copper wire amperage derating with temperature - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: September 07, 2011, 06:44:23 am
content: We have some returned cooling fans at work. They are burnt out - literally. We opened one up and found that they are 4 brush motors and that each power supply wire has an inductor made of 1mm wire in series with the power supply. I have no idea why, the equivalent motor from an equivalent fan from a better supplier does not have these coils.And it is these coils that have failed, they have literally melted the wire. The fans use up to 15A and the motor runs at 80-90C inside. isn't 1mm wire too small for this job ?

author: Unknown
author_rank: Unknown
timestamp: September 07, 2011, 07:16:39 am
content: Rating for resistive load at 90 deg. would be 14 AWG which is around 1.6 mm diameter.Here it also says you need to derate to 30% of rated resistive load for inductors.http://www.interfacebus.com/Component_Derating_Guide_line.htmlwhich places you near the 7 AWG - 3.6mm diameter mark which is quite absurd actually.

author: linc1
author_rank: Newbie
timestamp: September 07, 2011, 08:40:33 am
content: Something here is really wrong, can you get all the nameplate details of the fan motors, am i right to presume these motors are not DC and should be AC ? even though being a brush motor they should be DC.The inductors i believe are their to limit current or limit harmonics ?RegardsStephen

author: Jimmy
author_rank: Regular Contributor
timestamp: September 07, 2011, 08:56:02 am
content: thats a big fando you have any photos or other info because no one in there rite mind would use 1mm to do 15 ampis that each or 3.75amp each if so run 2 fans off 1mm with no greater than 16 amp fuseHow many volt dc is it

author: Simon
author_rank: Unknown
timestamp: September 07, 2011, 09:58:55 am
content: I'll upload photos in later. these are 1mm inductors getting up to 15A each, I have also found after 4.5 hours running that carbon from the brushes is ending up on them and suspect the brushes are wearing too fast too. it is a 24V DC motor for vehicles and used to cool radiators on mining vehicles

author: Simon
author_rank: Unknown
timestamp: September 07, 2011, 11:42:55 am
content: photos, the first 5 are a new motor after 4.5 hours running it reached a maximum of 85C on the back of the caseThe others are some of the returned ones. I'm going to put a temp sensor inside the motor next to get a more accurate picture

author: Jimmy
author_rank: Regular Contributor
timestamp: September 07, 2011, 12:20:42 pm
content: Ok coolHow long are the cables supplying the fans and what size are they. 15amp on 1mm cable is fine for about 2m but you need bigger cable the longer you go otherwise the volt drop would be too much and would cause excessive arcing of the brushes. 1mm cable branching off inside a motor for 15amp is bad bad bad at 90deg c Most people get the calculations wrong when calculating size of cable.(for the Kiddies With long cable runs you need to account for voltage drop for the maximum demand of the cable. A V75 cable (most) i rated from ambient temp to rise to 75deg C under a curtain fault current and length and should not be operated above this temp as the pvc breaks down. You can get V90 Cable.The Min I would use if specing the Job (I Had to open a Pharisaical book to check) would be 2.5mm.)edit after seeing PICThat is simple insulation breakdown from overheating and shorting the windings. Giving the dreaded dirty brown smell.Do you know what brand type of wire used to create the coil to check temp specs.

author: PetrosA
author_rank: Frequent Contributor
timestamp: September 07, 2011, 12:48:42 pm
content: Looking at the photos, I'm not sure I see how the coils are in series. It looks like the spot weld on the first brush in either direction is through both plates with the inductor tacked on in parallel. If that's how it is, I'd be checking the spot welds for integrity to see if they aren't coming apart and shunting the current to the other two brushes through the inductors.I'd also check the brush springs in a burned motor to see if they've lost pressure relative to a new motor, and check the commutators for out-of-roundness. That's an awful lot of carbon dust for four hours of spinning...

author: Simon
author_rank: Unknown
timestamp: September 07, 2011, 03:20:35 pm
content: each inductor is in series with one of the power feeds (so sees full current), the copper bar they are spot welded to has a slot cut through it, if not the inductors would be bypassed. The motors rotor setup is in two pairs in parallel. The welds of the inductors onto the bar is fine, infact the failure is in the middle of the inductor which in a worse case melded to ferrite core part of which I found in the motor with lumps on melted copper in there too.

author: tecman
author_rank: Frequent Contributor
timestamp: September 07, 2011, 03:27:15 pm
content: Inductors are usually used on motors to reduce EMI from the commutation.paul

author: Simon
author_rank: Unknown
timestamp: September 07, 2011, 05:28:24 pm
content: well looks like they went cheap and used home made fuses instead haha. like i said their competitors motor does not have these wank inductors and they are a very well respected company

author: Jimmy
author_rank: Regular Contributor
timestamp: September 08, 2011, 06:41:39 am
content: oh well throw them in the bin and buy better quality ones

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 01:09:29 pm
content: Quote from: Jimmy on September 08, 2011, 06:41:39 amUnfortunately I can't make that decision, but yes agreed, however i still want to get to the bottom of it all

author: Unknown
author_rank: Unknown
timestamp: September 08, 2011, 01:38:26 pm
content: Why not simply bypass the inductors?

