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Author Topic: Dissipation rating of balun cores  (Read 904 times)
VK1OD
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« on: October 19, 2009, 02:41:11 PM »

A number of recent threads have included mention of the power rating of baluns, and balun cores.

I performed a simple experiment by heating an FT240 toroidal core in an oven for a couple of hours at about 100°C.

I then took the core out and suspended it on a string, oriented with its axis horizontally, in still air at 20°C. I measured the temperature of the core surface from time to time, and plotted the data points, then fitted an exponential curve to the data points.

The system has a time constant of 1210s, ie the time for the temperature difference between the core and ambient to decay by 63% is 1210s.

Given that ferrite has a specific heat capacity of 800J/kgK, the mass of the core at 213g, and the time constant discovered above, Rth is 0.14W/°C, and dissipation of the naked core at 250°C in still air at 40°C is about 30W. (250°C is about the safe limit for #52 or #61 material, it is much lower for #43.)

Of course, the things we do when winding the core may thermally insulate it, and in a small enclosure, continuous dissipation may be quite a deal lower.

The time constant discovered above implies that if the core was heated internally (eg RF losses), that the core suspended in still air would achieve just 63% of its final temperature rise in about 20 minutes, or in 5 minutes would achieve just 12% of its final temperature rise.

Owen
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HFRF
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« Reply #1 on: October 19, 2009, 04:35:17 PM »

<<<250°C is about the safe limit for #52 or #61 material, it is much lower for #43.)>>>

This so called experiment is stupid.  No safety rating agency like UL would approve anything that got hotter than about 60 degrees C.  It would burn people, it would catch mounting materials on fire if anything got 250 degrees C or anything half near it.
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HFRF
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« Reply #2 on: October 19, 2009, 04:35:49 PM »

<<<250°C is about the safe limit for #52 or #61 material, it is much lower for #43.)>>>

This so called experiment is stupid.  No safety rating agency like UL would approve anything that got hotter than about 60 degrees C.  It would burn people, it would catch mounting materials on fire if anything got 250 degrees C or anything half near it.
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W8JI
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« Reply #3 on: October 19, 2009, 06:02:57 PM »

I'm not sure what your obective is Owen. The original topic was measuring loss, not having a core reach the ultimate temperature in a test.

What are you trying to do, and why do you think a core has to get that hot or reach full teperature in order to determine the quantity of heat released in an enclosure?

Tom
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K4DPK
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« Reply #4 on: October 19, 2009, 06:40:26 PM »

Owen...

If I understand the test, your experimantal balun would have to be at least 98% efficient at the 1500 watt level, else it might suffer permanent damage.  I suspect the use of a layer of glass tape would make this much, much more critical?

I recall once tracing some TVI and BCI to a damaged ferrite balun core.  It was then I began using only iron powder cores in antenna baluns.

Phil C. Sr.
k4dpk
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WX7G
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« Reply #5 on: October 19, 2009, 06:47:09 PM »

HFRF: The CE and UL limit for an external surface is 70 deg C. For an internal component it is not uncommon for a magnetic component (ferrite) to be allowed to operate at 130 deg C. Semiconductors are often operated this hot and hotter. And glass vacuum tube envelopes reach 300 deg C; even hotter for old sweep tube rigs.

I think the data provided by our VK friend gives us useful data. We now have a good idea of how long a balun needs RF applied to approach thermal equilibrium (3 time constants).

Another useful fact is that for free air convection cooling of an item such as a balun we can figure on a 35 deg C/W thermal resistance.

How does UL determine the 'hot spot' temperature of a magnetic component? The usual method is the 'change of resistance' method where the temperature of the copper wire is measured indirectly via the change in resistance. The tempco of the resistance is 3920 PPM/deg C.
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VK1OD
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« Reply #6 on: October 19, 2009, 07:50:08 PM »

Sorry, I gave thermal conductance, the inverse of Rth. Rth is 7°C/W.

These figures are for a balun core, bare in still air.

Of course, enclosures make a difference, and windings and insulation make a difference. In most cases, the net effect will be that an assembled balun will stand less power, probably much less power for an acceptable temperature rise.

A balun inside an ATU might be permitted to operate at quite high temperature safely, subject to insulation ratings etc. As I mentioned, some mixes are not suited to such high temperatures, eg the Curie point of #43 is around 130°C.

In reality, a balun assembly based on one of these cores might have a continuous dissipation rating somewhere from 5 to 30W.

