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

[J57ltr]

A little over voltage is fine but During Regen if the batteries can't handle it Its going to about 3 times the rated voltage of the FETS in which case the very thin layer of Si is damaged and the FET will short out, usually just after it's called into action again (when you take off), once one goes then the rest are sure to follow. At least that has been my experience with overvoltage and mosfets.

Jeff

[BrianG]

Originally Posted by J57ltr

A little over voltage is fine but During Regen if the batteries can't handle it Its going to about 3 times the rated voltage of the FETS in which case the very thin layer of Si is damaged and the FET will short out, usually just after it's called into action again (when you take off), once one goes then the rest are sure to follow. At least that has been my experience with overvoltage and mosfets.

Jeff

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.

[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.