probably spectacularly).


You can try it out and see what happens, but I don't recommend it.


Now, if you were to disconnect the gate drive of one controller from it's MCU, and connect it to the other controller's MCU, so that one controller drives both power stages, it might work. But if there is enough delay (long enough wires, enough inductance and/or capacitance in the path, etc), that could still have enough difference between power stage timings to cause problems. If there's enough dead time between switchings it won't cause a shoot-thru (battery short) but it could cause excessive heating.



You can't use diodes anywhere in the phase wires to or from the controllers, because current has to flow both directions thru each phase to be able to drive the motor.


there's other threads about two contorllers and one motor, if it helps, though there's no testing in them to verify that I recall. This search
https://endless-sphere.com/forums/searc ... mit=Search
finds some of them; most results aren't relevant but you can see from the titles which ones are.

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Re: 2 controllers in paralell for one hub motor, is it possi

Post by fechter » Nov 14, 2017 11:01 am

I've seen it done before without smoke, but I didn't think it would work. The PWM timing between controllers will never be in sync unless you run a sync line between them and use one as a remote. Better to just get a bigger controller.
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Re: 2 controllers in paralell for one hub motor, is it possi

Post by endonuclease » Nov 15, 2017 8:54 am

Yes, by a second thought, I totally agree..... Just a de-synchronization of a few milli-seconds will perhaps be tolerated by the controllers, but the battery will be also shorted in those fractions of milliseconds - something that can't be good at all for the battery. Especially when you think of the rpm of perhaps 600-700 at max speed (82 km/h). The the battery will short-out more than 700 times each minute.....That battery won't last for a long period (shorting out 10 times each second)....Then perhaps it would be better to modify the Controller...(?) I've even been reluctant to use regenerative breaking because of the battery life-time....

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Crystalyte H4080 motor. 72v 20Ah LiFePO4 battery, 1500-2000 charges (capable of 65A). Crystalyte sensorless 72v 45A Controller.

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Re: 2 controllers in paralell for one hub motor, is it possi

Post by fechter » Nov 15, 2017 9:14 am

Depending on the controller, it's often possible to increase the current capacity by installing better FETs and beefing up the traces.

Most controllers that support regen can be programmed for regen level. Just use a lower level if you're worried about it. Even at a low level, it really helps the brakes.
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Re: 2 controllers in paralell for one hub motor, is it possi

Post by endonuclease » Nov 16, 2017 2:16 am

Yeah, I can experiment with the oldest controller and see if I can change the FET's into those Crystalyte are using in the controlller for the CROWN motor (60A or 90A). And I can also check out if some of the capacitors need to be changed.

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Re: 2 controllers in paralell for one hub motor, is it possi

Post by John in CR » Nov 16, 2017 4:34 am

You could do it by separating the individual strands of the windings at both ends, and terminating both ends as 2 groups of separate windings. Seems like a waste of time to me when I can easily get a 90A controller. Plus it's a bad idea anyway, since 90A is too much for that motor anyway unless it's pushing a light load on flat terrain avoiding repetitive hard launches, so peak current is very short in duration. Heat in the windings goes up by the square of current, so if you double the current, heat goes up 4X.

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Re: 2 controllers in paralell for one hub motor, is it possi

Post by Buk___ » Feb 10, 2018 5:59 pm

fechter wrote:
Nov 14, 2017 11:01 am
I've seen it done before without smoke, but I didn't think it would work. The PWM timing between controllers will never be in sync unless you run a sync line between them and use one as a remote. Better to just get a bigger controller.
I don't understand why. (Assuming two ~identical controllers.)

If you feed them the same voltage (same pack), the same demand (throttle) signal, and the same timing (hall sensor) signals from the motor, then (within the tolorances of their components) they should produce the same output -- same voltage, ~same duty cycle. Connect the U,V&W outputs from both controllers and their amperages should add. (at least,that's what happens if you simulate it.)

