Auto transformer help

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bluemoon7799

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Hey all,
I am in need of some help. We have a project where I need to step down a 3 phase 230V, 60hz wye supply (although there is no 4 th wire [secondary voltage used]) to 115V 3 phase supply. But the catch is that because of space restraints in the cabinet the PM wants to use auto transformers (UGHH) to acheive this because of the space savings. From what I have been reading you cannot have a 3 phase delta (or wye) to delta auto transformer. Is there a configuration that can be used to acheive the desired voltage supply with auto transformers?

Thanks
 
110117-1632 EST

Your question is not clear. If you have a Y supply, then you can put any one or more of the following loads ---Y, delta, or single phase on that supply. And there is more than one way to attach single phase loads.

You can use three identical auto-transformers and connect these from the lines to neutral of the Y source and get any desired output Y supply voltage you want within reason. Then that new Y output can have any one or more of the above said types of loads on this new output.

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bluemoon7799 said:
We have a project where I need to step down a 3 phase 230V, 60hz wye supply (although there is no 4 th wire [secondary voltage used]) to 115V 3 phase supply.
Click to expand...
By "115v 3 phase," do you mean a Y, giving you a 208Y/120source (implying several loads), or a 120v line-to-line Delta source (implying one load)?

Either way, a 3ph 240v Delta primary to 120v secondary unit would work. Unless a very small single load, I wouldn't even consider auto-transformers.
 
Sorry i will try to be clearer.

Sorry i will try to be clearer.

Thanks all for trying to help me. I will try to be a little clearer.

The 230V source is a wye secondary from the main 20KVA xfmr with a primary 460V 3 phase delta. The need is for this secondary to be stepped down to a 3 phase 115V delta supply for our controllers. There are 3 cabinets that have our controllers in them and will require the auto xfmrs in them. The branch circuits to these cabinets are protected to 50A because we will need 40 A or less to power the controllers. Even though there is a wye for the secondary of the main xfmr, no 120V single phase circuits are used.

I don't know if this considered a small load. I really don't to use auto xfmrs, but like I said my PM wants to use them and unless I can come up with a solid good reason not to use them, they are in.

Thanks again!!
 
Okay. Now, I would recommend a 480v D to 120v D transformer, and not cascade transformers, if you have the choice.
 
A 120V delta secondary transformer is not a standard device.
Commonly available autotransformers have less than a 4:1 ratio.

I would look into using two 1-phase 480:120V transformers connected into an open delta.
 
If these controllers are single phase I'm not sure of the need of 3-phase?

I know of no 3-phase loads that operate on 120 volts here in the states, I heard of some in other countries but even then are rare, the main reason we install transformers for control cabinets (even when 120 volts is available) is the ability to disconnect all power when the main cabinet handle disconnect is opened, I have installed a few 3-5 kva single phase transformers in control cabinets to run other loads such as humidity heaters, very small cooling system for PLC electronics, and or connivance receptacles, for outdoor installations, but for the most part controls require very little power, generally around 100w and these single phase transformers are very small.

Most auto transformers are used as buck and boost for boosting 208 to 230 or visa versa, they generally are nothing more then a 240/120 in and 12, 16 or 24 volts out transformer, wired in an auto transformer configuration to allow the output to be an adder or subtracter to the original voltage your not going to get from 230 volts to 120 with them.

Like Jim said, finding a manufacture to make such an animal will be the hard part.
 
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110119-1331 EST

As I have reread this thread there is something very strange about the requirements. What is this 230 V 3 phase Y. Is it a 208 line-to-line and thus 120 line-to-neutral source or something else? What would be a 115 V delta load? Meaning 115 V line-to-line, and 66 V to a theoretical neutral, or does it mean 200 V line-to-line.

What is the content of this delta load that needs to be supplied? Does it require 120 line-to-line, or 208 line-to-line, or something else? Is it a diode rectifier load of some sort, and thus probably floating off of ground?

I do not believe 115 and 230 are considered nominal voltages today. What actual line-to-line voltage is required by the load, and does it need to be 3 phase? In what country is this equipment to be installed?

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more info

more info

Hi all again,
I know the requirements sound strange but that's really what is needed. This is for a welding system going on a barge. The direction of the design is that, at sometime in the near future our new controller (which will operate on 230V [actually 208-240V]) can be retrofitted into the cabinets and the autoxfmrs will be removed. That is why management does not want the main power distribution for the system to be 3 phase 115V.
Our controllers (a rectifier/capacitive load) do run on both 115V single phase and 115V 3 phase.
Believe me I know this is unusual and when we have built systems in the past, that required the 115V 3 phase we had to have custom xfmrs built. Most of the time single phase works fine but not in all cases.
I can only hope that the new model comes out soon and it is only the first system that has to go through this.

Thanks again
 
110119-2108 EST

bluemoon7799:

You provide a lot of peripheral information, but are not making it clear what voltage you require from a 3 phase source for your welding load. You imply that your load is delta. And you state it is 115 V. Do you want 3 phase with a 115 V line-to-line which is 66 V line-to-neutral?

I believe you also state your source is 230 V line-to-line in a Y configuration. Could this actually be a 240 V source?

These are unusual numbers.

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gar said:
110119-2108 EST

bluemoon7799:

You provide a lot of peripheral information, but are not making it clear what voltage you require from a 3 phase source for your welding load. You imply that your load is delta. And you state it is 115 V. Do you want 3 phase with a 115 V line-to-line which is 66 V line-to-neutral?

I believe you also state your source is 230 V line-to-line in a Y configuration. Could this actually be a 240 V source?

These are unusual numbers.

.
Click to expand...
I've heard that is fairly common on ship-board power. APC makes special dual pole controlled 120v UPS specifically for ship-board power having 120v line to line system.
Shipboard power is nothing like common land power configuration.

This article explains it.
http://psalserver.tamu.edu/main/papers/221 Zhang Butler.pdf
 
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