Given: 120/240 volt, 3 ph., 4 wire, open delta electrical service, consisting of one 10 KVA transformer and one 15 KVA transformer with a center tap. Lets say that phase A and B are at each end of the 15 KVA center-tapped transformer and phase C is at the other end of the 10 KVA transformer. That makes phase C the high leg (208 volts to ground).

I know you can supply 240 volt 3-phase loads and you can supply 120 volt loads from A-N and B-N, but not C-N (since its 208 volts to ground).

My question: Is it OK to supply 240 volt single-phase loads from A-B, B-C and C-A? or can you only supply these loads from A-B and B-C?

You didn't say whether the 10 kVA transformer was connected to phase A or phase B. But your final question suggests your concern about the C-A connection, so I presume this is the open end.

Yes, you can connect to C-A and get 240 volts. But this load will act on both transformers with a 0.5 power factor, effectively derating them. If you have a 30 amp load on C-A, that's 30 amps running through both transformers, but you get half the useful power from one and half from the other while taking up full capacity from each.

Re: Open Delta Transformer#129520 04/07/0510:29 AM04/07/0510:29 AM

For 3ø-transformer specification, see C57.105-1978 …Application of Transformer Connections in Three-Phase Distribution Systems Excerpted..

§6.2 …The effect of load unbalance is most easily explained in terms of one single-phase load… With delta-connected secondaries, all three windings are involved. The winding connected to the two load phases supplies 2/3 of the total load, and the other two windings are each loaded to 1/3 the total load as shown in Fig 11B. (If the connection is Δ·Δ, this statement is true only if each phase has the same transformer impedance. See 8.2.)

§8.2.2 …When the three legs of a Δ·Δ bank have different impedances (either ohmic or referred to a common kVA base), the currents within the Δ are not balanced even through the external currents are balanced. The asymmetry of the impedances may be explained or calculated in terms of an EMF which gives rise to a circulating current superimposed on the load current…

Is there impedance dissymmetry in an open·delta set? Aside from loading—voltage imbalance and individual-transformer temperature rise may be critical present and near-term considerations. Do not forget C84.1 annex-D impact.

[This message has been edited by Bjarney (edited 04-07-2005).]

Re: Open Delta Transformer#129522 04/07/0511:00 AM04/07/0511:00 AM

In a closed delta configuration, with a 240 volt single-phase load, load current = I, one transformer current = I/root3 and the other two transformer have current = .732*(I/root3). So the current circulating in the transformer is not balanced between each of the three transformers.

With an open detla the transformers are different sizes (10kva and 15 kva) so the impedences will be different. But, since there are only 2 legs on the open delta (no circulating currents as in a closed delta), the currents will be the same thru each transformer (Kirchoffs current law). Essentially a simple series circuit inculding two transformers and a load.