Banebots RS-775 Case Short

[MrForbes]

Using a better ohm meter, although the test leads themselves have a bit of resistance (about 0.1 ohms when shorted).

[Ether]

Hey Jim,

That is one fine voltmeter ya got there. Have you tried getting it assessed on Antiques Roadshow?


Oh, BTW: I think the calibration date is a bit overdue :-)

[MrForbes]

I was just noticing that it was last calibrated when our youngest sons (twins) were still a twinkle in my eye...and they're in college now....

[Andrew Bates]

Originally Posted by sandiegodan

If anyone knows if Banebots is replacing these it would be nice to know.

Dan Glenn
Mentor
Team 1622 Spyder, Poway High School, California

We got a response when we sent in an RMA request that this was a known issue and that they would not replace the motor. They simply said to apply some voltage across the case and a motor lead to burn off the manufacturing debris.

[Dad1279]

Originally Posted by corpralchee

We got a response when we sent in an RMA request that this was a known issue and that they would not replace the motor. They simply said to apply some voltage across the case and a motor lead to burn off the manufacturing debris.

Same here: "If it is an issue for your application it can easily be eliminated by passing a modest amount of current from the terminal to the case."

As far as I'm concerned, this is not a FIRST legal motor if it conducts current to ground. Worst customer support of any FIRST KOP supplier. I hope First thinks twice before using these motors in KOP again.

[Danny Diaz]

Our kit 775 also had 2.3 Ohms between terminal and the case on BOTH terminals, and we'd been using it off and on for a few days. We shorted the terminals to the case with the battery (one at a time) and now we're seeing 126kOhm between terminal and case on both terminals. Our other 775 that we were "gifted" from 3320 (The M&M's) had 3MOhm between terminals and case.

I was disappointed that there wasn't as much of a fireworks display as others seem to have experienced when we did it.

-Danny

[sanddrag]

Originally Posted by Dad1279

Worst customer support of any FIRST KOP supplier. I hope First thinks twice before using these motors in KOP again.

Show me where else you can get these motors and decent planetary gearboxes for them at the same price. If you want great support behind the products, would you also want to pay double the price for them, so they could hire more people to support them? I'm sure they probably are not a large company. Yes, they've had long lead times and a few issues, but give them a break. We could be using Bosch drill transmissions... You know back around 2002, there was no BaneBots, there was no AndyMark, there were no CIM motors with a nice keyed shaft, and we were restricted to purchasing parts only from one very overpriced supplier. Teams still made great robots.

Yes, the BaneBots products have a few flaws. But, it's up to the designer to consider those shortfalls, and design around them. For the price, their products are great. I was quite happy to see the BaneBots motors (especially the 775) in the KOP this year, and I hope they are here to stay.

[Chris is me]

With the exception of this large design flaw, the 775-18v is a great motor that I hope we get to use again and again. The decision to include the 18v version and to run that at 12 volts was really smart!

[Ether]

Originally Posted by Chris is me

With the exception of this large design flaw,

We don't know yet whether it is a design flaw or a manufacturing (quality control) problem.

[sandiegodan]

Originally Posted by Ether

We don't know yet whether it is a design flaw or a manufacturing (quality control) problem.

I think we do know that it is a manufacturing problem:

Originally Posted by dyanoshak

I just got off the phone with BaneBots.

They believe the problem is caused by a small amount of debris in the motor possibly introduced during manufacturing.

They have had success with several motors in house when they apply 12V between the motor lead and the case. This zaps the debris and you should be good to go. You might have try this with both leads depending on where the debris is.

I will try this on our 2 shorted motors tonight and post our results.

-David

If you do execute the fix, the motors are fine. Our problem is, we opened them up before we read about the fix, now we are left to buy new ones or design around the motors.

Dan Glenn
Mentor, Team Spyder 1622
San Diego, CA

[Ether]

Originally Posted by sandiegodan

I think we do know that it is a manufacturing problem:

Originally Posted by dyanoshak
I just got off the phone with BaneBots.

They believe the problem is caused by a small amount of debris in the motor possibly introduced during manufacturing.

I guess my point was this: Just because the debris is introduced during manufacturing doesn't necessarily mean it is a manufacturing problem.

If the design is such that no reasonable (cost effective) manufacturing process would be capable of avoiding unacceptable debris, then it is a design problem.

[MrForbes]

The insides look like countless other motors....but without seeing the complete design documentation, and seeing what the actual problem is, it's guesswork. My guesswork says manufacturing problem, either with the process or materials not meeting spec.

I should unwind the thing and see if there's really any debris, or if the insulation on the wire is damaged. Although it's probably gonna be hard to tell.

[MrForbes]

I cut all the windings at the commutator connections. The first winding (there are 5, overlapping) is the one that is shorted. So I guess I'll take them all off and see what I can find.

[MrForbes]

I found the short. The first winding on the motor was touching the amature core, at this corner. I found it by connecting my trusty 77 DMM to one end of the wire and to the shaft, and it beeped until I got this part of the winding loose. It was very difficult to remove the wire here. As soon as I pulled it loose, the beeping stopped.

My take on it: I did not find any "debris" in this motor, but it also had a complete short of winding to armature core. If you have a completely shorted motor (less than 5 ohms resistance from either terminal to case), then I'd suggest you do not use that motor. If the resistance is considerably higher, then you might possibly have a "debris" problem, and if you "fix" it by blowing it with high current from case to winding, the motor might be fine.

btw the white stuff is glue that is used to hold the plastic fan in place.

I hope this little forensic exercise was helpful!

[billbo911]

Originally Posted by squirrel

I found the short. The first winding on the motor was touching the amature core, at this corner. I found it by connecting my trusty 77 DMM to one end of the wire and to the shaft, and it beeped until I got this part of the winding loose. It was very difficult to remove the wire here. As soon as I pulled it loose, the beeping stopped.

My take on it: I did not find any "debris" in this motor, but it also had a complete short of winding to armature core. If you have a completely shorted motor (less than 5 ohms resistance from either terminal to case), then I'd suggest you do not use that motor. If the resistance is considerably higher, then you might possibly have a "debris" problem, and if you "fix" it by blowing it with high current from case to winding, the motor might be fine.

btw the white stuff is glue that is used to hold the plastic fan in place.

I hope this little forensic exercise was helpful!

First off, Jim, thank you for taking the time to look into this issue in such detail and giving us this information.

Here is my concern based on what you found:

If this is in fact the root cause, and common to all the 775 motors showing this fault, then the "fix" is only truly is a poor solution.

Here is why I say that. For the "fix" to work, the current path has to be through the terminal(s), through one of the brushes into the commutator, through the short and into the armature then finally through the bushings/bearings to the case. The current that can possibly blow this short open, also has the potential to damage the brush/commutator contact point as well as the bushing/bearing assembly. Yes, the brush/commutator are designed to carry high current, but the bushings/bearings are not. (I use the term "bushing/bearings" because I am not sure of exactly how this motor is actually assembled.)
Now, if enough current is passed to open this short, then I can almost assure you that it not passing through the bushing/bearings without having a detrimental effect. How much damage is being done is indeterminable at this point. How long will these motors run without failing is also an unknown.

I for one would rather replace the motor with a "known good" one, than risk it failing at the most inopportune time.