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I have a nice potted transformer I got recently at a hamfest that I think might be a heavy duty filament trans. Not a single bit of writing on it any where other than the terminal numbers.
There is continuity between terminals 1&2 and 3&4, but no continuity between the two sets; they are separate windings I think.
Terminals 5,6 & 7 have no continuity between them or between any of them and the first four terminals.
Assuming 5, 6 & 7 are most likely the primary terminal I hooked up the mains ac to 5 & 7 and put a dvm on 1 & 2. I got 5.3 volts when the variac was at full mains. I then the clips to 5 & 6 and got 8.8 volts at full power, and also got 7.5 at full power when clipped to 6 & 7.
I then switched the dvm to terminals 3 & 4 and got exactly the same results.
Has anyone every seen anything like this before? If so, what was it used for?
Follow Ups:
While 5-6-7 seems to be a center-tapped winding, the voltage measurements are not consistent - you should have gotten 10.6v with 120v on 5-6 or on 6-7. So that winding was probably saturated when using only half of it. The fact you got different output voltages with the half windings further suggests the tap is not at the center of the winding, but the identical resistances suggests the opposite conclusion.
The high resistance of 5-6-7 should correspond to a high impedance and high voltage (unless it's really small, a couple ounces or so?). The apparent saturation at 60Hz further suggests it is not designed to handle frequencies that low. Definitely an oddball (i.e. paperweight).
I agree, I just wish I could figure out the reason for the two sets of what appear to be identical windings on the primary side. It is far from being a small transformer, it's quite heavy and about 4" square by 5.5" high.
Edits: 07/25/15
Dual primaries for 115/230V were common in military and industrial equipment. But it could be a 400 Hz transformer, or could have some shorted turns. Have had a few like that - local recycler will give me 10 cents a pound for "copper bearing" scrap.
I suggest getting a bell transformer or using some other source of low voltage AC as the input rather than 120 volts. That's safe, and with a voltmeter and numerous readings you can ratio it out.
It's doubtful that a transformer with more than about 7 taps is for audio output (primary common, primary side 1, primary side 2, secondary common, secondary 4, 8 and 16 ohms). With a signal generator, a load resistor, and either a meter or (better) a scope you'll be able to tell whether it's a power transformer or an audio output transformer and get a pretty good idea of its frequency capabilities.
I have all that gear, so one of these days I will have to try doing a thorough analysis and try to figure it out. I was just hoping someone had run into something similar and knew what it was.
Edits: 07/25/15 07/25/15
Don't let TK rattle your cage. I hope this helps;
Seriously, power transformers have a primary and secondary, and the 2 sides are separate. The primary will have low resistance usually lower than the secondary except if its a filament trans. Usually, the AC primary will be 1 and 2 and sometimes 3 and 4 depending on if its dual primaries as in 115 (1,2) and 230 (3,4). Sometimes it has multiple tapped primaries as 1,2= 115, 1,3 = 120, 1,4 = 125. If your primary is 220 or 240 I don't know how you would determine that if it is unmarked.
The secondary may not measure exactly at the filament voltage. Mostly, they were made to output higher than 6.3. I have found that the designers would often have an unloaded voltage at 6.5 or even higher.
Also, posting a pic of the terminals may help with determination.
Cheers, Dak
Edits: 07/25/15
Here's a photo of the transformer. The top four terminals are numbered 1,2,3,4 going from left to right. The bottom three aere numbered 5, 6, 7 going from left to right. The resistance between 1 & 2 is 2 ohms, and it is the same between 3 and 4. There is no continuity between the two pairs.The resistance betweem 5 and 7 is about 125 ohms and between 5 &6 and 6 & 7 it is about 60 ohms. My guess is those three are the center tapped secondary terminals. Problem is there is no continuity between any of them which is hard to understand considering the resistance readings I got.
When I tried hooking up ac to 1 & 2 it blew a fuse on the variac after not too much voltage was applied.
I have found a lot of different transformers at hamfest over the years but never had one act like this. Is it possible for it to be kerflooey and still get the resistance reading I did?
Edits: 07/25/15 07/25/15
A resistance of 2 Ohms between terminals 1 - 2 and 3 - 4 is about right for a dual winding transformer. It's too high to be a low voltage filament winding. A resistance of 127 ohms between 5 and 7 is again about right for a high voltage "plate" transformer.
Is the transformer heavy (over 4 lbs)? If so, it is likely to be designed for 60 Hz applications. If not, it's probably a military 400 Hz transformer, although I haven't seen many potted 400 Hz transformers (they are more usually open frame with a icky epoxy material covering them).
Given that it blew the fuse when less than 117 volts was applied to terminals 1 and 2, it's likely to either have a shorted winding or to be a 400 Hz unit.
Well, it's a heavy sucker, definetly more than 4 lbs. Guess it probably has a shorted winding. Maybe I will try terminals 3 and 4 and see what happens as I have not done that. If that is no good i guess I have a door stop.
I have a few PT which statically test ok, but when powered up have poor performance. Probably a short to ground. regards, Dak
nt
I'm pretty sure those were used as wheel chocks for Grumman F4F Wildcats during WWII. It's a rare item; most eventually slid (or were thrown) off the decks of the carriers.
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Buy Chinese. Bury freedom.
Thanks for your most informative response TK. Hopefully I can return the favor someday.
Edits: 07/25/15
You never get the message. Unmarked transformers are paperweights. There's no way to know if it's a power transformer, let alone what frequency it's for. Many surplus transformers are 400 Hz. Of course, it could always be an audio transformer with a bandwidth of 300hz to 8kHz. Will that be useful? Your question is analogous to finding an unknown screw on the road and asking folks on the auto repair forum which car it came from. Wanna take a guess and plug it into a wall socket? A Darwin Award is just around the corner.
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Buy Chinese. Bury freedom.
You never get the message either.... everyone likes a little ass, but no one likes a smart ass. I am using a fused variac and asking questions. I have managed to salvage a lot of good iron that way.
Edits: 07/25/15 07/25/15 07/25/15 07/25/15
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