Fixing Burnt Motors
 

Hey,
We kinda sorta burned out two of our motors for the minibot earlier this week, and we can't get any more of them because they're currently stuck in the Iowa snowstorm.
Could we open up our motors and fix them, and still compete with them?

[Team 2035]

Re: Fixing Burnt Motors
 

I don't think you could fix them,even if it was legal

Re: Fixing Burnt Motors
 

For future reference, Team Update #5 specifically allowed teams to replace the 20A fuse that the Tetrix motors come with with an equivalent type of lower amperage. I guess there's a part that likes to fry at about 20A, so they're allowing that to be protected.

Re: Fixing Burnt Motors
 

The Tetrix motors don't have a fuse - what are you talking about?

Re: Fixing Burnt Motors
 

OK, so it's the battery. But it's the motor part that is the problem, as noted in this sentence from the same Update: "Additionally, it has come to our attention that teams have burned out their Tetrix motors by stalling them while directly connected to the battery."

The motor has a part that lets the smoke out at about 20A. As such, teams are encouraged to replace the fuse on the battery pack or use the thermal protection wiring (allowed later in the rule as updated in TU#5).

Re: Fixing Burnt Motors
 

He means the fuse that is on the battery, it comes with a 20Amp fuse but by lowering it you can fix the issue with the motors burning out, in response to the fixing question the issue is a 3.9 ohm resistor that burns out at about 20amps. The resistor can be replaced but it is illegal to compete with in such a state, also to remove it it cause significant damage to the motor housing in the form of destroying the tabs bent to hold in the actual components. However if you can solder a bridge in and solder in a replacement resistor to outside of the motor for easy access, the motor will perform identically to a normal one for testing, so that you don't risk new ones. I'd advise using those for testing and running at most 15Amp fuses to fix the issue. good luck !

Re: Fixing Burnt Motors
 

Doesn't really solve the poster's problem or answer their question, though. Their motors are burnt out.

Anyway - burnt out motors can't be repaired and remain competition legal, sorry.

Re: Fixing Burnt Motors
 

If burnt motor means the "magic smoke left it" then I don't think its fixable.

Re: Fixing Burnt Motors
 

It's easy, just get some magic red smoke, and put it back in!

Re: Fixing Burnt Motors
 

A little clarification-

No, you can't modify the motors and still compete with them.

Additionally, you can't really fix a fried motor, at least not in the traditional sense. Whats burning is the insulating enamel on the wire coils of the armature. Once that coating is burnt off the wire starts to short against its self, decreasing the effective strength of the magnetic field it can generate. Torque decreases accordingly. The only way to 'fix' the motor is to either rewrap the coils with new wire (nigh impossible to do by hand) or replace the armature. For a small DC motor like this neither option makes any sense.

Mark the fried motor and use it for R&D purposes that don't require full power, or toss it.

You can buy new motors from Lego and Pitsco. $30 each and make sure you get the same part number!

Re: Fixing Burnt Motors
 

RED????? Since when does magic smoke, or at least the type found inside electronics, come in such a color? Also, once the magic smoke is let out, it cannot be put back in, so even if you can get some, it is useless.

Re: Fixing Burnt Motors
 

Well, if the magic blue smoke comes out....it only makes sense to put the red stuff back in.......

Re: Fixing Burnt Motors
 

The rules basicly say no modifications. Repair therefore should be legal. I think if you realize what the spirit of the rule is and do not push these limits you will be OK. You can absolutely repair it for your developement and testing and you can hope your new ones show up before the regional. You may carry in your minibot.

Re: Fixing Burnt Motors
 

We fixed our burnt out motors only for use in prototyping, the fix we used would not allow the motor to be legal in competition.

This thread shows what we did.

http://www.chiefdelphi.com/forums/sh...ad.php?t=89848

Re: Fixing Burnt Motors
 

komeiji,
***Note: Under current rules, this is not a legal modification of this motor!***

The method you would use to open these motors is first remove the transmission and adapter plate. Then I use a wire cutter to pull back the outer case at each point it has been crimped over the end plate. Using a long, slender tool, i.e. screwdriver, push gently on the gear and shaft to push the end plate and armature out of the case and magnet assy. Once you have The armature and brush assy open, you can gently move the brushes to release the armature. Inspect the armature to see if the wires are burned or are dark in color. If so, dispose of the motor it is not repairable. If everything looks normal, set the armature where it will not roll off the table. Look at the endplate with the brush assy. (The picture in the linked thread will help.)
You will note an axial leaded component that resembles a coated resistor. This is an inductor that is likely burned open. If you carefully unsolder this component, replace it with a short wire. Assemble the endplate and armature being careful to avoid damage to the brush assy. Then using a long slender tool, reinsert the endplate/armature assy back into the case. Try connecting a battery to the motor and see if it spins. If it appears to be working correctly, then using a hammer and some other tool, recrimp the case to hold the endplate in place. This will give you a working motor that is not legal for competiton.

