Team Update #11

[jskene]

This may be a closer match to the stock inductor:

http://www.mouser.com/ProductDetail/...ZBsTfTG7 Q%3d

It has a maximum current rating of 0.8 amps, still well below what I would have expected the design to require.

There is another Bourns inductor of the same inductance but with a 4 amp capability:

http://www.mouser.com/Search/Product...42-5900-390-RC

I believe this is physically larger than the stock one, although I haven't measured the stock one yet.

[GaryVoshol]

Has anyone seen the rules files with the Team Update 11 changes in them? The said they would be published on Wednesday; it's now Friday and a new Update should be appearing.

[Al Skierkiewicz]

Jerry,
You might want to check out the data sheet. That 4 amp Mouser inductor is almost 1/2" in diameter.

[vamfun]

Originally Posted by Bill_B

That's a big help in telling whether a motor is repairable. My armature coils measured much less than 20 ohms. That is further support for my presumption that there is invisible damage to my coils internally.

I'm confused about your 6 ohm measure though. Why would shorting the inductor cause less than the coil winding resistance?

So, my advice now about trying to repair a motor is to measure your coils first, across all three pairs of commutator contacts. If you find a coil pair that is significantly less than 20 (or 6?) ohms, don't bother trying to replace the inductor. Even if you were to get an operating motor going, its work and power characteristics will be different than a stock motor. That would make the repaired motor questionable to use for prototyping. It might still be useful as an arm mover on an FTC robot though.

Thats a good idea... I was wondering about the possible failure modes of the choke. If it fails open, the motor should be a brick which I suspect most people see that can revive the motor by replacing that. If the choke can have a partial failure mode and still allow current to flow then we should see a partial power motor... like all three of the ones we smoked. But if the choke can typically only fail open, then likely the windings are shorted and should be tested like you propose.


What confuses me is the motor R should be 12v/Imax = 12/7.5 = 8/5 ohm.
So I suspect that the shunt capacitor may be interacting with the ohm meters to corrupt the reading.

[Al Skierkiewicz]

Chris,
The stall current should occur when two windings are in contact with the brush at the same time. If you happen to measure when only one winding is contacted, you will see a higher reading. The failure mode on the inductor is open if stall occurs often or for an extended period, accompanied with a little smoke and possible debris. Partial short with smoke if running extended over several amps. The cap is so small that you likely can't measure any resistance even if it was outside and not connected. The choke is so small that high heat causes the coating to crack and the in some cases the core cracks as well.

[Justin Montois]

So does anyone have a distributor for the stock inductor?

[pfreivald]

Justin, one of my people found the exact inductors today -- but I don't know the URL to order them. I texted him and will let you know where to get them. If you don't hear from me by, say, 11am tomorrow, have Rees call me!

[Justin Montois]

Originally Posted by pfreivald

Justin, one of my people found the exact inductors today -- but I don't know the URL to order them. I texted him and will let you know where to get them. If you don't hear from me by, say, 11am tomorrow, have Rees call me!

Sounds good! Thanks!

[pfreivald]

Sorry so late:

http://www.mouser.com/Search/Product...lkey434-22-3R9

[diviney]

All:

Based on everybody's work here, and my own experience, I would suggest the following:

If you still have any good motors, protect them with the "fuse protected" cable (really a thermal switch) sold by Pitsco: http://shop.pitsco.com/store/detail.aspx?ID=6122. These should be acceptable for use in competition, but minimally use them during test to prevent motor damage. I have not tried them, but they automatically reset and presumably can protect the motor (will they trip under "normal" load conditions though)?

If you have motors that are already burned out, it is indeed possible to repair them. You can carefully open them up by un-crimping the small tabs on the back cover. The hardest part of this is re-assembly because the brushes spring closed and you can't safely get them to go back over the commutator when trying to install the lid after repair. I found a good way to do this is to drill two small holes through the back plate 180 degrees apart next to the brushes (See attached diagram, thanks to Colin for the photo):

These holes allow you to insert small pins (safety pins for example) from the outside, holding the brushes in the open position while you re-install the cover plate. Once the cover is in position, then remove the pins and presto, the brushes are back in contact with the armature. I used a 0.036" PC board drill. You could use a more common 1/16" drill too.

