Re: Victor Burnouts
 

Actually the unloaded voltage is probably the best way to tell the state of charge for a sealed lead acid battery and is a technique used in electric cars. The relationship for charge and voltage is linear. Granted you can't run a load off the battery for at least fifteen minutes or else you can't get an accurate measure of the charge but otherwise this is a perfectly valid technique. Nickel metal hydride batteries on the other hand only show a measurable change in voltage level when you've pretty much destroyed the battery. You can't use this technique for that battery chemistry.

Re: Victor Burnouts
 

The state of charge is very close to linear, but SoC (state of charge) and battery health/capacity are two different things. Also SoC tables for flooded cell (e.g. car batteries) and starved glass mat cells (e.g. the FIRST MK batteries) are slightly different.

For battery health/capacity we bought a pretty cheap static 50A battery tester from www.harborfreight.com ($10, item 93784). We added a set of anderson connectors so we can simply plug in the battery and read the voltage which provides an indication of SoC. A flick of a switch provides a short 15 second test to determine health/capacity.

For example, we had a battery from two years ago that showed good SoC after charging overnight. However it failed the load test. Its voltage dropped to 2-3v with a 50A load. This could result from a high internal resistance which may occur due to large sulfate crystals forming. Large crystal size is associated with storing the battery with little or no charge for a period of time. Who knows, all we do know is it passed the SoC test but really failed the load test.

Often after charging there is a surface charge which results in an elevated voltage and an incorrect SoC evaluation - a quick 15 second load test removes most of that and gives us a rough indication of battery health at the same time.

Team 1073 Battery Pamphlet (chiefdelphi white paper section)

Re: Victor Burnouts
 

could it be possible that the MOSFET drivers are being damaged when hooked up backwards? if these drivers are damaged, the 4 quadrants of the H bridge could easilly short from statics charges.

Re: Victor Burnouts
 

Team 1625 Winnovation had atleast 5 victors (if not more) burn up this year... Does anyone know why this would be happening?:confused:

Re: Victor Burnouts
 

We had a Victor burn out while the robot was off and pushing the robot. The team member said the victor started smoking while he was pushing it, and when he turned it on, sparks shot out. He said he wasn't pushing it very fast. I always push it until the robot has enough power to turn on the dynamic brakes. Any reason that this occurred? There weren't any signs of metal shavings and the plastic casing on the victor around one of the FETs was melted, and then the middle FET had a blow out as well. The FET on the side that melted lost its heatsink very easily when i removed it. After I replaced it, everything was working like normal.

Re: Victor Burnouts
 

Ryan,
Pushing the robot would normally not cause a failure. Components do fail from time to time and this is likely one of those. It might have been stressed during construction or had some contaminents in the semiconductor or the package cooled a little weird and pulled on one of the internal wires. My guess is the Victor was smoking before this occured and no one noticed.

Re: Victor Burnouts
 

It may have been a couple of victor generations ago, but back when I was involved in a robot competition that is know for slightly higher physical loads, there was lots of talk about the FETs in the two corners shorting. Pop of a fan and look at the FET arrangement, 2 corners have "close together" FETs and 2 have "not so close together". Some people used to place a small zip tie through the mounting hole of one each of the close together FETs to mitigate this problem.

I always use Nylon screws to mount Victors. Although they provide a plastic plate between the contacts, combinations of large ring terminals and large mounting screws can short to the chassis. It would take a combination of shorts to kill a Vic, but it is an easy fix for the peace of mind. I feel non metallic screws have more that enough strength for this job.

Travis

Re: Victor Burnouts
 

...Maybe you pushed it so fast that there was a high enough backdrive voltage into the junction that it crossed an insulator within the FET itself? :confused:

-q

Re: Victor Burnouts
 

I'm not really sure still... Here are some pictures of the charred remains of the FETs.

Re: Victor Burnouts
 

I think that the color of the FET heatsink tab tells all. It indicates that a lot of current was passing through that device i.e. lot's of heat. Since there are three FETs in parallel to share the current, it is obvious that that side was not sharing. Two things are possible: 1) either the device was defective and started to produce a lot of internal heat or 2) the other transistors were not pulling their weight. They may have gone open or there is another failure on the board.
There is always an outside chance that a stray wire came in contact with the tab on that FET. It happens more often that you think. Examine the tab closely for signs of a high current entry point. It will be rough in texture and exhibit more discoloration than the rest of the tab.

