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Two phase to three phase conversion in New Zealand


Brian from New Zealand here

I have read all the rotary phase converter stickied posts but am struggling to find a solution relevant to my situation.

First of I live in New Zealand which has the same power supply as the UK, Australia and most other former British colonies.

That being 230v phase to neutral and 400v between phases. All three phase motors in New Zealand expect to see 400v between phases. In New Zealand most domestic property's simply have one phase coming in and a neutral, as I live on a rural property I have two phases coming in.

I work at a large factory in New Zealand, our corporate bosses are looking to scrap the entire factory so I will be purchasing some of the machine shop and building a shed at home to work from.

I live on a rural property and we have two phase power and neutral coming in (I believe there has been confusion in the past as two phase means something different to you Americans. When we talk about two phase it is as below (2 legs of 3.

My two phase is from the same service provider,s 3phase line.

-----------------------phase-1-------------I DONT HAVE------

-----------------------phase-2-------------I GOT-------------

-----------------------phase-3-------------I GOT-------------

-----------------------Neutral--------------I GOT-------------

Even better just imagine I have a 3 phase line and one of them just vanished
Now how can I create that missing third phase?

Some people in the UK that only have one phase use an old choke welder as a transfomer to jump 230 volts up to 400v the use a RPC. I believe in my situation as I have two phases coming in I wont need to do this.

As I will be pulling an entire factory apart over the next few months I can basically grab any large three phase motor that I want for scrap value.

What would be the best motor in terms of poles, rpm, construction to grab? most motors in New Zealand over 5hp are wired in Delta.

The largest machine I need to power is a lathe with a 7.5hp motor, it will only be myself working in the shed so only need to run one motor at the time.

I have looked in to getting mains three phase in to our property but the price is over $20 000 nzd.

Cheers for reading


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You have single phase supply when speaking with us Yankees.

A RPC will work fine for you, As will any decent and correctly rated VFD. These to CONVERT single phase to Three phase

You don't have any issues not covered by matching VOLTAGE of supply and motor.

Lots of information on these forums. Do your homework, Just drop the term "two Phase" from your vocabulary ;-)


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Good point, I also have 2 lines in but they are on the same phase.

What you describe indicates a condition whereby when BOTH lines are connected TOGETHER, Nothing bad happens, You are just capable of carrying double the current at the same LINE voltage.


Think of it, "on the same phase". visualize an AC wave form. As the voltage rises on one line, it also rises on the other. There would be no potential difference, and no current would flow between the two lines.

In AMERICA, we would just use a heavier gauge wire. Although, I have seen doubled up wires to carry the amps. In fact, I've done it myself.

If that is your case, You still need a single phase to three phase converter. Line to Ground/ neutral is your only option.

Put a VOLT meter between the two energized lines in. Tell us what the meter reads.

Do the same Line to ground and line to neutral. MEASURE the voltage, NO ASSUMPTIONS!


If the OP has 2 lines which are 400V between them, he is all set up. A 400V motor can run on that single "phase", and generate the third wire and the two added phases that go with it.

It will look very much like a standard US RPC, except at a higher voltage, and requiring smaller, but higher voltage capacitors, if a self-starting RPC with balance capacitors is wanted.

If a "Jim Rozen style" converter is done, having a large idler with pony start and no "balance" capacitors, then that is even easier.
Hi Guys

Thanks for the replies, I just grabbed my multi meter and confirmed I have 230v between phase and neutral and 415v between the two phases, so lucky on that front.

The two phases I have will be 120 degrees apart. Are the domestic American phases also 120 apart? Im just trying to figure out which plans I can go off and which I should discard.

I have watched part of the UK series mentioned above, he seems to be turning 1 leg into 3 legs so was unsure how much of it I could copy given I have two legs to start with.

Is there any reason he has a sketchy contraption to disengage the starter motor? couldn't the starter motor simply be turned off and left to rotate with the larger 3 phase idler motor running?
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LexD That is similar to what we have in in the US, but with 120 between each line and neutral, and 240 between the two lines. There is 240 between the lines because they are 180 degrees out of phase. If both lines were in phase they would not have a voltage difference between them.


As you see in the answer above, our 240 volt comes from the two lines being 180 degrees out of phase. Of course the voltage itself is just a difference in potential. It does not "know" where it came from. In other words, you can "see" the phase being 120 degrees or 180 degrees different only when you look at the voltage between the lines and neutral over time.

As JST indicated, you can set up an RPC in the same way we do, you just need to use capacitors rated for the higher voltage if you are going to use capacitors. Capacitors are often used to start an RPC turning, though the HP gets above 10 HP the more advantages there are in a pony motor start. Capacitors can also be used to "balanced" the generated leg- but if the idler is large enough relative to the load, this does not have to be done.

RE the video I linked. He is using a wye connection so he can use his 230 volts single phase to generate 415 three phase. You have 415 volts, so can use delta to generate three phase. Any yes, the starter motor can just be turned off and left connected. There will be some friction from the belt and the bearings, but not that much with ball bearings.


