Victor Burnouts
 

This year for the first time we had recurring problems blowing victors.

The (6) FETs on one direction of the “H” would all burn out for some reason. Luckily no underlying damage was done, so it was only a matter of replacing 90 cent FETs, but we haven’t come up with a definitive reason for the failure.

The obvious suspects are metal chips, but the victors seemed to be clean when they are pulled apart.



Any of you have unusual victor issues this year, or have other interesting theories on blowouts?

Re: Victor Burnouts
 

When do they blow for you? What where you doing when they failed? High amperage draw or just some 'fun driving' out-n-about with the robot...

I don't think we had any problems....

Re: Victor Burnouts
 

Oddly enough nothing in particular. One on our wench blew as we were doing a systems check before it went into the crate on the ship date and pulling the wench cable out by hand. Luckily, only one blew during actual competition, so that one on a drill was probably stressed, but not unusually so. It wasn't during a pushing match.

We didn't have any problems with victors we'd rebuilt, so I suppose it could have been a bad batch. I found it interesting that all the FETs would be burned out, but that may have been because we weren't noticing when just one FET went and only recognized it when power to a motor was gone completely after the remaining FETs were overloaded.

Re: Victor Burnouts
 

Were you powering the fans and were they running at the time of failure? What exactly was the failure? Did the FETs short or open? Did the circuit breakers click prior to the failure?

Re: Victor Burnouts
 

We had at least two Victors fail on us. We had a few extras and didn't have time to mess around with figuring out what was wrong, but, yes, we did have problems with them... :)

Re: Victor Burnouts
 

My team had no problems with the Victors this year. We have run the robot even harder than we did at competition once we got it back. Today we ran it for almost 2 hours straight with breaks every once in a while, pushing objects around, showing off, etc. and we still havent had any problems..........even after carrying me around. I do know that we check to make sure our fans are running before we start up the bot for an event but they have been fine. I still havent had a Victor die on me (883's or 884's) though i have melted the pwm cables going to them :cool:.
-Aaron
[edit] Also what type of drive system are you running? Are the Victors dealing with high current loads often? [/edit]

Re: Victor Burnouts
 

We had no problems with the victors (or fuses) at all this year. On the other hand, our driving was done with 2 drills, the bosch gearbox and a 2wd setup - According to what I've been told, that setup puts a fraction of the strain 4wd and 6wd bots on the motors. We ran the same 2 motors for almost our entire build season, the sacramento regionals (9 seeding rounds, and a few quarterfinals), and the practice day at atlanta. We saw the bot was moving slower than usual, and the driver said it was reacting slower. We had 4 extra drills in boxes, and they didnt take too long to swap.

We had spare victors ready to go, because we were told (and read on CD) about the magic smoke. Hopefully innovationfirst doesnt change them for the 2005 season .. and we can use the spares if we need to :D

Re: Victor Burnouts
 

Fans were always powered, we've had no circuit breaker's popping.
I believe the FETs opened, but I'll have to verify and get back on that.

Re: Victor Burnouts
 

We had a lot of problems with them at Chesapeake I believe. Twice I think, we extensively went back over the code and wiring and then it was like "Maybe we should try a different speed controller." and the new one worked. I dunno if we ever figured out what it was though. Too busy trying to get the robot ready.

Re: Victor Burnouts
 

our fan blow on us once. It was very mysterious. We were testing out a camera on the robot by driving around just the day before. when we came in the next day, the fan just wouldn't work. Nothing serious, just replaced it, but its very puzzling.

Re: Victor Burnouts
 

Which FET's did you use to refurbish your Victors, seems the guys repairing them on the Battlebots forums are finding the matching replacements costing MUCH more than .90 ea??

Re: Victor Burnouts
 

Were the cases damaged? i.e. blown off or burned. Simple electrical failure of some of the FETs without case damage, may indicate a static problem. Were the same FETs involved in all cases with the same motor? That might be an indication of a bad brush assy. or a bad drive part in one direction.

Re: Victor Burnouts
 

No case damage at all. All damage was limited to the FETs alone. The underlying substrate was unaffected.
It almost always involved different motors/speed controllers each time it happened (globe, drill, CIM). We never had a problem with a rebuilt Victor, and we didn't replace any of the motors this year. By Nationals all Victor problems had gone away, but we'd replaced/rebuilt a half dozen by then.

