My brushless Revo - Page 3 - RC-Monster Forums

BrianG · 08.12.2006, 08:58 PM

Lol. Those little things can bite ya. :)

If you think it wheelies good at ~9.6v, just wait until you get it working right at 19.2v!

coolhandcountry · 08.12.2006, 09:13 PM

If you got 25 like that you should hit the 45 mark.

Finnster · 08.13.2006, 03:33 PM

Hmm mixed bag of results... I made a y and wired it correctly. Lots of power now. :017: Surprizingly it doesn't want to wheelie now?:032:

Part of the prob is that it still stutters from a start. It surges *vroom...vroom vroom...vroom* then get to ~10 mph then just bolts to full speed. The back end def squats down and takes off. I would guess top speed to be high mid 30's currently.

I wonder if that cheap Ratshack cap is not upto the job. Seems like its missing a phase or something. The basic specs are the same 220uf, but it only 35V and 60*C rated, and non-fast switching. I've ordered some better caps from allelectronics.com, but I don't see them listed as fast-switching. I hope they can do better. I can't find a replacement Rubycon cap in a quant less than 400, so I'm a bit stuck. Also, the esc is still heating way up. It got to ~200F again within a pack (4-5 min really.) I may have already partially killed the RS cap with the temps. Perhaps that's why its misbehaving more badly at low speeds (was ~ok first run.) I should have just bought the esc new. Thought that as soon as I won it and have been regretting since. Maybe I'll get it working, its almost there.

May have to wait till mon/tues when the new caps come and see what happens. I will def need a roll bar. Got too used to flinging the rusty in the turns, and you just can't do that on such a big MT and expect to stay on the wheels.

BrianG · 08.13.2006, 04:17 PM

I don't see why those caps don't work right. Maybe it is the temperature since the traces may get quite warm and the heat will tend to be sinked through them.

High speed caps would be more of an issue for oscillator or high frequency filter, but I think (please correct me if I'm wrong) those caps are actually to provide a little amperage oomph during high current spikes.

Anyway, it will be interesting to see how those new caps work. If you are concerned about the working voltage, I found some inexpensive ones here. Temp ranges from -40 to 85C. These are from partsexpress so they tend to use the higher quality caps for use in audio crossovers so they may be faster switching as a result even though they don't specifically say that.

Finnster · 08.13.2006, 07:45 PM

Quote:

Originally Posted by BrianG

I don't see why those caps don't work right. Maybe it is the temperature since the traces may get quite warm and the heat will tend to be sinked through them.

High speed caps would be more of an issue for oscillator or high frequency filter, but I think (please correct me if I'm wrong) those caps are actually to provide a little amperage oomph during high current spikes.

Anyway, it will be interesting to see how those new caps work. If you are concerned about the working voltage, I found some inexpensive ones here. Temp ranges from -40 to 85C. These are from partsexpress so they tend to use the higher quality caps for use in audio crossovers so they may be faster switching as a result even though they don't specifically say that.

Honestly I don't know why they are used as such. But pulling the Rubycon part # off the caps, the spec sheet lists the caps as high freq switching (or whatever exactly.) I'm just trying to replace the caps with as close a match as possible. The whole spec was 220uf, 50V, 105C. The allelectronics caps I bought are exactly that. If that does not correct teh problem (I'm worried it may not) I will have no choice but to sent it off. My best hope is that it is the caps as its something I can repair cheaply.

I know what you are saying about the amp spikes, but on the other hand I don't know why BK would have used 4 small caps for that. It seems easier and more reliable to have used 1 large cap. The caps are only 220uf, so they aren't really that large at that. When one of the caps was bad, the esc did not work properly at all.

I'm concerned about the heat build up, something is casuing a high amp draw.I don't see it being the setup. It got warm even whenI tested it in a 2 min drive in my rusty. Worst case would be some internal short Would a bad FET do something like this? Dunno, I don't see a whole lot else there. Nothing smaoking or burning, so not a whole lot for me to go off of. I have a feeling there is more power there too which is not getting to the wheels. I would think the 8xl/16 cells could break 40. Its still off that by a bit. We'll see what Monday brings I guess.

