Autotransformers ?

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65535

65535

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I bought a GE autotransformer on ebay.com. Anyways it's a 1KVA 120/240V primary with a 12/24V secondary. From what I have read these were designed for power correction to boost lower line voltages up to equipment needs. I though plan on using it to get the 12/24V secondary output and use it as a low voltage source. I am hoping that it will work that way at up to 1KVA on the output. We'll see I suppose, but what do you guys know abou these units? It's epoxy filled.

More info and links in 4th post.

Added pic of cleaner label in 9th post.

Another link to PDF in post 25.
 
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Autotransformers do not have an isolated secondary, so they are not good for low voltage supplies. What they really are is a single winding with various taps on it. From your description, I suspect the intended use might be like this:
Code: 
           3---o 132 V (= 120 + 12)
           3
 120 V o---3
           3
           3
           3
           3
           3
   0 V o---3---o   0 V
           3
           3
           3
           3
           3
 120 V o---3
           3
           3---o 132 V (= 120 + 12)
If I'm right, it has a 120-0-120 V primary and a 132-0-132 V secondary, and you are not really meant to get 12 V out of it. Essentially, if the supply voltage was low, like 100 V instead of 120 V, then the output would get boosted back up to a more useful voltage. The 1 kVA would apply to the configuration shown, and does not imply you can get 12 V at 83 A. But I could be wrong.
 
02146.png
 
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Maybe from that picture you can see my confusion. The diagram clearly shows 4 separate windings within the sealed unit. 2 120V windings and 2 12V windings which depending on configuration would yield 12/24V when properly wired in either configuration. While you COULD wire it up like a standard autotransformer like MR Happy showed, this does seem to me to be a case of mistaken identity. The primary and secondary appear separate. Which is what I am hoping they are. A true autotransformer wouldn't have a true secondary nor would it have a rated 12/24V secondary. I'm confused but hopeful.

http://i22.ebayimg.com/04/i/001/3c/5a/1cc0_1_sbl.JPG

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=120396302265
 
Take a multi metter and check for continuity bettween the x leads and the h leads. That should just be a regular transformer. I dunno why they call it an auto transformer.


Now I remember. It is an auto transformer and it isn't. It is rated to be used as an Auto transformer. They are separate windings but the common use is to use the secondary to buck or boost voltage i/e you hook the secondary in with the primary.


Now that I look at it some more it says next to the autotransformer a max voltage. Should you use it as a buck transformer don't go over that voltage.


If you are wanting this to make a dc power supply at a true 12 or 24 volt you are going to have to put some regulation in there also. A simple bridge rectifier will get you close but not that close or very clean.



And for my final edit. Yes it is an isolated secondary. Yes it will work for 12 or 24 volt. DO NOT exceed 41.6 amp even at 12 volt! You will not get 1kva at 12 volt without burning it up. IF you do use it at a decent load it is going to get very hot. IT is rated to go 115 degrees c over ambient temperature. A majoraty of that will come if you are using it at capacity. You will catch things on fire if you are not careful.
 
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I am very not careful (cuz where's the fun in that) but I am also very good at keeping myself and my property safe. When the unit arrives first thing I'll do is strip it down to bare bones and test continuity. I'll need to get a 4A circuit breaker on the input it would seem.

I have myself coming a pair of 60A 1000V bridge rectifiers and am in progress on winning a 20 unit lot of 1100uF 250VDC caps. which will allow me to make a rather stable DC supply at a range of voltages. It's mad science crazy.
 
IT seam like the transformer will bee the weak ling in the chain then. I duno what you are going to do with 40 amps of dc power but have fun.
 
You did not say what use you have planned for the low voltage output. It will not be precise as the output voltage will follow the input voltage in direct proportion. This is an interesting autotransformer as the I/O ratio is very high at 10 to 1. Most have a much closer relationship.

Although we can't quite read the whole wiring diagram due to junk on the label, I do not think you will have a choice of 12V or 24V. I think it will give you input (120 or 240) over 10 to output 12V or 24V.

You could add a regulation circuit to the output but at that current level it would have to be quite substantial.

Mark
 
Well according to the diagram and the labeling instructions it's dual primary dual secondary with ability to wire for 120-12/24 or 240-12/24.

Here's a better picture. None of the poles are connected by default.

27793-B.jpg
 
That's got me completely tossed.

The versatile wiring options, the circuit diagram and the reference to "Insulation System" on the label all point to this being a conventional, double=wound transformer.

And yet it explicitly says "autotransfomer".

I give up. Just try not to kill yourself, OK?
 
