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#129284 - 01/03/05 07:33 AM single phase from three phase
randyagr Offline
Junior Member
Registered: 03/07/04
Posts: 6
Loc: Lanark, IL US
Let's say I have a v-phase primary (2 hots plus a nuetral) with a 3 phase center tap open delta service drop(3 hots plus a nuetral) Each phase is 120 degrees apart, but if I take 2 leads for a sigle phase application, the phases are 180 degrees apart. How does this happen? Also, how many degrees apart are the phases on the primary? Thanks. Randy.
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#129285 - 01/03/05 07:37 PM Re: single phase from three phase
winnie Offline
Registered: 09/15/03
Posts: 649
Loc: boston, ma
I just about wrote an essay related to this topic last week:

You cannot define phase angle with a single circuit. You only get a phase angle measurement when you have _two_ or more circuits, and can describe the phase angle difference between them.

If you supply a single phase line to line load from a three phase source, than as far as that load is concerned, there is no phase angle. You only have two terminals, with first one then the other alternatively being relatively positive.

Once you have two circuits defined, _then_ you can find a phase angle difference. With your 'V' primary, you have three circuits; neutral to A, neutral to B, and A to B. If we _define_ the phase of the neutral to A circuit as zero degrees, then the neutral to B circuit is 120 degrees. I believe that the A to B circuit has a phase angle of 150 degrees with an amplitude of 1.732 * the amplitude of the neutral to A circuit, but I'm not going to swear to that without doing the math out on paper.

#129286 - 01/03/05 08:25 PM Re: single phase from three phase
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: West-Southern Inner-Northeast ...
For an open-delta secondary, there are two possible configurations for the primary side of the transformer bank. One is open-wye, with 120° between two phase-to-neutral primary connections. The other is open-delta, with 60° between two phase-to-phase-connected primary windings, that are typically symmetrical with the two secondary windings.

[This message has been edited by Bjarney (edited 01-04-2005).]

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