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#61107 - 01/16/06 11:05 PM Another brain teaser....480 on a 240 volt delta  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
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.


Work Gear for Electricians and the Trades

#61108 - 01/16/06 11:12 PM Re: Another brain teaser....480 on a 240 volt delta  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
Did the transformers end up on the same input phase?


Greg Fretwell

#61109 - 01/16/06 11:14 PM Re: Another brain teaser....480 on a 240 volt delta  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
Dammit! Dammit! Dammit!
Didn't even get past the first guy. [Linked Image]


#61110 - 01/16/06 11:15 PM Re: Another brain teaser....480 on a 240 volt delta  
caselec  Offline
Member
Joined: Apr 2002
Posts: 558
San Jose, CA
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

#61111 - 01/16/06 11:17 PM Re: Another brain teaser....480 on a 240 volt delta  
caselec  Offline
Member
Joined: Apr 2002
Posts: 558
San Jose, CA
I’ve got to learn to type faster.


Curt Swartz

#61112 - 01/16/06 11:44 PM Re: Another brain teaser....480 on a 240 volt delta  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
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.


#61113 - 01/17/06 02:08 AM Re: Another brain teaser....480 on a 240 volt delta  
mxslick  Offline
Member
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
Oh, man lookit what I started.... [Linked Image]


Stupid should be painful.


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