Static is a possibility.
Any ideas on tests we can run in the off-season? I'd like to make it into a research project for the students, but I don't want to risk expensive mistakes.

Re: Victor Burnouts
 

The type of failure you describe is unusual from my experience. Even the number of devices replaced is unusual. Is there a possibility that your controllers were mishandled before install? i.e. students walking across carpets and picking up the controllers. Were they ever used in another application, testing or demo? Ever installed near bare metal or wire? Ever connected directly to a battery and motor outside of a robot without circuit breaker protection? The controllers I have experienced with failure due to over current or foreign material show charred and blown apart cases, high current types of damage and burned circuit boards. On our team we regularly use 8 or more controllers per robot. Over the years with both a competition and proto robot running near year round we have only killed 2 or 3 controllers in the years since they were introduced. (Wildstang has been around since 1996 but there were Tekins then and they failed all the time.) At least one of those was due to drill filings falling into the case. Have you disabled the dead zone in software? Is there a possibility that the controllers were always "on" just a little? Do you calibrate the controllers? Can you show a picture of how they are mounted in your robot?

Re: Victor Burnouts
 

There's no sure way of always knowing how students handled the controller's while the engineers weren't in the lab. While I was there they were being careful, but there were a lot of new students this year. The lab has no carpet, but clothing could still generate static. As a related measure we didn't have trouble with the IR sensors being blown by static electricity as some of the other teams I was helping did, but that was a different sub-group with a much more limited number of students.
The electrical layouts were reworked many times as mechanical designs changed, so there was a lot of controller handling (but no powering up).
Some of the controllers were taken off last year's robot when we started to run shy, so those were reused.
They were never used or installed without circuit breaker protection, or near bare wires.

We've never blown victors before, so initially we thought it was foreign object debris, but your description of the damage caused to the Victor case and the lack of other evidence seems to reduce the probability of that as a cause.

The dead zone is restricted by software with a smooth increase in power outside the dead zone compatable with the measured Victor deadzone. We verified via the dashboard the PWM output as neutral when it should be. The new joysticks didn't usually center properly (weak springs). The blown Victors were a mix of factory calibrated and team calibrated. We did calibrate all later ones, because we weren't measuring full power out of the Victors in bench tests with the factory calibration.
We had an issue early on with the drill/CIM victors receiving 127 according to the dashboard, but sometimes passing low power through to the motors anyway. That could be a related symptom. That problem also seemed to be gone by Nationals.

The wiring photos I have don't show the Victor mounts clearly. I'll have to wait for the robot's return from Nationals.


I'll check on the actual FET price and source (retail or wholesale) and get back to you.

Re: Victor Burnouts
 

Mark,
This is all very puzzling...Is there any chance that the controllers that failed were installed backwards or two were wired to one motor? I know I sound strange on this but I am grasping at straws. Everything seems in order except the unusually high rate of failure. In my regular job I repair and maintain electronic broadcast equipment. Finding a smoking gun (cause to the effect) is what I am trained to do.

Re: Victor Burnouts
 

Thanks Al, I really appreciate your help with ideas, maybe especially the "straws!" We too are grasping at straws. I'm only leery of straws when implementing potential solutions starts adversely affecting the operation of the robot. I had to fight to keep the Victor's on "brake" this year, because we used coast exclusively the last few years and that change was a natural suspect. Any issues with "brake" that you know of?

From your experience if one FET goes would the others naturally follow from the increased workload?

No miswiring that I'm aware of, all replacements were dropped in and wired the same. Backwards would be more likely than two to one motor which wasn't physically possible.

In the build season before we tightened up our construction practices and student (& engineer) training, metal filings could have gotten into the Victors, so that's what we blamed initally. We haven't uncovered any evidence to that effect and that shouldn't have been the cause on later replacements, but you never know for sure.

It's really a concern of course for next year when we won't want a repeat of the issue. I'll get together with the elec. students and other engineers to see if we can add to your straws, and get any stories of static discharge while handling any of the equipment.

