Lower the voltage of a three phase 220v delta wiring system with auto transformers

[Roman Ceglua]

Hi all, I have a three phase 220v delta wiring system but the real voltage is about 240-245V by measuring between the pairs RS, RT, TS. I want to lower it to about 220V so I was thinking to use three auto-transformers 240-220V wired like this:https://i.Upload the image directly to the thread.com/XL6dO2N.png
I expect to find 220V between the transformer outputs, is it right or there is something more to consider?
Thanks Regards

 

[dmxtothemax]

Your gunna have one hell of a turn on surge,
Is the supply up to the job ?
And are the breakers the correct type and size for the new load ?

 

[Shoei]

Unfortunately I don't think that works, it'll shift the phase but not the voltage. It may work if you connect the transformers in star to create a virtual neutral. I'll have a think tomorrow when I've had less beer and more sleep.

 

[dmxtothemax]

???
Why would it not change the voltage ?
Where are you taking the load from ?
What is the load ?

 

[marconi]

What is the 220V supply to be used for? How many kVAs? Mainly 3 phase loads or single phase? Are you bothered about the hum from the transformers? And remember the heat they will produce and thus energy they will consume 'just being a transformer' and when transforming. Are the loads always on or only as required?

 

[static zap]

RE TOP diagram.
Think like a graph scale -- you have moved some of the voltages but they are still 240 apart = FAIL.
(In theory need neutral point -- so common points of transformers can meet)
In practice .. not a standard install so unwise !

 

[Shoei]

Ignore my drunken rambling, there will be voltage transformation and a bit of phase shift. dmx is right though, switch on surge would be interesting.

 

[marconi]

Re #1 - Yes you will produce a 3 wire 220V(line voltage) delta supply with this configuration provided that you connect the auto-transformers exactly as you have drawn out.

But as others have asked do you want a 4 wire star supply which will provide you with a phase voltage of 220V with respect to the star's centre node? If you do then if you used auto-transformers/transformers they would have to be connected in a star (all 0 terminals connected together and each 240 primary terminal to a line) and be step-up 140V to 220V (where 245 divided by square root3 = 140V) or a primary to secondary turns ratio of 220/140 = 1 : 1.6

Electrically and economically it is best to use one 3 phase auto-transformer/transformer not 3 separate single phase ones.

 

[marconi]

Here is a firm - one of many if you google 'three phase power transformers' - which might be able to provide you with a suitable transformer:

https://am-transformers.com/buy/5kva-to-1-5mva-transformers/