How to estimate the cw currrent rating of a choke spec'd for intermittance ?

[GrimJosef]

I have a vintage choke (Parmeko Neptune style) which, as the rating plate says, is specified for 360mA DC operation assuming that it runs intermittently - 5 minutes on, 10 minutes off.

I'm curious to know what the current rating would be if I were to run the choke continuously. The rating plate timescales are long enough that the manufacturer must be worried about thermal effects - presumably due to copper losses and iron losses.

The copper loss contribution to the heating is quite easy to work with. Neglecting the winding's resistance change with temperature, the ohmic heating power will just scale as the current squared. So to go from 1/3 duty cycle to continuous operation while keeping the average heating constant I'd just have to drop the current by the square root of 3 i.e. down to 200mA or so.

I'm not familiar though with how important iron losses might be, nor how they scale with quasi-DC current. If the losses are important and if they scale more slowly than I squared then I might have to drop the current by more than a factor of root 3 to keep the average heating constant.

Does anyone know what the practicalities might be here (any explanation much appreciated !).

Cheers,

GJ

[RogerEvans]

You could try attaching a small thermistor to the iron and then to the winding and measuring the steady state temperature as a function of a) DC current and b) 100% ripple current from an unsmoothed rectified AC supply. I have to admit this sounds more like an undergrad
physics experiment than a pragmatic answer!

Roger

[kalee20]

Iron losses will depend how much ripple there is, which I guess you don't know! But, it's probably negligible anyway.

The choke will be gapped to not saturate at the 360mA. But it may overheat with that 360mA running continuously. Personally I'd put DC through at 200mA, assess temperature rise after 3 hours, if OK start increasing. After a day you should have a good idea.

One way to assess temperature is measure the resistance cold, measure it hot, and use the temperature coefficient of copper to work out temperature (0.39% per degree C assuming you start at 20 degrees C).

[Radio Wrangler]

You have two current limiting factors. The first is saturation. If the choke is rated for 360mA even for 5 minutes, then it ought to be OK on keeping its inductance, so we don't need to worry further about saturation, unless the temperature of the core starts to approach its Curie point.

Heating-wise, root-three sounds sensible to me.

Heating due to core losses is a function of the AC component (frequency and magnitude) expressed across the choke. This isn't mentioned in the 360mA DC bit. So assumptions abound.

What will be the temperature limiting factor? Well for transformer/choke lams I think you're going to run into trouble with enamel and formers before the core gets too hot.

If we assume it's for a power supply with full wave rectification and 100Hz ripple, then the AC relted core dissipation is likely included in them specifying the DC current, meaning the choke is intended for use in a supply with 360mA DC output.

If you want to do it formally, you can look at the peak stored energy (1/2*L*Ipeaksquared) you can then look at inductor data from saturation flux and permeability, dimensions and arrive at peak energy capability. I had to do a lot of this for a power supply scheme using choke input filters at 40kHz and it was a case of designing for enough L that that rectifier currents never cut off even at the minimum load impedance. Most people design simple reservoir rectifiers, so the calculations involved with choke input filters can fool your intuition.

Measuring the temperature in use can either be seen as a bit of a cop out, or a necessary double check on the finished work, so you have to do it anyway. It depends on how much money/effort gets invested before you try it. It seems to me that your chances are good.

David

I've just seen Peter's post. Using the copper thermistor trick with the thing hot, gets you the average temparature of the primary heating element, the copper, which is just what you want to know.

[Ed_Dinning]

Hi GJ, plenty of suggestions there, the tempco of copper measurement will be most accurate and given long enough the rise should even out with the additional heating from the iron losses. Being oil filled that will help the thermal transfer.
I see it is called an AF choke, rather than a DC one, so may have slightly different characteristics to the conventional smoothing PSU smoothing choke.

Ed

[GrimJosef]

Thanks for your thoughts everyone. I should have said for those who might not be familiar with the Parmeko Neptunes, these were a range of wound components, each sealed inside a can which contained insulating oil. So cooling to the oil is good but physical access to the iron and windings is impossible. The variation of copper resistance with temperature should still be a reasonable measure of temperature rise though.

I fear you're right David - the iron losses will depend on the properties of the time-varying component of the current. I don't even know if the choke is meant to be used in a choke-input supply (so an LC filter after the rectifier) or a capacitor-input one (CLC filter). It sounds like a measurement in the actual supply might be the only way to get a firm answer.

In fact I'm hoping to turn the choke into funds for other projects, so it'll have to be someone else who does any measurements.

EDIT: I wasn't quite sure about the 'AF' reference either Ed. I take it no-one would need telling that this isn't an RF choke. But as you say it may not be designed primarily for a PSU application.

Cheers,

GJ

[Ed_Dinning]

Hi GJ, the fact it has a 2.5KV rating and is specified as an AF choke with a duty cycle seems to rule out it being designed for PSU use. Possibly a mod choke in a TX, or some even more exotic application in a servo drive etc.
Any luck with a google of the PLA number etc?

Ed

[Radio Wrangler]

Choke mod is what it suggests to me. 2.5kV would suit 813(s) and a good number of other bottles

David

[kalee20]

It's possible it may have been designed as a modulation choke for an AM transmitter, in which case it would have to carry the quiescent current of the modulating valve (single-ended) as well as the RF PA stage current. The 2.5kV voltage rating does somewhat support that, as does the 'AF'.