208 range on 240 delta

AC\DC

Senior Member
Location
Florence,Oregon,Lane
Occupation
EC
How "hot" is your 240 volt supply? If it operates at lower end of what is acceptable it may not be too significantly higher than the upper end of what is acceptable for a 208 supply. In that case maybe just use as is if only going to be six months, that may also depend on intended usage.
I am actually running a little high. 254 /248volt
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If you need to go from 240V to 208V, and you are absolutely certain that a) the load does not care about line-ground balance and b) the load does not require a neutral, then the most efficient approach is to use _two_ single phase autotransformers in an 'open delta' configuration.

Given your 'hot' 254V supply, I would use a pair of 240V : 48V single phase transformers, connected in a buck autotransformer configuration, to give you roughly 212V L-L. A pair of 1 kVA transformers configured in this fashion could supply a 7.5 kVA load.

The resulting voltage to ground of each of the phases would be extremely unbalanced; something like 120V, 92V and 184V. But the L-L voltages will be correct.

The _simplest_ way to supply this load is with a 240V:208V delta:wye isolation transformer. But this transformer will be larger, be more expensive, and have more losses. Always a trade-off.

-Jon
 

retirede

Senior Member
Location
Illinois
but still within range that most utilities are allowed to deliver.

Most would rather see 254 vs. 230, which is similar over/under percentage of nominal 240, especially for a no load voltage.

I thought ANSI spec was +- 5%. That would be 228-252.
254 is 5.8% above while 230 is 4.2% under.

It’s true that induction motors prefer slightly high to slightly low. And due to voltage drop, it’s better to start high than low.
 

kwired

Electron manager
Location
NE Nebraska
I thought ANSI spec was +- 5%. That would be 228-252.
254 is 5.8% above while 230 is 4.2% under.

It’s true that induction motors prefer slightly high to slightly low. And due to voltage drop, it’s better to start high than low.
You probably right, for some reason I thought 10% was allowed.
 

AC\DC

Senior Member
Location
Florence,Oregon,Lane
Occupation
EC
If you need to go from 240V to 208V, and you are absolutely certain that a) the load does not care about line-ground balance and b) the load does not require a neutral, then the most efficient approach is to use _two_ single phase autotransformers in an 'open delta' configuration.

Given your 'hot' 254V supply, I would use a pair of 240V : 48V single phase transformers, connected in a buck autotransformer configuration, to give you roughly 212V L-L. A pair of 1 kVA transformers configured in this fashion could supply a 7.5 kVA load.

-Jon
Question.
Why would I use two single phase transformers vs post # 11 3phase transformer.
Cost saving??

Originally I was planning on using what you proposed from readying several other post about a similar issue.
 

kwired

Electron manager
Location
NE Nebraska
Question.
Why would I use two single phase transformers vs post # 11 3phase transformer.
Cost saving??

Originally I was planning on using what you proposed from readying several other post about a similar issue.
Again as mentioned it may not be possible if there is line to neutral loads as those will not be all be ~120 volts. For straight 3 phase 3 wire loads all the load will care about is seeing the right line to line voltage the open delta autotransformer arrangement is the least costly way to do this.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
but still within range that most utilities are allowed to deliver.

Most would rather see 254 vs. 230, which is similar over/under percentage of nominal 240, especially for a no load voltage.
No… utilities aren't allowed to go that high for any length of time

5D6571A7-96B2-4FD7-9E7F-ED86E87388E5.jpeg
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Question.
Why would I use two single phase transformers vs post # 11 3phase transformer.
Cost saving??

Originally I was planning on using what you proposed from readying several other post about a similar issue.

You pretty much have 3 choices:

2 single phase transformers in an autotransformer configuration
1 three phase autotransformer
1 three phase isolation transformer

The reason for selecting an autotransformer for this application is lower cost and greater efficiency.

I don't have a strong preference for the 2 transformer 'open delta' autotransformer or the three phase autotransformer. I'd go with whichever is cheaper or more quickly available.

-Jon
 

AC\DC

Senior Member
Location
Florence,Oregon,Lane
Occupation
EC
Well I guess owner wants to swap elements out since it will cheaper. Manufacture said that is okay. Though I got to what inspector says since its nameplate is 208. Seeing if they can send me a new nameplate I can then install.
 
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