Higher pinions = safer MMM's? Or no? Someone's lying....

[J57ltr]

Originally Posted by Pdelcast

That's close -- but the actual FET damage during braking is actually caused by current, not by voltage. It's an effect of voltage, but the damage is done by current.

What happens is this -- the voltage rises until the FETs avalanche -- that is, they start acting like a zener diode. On the MMM the FET avalanche voltage is around 44V-48V. When the FETs avalanche, they provide a low resistance path for current, and the instantaneous currents can be thousands of amps. This can (rarely) be high enough to damage the interconnects inside the FET itself.

We RARELY see this type of damage on an MMM though-- because the FETs on the MMM are tough enough to handle repeated avalanche currents without failing.

Much more often we see a power supply issue, where a part in one of the power supplies fails from voltage stress. We have been (and continue to) toughen up the power supplies on the MMM to help prevent these issues.

Patrick

And by power supplies are you talking about the drivers for the fets or the BEC, or something else?

Thanks,

Jeff

[Pdelcast]

Originally Posted by J57ltr

And by power supplies are you talking about the drivers for the fets or the BEC, or something else?

Thanks,

Jeff

No, I'm talking about the power supplies themselves. There are three on the MMM -- one for the processor (3.3V), one for the FET drivers (12V), and one for the BEC (6V).

[J57ltr]

Ok kinda what I thought.

Now on the large pinion smaller pinion issue is it safe to say that having a numerically higher gear ratio will cause more back emf which could exacerbate the problem if the batteries are sub par?

Thanks again,


Jeff

[Pdelcast]

Originally Posted by J57ltr

Ok kinda what I thought.

Now on the large pinion smaller pinion issue is it safe to say that having a numerically higher gear ratio will cause more back emf which could exacerbate the problem if the batteries are sub par?

Thanks again,


Jeff

In 99% of the cases, that is exactly true. Smaller pinion = less ESC stress.

[J57ltr]

Originally Posted by Pdelcast

In 99% of the cases, that is exactly true. Smaller pinion = less ESC stress.

Lol! By numerically higher I meant using the smaller pinion could cause a problem on subpar batteries due to the motor spinning faster from the start of braking.

Jeff

[BrianG]

Originally Posted by Pdelcast

In 99% of the cases, that is exactly true. Smaller pinion = less ESC stress.

Which brings us back to the original question in the thread title. See Freezebyte, we get to the answer eventually.

Patrick, I edited an earlier post apparently after you responded. Any ideas on offering a TVS/capacitor module one could solder to the MMM power wires (not to the PCB)? Definitely not a replacement for good quaility batteries, but would provide some peace of mind.

[Pdelcast]

Originally Posted by BrianG

Which brings us back to the original question in the thread title. See Freezebyte, we get to the answer eventually.

Patrick, I edited an earlier post apparently after you responded. Any ideas on offering a TVS/capacitor module one could solder to the MMM power wires (not to the PCB)? Definitely not a replacement for good quaility batteries, but would provide some peace of mind.

Pretty good idea. I'll look into it.

[J57ltr]

Originally Posted by J57ltr

Ok kinda what I thought.

Now on the large pinion smaller pinion issue is it safe to say that having a numerically higher gear ratio will cause more back emf which could exacerbate the problem if the batteries are sub par?

Thanks again,


Jeff

I asked if having a smaller pinion could cause more back EMF that could cause a failure if the batteries are up to the task, to which the asnwer was

Originally Posted by Pdelcast

In 99% of the cases, that is exactly true. Smaller pinion = less ESC stress.

So basically to me he is saying that in 99% of cases having the smaller pinion can cause this problem with the regen braking. But he ends with the statement of smaller equals less stress.

Those two statments given that Patrick are opposed to one and other unless he meant 2 different things.

Small pinion+bad batteries= damaged ESC

And Small pinion= less stress on the ESC under load.

Or he missunderstood higher numerical ratio.


Conclusive answer?

[Pdelcast]

Originally Posted by J57ltr

I asked if having a smaller pinion could cause more back EMF that could cause a failure if the batteries are up to the task, to which the asnwer was



So basically to me he is saying that in 99% of cases having the smaller pinion can cause this problem with the regen braking. But he ends with the statement of smaller equals less stress.

Those two statments given that Patrick are opposed to one and other unless he meant 2 different things.

Small pinion+bad batteries= damaged ESC

And Small pinion= less stress on the ESC under load.

Or he missunderstood higher numerical ratio.


Conclusive answer?

sorry -- I guess I misread the original statement. Higher gear ratio (smaller pinion) = less stress (lower reverse currents to the battery, lower ripple voltage.)

Any time you let the motor rev more freely, it is easier on both acceleration currents and braking currents.

[J57ltr]

Originally Posted by shaunjohnson

eagle tree can be set to below 1 second if needed i think...deffo not 10secs though!!

That's 10 times a second or 10 Hertz or every .1 second or 100 milliseconds. It's still not fast enough.

Mike,

I guess what I am trying to say is that the people that are having these problems more than likely don't make any other changes than the pinion. They don't change braking force all they do is slap in the new pinion and go for it.

I see it as downshifting, your rolling down the street at 40mph and shift to 3rd then you slow to about 30 and drop it in second the engine reves much higher than it does when your at 30 then you were at 40, basically you are changing the gear ratio.

I never missed that they are traveling at different speeds but when I ran some numbers I didn't some up with enough of a difference.

Could you show your math? I would be interested to see what you came up with. I (according to the speed calculator Brian has made) I only say an 8.62mph difference between the 2 pinions. I would really like to see what you have for total energy that is being dissipated to come from top speed to a complete stop from the 2 different gear ratios with the weight involved.

I also see times where the RPM of the motor is not the same given only the gear ratio change I see a lot of times if the motor is slightly overgeared that it doesn't reach it's calculated rpm.

"Likewise, a smaller pinion makes the motor spin faster per mph, and when braking the motor spins faster creating more back-EMF. I can understand the thought process, but the few tests I've run show that low gearing creates the highest back-EMF voltage, but these bursts are short and current is lower; while high gearing creates lower back-EMF voltage, the highest back EMF current, and the pulses lasts longer."

Brian, is that low gearing (numerically high) and high gearing (numerically low)? Also what equipment were you using to make these observations?




Granted I don't know what the ratios are in the Flux and haven't really looked, I just used the Savage as the example with all things being equal except the pinion size.



As Brian pointed out that the MMM is to close to it's limits on 6S which it is rated for. Patrick even said that adding an external TVS and cap setup would be a great idea. This is something that tells me that the unit is not sturdy enough for the general population (I feel for Castle tech, I really do).

That's one of the sticking points I have with the MMM. Not enough real useful information on operation. At the onset before the MMM came out there was no mention that the 2650 should be used for 4S and the 2200 for 6S, there wasn't anything about the batteries other than A123. Granted we already went over this and I know that Castle can't test every battery but there isn't anything on their website as what are good readings and what are bad readings for batteries. All you see is what the MMM is used with in the RC rags. There simply is not enough technical information to be had. I know that Patrick has said that people glaze over and they might lose a sale to China. What? I can't even understand the manuals since they are in Engrish, if I have a warranty problem do I really want to wait for 3-4 months for a replacment shipping overseas. No I don't.

I think Castle really needs a better manual even if you alienate a few people.
I have said it before I have a ton of experience in many different fields, but if I can't get a handle on everything involved, what makes you think Joe 12 pack is?

Patrick,

Thanks for the clarification.


I have asked for clarification on many points, not because I doubt you guys, but the old "trust but verify" is something I live by.

Jeff