I think "autotransformer" means "when used as an autotransformer". In other words, you can put the 24 V secondary in series with the 240 V primary to get up to 264 V at 41.7 A...?
 
That's the other thing, if it is rated to 264V AC max and 41.7A AC max that's pushing 10KVA. I suppose it is possible that both the primary and secondaries are 10 or 8 gauge wire. Which would be what a autotransformer would normally have 1:1 winding guage.

I think it's implying by the phrase "Autotransformer Max Volts 264 Max Amps 41.7" that if wired H-X as a normal single wound autotransfromer it will handle 10KVA, but it is limited to 1KVA on the secondaries alone. I think it should be safe to pull 80A off the 12V bridged windings. I'll do some checks for continuity and resistivity. I don't know if I would ever pull near 80A but maybe for some carbon arc expirements. Which I can now do at 28V AC or DC. Oh I forgot how much I liked carbon arc. HAHAHAHAHAHAH this is better than I expected.
 
65535 said:
That's the other thing, if it is rated to 264V AC max and 41.7A AC max that's pushing 10KVA.
Click to expand...
No, it's still 1 kVA.

If you wired it as a normal transformer, the primary would be 240 V and 4.1 A giving 1 kVA, which would correspond to the secondary delivering 24 V and 41 A.

If you now put the secondary in series with the 240 V supply to the primary, it can still handle 41 A, and the primary would remain at 4.1 A. What would happen is that the 240 V supply would provide 41 + 4.1 = 45.1 A total. Of this 45.1 A, 4.1 A would be diverted through the primary winding and the remaining 41 A would pass through the secondary and be delivered to the load.
 
Following on, if you wired it as a normal transformer with a 120 V, 8.2 A primary and a 12 V, 82 A secondary, that would be a pretty neat 12 V power supply. But, that 82 A is merely the rated sustained load and not the peak output available.

What you have to watch out for is that if you happened to short the secondary, for example with arc welding experiments, you would draw way more than 8.2 A from the mains supply. That transformer is a heavy duty industrial item and is in no way current limited. If you are not careful with it you are going to trip breakers and blow fuses in your house. :)
 
I have myself 3 nice pop type 10A breakers I bought. 2 are for the variac I have coming and 1 is for the input of the transformer. Things like this are nice to have around when you do a lot of low voltage things. I figure I can make a pretty damn fine LED lighting system with this. Not to mention potential for low voltage lighting systems. With an epoxy encapsulated transformer with 115C rise it should sustain the 1200VA that the breaker will allow.

Did I mention the 250V capable 22000uF array I'm bidding on. Or the 144,000uF 400V array I'm considering?
 
NO NO NO Hold on here a minute. Lets back up on the rated amperage. 41.6 amps PERIOD!!!!!!! At any voltage, any configuration. You WILL burn it up if you even try to run 80 amps out of it. There are 3 constants that you can never exceed with the thing. 41.6 amp 264 Volts AND 1kva. Exceed either one of those and transformer goes poof. NO matter how you hook it up you must stay under those three things.


You can hook it up to attain higher numbers but the transformer will not take it. Theory and actual real world are far different. A burnt potted transformer is a smell that you will never get out of your house.
 
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jhellwig said:
NO NO NO Hold on here a minute. Lets back up on the rated amperage. 41.6 amps PERIOD!!!!!!! At any voltage, any configuration. You WILL burn it up if you even try to run 80 amps out of it. There are 3 constants that you can never exceed with the thing. 41.6 amp 264 Volts AND 1kva. Exceed either one of those and transformer goes poof. NO matter how you hook it up you must stay under those three things.
Click to expand...
Can you expand further on this?

If the two secondaries are wired in series they will be rated at 24 V and 41 A maximum, with each secondary winding individually producing 12 V and carrying 41 A. If the two secondaries are instead wired in parallel, each winding will still be producing 12 V and carrying 41 A as before, but the sum of the outputs will be 12 V and 82 A. It seems to me that the windings internally are not aware of the external change in configuration. Where does this logic fall apart?
 
Don't forget about the transformer core - overloading the transformer will cause the core to saturate. Once the core saturates the inductance of the primary will significantly reduce and the current in it will rise.

Power transformers are big and heavy for a reason - they need large cores (at mains frequencies) to be able to effectively transfer the energy between primary and secondary windings.

In this case the windings are not aware of the external configuration, but the transformer core will be accutely aware of how much power you are asking it to cope with....
 
BillyNoMates said:
In this case the windings are not aware of the external configuration, but the transformer core will be accutely aware of how much power you are asking it to cope with....
Click to expand...
I get that, but in this case the power is the same in both cases too; either way the power is 2 x 12 V x 41 A = 984 VA?
 
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