Rolling Out A Slick Rotary Phase Converter

Home machinists can often find great deals on used industrial equipment, and many a South Bend lathe or Bridgeport milling machine has followed someone home. Then comes the moment to plug it in, and the new owner discovers that the three-phase plug needed to power the new beast is nowhere to be found in the shop. Thus commences the weeping and the gnashing of teeth.

Luckily, [Handmade Extreme] is ahead of the curve in terms of shop power, and built a rotary phase converter to power his machines. Industry generally runs on three-phase AC systems, mainly because three-phase electric motors are so much more efficient and compact than the equivalent single-phase motor. But residential electrical service is either split-phase or, in the UK where [Handmade Extreme] is based, single phase. A rotary phase converter is an electromechanical device that can generate the missing phases – in essence a three-phase motor that can run on one winding and generate the missing phases across the other windings. It needs some supporting control circuitry to do so, such as timers and contactors to switch the winding connections once the motor starts, plus capacitors for motor starting and for balancing the voltage across the phases. The control gear is DIN-rail mounted and neatly wired to a smart-looking control panel. Everything is housed in a sturdy enclosure that’s big enough to serve as a mobile tool cart. It’s a really nice job – watch the whole build in the video below.

If you’re interested in power distribution, we’ve got a primer that covers the basics. And if you’re in the market for machine tools, [Quinn]’s machine tool buyer’s guide will let you decide if a three-phase machine is worth the extra effort.

46 thoughts on “Rolling Out A Slick Rotary Phase Converter

    1. In the video comments he says he needs to use it for multiple machines and this way was cheaper. A VFD is tuned for the specific machine it’s connected to, which makes swapping it around tricky.

      1. “A VFD is tuned for the specific machine it’s connected to”… but NO. Most VFD’s come as a generic 3-phase PWM generator, not worrying wether you connect it to inductors, motors or even resistors to its output. Comes to the user to adjust the VFD to better serve them, but if you don’t care about adjusting, the VFD will work as well.

        1. Indeed, you may want to add a big low pass filter if you will distribute the output throughout your shop as it is usually very rich in harmonics that will cause a lot of EMI if not carried over a shielded motor cable.

      2. A VFD can be tricked to produce steady 50/60 Hz output by giving it a suitable feedback, or no feedback at all and running it in open loop mode. Depends on the unit in question and how it’s designed.

    2. Also sometimes you have machines that have more than one three phase motor in it, like coolant pumps, dust collectors, or table feed motors and it is pretty inconvenient to run them off a single VFD. I have a tool grinder that has three 200v three phase motors in it, I ended up replacing all the internal electronics with a VFD for each motor because I needed to be able to reverse two of the motors. I had run it off a RPC at first but ditched that when I redid the drive in my lathe.

        1. In the case of his lathe it looks like the fwd/rev lever is connected to switches which control the reversing contactors. VFDs wont tolerate this kind of situation, they need to be connected directly to the motor. They will fault out at best, fry at worst. He could go in and remove all the contactors and install a VFD in its place. This would also give you braking and much finer speed control, which is what I would do. The issue here though is that if he has a 3 phase coolant pump he will either need another VFD for it or a small static phase converter to run it off single phase because although you can have multiple motors on a VFD you dont want the coolant pump reversing or turning on/off with the spindle.

          This applies to mills that have three phase motors driving the power feeds, you dont want the feed motor tied to the spindle.

          If you dont mind spending the money for a VFD for each motor, VFDs are totally the way to go. But there are people that have a bunch of stuff and dont have the skills to rewire machines to install a VFD. Thats where most people just buy a ready made RPC.

          1. If you have dummy load for the VFD, like a ventilator fan to work as a “minimum load”, then you can have loads parallel to the motor and the drive unit doesn’t see a motor fault condition with disconnected phases.

            But you’re right that VFDs aren’t desinged for asymmetric loads, and they’re too smart to work as simple three phase inverters although that’s what they are.

          2. If you select the correct VFD, you can put the VFD on the power source and motors on reversing contactors and it’s not an issue. I say this having, just this year, put my hands on a system where two irrigation center pivot systems were running on a single VFD acting as a rotophase because the site only had single phase power. Each center pivot had five 1HP motors which somewhat regular switched on and off (on about 1 minute intervals) and reversing contactors on both systems. The VFD hummed along just fine, even when no motors were on or I reversed the machine and the reversing contactor threw for all motors at once. The only thing the VFD really hated (and was the reason I was sent to work on it) was the capacitive load that was on the front end of line filters installed between the VFD and the motors to reduce the high frequency noise that was interfering with the power-line communication system that machines had for their control devices.