Re: Victor Burnouts
 

Mark,
Foreign objects leave little parts of themselves in and on components. An inspection under a bright light will usually reveal an arc mark (burned deposit) on a component leg or on the board.
Brake mode should not have any adverse effects unless a controller was in brake for a full match while other motors were back driving it while running. To my knowledge, brake mode turns on the lower half of the "H" bridge thus putting a short on the motor through the two bottom sets of FETs. As you suggested earlier, the FETs died in one line, I took that as one set of positive FETs and one set of negative FETs, the two of which are in series through the motor.
If the controller is trying to push a lot of current, then there might be a domino failure. The first FET just pops and then the second and third have to share the current. When one of those fails, the last one is suddenly running all the current and blows it's case apart as there is no place for the magic smoke to go. If the controller were wired backwards, it is possible but I think unlikely, that FETs could be turned on across the battery input. They might be able to fire up in reverse polarity under these conditions and the failure might be instantaneous vaporization of the internal wiring causing an open FET. I think that the power input is reverse polarity protected but the motor output cannot for obvious reasons. That would also explain why only two sets of FETs were failing and not all four.

Re: Victor Burnouts
 

Open FETS would lead you away from static as a cause of the failure. Static or other overvoltage conditions tend to result in a short.

Re: Victor Burnouts
 

We (one of our electrician mentors) reinspected the blown FETs and in our cases, the FETs shorted, which means it wasn't a case of one going then the others sharing current, etc. Something, and the backdriving or motor brush theory is a real possibility, causes all three in any one quadrant to blow simultaneously. We didn't notice any burn or flash marks, but that doesn't mean that it didn't happen.

Re: Victor Burnouts
 

Now that we know they were shorted. Kris Verdeyen mentioned a closed FET as a sign of static electicity. Is it possible your wheels are generating static electicity, and the high voltage made it's way to the wheel's speed controller's FETs? Also, what were the speed controlers connected to when they failed? Just the wheels or an arm or roller?

Re: Victor Burnouts
 

Could be. I never heard reports of static discharge from the drive teams until Nationals (when we didn't have problems), but I'll ask them. I never encountered static when the robot was driven on the carpet before shipping though. This is a theory that could easily be tested at home when the robot gets back.

The controllers that blew were on both the drive and winch motors, I don't believe the arm (van door) controller was one of the victims.

Re: Victor Burnouts
 

Guys,
Static of sufficient energy to kill FETs would have been causing problems everywhere else, too. These FETs have internal protection to prevent damage (zener diodes) and are part of a circuit board where 12 FETs live. For only one quadrent to die from static is unlikely. It is still a big puzzlement though. For only one quadrant to fail (Not two opposite quads) then there had to be something wired incorrectly in the external circuitry. Something like one of the battery leads going to the motor and the other going through the controller. This could put full battery voltage across one quad set but not destroy anything else. It seems so unlikely though since the wiring error that would cause this would be easily diagnosed. It could happen, just not high on my list of possibles. It may remain a mystery for the rest of eternity.

Re: Victor Burnouts
 

If we discover anything new through testing during the off-season I'll post the results. I think I'll take one Victor (maybe an 883) and run controlled tortures. I might replace the FETs with lower amp models so they self destruct more easily (and are cheaper to replace).

I want to thank everyone for their thoughts and ideas!:)
FIRST people are just so VERY nice!

Re: Victor Burnouts
 

Re: Victor Burnouts
 

Bob,
The best way to handle the pins is this...
1. Use a solder sucker or solderwick to remove as much as solder as possible.
2. Heat just the pin with your iron and wiggle the pin with a small screw driver. While the pin is moving, remove the iron and the pin will not solder itself back in place.
3. Remove the device.

Any pad that lifts off is likely part of a plated through hole. When you install the new FET, just add solder to the bottom of the board and watch to see if the solder wicks up inside the hole. What is unknown is whether the board has more than two layers. Although some teams report that replacing FETs works, they can take out other devices. For the cost and reliability issues, I recommend a replacement. Anything that is stressed enough to take out FETs has other problems waiting to happen.

Edit...BTW a square pad is usually an indication of a particular pin (often pin 1) to assist with part insertion. Watch which way you put the FETs back in, they don't like to be reversed.
You also may need to clean out the holes if the new one doesn't go in easy. Forcing it may push out the plated through hole. If this is a multilayer board, pushing the hole out kills it in most cases.

