|
1 members (Scott35),
464
guests, and
12
robots. |
Key:
Admin,
Global Mod,
Mod
|
|
|
Joined: Sep 2005
Posts: 202
OP
Member
|
I work at a utility. We got called out on a customer complaint to a grocer who had burned up his 3 phase motors.
His service was a 120/240 volt open delta secondary fed from two phases of a 4 wire wye, 12.5/7.2 Kv primary.
When we got there, we read across the "lighting" legs and got 120 volts from A to neutral and 120 volts from B to neutral. A to B read 240 volts, as did B to C.
So far so good. So.....
A to C read 480 volts (instead of 240) and C to neutral read 360 volts (instead of 208). Neither transformer fuse was blown.
Hint: There had been many outages due to storms a few days before and lots of backfeeding had been going on.
|
|
|
|
Joined: Jul 2004
Posts: 9,928 Likes: 34
Member
|
Did the transformers end up on the same input phase?
Greg Fretwell
|
|
|
|
Joined: Sep 2005
Posts: 202
OP
Member
|
Dammit! Dammit! Dammit! Didn't even get past the first guy.
|
|
|
|
Joined: Apr 2002
Posts: 558
Member
|
I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see.
Curt
Curt Swartz
|
|
|
|
Joined: Apr 2002
Posts: 558
Member
|
I’ve got to learn to type faster.
Curt Swartz
|
|
|
|
Joined: Sep 2005
Posts: 202
OP
Member
|
Quote: "I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see."
Right on the second half. You CAN feed a 3 phase open delta bank with only two phases and neutral. I didn't mean to give the impression that they were going phase to phase on the primary with the common floating. The common is bonded to the neutral.
Quote: "I’ve got to learn to type faster."
Indeed. The second response was going to win a brand new Porsche.
...oh, wait. I was the second response.
|
|
|
|
Joined: Oct 2004
Posts: 806
Member
|
Oh, man lookit what I started....
Stupid should be painful.
|
|
|
Posts: 7,381
Joined: April 2002
|
|
|
|
|