Why do wires from battery to motor get hot? - Page 1
Forum section: electronics / beginners

author: niemand
author_rank: Contributor
timestamp: September 02, 2022, 03:54:35 am
content: I have a small DC motor from an old electric toothbrush that struggles to run. I tried connecting a fresh 1.2V NiMH "A" cell directly to it, and I felt the wires get very hot within seconds while the motor struggles. What do you suppose is the cause? Could worn brushes cause this? I haven't been able to take the motor apart yet.

author: amyk
author_rank: Super Contributor
timestamp: September 02, 2022, 04:04:45 am
content: Everything is a resistor and dissipates power. The wires may be too thin to carry the current.

author: niemand
author_rank: Contributor
timestamp: September 02, 2022, 04:18:07 am
content: The wires I used were as thick as the ones used to make paperclips. All I did was use those wires to connect the same battery used in the toothbrush directly to the motor itself, bypassing the circuit board. In fact, come to think of it, before I removed the battery to conduct this test, the toothbrush itself got very hot when the battery and all the parts were assembled as the toothbrush should be while the motor was struggling to run.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2022, 04:45:07 am
content: The motor is broken and is acting as a short circuit on the battery. An NiMH cell can supply a lot of current, and it can easily make wires get hot with a short circuit. If you leave it connected too long the motor windings will burn out.

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2022, 05:13:32 am
content: An alkaline battery quickly goes into current limiting lower voltage mode and the severity of short circuits related to them are much lower then short circuits related to NiMH. I made a big lab NIMH battery pack, and I actually bothered to fuse it since it can put out a impressive amount of short circuit current (I can bypass the fuse when I need a 'dumb' power source). This is a concern when replacing alkaline batteries with NiMH, many things can tolerate a shorted alkaline but possibly not a shorted NIMH battery. And some things can be over powered by the NiMH battery also. Have you tried it with a alkaline ? (if originally made for one).

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2022, 06:23:51 am
content: If the motor is struggling to turn it is faulty. A healthy motor will spin fast and freely. Trying to use an alkaline cell will make no difference if the motor itself is at fault.It is the back EMF generated by the motor spinning fast that limits the current through it. If the motor doesn't spin fast there will be insufficient back EMF and an over current situation will occur.

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2022, 06:50:34 am
content: I wonder if a NIMH battery can run a motor too hard with too hot arcing and wear it faster (if designed for alkaline). that was my train of thought. It seems that NIMH batteries are popular for powerful flashlights that have a 'turbo' setting where they can only run for a minute or two before over heating, since primary cells are not capable of supplying the amount of current they require.

author: niemand
author_rank: Contributor
timestamp: September 02, 2022, 09:17:02 am
content: Quote from: coppercone2 on September 02, 2022, 05:13:32 amThe toothbrush originally uses a NiMH battery of the same spec from the factory. I did not try with an alkaline battery.Here's the toothbrush in question:(https://www.ifixit.com/Guide/Braun+Oral-B+Professional+Care+9900+Triumph+Ni-MH+battery+Replacement/28633)The motor can be seen at the top of this pic:That was the part that got very hot rather quickly--like within a minute. A standard brushing cycle on this toothbrush is two minutes.I bought the toothbrush around 2005-2006.

author: macboy
author_rank: Super Contributor
timestamp: September 02, 2022, 01:02:24 pm
content: You say that the motor is struggling to run. I suggest that this is due to some mechanical reason, maybe with the motor, more likely with the stuff that it drives. The battery is fine, the wires are fine, and likely the motor is fine too, as long as it hasn't been damaged from this hard work.The wires are heating up because the motor struggles. A bushed DC motor pulls a lot more current when it slows down due to mechanical resistance. This extra current is happily provided by the NiMH cell (which can easily deliver tens of Amps continuously) but the thin wires aren't intended for that much current and are getting hot.You will need to look at the mechanism. Likely some water/toothpaste has gotten through the seal and fouled it up, causing a lot of friction. You will probably need to clean and re-lubricate it.

author: mikerj
author_rank: Super Contributor
timestamp: September 02, 2022, 01:05:12 pm
content: That isn't a standard brushed DC motor, it's just a coil that gets energised by an AC drive signal to vibrate the brush. Connecting a battery direct to the coil will just cause a lot of current to flow, as you have discovered.

author: Old Printer
author_rank: Frequent Contributor
timestamp: September 02, 2022, 01:25:37 pm
content: Quote from: coppercone2 on September 02, 2022, 06:50:34 amIt is a cells voltage that governs how hard it will push against a given load. Making more current available, by itself, will not cause the load to draw more unless the previous cell was already at it's current limit.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2022, 02:08:27 pm
content: Quote from: macboy on September 02, 2022, 01:02:24 pmThis. The motor does not struggle to run unless something is wrong with the motor or mechanism. Fix the reason for the motor struggling and everything will be fine.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2022, 02:10:06 pm
content: Quote from: mikerj on September 02, 2022, 01:05:12 pmNo, it is a motor that spins. I own and use a similar toothbrush. The brush does not vibrate, it turns in an oscillating manner.

author: niemand
author_rank: Contributor
timestamp: September 02, 2022, 03:30:35 pm
content: Quote from: mikerj on September 02, 2022, 01:05:12 pmIt is a motor that spins, not a coil that vibrates. Here are photos to a lower/later model with a similar design that uses the same motor:(captured from YouTube video by brankly:)

author: james_s
author_rank: Super Contributor
timestamp: September 02, 2022, 04:46:22 pm
content: The motor either has bad bearings, dried up lubrication or other mechanical binding, or the commutator is worn out or clogged with carbon. I can't say without seeing but my first guess is worn out brushes and clogged up commutator.