I wanted to speak to the no load voltage measurement for batteries. One must pay homage to Ohm' Law when making measurements with modern equipment. A typical Fluke meter has an internal impedance in the Megohm range. Ohm's Law solved for voltage drop with that kind of load, will produce an imperceptible voltage drop across the internal impedance of the battery. (what is the current supplied by the battery into a 1 Megohm load?) Hence, even a depleted battery when cool can measure nearly full voltage with that type of meter. Add to that, the varying resistance of the connection of the meter probes to battery introduce significant error even in fully charged batteries. I have checked batteries from various teams that show good when fully charged but have one or more cells that have a reduced capacity after delivering current for a few minutes. This indicates to me that a plate(s) internal to that cell have become disconnected and therefore no longer supply current under load. The CBA II battery analyzer from West Mountain Radio is a simple device to check batteries under constant load (about 7.5 amps) and performs a nice plot of time versus terminal voltage. The data can be stored for each battery, recalled and compared with the subsequent tests of the same battery over a period of years. Data can be overlayed for several batteries as well. West Mountain also has a higher current addition to their line of products but we have not purchased one yet. http://www.westmountainradio.com/CBA_ham.htm

Re: Victor Burnouts
 

You must remember that the robot was off when it started releasing smoke. I believe it was probably a defective FET (the only one that is not burnt to a crisp) that caused the problems, and while pushing the robot back to the classroom, the two FETs that were being back-driven and power the robot failed catastrophically releasing the all to familiar "Magic Smoke." It was one of the new Victors (with the blue Victor 884 label) I feel that the build quality of the new ones aren't up to par with the older ones that actually used screws. I still like the old 883s, and for just a little less tolerance, I would still like to see the 883s back in the robots. They are rated for more, and I think that's the best thing about them. It doesn't make sense to me to have an 884 Victor that can only handle 40A continuously with a breaker that can handle 40A continuously. I feel that the defective victors have the weak point and will fail before a breaker trips. The 883s would have a little leeway before they burn up. Personally, I've never seen an 883 fry, but I've seen plenty of 884s fry. I'd go for an 884 that can handle 60A any day, and maybe we'll see a speed controller in 2009 that does exactly this.

Also, I'm not sure where the stuff about the battery and the FLUKE meter comes into play in this thread... Just to let you know in case you wanted to put this in some other thread.

Re: Victor Burnouts
 

Remember that circuit breakers are meant to protect the wiring from failed components. They're not intended to protect components from failing.

Re: Victor Burnouts
 

Ryan,
Don't be fooled by the spec sheet. There are many improvements in the 884 that make it a great device. The FETs in the 884 are rated at 64 amps each so in parallel they can handle continuous 130 amps easy. Remember that the Chalupa motors were not part of the kit when the 883 was available. The small motor is rated at 129 amps in stall. You can fudge the specs a little since the FETs have no heatsink but do have a fan and are only reguired to supply current intermittantly over a two minute match. Heatrise in the devices is the more critical event in using these little wonders. (derate the 64 amps to 45 at 100 degrees C) For most cases there is no problem.
With today's technology, even manufacturing defects are rare. The vast majority of failures I have been able to inspect have turned out to be user induced. i.e. metal shavings, a foreign body contacting the tabs of the FETs or continuous driving with an inefficient drive train. As I have pointed out in many threads before, tank style driving produces incredible loads on the motors when turning, near or at stall for the motors. By far and away, I have seen more smoke in a turn (or pushing in a turn) then at any other time.

Re: Victor Burnouts
 

hey fred- its always good to have a trouble shooting list (for everyone not just fred) so that you can check for the simple stuff first then progress from one to the next and narrow it down to what you think it is and keep testing testing testing. and yes replacing the SC is the obvious one but we had a wire slip 1/2 way out of its contact and we had no idea why the SC kept cutting out and shuddering plus getting very hot. so another step i have for you is to check your contacts!!! among other things- but ill leave it to you to create your own list- trust me its different for all of us

Re: Victor Burnouts
 

ok- time to make myself sound like a noob- whats an FET? ive been in FIRST for 3 years and im a senior and ive seen some pretty crazy electronics systems. for instance- 357 has a self designed omni drive that is some of the best engineering i've ever seen. but part of that engineering was to tear a huge hole in the RC and add their own circuit of an original design into the system. pure genius. I digress, if anone could help me with my question(s) that would be great