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got an angle grinder that runs backwards!

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  • got an angle grinder that runs backwards!

    Standard 5/8-11 threaded spindle; should turn so that resistance on the rim of a wheel tends to tighten it.

    Well, this one doesn't. It runs backwards. How to fix it?

    I already opened it up and swapped the brush wires. That worked on another one years ago. Didn't change anything.



  • #2
    Can the core (block with a hole for the rotor to pass through) be flipped end-for-end?

    Sent from my XT1053 using Tapatalk


    • #3
      Swapped the brush wires and it didn't change- hmm. The only thing I can think of that would cause that is if the motor has a built-in rectifier. It would likely be permanent magnet, so you could turn the magnet housing 180 degrees if by chance it's a separate part- or you could remove the brush end bell and rotate that 180.

      So far this is assuming that there is an internal rectifier in the motor, and that must mean that it's a dc motor and the above would apply. I've never yet seen an angle grinder with a dc motor.

      There could be an issue with the gears- they are designed to work in one direction and will not like going the other way. It might work fine, or it might work for awhile, and it would likely be noisy. I wonder about your particular machine, has it been modified previously?
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #4
        It's dangerous to use with any threaded wheel so I'd get rid of it.

        Or you could run it on a battery with a transformer to get the voltage up and swap polarity.


        • #5
          Originally posted by Andre3127 View Post
          Can the core (block with a hole for the rotor to pass through) be flipped end-for-end?
          Hmm. Haven't tried that yet. Will give it a shot. Thanks!

          BTW I really don't think this is a PM motor.



          • #6
            Take it back to the store!


            • #7
              Swapping the brush wires only works for a simple series motor, it might not depending on how the thing is wired. You need to reverse either the field or the armature, but not both.

              That will work for EITHER AC or DC applied to it, so long as it is a standard "universal" motor, which is a series AC/DC motor.

              Did you see field windings inside it?
              CNC machines only go through the motions


              • #8
                J Tiers got it right. Angle grinders are universal motors, same (well, very similar with minor modifications) to DC motors with armature and field in series. You have to swap one of them to change direction, if you change both it will have the same magnetic forces so won't change the direction.


                • #9
                  I assume you just bought the thing. Mail order or a local store? If local store just drive back and ask for a replacement.
                  Chilliwack BC, Canada


                  • #10
                    I've only heared Halloween stories of the Austrailian angle grinder, it won't be satisfied till it sinks a 1mm slitting disk into your jugular, then it will revert to correct rotation.


                    • #11
                      Not related to this thread, but....

                      I have an estate sale Craftsman bench grinder that ran backwards. It was so cheap I couldn't pass it up. The seller said he got it from his workplace and someone had accidentally mixed up the hot and neutral when they put a new plug on the power cord. Of course, that was ridiculous. I figured there must be an easy way to correct the rotation so I bought it.

                      The grinder is one of those flat front and back style that Sears has sold for years.

                      Taking the bottom plate off to check for some combination of wires that could account for the reverse rotation. Nothing...

                      Taking the end bells off and checking the armature it appeared symmetrical end for end. Sure enough, flipping the armature 180 degrees corrected the rotation. Inspecting one of the end bells there was some scratching in the casting indicating which way it rotated. So, for some unknown reason the rotation had been purposely reversed.


                      • #12
                        A bit more .. these came to me free. They are used and came from the same place. One spins correctly the other backwards.

                        I tore them down and physically compared the wiring. The backwards one had had one of its wires spliced and I went ahead and heatshrunk some insulation over it. Both brush wires got nicked (the insulation) when I r&r'ed it before, so I heatshrunk some insulation on those too. Other than those I cannot see a difference.

                        The stator is wound, yes. See:

                        I couldn't make my stupid phone focus down very deep inside the stators, sorry. Here are two more pictures that show wiring.

                        In the pictures the reversed motor is on the left.

                        I can see how they assembled the stator inside the body of the tool. I could probably get it out. But I strongly doubt the wiring would allow me to turn it end for end and reinsert the stator.

                        And by the way, the gears feel the same to my hand and ear when I spin the rotors by hand in either direction.

                        Last edited by metalmagpie; 11-11-2016, 07:28 PM.


                        • #13
                          Can't really follow the wiring from the pics, but.....

                          I thought maybe that what appeared to be a brush connection might actually be a connection to a coil which continues to a brush. In other words, that you do not really have access to the connection from a coil to the brush. That may still be the case. Some motors have the wiring through one coil to a brush, and from the other brush to the other coil, and so back to the switch. In those cases, it can be hard to dig out the real brush connection.

                          This one I am not certain about, but it appears that white wire goes from the switch to a coil, and another white wire comes back from the coil and may go to a brush, or maybe to the other coil and then to a brush. It's difficult to determine from here, best to check with ohmmeter, etc.

                          In any case, if you reversed the brush wires and it did NOT reverse, then you reversed more than you think you did, because it HAS TO work..... reversing the relation of armature and coils HAS TO reverse the motor. So for whatever wiring related reason, you actually did NOT reverse the relation.

                          Did it seem to run OK after reversal? because if you reversed ONE coil, I would expect it to run oddly if at all.
                          CNC machines only go through the motions


                          • #14
                            It's a rednirg elgnA. Look in McMaster-Carr to buy the appropriate sleehw gnidnirG.


                            • #15
                              We had one at the shop a long time ago that ran backwards. One of the guys pressed the shaft out of the armature and pressed it in the other end and it ran in the right direction. So basically what he did was turn the armature around end for end.
                              All we could guess was it got messed up at the factory somehow.