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

[Pdelcast]

Originally Posted by BrianG

Exactly. Some people don't seem to realize failures sometimes don't come from just excessive current or excessive voltage, it's a domino effect. The high voltage "punches" through and damages dialectrics. And once that is done, even if the high voltage condition goes away, high currents can flow damaging it even further, and usually much more noticeable via fire/smoke.

Of course, damage can be done just by high currents without high voltage.

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

[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.