    3. A rotary phase converter can be a lot healthier for old and/or cheap motors.
      A VFD can shorten the life of a motor if it is not VFD rated. VFDs rectify the incoming main voltage to DC which makes a high rail of 1.7 times the rated voltage. (480VAC becomes 830VDC) Then the VFD’s IGBTs chop it into something that resembles 3 phase sine waves of the correct magnitude, but you still can hit very high voltages on the edges of the individual square waves from the IGBT switch on/off cycles.
      Motor windings have insulation classes rated for temperature and for voltage. If you exceed the voltage rating of the insulation with your peaks from the VFD, you can eventually generate pinholes in that insulation. Then you need a new motor.
      Yes, I’ve oversimplified a bit, but you can google for all the gory details.

    4. Because “man hooks up VFD according to instructions” is not really a hack. “Man builds rotary phase converter” is. Budding EE enthusiast builds a clock/power supply/nixie/arduino thing is fun and interesting. Person bought already made gizmo and plugged it in isn’t. It doesn’t always have to be the better use of time or money, it could be about the journey and the learning that goes along with it.

    5. VFDs aren’t ideal for many 3-phase applications. I have a handful of VFDs literally collecting dust as I’m trying to figure out how to build a RPC for my shop. I have multiple machines that require 3-phase power. On top of that, some of the machines have multiple components/motors. RPCs are superior when powering multiple machines/components/motors. VFDs are superior when powering a single motor or device – especially one that you might want to vary the power to. Now as to WHY a RPC is superior to a VFD that you set to the same output as the RPC and locked it in – I really don’t know. I was told the RPC would be better for my devices than the VFD. I didn’t have the time to clarify and verify that point to the degree that I would have liked. I need 3-phase power for multiple machines (with multiple components) ASAP. So I started building my RPC – and when I was about done, I got new machines that substantially grew my 3-phase power needs (I need roughly 5x the 3-phase power now than I did when I began building my rotary phase converter), luckily I also acquired a 5x larger 3-phase motor to make it from.

      I’m no expert, just a guy that needs 3-phase power in his home shop for some tools/machines that require it, so I’ve been trying to learn all I can on the subject. So take my input with a grain of salt. I’m only telling you what conclusion I came to after asking the same question online as well as to highly regarded electrical engineers that I know

  1. I’m lucky enough to have 3phase (415v) to my house but that video was all made extra worth while at 9:45 where he makes his hinge pins different lengths. I pine for that kind of attention to detail every time I take a door of an equipment rack. Something so simple I often wonder why rack manufacturers don’t do it as a matter of course.

      1. Honest question: only one fuse fitted or really only two wires reaching the fuse-box from the curb? Usually the electric stoves use three phases and thus I thought that maybe those houses where gas is used for cooking might spare the extra fuses, but I expected the wires to the fuse box being there.
        Even our old house (built 1927) had all three phases connected. And that was a set-up with only three fuses for the stove and another three fuses for everything else. If it were a car, it yould have been a Ford T-model.
        Why handle one house different from the other when it comes to burying copper?

        1. In my block (in poland) there is 3 phase going to main fuse box, then those phases are split into apartments as one phase for each apartment. So typically each building has 3 phases and it’s internally distributed as needed. If I wanted to have 3 phases in my apartment, I would need to redo the installation and install new cables and new meter.

    1. The house I live in has three phases in. Unfortunately, in an attempt to save a little money, only two phases are run up into the apartments… (Well on a four conductor cable, L1, L2, N and PNE are run up… The five conductor cable would have run them out of money I suppose)
      There is a regular three phase plug under the sink so I bought a used coffee grinder (Mazzer Major) with a three phase motor when it came up cheap and measured after acquisition where I found that one phase was missing…
      I ended going with a VFD for my coffee grinder.

      1. If you have two of the phases, you implicitly have the third. I used to work in a shop that had two-wire, two transformer three phase in ope delta (really only two wires… the earth was used for the neutral/return. A month without rain made voltage stability quite poor). A building an old friend lived in in New York supplied 2 of 3 phases from a wye source. Same deal. He ran his shop fin with that, using a small transformer set to get the third phase.

        For the coffee grinder, a VFD is probably the cheapest/most compact, these days.

      2. It is either L1,L2,PE,N or L1,L2,L3,PEN

        You can not have PEN *and* N together. Once PEN is split to PE,N it can not join afterward (ever).

        So if the cable’s cross section allows, you can have three phases in 4 wires *if* the cabinet where the power is coming from has PEN as input (and not PE,N).
        PEN wire has a minimum cross section requirements depending on the material (aluminium vs. copper) .
        So eg. if your wires have 4mm2 cross section you can not have PEN only PE,N (two separate wires).

