240/139 Volt
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kwired
Electron manager
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- NE Nebraska
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- EC
The only way to derive that voltage with simple transformation is to utilize three phase wye connected systems where line to neutral is ~139 volts.
It typically would be used where there is no line to neutral loads and only 240 volt line to line loads are expected, as 139 is not a common utilization voltage .
The advantage over going with delta systems is you would either have to corner ground those making it 240 to ground on the two ungrounded phases or use a high leg system with mid point of one phase grounded which does give you 120 volts to ground on two leads but is 208 to ground on the remaining lead.
Where might this really matter much? Surge protection on say a variable speed drive that operates on 240 volts line to line to line, but the surge protector is not designed for up to 240 volts line to ground on any input lead. This really is more of a problem with 480 volt delta supplied applications though but not impossible for 240 delta applications.
It typically would be used where there is no line to neutral loads and only 240 volt line to line loads are expected, as 139 is not a common utilization voltage .
The advantage over going with delta systems is you would either have to corner ground those making it 240 to ground on the two ungrounded phases or use a high leg system with mid point of one phase grounded which does give you 120 volts to ground on two leads but is 208 to ground on the remaining lead.
Where might this really matter much? Surge protection on say a variable speed drive that operates on 240 volts line to line to line, but the surge protector is not designed for up to 240 volts line to ground on any input lead. This really is more of a problem with 480 volt delta supplied applications though but not impossible for 240 delta applications.
brantmacga
Señor Member
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- Georgia
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- Former Child
I’ve never seen that. But sounds like it would be for Y service where all 240v is needed.
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Ok so what the advantage of 240/139 over a regular high leg Delta? The Delta still gets you 120 which seems like you're going to need for at least something 99% of the time. 240/139 wye could still not use slash rated breakers.
kwired
Electron manager
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- NE Nebraska
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- EC
Which is generally only a problem with two pole breakers. Three pole are pretty much always straight 240 rated, in the common 240 volt max rated panelboards that are out there.
single pole breakers isn't a problem unless you happen to come up with a 139 volt rated load to supply.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
It provides the delta voltage of 240v, but with the equal line-to-neutral voltages and simpler bonding of wye systems.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
That combo is somewhat common on portable generators- often you can get three-phase at 208/120 or 240/139, or single phase at 120/240. Really only useful if you need 240 delta for temporary power.
SceneryDriver
Senior Member
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- NJ
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- Electrical and Automation Designer
Balanced L-N voltages are the main attraction. IT power supplies are 240V capable, but most won't accept 277V. Those that need 3-phase power like to see balanced voltages. VFDs are the same.
SceneryDriver
- Location
- Illinois
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- retired electrician
Seen that voltage specified for drive isolation transformers a number of times....no line to neutral loads.
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- New Jersey
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- Journeyman Electrician
I've seen it but it was not by design it was due to an engineering error. We were building a testing lab for Good Housekeeping Magazine and they needed 240 volts to test ovens, ranges, and cooktops. The engineer had us install a very expensive monitored power conditioner to get 240 volts (3Ø, 480-240). They specified all 2-wire, 50 amp, 240 volt branch circuits to each receptacle. So far so good until they realized that each receptacle needed a neutral. Then the branch circuits ended up being 1Ø, 240/139 not very good for testing equipment.
They also ran the 120 volt appliance receptacles from the power conditioner because they were monitoring every circuit for amperage, power consumption, etc. I got a call one day and they were asking me how could they possibly have 140 volts to ground on one of the circuits. I told them call your engineer.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Many 3phase input power electronic devices, like VFDs, DC drives, servo amplifiers, large computer supplies etc., are designed expecting a consistent voltage referenced to ground. So for those designed to run on 240V, they make what are called “drive isolation transformers” where the 240V secondary is 240Y139. The neutral is grounded at the transformer but is not run out of it because it is never used, but the grounded Wye secondary gives the electronics that consistent reference to ground on all 3 phases.
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