240V from 120V generator

[Groover]

I would love to be able to use the 6KW quiet diesel in my motorhome as a backup generator at the house. One problem is that the house needs 240V while the generator makes 120V. I have not had good luck finding 120V to 240V transformers. I am not an electrician but I remember something about isolation transformers changing the phase of the current but I don't remember if it is 180 degrees which is needed to make to 240V or only 90 degrees. Anyway, is there anyone out there with experience on this that has a good solution? I would like to be able to supply 4kw at reverse phase in case the load is off balance in the wrong direction.

[mikestock]

You can run everything in your house that doesn't actually require 240 volts by joining the two home circuits in your jumper cable or by joining your circuits in the transfer switch. I use the second method and the only functions I live without are central air and clothes dryer. My range, water heater and central heat are all natural gas. I do have a supplemental, high efficiency, 10,000 BTU window AC that can cool a couple of rooms.

[NRALIFR]

“A totally separate rig only used during outages would require a lot of diligence to make sure that it runs when needed. ”

Probably true for a portable unit that uses liquid fuel, but I’ve been quite impressed with the reliability and minimal maintenance required on my home standby unit. I change the oil and filters once a year, and that’s it. The fuel (natural gas) is always fresh and available, and the fact that it’s totally automated is priceless. No extension cords, every critical circuit is powered by it, including the garage doors. We once came home from a long trip during a power outage and didn’t even realize the power was out until we got inside the house. :B

:):)

[Groover]

DrewE wrote:

allen8106 wrote:

Groover wrote:

AC, furnace and well pump. I don't have any built in fossil fuel heat. Giving out of water would be the most painful.

I live out in the county also and this is the very reason why my well pump runs on 120 volt. I doubt your furnace is 240 volt. Your AC certainly is but not the furnace.

It may be an electric furnace, in which case it would certainly be 240V...and quite a bit of power, as well; or a heat pump. The poster did mention no fossil fuel heat, so it's not a propane or natural gas or oil furnace, and that pretty much leaves only electric resistance heat, a heat pump, or I guess perhaps wood or wood pellets.

Yep, it is a heat pump with resistance backup. I would have to disable the resistance part when it is powered by a generator and I am thinking of putting in a mini-split that would be easier to start. Between putting the motorhome away with a full tank and tractor fuel kept on hand I should be able to come with 90 gallons of diesel fuel in a pinch. And I use the generator often enough that I expect to keep it running properly. A totally separate rig only used during outages would require a lot of diligence to make sure that it runs when needed.

[DrewE]

allen8106 wrote:

Groover wrote:

AC, furnace and well pump. I don't have any built in fossil fuel heat. Giving out of water would be the most painful.

I live out in the county also and this is the very reason why my well pump runs on 120 volt. I doubt your furnace is 240 volt. Your AC certainly is but not the furnace.

It may be an electric furnace, in which case it would certainly be 240V...and quite a bit of power, as well; or a heat pump. The poster did mention no fossil fuel heat, so it's not a propane or natural gas or oil furnace, and that pretty much leaves only electric resistance heat, a heat pump, or I guess perhaps wood or wood pellets.

[allen8106]

Groover wrote:

time2roll wrote:
What do you have that is 240 volts and critical? I would thing the fridge a few lights and a few outlets would be fine in an emergency. Furnace should run on 120 power. Retreat to the RV if you need air conditioning or to cook. +1 to get an electrician to look at it.

AC, furnace and well pump. I don't have any built in fossil fuel heat. Giving out of water would be the most painful.

I live out in the county also and this is the very reason why my well pump runs on 120 volt. I doubt your furnace is 240 volt. Your AC certainly is but not the furnace.

[Groover]

Dutch_12078 wrote:
Try this link for transformer phasing help:

Phasing - Chapter 9 - Transformers

Thanks! It looks like my plan should work if wired correctly.