In the meantime I've got to figure out a roll bar. I don't want to crush the motor or esc on a bad flip or landing. The stock body is pretty light& flimsy and doesn't offer a whole lot of protection in a crash. When I get everything together I'll put up some more pics.

BrianG · 08.13.2006, 10:05 PM

Quote:

Originally Posted by Finnster

...I know what you are saying about the amp spikes, but on the other hand I don't know why BK would have used 4 small caps for that. It seems easier and more reliable to have used 1 large cap. The caps are only 220uf, so they aren't really that large at that. When one of the caps was bad, the esc did not work properly at all.

I'm concerned about the heat build up, something is casuing a high amp draw.I don't see it being the setup. It got warm even whenI tested it in a 2 min drive in my rusty. Worst case would be some internal short Would a bad FET do something like this? Dunno, I don't see a whole lot else there. Nothing smaoking or burning, so not a whole lot for me to go off of. I have a feeling there is more power there too which is not getting to the wheels. I would think the 8xl/16 cells could break 40. Its still off that by a bit. We'll see what Monday brings I guess.

Actually, several smaller caps in parallel have a better transient response (charges and discharges faster) than a single large cap. Plus, multiple smaller ones are usually cheaper and easier to mount, but may take up more physical space than one would. Those four 220uF 50v caps have the same capacity as one 50v 880uF cap, if in fact they are all wired in parallel.

As to your heat question; FETs have an "on" resistance. Having several in parallel (like these ESCs do) effectively decreases this total "on" resistance. If one FET has, say, 0.1ohm of resistance, then ten of them in parallel will have 0.01 ohms of resistance. Higher resistance creates more of a voltage drop which, combined with the current, generates heat. Less resistance, less heat. The point to this "lecture" is to say that if you have one or more burnt out FETs, then the total resistance goes up, creating more heat for the same current. I don't know if this is the case in your situation, but it is a possibility.

Something like the Eagletree system would record your battery current for you and tell you if you had too much amp draw.

Could be one or more phases on the motor itself is bad but I doubt it's that since it ran perfectly fine with my Bk 12020, and I really didn't use it all that much anyway. If you had a meter with a "conductance" mode, you could get accurate readings of the windings. With a regular meter, the winding resistance is so low that it will look like a short when it isn't. The most you could do here is check the resistance from each terminal to the motor case - it should be an open circuit for all phases.

An interal short would definitely blow something, but how would that happen? Even water doesn't short electricity unless it is very high voltage, it's the corrosion that gets ya. Most semiconductors usually open when they give up the ghost as opposed to shorting. Kinda like a fuse; too much current will burn up the small parts inside the devices.

Sounds like the ESC to me, hopefully the new cap will help. While you are at it, you might want to replace all of them. If they are all in parallel, what happened to one happened to them all. Maybe only one totally crapped out, but the others may be weakened.

Finnster · 08.15.2006, 06:54 PM

Well I had bought a number of caps just in case of that very reason. Re placed all the caps and still no luck. The caps I got from Allectronics are very nice, looks like a direct replacement, brand is Tesco.

Unfortunately that did not solve the problem. Initially it was a bit more responsive, but got very hot very quickly. Even the caps were very hot. This was just trying to make it go back and forth in my basement. All I can think of now is that something is worng with the fets or something less replacable like that. The Bec jumper is def pulled BTW.:019:

I think I am just going to go with another esc. This thing is a pain, and I'm not sure worth the effort. The rest of the truck is really good and its lame to let the esc hold it all up.

squeeforever · 08.13.2006, 07:47 PM

Finnster, a wing works wonders :p.

Finnster · 09:21 PM

Yeah I am curious how much it would be to fix. I did get it really cheap knowing it wasn't working properly. It was hoping it was something easy like a busted cap, but planned for the worst case if it had to send it back. Either way it ends up being a decent deal, just one way better than the other.....

I heard ~$75 at some point but RCMMike said it would depend on the damage and I wouldn't know until I sent it in. Was just going to do that, but then was worried they'd quote me $100+ after I've shipped it all the way to Germany.

