Fixing Burnt Motors

[jskene]

Has anyone found an equivalent part?

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

[Dad1279]

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.

[Torbots 1197]

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.

[Siri]

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.

[jskene]

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

[Bill_B]

Originally Posted by Torbots 1197

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.

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.

[DarrinMunter]

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.

[Al Skierkiewicz]

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.

[philso]

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.

[DarrinMunter]

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.

[Al Skierkiewicz]

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

[scooperman]

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.

[Bill Beatty]

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

[Jimmy Nichols]

Originally Posted by Bill Beatty

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

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.

[Al Skierkiewicz]

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.