Off Grid 3 phase

A fellow I work with asked me to solve a problem for him.
Single to three phase to run a motor is not that big a problem actually but the need to be efficient about it is.

I have been trying to find a book called with a rotary converter that also uses a boost transformer for some additional shift and improved voltage regulation.
I can find it but I kind of know what I am looking for.

So what the hell is this?
In my minds eye I can see some possible uses for a pari of transformers in a Scott T config.
I have seen motors with an internal scott T for running a 2 phase machine on 3 phase but that connection internal to a rotory machine and its not single phase powered.

So what’s this?

I can;t say but its curious and it looks like it might be something related to an IEEE paper eluded to in this.

And the basic rotary converter I am familiar with.
Non of the videos on Youtube make this easy to see and understand but trust me this is not that complicated to build and wire.

The transformer thing…
I can’t answer to but there is something in the IEEE paper that might explain stuff and might be worth investigating for an off grid wood gas powered systems where there is not a lot of surge power available.

Your mileage may vary…


The video comes up as disabled by the owner.
The quick and dirty way to run a 3 phase motor when you have a single phase power source is to couple a single phase motor to a 3 phase motor turning the 3 phase motor into a generator. It is old school and reasonably efficient as electric motors have low losses. The more modern better solution is a Variable Frequency Drive which will also give you the ability to do electric speed control on the 3 phase motor. I don’t believe a VFD is cost prohibited they should be pretty common now.


Figuer #4 has a single phase to three with a DC bridge phase converter[( I am resistant to call that a VFD because I don’t know for most applications you want or need a variable frequency output and if these are able to do that… Unless you bought a true drive that can and it might be more expensive )

I wonder why the video does not work for you?
Hey does not work for me now, that’s odd.
If it says watch on youtube in a window that works

I can see what he has done.
He took an induction motor core and turned that in a toroidal transformer core.
He’s crudely wrapped some machine tool wire around.
It looks to have three distinct windings but nothing else is shown as far as connections or capacitors

Oh well mostly a thought exercise here but I thought it might be interesting to talk about it.


Well if you convert to DC you will have to use MOSFETS and a switching circiut to recreate the 3 AC waves. This could be done with either a 3 phase inverter or a VFD. The VFD has the advantage of being designed to handle the inrush current of the motor and will have some motor tuning functions to improve the smoothness of the motor to load performance. Yes it might not be needed for most applications but i suspect the cost benefits tradeoff comes out in favor of the VFD. In anycase i would not try to build my own phase converter the problem has been solved with the efficiency of scale that comes with mass production.
The motor to motor solution is the only cheap anolog solution i know of.


I have heard that you could use a VFD for single to three phase conversion, as you say. I would like to try that. I do know that we, at WPTA TV, are already on our second generation of 3 phase equipment with a VFD, So the surplus / used ones should be out there available. I saved a small one back somewhere, If I can find it!! :rofl: :cowboy_hat_face:
Here is that video creators’ YouTube channel, lots of good stuff!

Mike the first question is always how much 3 phase do you actually need. If you managed to get a single old industrial machine like a lathe and you want to run it on a single phase service then a VFD is the perfect solution. Simply wire it in as a load on the single phase panel and wire the motor to the VFD configure a few setting and you are off and running with the advantage of electrical speed control.
But if you have multiple motors or peices of individual equipment for example you bought out a small 3 phase machine shop then a generator of some sorts will make more since as the cost of multiple VFD will add up and you don’t really want to try and configure it to different loads each time you use something.
In that case my first thought at the moment is an inverter off a battery bank. But i think that way because to me solar plus storage is a wise move given the current energy costs around the world. I know our friends in Europe are suffer much worse but our rates here in NH are also going up making the alternatives look better cost wise.
But those are my quick thoughts. If i was going to plan for grid based single phase power to a small machine shop where one person runs many different 3 phase tools one at a time i would simply hook 2 motors together or find an old 3 phase generator head that i could couple to a single phase motor. If i was off grid i would simply buy a 3 phase inverter that could function on a shared battery bank with the single phase inverter. If i only had one lathe i wanted to power i would buy the VFD to match the motor. That is just my two cents like many things in life there are many solutions that will work which fits best depenens on the situation and what is available.


Circle back on this link for moment Mike.
He’s the guy that wound the toroidal transformer on an old AC motor cor from the video I posted ( that does not work unless you open in a new window ).
Made that odd static converter that I think is based on the ideas in the IEEE paper.

I’d like to dissect that thing and play with myself to see what he has there, but I’m not that interested in to buy his 29 dollar book on Amazon

I need to fix something up for some folks.
I’m leaning hard into a small frame 2 pole motor and rewinding it myself to at 10% more turns one phase and some balance caps ( also starting cap ).
But I have seen a Scott T connection in a winding before.
It would not be hard at all to put some offset into the winding in the form of some taps I could play with so I can tinker with the inductance as well as adding external caps…

This is a serious thing however.
The application ( sit down for this ) is a small artisan bakery

I looked at the prints for some of the equipment she acquired as the business has grown,
European 220 three phase stuff.
I rather not rewire this stuff to run single phase.
a large converter will draw a lot of power and I have restrictions in her place of business.

Done correctly and running not so much equipment at once will be the rule of the day until she moves ( if she moves ) to a larger place with proper 3 phase power.

These problems however are not unique to just her a lot of people find they have no cost effective means to run things like this.


I think what he is doing is lagging the circuit of one of the legs 30 degrees. Instead of 2 wires offset by 90, if you lag one, by 30, it is 120 degree offset which ends up to be a multiple of the 60 degree offset used for 3-phase.(instead of the first and second offset by 60, and the second and third offset by 60, it is leg 1 and 3 which are offset by 120.) Essentially he now has 2 legs of 3 phase which of course uses 2/3 the amps. :stuck_out_tongue:

Alternatively, a transformer, could be used you energize the secondary and get the 3 phase out the primary.