Re: Victor Burnouts
 

Did you replace all of the FETs or just the blown ones? What part # did you use to replace them (was it the same as the ones IFI uses)? I've had some trouble finding fets with the same part # as the ones in one of our blown speedcontrollers.

Re: Victor Burnouts
 

I believe our electronics group typically replace only the blown ones, then bench test the rebuilt unit.
The FETs used are IRL3103 (International Rectifier) or an equivalent like FDB6035AL (Fairchild Semiconductor) for our competition victors, but close matches have been used for ones on the practice or older robots.

Digi-Key
http://www.digikey.com/scripts/DkSea...321579&Site=US
$1.63 individually or $1.16 ea. for 10

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid= 199954&e_categoryid=277&e_pcodeid=51229 Mouser has them for $1.19 each if ordered individually or .90 each for 25

Re: Victor Burnouts
 

Al,
I've unsoldered parts from a lot of non-USF boards, including packages like these FETs, but the solder seems very difficult to melt out on these. My always reliable heated solder sucker can't melt it. I put a larger 1/4" chisel point iron on all three pins and it took a while to melt. I guess the traces must be huge since these just pull the heat away. Parts of the board and the screw terminals get pretty hot. I seemed to lift a pad on one side or the other on just about all of them. Then I started snipping the FETs off and taking out pins one at a time. Still lifted pads. I guess you need a very hot iron. Since my sucker wouldn't work, I ended up using a PC board drill to clean the holes out. Of course, if they are plated through, I may have partly or completely removed that. I tried soldering in a new FET, but there wasn't anything for the solder to stick to on some pins.

I am sure if I play around longer I can get the knack, but I don't think I will. You are right about the stress probably shortening the life of the rest of the board. I will either have them rebuilt (I see http://www.rotordesign.com/victor.html is doing them for $75 now plus shipping) or we will just get new ones. The rebuilts will be good for practice and we'll save the new ones for competitions.

I guess we have a motor problem that is blowing them. We lost one Vic and the replacement also went. I wish the breakers 'broke' fast enough to protect the Vics. As someone mentioned, the Vics 'sacrifice' themselves to save the breakers. Well, for my sake, they don't have to be so selfless. I wish they would let the breakers do their job.

Since I have trashed this board, I will probably rip it apart and see what I find in the way of layers.

BTW, I have now 24 brand new (and one slightly soldered) IRL3103 FETs from Mouser that I probably will not be needing any more. I don't know if its legal to offer things for sale here so I won't do that, but if anyone wants to discuss FETs, let me know.
Bob

Re: Victor Burnouts
 

Mark,
When one of our Vics went, it just smoked. It was running a match at the time. The crowd loved it. But when its replacement went in the pits, it was spectacular. We and another team saw a huge spark. I was about 50 feet away up in the stands and it sounded like a firecracker. All three FETs were cracked, of course, but their damage was incremental. One just cracked, another had a blob of white stuff on it, and the third had a larger blob of white. Must be vaporised innards. It could be that they went one at a time, but very close together. I am guessing the destruction got worse as the load on the remaining ones increased. As I mentioned earlier, we likely had a motor problem that caused it. Haven't opened the motor yet.
Bob

Re: Victor Burnouts
 

Bob,

A good solder should go "though" the board and both sides will show a clean and shiny solder wick connecting pad and pin.

The higher the current, the wider and/or thicker the copper trace needs to be on the PCB. This larger copper will leach away any heat you are attempting to apply to the connection and frustrate your efforts.

About 3 years ago, I did a power electronics design PCB able to carry over 200 amps. The soldering process was to have two technicians simultaneously heat the pin/pad from both sides of the board and then to apply solder to one side and verify that it flowed through to the opposite side of the board and wicked to both pin and pad.

De-soldering was nearly impossible as a one person operation...

Based on my experience:

1. Cut all FET pins with a good pair of transverse cutters. You will be removing each pin individually...

2. Heat both sides of the PCB (a two person operation) and pull the pin out with a good pair of needle nose pliers or a small spring hook (recommended for low thermal mass). Note that most pad delamination happens at this step...

3. Note that there will probably be solder left in the hole... Heat from both sides with soldering irons with very thin tips.

4. Remove the soldering iron on one side and then place a vacuum solder sucker on the hole. Note that a professional desolder station is highly recommended as the solder sucker tip is also the heating element...

