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  • 3 phase power

    I have now had some time to use the new 3 phase installation with the 10" Logan.

    I can unreservedly recommend going three phase. HUGE difference.

    1) I have yet to experience any chatter using 3 phase. Apparently the pulsing torque of single phase was the main reason for chatter previously.

    On a lark, to see if I could do it, I made a part like one someone had asked about on another forum. it involved trepanning a slot in the face with a fairly wide cutter. After I got done with the part, I realized that I SHOULD have had rather bad chatter with that cutter, as I always have had in the past. But I never had even a hint of it.

    2) The Logan is a flat belt machine. I always have had problems with the flat belt losing traction on a regular basis, stalling the cut.

    I had exactly ONE instance of it, when I was plunging the trepanning cutter faster than I had a right to. The belt came "loose" with a sort of "snick", indicating that it had had good traction right up to that point.

    In the past, I have noticed it acting "soft". Apparently the pulsing torque of single phase tends to "break it loose" much faster.

    3) I had the general feeling of considerably more power available. The machine just "pulled through" things that I would have expected to slow it down .

    While the actual motor power is identical as far as ratings, the continuous "inherent" torque of 3 phase seems to provide a much more powerful drive than single phase.

    If you get the opportunity, I would highly recommend 3 phase.

    What I put in is an Arco Roto-Phase RPC. I suppose that a VFD would give much the same effect. But I got the Roto-Phase for considerably less than the VFD would have cost, and it will power several machines without fussing with moving the controller, etc..
    2730

    Keep eye on ball.
    Hashim Khan

    Everything not impossible is compulsory

    Birds are NOT real, they are spying on you

  • #2
    Seems to me that a flywheel on the motor shaft would have the same effect on single phase. Wouldn't stop as fast though.
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    • #3
      I read an analogy somewhere on the internet that likened single phase to one guy swinging a sledge hammer. 3 guys swinging in concert is gonna get a lot more done.

      SP

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      • #4
        Not quite. There isn't more power with 3 phase. It's just delivered more smoothly. The difference is in what's called torque ripple.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          And that's the big difference.

          With single phase, twice per cycle the driving torque goes to ZERO.

          At all other times it is between zero and max.

          With three phase, the torque is never zero, never even CLOSE to zero.


          Using you guys with hammers analogy, which isn't THAT bad, The one guy has a considerably bigger hammer. the three guys taking turns have smaller ones.

          That means that to AVERAGE the same torque, single phase has to have roughly 1.5 x more PEAK torque.

          That means far more tendency to break loose the belt, excite chatter, etc.

          With a sufficient flywheel, you might get a similar effect. However, the flywheel must slow down to give up energy to drive the load. then it must speed up to get that energy back. So you are not really eliminating anything, just somewhat mitigating it.

          Three phase has almost constant power transfer. At all times there is positive torque, and torque is "inherent".

          Single phase is somewhat like a steam engine..... there is pull, then no pull, then pull, etc. The only thing preventing the motor from stopping is the inertia of the rotor and driven parts.
          J Tiers
          Senior Member
          Last edited by J Tiers; 03-31-2007, 12:15 PM.
          2730

          Keep eye on ball.
          Hashim Khan

          Everything not impossible is compulsory

          Birds are NOT real, they are spying on you

          Comment


          • #6
            "1) I have yet to experience any chatter using 3 phase. Apparently the pulsing torque of single phase was the main reason for chatter previously. "

            Was there actual 100% verifiable chatter caused by the single phase motor? Was the single phase motor reasonably good quality?

            I'm some what of a skeptic on this subject. Not that I don't recommend using three phase where ever possible.

            I have a fairly high dollar tool and cutter grinder that has a single phase on the spindle. Would they have done that if this was a major cause for concern?

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            • #7
              Originally posted by DR

              Was there actual 100% verifiable chatter caused by the single phase motor?
              it went away with 3 phase. was there previously. The frequency was always one which appeared to be 60 Hz related. If it looks like a duck.........

              Was the single phase motor reasonably good quality?
              Yes. Nice older GE

              I have a fairly high dollar tool and cutter grinder that has a single phase on the spindle. Would they have done that if this was a major cause for concern?
              A T&C grinder has far less chance of chatter, and the ratio of rotating mass to forces imposed in cutting is far different. You don't tend to load down a T&C like a lathe.

              I'd call that almost apples and pencils.........
              2730

              Keep eye on ball.
              Hashim Khan

              Everything not impossible is compulsory

              Birds are NOT real, they are spying on you

              Comment


              • #8
                Originally posted by J Tiers
                ...
                the ratio of rotating mass to forces imposed in cutting is far different. You don't tend to load down a T&C like a lathe.