The motor is demanding a given number of amps, and the two controllers should -- within the tolerances of their components -- split that demand equally between them.

Given that the heat generated in a mosfet is down to the switching transitions -- not the time hard on or off -- then delivering half the number of amps during those transitions should result in ~half the heat generation within each of the controllers.

And given the reluctance of the windings will act to impede changes in current, and the capacitors in the controllers will act to supply voltage on demand, the variations in tolerances between the components within the controllers will be smoothed.

What did I misunderstand?
Last edited by Buk___ on Feb 11, 2018 4:48 am, edited 1 time in total.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by amberwolf » Feb 11, 2018 2:25 am

One possible problem:

Clock cycle start and end times won't likely match even if everything is identical in the contorllers, so PWM of FETs won't match. Exactly what that results in depends on how the controllers do their PWM.

Frequencies won't match perfectly either, so after enough clock cycles they'll drift apart far enough so that even if somehow you got the clock cycles to match up at first, they won't later.



If timing is far enough different, then even with both controllers reading the same hall signals, the first PWM cycle of one phase of one controller might overlap the last PWM cycle of the previous phase of the other controller.

That means you may have some top FETs being switch on while other bottom FETs are being switched on, and if it happens on the same phase wire then the only resistance there is would be the wire from one controller to the other (meeting at the motor phase input).

That resistance is likely to be low enough to essentially be zero, resulitng in what amounts to a shoot thru from battery positive to battery negative thru the FETs of both controllers.

If it's a short enough event, it just results in heating those two sets of FETs more than others (possibly a lot more), but once the desynchronization starts, it will probably last long enough to create enough heat inside teh controllers from that sort of event that something could overheat and fail. It might never get taht far...but it could, under the right (wrong) circumstances.

You can test the theory easily enough if you have the money to possibly throw away (I would, but I don't have controllers I can sacrifice).

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 11, 2018 4:40 am

amberwolf wrote:
Feb 11, 2018 2:25 am
One possible problem:

Clock cycle start and end times won't likely match even if everything is identical in the contorllers, so PWM of FETs won't match. Exactly what that results in depends on how the controllers do their PWM.

Frequencies won't match perfectly either, so after enough clock cycles they'll drift apart far enough so that even if somehow you got the clock cycles to match up at first, they won't later.



If timing is far enough different, then even with both controllers reading the same hall signals, the first PWM cycle of one phase of one controller might overlap the last PWM cycle of the previous phase of the other controller.

That means you may have some top FETs being switch on while other bottom FETs are being switched on, and if it happens on the same phase wire then the only resistance there is would be the wire from one controller to the other (meeting at the motor phase input).

That resistance is likely to be low enough to essentially be zero, resulitng in what amounts to a shoot thru from battery positive to battery negative thru the FETs of both controllers.

If it's a short enough event, it just results in heating those two sets of FETs more than others (possibly a lot more), but once the desynchronization starts, it will probably last long enough to create enough heat inside teh controllers from that sort of event that something could overheat and fail. It might never get taht far...but it could, under the right (wrong) circumstances.

You can test the theory easily enough if you have the money to possibly throw away (I would, but I don't have controllers I can sacrifice).
Thanks for grounding me AW.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 11, 2018 6:05 am

Sorry for coming back. I know you are (probably) right, but I'd like to understand this a bit more if you have the patience :)
amberwolf wrote:
Feb 11, 2018 2:25 am
... if it happens on the same phase wire then the only resistance there is would be the wire from one controller to the other (meeting at the motor phase input).
Aren't there (electrolytic) capacitors in the output circuit after the mosfets; and wouldn't their ESR and /or ESL act to resist shoot through?
amberwolf wrote:
Feb 11, 2018 2:25 am
You can test the theory easily enough if you have the money to possibly throw away (I would, but I don't have controllers I can sacrifice).
I'm saving my pennies to buy a couple of RC ESCs and matching small motors to play with, specifically to try and wrap my brain around some stuff that currently leaves me cold; but I'll be trying to only try things that I have a reasonable assumption won't blow anything up, cos I can't afford to.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Alan B » Feb 11, 2018 7:32 am

At first thought it seems like paralleling controllers on sensored brushless motors is nuts. This turns out to be incorrect thinking.