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On thinking on it a bit more, if your phases are 120 Degrees apart, I believe you could hook up your lines to two of the 3 connections for a wye motor and have it start on the application of power ( no capacitors or pony motor needed), and then generate the third leg.

Its late here, I am tired and may be wrong about the above. I am sure someone will correct me if I am.
Hi Thiele

Thanks for your detailed response it is very helpful, as I will have access to a selection of large three phase motors and am more skilled in fabrication than electrical I will choose a large delta 3 phase motor (15hp plus)and then have a single phase pony motor to start it hopefully avoiding the need for capacitors (or minimizing the need for them)

If I machined a large flywheel for the idler motor (say 500mm in diameter) then simply spun it up by hand then turned on the power to the large three phase motor would this get it going? or does it actually need to be spun up to its running rpm to start? If I went the flywheel route I would have guards etc to make it as safe as possible.

Another question I have is about a floating neutral, in the UK video series mentioned above he talks about how other RPC are bad designs as there neutrals float and this causes a hazard as there is potential between the neutral and earth.

Would using a large delta motor as described above cause a floating neutral?


Active member
I have two phases coming in.

No. You do not. You have two CONDUCTORS (wires).

And that is not a "language barrier" issue. It is basic PHYSICS.

I have been having this "discussion" on and off for right about forty years with two of my old Cable & Wireless mates with that UK misconception.

Resolution is dead-easy, and for the simplest reason of all:

TWO PHASE power actually EXISTS.

And it requires FOUR wires, nothing LESS...

... whereas THREE-phase (Delta) needs but THREE.

Seems bass-ackards? Has to do with angles. Plenty of info online.

What you have is single-phase.

IF it is delivered off a centre-tapped transformer?
We call it SPLIT phase.

But even so, there IS NO "extra" phase derived from that arrangement. Just put it on a 'scope and see what transpires over TIME.

So where Cal said "American" as to terminology?
Actually it is the Laws of Physics.
Universally so.

Do your research on the basis of:

"I have XXX VAC SINGLE-PHASE" available.

OK. That's common enough.

You need a rotary phase converter AKA 3-Phase motor used as a "pilot" or "idler" ..IOW no load on its shaft... if even it HAS an external shaft. Some store-bought idlers don't have an exposed shaft.

The "idler" is actually functioning as a rotating field transformer - shifting energy from the one "directly powered" phase winding through its Iron and Copper (or Aluminium) wire structure via CEMF.. to the windings at the other two sides of the "triangle."

Annnd you will need any one of several well-known and well-proven means to START it.

TOO LARGE an idler for your service ampacity? You won't be ABLE to start and run it! Plan ahead. Do not ass-u-me that bigger is better.


OPTIONALLY run/balance capacitors to improve "generated leg" balance and stability.
But "optional" is all they are.



Single-phase step up or down, one set of windings, supply LINE side of the idler.


Three Phase step-up or down, three sets of windings, one per phase, LOAD side of the idler.

Idler has to be happy with "whichever" Voltage.

That's the basics.

Hz and Voltages may differ, globally.
The physics do not differ. At all. Anywhere.


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There are people who have motors they can spin with their foot or a rope, apply power and the motor will run whichever way they spun it, and generate 3 phase power for their machines. If somehow they spin it the other direction next time, their machines are running in reverse... When starting by such means, or with a capacitor start, there is a significant inrush current on staring as the motor accelerates. This will be some 5 or 6 times the motors FLA (full load amps).

A 15 HP motor at 415 volts has a FLA of about 19 amps. Six times that is 114 amps of 3 phase power. We are generating that from single phase, so the RPC will draw 114 x 1.732 = 197 amps for a short time. Some locations will have enough electrical "reserve" to allow this and not dim the lights- but some will not.

Spinning the idler up to speed before connecting the power to it greatly reduces the inrush current, which is why they are used more as the idler HP goes up. That and the cost of start capacitors and the contactors to disconnect them.

Flywheels will add to the mass that has to be accelerated, so will increase the inrush current unless you are pony motor starting the idler. From what I have read they will not help the RPC in starting a larger motor, though many assume they would.


For the OP, I have found that RPC’s work better at a higher RPM. For instance, if for your phase converter you have a choice between a 15hp 1725 rpm motor and a similar hp 3450 rpm motor, go with the higher rpm motor. All things being equal it will start a larger load.
Actual NZ person here. There are two options for what OP has. Both are moderately uncommon but there are still hundreds if not thousands of properties fed by them, and we're not a large country.

1) 230/460V split single phase. This is essentially what the US uses, but double the voltage. Common in places feed by SWER (Single-wire Earth Return) or very old rural construction. It is a single phase transformer, wired for centre-tap. Typically only used where they're expecting a lot of volt drop.

