Effects Of Dropping A Phase On A Generator

[Seven-Delta-FortyOne]

2 projects just came up, where they want to use a 3 phase generator on a single phase service.

One is a municipality, they’re switching from an existing 3 phase to single phase service, but they have several 3 phase generators on site, so the plans show a 3 phase power inlet, with C phase not connected.

Second is a residential customer who has a 3 phase unit and wants to use it at his house.

I’m not questioning a municipal engineer, it just got me thinking; How much can you load the A and B phases, with zero load on C phase, before you would run into voltage or current imbalance issues, and what would that look like?

 

[don_resqcapt19]

Some generators have 12 leads and can be reconfigured for single phase operation, but only at 2/3s the name plate capacity. The advantage of reconfiguration is that current flows in all of the windings and you can get 120/240 and not be stuck with 120/208.

 

[Joethemechanic]

Some generators have 12 leads and can be reconfigured for single phase operation, but only at 2/3s the name plate capacity. The advantage of reconfiguration is that current flows in all of the windings and you can get 120/240 and not be stuck with 120/208.

I've come across these Zig Zags now and again

 

[Seven-Delta-FortyOne]

12 Lead Generator

I have some experience reconfiguring a 3 phase generator from 208Y to 480Y or the other way around. The other day I was off on a favor job, make this old three phase 20 KW generator work on this guys house. Now I expected I would be supplying the house single phase 208/120 leaving one leg...

forums.mikeholt.com

This was an interesting thread about how that works with a 12 lead generator.

I don’t think the municipality would want the unit re-wired, but I will look into doing it on the residential one.

 

[hillbilly1]

Some generators have 12 leads and can be reconfigured for single phase operation, but only at 2/3s the name plate capacity. The advantage of reconfiguration is that current flows in all of the windings and you can get 120/240 and not be stuck with 120/208.

Hopefully it is a Kohler or other major brand, not a Generac. Generac tends to use the cheapest, so not usually 12 lead.

 

[Seven-Delta-FortyOne]

It’s a WhisperWatt by MultiQuip. Diesel unit.

 

[Joethemechanic]

It’s a WhisperWatt by MultiQuip. Diesel unit.

Trailer mounted? Usually they have 12 lead when they are portables like that. Probably has a Zig Zag diagram in the documentation that came when they bought it.

 

[qcroanoke]

Hopefully it is a Kohler or other major brand, not a Generac. Generac tends to use the cheapest, so not usually 12 lead.

We had a Generac that we used for testing non USA buildings. It had several boards and switches we could configure and change to get the voltage we wanted.
Never had to make or break a splice.
And we could andjust for 50 or 60 hertz.

 

[hillbilly1]

It’s a WhisperWatt by MultiQuip. Diesel unit.

The whisper quiets we rented had a selector switch that could change it to just about anything.

 

[hillbilly1]

We had a Generac that we used for testing non USA buildings. It had several boards and switches we could configure and change to get the voltage we wanted.
Never had to make or break a splice.
And we could andjust for 50 or 60 hertz.

Probably a custom build. Of the few commercial Generac’s we installed, you were stuck with whatever they shipped it as. Their controller sucked too, you could do two wire start, but the controller would wait 10-20 seconds before it would actually start. Would not pass the fire marshal inspection even though Generac swore up and down it would. Then had the gall to want I think was $5000 to change the controller out to one that would.

 

[qcroanoke]

Probably a custom build.

That is very possible. Was a while back.

 

[Seven-Delta-FortyOne]

Finally got time to look into this. Been trying to run my business from my phone in the cab of my truck while driving from the job site to the supply house.

Apparently this unit, DCA-25USI, according to the manual I can find online, has a selector switch to go to single phase.

I still wonder to what capacity a 3 phase generator could be loaded to on only 2 phases. Or a transformer for that matter. I’ve always tried to balance loads as best I could. I’ve taken readings on existing panels with close to 100A difference on a 200A service, but I doubt it was planned like that. More likely it was just a condition at the moment.

 

[Birken Vogt]

I still wonder to what capacity a 3 phase generator could be loaded to on only 2 phases. Or a transformer for that matter. I’ve always tried to balance loads as best I could. I’ve taken readings on existing panels with close to 100A difference on a 200A service, but I doubt it was planned like that. More likely it was just a condition at the moment.

This is what I do for a living and I can tell you what you want to know, but the answer is long and complicated.

Kohler has two slightly conflicting white papers on the subject, and Cummins has another good one. Do a search because I have not bookmarked them in my computer.

A Multiquip 25 (which is actually a 20) is rated at 14 on single phase, I know that from memory.

