120V to 120/240V. Transformer or other ideas?

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fastline

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Going to be employing a Honda EU3000 generator as a backup charge source for an off-grid DC inverter system. I must provide the inverter with 120/240V, 180* power to make it happy. There is little sense in running a bigger generator that has 120/240V and it cuts into efficiency.


the EU3000 is an inverter genny. I have a little transformer to step up to 240V when I need it but I have never seen a transformer like what I am looking for. Ideas? I was trying to think how I might consider a 120/240:240/480 transformer for the job, or possibly a pair of buck/boost and make the neutral?

trying to avoid using two transformers going 120 to 240, then 240 back to 120/240 with center tap.
 
If I was having to use a transformer, I would probably just setup for worst case which might be 3kw on each leg so that would be 6kw at 240V. Obviously I cannot get that from the genny but it would prevent transformer damage if one one leg was loaded to max. I could probably back that out a touch as I know any such load would be minimal duty cycle.
 
120/240Pri x 120/240Sec

120/240Pri x 120/240Sec

Any maker can build what you need, but an off the shelf unit is available from Hammond. Others may have one, but this was easy to find.

Hammond_1Φ 120-240Px120-240S.jpg

MTW Ω
 
Ha, I didn't even check the books assuming this was something special because I have never seen one. Thanks for the help guys!

Whoops, I forgot something! Having not played with split phase secondary transformers much other than pole pigs, can I expect on a 3kva transformer to only be safe for 1.5kva per 120V half leg on the secondary side? Or would I be able to pull the full 3kva at 120V?
 
Last edited:
190809-1142 EDT

fastline:

All you need is one 120 to 120 isolation transformer, or a 120 to 240 autotransformer.

With an appropriately sized transformer you can load either phase to the full generator capability, but total load must remain within the generator capability.

.
 
Gar, as I understand, the inverter we need to connect to is going to need L1, L2, and N, and it will be checking that phasing of L1, L2 is 180*. Inputs from 3phase power will cause it to error out for example.

I am not following how you will get full capacity from all legs? explain?
 
fastline said:
Whoops, I forgot something! Having not played with split phase secondary transformers much other than pole pigs, can I expect on a 3kva transformer to only be safe for 1.5kva per 120V half leg on the secondary side? Or would I be able to pull the full 3kva at 120V?
Click to expand...
1.5
 
190810-0937 EDT

fastline:

If you have a 3 kVA 120 V generator, then max load on the generator is 3 kVA.

If I connect a 3kVA 120 V to 120 V isolation transformer to that generator, then I can load that transformer secondary to 3 kVA.

Next if I connect one lead of the transformer secondary to the appropriate (meaning phasing) primary lead, then I can produce 240 V.

Then I can do any loading to this combination that does not exceed 3 kVA loading to the generator. One loading possibility is 240 V at 3 kVA.

.
 
gar said:
190810-0937 EDT

fastline:

If you have a 3 kVA 120 V generator, then max load on the generator is 3 kVA.

If I connect a 3kVA 120 V to 120 V isolation transformer to that generator, then I can load that transformer secondary to 3 kVA.

Next if I connect one lead of the transformer secondary to the appropriate (meaning phasing) primary lead, then I can produce 240 V.

Then I can do any loading to this combination that does not exceed 3 kVA loading to the generator. One loading possibility is 240 V at 3 kVA.

.
Click to expand...


You are going to have to draw that up. Remember I said the inverter needs to see 180* relation between 2 120V phases.

It sounds like you are making an autotransformer for the 240V, which I can see (and just lose isolation), but the 180* split 120?
 
fastline said:
You are going to have to draw that up. Remember I said the inverter needs to see 180* relation between 2 120V phases.

It sounds like you are making an autotransformer for the 240V, which I can see (and just lose isolation), but the 180* split 120?
Click to expand...
Ignore that there are dual coils in series, and pretend that H1 to H4 is 120v and that X1 to X4 is 120v. The wire connected to H4/X1 should be brought out as the neutral between H1 and X4 on the HV side.


attachment.php





To help you see it differently. Again, pretend each winding is 120v.



t1.jpg
 
Last edited:
190810-1654 EDT

fastline:

You are listed as an engineer. What does that mean? Electrical, civil, chemical, or what?

I will assume electrical.

Thus, if you apply a sine wave to a transformer primary, and pick one terminal as your reference point, then relative to that reference point your secondary voltage will have a phase relationship of 0 or 180 degrees depending upon which of the secondary terminals is connected to that primary reference terminal.

You pick the one that gives you the 180 degree output, and thus get your 240 V with the said reference point of the primary being your neutral. Effectively you built an autotransformer from an isolation transformer.