2) 230/400V with two-of-three phases. There's a three phase transformer, but only two phases are connected to your house. Perhaps the transformer feeds three properties, and each get two phases, with two properties connected to each phase. Similar to the two-hots 208V found in some larger apartment buildings in the US (NYC?). Again, mostly only used to reduce volt drop on farm supplies. Unlike the US, this will be a three phase transformer in a single tank, with three MV fuses and bushings.

Both will give two 'phases' with 230V nominal to neutral/ground. As that's all most appliances need, it "just works" regardless of which configuration is used.

If it's really expensive to get three phase, I would expect it to be option 1. But OP's measurement of "230v between phase and neutral and 415v between the two phases" implies option 2, although not quite exact.

Can we have a picture of the pole-top transformer, and exact voltage measurements (not rounded)?

PS: Here in NZ, the term 'hot' isn't really used except for live-line work. 'Phase' typically refers to 'phase conductor', the common name for what's also known as an 'active conductor', regardless of its actual phasing. 'Live conductor' refers to every conductor carrying circuit currents and that could be hazardous in the event of a single fault, including neutral conductors.
Thanks for the input guys, I will look into a way to get the motor up to full speed before turning on then to reduce current draw.

As for terminology I cannot comment on it as Im simply going on what the linesmen and electricity generators call it here in New Zealand. We have never had a two phase system in New Zealand requiring four wires so there is never any confusion locally.

To put it simply my neighbor has three red wires coming in all 120 degrees apart with 415v between them and a neutral, he can run any three phase motor available in NZ

I have two red wires coming in 120 degrees apart with 415v between them and a neutral and I cannot run three phase motors currently without putting it through a RPC.
Im confident I have the option 2 listed above as I have asked both the lines company and also a technician who recently installed a smart meter on our property. The voltage between phases was 415v give or take a volt

My real question is how I put this through a rotary phase converter and what motor would best be suited to this.


Active member
Another question I have is about a floating neutral, in the UK video series mentioned above he talks about how other RPC are bad designs as there neutrals float and this causes a hazard as there is potential between the neutral and earth.

Would using a large delta motor as described above cause a floating neutral?

You don't always NEED a "Neutral" off an RPC. You MAY "corner ground" the Delta so it is fixed, not floating, but ... that does not create a Neutral!

RPC are inherently displaced as to phase from their single-phase source.
So... the "output" side no longer has the same each-leg to Earth balance to the supply side Neutral.

If you NEED a true Neutral?

You "re-derive" it by connecting the Delta output of the RPC to the Delta windings of a Delta-Wye transformer.

The Wye side winding are in "float".. no reference to Earth. until you Earth that side's own Neutral tap. Floats no longer.

NOW you have "re-derived" an Earth-referenced"Neutral" annnnd all three legs are once again at the same potential above Earth.

Or near-as-dammit.

I use a 1:1 ratio 27 KVA EGS/Hevi_Duty "drive isolation" transformer for exactly that reason. I DETEST "corner-grounded" or "high-Leg" Delta.

If I am meant to be electrocuted? I insist on equality!
Any leg to ground the same potential!


But... having It adds the cost of the Delta-Wye transformer and frieght.

That ain't cheap. Cost with a really clean used-but-damned-good transformer and reasonable freight was nearly USD$ 1,000 for my OCD fetish. Kinky sex might have been cheaper? Or so the grownups say. Mind ...my Diesel is "Wye" so I wanted THAT compatibilty, too.

IF the plant being shuttered HAS a suitable Delta-Wye on-site? You could do much better?

But even then, "only maybe"

CAVEAT: More common Delta-Wye 2:1 ratio may have the "sides" bass-ackwards of what you need to do the step-up/down AND re-derive a Neutral in one go.

Most are built to DROP 4XX Delta to 2XX Wye.

US powerco Utilities do this a lot as Wye is considered to have lesser electrocution risk vs Earth. Or at least fewer "surprises" as to a high-leg.

Sounds a lot like option 2; two of three phase. Definitely possible and reasonably common. It's why old NZ ranges can be split into two loads, both running at 230V.

If you get a star-connected 400V motor, connect the two phases, and connect neutral to the star point, you should get the third phase back out, with the usual issues around voltage drop.

As only motors up to 4kW are typically set up for 400V star, you might consider using 2-4 of these in parallel. Advantage is no starting issues; you should be able to just dump it DOL.

Alternatively, I *think* you should be able to re-generate the third phase with a zig-zag transformer. But they're not really common in low voltages. You might be able to make one from 3x 230:2x115V transformers, or contact a transformer manufacturer and see if they stock or can build one.

Honestly, paying the $20k is probably your best option. See if you can negotiate them down and find out what work is involved.

Do you know how large your pole fuses are?
I have a 63a fuse on each of the phases coming in. One minute a RPC for this setup sounds like a walk in the park the next minute very complicated.

I could also just use one phase and copy the uk version on you tube but thought being 2/3 the way to three phase would make it easier. Scraping a whole factory I will be able to get almost all the parts for scrap value, the electrical workshop and spares are also being binned.