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single to 3 phase power converter

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  • single to 3 phase power converter

    I've done a ton or reading on running a 3 phase machine off of my single phase power and am still a little confused. ( ok.. a lot confused, but I'll limit my questions to a couple)

    Please tell me if any of this is incorrect:

    -- There are 3 types of converters, a Rotary Phase converter, Static phase converter, and a VFD

    -- Rotary phase coverter the most expensive option, but it will allow you to run your equipment at their full horsepower rating.

    -- The Static phase converter is the cheapest solution, but runs the equipment at 66% - 75% of it's rated HP

    -- VFD pricing seems to be in between the Static and Rotary phase converter solutions. A VFD will allow you to control the RPM of your motor from the VFD itself.


    Does a VFD allow you to use a motor at it's full horsepower rating ?

    I just received my latest Enco catalog, and they have a Static and Rotary phase converter listed, but no VFD. In looking through various catalogs and websites that deal in machinery and accessories, few list VFD's, is there a reason ?

    When buying any of these types of converters, does it help at all to run a larger unit then necessary ? ( i.e. Enco lists a 3/4HP to 1.5 HP model and a 1-3 HP model ( both statics ), I've got a 1HP Bridgeport I want to power)



  • #2
    The static converters are just a couple of capacitors and grossly overpriced for what they are. Google on the term for some articles that explain what they do. Statics have to be matched fairly closely to the motor- within +/-
    50% of the motor rating as the capacitor sizing relates to the HP rating of the motor. Rotary sizing: size of the rotary part of the converter should be at least 2x size of the motor being driven by the converter, other than that larger is not a problem, except for forklift sizing to move the monster and for sizes above 7-10HP,amp draw can be pretty impressive on 220v circuits.
    Rotaries are easy to make, a google on the term will give several articles on the basics and others on 'automatic' rotaries. For the HSM with several machines, converting a 3-7.5hp used 3ph motor into a rotary converter is the cheapest route to 3ph and can be done for far less (10-30%) the cost of buying one. VFD are a bit more in the real world than DIY statics and rotaries, but 1-2hp VFD can be had for $50-100 on ebay, or $120-280 new depending on the design and are the preferable route for single machines where variable speed is desirable and the original machine doesn't have 300# of '30s era wiring and ferrous/copper mixes to provide variable speed (eg Monarch lathes) AND the 3ph motor is not 440-460v only. Incidently, for simple rotaries and statics, the only difference is the addition of a 3ph motor to the static convertor to generate the 3ph output. Statics reduce 3ph motor HP by 30-40%, rotaries do not derate the motor. Better rotaries have additional circuitry that doesn't add much complexity but makes the device safer and easier to use. See Matt Isserstedts' article when you google on rotary 3ph converters.

    VFD must be rated at or above the motor it is driving amperage draw. VFD are rated at current supplied as well as HP. For most of these setups, motor HP delivered to the shaft is a function of motor speed, and the slower the motor goes, the lower the HP delivered. My 1.5hp Van Norman can be stalled at 5hz into the motor by grabbing the spindle by hand and squeezing but not at 10hz. VFD are made that can compensate for this and increase drive to the motor at low speeds, they cost 25-50% more, several different designs exist. is one source for VFD. Drill down til you get to the TECO FM50s for the least expensive good VFD (they have some rated at 1hp for $152, just checked). The FM100 is a more elaborate design, not really any better for your use. Both are 'older' designs soon to be replaced, but more than adequate for your use.
    Last edited by sch; 09-27-2006, 10:54 AM.


    • #3
      The static converter can be a simple as a on/off switch, a start cap, a run cap and a push button.

      To make it a rotary converter you just add a three phase motor called an idler.

      How much it will cost will depend on how much you can scrounge vs buy. I built my rotary converter for what would have been zero dollars if I hadn't blown one of the caps through a stupid wiring mistake. I do have the good luck to have a friend who is an electrician who gave me a 3hp 3ph motor to use as an idler.
      Free software for calculating bolt circles and similar: Click Here


      • #4
        Originally posted by 67chevelle


        In looking through various catalogs and websites that deal in machinery and accessories, few list VFD's, is there a reason ?


