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  • Lathe motor soft start

    Nothing big, just a 1/2 hp 120 volt induction repulsion cap start motor on a 9" south bend.
    The motor is the under mount design on a swinging mount that provides the belt tension. When I start it the inrush will chirp the belt and give a resounding clunk from the motor mount so I'd like to implement a soft start to ramp up the power in about 1.5 seconds. I don't want a VFD as the start caps don't like the low freq. Just something to ramp the power in. Any suggestions appreciated.
    Last edited by I make chips; 01-16-2021, 10:59 AM.

  • #2
    if you had a 3 phase motor you would have a lot more options, but a soft start single phase 120 volt starter is going to be a challenge to find. Look at what is available on Factorymation.com or directautomation or similar suppliers and you will have a good idea whats out there. Jim

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    • #3
      If you have start caps you do not have an "induction/repulsion" motor. You have a plain capacitor start induction motor.

      A soft start is not going to "ramp up the power in about 1.5 seconds". A soft start typically reduces the voltage going to the motor for a short time, then applies full voltage. Usually they have been 2 step like that, but some newer types make a more smooth increase.

      They are really intended to reduce the inrush current, but can have the effect of reducing motor torque on start. That could avoid what you are complaining about.

      One type inserts resistance to "drop voltage" due to the current through it, switching out the resistor after a delay. Another type uses an autotransformer to reduce voltage, and then switches the transformer out to apply full voltage.

      A third type uses SCRs to apply partial voltage to the motor. That type "may" change the firing point for the the SCRs to rapidly but reasonably smoothly increase the effective motor voltage. They are used in HVAC systems and may be more available
      2801 3147 6749 8779 4900 4900 4900

      Keep eye on ball.
      Hashim Khan


      It's just a box of rain, I don't know who put it there.

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      • #4
        Viscous drive coupler between the motor and machine, this is how rapid motor starts were controlled before VFD became cheap enough for most applications.
        Think of it as a less complicated automotive torque converter.

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        • #5
          How about a belt tensioner? Maybe an over center lever with a spring so belt changes wouldn’t be made more difficult

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          • #6
            Using the motor's weight to provide tension is just a bad idea.You always get the pulley climbing up the belt and falling back down.

            Before doing an electrical soft-start fix, try locking down the motor. That is, anchor the motor so that it can't climb up the belt. There's a good chance that it will allow the lathe to start smoothly and quietly. If so, you can create a linkage that allows the tension to be relaxed for speed changes (I assume that the motor has a multi-step pulley). I did this on my Jet lathe & it works perfectly.

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            • #7
              You have something loose. Tighten it and then your problem will go away. Thousands of SB owners use regular induction motors that start just like yours. Repulsion start motors in fact do start as soft as any; once they come up to speed and switch to induction run, then they run just like squirrel cage.

              You might also try running your motor on the bench, to see if it's behaving. Most R-I motors are quite old now. You can learn a lot with an hour or two on YT, lots of good videos on there about fixing them.

              metalmagpie

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              • #8
                What Bob E. said. This is not an electrical problem, but a mechanical one.
                "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                • #9
                  https://www.raymondinnovations.com/c.../products/gs11

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                  • #10
                    Originally posted by mickeyf View Post
                    What Bob E. said. This is not an electrical problem, but a mechanical one.
                    Mechanical, the belt might not be tight enough. Starting a south bend 9 is usually not a problem, but everything needs to be adjusted properly.

                    Just a thought......If you want to slow the start can you swap out the motor for one with similar electrical specifications, but is a double shaft motor? If so, then maybe adding a small flywheel on the motor would soften things up.

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                    • #11
                      why are soft starters so expensive? i wanted one for the lathe and bought a vfd instead.

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                      • #12
                        I Think something is loose in the motor and countershaft assembly. There should not be a noise other than the belt "chirp". On my SB9 the belt would "chirp" but no other noise. It turned out the countershaft bearings needed oil.
                        Things to think about and check 1) Does it chirp in all the different speeds? 2) Is the belt needing replacement 3) Does it chirp with out the chuck attached 4) Is the Chuck plus the work really large for the size of the lathe 5) Is every bearing in the system properly oiled? This both the underdrive system and the lathe 6) What is moving to create the thunk noise?

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                        • #13
                          If the motor weight is the tension, that can work OK, but usually the motor will climb the belt and chirp the pulley. Might easily be the whole problem.

                          My big Atlas-Clausing drill press uses motor weight to tension the belt, and so does the Benchmaster, but there is extra leverage, since the belt is horizontal and the motor shaft vertical. the belt gets the short leg of the lever.
                          2801 3147 6749 8779 4900 4900 4900

                          Keep eye on ball.
                          Hashim Khan


                          It's just a box of rain, I don't know who put it there.

                          Comment


                          • #14
                            If I recall my buddy's cabinet drive the lever that lifts the motor to remove tension to allow the speed changes is also the lever with arm that tensions the belt and holds the motor down. I'm with the others in thinking that this is a mechanical adjustment that has come loose or was not adjusted correctly. That and oil the countershaft while you're down in there looking it over.
                            Chilliwack BC, Canada

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                            • #15
                              Originally posted by BCRider View Post
                              If I recall my buddy's cabinet drive the lever that lifts the motor to remove tension to allow the speed changes is also the lever with arm that tensions the belt and holds the motor down. I'm with the others in thinking that this is a mechanical adjustment that has come loose or was not adjusted correctly. That and oil the countershaft while you're down in there looking it over.
                              Yes. On South Bend under drives the tension lever lifts the motor for pulley speed changes. When adjusted properly the lever linkage will cam over when the belt is tightened, locking it into place. If you don't feel it cam over then the belt is not tensioned properly
                              Last edited by tom_d; 01-17-2021, 06:06 PM.

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