But for your application, I would look at a solid state. a 1hp 3-phase box is 100 bucks, and you might be able to find them for less. The 10Kw fronius solar inverter is $2500.

The price has dropped over the last several years. The last time I looked prices were all over the place in a large part due to legacy prices for components.

1 Like

The box has start and run capacitors he says.
So if one leg is lagging because of added inductance like you say, one leg is leading because of the the start and run caps it might make sense.

Thats a really crude transformer.
I wonder if its also limiting inrush current…
Sort of inductive reduced voltage starter?
I have never seen that done in the real world but its not unreasonable to think it would not work.

That “root of 3 thing” comes into play here with only two operational legs but instead of 2/3 like you said I think its more like .58.
Thats a big reduction in starting amps and torque.
Probably good for a high inertia load like a blower.

I’m pretty stubborn in not wanting to use electronics to solve this if I can, its pretty easy to get a 208 motor, and I can wind a whole lot nicer looking transformer and re stack something if needed or rewind a motor if needed.

Your thoughts are appreciated

That is more what I’m thinking.

You are probably best to get that 5ton elevator ac to DC thing that you said you always wanted. I am not sure running transformer backwards works with all the phases now that I think about it…

Which leaves a rotary phase converter

Unless you run a dc motor with a wheel that rotates at 60hz, have a wheel or lobe (like a camshaft) that flips a mercury switch. A series of 3 would give you three phases of pulsed dc which you can obviously run a squirrel cage with because that is literally a VFD.

That wasn’t me…
But now that you mention it, I think I want one of those too, and a wind mill, and an overshot water wheel, and a servant named Friday to cut palm trees and build stuff from it and cook my meal…

Maybe I just stick with an elevator or stare lift for now because my knees do make the steps kind of hard now lol

The stairwell can probably fit an overshot water wheel, and put a hot tub at the bottom of the stairs. :slight_smile:

You would have better luck building a robot named Friday.

1 Like

I built a rotary 3-phase converter for a large lathe. I used capacitors to adjust the phase angles and start capacitors to get it spinning. No details (It’s been a few years ago) but it worked a treat and was a very fun project.


I like the way you think.

That amplidyen and MG set were sent to the scrap yard sadly

Oh man! I thought I had the perfect excuse for you to acquire those. :frowning:

Anyway. here is some old skool tech since for your SHTF scenario.

The only OTHER diode I found was a wet cell type, but those don’t work as well and have issues. Althought I did stumble on to how to make a mosfet at home, but that was several steps and some serious chemicals.

It’s really shocking the speed of change

I still encounter selenium rectifiers and so many things that have technically gone obsolete

But they struggle on un noticed until a component failure reminds you they are there

Generations Of technologies can be compressed into an old plant

End of the shift I will try and get some pics of what I mean


I’m actually not surprised. “If it ain’t broke, don’t fix it”. that goes hand in hand with another old addage of ‘it is cheaper to waste energy, then to conserve it’. Some of the old technology actually works better in very specific use cases. I don’t think that is the case for rectifiers, but you have to account for the rest of the circuit as well, and in some cases if you change the design, it falls out of code so you have to fix a lot more then you intended to fix.

1 Like

Not so long ago I could disconnect and remove heating elements in a forced air air electric furnace
Now they don’t want you to do that, and if you do you need a field inspection

What counts as built or bought gets gray and the clouds get very dark if you modify things like you say.

I think it’s it’s over reach on the side of inspectors and csa
Technically I’m not sure how much is allowed
Is a rewind allowed if you add modern insulation and then take advantage of this by making a motor stronger?

I don’t know
Better to just keep my head down and not get caught saying I fixed that but altered it with a new part

I guess I picked up some attitude from my boss back in the day when I worked in the motor repair shop
I learned a hydro inspection doesn’t know much except the code and when you corner him with math and physics he gets very nervous if he thinks he’s going to lose face and seen as not know electricity as well as he knows codes

BaAck in the days before me when we switched from 25 to 60 cycle lots of washing machine motors had to be replaced.
But really they did not
You just had to split the winding and reconnect it it.
You turned a 1450 rpm 1/4 hp motor into a 3500 rpm motor that was now close to 1/2 hp
Do a pulley change and maybe he house wife was happy for less money than a new motor

I don’t t think you could do that with today without somebody telling you it’s a code violation or something

By 1990 I was rewinding the last of the industrial motors for 60 hz ( going add to this at a later date maybe 95 we stopped generating our own a few years after hydro stopped days are hard to remember )
I don’t know if they would let me just restrike a new data plate and send that stuff back out like the old days

And of an inspector called me out on it but just might want to tell him to get stuffed
I’m not sure how I feel about effectively seeing much higher levels of enforcement.

On the one hand it’s probably made electricity in Ontario safer
On the other it means I’m not special and I should follow them too


This is a common problem. People enforcing the codes aren’t experts. Neither are most people that make changes. You can convert the washer, and it might spin fine, but is it designed to handle the extra heat? and the rest of the components the extra torque? I am guessing in some cases yes, but I am also guessing in some cases no and the extra heat could cause a fire or stuff will break, and then people get mad and sue. In a number of cases rules are set, not because you can’t make it work, but so things aren’t crazy that you need an engineering degree figure out what was done when someone else comes back in 30 years.

1 Like

Here’s some stuff

80 years ago a guy would have stood here and pull three levers

Now it’s down to two joystick and it’s automated
But the clutch handle is still original

Modular starter?
Nope antique from the 40

U frame motors lol