Victor Burnouts

[Al Skierkiewicz]

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.

[Matt Reiland]

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).

[Sparks333]

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.

[Al Skierkiewicz]

Originally Posted by Sparks333

Hey all!
Been awhile...
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.

Sparks

Oh, and Bob? That white stuff, I'd bet, is electrolyte.

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.

[Sparks333]

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

[Al Skierkiewicz]

Originally Posted by Sparks333

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

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

[ConKbot of Doom]

Originally Posted by Sparks333

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.

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.

[Andy A.]

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.

[Al Skierkiewicz]

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.

[Sparks333]

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:

Do not reverse the input polarity on a Victor. Doing this will permanently damage the unit and void the warranty.

Is it such a jump to say that using an output as an input would have much the same affect?

[ConKbot of Doom]

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.

[Al Skierkiewicz]

Originally Posted by Sparks333

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?

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?

[ConKbot of Doom]

Originally Posted by Al Skierkiewicz

The brake should only cut in during a PWM 127 +/- the deadband right?

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...

[Sparks333]

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

Sparks