Other than the BKs, really only the Q125 fits the bill. I'm just miffed at the $$$ price. Like I said, I only paid $280 for the truck, but it was brand new (couple tanks) and w/ a never-started 2.5R. After I sell all the xtra nitro stuff the roller would only be ~$150 or less. Silly thing would be the most expensive thing on the truck... I dunno. Maybe I'll just sulk about it a while... :( Although the few mins it does run its absolutely awesome.

Finnster · 08.15.2006, 09:26 PM

Oh, BTW Brian, I do ahve a nice Fluke multimeter. I'm not sure about "conductance" mode, but I did measure the resist of all motor leads to the case, then to each other. To the case all the readings were "OL" (over limit) and between all the leads to each other was .2 ohm. Not sure what that tells me tho. I don't think its the motor tho. I had hooked the 9920 to my 380c and it was behaving badly with it as well.

BrianG · 08.15.2006, 10:37 PM

Conductance is just the inverse of resistance, but would definitely be another mode on the meter. I have a Fluke 87 and it doesn't have it. Also, you'd need near-zero loss meter leads to accurately measure really small resistances (or large conductances). The .2 ohm coils was more than likely just your meter lead resistance. Short them together for a baseline value - it'll probably be about the same. An "open" from the wires to the case is good. At least you know there isn't a phase or two shorting to the can.

Finnster · 09:08 PM

Well, I went ahead and tried a CC Mamba Max ESC and tossed the BK aside for repair. I pulled the BEC and and put it 14 cells. My impressions? :004: :027:

It ran really nicely and thru 2 packs it did not thermal. The Esc was running ~150, but the motor did get up to 160 by the end. It had been hovering around 140-150 most of the time. Some hill climbing, high speed stuff, running around in grass, and a bit of psuedo-crawling.

It was a nice little bash session, but nothing too crazy, I was just really testing the setup and the esc. The CC esc is absolutely amazing, what a little power house. I can't wait for the monster maxx. Before anyone jumps in, yes, I know it is running the ESC above spec. But I've done things like this before with success. I'm taking it easy on the esc, and so far it is doing well. I do have a 25mm fan I want to mount, but allelectronics sent me the 20mm fan instead, so I need to exchange it.

Top speed was somewhere just south of 40 I would guess. I didn't have my gun on me, but I did enough testing with the Rusty that I would say this is a pretty close number. The truck handles awesome. Much better than my maxx ever did. It was gripping well and would tend to slide the rear end out on most surfaces and only rolled at high speeds on high traction asphalt. Ac ouple crashes were a bit ugly. I really need to get a roll bar in there. :P I got a couple ideas.

The only real worry is the motor temps, it getting hotter than I was really expecting, so I'm wondering if I need to go down a tooth to 14t/51. Of course its the one I didn't buy... Could take a little pressure of the esc too which wouldn't hurt. The esc is really smooth and doesn't cog. The revo will wheelie with ease. I'll post some mroe pics later. Thanks to everyone, esp Brian for all your help.

BrianG · 08.20.2006, 10:12 PM

Wow, great. Although you are braver than I by using more cells than specified. I know manufacturers build in a safety zone, but I hate running higher voltage/current than rated. That little ESC must have been lost on a Revo - it's about the size of a receiver module!!

BTW: glad to help!

Finnster · 08.20.2006, 10:32 PM

Quote:

Originally Posted by BrianG

Wow, great. Although you are braver than I by using more cells than specified. I know manufacturers build in a safety zone, but I hate running higher voltage/current than rated. That little ESC must have been lost on a Revo - it's about the size of a receiver module!!

BTW: glad to help!

Haha, I'm not sure how brave I was considering a CC engineer said it could be done (voltage wise anyway.) I don't think I'd take this thing racing, but so far so good, lets see how it holds up.

Serum · 08.21.2006, 12:29 PM

I heard more of the BEC issues with more than 12 cells. and if i recall correct, the mamba maxx esc was initially meant for 14 cells.