        Generally, VFD's have not reached the "consumer" market, although they're beginning to make inroads, mostly through internet sales.

        Could be a few reasons. They require a bit of knowledge to setup (getting better though). They're "fragile", hook them up wrong and you could fry the unit. Potentially dangerous, most have ability to overspeed the motors which could cause a dangerous situation with the driven machinery. They require dismantling/bypassing the controls on machine tools, that's beyond the electrical knowledge of the masses. They only can safely run one motor at a time.


        • #5
          Like Evan, I wouldn't recommend the static converter solution. About all that can be said for it is that the motor runs. It doesn't run particularly well. I used one for quite a while on my milling machine, and it did work, but it was pretty easy to stall the motor when drillling big holes.

          A few years ago I switched over to a VFD, and it's great. The motor runs much better, and it also gave the mill the low speed capability it was lacking.

          If you're looking for a cheap/good solution, a homemade rotary converter is a good way to go, but of course you don't get the variable speed capability the VFD gives you.

          Search the archives for VFD for a ton of information on them and other places to buy them.
          Try to make a living, not a killing. -- Utah Phillips
          Don't believe everything you know. -- Bumper sticker
          Everybody is ignorant, only on different subjects. -- Will Rogers
          There are lots of people who mistake their imagination for their memory. - Josh Billings
          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
          Don't own anything you have to feed or paint. - Hood River Blackie


          • #6
            There is one option that wasn't listed, digital phase converters. A digital phase converter can run many machines from one unit, provides very clean 3 phase power, dosen't take up much room and make very little noise when in use (just a cooling fan that cycles on when the unit is warm).

            The downside is digital phase converters are expensive, but if you have the money its a good option.

            Mark Hockett


            • #7
              I have been running my Bridgeport on a rotary that I put together. A year ago I was in the same boat as you.
              I had a friend give me a 3hp 3 phase fan cooled motor. I mounted it on a board
              along with a 1/4hp single phase that I use via belt to get the 3 phase turning.
              A 3 phase motor will run on single phase but it won't start on it. Once running on single phase the 3rd leg of the phase will be generated by the motor.
              The only thing I had to buy was a 20amp disconect that I turn the 3 phase on and off with.

              I took 220v and ran to my 3 phase. Of course the ground went to ground and the two hot wire went to L1 and L3. I then took a third wire, connected to L2 and ran the same 220 and the 3rd (L2) wire to my mill.

              Here is the starting process. Turn on the 1/4hp motor with belt running to 3 phase (I mounted this motor with hinges so the belt can be loosened or tightened by moveing the motor up and down). I then hit the disconect putting power to the 3 phase. I loosen the belt, knock it off and turn off the 1/4hp. With the 3 phase setting there running I now have 3 phase power to my mill. The mill will start and stop normally. I saw pictures somewhere of a rotary just like this but I can't recall where.
              Hope this wasn't too confusing.


              • #8
                As a real simple solution, I've seen 2 or 3 setups where a pull cord (like on the old lawn mowers) was used to get the 3ph motor spinning, while the power was simultaneously switched on to it. Requires close coordination of the cord yank and hitting the switch, but works nicely.
                Lynn (Huntsville, AL)


                • #9
                  I use a similar set up to what Bill in KY has. In my set-up the 3 phase and single phase motors are mounted in afixed location. Once I have the 3 phase motor running at speed I just shut off the "pony" motor and let it spin being driven by the 3 phase motor.

                  The old timer that I was refered to, at (business name here) explained how to figure out the proper motor specifics for the maximum efficiency. I was lost while it was explained to me. Figuerd out the sheave sizes for both motors to get the 3 phase motor up to the proper speed in his head. It was like second nature to him. He then drew out how to do the wiring to make it all work. I still have that drawing.

