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slowing down single phase motor

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  • slowing down single phase motor

    in the book mentioned in the other thread:

    it sais on page 23:

    "one running winding, one starting winding- consequent-pole connections

    as explained above, when poles are connested so that adjacent ones have the same polarity, the magnetic effect is to produce twice as many magnetic poles as there are wound poles. ..."

    (unfortunately i cant find a way to copy that and the pictures are missing.)

    without really understanding what he is saying, does that mean there is a way to slow down my drill press or would the motor have to be wound in a special way in the first place? would that method double the torque?

  • #2
    If the winding is done to do this, yes, you can double the pole count by phasing all wound poles the same. Then the opposite poles appear in between, and the pole count effectively doubles.

    Not all motors will do this well.
    CNC machines only go through the motions


    • #3
      Bite the bullet. There is no simple effective way to alter the RPM of a single phase induction motor that results in good RPM regulation over changing load. You can slow it down by reducing the voltage by various means but this is wasteful of energy, tends to heat the motor, and is only effective on fixed loads like fans and centrifugal pumps.

      If the single phase motor is built into the equipment as a pemanent part you are stuck with it.

      If the motor has a standard NEMA or IEC C face or base mount frame replace it with a three phase induction motor with a VFD control and be done with it. This is one case where throwing money at a problem is a good thing. If you are a good scrounger it doesn't have to be that much money.

      What size motor? What application? What load will it drive?


      • #4
        That might be an answer at cross-purposes....

        Pole changing DOES give perfectly fine speed regulation, but both speeds typically are not at the same power. Any common two speed motor does pole changing just as the OP describes, but they need to be designed for it.

        OTHER means of regulating a motor which is designed to run at a fairly closely set speed related to the AC input frequency are indeed doomed to poor performance. That is, if they do not involve frequency changing of the incoming AC i.e. a VFD.
        CNC machines only go through the motions


        • #5
          The simplest way to lower speeds on a small drill press is to add a jackshaft and third pulley into the drivechain on top of the DP head. If it already has one then there isn't any practical way to slow down a single phase motor for this application.



          • #6
            There are VFDs with single phase output, but are about $300-$500:

            Probably much cheaper and better to replace the single phase motor with a three phase equivalent (about $100 for 1-2 HP), and a VFD which will be under $200 and possibly under $100 for 1 HP or less.
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030


            • #7
              Originally posted by PStechPaul View Post
              There are VFDs with single phase output, but are about $300-$500:
              Those VFDs will work only with certain types of single phase motor, not with a standard split phase or capacitor start type. I am very familiar with those drives, which are an OEM "badged" version of a european based manufacturer.
              CNC machines only go through the motions


              • #8
                so does it increase the torque or not? is there any sence in taking apart a double cap motor and try to figure out if and how this can be done?

                (as i posted before, i have a vfd that will run a single phase motor with no problems. however you start loosing torque very soon, if you slow the motor down.)


                • #9
                  Quit fooling aaround with exotic posibilities. Yes you can tinker with a single phase motor rewiring the coil sets but what use are you making of productive time? I stopped at a local HVAC shop and prowled their scrap pile. In a half hour I found a junked commercial air handler with a 2 HP three phase motor and a VFD with a 5 - 20 ma card. It was a Hitachi I think but it had a thick layer of dirt obscuring the nameplates. Looked like a recent model though. The motor was pristine except for a little dust.

                  The man said he wanted $50 for the motor and drive and his apprentice would take it off for me. Cheap. I passed but said a buddy would contact him.

                  Quit pecking on keyboards and head out scrounging. They make little air handlers too - suitable for a small drill press and a hell of a lot easier to swap out.

                  Unless you're reconnecting an existing motor for two speed consequent pole operation just because. Hell, I can understand a challenge.


                  • #10
                    well as i have two identical motors (4 pole) i dont need, i thought i would sacrifice one of them for my education.

                    unfortunately there is nowhere to go "scrounging" in this part of the world, no surplus stores or techticalyy oriented fleemarkets. the local ebay is full of crap at prices higher than if you go and buy it new.

                    of course if its a futile attempt, i would like to be told. as well about the torque issue. if its only going to slow down the motor with constant torque, its useless. also the drill press is in a location, where i cant easyly connect it to 3 phase power.


                    • #11
                      You will likely run into problems with the starting circuit. I think most single phase motors rely on centrifugal force to disengage the starting circuit at around 75% of full rpm. If you reduce the motor speed by 50% the starting circuit will never disengage. So you could reduce the strength of the springs by 50%, I suppose....

                      If you want to experiment, have at it. Tinkering is a most worthy way to occupy one's time. If you want to solve your drill press speed problem though, the two ideas suggested -- a center jackshaft with reduction pulleys, or a 3-phase motor with a VFD to convert single-phase to 3-phase -- are eminently more practical.
                      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


                      • #12
                        Originally posted by dian View Post
                        the drill press is in a location, where i cant easyly connect it to 3 phase power.
                        "most" little VFDs are normal 1 phase in, 3 phase out for just the kind of thing you're trying to do. Be careful as there are some 3ph in / 3 ph out models out there, sometimes the different is only 1 digit in the model number.

                        Totally respect the educational purpose, but go into it with the idea that you'll probably be accepting scrap price for the old motors and doing the VFD thing in the end.

                        I was recently surprised to discover Hitachi is now selling a 1/2 HP 220 1ph in, 220 3ph out vfd for a bit over a hundred bucks brand new. I do believe that we're probably going to see plain old cordless drills and plain old woodworking tools with little full featured VFDs inside them, relatively soon.

                        Forest's answer is correct that used VFD of unknown problems and unknown model and no manual and no support is cheap, but it might be a overwhelming place to start, especially since a known working, new one with manuals and a support phone # and web sites is not much more money.


                        • #13
                          The VFDs specifically mentioned are single phase output. But ONLY for "PSC" and shaded pole motors. NOT for split phase types. While they might work on a split phase motor that is already spinning, only the PSC and shaded pole can be started by the VFD.

                          As for torque, it may depend on the motor. But torque can be the same, or even more.

                          Many 2 speed motors have lower current draw in slow speed mode, which will certainly make the power less, and possibly the torque less as well. Power will depend on the speed and torque, so at constant torque, the power will reduce in proportion to speed. If the motor has lower power by more than the speed proportion, then the torque must be lower as well.
                          CNC machines only go through the motions


                          • #14
                            Question then: What's different about a single phase motor that runs at 1125 rpm? Most are 1725 or 3450...


                            • #15
                              An 1125 rpm motor is wound with 6 poles, the other speeds are 4 and 2 pole respectively.