5. Remove the other soldering iron and engage the solder sucker.

This should clean the hole without lifting the PCB pad.

Note 1: The above is augmented if you have design information on the PCB (at what temperature will it delaminate?) and solder/desolder stations with adjustable temperatures (as design engineer, I had both).

Note 2: My experience is with 60/40 lead/tin solder. Newer "lead free" solders melt at an even higher temperatures...

At any rate, good luck...

Mike

Re: Victor Burnouts
 

Bob,
Mark has some good advice. Although I have never changed the FETs, what you describe sounds like a combination of a large thermal mass and holes that are too small for the leads. It is possible that the board was drilled for the right lead size but never compensated for the plated through hole.
As far as the fire and flame from the controller, let us know if you find a bad motor. Most often, it would have left evidence in some other way like driving problems. You may have a thermally induced short. This is where wiring breaks down under high temperature. You may have a wire in the motor that shorts when the motor gets hot but is fine under test in the pit. The phenom of one FET blowing after another is normal as the shared load gets dumped on the remaining FETs or the dead one produces a short on the other two. High junction temperatures inside the package cause the case to crack open. The light (arcing) you see is the FET trying to turn itself into an LED as the silicon junctions break down one after another. I love the smell of burning silicon in the morning.

Re: Victor Burnouts
 

For more info on replacing bad FETs on the Victors, you can go to www.delphiforums.com and go to the Battlebots forum. They have a few people in threads that fix a ton of them (even upgrade them).

Re: Victor Burnouts
 

Hey all!
Been awhile...
Just my thoughts: I have seen a couple vics blown out in several ways. The most common I have seen (and yes, I am ABSOLUTELY SURE this is what I saw) is people pushing their robots when they are off. This creates juice from the motors, which goes where? the victor. (I really don't recomend this, but if you backdrive the motors and look at the victors, sometimes the lights will turn on. Again, because of the previous point, I don't recomend this.) Push it hard enough, they can zap themselves. This is what I think happened with the hand-cranking the winch, and also it makes sense that only one set of FETs would blow out if you're only wheeling it one way, so the other direction should still work. Also, check the main capacitor in the middle. Another man I once met inspecting the Vics noticed some sticky stuff on one of his, and said it's electrolyte. I don't know how much truth there was to this, but if your capacitor's top is bulging, then it's gonna blow. Replace it before bad things happen.

Hope I am 1)helping, not hindering, and 2)not going over old territory.

Sparks

Oh, and Bob? That white stuff, I'd bet, is electrolyte. I could be wrong (you're not supposed to see it, so I don't know what color it is) but that seems to be the best guess I have.

Re: Victor Burnouts
 

Sparks,
I thought we lost you, glad to see you back. We have been back driving Victirs for years with out any ill effects. The motors do act as generators and there is enough leakage current that backs up through the controller to turn the LEDs on but unless you are pushing the robot at highway speeds in is unlikely you can genreate enough current to fry the controllers. The main cap in the middle of the FETs is rated pretty well for the voltage we use but where it is located subjects to very high heat when the robot is working hard, like during practice. The heat causes early failure and the case has a pressure relief that leaks the electrolyte out of the can. This is nasty stuff so don't touch and then put your finger in your mouth. When it has dried for several days, it does turn into a white, crusty almost lime looking (as in hard water deposit) gunk and it eats copper and other metals. The white smudges are really the result of the magic smoke leaving the device at high speed. They are mostly the ashes of the silicon interior that vaporized during the event. If everything looks normal except the white residue, know that a closer look will show a crack in the case. In some of the more spectacular deaths, there has been loud cracks followed by microscopic plastic shrapnel as the cases of the FETs blow apart.

Re: Victor Burnouts
 

Al,
That sounds reasonable. My only point here is that some teams really gear their motors down, so it is sorta kinda maybe possible that by backdriving they are spinning the motors much faster than the wheels. I'm drawing at straws here, aren't I. What I saw was a team pushing their robot in a parking lot, then complaining of a burning smell. On closer examination, the victors wouldn't turn on. Since it was another team's, I couldn't get a close look at the vics, but a couple mentors blamed it on backdriving. I have avoided it ever since.
Thanks for setting me straight!

