The AM Forum

I had the best intentions  - I was going to put a plate transformer in the car for a few days, and let the sun bake it for me, and then into the oven while the XYL was at work. But, the weather quickly went towards the cold side and she has several days off.  No oven time until I go to work Sunday, and lug the thing with me.

This iron has been packed up for ~10 years, and I want to be sure that she'll live (yes, the VOM says the windings don't appear to be shorted). So, Powerstat on the bench, I intend to warm things up a bit. What I have a problem with is the advise to short the secondary to help warm the core up.

My question(s)... do I throw a load across, or do I short the windings on the output?

Freed Grade 1 Class A power transformer.

120/230 V primary, 1600/1750/1900 V  0.3 Amps

Any help is appreciated,

klc

Hi Kevin,

Here I thought you were pregnant... :o

I'd use a resistance load on the secondary, something that will pull about 25% of the rated Wattage, with your Variac set to get just a good temp rise.

A cheapie IR thermometer is the ticket for seeing just where the kWs are coming and the BTU's are going.

73DG

I wuz given a 3 phase 17 KVA transformer by Tim, WA1HLR that had been under water. Till this day I can still see mud in some of the windings, anyhow he told me to short out the primary windings and feed the secondary with 208 three phase to dry it out. It took a very long time just to get warm but I pressed on getting it up to feel warm and then let it cool down and then run it until it got warm again. Each time when the transformer was cooled down I gave it some tests with the highest setting on my ohm meter looking for leakage. A number of times it showed leakage but would show less leakage each time I ran it thought a cycle. Since it has been over 15 years ago that I did this treatment, I cannot remember how many times I cycled it but it eventually came down to zero leakage. I then put the 208 volts on the primary and fired it up and it did not flash over and used it for more than seven years without a problem. I guess I got lucky?

I gave a huge open frame SINGLE PHASE plate transformer that also got wet to a friend and he did the same procedure but in his case the transformer arced over. His best bet would have been to take it to a large motor repair shop and have them dry it out in their large industrial oven. Maybe that would have saved it? I'm just passing this along because it worked for me but I never heard of anyone else drying out a transformer this way?  

Drying out iron is easy when one lives in an oven... 8)

73DG in the AZ desert.

Without commenting on the merits of electically heating the transformer, it may be likely that the idea behind shorting the secondary is that the desired portion of current can be had at quite low applied primary voltages, lessenning the change of arc/burn due to moisture.

Using a large/oversized variac on the primary, the secondary current can be adjusted easily. If the variac adjustment is too ticklish when setting secondary current, a step down transformer can be used after the variac to give a lower max voltage thereby giving a much wider degrees of variac shaft rotation for easier adjustment.

A resistor can still be put across the secondary to do some limiting, but then some energy is wasted heating up the resistor instead of most of it going to heat the transformer via the resistance of its internal windings.

It is much easier to do this with a variable DC power supply.  With their typically available current limiting, you can put a few volts on the primary and still get the same I^2R heating as AC with no danger of flashover.  You want to do it with all windings in series so they all get hot, but depending on the secondary resistance that may take a few dozen volts instead of a few volts…

Ed

I wonder if some trans would develop residual magnetism from a long DC exposure?

73DG

If that’s a concern feed the HV center tap, connect the outer ends together, then feed that to opposite ends of the two primaries, and the the other opposite ends back to the psu.  Fields are then all opposite and nearly (a few turns/manufacturing variances/etc.) equal.

Ed

True, but my 3-phase iron I use (have 3ph here) don't have CT's...


73DG

Thanks everyone.

I ended up putting a few 18K resistors on the secondaries at 400 V overnight. Later today, I wanted to draw a bit more juice through this thing, so I loaded her up with  two 100W lamps sitting at ~70 volts.... My powerstat didn't like the low output, and began to smell a bit.. (actually, it can't smell but I can). Pat, I guess I cudda tried using another transformer to lower the primary voltage on the powerstat, but I'm too lazy to try drying out *another* piece of iron.

So, I know that each of the windings can put out ~500 Vac, nothing arced over, and I'll see If I can bring this thing into work and bake it for a while.

Ed, good idea. My only 'variable' DC supply is good for ~15V, so probably a no go for now.

Dennis -  just fat.

Terry, I don't have 3Φ .... ..    I guess it's a bit safer loading the xformer backwards.

Again, thanks

klc

Put ya a couple lights in the primary in parallel and that will limit the current while also letting your variac work at higher impedance.

--Shane
KD6VXI