Single-phase from of 3 ph transformers

[HughL]

I am following on from some earlier discussions earlier this year.
It is actually easy to provide 120/240V from a 3-phase source, in which the voltages are correct and the loading can be close to 100% and spread evenly across all three supply phases.

1 ph loading of 3 ph transformers. [ previous discussion thread ]

" As I recall from past training I believe that a 3 ph Y connected transformer can be loaded to 58% with single phase loads if there are no 3 phase loads. " .. And
" Providing 120/240V supply with 3 single phase transformers. " etc.

There is an easy way to provide 120 / 240V from a 3-phase supply, using a simple, but special transformer arrangement that can be done with a special 3-phase transformer if available, or by using three standard, off-the-shelf readily available single phase transformers. Their primaries being connected in the typical or standard way. Their secondaries being connected to each other in a special way, so as to provide two 120V outputs, L1 & L2 relative to Neutral and with 240V between L1 & L2. These connections are easy to do, but the electrician does need to know which connections to make. This is explained in a paper ?Transformer for 120-240V from 3-phase? : https://dl.dropbox.com/u/88752751/Transformer for 120-240V from 3-phase - Detailed A2.doc

 

[kwired]

I am following on from some earlier discussions earlier this year.
It is actually easy to provide 120/240V from a 3-phase source, in which the voltages are correct and the loading can be close to 100% and spread evenly across all three supply phases.

1 ph loading of 3 ph transformers. [ previous discussion thread ]

" As I recall from past training I believe that a 3 ph Y connected transformer can be loaded to 58% with single phase loads if there are no 3 phase loads. " .. And
" Providing 120/240V supply with 3 single phase transformers. " etc.

There is an easy way to provide 120 / 240V from a 3-phase supply, using a simple, but special transformer arrangement that can be done with a special 3-phase transformer if available, or by using three standard, off-the-shelf readily available single phase transformers. Their primaries being connected in the typical or standard way. Their secondaries being connected to each other in a special way, so as to provide two 120V outputs, L1 & L2 relative to Neutral and with 240V between L1 & L2. These connections are easy to do, but the electrician does need to know which connections to make. This is explained in a paper ?Transformer for 120-240V from 3-phase? : https://dl.dropbox.com/u/88752751/Transformer for 120-240V from 3-phase - Detailed A2.doc

Looks like it should work. Is it more cost effective than other options? Looks like from the example they needed a 75KVA transformerr bank to get the output of a 48KVA single phase transformer.

 

[HughL]

Hi kwired,
Not only should it work, it does work. I have had 50 KVA systems working for 6 years.
Originally, after devising the configuration theoretically, I bought $1,000 of transformers to prove it.
We then installed several systems. Only later, when working on the Generators, did I find that the Generator people have been doing exactly the same thing for many years, 20 years or more.
Rgds Hugh.Leyton@gmail.com

 

[HughL]

Looks like it should work. Is it more cost effective than other options? Looks like from the example they needed a 75KVA transformerr bank to get the output of a 48KVA single phase transformer.

You are correct, it requires 3 x 25 KVA for a 50 KVA system. .. Generator manufactures, these days, make all 10 KVA to about 1,000 KVA in the same way... That is they are all 3-phase Generators, with 6 windings, which can be connected in different ways, to provide a whole verity of outputs from 110V Single phase, up to 500V 3-phase, just by the way the 12 tails are connected, this includes the 2-phase or split phase system of 120 / 240V used in America. .. When a 120/240V Generator is ordered, they will use a 20 KWe Engine and fit a 30 KVA Alternator.

Cost effective ? .. What is the cost of damaging an Engine with heavily unbalanced loads causing vibration and excessive bearing wear ? .. What is the cost of damage to equipment because the Generator voltage is all over the place, because the Generator can not maintain good voltage control if the voltage drop on different phases, due to different and varying loads is unequal ? .. What is the cost of regular Air-conditioner compressor motors failures, when running from a too low supply voltage ? .. A 208V Supply, even when wired to standards, will result in only about 195 - 185V at the Air-conditioner terminals at start-up, when it should be about 220V ?

 

[HughL]

Looks like it should work. Is it more cost effective than other options? Looks like from the example they needed a 75KVA transformerr bank to get the output of a 48KVA single phase transformer.

If, for example, your Intake Transformer and or Generator are 100 KVA and your total 120/240V are 10 KVA or less, then stick with the Single phase transformer across two phases.

If, on the other hand, your 120/240V loads are highly variable, and in the 20 KVA to say 80 KVA range, then this 3-phase to 120/240V transformer arrangement is the Best solution.

 

[kwired]

You are correct, it requires 3 x 25 KVA for a 50 KVA system. .. Generator manufactures, these days, make all 10 KVA to about 1,000 KVA in the same way... That is they are all 3-phase Generators, with 6 windings, which can be connected in different ways, to provide a whole verity of outputs from 110V Single phase, up to 500V 3-phase, just by the way the 12 tails are connected, this includes the 2-phase or split phase system of 120 / 240V used in America. .. When a 120/240V Generator is ordered, they will use a 20 KWe Engine and fit a 30 KVA Alternator.

Cost effective ? .. What is the cost of damaging an Engine with heavily unbalanced loads causing vibration and excessive bearing wear ? .. What is the cost of damage to equipment because the Generator voltage is all over the place, because the Generator can not maintain good voltage control if the voltage drop on different phases, due to different and varying loads is unequal ? .. What is the cost of regular Air-conditioner compressor motors failures, when running from a too low supply voltage ? .. A 208V Supply, even when wired to standards, will result in only about 195 - 185V at the Air-conditioner terminals at start-up, when it should be about 220V ?

I'm not trying to say anything is wrong with the set up. Even with the generator you have more component to purchase than what is needed to get the job done, maybe it is still cost effective from the view of the manufacturer in that they don't have to make as many different alternators but instead connect them according to what is needed.

In a small commercial building where maybe the AC is the only three phase load - we are often times switching them to single phase service if they are doing major rennovations. The AC is often older and an newer more efficient one is a good investment, and single phase 200 amp and less loadcenters and breakers are much less expensive than three phase as well as the need for a transformer. Now load balancing the three phase system is the POCO's problem.

 

[kwired]

Curious, what happens if it it back fed with single phase voltage?

 

[HughL]

Curious, what happens if it it back fed with single phase voltage?

Hi Kwired,

Interesting ! - You would not get 3-phase out of it unfortunatly. Not sure what would happen though.

I have now put together a 3-page paper giving much more details of Connections and Drawings.

I am suggesting that manufacturers shoud have available a 3-phase Transformer with Six Secondaries, that is two 120V on each of the three phases.

See : http://dl.dropbox.com/u/88752751/Transformer 3-phase - Six 120V Secondaries - Connections A2.doc

The original explanation paper has been slightly updated.
See : http://dl.dropbox.com/u/88752751/Transformer for 120-240V from 3-phase - Detailed A2.doc