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  • Motor Stutter - Cause?

    The Dumore tool post grinder was whizzing right along then all of a sudden sounded like it was running at half speed. I took it apart and found one of the soldered field wires came loose. Fixed that, cleaned up the commutator and reassembled.

    Now it stutters. It's like one or more of the comm segments are dead but I checked each contact. They vary from .6 to 1.4 ohms as measured from one to around the others. None are grounded or shorted. I checked the field coils too. It powers up and runs but with heavy vibration. When turned off, the vibration stops and the motor glides to a stop.

    Any ideas? Should the resistance in the armature be consistent? Each segment appears to have two wires connected from the armature winding. Is the armature shot?

  • #2
    Motor

    Ken
    The resistance should be uniform around the armature.

    JRW

    Comment


    • #3
      Don't you hate it when fixing one problem creates another?

      I think the problem may be related to "cleaned up the commutator". You may have gotten some conductive material worked into the insulation between the commutator bars. Hopefully, a little scraping will take care of it.

      Roger
      Any products mentioned in my posts have been endorsed by their manufacturer.

      Comment


      • #4
        Thanks Roger but I checked that...three times.

        Comment


        • #5
          Originally posted by J. R. Williams
          Ken
          The resistance should be uniform around the armature.

          JRW
          I'm assuming you mean at ~180 degrees?

          Comment


          • #6
            How about an intermitant in the cord?I had that one in a side grinder once,it sputtered as a result of vibration opening the intermitant.
            I just need one more tool,just one!

            Comment


            • #7
              brush bounce does it too.
              Time for a growler I think
              How did you "clean" the commutator?
              No sand paper or silicon abrasives please
              Did you wash the comutator with solvent? (Laquer Thinner/acetone)
              Did you clean the brush with a clean file.
              Oiled the bearings and it contaminated the area?

              just a few pointers

              rich

              Comment


              • #8
                Bar to adjacent bar, or any bar to any other at a given span, all should be the same regardless of position.

                If you DO have an open bar, you should get sparks trailing around, not just at the brushes. At least that is what I find.

                Other problems may have different symptoms.

                Sometimes with low resistance, it is hard to get a "real" reading. But the difference may be 0.5 ohm vs 0.3 ohm..... not easy to be sure of seeing on most ohmmeters and DVMs etc. You may need a low ohmmeter, or a bridge.

                If one wire to a bar is open, it may work poorly, but you may not be able to measure a difference that you can be sure of, due to the winding resistances.

                If you found a FIELD problem, it is unlikely to have extra problems, unless you have no history on it. I think most of those are series motors, so the field being open ought to have had more problems........like no run-y......

                Mine is wired through the field to one brush, from that thru armature to the other brush, and then I think thru the other field half.... that or both fields are in series on "one side" of teh armature..... switch is in sereis on one side, of course.

                So I am a bit puzzled by your field wire and yet it still ran..... Did you measure the AC to ground? Maybe there is a short in one field
                Last edited by J Tiers; 11-06-2006, 11:23 PM.
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  Thanks JT. I just assumed the power lead started the problem. When I took it apart the first time, one cord-to-field lead just pulled out of the shrink tube. (bad solder joint) I'm now wondering if higher resistance at that connection may have toasted the armature? But... I can't find an open bar and no shorts to the core or shaft.

                  Not to argue with the bar-to-bar resistance but I've checked two other armatures for different motors and get similar results. When measuring across adjacent bars, I get the lower reading. Leaving one probe in place and moving the other probe another bar over, the resistance increases. The resistance increases in steps with each bar until the moving probe reaches 180+ degrees from the stationary probe. After that, resistance starts going back down in steps with each bar. Measuring the resistance on any two opposite (180*) bars gives a stable reading--The higher resistance.

                  Whut up wit dat?

                  Edit: Forgot to mention- The motor plate says "DC to 60hz" if that makes a difference.
                  Last edited by CCWKen; 11-07-2006, 01:03 AM.

                  Comment


                  • #10
                    Your measurement is correct.

                    From ANY bar to ANY other one should be the same in any position..... meaning from bar at 12:00 to bar at 5:00 is the same no matter how you turn the armature.... or from one to the next is same for any pair.

                    Sounded like you had some that were not the same, but your method as explained is correct.

                    If it is DC to 60Hz, it should be a series "universal" motor wired similar to what I suggested. If the two field coils are parallel, then it could work with one out, but badly.... maybe they are.

                    What model is it? I have the very small one.
                    1601

                    Keep eye on ball.
                    Hashim Khan

                    Comment


                    • #11
                      It's the older Model 44 with the switch on the cord.

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                      • #12
                        The armature is a closed series of coils separated by taps to the commutator bars. Each bar is connected to two coils, the end of one and the start of the next one. The coils are usually offset from the bars they are connected to.

                        Is the vibration or stutter constant at a particular frequency, or does it vary as the motor runs. If it varies in an irregular pattern, I'd look for a loose connection or a bad brush spring.

                        Roger
                        Any products mentioned in my posts have been endorsed by their manufacturer.

                        Comment


                        • #13
                          Any one have the resistance measurement for the field coils? I got about 1.5 ohms and that seems low but both measured the same. After about the sixth test on this thing, the field coils got so hot, they were smoking. I'm wondering if the windings are "short" from an insulation breakdown?

                          The motor speed is way low--Maybe 1-2K rpm. Not the whine of 13,000rpm I used to get. And still a vibration under power. Now I'm thinking the field heat may be from the fan not moving enough air.

                          Roger - Just one frequency. 60Hz from the line cord. (~120vac)

                          Comment


                          • #14
                            Low field resistance is not surprising, for a series motor. All the motor current flows in them, so each dissipates almost 19W if the current is thru each in series.

                            If the motor is running SLOW, then the current is way high, since the back EMF will not be there as it would at normal speed. Normal current would be 3.6 A.

                            At slow speed, it could be nearly anything up to 30 or 40 amps.

                            If it is going slow, did you have any choices for the way you soldered the wire back?

                            If the two coils got wrongly polarized, i.e. one is reversed, there might be no net field, and so no torque for rotation to speak of.....
                            1601

                            Keep eye on ball.
                            Hashim Khan

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