On more careful analysis one realizes that the necessary synchronization is provided by the hall sensors. There is no requirement for the PWM to be in phase. The voltages are identical since the same power source is used and the FETs are merely switches that allow current to flow.

The increased current is provided by the lowered resistance of the "paralleled" FETs, each provides part of the current based on the PWM and their resistance. The inductance smooths the PWM current as usual.

It is not a perfect situation, but it has been demonstrated to work. It does not catastrophically fail.

If you think it is going to explode, or short the battery then analyze it deeper. The three phase motor's operation as it changes states is key here. The adjacent states in the six state phase cycle are compatible. At each switch point one phase goes open, and the previously open phase goes high or low. Being simultaneously in both states is not creating a short, so being in both states for a short time doesn't create problems. This is not like having both high and low FETs in the same phase driver on which creates a low resistance low inductance supply short.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 11, 2018 8:26 am

Alan B wrote:
Feb 11, 2018 7:32 am
At first thought it seems like paralleling controllers on sensored brushless motors is nuts. This turns out to be incorrect thinking.

On more careful analysis one realizes that the necessary synchronization is provided by the hall sensors. There is no requirement for the PWM to be in phase. The voltages are identical since the same power source is used and the FETs are merely switches that allow current to flow.

The increased current is provided by the lowered resistance of the "paralleled" FETs, each provides part of the current based on the PWM and their resistance. The inductance smooths the PWM current as usual.

It is not a perfect situation, but it has been demonstrated to work. It does not catastrophically fail.

If you think it is going to explode, or short the battery then analyze it deeper. The three phase motor's operation as it changes states is key here. The adjacent states in the six state phase cycle are compatible. At each switch point one phase goes open, and the previously open phase goes high or low. Being simultaneously in both states is not creating a short, so being in both states for a short time doesn't create problems. This is not like having both high and low FETs in the same phase driver on which creates a low resistance low inductance supply short.
Thank Alan!

It's nice to know my intuition isn't completely out of whack, even if I don't know enough, to know if it is or not :)

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by amberwolf » Feb 11, 2018 12:45 pm

@AlanB:
I see your point, but if there's no problem with doing this, I would have expected *someone* here on ES to have tried it and it to be a common method of getting more power with cheap controllers.

There's so many "cowboys" here ;) that I have a hard time imagining it hasn't been done just because a problem *might* happen, unless it was already done and shown a problem *does* happen. :lol:



Buk___ wrote:
Feb 11, 2018 6:05 am
Aren't there (electrolytic) capacitors in the output circuit after the mosfets; and wouldn't their ESR and /or ESL act to resist shoot through?
There are no caps *after* the FETs, because that's the motor windings (phase wires). (There are some *before* the FETs, on the battery side, but they act like extremely low resistance batteries in parallel with the "real" ones as far as the circuit is concerned).

Even if there were, they still wouldn't, because shoot-thru is a direct path from battery positive to battery negative, when both top and bottom FETs of the same phase turn on at the same time. That should never happen, and generaly results in loss of magic smoke containment. ;)


Imagine putting a pair of switches across your battery terminals. You never want both of them on at the same time, even for an instant, because no matter what else is in parallel with them, it's still a direct short.

It does take time to build current thru that short, but it's very low resistance, and really only the wires from one controller to the other, meeting at the motor phse connection (not going thru it) create any significant inductance or resistance to slow that short down. Given the small time slices we're talking about (part of a PWM slice of a phase commutatin pulse), it might not be long enough to directly blow anything up. I won't pretend to be good at math and calculate it; I'll just avoid the chance entirely. ;)

Honestly if I did have the extra controllers I could afford to lose, I'd do the experiment, instrumented as well as I could, just to see what happens. (explosions can be fun, and if nothing bad happens, a myth could be debunked and a new easy way of getting more power is born. ;) )

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by fechter » Feb 11, 2018 1:45 pm

The controller is using the motor windings as the inductor in a buck converter topology. If the PWM isn't synchronized, sometimes they would be in phase and sometimes they would be out of phase. If the controllers aren't using synchronous rectification, then they shouldn't blow up from this but you could possibly get some surging in the output.