Sparks

Re: Victor Burnouts
 

Sparks, If you can remember the team I would like to find out exactly what they found.
Thanks,

Re: Victor Burnouts
 

On our robot, driving the arm at any decent speed, (its powered by a geared down Van Door Motor) will provide enough juice to turn on the RC, and our LEDs will continue to blink running off the 7.2v battery after the motor stops providing power. I've even seen the fans on the victors spin up from pushing the bot on the floor. Not the big fans though... I think the major source of problems is if there is somewhere for the current generated to go or not. Because if the current cant flow, the voltage generated increases. That is what could possibly cause problems.

If this were the problem, it would be very possible to provide something to sink the current and keep voltage spikes down. A 120mm fan or two would do the job, or maybe some sort of lighting. Just make sure that the load is wired so that it is on when the main breaker is on. Can I say for sure that this is the problem? No. Is it a possibility? yes. Look at the schematic symbol for some FETs and you see a reverse biased diode, with the H-bridge, that would act as a rectifier, and power up the bot.

Re: Victor Burnouts
 

I'm with Al here on the backdriving. I've seen some LED's light up, and the fans twitch, but there can't be anywhere near enough current flowing to mess up the fets.

Or is there?

Could it be that with dynamic braking on, the current on some victors just ends up as heat as opposed to getting sent onto the rest of the harness?

I'm just wondering that maybe if dynamic braking is not on, all that backdriven current gets disapatied through all the victors, RC, etc., causing little if any problems. If braking is on, that current would 'stay' in the victor, right? Or am I just missing something key about how braking works on a victor?

Just grasping at some more straws.

-Andy A.

Re: Victor Burnouts
 

Andy,
I am guessing on this, but I don't think the jumper keeps the brake on when the controller is not powered. If that was the case, you wouldn't be able to push the robot with the wheels on the floor. Also, as I remember, IFI started including a varistor across the output. This was something we previously had to add to the output terminals. The device is designed to eat up spikes from brush noise and back drive so I think it limits the voltage to 40 or 50 volts. I am going to have to look back at the specs on the devices we had to add in the past.

Re: Victor Burnouts
 

Al,
I can't remember the team number, but I will see if anyone else does.

Sparks

PS: Are FETs sensitive to polarization? I think they are, but I don't want to make false assertions. IFI Robotics has a notice on the vics page that says:
Is it such a jump to say that using an output as an input would have much the same affect?

Re: Victor Burnouts
 

Like I was saying, the body diodes in the FETs rectify the power from the motor and, which then puts it back through the circuit breaker, and makes it available for everything else. There is a diode formed in the process of making the FET, but some diodes may even have an additional diode on the die for extra protection.
But if you hook up the victor backwards, on the inputs, the body diodes on the FETs all of a sudden point from the line with +12v on it to the gnd line. So you then have a dead short through 2 diodes. (the top and bottom halves of the H-bridge)

The attached schematic is a typical H-bridge with 3 FETs in parallel for each "quadrant" When you power up a victor with the inputs reversed, a massive amount of current can flow, and you could burn up the FETs. That isn't even worrying about the control circuitry. Reverse polarity on that could just kill a chip and the victor would be nuked, simple as that.

But the rectification provided by the body diodes, directs the power put in on the output to the correct side on the input. So if you hooked up a victor backwards, (inputs as an output, output as an input) when you turn on the circuit breaker, you would get +12v on the +12v connector, regardless of if the battery was hooked to the m+ or m-.

As for braking while powered down. The victors don't even brake when they are powered up, have the jumper set to brake, and they aren't receiving a signal. I know this for a fact. On our bot this year, you can move the arm around with one hand when the bot is off, and when you have no link or it is disabled. But if you have a link and it is on, that arm is not moving (not with one hand anyway...) IFI says that dynamic braking still works while it is not receiving a signal, but not from what Ive seen. Even if it did, most of the power would be dissipated in the motor windings, and not the FETs. Looking at the datasheet for the FET that Mark McLeod linked to, the on resistance is 14 mOhms . With 2 banks of 3 FETs in parallel, in series, that would be 9.33 mOhms. That would be almost insignificant compared to the resistance of the motor windings. So we know that braking can't really hurt the FETs while it is on.

