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Trying to identify something that resembles a transformer!

Benny7440

Jan 14, 2016
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I don't know from where it was taken nor when, but what surprises me is that it shows only 3 cables but physically looks like a typical xformer. What it might be and what possible uses it might have?

See attached photo... The characters shown are (from top to bottom): RD 220 V \ BN 115 V \
BL 0 V \ 259 VA. There're no other markings. Thanks!

IMG_20181101_104056.jpg
 

hexreader

Apr 21, 2011
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1 Amp (approximately) auto-transformer.

Can convert 220V to 115V or 115V to 220V.

1A Maximum load (approximately)

My advice is to bin it. Looks to be in sorry state and provides no isolation. Plus you do not know where it has been.
 

kellys_eye

Jun 25, 2010
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@hexreader has it right. Unless you're familiar with auto-transformers and the issues they create then it's not worth your life to put it to any reasonable use.
 

Benny7440

Jan 14, 2016
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Thanks for replying, hexreader & kellys_eye!

It looks then very useful but a little dangerous. Where can I find more useful info on them?
 

kellys_eye

Jun 25, 2010
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Just Google auto transformer. They all work on the same principle regardless of size.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Auto-transformers are pretty nifty, especially variable auto-transformers (which your example is not) also known as Variacs™, but those that operate directly from line (mains) voltage can be very dangerous, even lethal, for the newbie to "play" with.

Newbies should stay away from any circuits, except wall-wart type power supplies with internal transformers, that involve direct power line connections. Learn something (a lot!) about electricity first, and electricity best practices for safety.

First safety rule: never use both hands to explore/probe/make connections to "live" circuits. Keep one hand in your pocket at all times when working around "live" circuits.

Second safety rule: never stare into a laser beam with your remaining eye.
 

Benny7440

Jan 14, 2016
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Thank you hevans1944 for replying!

I took some measurements expecting some serious symetry but found that it measures as follows:
Blue to Red : 15.1 Ohms
Brown to Red : 11.9 Ohms
Blue to Brown : 7.3 Ohms

Does it makes sense?
 

duke37

Jan 9, 2011
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The voltage depends on the number of turns.

The resistance depends on the number of turns and the length of each turn. Outer turns are longer than inner turns.
The label shows blue as 0V
Brown is 50% up and red is 100%
Blue to brown = 7.3
Brown to red by subtraction = 15.1-7.3 = 7.8
Not far from symmetry. I would think the brown to red measurement is in error.
The blue is the inner connection.
The brown is halfway.
The red is the outer connection.

All this assumes that the wire diameter is the same in all three windings which is likely for a 50% auto transformer.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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It is also possible that part of the winding is made with different gauge wire than another part

I have a 4.5kVA autotransformer designed for 240 to various lower voltages like 220V, 230V etc. that is wound in this manner.
 

Benny7440

Jan 14, 2016
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I want to test the thing before attempting to give it a real useful usage, so thought of connecting it to a 110 VAC between Blue & Brown but with a 110 V light bulb in series and then check what voltages are read between Blue & Brown and what between Blue & Red. Think this test might give some info on the transformation and reliability of the unit. Have read some info on them and my proposed test seems to bring some useful insight.

What do you think?
 

duke37

Jan 9, 2011
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Lamp limiters as you propose are often used to start old valve radios and will limit the current.
I have just used one to find out why a radio/ampmlifier was blowing fuses. It was a shorted diode in the diode bridge. I was very pleased at how easily the fault was found.

The connections appear to be right. It will not give any information on the reliability of the transformer. This will depend on whether it is overrun or kept in a damp atmosphere where corrosion will rot the wires or give shorted turns.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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What do you think?
Good idea. Take the time to wire up a standard Edison-base lamp socket so you can swap in incandescent lamps of various wattages. Start small, say, 25 watts or so and work your way up towards 200 to 300 watts. Most of the applied AC line voltage will be dropped across the low-wattage lamps. If you can measure reasonable voltages (not zero) between the blue and the brown leads and the blue and the red leads and the ratio is about 2:1 then the auto-transformer is probably working. A good transformer will not light up the larger wattage lamps.

As for its reliability... who knows? A "good" transformer will hardly make a 300 watt incandescent lamp glow, and most of the 115 VAC line voltage will be dropped across the blue-brown transformer winding, with 230 VAC appearing between the blue-red leads. Allow it to run for awhile with the high-wattage lamp in series (safety precaution) and make sure the transformer case doesn't get hot (shorted turns would cause this). After that you can probably safely use the auto-transformer for its original purpose... either a 230 to 115 VAC step-down auto-transformer, or a 115 to 230 VAC step-up auto-transformer. In either case, suitably fuse the connection to mains power and stay within the volt-ampere (VA) limitation (barely readable on the transformer) for the load, whether that be 115 VAC or 230 VAC. Appears the VA rating might be something on the order of 28 VA or 28 watts with a resistive load.
 
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