Current Division In Three Phase Generator Windings Connected for Single Phase

[JohnMcNutt]

I came across an interesting generator configuration that I would like to run by you guys.

It is a 12.5 kW generator set up to deliver 120v only. The windings are just a regular 12 lead but they are connected as a 120v parallel (low) delta, corner grounded.

The scant documentation I have ever seen on getting 120v out of a 12-lead delta shows the power all connected between the ends of the U winding. The V and W are out-of-phase connected in series across the U winding and presumably carry half the load with the in-phase U winding--right?

Let's label the corners of this delta RST for argument's sake since delta makes this confusing with U1=R=Neutral.

In this installation it appears that they built it so that half the 120v loads are connected R/N - S and half are R/N - T. I don't think there are any loads between S - T.

It seems to me that in this case, spreading loads like this might be counterproductive because if you kept them all connected across the U winding, the current would divide equally across the two current paths, but now that you have moved stuff to half of the alternate current path, the balance would be messed up.

I suppose this would be easy enough to check with an amp-clamp and a load bank if there is enough slack in the leads.

Thanks for looking.

 

[waross]

You are correct. In the normal connection all the phase windings carry equal current, but in the out of phase windings the power factor is 50%, lagging in one winding and leading in the other winding.
As an example consider a 12 KVA delta generator with an 8 kW single phase load.
The in phase winding will supply 4 kW and 4 KVA.
The out of phase windings will each contribute 2 kW and 4 KVA.
The total load on the generator will be 8 kw and 12 KVA.
It will be more accurate to use the term "Grounded conductor" than the term "neutral". The term neutral in description of one conductor of a 120 Volt two wire circuit is only accurate when the circuit is derived from a center tapped 120:240 Volt supply or a 120:208 Volt three phase supply.

The delta connection may be used to supply 120 Volts from a standard generator.
Th supply 120:240 single phase power, there are three connections:
The Double Delta,
The Zig-Zag.
And, for 10 lead generators, the Bar Diamond, or Collin connection.

When a generator is reconnected from three phase to single phase, the kW capacity drops to 2/3 of the original kW capacity.
There is still the same prime mover power available so the PF jumps from 80% to 100%. (Based on the new kW rating.)
Now for your unique connection; A clamp meter reading may be confusing because of the phase angles of the various loads.
You can consider the loads across R/N-S and R/N-T individually. Do vector sketchs for the loads R/N-S showing the currents in R-S, R-T, and S-T. The three sketches will each be a straight line, and all three will be at the same phase angle as R-S.
Now do a sketch for the currents for the loads R/N-T showing the currents in R-T, R-S and S-T. The three sketches will each be a straight line, and all three will be at the same phase angle as R-T.
Now combine the currents for each phase and you will see the resultant currents and phase angles.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Draw winding arrangement out. If you resolve vectors you'll find that the S and T windings each contribute 0.5 PU voltage in phase with the R winding voltage. In essence you have two voltage sources in parallel, each through an R-L impedance. Current will split between them in inverse proportion to the impedances.

 

[JohnMcNutt]

Draw winding arrangement out. If you resolve vectors you'll find that the S and T windings each contribute 0.5 PU voltage in phase with the R winding voltage. In essence you have two voltage sources in parallel, each through an R-L impedance. Current will split between them in inverse proportion to the impedances.

That is how I always figured it would be as well.

If it was strictly true then wouldn't the impedance of the out-of-phase string of windings be roughly double to the in-phase winding, therefore the current through them be theoretically half the in-phase winding?

Therefore a hypothetical gen-head with a 20A rating per winding would have 20A on the in-phase winding and 10A on the out-of-phase windings if loaded single phase without exceeding the rating of any winding.

But since generator manufacturers allow the same rating on single phase in the double delta and zig zag connection, I had always just assumed that the current divides equally between the in-phase and out-of-phase parts, otherwise, the in-phase winding would be overloaded on double delta.

Am I doing this wrong?

 

[waross]

You are sort correct that the impedance of the two windings in series is greater. This may be complicated a little by the phase angles. However the combined EMF of both windings increases in the same proportion. The result is that the greater EMF and greater impedance allow a current flow equal to the current in the in-phase winding.
This is very similar to the current division in a delta transformer bank with a single phase load.
The KVA capacity of a delta bank supplying a single phase load on one phase will be 2/3 of the capacity of the tree phase capacity, the same 2/3 KVA ratio seen when a generator is reconnected for single phase use.

Bill
--------------------
"Why not the best?"
Jimmy Carter