208/120, 3 phase Buck Boost Transformer

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AJones

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Does the high leg affect the ability to stabilize my voltage on the load side of the transformer?
I was going to purchase a 240 to 220V step down transformer but am concerned about B phase.
Line to line reads 245V equipment requires 220V.
 
Your thread title says 208/120 but you're saying that your equipment requires 220 volts? The 245 volts is from a 3Ø, 4 wire delta system?
 
Yes, it is a 208Y/120V pane. B phase is 216V line to neutral. Th equipment needs to be fed by 220V 3 phase.
Their technical adviser had said that my 246V is not with-in their equipment's tolerance.
 
I think you either misunderstand your situation, or you have a much deeper problem to deal with.

Do you have a 208/120V wye system that is running very hot, or do you have a 240/120V 'high leg' delta?

Do you have a 208/120V panel that is misapplied on a 240/120V 'high leg' system, or do you have a 240V (useable for 208/120V _or_ 240/120V) panel with properly rated breakers on the high leg?

Are you asking about a 3 phase autotransformer requiring a neutral to buck 240V to 220V? Or are you asking about a 3 phase isolation transformer with 240V input and 220V output?

-Jon
 
winnie said:
I think you either misunderstand your situation, or you have a much deeper problem to deal with.

Do you have a 208/120V wye system that is running very hot, or do you have a 240/120V 'high leg' delta?

Do you have a 208/120V panel that is misapplied on a 240/120V 'high leg' system, or do you have a 240V (useable for 208/120V _or_ 240/120V) panel with properly rated breakers on the high leg?

Are you asking about a 3 phase autotransformer requiring a neutral to buck 240V to 220V? Or are you asking about a 3 phase isolation transformer with 240V input and 220V output?

-Jon
Click to expand...
All the B spaces on the panel are blanked off for the high leg. I am asking about a 3 phase isolation transformer
with 240V input and 220V output. As for the neutral the equipment does not require one but of course if the transformer
does it will be added. The system is running a little hot as leg to leg it averages about 246V. I believe I may have misstated
the configuration as a Y when actually it's a delta.
 
AJones said:
Yes, it is a 208Y/120V pane. B phase is 216V line to neutral. Th equipment needs to be fed by 220V 3 phase.
Their technical adviser had said that my 246V is not with-in their equipment's tolerance.
Click to expand...
Does the load require a neutral? If not chances are you can ignore the fact you have a high leg, the one exception being if there is surge protection devices that want to see nominal 120 volts on all three lines to ground. Then a delta supply isn't feasible at all, corner ground is full 240 volts to ground, high leg is 208 to ground, ungrounded - can be anything up to full 240 to ground. You mostly run into this with things like VFD's or servo drives.
 
AJones said:
All the B spaces on the panel are blanked off for the high leg. I am asking about a 3 phase isolation transformer
with 240V input and 220V output. As for the neutral the equipment does not require one but of course if the transformer
does it will be added. The system is running a little hot as leg to leg it averages about 246V. I believe I may have misstated
the configuration as a Y when actually it's a delta.
Click to expand...
If you use buck/boost you won't necessarily have equal voltages to ground especially if a delta supply or even with open delta buck boost configurations on a wye supply. You will have equal or very near equal line to line voltages though. If you must have 120 to ground on all three lines then you about need a true step down that will have a wye secondary that is a separately derives system
 
kwired said:
If you use buck/boost you won't necessarily have equal voltages to ground especially if a delta supply or even with open delta buck boost configurations on a wye supply. You will have equal or very near equal line to line voltages though. If you must have 120 to ground on all three lines then you about need a true step down that will have a wye secondary that is a separately derives system
Click to expand...
Thanks. No I goofed... It's a delta system.
 
ActionDave said:
What does the nameplate on the equipment say?
Click to expand...
Unfortunately, I don't have the CNC table on site. So, I am basing my pre-installation on what their sales/technical staff has
provided and they call for 220V 60AMP 3 phase and 246V is above the tolerance.
 