                  I bought a 3 HP three phase motor, a 1/4HP single phase motor, belt, and the necessary sheaves from his business. He wouldn't let me pay for his help and the advice. The only item he did not have on the shelf was a 30 amp disconnect, which was not a big deal, he called another elecrtrical supply house and asked them to hold it for me.

                  My (about 10years ago) of pocket was about $150.00 for everything I had to buy.

                  I am going to have to upgrade the RPC as I (hopefully) will have a larger motor to drive than the motor on the RPC.
                  Paying Attention Is Not That Expensive.


                  • #10
                    Originally posted by sch
                    Rotary sizing: size of the rotary part of the converter should be at least 2x size of the motor being driven by the converter,
                    Doesn't have to be anywhere near that big.


                    • #11
                      I keep seeing folks describing the "simple" solution as one that uses a pull rope or pony motor or something else to spin up the idler motor when building an RPC. The pony motor only makes sense when using a ridiculously oversized idler motor as they draw huge amounts of current when starting, making for a big bank of starting caps.

                      Otherwise, the "simpler" solution in my book is to spend just a small amount of additional time and effort and build it so that the push of *one* button starts the idler motor and the entire phase converter function running. That's *simple* in my book Remember, it is annoying enough to have to have something else running in order to run a piece of equipment. Making it so you can push a button and then go use your mill as normal is sure slick as snot.

                      Here's what mine looks like:

                      and here's the wiring internally....not too complex really:


                      I used a used magnetic motor starter, a potential relay to drop out the starting cap after the motor reaches speed and I have two buttons--one to start it and one to stop it. Now that is simple The parts are all surplus stuff and I think I have around $100 in it including the motor, panel, starter, etc.

                      Paul Carpenter
                      Mapleton, IL


                      • #12
                        well i just dont know I use static converters on my lathes and mills have been for years with no trouble . Have a rotary conveter too but took it out puts out to much heat and to noisy . I pefer the static type just turn switch on and off as needed no motors to start and have running when not needed. GO static .
                        Every Mans Work Is A Portrait of Him Self


                        • #13
                          Thank you all for your thoughts.

                          DR, Thanks for your specific comment on VFD advertising, this was helpful in giving me a piece of the puzzel I was missing.

                          I've got a 1 HP 1962 J head Bridgeport that I need 3 phase for. Given the comments and information I've gathered, I've decided on a VFD. I need it for only this machine.

                          Reccommendations? Here are two I've found on-line, the 1st for $125 and 2nd for $172. Anyone got a favorite that installed easily, worked well and was as easy as possible on the tool budget?




                          • #14
                            Well i just put in a 440 volt 10 HP Hitachi L100
                            and was impressed with it's adaptability and sensing features.
                            I also appreciate the extensive manuals they have online.
                            So they do support older models..
                            When using VFD's, the motors sing a little due to the frequency
                            they run at. The Hitachi is very quiet compared to my other VFD's.
                            Green Bay, WI


                            • #15
                              Parsing out the huge number of parameters on a VFD is a bit daunting, a hint of these is in the Hitachi list "table of contents" style starting about 6.0. From that point down every one of those parameters is an option you can program into the VFD. I used a TECO FM50 which has 40-50 "FNs" (functions?) and a friend bought an FM100 which has over 100 FN. I was pleased to find that limiting the upward excursion of frequency to 90hz did not compress the operating range of the freq selection potentiometer. The FM50 native range was upwards of 160hz. Most VFD go up to 400hz these days which makes very little sense in a machine tool. Running a 3600rpm motor above 120hz seems a recipe for disaster. Another parameter is accel/decel speeds, how fast the VFD ramps down the frequency on speed change or turnoff. On mine the Estop turns off the motor at the decel rate (0.5 sec or so). Since the VFD has to absorb the energy dumped back into the electronics by the motor you can't be too fast on the decel rate without extra braking resistors. One other thing, VFD have large capacitors inside and this means that complete disconnect of the input power to the device
                              is followed by about 30 sec delay before it actually powers off, of course if it was running a motor it would power off faster. This is a consideration for those that use one VFD to power several machines (serially).