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  • Question for the motor/electrical experts

    A gear motor followed me home. "B&B", apparently now Baldor, but looking up the model and serial got me nothing - presumably too old. It has 4 of wires coming out. The guy I got it from said he was not able to figure out how to wire it to make it run, but I think he was using straight trial and error, not science. It appears to be a permanent split capacitor run motor - it even specifies "15 MFD" on the name plate. I don't think he ever tested with a capacitor in any way. By observation and ohmmeter I confirmed that two of the wires are immediately connected after entering the housing, so it effectively has 3 wires, not 4. I believe I have identified the start and run windings by lower and higher resistance. I did not see any evidence or overheating, etc inside. If I'm understanding this design correctly, I should be able to put an appropriate capacitor in series with the start winding, supply power to both windings, and it should run.

    So my question is... Before I go out a find a correct motor grade capacitor, what can go wrong if I test by *briefly* connecting it with a capacitor of the correct capacitance and adequate voltage rating from my electronics junk box, but which is not intended for motor use? Presumably this may mean polarized, higher (?) esr, and I don't know what other differences.

    Further question. Since this design relies on an on-going difference in phase between the two winding, as set by the capacitor, it is not dependent on the frequency of the AC the way a synchronous motor would be, and there should be other options for varying its speed besides what would be done using a VFD. Can I realistically expect to do this by varying the the effective power input, say with a triac as is done with commutated motors, or ...?
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

  • #2
    You'll hear shortly from the EE types here, but I don't think a polarized cap will last long at all. I've use household dimmers to control shaded-pole motors on small blowers, but I don't think they will work on your motor,
    Southwest Utah

    Comment


    • #3
      You can make a 15 uF bipolar capacitor by putting two 30 uF electrolytics in series, "back to back". If the motor is rated for 15 uF, it will probably work with values from 10 to 20 uF. If the value is too low, it will just have reduced torque. If too high, it will draw more current and may eventually overheat. I doubt that you will be able to get any significant speed control by adjusting voltage or capacitance. I think variable speed motors such as those used for fans are wound with high impedance so that operation with a large degree of "slip" will not draw excessive current.
      http://pauleschoen.com/pix/PM08_P76_P54.png
      Paul , P S Technology, Inc. and MrTibbs
      USA Maryland 21030

      Comment


      • #4
        Edit: Paul beat me to it.

        You need a non-polarized cap. A polarized cap will explode.

        -js
        Last edited by Jim Stewart; 09-14-2020, 09:24 PM.
        There are no stupid questions. But there are lots of stupid answers. This is the internet.

        Location: SF Bay Area

        Comment


        • #5
          Do you know anything of its original application?

          I'll make one guess- if it's a gear motor, it might have been reversible. In that case the junction of the two wires would probably go to 'hot', then one of the other wires would go to a cap and the other to 'neutral'. Swap those wires for reverse operation.

          An ohmmeter test could shed some light. If both separate wires show the same resistance to the two coupled wires, then this scenario could be likely. If they show different resistances, then one is probably a start and the other a run- but I don't think this is a common theme for a gear motor. I could be wrong.

          You say it specs a 15 mfd cap- that must be a non-polar as everyone is saying, and with that spec it's a run cap, not a start cap. Depends on the size of the motor though. Is there any other info on it at all?
          Last edited by darryl; 09-14-2020, 09:28 PM.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

          Comment


          • #6
            With 3 wires, it may be reversible, if the two windings are identical (common for gear motors).

            What is done in that case is to connect the cap between the two separate wires. Now the power is connected to one end of the cap (one of the windings) for "forward", and to the other end (other winding) for reverse. Very simple.

            The double wire is not so typical of that, so it is not clear what the deal is with that motor. however, a capacitor goes in series with one of the separate wires, and its other terminal either gets connected to hot or neutral, depending on how the internals are set up. The extra wire tends to suggest the cap may connect to one of the other wires and to the "extra" wire of the two.

            A PSC can be used with a triac type controller, or a single phase output VFD.

            The triac will only control speed well if the load varies with speed, like a fan, because the frequency stays the same. The motor is an induction motor, so as power to it is reduced, it will not change speed unless the load changes with speed. If so, then it will run slower (more slip) at lower power.

            The VFD will control speed regardless, but the range is not huge, because the capacitor does not pass much current at low frequency. However, the speed will be much less dependent on load, because it is mostly set by the frequency.

