Two Honda Inverter generators in Series for 240V

[jaggedben]

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As for current sharing, that's only an issue for 240V loads on this 120/240V system that is trying to be created, but at first glance I haven't wrapped my head around how that is going to work out. Any 120V load will be on one generator or the other.

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Cheers, Wayne

Well, for any 240V load, physics will force the amount of current to be the same through each generator, and the question is how their control systems will react to that.

As far as 120V loads, I don't see why both generators wouldn't power the load (unless one or both inverter controls see something they don't like and cut out). One directly, the other through the transformer. The current from the 'opposite side' generator would be lower for the reasons you explained earlier. But it's not clear when this becomes a problem, which was the question from the start. The more I think about it though, I think this all means the transformer has to be rated for at least the larger generator's output. It can't just be a small transformer to provide a reference, because it will actually transfer power for unbalanced load. Right?

 

[winnie]

Having followed this thread for a while, I believe that if the generators are matched and the transformer has a rating equivalent of one generator, then this approach will work.

The question is if a smaller transformer could be used. If the slave generator has 'sense' terminals which are not simply parallel with the output terminals then a smaller transformer is possible. But if the slave generator is sensing the master and supplying the load via the same terminals then the impedance between master and slave will need to be pretty low.

Jon

 

[synchro]

Having followed this thread for a while, I believe that if the generators are matched and the transformer has a rating equivalent of one generator, then this approach will work.

I agree that this is likely the case. But if one has a transformer rated for one generator, then a safer bet would be to parallel the two generators directly, and then use the transformer to provide the 180° phase.

 

[wwhitney]

But if one has a transformer rated for one generator, then a safer bet would be to parallel the two generators directly, and then use the transformer to provide the 180° phase.

I'm confused by this statement. If the generators are rated, say 3 kVA each, how do we use one 3 kVA transformer to turn a 6 kVA 120V source into a 6 kVA 120/240V source?

Also, seems to me we need more info on how the inverter generators do this paralleling. Although I guess for any speculated way in which the paralleling is controlled, we should be able to figure out what shenanigans are possible.

Cheers, Wayne

 

[jaggedben]

I'm confused by this statement. If the generators are rated, say 3 kVA each, how do we use one 3 kVA transformer to turn a 6 kVA 120V source into a 6 kVA 120/240V source?

Yeah, he glossed over that, but I think the point is that shelling out for a relatively common 6KVA 120 to 120/240 transformer probably costs no more than a less common 3kVA 1:1 transformer, especially accounting for not having to worry if it will work or void a warranty.

 

[wwhitney]

Well, for any 240V load, physics will force the amount of current to be the same through each generator, and the question is how their control systems will react to that.

I guess so, the transformer is just confusing me a little. I mean suppose that the two generators use a side cable to carry some unspecified signal that ensures the generators will keep their outputs in sync, but that each generator's power output terminals are floating with respect to everything else (the other's outputs, the signal cable, any chassis). Then no transformer is required, and if the generator outputs are N1, L1, N2, L2 (where the N=neutral designation is nominal only), we could just connect L1 and N2 to get 240V between N1 and L2, with L1 and N2 as the actual neutral of the system. That seems clear enough.

As far as 120V loads, I don't see why both generators wouldn't power the load (unless one or both inverter controls see something they don't like and cut out). One directly, the other through the transformer.

So what happens if the transformer connection is intentionally high impedance, so that any current sharing via that path is minimal? The 240V current path doesn't go through the transformer.

Cheers, Wayne

 

[wwhitney]

Yeah, he glossed over that, but I think the point is that shelling out for a relatively common 6KVA 120 to 120/240 transformer probably costs no more than a less common 3kVA 1:1 transformer, especially accounting for not having to worry if it will work or void a warranty.

Not so familiar with the transformer market, but I was under the impression that a readily available, cost effective style has split coils on the primary and the secondary, so you can wire the primary to 120V or 240V 2-wire, and the secondary to 120V 2-wire or 120/240V 3-wire. In which case 3 kVA 1:1 is easily obtained, and not a special or more expensive option than other 3 kVA transformers.

Anyway, the OP's diagram indicates that the goal is a solution with a "low cost, low power" transformer (if necessary), so the goal would be a solution in which load current doesn't (significantly) pass through the transformer.

Cheers, Wayne

 

[winnie]

I'm confused by this statement. If the generators are rated, say 3 kVA each, how do we use one 3 kVA transformer to turn a 6 kVA 120V source into a 6 kVA 120/240V source?

Start with a 6 kVA 120V source. The 3 kVA 120:120 transformer makes 3 kVA available on the 'opposite leg'. The remaining 3 kVA is directly supplied by the 120V source.

-Jon

 

[synchro]

I'm confused by this statement. If the generators are rated, say 3 kVA each, how do we use one 3 kVA transformer to turn a 6 kVA 120V source into a 6 kVA 120/240V source?

In the OP's schematic you can disconnect one generator and then connect it in parallel with the other generator. No other wiring changes are needed.

I see that winnie has already said the same thing.

 

[topgone]

Right, but we know these inverter generators are designed to be paralleled, and on startup they check for the presence of another generator to sync to. So the question from the OP is how that syncing works, and whether the inverted voltage waveform provided by the transformer is something that the second generator can sync to.

As for current sharing, that's only an issue for 240V loads on this 120/240V system that is trying to be created, but at first glance I haven't wrapped my head around how that is going to work out. Any 120V load will be on one generator or the other.

This has been an interesting question, as prior to this thread I just assumed that parallelable 120V inverter generators would support this inverse syncing configuration to create 120/240V split phase right out of the box. Just because it seems like an obvious application and I assume the electronics to do it would be simple.

Cheers, Wayne

On the current sharing thing, most electrical equipment are going to share the load well if there is cross-compensation. The droop characteristics of each generator are also important.
Though not an experience with low-voltage generators, we have seen two generators fighting over the load when a new PLC-driven energy management device was used. PLCs are just too slow and resulted in power swings back and forth.