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 02:54:25 pm
content: well we can't do it on production, only as a test

author: Jimmy
author_rank: Regular Contributor
timestamp: September 08, 2011, 03:13:47 pm
content: Np SimonThe commutator needs to be more cylindrical it looks more like a cheese greater and needs to be smoother.The inductors may be overheating because they are too close to the magnets or the magnets are stronger than normal which would cause more back emf in the inductor. Is this a new design or an old one that has changed

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 03:29:13 pm
content: well the inductors are as far from the permanent magnets as they can be, are you saying that putting then in a variable magnetic field which will be produced by the rotor could be causing havoc in the inductors. I would like to get a scope over an inductor as it has occurred to me that there much be an AC component here as well as the rotor commutes and instantaneous power consumption is never the same.i will measure the commuter for roundness

author: Unknown
author_rank: Unknown
timestamp: September 08, 2011, 03:52:41 pm
content: So two coils connected in series are magnetically coupled in such a way as to cause more current to flow? I think it is impossible...Any magnetic interaction should cause the current to drop, not to increase.

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 04:37:19 pm
content: well if the magnetic field induces an EMF in the inductors that does not agree with the supply power going through (or adds to it) could it cause a bigger voltage drop and over heating ?

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2011, 04:45:58 pm
content: Quote from: HAL-42b on September 08, 2011, 03:52:41 pmIt depends on the phasing.Think of an isolation transformer with two 230V windings. If they're connected in parallel, in phase then hardly any current will flow but if one is anti-phase there will be a huge short circuit current. This obviously isn't as bad with loosely coupled inductors but it can be a problem nonetheless.

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 05:24:58 pm
content: the inductors are on oposite sides of the motor and I don't think they can affect each other that much as we are talking less than 20 turns of wire. They may of course be interacting with the rotor windings. I'm going to take my scope in tomorrow and hopefully get hold of the blasted company power supply that everyone wants when I do

author: Unknown
author_rank: Unknown
timestamp: September 08, 2011, 07:17:47 pm
content: Quote from: Hero999 on September 08, 2011, 04:45:58 pmIsolation transformer is a case of parallel coils and alternating current. I get the notion that connecting them anti-phase would make them behave essentially as a low value resistor. But what can one possibly do to the two coils connected in series passing DC to cause them to flow more average current? If there is an answer, it is basically the key to room temp superconductivity. This is why I think it is not possible.

author: Simon
author_rank: Unknown
timestamp: September 08, 2011, 08:45:04 pm
content: my thought is that the magnetic field may induce an AC voltage superimpossed and adding to the coils power dissipation, it takes 1083.62C to melt copper and that copper definitly melted, the ferrite cores did too, what happens to the magnetic properties of ferrite with heat ?

author: IanB
author_rank: Super Contributor
timestamp: September 08, 2011, 08:48:46 pm
content: Quote from: Simon on September 08, 2011, 08:45:04 pmMost likely the motor stalled, drew a short circuit current, and things got red hot and burned out. Check for things that could stop the fan from turning, like dirt in the bearings, obstructions to the fan blades, worn out brushes on the commutator, something like that. Before getting into exotic causes, look for some mundane causes.

author: gregariz
author_rank: Frequent Contributor
timestamp: September 08, 2011, 08:56:40 pm
content: Quote from: HAL-42b on September 08, 2011, 03:52:41 pmIn the early days of coils and transformers, often windings were split with one half being on a rotatable shaft. In one direction inductances added and in the other direction inductance was subtracted (variometer). This was used most commonly in early radio as a means of inductive tuning, before variable capacitors were commonly available. Likewise early RF resistors were wirewound using two layers with the second layer opposing the first layer in order to provide resistance but cancel inductances.

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 05:38:24 am
content: Quote from: IanB on September 08, 2011, 08:48:46 pmapparently stalling the motor does not cause any extra current draw, however I assume it would still make more heat

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2011, 06:24:54 am
content: Quote from: Simon on September 09, 2011, 05:38:24 amReally? That would be different from the behaviour of most electric motors.I believe that the current through DC motor windings is restricted normally by the back-emf generated by the turning armature. Essentially the windings are an inductor and the motor is manufacturing AC current out of the DC supply through the commutator, and inductors have impedance to the flow of AC current. The windings themselves will tend to have a low DC resistance for efficiency, as higher resistance to current flow will just waste energy as heat and increase the cooling requirements.Therefore, if you stall the motor you now have a low resistance DC path for current to flow through the windings. The current may increase dramatically in this situation and could certainly destroy the motor if there is no fuse or cut-out to save it. This is also the reason why motors tend to have a very high surge current on start up (why big motors make the lights go dim when you switch them on).

