Seeking solve for 2 of 3 legs on a 208 system

[3phaseMarty]

Hi,

I'm helping a business purchase and install an auger filler machine.

They're in a building with 120/208 3 phase provided, and are currently have a setup with 120 & a 240 option - on these 2 leg circuits line to neutral measures 125, but between two lines reads 217 as the two legs are 120 degrees out of phase. They don't have the 3 legs set up for 3-phase on anything.

They want to install a machine requiring on 240 V, 3 phase, but don't want to have the electrical work done to connect the 3rd leg. Instead, they want to convert the electricity up with a 240 1 to 3 phase transformer, even though their electricity is actually provided on 2 legs of a 3 phase system. They don't want to do electrical work on the building to put them on their existing 3-phase, believing the manufacturer's claim they can use a transformer to provide the 3 phase.

Two questions:
-I'm looking to be confirmed in the following: Installing a 1 to 3 phase transformer won't work as they wouldn't be cleanly converting 1 phase to 3, but 2 out of sync ones. If anything they should install the 3rd leg into their existing 3 phase system and boost the voltage if necessary. Is my understanding correct? Does their solution actually work? Has anyone had a similar situation to this?
-Is there a simple way to explain to a customer what's going on with 2 out of 3 legs of 3 phase? The bicycle analogy is all I have here.

Nice to be on the forum!

Thanks,
Marty

 

[jaggedben]

There's no such thing as 1 to 3 phase transformer. You'll need a phase converter.
Alternatively they could ask the utility to change the service to 3 phase, but then they'd still need a 208 to 240 transformer.

 

[gar]

181213-1951 EST

3phaseMarty:

You have two phases of a three phase system. From these two phases you can create the third phase.

If the device can operate from 208 V, then you need only two isolation transformers to get the third phase. If 240 is required, then an additional two autotransformers are needed.

.

 

[3phaseMarty]

Thanks!

Thanks!

Thanks for the help!

With the machine in question, I suspect they would need 240 rather than the 208.

jaggedben: You're right, I meant phase converter.

Cheers,
Marty

 

[kwired]

They can derive three phase using open delta with both lines and the neutral as input. It will need two 1 phase transformers, those and the conductors will carry more current than if you had three "phase " conductors, you also have that issue with a phase converter though.

If you do have access to the third phase might be simpler or even less cost to run the third conductor, if you need 240 instead of 208 volts maybe use a buck boost transformer.

 

[GoldDigger]

They can derive three phase using open delta with both lines and the neutral as input. It will need two 1 phase transformers, those and the conductors will carry more current than if you had three "phase " conductors, you also have that issue with a phase converter though.

If you do have access to the third phase might be simpler or even less cost to run the third conductor, if you need 240 instead of 208 volts maybe use a buck boost transformer.

IMHO either an open delta arrangement with the two 120 volt phases stepped up to 240 will provide far better waveform, voltage regulation and stability than a phase converter driven from any single phase source.
Running an idler motor with a fair sized flywheel on the open delta might give close to closed delta performance if the open delta is for some reason unsatisfactory.

 

[kwired]

IMHO either an open delta arrangement with the two 120 volt phases stepped up to 240 will provide far better waveform, voltage regulation and stability than a phase converter driven from any single phase source.
Running an idler motor with a fair sized flywheel on the open delta might give close to closed delta performance if the open delta is for some reason unsatisfactory.

True. Also depends to some extent on what is needed to be powered.

Lightly loaded motors - you can get away with unbalanced phases that result from rotary and capacitor type phase converters easier than with motors loaded to full load rating.

Only supplying a single motor - VFD is also an option, it will need more capacity than it would if supplied with all three phases.

Again depending on circumstances may be more cost effective to run the third phase to where it is needed though.