                I'd call that almost apples and pencils.........
                Do you think it would be fair to conclude a machine with single phase exhibiting chatter type symptoms was under "motored' for the application?

                By under motored I mean not enough hp and/or lack of rotational inertia.

                For the 3phase-phobics could they achieve the same result by changing to a bigger one phase motor? For instance, I have an open frame 1/2 hp motor (pick it up with one hand) and an older 1/2 hp cast frame motor (an effort to pick up with both hands). Logic and experience tells me the older motor would be a better choice for a cutting application. The armature alone of the older motor weighs more than the newer motor

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                • #9
                  With a sufficient flywheel, you might get a similar effect. However, the flywheel must slow down to give up energy to drive the load. then it must speed up to get that energy back. So you are not really eliminating anything, just somewhat mitigating it.
                  The flywheel only need be big enough damp the torque ripple so that it is equivalent to the torque ripple of a three phase motor. A 4 pole single phase motor has much less torque ripple than a 2 pole motor and should be close to a 3 phase motor in torque ripple as the poles are excited 90 degrees apart. The use of skewed pole rotors reduces the torque ripple even more.

                  The primary advantage of 3 phase motors is starting torque. They naturally develop close to full torque (at least 50%) at all rpms while starting.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    Originally posted by Evan
                    The primary advantage of 3 phase motors is starting torque. They naturally develop close to full torque (at least 50%) at all rpms while starting.
                    I presume you mean the primary advantage.... compared to a single phase motor with appropriate flywheel in the drivetrain ?

                    Of course the flywheel idea is good in theory, but in reality how many folks are going to actually intall one ? Almost no one I suspect....most HSM will "get by" with the status quo single phase until they run across another machine that already has a 3 phase motor or get a free 3 phase motor somewhere.

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                    • #11
                      I suppose one other mentionable advantage to three phase motors is their relative simplicity. Starting torque is higher as mentioned. No special starting or rotation reversing scheme is needed either.

                      That simplicity coupled with ready availability of surplus three-phase motors due to their prevalence in industry can end up making them cheaper for us home shop types once you get maybe two or more machines...even if you have to spend say $100-$150 up front to build an RPC. Add in the smooth power provided and they are not a bad way to go. I don't know that I fall into the "convert everything to three phase" camp, but I sure hate to see guys swap out a three phase motor on a new-to-them lathe or mill because they start multiplying the costs of VFD's times several machines. There are really reasonable ways to get to three phase power for revived industrial machine tools, inexpensively, and the smoother power cycle is just one of several advantages.

                      Paul
                      Paul Carpenter
                      Mapleton, IL

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                      • #12
                        My SB9 has a 4 pole motor and several flywheels in the drive train as well as two rubber belts. The motor drives the jackshaft with a ~ 8 to 1 reduction and the Jackshaft pulley is 10" in diameter.

                        The chance of any torque ripple getting to the spindle is ~0.0.

                        After taking into account the ratio of the motor rpm to the spindle rpm the ripple frequency of a 4 pole motor starts at 4 pulses per rev and finishes up at about 40 pulses per rev at the spindle. The spindle and chuck have enough mass that they can't possibly respond at that frequency even if any ripple did make it through the drive.
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                        • #13
                          Originally posted by Evan
                          The spindle and chuck have enough mass that they can't possibly respond at that frequency even if any ripple did make it through the drive.
                          F=ma. If F is finite, there will be an a, which integrates to a distance (a displacement) no matter how big the m is. If m is large, a is small, but it's never zero. The idea is to get it small enough so that in a particular application it's close enough to zero.

                          In rotary terms, T=Ia - same idea, but a bit harder to type, as those omegas get in the way.

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                          • #14
                            I really cant relate to all the technical jargon, --- but I DO know that after installing a VFD/3ph set-up on my lathe, I am just really blown away with the difference in performance,--- improved finish, quietness, torque, etc, ---and of course ..... that wonderful variable speed (with that torque even down in the teen rpm's!)

                            I CAN relate to J Tiers excitement over his improved performance, cause I got the same thing!
                            If everything seems to be going well, you have obviously overlooked something........

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                            • #15
                              F=ma. If F is finite, there will be an a, which integrates to a distance (a displacement) no matter how big the m is. If m is large, a is small, but it's never zero. The idea is to get it small enough so that in a particular application it's close enough to zero.
                              That would be true for a perfectly rigid body. In the real world no such thing exists. More importantly, the spindle and chuck are driven by an elastic rubber drive belt which serves as a damper. Rubber belts in particular have excellent damping properties.
                              Free software for calculating bolt circles and similar: Click Here

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