Why do wires from battery to motor get hot?

[james_s]

The ones I had that failed were the ones that take AA cells. Like you, I put rechargeable NiMH cells in them. They all failed the same way, where eventually they wouldn't turn on anymore. I broke one open to look inside and found condensation all over the circuit board. Drying them out didn't seem to help, so I assume something got corroded.

I wonder why I've had so much better results with mine. I do smear a little silicone grease on the o-ring periodically, and despite ostensibly being watertight I'm careful to  avoid getting the handle wet. My friend has the same toothbrush and leaves the battery compartment open when it's in the cabinet, I haven't personally found that to be necessary.

[mathsquid]

Is it just me, or does this look like a cutaway of a Saturn rocket?

[coppercone2]

my sonicare tooth brush split and I have been using it without fixing it for 1 year in the shower and some how its still OK. I think its so long the seal on TOP that lets dribble get into the mechanism does not fail, the bottom seal does not matter (you can open it up like a butterfly knife). I should fix that

[Jeff5]

So from what I understand of your posts is that the battery contacts get hot between battery and motor while the motor turns slow but does turn? 
Your battery contact is where all your resistance is for the current that is flowing.  If your brushes were very worn then by all rights that's where your resistance would be causing less current flow. This would not allow enough current through the battery contact, therefor no heat there.  I recommendation is to remove motor and spin by hand.  The motor should spin quietly and smooth ish.  You may feel a little clunkyness from the motor but as long as it is quiet then the bushings are fine(clunkyness is from the magnetic field).  Next would be to pull out and push in on the pinion gear to feel the thrust play, you want some movement but not too much.  Wish I could be more descriptive but I'm not familiar with that exact motor.  After this grab the pinion and try and move it side to side, there should be vertually not movement, there needs to be a very slight play(may not be able to feel any on a motor that small).  Then the next step is to have motor and battery on bench, run good jumpers and try to get motor to spin.  If motor does not spin then motor is broken, probably a shorted winding if all other tests appear good.   Let me know!!

[niemand]

So from what I understand of your posts is that the battery contacts get hot between battery and motor while the motor turns slow but does turn? 
[...]

After I installed the new battery and charged it fully, the motor ran smooth-ish, but slowly, at the first test run lasting just a few seconds. I switched it off and back on, and it just made a buzzing noise and did not run at all. I tried a couple more times, each lasting just a few seconds, and I got the same buzzing/no spin result and noticed the area where the motor is located got very hot. I noticed the display was now telling me to recharge the battery (even though it had just finished recharging just a few minutes ago). I then took off the housing, and indeed the motor was hot. With no housing, I switched on the toothbrush and saw it struggling to spin in fits and starts. When I helped it along using a finger to spin the pinion, it would spin one or two turns only. I decided to test the battery strength by directly connecting the battery to the motor terminals by fashioning a couple of pieces of jumper wire that are a little thicker gauge than that of a small paper clip, bypassing the circuit board entirely. Within a few seconds of making the connections, the wire I was holding burnt my fingertips. Using a multimeter, I tested the battery to have 1.29 v. (Today, with the battery fully charged, I tested it to have 1.45v. It is an "A" cell Ni-MH with 2700mAh.)
As you can see from the video, removing the motor from the frame would be quite a challenge, which I have yet to figure out.

[james_s]

So from what I understand of your posts is that the battery contacts get hot between battery and motor while the motor turns slow but does turn? 
Your battery contact is where all your resistance is for the current that is flowing.  If your brushes were very worn then by all rights that's where your resistance would be causing less current flow. This would not allow enough current through the battery contact, therefor no heat there.  I recommendation is to remove motor and spin by hand.  The motor should spin quietly and smooth ish.  You may feel a little clunkyness from the motor but as long as it is quiet then the bushings are fine(clunkyness is from the magnetic field).  Next would be to pull out and push in on the pinion gear to feel the thrust play, you want some movement but not too much.  Wish I could be more descriptive but I'm not familiar with that exact motor.  After this grab the pinion and try and move it side to side, there should be vertually not movement, there needs to be a very slight play(may not be able to feel any on a motor that small).  Then the next step is to have motor and battery on bench, run good jumpers and try to get motor to spin.  If motor does not spin then motor is broken, probably a shorted winding if all other tests appear good.   Let me know!!

What happens is with motors that have carbon brushes the carbon that wears off clogs the gaps between the commutator sections and this dust is fairly conductive so that results in excessive current draw. For smaller motors that have plain metal brushes these wear down and results in the timing being off and/or the commutator wears down and clogs with copper particles and you get excessive current.

[BeBuLamar]

I don't think the battery is the problem. I am pretty sure the motor is binding somehoe.

[coppercone2]

yeah and really cheap motors like a dremel just have bent pieces of metal (like leaf springs) that make the contact. Better quality smaller motors have like some kind of thicker copper thing (like a machined piece) that looks a little bit like a pork chop with a moon cut out of it, and bigger motors have brushes.