If the controller are using synchronous rectification, then you could get shoot-through and destruction.

It would be possible to synchronize the PWM by running the signals from the gate drives of one controller over to the other one. Not super hard, but would take some work.
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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Gregory » Feb 11, 2018 10:22 pm

Samd was posting that he'd had some successes using two controllers a couple of years ago, but no follow up on actual road use and longevity. Personally I'd just get a bigger controller. :D

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1) x5305 Hub Motor in a 24" Sun rim with 10G spokes, Kelly 72601 controller, 74V 10Ah Turnigy LiPo 20C Battery and CycleAnalyst
2) Mac 10T rear hub in a 700C "comfort bike" 15S 5Ah LiPo, stock 28A Xie Cheng controller
3) 38" Longboard, Turnigy 6374, CC Mamba XL2 ESC

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 11, 2018 10:29 pm

fechter wrote:
Feb 11, 2018 1:45 pm
The controller is using the motor windings as the inductor in a buck converter topology. If the PWM isn't synchronized, sometimes they would be in phase and sometimes they would be out of phase.
There are two possibilities for the PWM being in or out of sync. The start/end of each sine wave (or tapezoid); and, the start/end of the individual pulses that go to make up those waves.

If the former (sine/trap waves) are slightly out of sync, the result is a proportional phase shift in the output and a small reduction in the peak current which (I think) would not affect anything much if the discrepancy was small, as I believe they would be, as they are both triggered from the same hall signals:
10degSync.jpg
10degSync.jpg (67.01 KiB) Viewed 107 times
20degSync.jpg
20degSync.jpg (67 KiB) Viewed 107 times
If the PWM carrier frequencies were slightly different and/or out of sync, then once fed out to the RC of the motor windings and smoothing caps, the difference would be almost imperceptible. (I think.)
fechter wrote:
Feb 11, 2018 1:45 pm
If the controller are using synchronous rectification, then you could get shoot-through and destruction.
This is where I move on to even more shaky ground -- which is not to say I have any great confidence in the above.

Alan B states this much more scientifically/electronically above, but my take on it is this.

Individually, each of the controllers -- assuming some base level of competence -- will have been designed to at least minimise the risk, if not totally preclude the possibility of shoot through due to circuit board and/or component variances. In my research I've seen a bunch of feedback/interlock subcircuits in H-bridge designs specifically prevent top and bottom fets being on at the same time.

So if we assume that neither of the two controllers allows shoot-through as a results of its own circuitry, then the fear is that a lack of sync between the two controllers would allow one to put a potential on the output stage of the other with a timing that might find an overlap between the top and bottom fets in that other controller and cause shoot-through.

But if neither controller is susceptible to shoot-through from the potential that continuously exists on it associated phase winding -- whether due to the reluctance of that winding, or the back-EMP being generated in it -- then why would it be susceptible to potential coming from the other controller?

I'm not sure that says anythng more, or more clearly than AlanB's statement above, but it makes sense in the way I think about the situation.

Feel free to tear it down or totally ignore it.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by amberwolf » Feb 11, 2018 11:24 pm

Buk___ wrote:
Feb 11, 2018 10:29 pm
then once fed out to the RC of the motor windings and smoothing caps,
There aren't any smoothing caps on the motor side of the FETs--those are only on the battery/gate side, on the controllers I've had.

The only thing out there past the FETs to do smoothing is the motor windings and the wires from the FETs to the windings.


If there were caps on the FET outputs, they'd have to be non-polarized because polarity reverses during commutation.

