Delta lost leg question

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S'mise

Senior Member
Location
Michigan
In a delta system, one leg can be lost because of a blown fuse and still deliver full voltage on all 3 legs.

Say, I have a motor running with a lost leg. What will actually happen?
The amperage the motor will draw, remains the same, but the kva capacity of the transformer is reduced by 1/3?
I have a feeling that pesky old 1.732 number is involved somehow.
Thanks
 

Smart $

Esteemed Member
Location
Ohio
In a delta system, one leg can be lost because of a blown fuse and still deliver full voltage on all 3 legs.
That is incorrect. You will only get full voltage across two legs. The voltage from either of those legs to the load side of the blown fuse is a result of feedback of the other two legs through connected loads.

What happens to a motor on loss of leg has a lot of variables. Hopefully the protection system kicks in.
 

S'mise

Senior Member
Location
Michigan
Ok, still not clear.
What happens when a primary fuse blows in either a delta delta or wye delta, isn't the secondary full voltage, but reduced by potentially 2/3rds because of the single phasing primary?
 

kwired

Electron manager
Location
NE Nebraska
Ok, still not clear.
What happens when a primary fuse blows in either a delta delta or wye delta, isn't the secondary full voltage, but reduced by potentially 2/3rds because of the single phasing primary?
Yes there are times when you can lose an input leg on a transformer and output still sees full voltage. If you have a wye primary/delta secondary this can happen pretty easily most POCO pole top transformer banks are wye primary - secondary can be either wye or delta. Take one of these systems with a full secondary delta system and inadvertently open one input leg and you have exactly what they intentionally build for open delta systems, you also reduce what capacity is available before overloading what is still connected and working, 2/3 capacity seems logical but I believe it is actually about 58% (which is the reciprocal of 1.732)of the full delta bank that is the new "full load".
 

Smart $

Esteemed Member
Location
Ohio
Ok, still not clear.
What happens when a primary fuse blows in either a delta delta or wye delta, isn't the secondary full voltage, but reduced by potentially 2/3rds because of the single phasing primary?
Your OP didn't say a primary fuse. I took it as losing a leg on the secondary side. I take it kwired's post has you sorted now?
 

kwired

Electron manager
Location
NE Nebraska
Losing a leg on secondary side can result in motors acting like a rotary phase converter and things continue to run - but will have less capacity may or may not be close to full normal voltage - can't give exact details and probably varies anyway from one place to another depending on load conditions.
 

S'mise

Senior Member
Location
Michigan
Yes, I understand what happens, but was wanting to better understand how a transformers capacity is reduced by single phasing.

The reciprocal of 1.732 does seem to make sense.
So theoretically, if a delta transformer is oversized by 1.732, the loads can continue to run with no adverse events?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Losing a leg on secondary side can result in motors acting like a rotary phase converter and things continue to run - but will have less capacity may or may not be close to full normal voltage - can't give exact details and probably varies anyway from one place to another depending on load conditions.
And if there are no large motors actually running when the phase is lost, it is unlikely that any three phase motors will successfully start. (That assumes that you have really gone to single phase rather than just an open delta situation.)
 
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