Either building an autotransformer with one single continuous winding with a tap at the midpoint, or using a one to one isolation transformer with the correct phasing you get the same result.

.

 
I have done it with a typical buck boost autotransformer, 120 / 240 primary, 18 / 36 secondary. Ignore the autotransformer secondary and feed 120 to one of the primary 120 V windings. From the two primary winding in series you will get 240 V out, but it is still an auto transformer connection and not a SDS.

Used it to feed a 240 V well pump from a 2 kW inverter generator. Transformer came from the collection in the basement. Voltage and running current were right where they should be, but I don't know if I would sell an off label use to a customer.
 
An option for the autotransformer mentioned by gar is either the 3 or 5 KVA unit below;
(and vendor is approved by Wile E. Coyote :) )

https://rexel-cdn.com/Products/Cata...i=FEC7CFC1-B714-4742-AF55-10DBDF42CF6C#page=7

https://www.mrosupply.com/electrical/transformers/885493_t253063_acme/
https://www.mrosupply.com/electrical/transformers/885495_t253064_acme/

With the 3 KVA unit you can get the full 3 KVA output of the generator on balanced 120V loads, a 240V-only load, or a 120V load on the phase that's directly connected to the generator. The derived phase at 180 degrees would support up to 1.5KVA.
The 5 KVA unit would support 2.5KVA on the 180 deg phase but for $216 more. The inrush current would also be higher, so there's a higher risk of the inverter generator having a problem with that. But it could be a non-issue.
 
__dan said:
I have done it with a typical buck boost autotransformer, 120 / 240 primary, 18 / 36 secondary. Ignore the autotransformer secondary and feed 120 to one of the primary 120 V windings. From the two primary winding in series you will get 240 V out, but it is still an auto transformer connection and not a SDS.
Click to expand...
And don't forget that the incoming conductor that also connects to the output neutral should be supplied by the grounded conductor of the source circuit. (H4/X1 in my first diagram)
 
synchro said:
An option for the autotransformer mentioned by gar is either the 3 or 5 KVA unit below;
(and vendor is approved by Wile E. Coyote :) )

https://rexel-cdn.com/Products/Cata...i=FEC7CFC1-B714-4742-AF55-10DBDF42CF6C#page=7

https://www.mrosupply.com/electrical/transformers/885493_t253063_acme/
https://www.mrosupply.com/electrical/transformers/885495_t253064_acme/

With the 3 KVA unit you can get the full 3 KVA output of the generator on balanced 120V loads, a 240V-only load, or a 120V load on the phase that's directly connected to the generator. The derived phase at 180 degrees would support up to 1.5KVA.
The 5 KVA unit would support 2.5KVA on the 180 deg phase but for $216 more. The inrush current would also be higher, so there's a higher risk of the inverter generator having a problem with that. But it could be a non-issue.
Click to expand...



RIght on. Thanks guys, this makes sense. I think with the smaller power range, we can just minimize any load on the 180* 120V connection. Plus, I guess we can have low voltage on tap for whatever.....lol

I would assume a buck/boost would be about the cheapest bargain barn solution here? I mean, really anything with a 120x240V primary should do.
 
Inverters are just the right choice on a generator. It would have been super easy for Honda to provide a 240V switch but I guess we can't get too fancy or the general public will find a way to destroy them. It would not change the IGBT pack one bit and just a simple processor adjustment but I guess I don't know what the alternator and DC bus voltage is on it.

Here is something "I" would like to try. OK, the way most inverters work is if there is an AC wave present (being fed) at start up, they will synch to that. If they don't sense one, they just start their own. So, if you have one running, plug two together and start the second, you just synched them and double your amperage.

But if I have them both running, and combine only the Neutral of each, I am just wondering if I can get 180*. I am guessing this won't work but would sure be nice. I think even with the small variance of an inverter, it would be too much and the phase relationship would be all over the map.


To be honest though, the only thing in a home I can even think of that needs true split phase is a dryer. I realize feeding a load center with backup power is much easier when you can just plug in and go. However, I have been feeding my personal circuit at 120V on both legs for 20yrs. I know anything that is 240V will have a potential of 0V so its not like there is any risk.


What WOULD be nice to see is one of the transformer companies build a special transformer for this. Delete the secondary entirely, and provide a beefed up 180* winding on the primary side so the entire KVA could be used on the 180* leg. The only other option to get full KVA from that 180* leg is double the size of the transformer. What I will probably do to protect the transformer is just use a 3KVA transformer and protect that 180* leg at 1.5KVA just so we don't roast any marshmellows.
 
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