RF transformers and chokes, how hot is too hot - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 10, 2017, 09:17:39 am
content: How hot is TOO hot for an iron dust or ferrite cored inductor wound with enameled wire? Is shining an IR temperature gun on the core and outer windings, assuming a multi layer coil, enough to get a sensible idea, once it's heat soaked? I am running a 50mm computer fan on the outside of the centre iron toroid cored inductor for a low frequency LPF as it got what I thought was uncomfortable hot (68 degrees C when I chickened out, with no fan). With the small processor fan in a milled window in the die cast alloy box the centre and two end toroids are in, with an exit port on the other side, the temps are near ambient. My worry is if the fan croaks....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2017, 09:28:28 am
content: What core, turns, etc. did you end up using?

author: digsys
author_rank: Unknown
timestamp: July 10, 2017, 09:58:21 am
content: We design all our transformers / chokes / inductors / of all materials to work toat least85-90C, which is some go to 150C, for harsh environments.But now it depends on who made it ? What country ? Any standards stickers ? Genuine brand labels ? Even in a worst case, 80C is no issue at all.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 10, 2017, 12:23:03 pm
content: Hi, thanks for the replies, it's a home made LPF assembly for a 136kHz solid state amplifier, about 2kW at full tilt. I couldn't find bigger cores in a suitable material in the UK, or anywhere else, so stayed with the original T225A-2 cores, the end ones have 52 turns and the center one 73 turns of AWG 14 enameled wire. The center one ran hot (to my mind, seems I am worrying needlessly) on long TX periods, the end ones stayed quite cool / cold, so I added the fan to just the middle one and now it stays pretty cool even on extended TX sequences. I just didn't really like the fact it needed forced cooling, but before adding the fan the cast alloy boxes were effectively sealed, so that probably wasn't doing the cooling any favours!

author: CJay
author_rank: Super Contributor
timestamp: July 10, 2017, 01:59:26 pm
content: Anything under the Curie point and you have an inductor but of diminishing value, how much it changes up per degree up to the Curie point should be in the datasheet but I suspect it's negligible for your temperature range.Seem to remember that clamping them or subjecting them to other mechanical stress can affect their performance too, I wodner if that may be similar to the old blacksmith's way of making magnets (wallop them on the anvil with them oriented N/S, and yes, doing that to a ferrite would make lots of spiky bits)

author: Nusa
author_rank: Super Contributor
timestamp: July 10, 2017, 02:55:58 pm
content: Somewhere betwen 100C and 250C, depending on the nature of the enamal on the wire. So either find the specs of the wire you bought, or test pieces of it at various temperatures.https://en.wikipedia.org/wiki/Insulation_system

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2017, 07:37:59 pm
content: Cores are affected by temperature. How much depends on type.Ferrite cores are immediately affected, by proximity to Curie temperature: the closer it is, the less flux is available in the core. It drops off hyperbolically, so most cores are okay at 100C, but it starts to get very questionable above there. Losses and permeability may also change.Powder cores are made with high temperature materials (pure iron's Tc is around 800C!), so the above isn't a concern. What is a problem is breakdown of the organic binder. All powder cores will eventually "cook" through, in an accelerating process that depends on time at temperature. As the binder burns up, losses go up, increasing dissipation, accelerating breakdown.Fortunately,Micrometalshas curves for their materials (they make #2 powdered iron, among others) so you can estimate lifetime.Keep in mind, the temperature in the center of the core may be higher than at the surface.You may also have more wire loss than core loss. #2 is quite low loss, at low frequencies. I wound this inductor,which runs at a similar frequency but much lower power level (it's also T120 size, I think?). With the 10 AWG equivalent Litz cable, I can't tell it's generating any heat at all.Wire size is very important for low frequency inductors. Every layer you wind on top of the base layer, pinches the current in the layers below (proximity effect). Losses go up rapidly. Litz staves this off by dividing the current into myriad fine strands, each strand being smaller than the "pinched" skin depth so each strand carries current evenly. Unfortunately, modest size Litz is also kind of hard to come by, and expensive...As for cooling, a fan is definitely an option. The conductivity of these core materials isn't terrible, so they can be cooled effectively without worrying too much about the hot-spot temperature in the middle. And if it's the wire generating heat, that's even better.Tim