But if neither controller is susceptible to shoot-through from the potential that continuously exists on it associated phase winding -- whether due to the reluctance of that winding, or the back-EMP being generated in it -- then why would it be susceptible to potential coming from the other controller?
Because shoot-thru prevention inside a controller is mostly a matter of timing things so that never happens, and/or ensuring the circuits to drive the FETs are interlocked so the bottom and top FETs in any one phase of that specific controller can never be on at the same time.

But there is nothing in software or hardware to detect *another controller* that's on the same phase wires that happens to be in the opposite state on the same phase, because it's not something anyone has any reason to design for. It could probably be done, but if I were designing (or redesigning) one, I'd rather spend my time making a higher-current powerstage to start with. :)


There *is* the battery-current detection, which will detect a spike in current if shoot thru happens, but it might not be fast enough to shut it down before bad stuff happens--that all depends on the specific contorller design and the situation.

Some more advanced controllers have per-phase current detection, which is probably more likely to be able to prevent this kind of problem, but the typical ebike controller doesn't have this. And I'd guess that the common ebike controller is the one more people would be likely to want to double-up. :) If they already shelled out for an advanced controller, they also probably got one that can handle the power they want.


Keep in mind that none of the events may ever happen, at least not in a catastrophic fashion, but the potential exists.




Something else not covered is electric braking: There are at least two general types, and each one probably has different effects on a paired-controller system.

The most likely to be found in common ebike controllers at present is regular regen in one form or another, where it switches the FETs in a way that causes current to flow back into the battery from the motor. The way it switches the FETs is different than when it's causing current to flow to the motor from teh battery, so I don't know if there's even any possiblity of shoot-thru in this case, but I suspect there is. Even if there's no battery-shoot-thru, perhaps there could be a form of shoot-thru from one phase to another.


Then there is the "EABS" (called various names) that's an active form of braking, where it powers the wheel opposite of the direction of motion, to force it to slow down much more quickly than by just turning it into a generator. Since this basically works the same way as running it forward as a motor, it probably has the same risk of shoot-thru.


The reason I bring up braking is that if you have a pair of controllers that aren't identical in their reaction times to start braking, it's possible that one will still be in motor mode when the other changes to braking mode.

Also, if they function differently during braking, (one with regen, one with EABS, perhaps), timing of things may be differnet and exatly what they do may be different.


I don't know what would happen in either of those cases, but I expect it would make some form of problem more likely.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 12, 2018 2:57 am

Hm. Gonna overstay my welcome with this post, but this (my misunderstanding or failure to convey my point) is niggling me.
amberwolf wrote:
Feb 11, 2018 11:24 pm
Buk___ wrote:
Feb 11, 2018 10:29 pm
then once fed out to the RC of the motor windings and smoothing caps,
There aren't any smoothing caps on the motor side of the FETs--those are only on the battery/gate side, on the controllers I've had.
The only thing out there past the FETs to do smoothing is the motor windings and the wires from the FETs to the windings.
If there were caps on the FET outputs, they'd have to be non-polarized because polarity reverses during commutation.
In every controller circuit I look at, from the simplest bare-bones example with its "Main capacitor", to (what I assume to be) the most well-designed and sophisticated (from here), where there are paired (polorised and non-polarised) capacitors per phase, labelled C15/C18, C16/C19 and C17/C20 respectively; there are capacitors to prevent spikes across the battery, parallel to the top & bottom mosfet circuit, that would provide current if the battery weren't able to, should both fets be open at the same time.

And for shoot through to occur, both fets have to be open. The current from the "other" controller would be seen in between the top and bottom mosfets, and would flow to either battery+ or battery-, but not through both.

Unless both were open within a single controller, when there would already be power from B+ to B- from the capacitors, or indeed the battery; but (I assume) any half descent circuit is designed to prevent that.