[Dutch_12078]

Try this link for transformer phasing help:

Phasing - Chapter 9 - Transformers

[Groover]

There are some good options here. What I am leaning towards right now is this:
30A isolation transformer

Bearing in mind that in a home 240V is created by using two 120V legs that are 180 degrees out of phase all that really needs to happen is use the raw output of the generator for one leg and then change the phase of a portion of the output 180 degrees for the other leg. About 30 years ago I sat through a roughly 15 minute lecture on isolation transformers and as I recall they change the phase the required amount. I have been trying to confirm my memory and make sure that this transformer is suitable but since might be considered an "off label" application information is scarce. I like the idea that only half the power gets transformed so transformer losses are reduced.

Plus, the Quiet Diesel has two 35A outputs so I could use one output for each leg without messing with the setup of the generator. As far as I know these two outputs are in phase and come from the same source. They are simply there to power the two different circuits in the motorhome but I can't get Cummins to comment on this and I would rather not find out the hard way.

[KD4UPL]

Wow. Those transformers that time2roll and htss linked too sure are pricey.
The Outback PSX-240 is only about $600.
Theoldwizzard, thanks for mentioning the interlock, they are a great alternative to a transfer switch. They only cost about $70 and are available for just about any panel.

[pianotuna]

Since this is going to be for emergency use, much as I dislike generators, a cheap contractor unit may be the way to go.

[htss]

You need something like this:

https://www.digikey.com/product-detail/en/signal-transformer/DU-7.5/DU-7.5-ND/1984857

[theoldwizard1]

time2roll wrote:
6kw $1,300

Might have to be dedicated to just the pump.

THIS (step up transformer) IS THE CORRECT ANSWER, BUT THERE ARE SOME ISSUES !

Step one
Have a generator inlet installed on the house and have wired it to a generator interlock in the breaker panel. NOT a transfer switch. Less expensive and it allow you select at the time of the outage which circuits get power.

Step two

DownTheAvenue wrote:
You can feed both 120 volt legs of your house by jumping the one hot leg over to both legs off the house. You will not have 240 volts to power things like the stove, dryer, A/C, but both legs of the house power will be charged with 120 volts.

More specifically, the voltage between the two legs will be ZERO !

Step Three
Size the transformer referenced at the beginning for your well pump. My SWAG is 3000W


Step four
Have an electrician wire a 240V DPDT relay (the coil voltage must be 240V) so that when you DO have 240V the pump receives power . When you DO NOT have 240V (power outage) the 120V runs through the transformer to the well pump.

I would add an additional 240V relay "upstream" from the transformer, wired so that the transformer does NOT receive power when 240V is available.

[KD4UPL]

You can absolutely use your 120v generator and have 240v. What you are looking for is called an autotransformer, sometimes shortened to autoformer. One I would recommend is made by Outback Power, the PSX-240. I'm sure there are others.
To answer your question about phase angle, the 2 hot legs of a 240/120v split phase service are 180 degrees out of phase.
By using the PSX-240 and a PROPER TRANSFER SWITCH you can have 240v available in your panel form your 120v RV generator.
As anyone with a deep well pump knows, often have 240v to run the well pump is one of the most important loads a generator might be needed for.
The suggestion of feeding both legs of the panel the same phase is not a good one if your house has any multi-wire branch circuits. These can be identified by a cable containing a red wire in addition to the black, white, and bare ground. Supply both the black and the red with 120v from the same leg of a generator can cause the neutral wire to carry excessive current leading to overheating.

[NRALIFR]

Generac home standby generators in a sound deadening enclosure with ATS is available from most big home centers and are reasonably priced. They can run on either natural gas or propane by simply sliding a fuel selector. I would install one of those before I’d mess with the gen set in your RV.

I’ve had a 10K NG unit installed on my house for over 10 years, and it has been a godsend. I’m not powering any 240 volt circuits with mine, but could if needed. I have 16 critical 120 volt circuits powered by the ATS. It powers up the gen and switches automatically after a power outage, then switches back to utility power when it comes back. The gen does a test run once a week on its own. It’s run as long as 5 days straight after a storm knocked our power out.

Propane and NG never go stale like liquid fuels.

:):)