Also, looking at the datasheet, the D-S diode has a maximum continuous current of 60A. Remember, there is 3 FETs in parallel. I really don't think you are going to turn your motor fast enough to make 180A for an extended period of time. I seem to remember that the maximum discharge for the ES18 batteries that we use is 230+ A so it would be possible to fry the victor in the manner that I mentioned earlier, reversed polarity in the inputs. But I'm sure the diodes wont exactly share evenly either, making a cascading failure.

Sorry to make this so long, but looking at the electronics, I would have to say that backdriving a motor hooked correctly to a victor in any state ( on, off, signal, no signal...) could not fry it.

Re: Victor Burnouts
 

It is not the FETs that are damaged by hooking up the power incorrectly, the drive circuitry is not protected from reverse polarity. These FETs do have protection zener diodes in the construction (as pointed out above) so reversing the power to the FETs will cause the diodes to conduct with a 1.2 volt drop. When the Victor is off and the motors are turned, the diodes conduct and the only path is through the rest of the driving circuit and back through ground. Since they are in a bridge configuration, the direction of the motor doesn't matter. Theoretically, there is nothing to limit the voltage produced by the motor, but with two diode in series, there is a 2.4 volt drop before anything else comes into play. I agree that it is unlikely that damage will result from a backdriven motor in power off.
I agree that the brake will not be applied unless the controller is enabled now that I think about it. All PWM outputs are disabled unless the OI and RC are talking and enabled. The brake should only cut in during a PWM 127 +/- the deadband right?

Re: Victor Burnouts
 

Yup, and if your OI is on, and you actually power up the RC, they get the link, and give the victor the signal, it will short out the motor, which happens to be its power source at the time, so for the 5 seconds or so the backup battery powers the RC, it will brake.

I could actually see having a fun time getting freshmen to push the robot in this condition. They push it, it brakes, they stop to see what is up, and by that time, it has turned back off. They continue to push... :D

Re: Victor Burnouts
 

All,
Okay, I'm convinced. Backdriving doesn't hurt a vic. Unfortuantely, this makes this mystery even harder to solve...

Sparks

dummy load / test load for Victor speed controller
 

We'd like to build a test setup for the Victors. Anybody have a suggestion for a cheap fixed resistance dummy load? I don't want to use a motor because the motor is not fixed resistance and it's hard to measure motor power output. Would like to have the ability to set the resistance to about 0.32 ohms to get a 40 A max current test, and also to something like 2.5 ohms for a 5 A max current test. Would like to use a fixed resistor that would spread the heat dissapation over a large area so it doesn't get dangerously hot (500+ watts for the 40 A test).

I can put a big resistor in a can of oil like we do in RF power testing. Anybody got a better idea?

Bill

Re: Victor Burnouts
 

I passed your query on to one of our electrical sub-system mentors.
Our electrical group mostly use the ribbon-style power resistor with a fan blowing on it. The ribbon style are easier to get, cheaper and take a lot of abuse. Between Ebay and online surplus vendors, this can be done on the cheap.
http://www.milwaukeeresistor.com/pdf/ribwound.pdf

There is a chassis mount style which one of our electrical mentors has used at work as well. These are aluminum cased and can be mounted to a finned or water cooled plate, but are more expensive.

Re: dummy load / test load for Victor speed controller
 

How about a 500' roll of #10. An RF load would not get you to the current you want to draw but you could parallel 10 resistors, 5 ohm at 10 watt and drop them in the oil for cooling and be OK I bet.

Re: dummy load / test load for Victor speed controller
 

For discharging our Prius battery packs for our electric airplane project, we use 6 electric water heater elements in a trashcan full of water. Unfortunately, I think their resistance is somewhere around 10-15 ohms, which wouldn't be very doable to achieve .32 ohms.

Re: Victor Burnouts
 

A long time ago, we tested fuel cells by placing two electrical plates into a small swimming pool filled with water and then added rock salt until the resistance was what we wanted. I'm sure that a small garbage can could be used in this case. Some experimentation is required and must be done in a ventilated area (possibility of electrolysis).

Mike

Victor load testing
 

This sure provoked some interesting suggestions! My favorite is the salt water swimming pool...

