Current Division In High Leg Delta Genset Single Phase Only Loads

[JohnMcNutt]

You guys helped me understand the last one well so here is another I have not seen.

20 kW genset where the data plate says 120/240 single phase only. On looking at the gen head it definitely has 12 leads and they are connected in a 120/240 high leg center tapped delta configuration with load taken off 2 corners and neutral at the midpoint. Just as if it was a 3 phase unit.

I do not want three phase from this unit so I will continue to run it as-is but my curiosity is making me wonder how the currents would divide among the windings of this thing if it was loaded unevenly with 120v loads on one side or the other. Obviously 240v only loads would divide with no problem at all.

Thanks for looking

 

[waross]

50% current in the loaded 120 Volt winding.
50% counter current in the unloaded 120 Volt winding.
50% current in the delta legs.
The equivalent circuit may be though of as three 8.3 KVA transformers (at 0.8 PF)
The other two windings form a virtual winding in phase with the loaded winding. Due to the phase shifts the capacity of the virtual winding is the same as the other two windings, 8.3 KVA. Use any value that you wish for the KVA, the percentage will be the same.
You may model this as an auto-transformer with a load connected to the center tap, in parallel with another 8.3 KVA transformer.
This is for a single delta connection. For a double delta connection, one delta will take the load with a 50%/50% current split and the other delta will not have any current.

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

 

[waross]

I have to correct myself.
I did not consider the impedances.
If the loaded winding has an impedance of 1 unit, the unloaded winding will have an impedance of 1 unit.
The combined impedance of the other two windings will be 2 units.
So the current division through the loaded winding with 1 unit of impedance will be 75% and the current through the other path with 3 units of impedance will be 25%.

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

 

[melspuds]

John, I had the same problem about 38 to 40 years ago. The gen-set was for a radio repeater a for fire department. That gen-set was from some Asian country and all of the paperwork was missing. Someone finally got some new drawings, but had to be translated into English. Some of the wire tags were also missing. The key to get 120/240 Volt at rated kw, is connecting the other 4 coils together with the right polarity to make the gen-set output pure single phase. I do not remember how how a figured out. Good look!
ps: I also had to change the governor setting to go from 50 to 60hz. The volunteer fire department did the design and install themselves. The money they paid my boss for my help, they could have bought 10 gen-sets.

 

[waross]

When a three phase generator is reconnected for single phase it looses at least 1/3 of its KVA capacity.

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

 

[JohnMcNutt]

The funny thing is that the name plate of this unit says 120/240 single phase only, and this is the way they chose to build it. I would have at least thought they would use a dog leg or double delta but they chose to build it as a high leg delta instead.

Maybe the order came in for a single phase unit and all they had were 12 lead heads in the factory?

But that still does not explain why they used the center tap high leg delta.