Banebots RS-775 Case Short

[Jared Russell]

We were 3/3 with the "zapping" technique. We had to do it a couple of times to max out our ohmmeter on a couple of motors. We rotated the shafts by hand both while "zapping" and while verifying the open circuit.

[Tom Line]

I'm sorry, but working or not this is an extremely bad suggestion. What if that small bit of 'residue' is actually a sizeable chunk of metal? You can potentially spot weld the battery leads to the motor and cause someone serious injury. I've seen this happen when someone tried something similar with a fan.

We'll try something tomorrow, but we'll do it through a 40 amp snap action and see if we have any luck with that.

I will also add that this will likely be the swan song for the use of banebots hardware for our team (unless we're forced into it by FIRST continuing to use their motors). After '07, the transmissions, and now incredibly poor quality control on their motors, we've had enough.

We have enough build season issues without having to RMA things we've bought and redesign major components because of a lack of timely shipping.

[Al Skierkiewicz]

Adam,
Was there fireworks when you applied the battery? Please note that TU11 was released tonight that modifies the 'do not modify' rule.

[Adam Freeman]

Originally Posted by Al Skierkiewicz

Adam,
Was there fireworks when you applied the battery? Please note that TU11 was released tonight that modifies the 'do not modify' rule.

Al,

I was working on another part at that time, but from what I saw there were no fireworks. It actually appeared "easy" to do....but I am sure they were taking precautions to attempt to limit the risks.

I do agree with Tom Line, its not the smarted thing to do...and that there are potentially some really bad things that could happen.

I would advise that people attempt to do this at their own risk.

Hopefully we can come up with a safer way to "fix" these motors.

[Jared Russell]

There were no fireworks when we tried this. For motor #1, there was a very quiet "pop" sound at first, then nothing. For motor #2, there was a bit of an arc when the motor lead was attached, then nothing. For motor #3, there was no visible or audible reaction at all. The suggestion to use a 40A breaker is a good one.

2 of the 3 775s on our first robot were affected by the case short. Prior to doing the "zapping", we were measuring ~8 megaohms between the negative battery lead and conductive parts of our chassis. After removing each affected motor and applying the fix, we now max out the ohmmeter (>30 megaohm resistance).

[Mike9966]

Hi all,
I checked our 3 motors last night and found 2 of them to be shorted, if you call 6 ohms shorted.
I set up a smaller battery, and a 40A breaker and proceeded to 'zap' them by putting one wire on the terminal and one on the housing.
One motor showed no sign of a spark and was healed and works fine now, and the other one showed a bigger spark on the first touch, but no sparks on subsequent tries, unfortunately, it was only semi healed in that it continues to have 300-500 ohms of case to terminal resistance.

While I'm happy to get one motor working, I have to say that this is the craziest fix, and I'm not happy with having to have done it.

If you try it, please be safe.

Mike

[Al Skierkiewicz]

Has anyone be able to see the debris? Can you describe it? What I am trying to determine is this a permanent fix or is there more of the debris just waiting to bite us in competition. We need a permanent and reliable solution.
For others, a 10K or higher resistance any lead to case seems to be OK, megohm is better. The six ohms reported is definitely bad, 500-600 ohms is still in the bad region as well.

[Jared Russell]

I did not see any debris before, during, or after the zapping. I will continue to monitor for case shorts as we continue to do testing and driving.

BTW, although in my prior post I mention 8 megaohm resistance to the frame (from negative battery terminal to the furthest point of our arm), resistance between the motor terminals and the motor case itself was below 1 ohm prior to the zapping (now it is >30 megaohms).

[Jim Wilks]

We had 2 of 5 motors with case shorts. One was 1k ohm and the other was about 2 ohms. We applied 12VDC from a current limited power supply set for 25A max. On the 1k ohm motor, the problem cleared immediately. The 2 ohm motor let loose with some nice blue sparks, but then was good.

We remeasured both motors and both still had some high resistance (~20K ohms) to ground. We than ran each motor for a few minutes while applying + to the case. We then repeated this with - to the case. Both motors are now showing >20M ohms. I guess that's a fix!

No debris was seen at any point in the proceedings.

However, I must echo the points above about being careful when doing this as a short which does not clear could be very dangerous, particularly if your voltage is not current limited to a "safe" level.

Banebots has really "missed the boat" on this years offerings to FIRST.

JW

[JesseK]

Thanks for all of the info -- we'll get our extra 775's in after ship date, so we'll have to test things then. The KOP 775 was fine though.

This makes me weary of BB. We didn't have the problems in 2007 that others had (custom plates), yet between this and our eaten 256:1 gearbox in 2008 I think we'll take pause on our gut reactions to use the BB motors in future years.

[dyanoshak]

It seems like our attempt at the zap was successful on one motor. We still have one other motor to try later tonight.

I ran the motor for some time and it is still operating as it should. Before the zap, 4 ohms, after the zap, open circuit.

-David

[Al Skierkiewicz]

Everyone,
I am attempting to find the type of debris that teams may encounter and where it might be located in your motors so I can pass this info to inspectors. Believe it or not, some teams don't visit CD for the latest discussions. Electrically neutral frames are a line item for inspections that will need to be dealt with during the season. Thanks for everyone's help on this issue.

[Tom Line]

I was not able to see any debris in the motors we fixed today.

However, one was a little more 'substantial' than the other, we got sparks about 12 inches out the front of the motor and the negative lead spot welded itself to the motor case.

The motor does appear to be fixed though. We got a nice round of 'oooooooooooo' from the audience when we did it too.

[Al Skierkiewicz]

Tom,
That was more the effect I was worried about. From the angle of the trajectory would you say that the flames came from the end of the motor (brush assy) or from the commutator? I am guessing that is the only two places where debris would collect and still short to frame. Or could it be excess solder where the windings are tied to the commutator?

[Tom Line]

Originally Posted by Al Skierkiewicz

Tom,
That was more the effect I was worried about. From the angle of the trajectory would you say that the flames came from the end of the motor (brush assy) or from the commutator? I am guessing that is the only two places where debris would collect and still short to frame. Or could it be excess solder where the windings are tied to the commutator?

Actually, if I were to guess I'd say neither. The sparks actually exited the front of the motor through the holes that surround the axle. Had this been a short near the brushes or commutators, I would have expected them to come out the back (where I had my hands...).

That doesn't make much sense to me, because I'd expect the wire to be covered in varnish and not be conductive except at the locations you mentioned.