***Note: Under current rules, this is not a legal modification of this motor!***

Re: Fixing Burnt Motors
 

My surgery on one of our two dead motors was done in a similar fashion. A wire replaces the inductor now. Incidentally, the burned hulk must have iron in it somewhere because it was attracted to the motor's permanent magnet as I cut it loose. When you reinsert the armature and brush-bearing end plate, you have to be careful that the magnets don't just snatch the armature away from you. It will separate the two pieces quickly. I devised an alternate re-assembly method, but that's another story.

After reassembly, a power test had the motor spinning for about 10 seconds before it blew a 5A fuse. Current measurement with another fuse in place showed over 7A current for the free-running motor. The armature coil wires looked OK or I wouldn't have attempted reassembly. My theory is that wires on the armature deep inside have cooked off their insulation making this motor useless for testing even in its "repaired" condition. siiiiigh. :eek: Maybe the other motor will be repairable while I await replacements from pitsco.

At least I now have gears to play with. :D

Re: Fixing Burnt Motors
 

Bill,
We have smoked 3 motors, but they all have partial power still. I measured one with a 4.5 volt input and measured the torque. It scales to about a 45% torque of a new motor. So I suspect that we cooked some windings.

Will a burnt inductor possibly allow partial torque or is the motor a brick with this condition?

If we do have partial torque, then is is likely any repair can be effected?

Re: Fixing Burnt Motors
 

Burned out motors can be fixed, as of team update #11.

See this thread. they are discussing the resistor value, which is most likely what you burned out.

http://chiefdelphi.com/forums/showthread.php?t=91959

Re: Fixing Burnt Motors
 

I'm pretty sure the internal part of the coil has shorted by burned insulation there. This is based on the high current observed by running my one attempt to repair a drive. I thought burned insulation would be more obvious by discoloration. My nose tells me something serious has happened to that motor.

I think overcurrent in the inductor has made its coating break down and might have been the source of the first noticed smoke. My current theory {pun noted} is that the motor failed in stages as more internal coil heating caused more insulation failure leading to more overcurrent in a vicious circle that eventually killed the inductor into open circuit failure. The circle ended but repairing with a new inductor, or a new wire in my case, will not restore the motor to original condition. It won't make it usable for practice either due to the suspected armature damage.

The partial torque you noted throws off the gearing calculations so any experiments you are trying mechanically will be skewed for a fresh motor. That is, you get your robot working well with a "repaired" motor, but changing to a new one inserts a different energy profile that effectively negates all your mechanical tweaking. In our earliest testing, I think we might have fallen prey to changing characteristics of partially disabled motors. That is, the first smoke only wounded the motor so we adjusted pole grabbing tension, for instance. As the motor became less and less capable, we thought there was fault in whatever adjustment we just made.

Bottom line {hate that term} I'll be playing with the gears from these motors, and maybe I'll try to compare the burned coils with a good armature, but the motors will never make it into even a testing robot.

Re: Fixing Burnt Motors
 

Thanks... I think I'm in full agreement with your reasoning.

It doesn't make sense to under design the choke unless you are using it as a protective component.
So lets assume two cases:
1) The choke is properly designed to take the 7.5 amps. The temporary stall condition then smokes the windings first. This shorts the armature and leads to higher currents. The higher current eventually smokes more winding and eventually smokes the choke. The motor will have residual degraded torque if the choke didn't self destruct open but the motor will not be usable for a minibot design.

2) The choke is undersigned and opens before the motor windings smoke. Then it protects the motor windings and can be replaced to restore the motor to original specs. But.... the motor will not have residual torque after this event. So..... This doesn't fit any of my motors.

3) Maybe something in between where either the choke or winding smoke first. If the choke just smokes and doesn't open , then its resistance will lower and the motor current will increase and probably lead to more power in the motor. This is probably a good thing unless the stall persists... then the extra power will smoke the motor faster.