Stated simply, I'm still failing to see how the presence of the other controller's output (detected or not) at this controller output, could cause shoot through within this controller, unless this controller is already susceptible to shoot through; in which case it would occur anyway, regardless of whether the other controller's output was there or not.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 12, 2018 3:26 am

amberwolf wrote:
Feb 11, 2018 11:24 pm
Something else not covered is electric braking: ... The way it switches the FETs is different than when it's causing current to flow to the motor from teh battery, so I don't know if there's even any possiblity of shoot-thru in this case, but I suspect there is. Even if there's no battery-shoot-thru, perhaps there could be a form of shoot-thru from one phase to another.
The only concise, non-technical description of regen I've seen (from a half descent source) is this one.

I mention it for two reasons.

1) It forms the basis of pretty much all I know about regen so far.

2) It contains this (rather long) quote and I've highlighted the bit that seems to entirely contradict your "The way it switches the FETs is different than when it's causing current to flow to the motor" above:
This type of circuit (where hi-side is turned on when the loside is off) is capable of sourcing current or sinking it. The way this works is that the reversed motor current is now a forward current to the flywheel MOSFET so when this is on it shorts out the motor - whose braking current rises during this period (arrow B, reversed). The Flywheel MOSFET now turns off, but this current must keep flowing - because of the motor's inductance. So it flows as reverse current through the drive MOSFET, recharging the battery as is does so. The extra voltage for this is derived from the energy stored in the motor's inductance. The process of switching from drive to braking is entirely automatic. Moreover it is done entirely by the motor's speed exceeding the drive voltage and without any change of state or switching within the controller. The regen braking is, if you like, a by-product of the design of the controller and almost a complete accident.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by amberwolf » Feb 12, 2018 3:29 am

Buk___ wrote:
Feb 12, 2018 2:57 am
In every controller circuit I look at, from the simplest bare-bones example with its "Main capacitor", to (what I assume to be) the most well-designed and sophisticated (from here), where there are paired (polorised and non-polarised) capacitors per phase, labelled C15/C18, C16/C19 and C17/C20 respectively; there are capacitors to prevent spikes across the battery, parallel to the top & bottom mosfet circuit, that would provide current if the battery weren't able to, should both fets be open at the same time.
Yes, those caps exist, but they are on the FET's power supply (what the FETs are switching to send to the motor), not their outputs. There are no caps on the FET outputs, with the motor phases.

Those caps only smooth the power input to the FETs; they don't smooth the PWM or commutation output of the FETs. (the motor's windings may do that)


Stated simply, I'm still failing to see how the presence of the other controller's output (detected or not) at this controller output, could cause shoot through within this controller, unless this controller is already susceptible to shoot through; in which case it would occur anyway, regardless of whether the other controller's output was there or not.

It doesnt cause shoot thru within a *single* controller--it would be from one controller into the other. If the top leg of any phase of controller A was turned on, and the bottom leg of the same phase of controller B was turned on, (however unlikely it might be to happen) and they are in parallel at the same battery and the same motor's phase connections, you now have a direct short from battery positive to battery negative, with only the FETs and the wires from the FET outputs to each other in the way.

No phase windings of the motor itself are involved in such a connection, so there's no reluctance or resistance from those to mitigate any current flow.

Even though the wires connecting the FETs are longer between the two controllers than they are within a single controller, it is still the same electrical path, and could (probably would) result in the same thing.

On a single controller, if you were to cut the traces that connect the two halves of the FET bridges, and add a couple feet of wire between them (or however long your phase wires would be to the motor input), it would create the same situation, if that controller had a situation that would cause a shoot-thru event (which it wouldnt, but two fo them in parallel theoretically could).