At lunch I went over to the electronics store and bought 10 3.6 ohm 25 watt resistors. We'll wire them in parallel, in two banks of 5 each, so we can test with either (approx) 450 or 225 watts load at full power (12.8 volts). This should give a max current through the Victor of about 36A, a reasonably safe 10% below max rating for it. These are rectangular concrete resistors, so we can cool them with a slab of steel laid across all of them that will have pretty good surface contact, and keep the full power tests reasonably brief, since at max the resistors will be dissipating about 46 watts of heat each, substantially over their 25 watt rated limit. The steel will also protect from the shrapnel if they blow up....

Thanks again for the ideas.

Bill

Re: Victor load testing
 

I remember seeing a DIY battery tester that used steel banding for the load. A suitable length of steel banding that is used to secure crates was attached to a piece of plywood in a zigzag pattern. The resistance might ba a little low for this project though.

Re: Victor load testing
 

Bill,
I would add a fan to your setup blowing under the steel plate. 25 watt resistors are conservative rating in an enclosed space so with the plate and fan you should be OK for the dissipation. Of course, if you wanted to go the "swimming pool" route, a trip to the store for a couple of gallons of distilled water should add some margin of safety. Pure distilled water does not conduct and is easier to dispose of than mineral oil. When you are done, just add some coffee or tea, sugar and cream to taste, donuts optional.

Re: Victor Burnouts
 

we had to change 3 Victor right be for a demo, we were getting shocked very badly we think think static was the problem. is there a good legal way to ground the robot

Re: Victor Burnouts
 

Colin,
There is no real effective way to do that. In 2003, the "ice" produced incredible sparks so we dragged a tail (the outer shield of a braided coax without insulation or center conductor) and that helped a little when the robot contacted the ramp. (made of copper plated steel) This method was determined to be legal by FIRST for that year only.

Re: Victor load testing
 

Bill;
Check out this link.
http://www.harborfreight.com/cpi/cta...emnumber=90636

This is an auto battery/charging system analyzer but cheap enough to modify for your purposes. You might even rescale the meter and get some sort of go/nogo test rig setup. Enjoy, Larry U.

Re: Victor load testing
 

Remember the old adage about " if it looks to good to be true it is"? I think that is the case here.

Re: Victor load testing
 

Chief Delphi has (or had a few years ago at least) one of these rigged with a standard FIRST battery connector. It is (was ) a good way to know if the battery was charged - it is not perfect but it is a lot better than going by unloaded voltage which is the way many folks try to tell if a battery is fully charged.

But to the matter at hand, they have a coil that literally glows like a toaster when enough current is passed through. I don't know what the current that makes the thing glow is, I but am pretty sure it is above the 36A you are trying to test. I would think that you could calibrate the PWM value of your Victor tester to give you the 36Amp test current.

Anyway, in my opinion, it possible that this tester could dispate the amount of electrical power you are looking for especially if you are only looking for short time periods. A toaster is typcially 1000W or more, you are talking about 36A at perhaps 12V, clearly under 500W.

For $20, it seems to be worth at least a shot.

For what it's worth.

Joe J.

Re: Victor Burnouts
 

We have fried 3 Victors on one motor this year, and the 40 AMP fuse has not tripped... I suspect it is because a short on the motor, because everything else seems fine.

There is another idea.

Re: Victor Burnouts
 

Neil,
It is clearly a problem with that motor if there are other motors on your robot with similar transmissions. It is entirely possible for there to be a mechanical problem that is causing the failures. However, do not rule out the possibilty that the mounting position for that Victor allows it to gather dust thrown up by the motor. I have encountered teams that had positioned the Victor directly in line with the main gear in their transmission which threw little bits of metal into the Victor every time they ran the robot.

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

Re: Victor Burnouts
 

FET stand for Field Effect Transistor. More accurately the Victors use Power MOSFETs or Metal Oxide Semiconductor Field Effect Transistors. The nature of these little beasties is a micro amount of drive current, fast on and off times, very low "on" resistance and the ability to leap tall buildings in a single bound! Effectively they act like a bidirectional switch that is controlled by voltage and can deliver 64 amps at less than .012 ohms between two current carrying pins. (i.e. source to drain resistance) So with three FETs in parallel for each leg, the entire package drops as much voltage as 4 feet of #10 wire (that's two conductors, one red and one black)