Also it is somewhat of a case-by-case basis. A three phase gen head will be running inefficient at single phase which necessitates a bigger gen head. But bigger gen heads are just a tiny incremental cost to the mfr, so often you will find what your unit came with is already oversized because that is what they had available. Therefore not much derating on single phase.

But often, we find that just running a house on 2 phases of 208 is good enough. What devices can you think of in a house that really need a full 240?

Running unbalanced on 2 legs out of 3 would only be a problem if those 2 legs were fully loaded and remained so for quite a long time. How often does that happen on a single phase installation?

On the topic of Generac and other "non reconnectable" units. There is always a way. Just how deep do you want to go. Might have to dig into the gen end and cut and splice winding ends and use fiberglass tape and such. Also have to make the controller happy. I have done it. Seldom is it necessary though. Usually there are simpler ways, see above.

 

[hillbilly1]

On the topic of Generac and other "non reconnectable" units. There is always a way. Just how deep do you want to go. Might have to dig into the gen end and cut and splice winding ends and use fiberglass tape and such. Also have to make the controller happy. I have done it. Seldom is it necessary though. Usually there are simpler ways, see above.

What I did was just put a step down transformer, no need to reinvent the wheel! LOL!

 

[Birken Vogt]

What I did was just put a step down transformer, no need to reinvent the wheel! LOL!

I hate the cost and inelegance of that, but I understand it.

 

[Joethemechanic]

There is always a way. Just how deep do you want to go. Might have to dig into the gen end and cut and splice winding ends and use fiberglass tape and such. Also have to make the controller happy. I have done it. Seldom is it necessary though. Usually there are simpler ways, see above.

You just need a wife like this girl.

Lin Guoer holds a degree in Water Conservancy and Hydroelectric Engineering from Kunming University, providing her with the necessary technical foundation for her work.

Probably can make a nice chicken and rice dish with some black bean cakes for lunch while doing it too

 

[Seven-Delta-FortyOne]

This is what I do for a living and I can tell you what you want to know, but the answer is long and complicated.

Kohler has two slightly conflicting white papers on the subject, and Cummins has another good one. Do a search because I have not bookmarked them in my computer.

A Multiquip 25 (which is actually a 20) is rated at 14 on single phase, I know that from memory.

Also it is somewhat of a case-by-case basis. A three phase gen head will be running inefficient at single phase which necessitates a bigger gen head. But bigger gen heads are just a tiny incremental cost to the mfr, so often you will find what your unit came with is already oversized because that is what they had available. Therefore not much derating on single phase.

But often, we find that just running a house on 2 phases of 208 is good enough. What devices can you think of in a house that really need a full 240?

Running unbalanced on 2 legs out of 3 would only be a problem if those 2 legs were fully loaded and remained so for quite a long time. How often does that happen on a single phase installation?

On the topic of Generac and other "non reconnectable" units. There is always a way. Just how deep do you want to go. Might have to dig into the gen end and cut and splice winding ends and use fiberglass tape and such. Also have to make the controller happy. I have done it. Seldom is it necessary though. Usually there are simpler ways, see above.

So what is the difference, if any, as far as the generator is concerned, between connecting A and B phase only, and setting it on 208Y/120, and setting it on 120/240? Is there any advantage to the generator one way or the other?

And I agree about the 208 in the house. I don’t have any issue with supplying a residence with 208 instead of 240. Other than the electric clothes dryer maybe taking a few minutes longer, I don’t know of many items that actually require 240V.

 

[Birken Vogt]

So what is the difference, if any, as far as the generator is concerned, between connecting A and B phase only, and setting it on 208Y/120, and setting it on 120/240? Is there any advantage to the generator one way or the other?

In the 208 position, 2 windings will take all the current, and the third will be idle. In the 240 position, the third winding is basically repurposed almost like a boost transformer tacked onto the end of the string to add the last 32 volts. It is because of the phase angle, does not translate well to a rambling text. Also when you have the proper 240 connection, the phase angle in the 2 out-of-phase windings will not be lined up with their "natural" angle any more.

There are also issues of rotor heating under some unbalanced conditions, but remember, none of this matters, unless you are running near sustained 100% loading of the copper and iron of the gen head. The engine doesn't care. The air flow is constant so low loads will not overheat anything.

And I agree about the 208 in the house. I don’t have any issue with supplying a residence with 208 instead of 240. Other than the electric clothes dryer maybe taking a few minutes longer, I don’t know of many items that actually require 240V.

Even the clothes dryer will not take much longer. The initial heatup will be longer but it won't make a difference after that, if on 240 the element runs 50% on 50% off then on 208 it may run 67% on 33% off but the total kwh consumed will be the same and dryer runtime probably indiscernible.