Step up Transformer

[theoldwizard1]

This is related to another post.

I find it hard to believe, but I can not find a 120V to 240V step up transformer with split phase output anywhere on the web.

Purpose: Take the output from a 120V generator and connect it (via a transfer switch) to a home load center (breaker panel).

Does such a thing exist ?

 

[mrb]

yep, theyre pretty common. i dont think youre going to find it for sale on some website but you should have no problem ordering it through an electrical wholsaler. To see such a transformer, look at this http://www.federalpacific.com/literature/drytrans/04redbulletin600volt.pdf first section on page 26 for 120x240 - 120/240 transformer and look at wiring diagram #5 on page 66

 

[theoldwizard1]

Yep, that was what I was looking for. Not cheap, but definitely "industrial grade".

I found some others that did not have split phase (center tap) output. They certainly did not look like "continuous duty" to me !

 

[mrb]

Yep, that was what I was looking for. Not cheap, but definitely "industrial grade".

I found some others that did not have split phase (center tap) output. They certainly did not look like "continuous duty" to me !

i saw that $100 on ebay you linked to, looks like a real pos. probably not ul listed, exagerated rating, and might even be a buck boost transformer vs a true isolation transformer which you need.

 

[nehog]

If you don't need isolation, an auto-transformer would perhaps work. But what you are asking for is a common thing, easy to find. Ask (search) for a transformer with a 120 volt primary, 240 volt center tapped secondary, with appropriate current ratings. Realize that these can be big, heavy, and expensive.

 

[lametec]

Any reason you can't just replace the generator?

 

[nehog]

Here is one example from eBay that does what you want:

http://cgi.ebay.com/Precision-Elect...uit_Breakers_Transformers&hash=item43a671e63f

Do note this is a 500 watt transformer, so won't power much big stuff.

Another, also 500 watt:

http://cgi.ebay.com/ACME-step-up-tr...uit_Breakers_Transformers&hash=item588dbb936c

Now, this one could be wired as an auto-tranformer, and would work. Much higher ratings too:

http://cgi.ebay.com/Square-D-7-5Kva...uit_Breakers_Transformers&hash=item5641a64ea6

So there are lots of options there for you, just gotta search for 'em! If you see one you think/hope will work, post the eBay listing number and I'll look at it for you. If it is not eBay, take a picture of the specification plate/tag and post that.

BTW, how big is that generator? Can't be too large if it is only 120 volts.

 

[MoonRise]

This is related to another post.

I find it hard to believe, but I can not find a 120V to 240V step up transformer with split phase output anywhere on the web.

Purpose: Take the output from a 120V generator and connect it (via a transfer switch) to a home load center (breaker panel).

Does such a thing exist ?

Ummm, you do realize that a generator that is supplying only 120V AC most likely does NOT have the amperage output to be able to do much after you run that voltage through a step-up transformer to 240V?

ex: Generator supplies 20 amp @ 120V. Ignoring all losses, running that through a 1:2 step-up transformer gives you 10 amps @ 240V. The power in watts (volt-amps) doesn't change when you run through a transformer (again, ignoring losses), just the ratio between the volts and the amps.

So, for example (example numbers only, but the general math and such is 'real world'), a little 2000W generator is only going to supply 2000W no matter what the output voltage is. Add in transformer losses in the real world, and you are then down to more like 1600-1700W after going through a transformer (big industrial transformers and power-company transformers would most likely be more efficient than that but at a bigger $$$$).

Add in the cost for a transformer and I think you would just be better off getting a generator that can supply the desired 240V (as well as 120V) power.

 

[mrb]

Ummm, you do realize that a generator that is supplying only 120V AC most likely does NOT have the amperage output to be able to do much after you run that voltage through a step-up transformer to 240V?

ex: Generator supplies 20 amp @ 120V. Ignoring all losses, running that through a 1:2 step-up transformer gives you 10 amps @ 240V. The power in watts (volt-amps) doesn't change when you run through a transformer (again, ignoring losses), just the ratio between the volts and the amps.

So, for example (example numbers only, but the general math and such is 'real world'), a little 2000W generator is only going to supply 2000W no matter what the output voltage is. Add in transformer losses in the real world, and you are then down to more like 1600-1700W after going through a transformer (big industrial transformers and power-company transformers would most likely be more efficient than that but at a bigger $$$$).

Add in the cost for a transformer and I think you would just be better off getting a generator that can supply the desired 240V (as well as 120V) power.

right. watts is watts. on the type of transformer i posted, you will see an efficiency in the high 90%. You arent going to lose 20% of your power in the transformer. the idea here is to be able to use 120v generator(s) to supply both phases of the loadcenter in your house. For example a pair of honda eu2000i generators is going to give you 4000w when paralleled, feed that through the transformer and you can power quite a bit of stuff in your house.

 

[lametec]

But why not just buy a 4000W generator with 240V output? Could certainly be cheaper, use less fuel, less of a "contraption" than paralleling two generators through a step-up transformer.

I use a 5000W genset myself, and it powers the whole house short of the electric water heater.

 

[mrb]

But why not just buy a 4000W generator with 240V output? Could certainly be cheaper, use less fuel, less of a "contraption" than paralleling two generators through a step-up transformer.

I use a 5000W genset myself, and it powers the whole house short of the electric water heater.

the honda eu2000i generators are very fuel efficient, quiet, output extremely clean power that isnt going to cause havoc with appliances and electronics, and with this type of setup you have two very portable generators for other uses.

