Team Update #11

[Mike Betts]

I would hope that FIRST publishes the recommended inductive component part number so that teams with limited technical resources can take advantage of this change.

[martin417]

Originally Posted by 340x4xLife

Does anyone have a supplier for the inductors?

This opens up a situation where gracious professionalism is going to be paramount. Even in an instance where a team makes an honest mistake an thinks they have replaced the inductor with an identical part, the performance difference can be huge.

My Electric Circuits theory is a bit rusty, but I am pretty sure that an inductor appears the same as a wire to a DC current. I don't think there can be any advantage gained by using different components in place of the inductor. The purpose of the inductor is to work with the shunt capacitor to make the motor less electrically "noisy". Unless the inductor has a resistive component as well (more than an equivalent length of wire) the only effect of substituting the component will be more electrical noise in your system.

___pause to check the specs____

I just looked at the specs for the 3.9 microhenry inductor listed, and it shows a 2.3 ohm DC resistance. It also lists a max current of 280 mA. at the max listed current, the inductor would drop 0.65 volts. I doubt that .65 volt drop would account for a "significant" difference in pole climb time, but it could make a small difference.

[martin417]

Originally Posted by martin417

My Electric Circuits theory is a bit rusty, but I am pretty sure that an inductor appears the same as a wire to a DC current. I don't think there can be any advantage gained by using different components in place of the inductor. The purpose of the inductor is to work with the shunt capacitor to make the motor less electrically "noisy". Unless the inductor has a resistive component as well (more than an equivalent length of wire) the only effect of substituting the component will be more electrical noise in your system.

___pause to check the specs____

I just looked at the specs for the 3.9 microhenry inductor listed, and it shows a 2.3 ohm DC resistance. It also lists a max current of 280 mA. at the max listed current, the inductor would drop 0.65 volts. I doubt that .65 volt drop would account for a "significant" difference in pole climb time, but it could make a small difference.

After talking with an EE, I have been informed that there COULD be significant gains by replacing the inductor with a wire. Possibly as much as a 10% power boost. has anyone tried this? An easy test for this is a resistance check across the motor windings. The motor winding resistance is normally quite low, so a quick check could show that someone has "mistakenly" replaced the inductor with a non-complying part.

[Justin Montois]

Originally Posted by martin417

After talking with an EE, I have been informed that there COULD be significant gains by replacing the inductor with a wire. Possibly as much as a 10% power boost. has anyone tried this? An easy test for this is a resistance check across the motor windings. The motor winding resistance is normally quite low, so a quick check could show that someone has "mistakenly" replaced the inductor with a non-complying part.

We found around 20ohms of resistant across the windings with a stock motor and 6 ohms with just a wire in place of the inductor. Pretty big difference.

Does anyone know if the inductor I listed earlier in this thread an identical part?

[Bill_B]

Originally Posted by 340x4xLife

We found around 20ohms of resistant across the windings with a stock motor and 6 ohms with just a wire in place of the inductor. Pretty big difference.

Does anyone know if the inductor I listed earlier in this thread an identical part?

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.

[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!

[Bill_B]

Originally Posted by martin417

My Electric Circuits theory is a bit rusty, but I am pretty sure that an inductor appears the same as a wire to a DC current. I don't think there can be any advantage gained by using different components in place of the inductor. The purpose of the inductor is to work with the shunt capacitor to make the motor less electrically "noisy". Unless the inductor has a resistive component as well (more than an equivalent length of wire) the only effect of substituting the component will be more electrical noise in your system.

___pause to check the specs____

I just looked at the specs for the 3.9 microhenry inductor listed, and it shows a 2.3 ohm DC resistance. It also lists a max current of 280 mA. at the max listed current, the inductor would drop 0.65 volts. I doubt that .65 volt drop would account for a "significant" difference in pole climb time, but it could make a small difference.

Your circuit theory cannot be much rustier than mine, however, the current to a brushed DC motor, direct as it is, still varies as the commutator redirects the current to the armature coils. This motor has a three coil armature, so two coils are powered at a time. The coils are inductors themselves, although they are not usually rated that way. One way to think of an inductor is a current sustainer. That is, it tries to maintain the same current flow across voltage fluctuation. (contrast to a capacitor that tries to maintain voltage in current fluctuations) A collapsing field in a 3.9 micro henry inductor will not be able to generate much current flow for this motor. The capacitor will be equally inadequate to maintain much voltage. The result is called a low-pass filter, because only the very high frequency parts of the direct voltage variation will be absorbed by this LC filter. Well into the radio range I'd guess without doing any math.

This brings me to the current rating of the inductor. Based on various other observations, we can be pretty sure that more than 280mA is being drawn by this motor in minibot operation. Probably more than that in FTC robot operation as well. So the mystery becomes, how can such a relatively low-rated component expect to survive these over-current excursions? by duty cycle? by conservative rating spec.? Another spec. I read mentioned a 40 degree C rise for their inductor at rated current. This setup won't allow for very much heat dissipation, especially being enclosed and in as close proximity to a plastic mounting as it is.

The one thing I do know is that replacement by a wire in one case allowed way too much current for the fuse I was using. There may be something else wrong with that motor, but it wasn't obvious visually. Meanwhile, I have replacement motors to use for testing using the thermal limiter device. I will postpone further repair attempts until time allows or more learned commentary here convinces me to have another shot at it.

Thanks for your consideration.

[AllenGregoryIV]

Does anyone know why they changed the rules about the floor protector for the tower? They removed the sentence about it only having a 1/4in ridge in the field but did not replace it with anything else. Does anyone know if this means the bump will be larger or smaller?

[Mr.G]

Originally Posted by AllenGregoryIV

Does anyone know why they changed the rules about the floor protector for the tower? They removed the sentence about it only having a 1/4in ridge in the field but did not replace it with anything else. Does anyone know if this means the bump will be larger or smaller?

Now we are going to get stuck on it and not be able to reach the tower with our 1 inch off the ground bumpers.......j/k

[Al Skierkiewicz]

Guys,
Although the Bourns page is similar it is not nearly the identical part. When I first started research on this a few weeks ago, I found an identical sized inductor with the same conformal package. I believe they were more like 900ma max DC current. The series resistance is much lower than the Bourns above. AT 900 ma used with a motor that stalls at 7.5 amps, you can see why the motors constantly burn open.

BTW, I started the discussion to allow opening Tetrix motors to replace the inductor. My recommendation was based on the purchase cost factor of replacing a motor that had failed simply because an under rated part had failed. I had hoped that teams could cut their losses by replacing the inductor. FIRST and the GDC was overly generous by allowing teams to open and repair other items besides the Tetrix motor and delineating "repairs" from the "modification" rules.