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Home built rotary phase convertor

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  • Home built rotary phase convertor

    I would like to build my own rotary phase convertor. I have a 5 hp 440 v three ph motor, and can probably find a pony motor. Problems is I have no idea how to connect the whole thing, and end up with three ph to my lathe. I guess I need making a rotary phase convertor for dummies. Anything out there? Stan

  • #2
    I based mine of the old Fitch Williams design.
    page down to FRW files.
    I use a start capacitor rather than pony motor.


    • #3
      At one time (quite a few years ago) Lindsay Books sold a pamphlet that explained how to build a rotary phase converter. I have no idea if they still offer it or not.

      Probably somebody on this forum can explain it as well. I'm afraid I can't, with any reliability of correctness.
      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


      • #4
        Booklets are cheap on ebay on how to do it.


        • #5
          Here's a article to start with ...


          • #6
            I did this years ago and it was quite simple. Once the three phase motor is up to speed, you put single phase input to one pair of the wires. Connect your three phase load to all three of the wires, and it will receive imperfect but passable three phase. It's really that simple.

            I used a pony motor and simply switched the pony off as I switched the converter on. My converter was pretty hefty (an ancient ~7 1/2 horse monster), so I geared the 1/2 horse pony motor down a little and it didn't come up to full speed, but the converter picked up the remaining speed all right. I used an old repulsion-induction motor for the pony, so it could take its time spinning up without burning out.

            Ideally you'd want a double throw switch with off in the center, so that you could switch over with one hand, but two switches work fine.


            • #7
              440 motor? is it dual voltage - 220/440?

              There's an entire forun on PM devoted to RPC/VFDs etc, and a sticky with designs. We'll worth reading.


              • #8
                It is more than likely it is a 220 440. If my memory serves me correctly 6-9 together 5-8 togehter 4-7 together 1-2-3 is the line, for 440v. Can't remember our anniversary date but that number stuck with me as well as my service number from the Navy for some reason. Thanks for the info I will check some of these sources. stan


                • #9
                  While you can sometimes get by with simply starting a 3Ph motor on single phase and using the raw output of the third winding, you'll have a unit that vibrates, is noisy and you'll also be getting a poor approximation of line voltage on that third wire. Putting a little time into balancing the output with capacitors will do a lot to reduce all of those problems.
                  Southwest Utah


                  • #10
                    5 hp RPC

                    I built a 5 hp self starting RPC about two years ago. I used the schematic below from a site called Electro-Tech. It works quite well. Line voltage (L1-L2) is of course 240 V and under load the other two legs (L1-L3 and L2-L3) are usually 242 V and 244 respectively.

                    Capacitors that I used for 5 hp were dual 250 mf in parallel for 500 mf and dual 50 mf run caps wired as shown in the schematic. The start caps make use of bleeder resistors of 15K ohm, 5 Watt to discharge them after startup.

                    Since you want to use pony start, which is a good alternative, you can leave out the Steveco potential relay. (In the schematic, this is normally closed and connects the start caps into the circuit. Once L3 is generated, it drops out and the RPC contuinues running)

                    These units are quite reliable. I recommend a 1,750 rpm motor since they are quieter. My first RPC was a Phoenix which had a 3,600 rpm motor and it was unpleasantly loud. I therefore sold it and built my own. It used the above schematic as well.

                    I also use a definite purpose contactor with a 120 V coil and fuses on each line. If you use 120 VAC for a contactor, you will need a neutral since you should't use one side of the hi voltage to ground for 120 V unless you have a neutral. ( I know you say you have 440 V) I added a control transformer to get 120 V with neutral so that I could have a red power light and 120 V with neutral.

                    Good luck on your build.

                    Last edited by gnm109; 10-22-2011, 10:08 PM.


                    • #11
                      I built one a year or two ago, it worked, I got the legs "balanced", but I was finally fed up with the seeming lack of power and bought a VFD, I will never go back.


                      • #12
                        Originally posted by Abner
                        I built one a year or two ago, it worked, I got the legs "balanced", but I was finally fed up with the seeming lack of power and bought a VFD, I will never go back.
                        VFD's are an alternate method. I used an RPC because I have a variable speed mill and I can fix any part of the RPC.