The capacitors in the controllers could actualy exacerbate the problem, as they can more quickly release their entire energy content thru this short than a battery can, although they have such a low energy content in total that it's not likely to be a big deal for most controllers.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by amberwolf » Feb 12, 2018 3:42 am

Buk___ wrote:
Feb 12, 2018 3:26 am

2) It contains this (rather long) quote and I've highlighted the bit that seems to entirely contradict your "The way it switches the FETs is different than when it's causing current to flow to the motor" above:
That is actually almost exactly what I"m talking about, in that it is deliberately shorting the motor phases (which because of commutation plus PWM in a brushless motor is more complicated than a brushed with just PWM, which is what the 4QD site talks about) momentarily to increase the voltage output above battery voltage, so that it can cause current to flow into the battery (and create drag in the motor to slow you down).

That kind of switching does not occur in normal motor operation, only in regen braking.

THe part at the end where he says it's a part of normal controller operation may be true (at least in his brushed controllers)--but it still makes a different current flow than in normal operation.

Remember that in the 4QD stuff, the brushed motors do not have any commutation perfrmed by teh controller--it's all mechanical inside the motor.

But in brushless ocntrollers such as those generally discussed here on ES (especially in threads like this one), the controller not only performs PWM to control the motor speed, but also performs the commutation by swithcing between different halves of different bridges for different phases.

It's still almost the same...but not quite. And that's where the problem can arise.


Keep in mind I'm not a motor expert, or a controller expert--and it's possilbe that the flow I see in my head for these things is wrong. I can't draw it; only see it in my head, or I would.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 12, 2018 3:51 am

Gregory wrote:
Feb 11, 2018 10:22 pm
Samd was posting that he'd had some successes using two controllers a couple of years ago, but no follow up on actual road use and longevity. Personally I'd just get a bigger controller. :D

https://endless-sphere.com/forums/viewt ... =2&t=75186
Thanks for the link.

Damn! I wish I had read that thread earlier, I could've saved myself some time and avoided making an ass of myself (again:().

That's the trouble with following threads by notification, you're (I) am apt to miss some posts :(

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Punx0r » Feb 12, 2018 3:59 am

Buk___ wrote:
Feb 11, 2018 10:29 pm
In every controller circuit I look at, from the simplest bare-bones example with its "Main capacitor", to (what I assume to be) the most well-designed and sophisticated (from here), where there are paired (polorised and non-polarised) capacitors per phase, labelled C15/C18, C16/C19 and C17/C20 respectively;

In the 4QD diagram the main capacitor is across the battery rails, not the output. In Lebowski's post (C15-20), I believe you are looking at teeny tiny electrolytic caps that are part of the gate drivers for each FET. I've seen these before in ebike controllers, but I've never seen an ebike or RC controller with caps on the phases.

The input caps on a controller limit voltage spikes caused by the inductance of the battery cables as the supply voltage is chopped on and off.

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Re: 2 controllers in paralell for one hub motor, is it possible?

Post by Buk___ » Feb 12, 2018 4:05 am

amberwolf wrote:
Feb 12, 2018 3:42 am
Remember that in the 4QD stuff, the brushed motors do not have any commutation perfrmed by teh controller--it's all mechanical inside the motor.

But in brushless ocntrollers such as those generally discussed here on ES (especially in threads like this one), the controller not only performs PWM to control the motor speed, but also performs the commutation by swithcing between different halves of different bridges for different phases.

It's still almost the same...but not quite. And that's where the problem can arise.
Very valid point. I tend to think of a brushless 3-phase controller, as 3 brushed, single phase controllers with interlaced and offset PWM.

The circuits for brushed are generally simpler to follow; I realise that leaves (lots of) room for missing the significance of the differences.
amberwolf wrote:
Feb 12, 2018 3:42 am
Keep in mind I'm not a motor expert, or a controller expert--and it's possilbe that the flow I see in my head for these things is wrong. I can't draw it; only see it in my head, or I would.
Understood, but I like your explanations of this stuff, because you seem to be able to write them at a level that (eventually) penetrates my thick skull. And I thank you for that!

I'm just trying to get to a level of understanding that I can think about a project I've had on the back burner for years now. It'll probably never go anywhere, but I put a lot of effort into it in other areas, and now I'm trying to tackle my bogey subject:electronics.

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