BACK FEEDING A 110 CTE TRANSFORMER

[flyingsparks]

IS IT POSSIBLE TO BACK FEED A 110V SITE CENTRE TAPPED TO EARTH (CTE) TRANSFORMER TO GET 230V OUT OF IT? i HAVE RIGGED IT UP TO A 110V GENERATOR TO GET 230V OUT, BUT EVEN WITH NO LOAD ON THE 230V SIDE OF THE TRANSFORMER THE GENERATOR STARTS WORKING HARD, AND THEN EITEHR CUTS OUT OR THE GEN TRIP GOES. PLEASE HELP. THANKS.
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lynda, moderaor

please note forum rule 6

 

[Spark123]

Generally no, the transformer has been designed and built to transform down, not up. You can buy transformers specifically to convert 110v to 240v.

 

[flyingsparks]

OH RIGHT OK. WOULD ONE OF THESE 110-230V TRANSFORMERS ALSO ALLOW ME TO CONVERT 230 BACK TO 110V? BASICALLY ITS A CAMPERVAN FROM USA BEEN IMPORTED TO UK. ALL THE ELECTRICS ARE 110V, INCLUDING THE GENERATOR. I HAVE CHANGED THE POWER LEAD TO HAVE A 230V 16A PLUG ON IT, AND THIS GOES INTO A 4POLE CHANGE OVER SWITCH, ALONG WITH THE GENERATOR SUPPLY. THEN I HAVE A NEW CU FOR 230V SOCKETS, AND THE EXISTING CU FOR THE 110V. SO WHEN YOUR USING THE MAINS ELECTRIC (230V) THE TRANSFORMER CONVERTS TO 110, AND WHEN USING THE GENERATOR, IT BACK FEEDS THE TRANSFORMER TO GIVE ME MY 230V AGAIN! HENCE YOU HAVE 110V AND 230V ALL THE TIME FROM BOTH MAINS AND GENERATOR! VOILA!

HOPE THAT MAKES SENSE!

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lynda, moderaor

please note forum rule 6

 

[Lectrician]

It is a standard isolating transformer, so in theory would transform up.

You would need to remove the centre tap - usually there are two secondaries in series, the join between these being the centre tap. You would need to remove this link to earth.

You would also need to consider earthing arrangements after the transformer.

It isn't an ideal situation really - can't you just get the van running for 240v, and get a new alternator fitted (not neccessarily the entire genset).

 

[Spark123]

You're up against the same problem again, unless you can get a tranny which is hard wired in and compatable with being used in both directions.
You'd maybe be better having a 240v generator and use a 240-110v tranny for the 110v?

 

[Lectrician]

And buying a 110v toaster and 110v hair straighteners

 

[Spark123]

I thought he has 240v outlets too, that would be the better way around imo, the lights are probably 110v fluoros or something daft which only take a tiny bit of power.

 

[flyingsparks]

I think what i may do is either change the 4 pole change over for a 6 pole, to disconnect the output of the transformer, or stick a DP switch in to do this, and put up with having no 230v while the generator is being used.

I cant see how the centre tap is causing the problem, in theory surely it should work??

 

[Lectrician]

If the gen has it's neutral connected to chassis/earth, then connecting to your 55-0-55 tranny aint good. You need to connect it to the 110 only.

 

[Spark123]

I can see where you are coming from Lectrician, the connecting the neutral to the chassis earth and hence causing a short on the CTE transformer. Would a portable genny have the neutral connected to the chassis? I thought this only happens if you purposely do it, then again I am unfarmiliar with the way the americans work.

 

[plugwash]

the following should work and be safe

open up the site transformer

disconnect all wires that are nominally earth going to the actual transformer (i belive the transfomer itself is usually potted but the connections to outside aren't) inside the site transformer and make them safe, add a new earth link from input earth to output earth if input and output had seperate earth connections going into the resin. This should leave you with a transformer with both sides floating.

drill a hole and add a flex to the 110V side using a cable gland to restrain the cable, this is nessacery as you really musn't feed though a plug/socket connection backwards. remove the plug from the 240V side.

then wire as follows, DO NOT use any plug and socket connections between the changeover switch and the transformer.

 

[plugwash]

That diagram shows the basic wiring but i'd also add some suitable MCBs in appropriate places to make sure the transformer can't be overloaded, i'd also make sure that both CUs have RCD/GFCI protection if they don't already.

 

[flyingsparks]

Yea that looks like it would work! Thanks for all your help. I'l give it a go. I already have RCD's and MCB's to make sure the transformer isn't overloaded so thats OK. Thanks again.

 

[bernardgreen]

Yea that looks like it would work! Thanks for all your help. I'l give it a go. I already have RCD's and MCB's to make sure the transformer isn't overloaded so thats OK. Thanks again.

When calculating the maximum for the transformer in the "reverse" transform use a value of 80% of the rating in the forward transform.

There is a limit set by heat dissapation in the windings.

If rated at 1000 watts limit to 800 in the reverse. You may be OK at 900 but it depends on the quality and construction of the transformer.

A few very good transformers will carry 100% in the reverse transform.

 

[plugwash]

to the original poster

did you try what i suggested and if so did it work?

its nice to know the final outcome in cases like this