4)???

Bottom line, smoke a motor and if it still operates...... use it for mini fan motor and not a minibot.

I hope someone can post a counter to this reasoning:)

Re: Fixing Burnt Motors
 

Chris,
The more obvious (at least to me) is that the choke is under rated in order to fit it in the motor that is extremely noisey. It was an afterthought. Remember that the motor is not designed for repair. It is just that some of us have figured out how to open it, diagnose, and repair. This is not a job a typical FTC or FLL team could accomplish.

Re: Fixing Burnt Motors
 

Thanks, Al, and others, for sharing information on the repair.

I've been trying to locate a part number for the equivalent choke but have been having trouble identifying a 3.9µH choke with the correct physical size. Is anybody able to provide a distributor (e.g. Digikey) part number?

Thanks,
--ken

Re: Fixing Burnt Motors
 

The ones that I found on the Mouser website are non-stocked items.

Re: Fixing Burnt Motors
 

It refers to the fuse thats on the Tetrix battery.

Re: Fixing Burnt Motors
 

No, you can't compete with a broken motor. You can, however, fix a motor, but it is not competition legal and should only be used with simple circuits, not with an NXT brick. Basically we use a dremel to remove the back cover, remove the inductor (looks like a blue resistor, this is what fries first) and the capacitor, and solder their contact points to wires protruding through holes drilled in the back of the casing. then we tape the back on.... not competition legal but it works for prototyping.... good luck!

Re: Fixing Burnt Motors
 

Yes it is.

Re: Fixing Burnt Motors
 

The part of your statement quoted above became invalid in Team Update #11, with the addition of <R55-M>, which reads: (Also note that should a repair be made, any warranties should be assumed to be voided--see the updated blue box.)

Also, the same Update added a second part to <R93>, which reads: Now, the repair you describe would in fact be illegal for competition, because the replacing of the capacitor and inductor with wire does alter performance and specs, and is not replacing the inductor with an identical part. But any repair that successfully replaced the inductor with an identical part (should such an identical part be identified) would be perfectly legal for competition.

Re: Fixing Burnt Motors
 

Also Eric, the drilling of the endplate is not a legal modification .

Re: Fixing Burnt Motors
 

So after going through a half dozen motors, we've taken the plunge and actually removed the back off a (seemingly) working motor to find out whether we can make a legal repair to our motors. This resulted in the following:



The closer inspection of the part showed that even under seemingly normal loads we're pushing the limits, as seen from the burning marks by the white band and the crack that stretches from one end to midway across the backside.

It seems like the inductor we pulled out is a 1.9uH inductor, with a resistance of 0.3 ohms. Tomorrow we're hoping that some tests with an o-scope will help with verifying the inductance value and get to the bottom of this mystery.

Re: Fixing Burnt Motors
 

The brown band you are interpreting as the digit "1" was originally orange. That is a 3.9 uH,

Re: Fixing Burnt Motors
 

Has anyone found an equivalent part?

We have a couple of motors that we would like to repair.

Re: Fixing Burnt Motors
 

Torbots, did that motor ever 'release the magic smoke' or smell burned?

We smoked a few last night, but they were minor incidents, and appear to still run.

Re: Fixing Burnt Motors
 

The motor we pulled the photographed inductor from did not smell burned nor did it have any visual release of "magic smoke". We have noticed with some other motors that even though some magic smoke was released they continued to run but at a degrated level. Some guesses within our shop is that some of the inductor's windings fused together, allowing current to still flow through the motor.

Re: Fixing Burnt Motors
 

Has anyone gotten official confirmation of what specific part numbers/vendors we're allowed to use for the inductors? The most common I've heard of is Mouser's 9310-26-RC, but I confess I don't know enough electrical theory to confirm anything.

Re: Fixing Burnt Motors
 

That inductor has only a 0.28 amp current rating. I doubt it would last long.

Re: Fixing Burnt Motors
 

Your report just adds to the mystery of the smoke. Here is my current theory about the failure of Tetrix motors. These motors have no ventilation or builtin fan cooling. Over current in the armature coils can cause them to overheat. Do this often enough and the shellac (like) insulation will melt (and smoke) in places and short some wires together. This will lower the impedance of the coil and increase its current demand. That is, the motor still runs, but uses more current to do it. More current means more possibility of insulation melting and shorting. Eventually so much current is required by the motor, that the inductor can no longer pass it without failure by overheating.

