Convert Generator to Double Delta?

[LightGuy48]

Hello, long time reader, first time poster to the forum.

I have a small 25kva Generac generator that I have installed at a transmitter site, when it was installed the unit was wired as a 4 wire delta to provide single phase 120/240v service to the building. The high leg is insulated and unused.

I was talking with a friend about this configuration as I maintain another unit at different location that is wired double-delta from the factory for single phase 120/240v usage.

My first question is, can any 12 lead generator be converted to double delta? There is nothing in the manual that shows a DD configuration as an option, I don't want to risk damaging the unit. There is a diagram for a zig zag configuration but it's listed for 85 - 200kW units, not for this smaller unit.

Second, the unit seems to operate properly as currently wired but a person voiced a concern as to whether operating long term with only a load across two legs of the delta might cause harm to the stator.

Thanks

 

[waross]

Approximately the same current will flow in all the windings in either a 4 wire delta or a double delta connection.
You should be able to use any generic DD connection diagram.
If there is any doubt about the numbers on the leads leave it as is. It is not that important.
The only real reason to change would be if you had a single phase load that exceeded 1/2 of the rating of the set. Then you would reconnect the set as 120 Volt delta.
When I have to reconnect a set I will generally use the double delta connection, but if it is already connected 4 wire delta I don't waste time changing it.
respectfully

 

[LightGuy48]

I'm pushing the 50% capacity mark on the genset, I'm running close to 9.6kW not including the inrush from an A/C unit cycling at the site.

I know you lose 1/3 of the KVA rating when converting from 3ph to single phase, but I'm trying to understand what the electrical benefit is by changing from a single delta to a double delta. I'm having difficulty understanding the difference because you still have L1 to N and L2 to N each across an individual winding, so how does separating the single delta into two delta's improve things?

As a sidenote, the leads are well labeled on the unit, it's marked not only the insulation but with wire labels at the terminals too.

 

[LightGuy48]

I thought about my post later and I think I did a poor job of wording it.

What I meant to say, is what is the electrical difference in having one large delta configured winding compared to having two smaller deltas?

Does this better distribute the circulating currents in the windings and lower the overall operating temperature of the stator?

 

[waross]

With unbalanced 120 volt loads, the voltage balance may be a little better with the double delta. Still, unless there was a problem or you have a bad 120 volt balance problem I wouldn't spend the time to change the connections.
respectfully

 

[LightGuy48]

Thanks, I appreciate the help.

 

[resqcapt19]

If it is 120/240 with a high leg on a single delta, what would the voltage be if you wired it as a double delta. It seems to me that you would only have 120 phase to phase.
Don

 

[waross]

The vector sketch for the standard double delta generator connection is two 120 volt deltas connected side to side.
respectfully