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2022, 04:53:14 pm
content: the poster is thinking of a sonicare tooth brush (my favorite) that does infact make a AC magnetic field. lots of arguements about which mechanism is better.

author: niemand
author_rank: Contributor
timestamp: September 04, 2022, 10:05:21 pm
content: I partially dissected the toothbrush. The motor is spot-welded onto the frame/cage. I wonder whether I can proceed further.

author: niemand
author_rank: Contributor
timestamp: September 05, 2022, 02:54:49 pm
content: Looks like a burn mark:" alt="" class="bbc_img" />

author: niemand
author_rank: Contributor
timestamp: September 05, 2022, 10:07:11 pm
content: Quote from: niemand on September 05, 2022, 02:54:49 pm

author: james_s
author_rank: Super Contributor
timestamp: September 06, 2022, 12:38:03 am
content: It's probably residue from the welding process.

author: RJSV
author_rank: Super Contributor
timestamp: September 06, 2022, 04:02:33 am
content: I would think, a 10X (current drain) factor comes into play, when stalled by sticky 'debris', for most motors. It's an extreme situation, but I've expected most 'Consumer' products to catch up with that reality. At very least, expecting a toothbrush, to have decent 'care' guidelines. So, yes, excessive current causes heating, but that's got to be considered a long with reasonable care and cleaning. (Maybe manufacturer should be clear on the 'debris' / stalling potential...and how to avoid.

author: IanB
author_rank: Super Contributor
timestamp: September 06, 2022, 07:06:30 am
content: Quote from: RJHayward on September 06, 2022, 04:02:33 amSometimes it is just carelessness by the manufacturer. I went through three Philips Sonicare toothbrushes, and all three of them failed from moisture ingress. Condensation formed on the inside and destroyed the electronics. After the third failure I gave up in disgust and went to a different brand.I would have thought it rather obvious that toothbrushes will come in contact with water, and therefore careful sealing to prevent moisture ingress is a fundamental necessity.I have had no problems with the Braun toothbrush that I have now been using for several years. I always rinse it thoroughly after use and leave it to dry in a ventilated place.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 06, 2022, 04:05:26 pm
content: It's mechanically jam. You should be able to turn the motor easily by hand and no power if not you have to find out what keeps it from turning. The motor can't turn then it would draw a lot of current and so the wire get hots. If it's a bad brush it won't get hot. The problem is mechanical.

author: james_s
author_rank: Super Contributor
timestamp: September 06, 2022, 06:47:01 pm
content: Quote from: IanB on September 06, 2022, 07:06:30 amI've had two Sonicare toothbrushes in 22 years. The first one I replaced after about 8 years when the batteries failed, they were permanently sealed in it so there was no practical way of replacing them. The current one uses standard NiMH AA cells that I swap out when they need charging. I've never had any issues with moisture getting in, the toothbrush handle doesn't get wet, only the brush head does for the most part.

author: IanB
author_rank: Super Contributor
timestamp: September 06, 2022, 07:13:47 pm
content: Quote from: james_s on September 06, 2022, 06:47:01 pmThe 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.

author: james_s
author_rank: Super Contributor
timestamp: September 06, 2022, 07:37:11 pm
content: Quote from: IanB on September 06, 2022, 07:13:47 pmI 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.

author: mathsquid
author_rank: Frequent Contributor
timestamp: September 06, 2022, 10:20:50 pm
content: Quote from: niemand on September 02, 2022, 03:30:35 pmIs it just me, or does this look like a cutaway of a Saturn rocket?

author: coppercone2
author_rank: Super Contributor
timestamp: September 06, 2022, 11:47:48 pm
content: 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

author: Jeff5
author_rank: Newbie
timestamp: September 07, 2022, 02:10:30 am
content: 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!!

author: niemand
author_rank: Contributor
timestamp: September 07, 2022, 02:53:04 am
content: Quote from: Jeff5 on September 07, 2022, 02:10:30 amAfter 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.

author: james_s
author_rank: Super Contributor
timestamp: September 07, 2022, 06:42:46 am
content: Quote from: Jeff5 on September 07, 2022, 02:10:30 amWhat 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.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 07, 2022, 08:21:34 pm
content: I don't think the battery is the problem. I am pretty sure the motor is binding somehoe.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2022, 03:08:32 am
content: 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.