You can replace the inductor with an equivalent (The best one I have seen was recommended by Patrick Freivald (Thanks Patrick)) --> Here:
http://www.mouser.com/Search/Product...lkey434-22-3R9

** Important ** -- Since these inductors burn out so easily, I would recommend repairing the motor by removing the inductor and replacing it with a jumper wire. That is what is shown on the attached diagram. Then install the new inductor EXTERNAL to the motor. You can put it in-line with the wire and cover it with heat shrink tubing. That way when it fails again, you don't have to open the motor to fix it. I cannot imagine any sane person disputing that this is an exactly equivalent repair. Deleting the inductor would not be legal (although perfectly functional for prototyping), but if it is replaced externally, that is exactly equivalent.

-Tom

[scooperman]

I previously repaired a broken terminal on one of these motors. It was broken off down in the plastic. After getting the motor apart, I found an old on/off switch which had little flat terminals the same size and shape of the motor terminal. I broke the switch open and stole one of its terminals, used it to replace the broken-off motor terminal. It did not have a locking tang, so after installing and soldering it inside, I gave the new terminal a quarter-turn twist on the outside to lock it in place. When I was trying to figure out how to get the motor apart, I decided to use a hobby motor gear puller, and then slide out the armature with the end bell still on it. That worked OK, next time I will try the method above with the holding pins.

Now it looks like I will be doing more motor work. We are purchasing new motors to replace two that burned out this week, and will get the motor-protecting cabling. The motor that had been previously repaired is one that burned, it is starting to look a little rough due to all the handling. I might experiment on this one, just to see if I can make a fault-protected practice motor. The companies that make polyswitch fuses make some for low voltage motors, the leaded ones are about the size of the motor's original inductor.

For some background, here is one mfg's site

http://www.circuitprotection.com/app...brushdcmotor=1

I have not picked a fuse yet, probably go for one with about a 30V rating, a hold current in the 3 to 5 amp range, trip current in the 5 to 10 amp range, trip time in the 1 to 2 second range. But these are expensive components, I may have to spend as much as 50 cents. If I can make it un-burnable, I will update this.

[pfreivald]

WARNING: We just got those inductors from Mouser, and they are NOT identical. They have much more internal resistance than the ones on the motors, and burn up under relatively nominal load.

Sorry, everyone, for giving you bad information... We were certain we'd hunted down an inductor with the right specifications.

[Al Skierkiewicz]

Originally Posted by diviney

IThe hardest part of this is re-assembly because the brushes spring closed and you can't safely get them to go back over the commutator when trying to install the lid after repair. I found a good way to do this is to drill two small holes through the back plate 180 degrees apart next to the brushes (See attached diagram, thanks to Colin for the photo):

Tom,
This is not a legal repair. However, if you assemble the brush/endplate assy to the armature prior to installing the armature in the motor housing, your step is not needed. For those who are attempting this repair, the magnets will try to draw the armature into the housing, so a device will be needed to control the attraction. I found that a wooden swab works for this purpose.

Pat,
The inductor you link to is only 400 ma, there are others in that package that are 900-950 ma. They are non-stock on the Mouser website so you will need to find another source.

[pfreivald]

Al,

In your opinion, is the suggestion to replace the inductor on the outside of the motor (so it is easy to replace if it burns up again) a legal repair? (I understand that your opinion is not everywhere "legally binding", but it is afforded a great deal of respect!)

Thanks!

[Al Skierkiewicz]

Patrick,
The TU makes repairs replacing defective parts with "identical" parts legal. Adding a replacement in a different location seems to disagree with this ruling.
Thanks for the kind words. I had originally suggested that teams just be allowed to remove the inductor from Tetrix motors, if they were making repairs in order to eliminate the cost of replacing the motor. The GDC felt very generous and allowed teams to make repairs on all components. I have to thank them for that.