Maximum Coil Temperature

[srnoth]

Hey there,

I recently built a high-amperage battery charger from two dead UPSs. Works great using the two transformers connected to an automotive alternator rectifier.

The charger works great, approximately 40 amps into a dead battery, or can power two 100W 12v lights. The problem is that the coils on the transformer get very hot after a couple minutes. I tried attaching a cooling fan to them, and that works to an extent.

My question is how high a temperature can such coils handle before they fry/melt-down etc? For the sake of discution, not just transformer coils, but the coils in alternators and such. And what happens when you go past that temperature? Do the coils literaly melt and short-circuit everything?

Yours,

Stephen.

[p0lizei]

The copper (depending on the thickness of the wiring you're using) probably won't MELT-- it's the insulation that you really have to worry about.  I did a google search a while back on "high temperature coil insulation" a while back and found some interesting information.

Here's what I have at the moment:

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http://www.nasatech.com/Briefs/Aug02/LEW17164.html

http://www.mwswire.com/inschar.htm

http://www.reawire.com/pyreml.asp

Those were on the first results page, I'm sure you'll find more on subsequent pages, and by moving some of the wording around in the original search query.  

Copper has a high melting point, but even at lower temperatures it will become very brittle, and I would imagine this probably does not do well for conducting electricity (not to mention maintaining a closed and reliable circuit), though I could very well be wrong here.  My SAAB had a major electrical crisis last year when the heat shield for the catalytic converter had rusted out, allowing intense heat to thoroughly cook a section of the central wiring harness.  The wiring that I was cutting out had become very brittle.  I don't know to what extent that heat was getting through the "floorboards" and into the harness, so I wouldn't be able to give you an exact temperature, but hopefully that kinda gives you an idea of the range you can work with (if you happen to know how hot a cat. runs and would be able to make an estimate from that).

[monte350c]

Hi,

(If you're lucky) you will have a part number or manufacturer sticker on the transformer and can use that to look up the class of insulation they used on those particular units.

There's a good page of transformer faq's at:

 http://www.hammondpowersolutions.com/faqlist.html

These guys (Hammond) have been at it in the transformer business for quite some time. If you see something like 105 rise that means 105 degrees C is the maximum allowable temperature rise in operation. BTW that is a quite common spec...

Good luck!

Ted.

[hiker]

yes the wire can melt!!!!!

picked up an old 2hp motor --inside one stator slot the windings were melted together.

[richhagen]

Yep, Hiker, but it is quite likely that the insulation failed first, causing the coils to short, causing them to heat even more and then melting them.  The insulation usually fails long before the wire glows bright red and reaches the melting point of the copper.  (I've inadvertently tested this theory on several occasions)  Rich Hagen

[old55olds]

Your biggest problem will be how to tell how hot the coil is in the center.  Most insulations for transformers are rated around 600 degrees but that is maximum.. Usually if it is too hot to the touch it will have a very short life. Most consumer goods are rated at the max. So just feel one that you know is good.

[finnsawyer]

Have you connected the two transformers in parallel?  If so, you might have a voltage imbalance causing a large current flow through one of the transformers on the output.  You may wish to add small resistors in the output of each transformer before the diodes and any common connections.  This would allow you to determine the current out from each transformer.

[srnoth]

Actually the alternator rectifier is designed for a two phase alternator, so I just hooked each transformer to a different phase. Both are about the same temperature when I touch them after 5 mins at 20amps. After about 10 mins at 20amps they get a bit too hot to touch. I did an experiment to see if a cooling fan would help, and it certainly kept the outside cool enough to touch. But will the core temperature be cooler as well. Even if the outside is cool, will the core still overheat?

I have already removed all that paper and plastic rap that surrounds the coil, partly because it makes a nasty smell when it gets hot, but more importantly it allows the air to flow directly over the coils. My idea is to put two high capacity computer cooling fans on either side of the transformer (for both). This should maximise cooling. Hopefully it will work.

[RC in FL]

Sounds like your are not sure what you have created.  Really be careful that you have not created a fire hazard.  For a charger, my criteria, if you can't touch it without getting blister burned then it is too hot.

First step is knowing what transformer you are starting with and if it appropriate for what you are doing with it.  Just because it came from a UPS does not mean it is right for your charger application.  Did it come from a 12v, 24v, or greater UPS system.  

Measure the open circuit AC voltage from the transformer.  If you are more then 15 to 16 vac it can be problem using it in the appearent charger config you are attempting.  This is assuming you are going after a single 12 v battery charger.

If you can, load the transformer with power resistors or water heater elements in attempt to get at least a 10 to 20 amp AC load on it if you think that is what the transformer is capable of.  Measure the vac with load.  Should not drop more then 3 or 4 vac from unloaded voltage.

Second,  If you claim you are delivering 40 amps then the alternator rectifiers are dissipating about 20 to 30 watts.  This is alot of heat to get rid off and requires a large heat sink.  In a car alternator the rectifiers are mounted to case of with the pulley fan blades running air through the alternator to help cool the rectifiers.

At the end, you have a very uncontrolled charger that without alot of careful supervision while charging a battery will likely damage the battery.

Anyway, to answer your specific question.  The dominate problem excessive heat causes in a power transformer is wire insulation degradation.  The enamel or Formvar insulation will get brittle and cracked when subjected to >100 deg C.  Eventually you will get a turn to turn short between the windings.  Assuming you have a fuse in the AC plug side, it will blow when you get a shorted transformer.