        I hope it clears up.

    2. Definitely not all houses in Europe… I’ve yet to live in a house with 3-phase power to it. Electricity board deliver a single phase to the consumer unit here in the UK (which is still – just about – part of Europe)

  2. In Uk, the most common is single phase power to residential houses. The grid is 3-phase of course and you can get three-phase to your house or business also if you pay for it. In most other European countries, like the Nordic, Germany, etc, three-phase is more common, albeit small apartments where power demand is not high can still be single phase.

  3. My own house has three phase (three 25A main fuses). This is the most common and can be considered standard in my country (Finland). I have never even heard of anyone considering creating additional phases. For a small cost the utility company will attach the remaining two if you happen to have an old house with single phase (as was actually the case when I bought the house twenty years ago).

  4. It’s interesting to hear that 3phase seems so common in much of the world in Australia it’s not that common. It’s easy to see when there is a black out its often only one phase that’s dropped and looking down the street every 3rd house will be out.

    I’m guessing as people shift from gas to electric appliances the take up of 3 phase will no doubt increase but it is rather expensive to get the extra wires run from the street to the premises.

      1. And hopefully all of the three phase motors shut down. Aside from the noise due to unbalanced drive, the noise of the fire signal because they overheat is really disturbing.

    1. Inside my house the kitchen stove, sauna stove and geothermal heat pump (house heating) use three phases. Outside there is a three-phase outlet if I need to run any heavy equipment. Otherwise the three phases are evenly distributed inside the house, so if one phase is dropped (which hasn’t happened in 20 years, except once when I blew one of the main fuses by accident), only about 1/3 of the house is black. Kitchen stove still works on lower power, but the heat pump is screaming about inverter errors (it needs all phases).

    2. 3 phase supply is common in Australia. Almost every house has access to it if needed, at the owners expense for installation or upgrade.
      Generally you need a reason to have it installed (As loose a reason as ‘I plan to install a 3 phase air conditioner’ or similar is enough).

      I used to install and upgrade 3 phase service all the time. Not every house has 3 phase installed, but most practically every house on the suburban/city grid has access to it if needed.

      1. In Hungary, Europe each citizen is granted 32A of power free of charge.
        Up to the power meter it has 3 phases (new installation in the past 20 year or so).

        If the user wants a single phase then he/she gets a single phase meter with 25 or 32A circuit breaker. (up to the user).
        If the user wants 3 phases he/she can get with 3 circuit breakers as 10+10+16A. And he needs to pay the additional 4 ampers.
        Above 32A each A is about 25EUR/A. So the above example (10+10+16 is 100EUR extra).

        I got 16+16+16 and payed the 16*25EUR fee at installation.

      2. In Finland standard (and minimum for new houses & apartments) is 3x25A. In some old block house apartments and summer cabins we can still have 1x25A. Quite often if we have big house we have 3x32A or 3x63A. I have 3x160A but that is not so common at normal house. I just like to have power at my lab ;) In Finland basically all devices over 3600W are 3phase devices. Except some of stoves are 2 phase but most probably because another countries. Usual fuses to devices goes so that it is 1x10A or 1x16A. And if device need more that 3,6kw it is 3x10A or 3x16A. If it needs more than 10kw then it is 3x20A or what is best suitable. Some times I have seen 1x25A heat pumps but just because they are imported from Italy and they have some developing country issues in their things…

  5. In the US industrial areas have 3 phase, homes have split phase.

    If you just have one device with one 3 phase motor, I would consider replacing the motor. If you can sell the 3 phase motor that is even more of a win.

    1. Yes but it is well know information that US is developing country what coming to infrastructure like electricity, internet and mobile networks. But I tought that UK is some kind modern zivilization. But maybe after they are not part of EU they can make union with US and Namibia.

  6. Nice Write-Up. Great Project!

    If you tricked a VFD to produce steady 50/60 Hz output and accommodate asymmetric loads of multiple machines it still isn’t as economically practical from a retail/secondary market cost analysis compared to a home brew RPC. A surplus/used/second-hand AC Drive VFD rated @ 25hp/18.7kW is more than $1000 USD. You’re more likely to find surplus 3PH 10hp & 15hp or one 25hp or 30hp motors FOB in your area relatively cheap by comparison.

    I know all over the internet there are people who have/would and will negate my argument of “economic practicality” that a RPC > VFD by asserting their ability to source the required hardware for the latter at a pittance. With this in mind if you’re employed in the industrial sector and leadership is apathetic to asset exploitation, atypical of detached and incompetent PP&E management, to boast it’s a perk of the job is blue-collar ego. In the end you’ll both have egg on your face.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.