As Phil has noted, such a balun rated for several kW continuous would have to be extremely efficient. For example, for a 1kW continuous rated balun to dissipate just 10W, it must have total loss (copper and core) less than 0.05dB.

Tom, the relevance of this to the other thread, is that at extreme loads where efficiency might be well below 50%, a 1kW rated balun may well be not able to withstand 10W continuous power. Some of the discussion perhaps is about what "continous power" means in balun ratings.

Owen
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W6RMK
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« Reply #7 on: October 19, 2009, 08:52:52 PM »

Interesting experiment.
As pointed out:"How does UL determine the 'hot spot' temperature of a magnetic component? The usual method is the 'change of resistance' method where the temperature of the copper wire is measured indirectly via the change in resistance. The tempco of the resistance is 3920 PPM/deg C."  

This is how we used to measure thermal properties of things like motors and transformers. We'd short one winding of the transformer or lock the rotor of a motor (same thing, really) and apply power with a variac, periodically checking the resistance.  You put it in a suitable insulated box and bring it up slow so that it reaches internal equilibrium.  

Then, you'd take it out of the box (or, more accurately, take the box off of it sitting on the bench fixture) and run it (to see how fast it cools down). Likewise, you do this to evaluate motor running temperature in the installed case.

Why do the test hot, as opposed to room temp? The idea is to get an idea of how much effect you get from thermally driven convection.  Of course, just about any external air flow changes the picture dramatically..it seems the slowest fans in the world move air faster than thermally driven convection.
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W6RMK
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« Reply #8 on: October 19, 2009, 08:53:01 PM »

Interesting experiment.
As pointed out:"How does UL determine the 'hot spot' temperature of a magnetic component? The usual method is the 'change of resistance' method where the temperature of the copper wire is measured indirectly via the change in resistance. The tempco of the resistance is 3920 PPM/deg C."  

This is how we used to measure thermal properties of things like motors and transformers. We'd short one winding of the transformer or lock the rotor of a motor (same thing, really) and apply power with a variac, periodically checking the resistance.  You put it in a suitable insulated box and bring it up slow so that it reaches internal equilibrium.  

Then, you'd take it out of the box (or, more accurately, take the box off of it sitting on the bench fixture) and run it (to see how fast it cools down). Likewise, you do this to evaluate motor running temperature in the installed case.

Why do the test hot, as opposed to room temp? The idea is to get an idea of how much effect you get from thermally driven convection.  Of course, just about any external air flow changes the picture dramatically..it seems the slowest fans in the world move air faster than thermally driven convection.
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KB4QAA
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« Reply #9 on: October 19, 2009, 09:42:42 PM »

I guess the practical application of this data would be to develop de-rating curves for a particular balun so the operator could avoid burning it up?

B.
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W8JI
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« Reply #10 on: October 20, 2009, 03:23:40 AM »

OK, I understand your point now.

What you are trying to say Owen is in short term use a balun's thermal lag can allow a relatively low power CCS capable balun to handle fairly high power.

There can be no disagreement with that since in nearly all cases, unless it is pulse or a very low average power or duty cycle, the limitation is heat build up and not magnetic saturation.

Tom
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WA3SKN
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« Reply #11 on: October 20, 2009, 04:26:29 AM »

Interesting experiment.
However, the usual concern is that of saturating the core, this is controlled by the current and magnetic fields involved.  And I would expect would be reached before the maximum allowable tempurature would be reached with the core.  I do see tempurature as a factor over a longer term and part of the overall power rating, though. And I doubt that by simply adding a fan more power could be handled.
But, more experimenting is indeed indicated.
73s.

-Mike.
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HB9PJT
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« Reply #12 on: October 20, 2009, 04:36:31 AM »

"And I would expect would be reached before the maximum allowable tempurature would be reached with the core."

I don't know for the ferrit but iron puder toroids reach the allowable temperatur much much earlier than saturation.

73, Peter - HB9PJT
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HB9PJT
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« Reply #13 on: October 20, 2009, 04:36:40 AM »

"And I would expect would be reached before the maximum allowable tempurature would be reached with the core."

I don't know for the ferrit but iron puder toroids reach the allowable temperatur much much earlier than saturation.

73, Peter - HB9PJT
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W5CBO
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« Reply #14 on: October 20, 2009, 08:28:23 AM »

If it gets too hot,get a bigger one. Now get on the radio and have fun.
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