            If the unit is made for reversing as above, the resistance from either wire to the double wire will be the same. If it is made to wire the capacitor from one wire to the double, then the resistance from the two will be different, and the wire with lower resistance to the double is probably the power wire.
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              If the windings are the same resistance, i.e. PSC motor, this shows a SPDT SW with centre off.
              An AC motor run cap can be had at a local rewinder.
              Max.
              Attached Files

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              • #8
                No idea what it was originally used for. One winding has roughly 3 times the resistance the other. I'm assuming the one with the larger resistance is "run" and the lower one "start". I'll fish around and see if I have caps I can reasonably test with. They only have to last long enough to indicate that it works - i.e., not blow up immediately, not damage anything else (including me) and show that everything else is apparently ok. It also says on the name plate "Torque 90 In lbs." and "Caution - do not load beyond stated torque".
                "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

                Comment


                • #9
                  No the lower resistance is the run winding, the higher one is the start/run winding,. This is the one that has the cap in series.
                  Max.
                  Last edited by MaxHeadRoom; 09-14-2020, 11:42 PM.

                  Comment


                  • #10
                    Check the resistance BETWEEN the two wires.

                    Possibility 1: The resistance between them is 4x (four times) the resistance of the run winding. In that case, each winding is independently wired to the double wire, and the capacitor goes from the top of the run winding to the other winding.

                    Possibility 2: The resistance between them is only 2x the resistance of the run winding. In this case, the two are both connected to the power wire, and the capacitor goes from the end of the capacitor winding to the "spare" wire of the double wire. That would explain why there IS a "spare" wire, so it seems slightly more likely.

                    I would not use the back-to-back electrolytic capacitor idea even to test. Get a proper run capacitor, which will be a film or "oil filled" type.

                    With the type of wiring that seems to be present, the motor is not reversible.

                    And, just as a pint of information, which is not that important, the capacitor winding is NOT a "start" winding.... it is always powered through the capacitor, and is needed to provide torque, especially if there is a variable speed arrangement such as a triac or single phase VFD..

                    1601

                    Keep eye on ball.
                    Hashim Khan

                    Comment


                    • #11
                      I found some discussion about using two electrolytic capacitors in series (+ to + or - to -), and it appears to be safe to do so. I had thought it would be best to add anti-reversal diodes across the capacitors, but apparently that is not necessary. The two capacitors actually both charge up in opposite directions so that when one is charging, the other is discharging, and neither one sees a reverse charge condition (except for a few cycles initially). As long as the capacitors are rated for the peak voltage, and have reasonably low ESR, they should be OK for the few seconds it takes to verify motor operation. But as is always prudent when dealing with mains power (and most machining operations), to use safety goggles and other precautions.

                      https://antiqueradios.com/forums/viewtopic.php?t=191536

                      https://electronics.stackexchange.co...r-electrolytic

                      .
                      http://pauleschoen.com/pix/PM08_P76_P54.png
                      Paul , P S Technology, Inc. and MrTibbs
                      USA Maryland 21030

                      Comment


                      • #12
                        Those 4 wire motors where 2 of them are connected together, are used to simplify the connections. 2 wires go to power and the other 2 to the capacitor. You can't reverse these easily. The only way to reverse them is to tap into the connections or dismantling the motor to invert the rotor (if the motor is symmetrically built). This is the way you change the direction of shaded pole motors.
                        Helder Ferreira
                        Setubal, Portugal

                        Comment


                        • #13
                          Nothing has been mentioned about whether there is a centrifugal switch in the motor. That would complicate things a bit That would mean a start cap, and if that is 15 uf, then it's a pretty small motor. How large or small is this thing? I suspect that it doesn't have a switch, and the cap is meant to be in circuit all the time. Then I think it's correct that it goes to the highest resistance winding- working to cause a phase shift so the main current can start and maintain rotation. In this case, the correct value of cap would make just the right amount of 'shift' for optimal power production, plus lowest wasted current and lowest motor hum. It might not like a 10 uf, and it might not like a 20 uf.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                          Comment


                          • #14
                            Originally posted by MaxHeadRoom View Post
                            If the windings are the same resistance, i.e. PSC motor, this shows a SPDT SW with centre off.
                            An AC motor run cap can be had at a local rewinder.
                            Max.
                            Or HVAC, plumbing supply.

                            Comment


                            • #15
                              I used to work next to an HVAC shop, and I scavenged all sorts of motors, capacitors, contactors, and entire blowers from the dumpster.
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

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