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  • My first 3 phase machine... questions.

    Hi group! I recently bought a new mill with a 2 speed, 3 phase motor, and a rotary phase converter to feed it. Got everything wired up, and kicked on the power to the RPC. Idler starts instantly and runs smooth and quiet. (So far so good...)

    Turn the power switch on the mill to low speed (I) and the motor comes to life, again smooth and quiet.
    Turn the power switch off, and the mill motor slowly coasts down and stops.

    Repeat the process again, first starting at low speed (I) and then switch up to high speed (II). Again, smooth running but a bit louder at high speed of course. OK, life is good!

    Then I turned the power switch back to low speed and got a loud gawd-awful screeching sound (approx 1 sec in duration) and the spindle speed dropped INSTANTLY back to the low speed (no coasting down in RPM).

    Again, since this is my first experience with a 2 speed, 3 phase motor - I have to ask... is this normal?

    Also, what would be the "normal" orientation for the power switch on a typical Bridgeport type step pulley mill? That is.... Forward spindle rotation: clockwise or counter-clockwise on the power switch?
    Last edited by Highpower; 09-11-2008, 11:39 PM.

  • #2
    Originally posted by Highpower
    Hi group! I recently bought a new mill with a 2 speed, 3 phase motor, and a rotary phase converter to feed it. Got everything wired up, and kicked on the power to the RFC. Idler starts instantly and runs smooth and quiet. (So far so good...)

    Turn the power switch on the mill to low speed (I) and the motor comes to life, again smooth and quiet.
    Turn the power switch off, and the mill motor slowly coasts down and stops.

    Repeat the process again, first starting at low speed (I) and then switch up to high speed (II). Again, smooth running but a bit louder at high speed of course. OK, life is good!

    Then I turned the power switch back to low speed and got a loud gawd-awful screeching sound (approx 1 sec in duration) and the spindle speed dropped INSTANTLY back to the low speed (no coasting down in RPM).
    Yep the noise is normal,nearly all three phase motors will make it when you plug stop,reverse or reduce speed suddenly.It may not be the best on your switch contacts though so you might want to let it coast down before switching.

    Originally posted by Highpower

    Also, what would be the "normal" orientation for the power switch on a typical Bridgeport type step pulley mill? That is.... Forward spindle rotation: clockwise or counter-clockwise on the power switch?
    Both,the mill has a backgear setup for providing high and low speed ranges.Whatever switch rotation gives you FWD in high range will give you REV in low range.

    Easiest way is to add a new face to the switch cover.Have the top half painted or marked for High range and the bottom half marked for Low range.

    Try to avoid reversing a spindle on any metal working machine while a cutter is in a cut,only real exception being power tapping.
    I just need one more tool,just one!

    Comment


    • #3
      Originally posted by wierdscience
      Yep the noise is normal,nearly all three phase motors will make it when you plug stop,reverse or reduce speed suddenly.It may not be the best on your switch contacts though so you might want to let it coast down before switching.
      That's just it... this mill is controlled by the power switch only. 2 forward speeds and 2 reverse with "off" in the center. To go from high speed to "off" requires turning the switch through low speed before you can get to "off". That is when I get the sudden speed change. (Dynamic braking?)

      Both,the mill has a backgear setup for providing high and low speed ranges.Whatever switch rotation gives you FWD in high range will give you REV in low range.
      Unfortunately this is not an actual Bridgeport machine, but an Enco semi-copy (8X36) and does not have a back gear. A full size Bridgeport would not fit in my basement with the low ceiling height.

      Thanks for the information.

      Comment


      • #4
        Oh,you mentioned "step pulley bridgeport" that makes a diffrence

        So basically you have a machine with just a drum switch for a control and no relays?

        If that's the case then unless it's missing something like a contactor set and some interlock relays you should be set.

        Do you have a picture of the machine or a catalog shot?
        I just need one more tool,just one!

        Comment


        • #5
          Originally posted by wierdscience
          Oh,you mentioned "step pulley bridgeport" that makes a diffrence
          Yeah, my bad... I was just curious to know which way to turn the switch on a Bridgeport to get "forward" rotation.
          So basically you have a machine with just a drum switch for a control and no relays?
          That is correct...
          If that's the case then unless it's missing something like a contactor set and some interlock relays you should be set.

          Do you have a picture of the machine or a catalog shot?
          Yes, the machine runs fine - forward and reverse - high and low speeds. I just didn't expect the transition from high to low speed to be so instantaneous along with the screeching noise. I didn't know if that was normal for all 3 phase motors or not. I assumed that dynamic braking was an optional feature for a 3 phase motor.

          Anyway, a pic of my new "money pit" can be found here:
          http://www.use-enco.com/HTM/2008/493.htm#100-1525

          Comment


          • #6
            Highpower,

            What you describe as dynamic braking is not exactly correct. A motor is a magnetic device. The speed of the rotating member of the motor is determined by the number of poles in the stator and the frequency of the source voltage. When you switch from hi to low you are changing the number of poles in the motor and the rotor will TRY to instantly slow down. Because of inertia this cannot happen so the motor slips magnetically. This is what causes the screech you hear. It will not hurt anything it just sounds bad.

            Robin
            Robin

            Happily working on my second million Gave up on the first

            Comment


            • #7
              I have that same mill (though mine is probably a little older - and dirtier).

              It really is a great piece of machinery. Very rigid for its size. Keep up on the automatic lubers and you'll have a friend for life.

              Comment


              • #8
                Originally posted by RobbieKnobbie
                I have that same mill (though mine is probably a little older - and dirtier).
                Oh reeeeeaaaly........! (Insert evil laugh here.)

                Well then - I have a couple questions for you if you don't mind...
                Do you have any problems with any gibs that allow free movement in one direction, but drag in the other?

                Also, do you have a DRO on your machine? I'm getting ready to install an Accu-Rite 3 axis unit on mine and looking for a good way to get the Z axis scale mounted to the quill. As you know, its a bit different than the standard Bridgeport quill.

                I'll take any help I can get.

                Comment


                • #9
                  Originally posted by rdfeil
                  Highpower,

                  Because of inertia this cannot happen so the motor slips magnetically. This is what causes the screech you hear. It will not hurt anything it just sounds bad.

                  Robin
                  So, if a rotor slips in the magnetic field, and there is no one around to here it... does it still make a noise?
                  (Sorry Robin - I couldn't resist.)

                  Seriously though, I had no idea a magnetic field could produce such a sound. I just assumed it was the inertia of the spindle/pulley causing it to slip against the drive belt when the brakes were slammed to the motor drive pulley....
                  All I know is, it sure surprised me!

                  Thanks.

                  Comment

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