MMM V3 problem with heatsink

[Pdelcast]

Originally Posted by BrianG

Huh? If the HS does happen to short against the HS, it would definitely matter. You'd be shorting the offending phases together and probably damage the FET(s). I don't think Castle is saying it's ok for the motor connections to touch, they are just saying they could not get the HS to actually short the phases.

But yes, I agree, get rid of those silly plugs and use wires instead. I think they want to use bullets because their CC-Nue motor will have wires with bullets on the end and will offer more of a plug-n-play setup as opposed to soldering anything.

Or maybe use solder-posts (like Mtronics) or thick tabs (like Novak) on the motor outputs instead since they will be smaller in diameter and have much less chance of touching the HS. That way, people wouldn't be soldering directly on the PCB...

Hey Brian,

Well, when you consider that our motors have lower resistance than the heat sink (by about 1/2) -- you can see why the heat sink shorting a phase really isn't an issue. The heat sink actually ablates (burns) away if the bullet manages to get pushed into the heat sink, without damaging the FETs.

One of the tests we do when we stress test our ESCs is to short the output phases during operation -- at low, mid and full throttle. They are designed to survive.

[BrianG]

Originally Posted by Pdelcast

Hey Brian,

Well, when you consider that our motors have lower resistance than the heat sink (by about 1/2) -- you can see why the heat sink shorting a phase really isn't an issue. The heat sink actually ablates (burns) away if the bullet manages to get pushed into the heat sink, without damaging the FETs.

One of the tests we do when we stress test our ESCs is to short the output phases during operation -- at low, mid and full throttle. They are designed to survive.

In another post somewhere, I do mention that the HS would most likely ablate (although I used a less fancy word ) if it does happen to touch. I'm glad that these things are tested so vigorously, but shorting phases on purpose can't be good!

And I understand your point about the HS having more resistance than the coils, but the HS does not have any back-EMF that coils do. So, does that rule still apply?

Also, if a motor has X ohms of resistance, and the HS has X*2 ohms of resistance, then the two (effectively in parallel during a potential short) has X*0.66 ohms of resistance (until the Al ablates). To me, that's just pushing it for no reason. Even if the FETs don't mind this, this can't be good for the batteries.

As far as people modding things based on advice from these forums, I can take partial blame I guess. But, that was for previous versions and I did provide disclaimers (more than once). The only mod I have suggested to a V3 is to place a thin piece of lexan between the bullets and the HS, which of course needs no disassembly or soldering, and provides a little piece of mind (to me anyway). Believe me, I don't claim (not even close) to know as much about this stuff as a true engineer such as yourself, I just get a little apprehensive when I see things I don't feel is right.

[Pdelcast]

Originally Posted by BrianG

In another post somewhere, I do mention that the HS would most likely ablate (although I used a less fancy word ) if it does happen to touch. I'm glad that these things are tested so vigorously, but shorting phases on purpose can't be good!

And I understand your point about the HS having more resistance than the coils, but the HS does not have any back-EMF that coils do. So, does that rule still apply?

Also, if a motor has X ohms of resistance, and the HS has X*2 ohms of resistance, then the two (effectively in parallel during a potential short) has X*0.66 ohms of resistance (until the Al ablates). To me, that's just pushing it for no reason. Even if the FETs don't mind this, this can't be good for the batteries.

As far as people modding things based on advice from these forums, I can take partial blame I guess. But, that was for previous versions and I did provide disclaimers (more than once). The only mod I have suggested to a V3 is to place a thin piece of lexan between the bullets and the HS, which of course needs no disassembly or soldering, and provides a little piece of mind (to me anyway). Believe me, I don't claim (not even close) to know as much about this stuff as a true engineer such as yourself, I just get a little apprehensive when I see things I don't feel is right.

Hey Brian,

Well, in the experiments we did here, the heat sink ablates so fast it doesn't even warm the FETs much. (short term surge currents of 1200A don't hurt the FETs or batteries, but it sure vaporizes some aluminum pretty quickly!)

[BrianG]

Originally Posted by Pdelcast

...short term surge currents of 1200A don't hurt the FETs or batteries...

Well, I wouldn't say that about all batteries. Some are, shall we say, optimistically rated...

[brushlessboy16]

*cough, cough* maxamps..

[suicideneil]

Facinating stuff- I still like the wires vs bullets option though- fewer issues, even if its purely all in the mind.

[hootie7159]

lol....crapsamps...