Fixing Burnt Motors

[Bill_B]

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. Maybe the other motor will be repairable while I await replacements from pitsco.

At least I now have gears to play with.

[vamfun]

Originally Posted by Bill_B

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. Maybe the other motor will be repairable while I await replacements from pitsco.

At least I now have gears to play with.

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?

[Alex.q]

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

[Bill_B]

Originally Posted by vamfun

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?

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.

[vamfun]

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

[Al Skierkiewicz]

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.

[Ken Streeter]

Originally Posted by Al Skierkiewicz

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.

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

[Al Skierkiewicz]

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

[androb4]

Originally Posted by EricH

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.

Originally Posted by Chris is me

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

It refers to the fuse thats on the Tetrix battery.

[dudefise]

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!

[Chris is me]

Originally Posted by dudefise

You can, however, fix a motor, but it is not competition legal

Yes it is.

[EricH]

Originally Posted by dudefise

You can, however, fix a motor, but it is not competition legal ...

The part of your statement quoted above became invalid in Team Update #11, with the addition of <R55-M>, which reads:

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

(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:

b) a burned out motor may be repaired by replacement of the burned-out inductor with an identical part.

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.

[Al Skierkiewicz]

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

[Torbots 1197]

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.

[Al Skierkiewicz]

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