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

[Unsullied_Spy]

Taller gearing will put more load on the ESC, I'd assume that would translate into more stress on the ESC and therefore not safer for MMMs.

My Muggy weighed in around 14 pounds at one point running on 6s and geared for (including tire ballooning) 90 MPH and I ran the crap out of it for about 12 minutes and it still works to this day. That's not being used intelligently and I just had the Muggy out again earlier today and the MMM is still kicking strong, they aren't as touchy as the internet drivel leads many to believe.

EDIT: I should mention this was on 20-30C Turnigy 5AH lipos and they were around 100 degrees after the run and didn't seem to cause any issues, not all Chinese packs are bad...

[lutach]

If I had something to do with this, I do apologize. Now if taller gearing or lower gear ratio is desired for a pure rush of sexy Ferrari speed, then everything must be up to par and that includes the batteries used which should be also mentioned here. The lower one goes on the ratio, the hungrier (Not sure if this is an actual word ) your motor will get and the ESC will have to provide the juice. The main cause of failure of a ESC is going over the AMP rating of the MOSFETs. A lot of engineers I speak with tells me a MOSFET will usually handle a little over voltage better then going above the AMP rating. Now the bigger pinion will make a vehicle go faster to a point the owner of the vehicle will have to let go of the throttle sooner letting everything take a breather or it might just be that the motor is not wasting precious energy in unloaded heat. Who knows, I'm just a noob at this brushless thing.

[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).

[RBMike]

Originally Posted by Unsullied_Spy

EDIT: I should mention this was on 20-30C Turnigy 5AH lipos and they were around 100 degrees after the run and didn't seem to cause any issues, not all Chinese packs are bad...

I think your missing the point on the cheap batteries. The point is, they don't screen out the bad cells so you could buy 3 of those packs & have 2 of them work fine but the 3rd one might have a higher resistance cell. It could cause a bunch of ripple to esc & kill it.

So your right, not all Chinese packs are bad but you will never know if your buying the "bad one" or not. The brand name companies (that charge more) are taking the time to test & screen out the bad cells & NOT assembling them into the packs they will sell. That's what your paying for.