                        If you had a seeming lack of power, perhaps your idler motor was not sized appropriately. I use a 5 hp for a 3 hp induction mill motor. It has plenty of power.

                        I know that VFD's are popiular, though.


                        • #13
                          i built a 5hp to run a 3hp lathe and now run a 1 1/2 hp mill with the same RPC. with an RPC the converter motor needs to be about 1/3 larger than your largest machine motor.VFDs are popular but you need one for each machine. with a properly sized RPC you can run multiple machines at the same time if your able to move fast. i simply unplug one and plug in the next.
                          Last edited by wooleybooger; 10-22-2011, 11:09 PM.


                          • #14
                            Can someone explain the power up and power down sequence when using a machine that is plugged into a rpc? Another word what do you do when you want to use the machine?


                            • #15
                              From an engineering perspective, the "balancing" is overrated, and probably most folks who swear it is required have no idea WHY.

                              If your idler is considerably larger than the load motor, balancing is pretty optional.

                              The more "matched" they are, the more important the balance capacitor is.

                              Now..... obviously, two lines are unchanged input lines....... the "manufactured leg" is the troublemaker.

                              As a motor, a three phase motor generates a "back EMF", which is nearly, but not quite, equal to the input voltage from the line. The difference between them is what makes current flow in the motor.

                              When a motor is unloaded, it generates a high back EMF, and has low "slip'. Speed is nearly synchronous.

                              The more heavily loaded it is, the slower it goes, the higher the "slip", and the lower the back EMF, which allows higher current to flow through the motor impedance to provide the added power.

                              Having "back EMF" is the same thing as the motor acting as a generator. Motors do that. In an RPC, the "generator" action is what provides the third, or "manufactured" output.

                              from the above, you see that that output will be slightly less than the input voltage, and equal to the "back EMF". If any current flows, the output will be even lower due to the "impedance" of the motor being in series with the output.

                              But, with a larger idler, two things are in your favor.

                              First, the full load current of the smaller load motor is only a fraction of the idler's full load, so the idler has only a small voltage drop in it's impedance.

                              Second, with the idler loaded only lightly, it is running closer to synchronous, with high back EMF and thus high output voltage.

                              With those two things working in your favor, you may have no need to balance to get a perfectly good performance from your machine motor. Slight imbalances only reduce the max power the load motor should be allowed to produce, limiting max loading of the machine a bit. If you have a flat belt lathe, you probably can't get full power at many belt settings, and it won't matter much. Ditto for other situations.

                              But there is nothing wrong with balancing the output, if you do it right.

                              Now, balancing can ONLY be done perfectly for ONE load. All other loads are going to be in a state of imbalance. Therefore you should pick your oad for best balance.

                              The worst idea is to balance at no load..... that guarantees poor balance at heavier loads, where you need decent balance.

                              balancing at full load can lead to a "reverse imbalance" at lower loads, where the manufactured leg is HIGH. Not terrible, but possibly not the best if you don't load to max very often.

                              Probably the best idea is to balance for your most common heavy load. If you do not know that load, perhaps about 2/3 power is best..... good even if not perfect at high power, reasonable everywhere else.

                              So, what is "balance" and how do you know it?

                              Balance just means the voltage between any pair of wires is equal, in whatever loading condition you have when measured.

                              Why does a capacitor "balance" the RPC?

                              The idler impedance is resistive and inductive. Adding a capacitor from the manufactured leg to neutral would cancel some inductive reactance, with the effect of raising the voltage. The amount of raise depends on how much capacitance is used.

                              You said "to neutral", but the rpc has no neutral, what's up?

                              that is true, but a capacitor to one of the other legs can act similarly, except that the value can be smaller by a factor of just under 2:1 (1.73 : 1), and that is what is often done.
                              Sometimes two smaller capacitors are used, one to each of the other wires, which does the same thing in a slightly more "perfect" way. One can show that the line-to-line capacitors are equivalent to capacitors to neutral.

                              If a smaller cap is good, how about a bigger capacitor?

                              You can end up raising the voltage too much if you add too large a capacitance, so you should use as much as you need, but no more. The voltage will usually be the highest with no load, or the lightest load.
                              Last edited by J Tiers; 10-22-2011, 11:26 PM.

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