Passive high-voltage dummy load

[Mark R]

That is a poser Al, not one I can answer, but I look forward to further input.

cheers Mark

[Skywave]

O.K.: temperature-rise measurements. The usual scientific approach: verify the concept with known-good test equipment and set-up prior to entering uncharted waters: in this case, try the Fluke kit first with a 'known-good' transformer.

Photo. 1: the general set-up with the thermocouple in use. (Sorry about the background clutter). The transformer shown is on a no-load / temperature rise test. This particular transformer is the one I used for the design & development work: it has a secondary winding of 235v – 0v – 235v. The current rating is not known (yet).
The result using this test transformer looked sensible.
[attachment=3536]

Photo. 2: A close-up view of the Fluke thermocouple accessory.
[attachment=3537]

So now for an unknown transformer: Photo 3.
[attachment=3538]

And that is when the fun began. On a no-load test, the temperature rise was so large that I didn't need the thermocouple to tell me something was seriously wrong. After 30 minutes in this test condition, the core was far too hot to even touch! :omg: I initially suspected that I may have configured the split-primary winding incorrectly and was over-running it, but the HV winding voltages read 330v - 0v - 330v . . . as stated on the transformer and 660v on the transformer test unit. (The LV windings were on no-load ~ and measured correct, too). If I had seriously mis-configured the primaries, then those voltages would have been different . . . and incorrect.
There appears to be one primary winding of 110v and another primary winding with taps labelled from 120v to 245v. I am sure that I correctly interpreted that and connected them in phase-additive to match a 230-v supply: linking the 110v point on primary 1 to the 120v. point on primary 2 and supplying input 230v across the 0v. of primary 1 and the 245v point on primary 2. That's much easier to follow in a drawing: the photos. below help clarify:
[attachment=3539] [attachment=3540] [attachment=3541]

Hence, question: Do I have a failed transformer or have I made a transformer configuration error?

Al.

[AlanBeckett]

Al,
The laminations look a bit loose, but ......
What was the magnetising current?
Alan

[Skywave]

Ah-Ha! Good question Alan. That's the primary current on no-load, isn't it? The current that is required to establish the magnetic field in the core prior to the transformer delivering power to any secondary winding.

I'll go and measure it now: don't g'way . . . .

Subsequent edit:

Cold, at switch on: 400 mA, dropping to 300 mA after 5 minutes.
That isn't right! 100 mA I might believe - but not that figure.
I reckon it's got a shorted turn on one of its primaries.

Al.

[AlanBeckett]

Upon my soul, that's high. I would have expected low tens.
Could you measure it on another transformer just in case my memory has shorted turns?
Alan

[Skywave]

"High?" Yes, agreed! I was expecting something in the range 10 → 40 mA at most.

In the above photo. 1, you see the black-coloured transformer that I used for the initial R & D.: I measured the mag. current on that: 100 mA. It's temperature on N/L rose by 10 °C after 15 minutes and seemed to flatten out. The ambient temp. in the 'shop was about 4 °C (brrr!). Although that current seems a bit on the high side, that temp. rise does not seem to be an issue for concern. Besides, it's quite an old 'former, and I expect that the lams. aren't as good in terms of B/H as the later (suspect) Partridge transformer.

Al.

[TheChipmunk]

Al, I may be being stupid, but have you measured for DC continuity between the 'two' primaries? I believe you said you have one labelled 0-110 and then 120-240 in steps. Maybe they're all ONE winding, and you've effectively shorted the 110 and 120 taps?

Hope this makes sense and isn't 'teaching my grandmother to suck eggs'!
Kindest regards, Martin

[Skywave]

Right now, off the top of my head, I can't be 100% sure: this was quite a while ago now. But I'm sure I would have done: two separate primary windings. Anyway, it's all a bit irrelevant now, since I've parted company with it.

Al.

[pwdrive]

Al, I had National Dog house PSU fry up but the HRO worked fine during the proccess ie: black pitch bubbling away, secondary voltages within spec, receiver working drawing its normal current.
This happened without any prior alteration of any of the primary tappings/combination etc. I put it down to a shorted turn(s) on the primary.

Lawrence.