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 06:46:43 am
content: yes, I'm aware of all that but i was told that the stall current does not go up. of course the person that did it may have done it wrong. I will have to verify.another thought i have is that the coppers resistance will increase with heat ? by how much ?

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2011, 07:09:09 am
content: Quote from: Simon on September 09, 2011, 06:46:43 amI don't think that would be it. The kind of destruction in the pictures suggests there must have been a massive over current; many tens of amps, over 100 A even, and vastly more than the normal running current. Copper wire doesn't vaporize like that through simple overheating.

author: Jimmy
author_rank: Regular Contributor
timestamp: September 09, 2011, 08:16:15 am
content: Quote0.00399 ohms per Deg CThe ferrite core may have cracked due to thermal shock.If you stall the motor it will heat up pretty quick. As IanB said motor stalling would cause these kinds of faults. Maybe it is just crap in the bearings.

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 09:02:25 am
content: well i did some research:delta P = alpha x deta T x resistivity@20Cwhere:delta P is the resistivity @ the given tempdelta T is the temperature difference between the new temp and 20Calpha is the temp coef.so that basically means that 100C-20C = 80 x alpha = 3.12 which multiplied by the resistivity makes the resistance 3x what it was originally thought to be. Is that right ? in fact I found information saying that wire is to be derated to 33% @90C (because the resistance has gone up by 3 times.

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 09:05:39 am
content: Quote from: Jimmy on September 09, 2011, 08:16:15 amcracked ? how about gone molten !

author: Jimmy
author_rank: Regular Contributor
timestamp: September 09, 2011, 11:22:15 am
content: Quote from: Simon on September 09, 2011, 09:02:25 amIn that case you need a bigger conductorAt 25deg C a 0.912mm copper conductor will melt at 70amps in about 10 seconds.If its derated by 33% it would be 47 amps and you only need 15Is this riteR of Circuit = 1.6and you are saying it raises to 4.8 ohmscan you measure the R of the fan just turn the blade slowly while doing it to see if it changes much.What size cable did they use on the rotor?My bet is still on a stalled motor or someone putting 240v into it

author: FreeThinker
author_rank: Frequent Contributor
timestamp: September 09, 2011, 12:12:13 pm
content: I would send samples back to the manufacture for a technical report. They are in the best position to analyse the evidence and advise. I suspect that it is caused by overheating either ambient or fault generated. My experience is that even moderate over temp in the brush box / commutator area will cause expressive brush wear (due to brushes expanding and sticking in the brush box) This then has a cumulative effect of generating more heat due to poor contact of the brushes causing more arcing of the commutator and so to eventual burn out. DC motors should not as rule run hot (unless designed too do so) so you need to fully check the specs of the motor and ensure you comply with them. Another thing to check is the rating of of the motor, ie is it continuously rated ? a lot of fan motors are not. The inductors connected across the brushes are for arc quenching and WILL fail just as in the photos due to excessive arcing at the commutator. Remember that Ambient temperature needs to be considered too, air cooling can only cool at best down to this temperature. Blocked filters or restricted air flow can raise temperatures significantly in localised areas way above normal parameters.

author: Jimmy
author_rank: Regular Contributor
timestamp: September 09, 2011, 12:19:09 pm
content: I think Simon is the manufacturer

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 12:22:58 pm
content: well today I debuked the science fiction findings that stalling the motor causes no increase in current (I should have known better than to believe it) and did a stall test: 66A, we have also noted signs of melted insulation on the rotor, with a stalled motor the temperature increases very quikly, in a matter of seconds by 4C so I am now looking to the possibility that the fans have been overloaded physically and the heat has built up. The poor rating of the inductors has caused and easy failure.We also opened up the 12V version of this fan. Of course the rotor windings are a little thicker but interestingly the inductors are not there. So why put the inductors on the 24V version ? EMC ?The inside of the motor reaches 126C in a 35C ambient. Monday I will try and get the test area cleared and used

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 12:25:33 pm
content: Quote from: Jimmy on September 09, 2011, 12:19:09 pmnope we just stick them on radiators

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 12:27:29 pm
content: Quote from: FreeThinker on September 09, 2011, 12:12:13 pmwe should end up doing that, but we are trying to get some information ourselves as I think they are a bit micky mousethe inductors are in series with the brushes not across them

author: FreeThinker
author_rank: Frequent Contributor
timestamp: September 09, 2011, 01:29:07 pm
content: Quote from: Simon on September 09, 2011, 12:27:29 pmMy bad terminology. The chokes are connected so (see attachment) and under excessive arcing the brushes short across the commutator, and as you can see all that limits the current are the two chokes, Poof!

author: Simon
author_rank: Unknown
timestamp: September 09, 2011, 09:04:49 pm
content: well these are imply in series, there is no huge effort on EMC, ther is a small cap in parallel with the input