AJones said:
Thanks. No I goofed... It's a delta system.
Click to expand...
And like I mentioned earlier, buck boost can give you equal line to line voltages but if you need 120 volts to ground input on any or all lines, you won't necessarily get that. Buck boost in a wye configuration on a wye derived input will have equal voltages to ground but voltage to ground will change at same ratio as line to line voltages changes.
 
AJones said:
All the B spaces on the panel are blanked off for the high leg. I am asking about a 3 phase isolation transformer
with 240V input and 220V output. As for the neutral the equipment does not require one but of course if the transformer
does it will be added.
Click to expand...

Okay. If all the B spaces on the panel are blanked off, then you don't have a problem there. The issue would be if you had 120/240V 'slash rated' breakers on the B space. So not a problem.

If you have a 3 phase transformer with separate primary and secondary windings (a common 'isolation transformer', usually with a delta primary and a wye secondary), then the fact that the source is a high leg delta won't cause any issues.

The title of your thread causes a bit of confusion, because 'buck-boost' generally implies an autotransformer arrangement, where one set of coils serves both primary and secondary function with different taps. Depending upon the precise load requirements, you might be able to use an autotransformer setup. The high leg will not cause any problems with the line-line voltage of the output, but will certainly mean the output is not balanced relative to ground.

In your OP you ask about voltage stability. This is worth investigating a little further. _Some_ high leg systems have a relatively small (high impedance) transformer providing the third leg. This means that its voltage will drop more than expected under load. So you may need to confirm that the voltage stability of your 240V high leg service is suitable for your load.

-Jon
 
AJones said:
Unfortunately, I don't have the CNC table on site. So, I am basing my pre-installation on what their sales/technical staff has
provided and they call for 220V 60AMP 3 phase and 246V is above the tolerance.
Click to expand...
IMO for simplicity your best bet would be a 3 phase isolation transformer with 240V input and 220V output mentioned by Jon in post #5
 
AJones said:
All the B spaces on the panel are blanked off for the high leg. I am asking about a 3 phase isolation transformer
with 240V input and 220V output. As for the neutral the equipment does not require one but of course if the transformer
does it will be added. The system is running a little hot as leg to leg it averages about 246V. I believe I may have misstated
the configuration as a Y when actually it's a delta.
Click to expand...
Chances are if going with an isolation transformer (separately derived system vs autotransformer) you will end up using one with 240 delta input and 208/120Y output.

Seems you probably have equipment rated for 208 volts with a maximum input of 220. A 208 volt system running on high end of tolerances will run about 216 maybe even up to 218, but same system will also run 124-126 line to neutral.
 
winnie said:
Okay. If all the B spaces on the panel are blanked off, then you don't have a problem there. The issue would be if you had 120/240V 'slash rated' breakers on the B space. So not a problem.

If you have a 3 phase transformer with separate primary and secondary windings (a common 'isolation transformer', usually with a delta primary and a wye secondary), then the fact that the source is a high leg delta won't cause any issues.

The title of your thread causes a bit of confusion, because 'buck-boost' generally implies an autotransformer arrangement, where one set of coils serves both primary and secondary function with different taps. Depending upon the precise load requirements, you might be able to use an autotransformer setup. The high leg will not cause any problems with the line-line voltage of the output, but will certainly mean the output is not balanced relative to ground.

In your OP you ask about voltage stability. This is worth investigating a little further. _Some_ high leg systems have a relatively small (high impedance) transformer providing the third leg. This means that its voltage will drop more than expected under load. So you may need to confirm that the voltage stability of your 240V high leg service is suitable for your load.

-Jon
Click to expand...
Thanks. Sorry for the confusion. I am looking at using an autotransformer and a power conditioner if they believe it will be necessary.
It's a shame the "other" 240V panel isn't 200' away and in the wrong place but that's how it goes over here.
Thanks again for your input.
 
in product overview check out "limitations".

Some of what is in there has been mentioned in this thread.

Main thing is do you need a neutral for the load being served? And would supplied equipment happen to require equal voltages to ground (or nominal of 120 to ground on all three lines) even if it doesn't need a neutral?
 
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