I suspect that the smoke in your case came from the armature and that your motor is still usable even though degraded as you report. Melting insulation does not necessarily cause windings to short every time. It is just that you cannot continue to melt insulation without consequence for very long. The difficulties we're having with this motor are not mirrored by the way they are used in the FTC because the FTC is using them via motor controllers with electronics. Perhaps the GDC envisioned more robot-like minibots climbing those poles with more electronics than is common in this year's machines. I think that would have been kinder to the motors, at least.

Re: Fixing Burnt Motors
 

There is a coil inside the motor to prevent electrical noise. That coil is only good to handle about 500mA worth of DC current. With the way were using them, we are easily exceeding that current rate of that little guy. Therefore it goes up in smoke, burning itself open. The motor wires themselves are heavy enough to handle that power, or at least the 3-4 second runs they have to go through.

Mouser has a 3.9uH that has a current rating of 640mA made by Bourns.
I don't have the part number now, I have it at work. - and its pretty close to the same size as the orignal part. Only major difference is instead of the body being green this one is blue - but the color bands are all the same.

Re: Fixing Burnt Motors
 

Darrin,
The inductor is more like the 900 ma version. Those are non-stock items at Mouser the last time I looked. Still under rated for our service with a motor that stalls at 7.5 amps.

Re: Fixing Burnt Motors
 

When choosing a replacement inductor, one should take into account the series DC resistance, or DCR on many of the data sheets from the manufacturers.

I measured the DC resistance of an unburnt inductor at 0.2020 Ohms and 0.1991 Ohms with a second meter. I used two brand new (one still had the protective plastic on the display) Fluke 8845A 6 1/2 Digit bench DVM's connected in the 4-wire mode to cancel the resistance of the meter leads. These meters are accurate to 0.006 Ohm on the range used. For comparison, the resistance across the metal back plate of the motor measured 0.0050 Ohms.

The power connection tabs on the motor had been broken off by rough handling and the inductor showed no signs of turning brown in the middle.

Re: Fixing Burnt Motors
 

This is what I found: 77F3R9K-RC from Mouser. This has a MAX DC resistance of .37 ohms, can handle 640 mA of current. And is the exect same size of the old ones.

Al, I think if you went with one rated at 900 mA it would be larger than the orignal.

Here is an orginal burnt one and the new one that I got in today.

Re: Fixing Burnt Motors
 

Darrin,
The non-stock item I was looking at was the exact same package size in the 900 ma+ range.

Re: Fixing Burnt Motors
 

Lots of smoke suggested that this motor would never make full torque again, so I didn't mind trying something different when repairing it. I picked a higher-current rated 3.9uH inductor, J.W.Miller (now Bourns) p/n 5800-3R9-RC, got it from Digi-Key, their number is M8276-ND. Its rated current is 1.28A, at which it will heat up 35C. Physically this part is larger than the original, but there is just enough room if you mill the groove/divot in the plastic, make the groove a bit longer. I got the motor to run again, but sure enough it uses a lot more current than a new motor even with no load. So this motor won't be going on any bots again.

Re: Fixing Burnt Motors
 

I heard that at the Boilermaker Regional they were openly removing the choke in the mini bot motor. There was even a guy who was doing it for anyone who brought a motor to him. He said that it did not improve the motor performance and therefore was perfectly legal to take it out and replace it with a piece of wire.

Has anyone heard anything more on this subject? Although our sampling is very small, we have seen a significant improvement with the choke shunted out.

"Mr. Bill" Beatty

Re: Fixing Burnt Motors
 

I would be extremely interested in this as we have repaired several motors with the most equivalent inductor we could find and have noticed significant power decrease, as well as the motor only running for about 30 seconds before burning up again.

Re: Fixing Burnt Motors
 

Bill,
I had made that recommendation to the GDC to eliminate the expense of replacing motors. However, the official response was to allow repairs with equivalent replacement parts only. As I read that, removal is not legal.

R55..
M. Devices may be repaired, provided the performance and specifications of the component after the repair are identical to those before the repair.

Re: Fixing Burnt Motors
 

As I was at the Boilermaker, I don't know any one doing this. I had two repaired motors using the coils I found from the above post. But never got the chance to try them. We were doing so good that I didn't want to change anything, and left the untouched motors in the mini-bots.