 

[sherlocktk]

Why would you not be able to power both legs of the house with 120v. None of your 240 appliances will work, but thoes will all take too much power anyway.

So wirenut the positive wire into a dual pole breaker and be done with it? No transformer, and everything in the house works, except 220v appliances.

10-15 years ago when my parents lost a leg into there house, the city just used a jumper wire to bridge the 2 hot legs together (before the meter), which makes the meter not work, but we got free electricity for 12+ hrs.

 

[malibu101]

10-15 years ago when my parents lost a leg into there house, the city just used a jumper wire to bridge the 2 hot legs together (before the meter), which makes the meter not work, but we got free electricity for 12+ hrs.

Well that is a sure way to overload the neutral on a multiwire branch circuit. Assuming both legs are drawing near capacity.

 

[sherlocktk]

Well that is a sure way to overload the neutral on a multiwire branch circuit. Assuming both legs are drawing near capacity.

Good Point. I know in my house I only have 2 such cicuits, Dishwasher/Garbage Disposal, and Heater/Washer Dryer Circuit. Not likely to have an issue running both of thoes at the same time when on genny power.

They are only used for "dedicated" plugs in my house, not sure if this is the norm. Personally I just wound not use both at the same time since its a known failure condition.

Also the generator is just for "emergency" so I would be willing to deal with a lower safety factor than a permamnent install, but this all depends on the size of generator you are using.

 

[dwilliams35]

Good Point. I know in my house I only have 2 such cicuits, Dishwasher/Garbage Disposal, and Heater/Washer Dryer Circuit. Not likely to have an issue running both of thoes at the same time when on genny power.

They are only used for "dedicated" plugs in my house, not sure if this is the norm. Personally I just wound not use both at the same time since its a known failure condition.

Also the generator is just for "emergency" so I would be willing to deal with a lower safety factor than a permamnent install, but this all depends on the size of generator you are using.

Did you wire the house? Back when I was doing that, we used those on a pretty regular basis: especially on the longer home runs. One cable for two circuits on the other end of the house....

count me in the camp of getting a new generator: by the time you get a transformer, get it set up, etc., it would be easier and cheaper to just run down and get a bigger generator with a 240 v output.

 

[theoldwizard1]

i saw that $100 on ebay you linked to, looks like a real pos. probably not ul listed, ...

Yep. Seller said no UL or other certifications.

That is why I asked here !

 

[theoldwizard1]

Ummm, you do realize that a generator that is supplying only 120V AC most likely does NOT have the amperage output to be able to do much after you run that voltage through a step-up transformer to 240V?

Yes, fully aware.

I wanted to run a 1 hp submersible well pump. I have not checked the current draw so it very well might exceed 1/2 of the rating of a 120V generator. Of course that assumes there is no other loads !

Add in the cost for a transformer and I think you would just be better off getting a generator that can supply the desired 240V (as well as 120V) power.

Being the cheap SOB that I am (note I was shopping eBay) I wanted to know what what were "good" (UL, CSA, etc certified) transformers and how much they cost new (wide range of prices).

Thanks for the warnings. They will be headed.

 

[theoldwizard1]

the honda eu2000i generators are very fuel efficient, quiet, output extremely clean power that isnt going to cause havoc with appliances and electronics, and with this type of setup you have two very portable generators for other uses.

No question that the Honda inverter generators are the Cadillac of the small generator world. I think the EU2000i would be too light for this job.

Possibly a EU3000iS or a Yamaha EF2800i.

The interesting thing is, you can pickup a good, used, non-inverter, "gently used" 4kW-5kW generator, capable of supplying the required voltage and current, for not very much money on CL. They are loud and use a lot more fuel, but used, are less than 1/3 the cost of a new Yamaha EF2800i or 1/4 of the cost of a Honda EU3000iS !

More research (and eBay shopping) required on the transformer option.

 

[theoldwizard1]

I know a little bit about transformers (not a EE, but had some classes), but I have seen some terms used in this thread that I don't understand.

• Auto-transformer
• Buck-boost transformer
• Isolation transformer

I thought ALL transformers provided "isolation" between the primary and secondary ?

 

[oleguy]

buck boost=say your supply voltage is 208 and you need 240.a 15% boost xformer.
isolation is usually a 1 to 1 match,line to line.
auto trans is a one winding xfmr with a tap to get desired voltage.

 

[mrb]

you can get two eu2000i for the cost of one eu3000i.

 

[theoldwizard1]

buck boost=say your supply voltage is 208 and you need 240.a 15% boost xformer.
isolation is usually a 1 to 1 match,line to line.

So a buck boost NOT provide "true" isolation like a step up/down transformer.

auto trans is a one winding xfmr with a tap to get desired voltage.

Looks strange to me !! Certainly not for this type of application

 

[theoldwizard1]

you can get two eu2000i for the cost of one eu3000i.

I did not know this !

I know they can be "slaved" together to provide more current.

Too bad no one has figured out how to slave them for 240V split phase (without a big external transformer).

 

[A_Pmech]

One thing to consider is when the generator excites it may trip out on the inrush current of the transformer which will be somewhere in the neighborhood of 25 times the rated current.

I have that problem here with my phase converter. Many of my machines have 1kw control transformers. I have to sequence the start of the converter and closing of the main contactor to prevent the single phase supply breaker from tripping out.

 

[ddawg16]

Pmech.....there should be very little inrush on a transformer at startup....as long as the load it not on.......if the load is also on....yea....big inrush.

BTW....A stepdown transformer will also work as a step up transformer....instead of the load being on the 120/121Vac side, it's on the 241.5Vac side.