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Thread: A question about AC motors...

  1. #31
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    Quote Originally Posted by Forrest Addy
    If you are content with the finishes you get at present there is no reason for you to spend the money. But you weren't going to anyway; your mind was already made up.
    I would like to improve my finishes but nothing I have read has convinced me that converting to 3 phase should be my next step in that direction. I said right up front what my thoughts are on the single phase issue.

    FYI, classic chatter is a self-excited resonance like a bow drawn over a violin string. In a machine tool making chips some part of the cutting force loop (generally the tool or the work) deflects slightly unloads part of its strain, recovers, loads again, deflects, etc repeatedly, this raises a resonance whose frequency is governed by the spring in the system and the mass deflected (tool or work). Quite often chatter will emit a musical note easily heard over the machine's usual sounds. Drills often chirp but this is not chatter bourne of edge interaction with the work but the flutes scraping the hole walls. Noobs and those who should know better are apt to refer to as "chatter" any rhythmic pattern in the work surface, shadowing from congugate gear action, vibrations from motors, out of balance, rhythmic surge in the feed motion, tool recovery from interrupted cut deflections, and so on. In fact this is not chatter but gear telegraphing , phonographing, surge, etc. There are technical terms for most anything in industry. It's misleading and confusing not to use them properly.
    I scarsely mentioned chatter, except to say that the effects of torque variation should not be compared with chatter.



    Bodger you've done it again. You successfully raised an issue, made it contentious, lured people into choosing sides, started hares, fomented rebellions, raised straw men, and in general was agent provacateur for a topic usually eliciting yawns. In the end you state "well, I'm not convinced" and abandon the field to leave the combatants staring around in confusion. The CIA can use a man like you. Drop you into Iran and soon the whole country will be squabbling over non-issues.
    No doubt the CIA or maybe the NSA have a nice file on me but I doubt they will be calling on my services.

  2. #32
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    Quote Originally Posted by The Artful Bodger
    I would like to improve my finishes but nothing I have read has convinced me that converting to 3 phase should be my next step in that direction.
    -Quite apart from any vibration issue, there's the fact that certain materials and certain cutters, notably various carbides, have optimal surface speeds and feed rates. An infinitely-variable VFD control makes it both easier to find that "sweet spot", and quicker to set it back to that speed when the next job comes up.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  3. #33
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    Quote Originally Posted by The Artful Bodger
    Thanks everyone for your contributions to this topic.

    Perhaps the effects of torque variations are detectable in some circumstances but having read the topic I have decided that I will not be spending money on a 3 phase motor and VFD in the expectation of being able to see a difference in the finish of my turned work.

    Well, you (and Evan) are only hurting yourselves, so go on bravely and suffer with your hair shirts, keep lashing yourselves with the cat-o-nine-tails........ WE are OK, and it won't bother us if you never use 3 phase.... some of us have seen night and day differences.

    (And I disagree somewhat with Forrest on chatter.... it has lots of variations... of course telegraphing isn't chatter, but chatter can look just like telegraphing. Forrest just hasn't used smaller more chattery lathes as much as some others who only HAVE them)

    PROVE it? To YOUR satisfaction?

    Why bother?

    Laboriously writing an explanation is time consuming and takes away from other activities.....

    And then the questioner dashes off a quick "well I'm not convinced, what about this other thing?" comment....... with the expectation that another long effort will be made by the damn fool writing.

    It is so easy to discount any argument and continue to stubbornly say "I'm not convinced, I don't believe it"...... it's like a game.... lead 'em on and make 'em work until they get tired this time, we'll get 'em again soon.....

    it only uses up willingness to answer legitimate questions, so it actually hurts everyone in the "community"..... if you do NOT want to know, just don't ask.

    it's just drawing water from the well and pouring it on the ground..... quit it, or otherwise the well will be dry for the next person.

  4. #34
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    Quote Originally Posted by Evan
    It doesn't mean it is being transmitted by the drive train. Vibration can be transmitted via the static parts of the machine from the motor. It also isn't necessarily a problem cause by single vs three phase. There are many possibilities. However, if torque ripple were to blame then it should show up on all lathes that run single phase motors. It doesn't.
    It worked fantastic for me, the only thing that was changed on my lathe was the drive. If it was the 3~ power or the motor or both, makes little difference to me.

  5. #35
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    If I were a younger fitter man so I could work from a grovel I would invite a few doubters to watch me start a cut in my lathe using the three phase motor at 60 Hz, disengage the feed back the tool away for safety, swap out motors (it would take a couple hours) resume the cut and in triumph point to the single phase motor signature in the feed marks on the work, then restore the lathe to three phase and point to the smooth superior finishes on either side of the corrugated looking finish resulting from a single phase motor. Same tool. same material, same feed, same speed, sdame depth of cut, - but different motor.

    And that's with the single phase motor mounted on the floor entriely unattached to the machine except by the V belt and the three phase motor direct mounted on the transmission.

    The disadvantage in the "single phase finish" is in part esthetic but the other part is practical.

    Esthetic: Nobody likes ugly. They like their work to reflect well on them and funny looking, unworkmanlike finishes do not enhance the shop's image. That's worth something but it's intangible and impossible to assess a cost for it.

    Speaking practically: In a smooth running lathe if you were turning a diameter on which a seal runs it would take about 30 secvonds to polish out the feed marks. If the lathe were prone to single phase vibration, it might take three to five minutes. Not a big deal but a PITA never the less. So what's a couple minutes? Maybe not much but sooner or later time accumlates into money.

    Move on to a ball bearing fit. Here you desire finish and an accurately sized, cylindrical fit on which to mount the bearing. Polishing does not improve roundness or cylindricity but if deftly done you can work from a small polish allowance to accurate diameters. It does add time.

    But why not go direct to size and bypass polish? Saves time and improves the fit. When your lathe is running smoothly, you can.

    More more than a few people, once they take a contrarian public position cannot be convinced against their will. In many cases public, physical, and undeniable proof can be set in front of a stubborn man and still he holds a position untenable to anyone with a shred of objectivity. Germ theory to the 19th Century medical community for example. To them incontrovertable proof and reprodicible results meant nothing. Preservation of their own rightness is everything, They would claim "My ship don't leak" as the water rises past their nostrils.

    So, I give up. You fellows with single phase lathes having finish problems stand on the line you've drawn in the sand.

    Those of you with single phase lathes having no finish problems probably wonder why the fuss. Indeed I do myself. I suffered disfigured finishes once, funished my lathe with a three phase motor and VFD and now my lathe produces work finishes rivaling those of a Monarch EE. Cause and effect.

    And for the record, I write to convey information I may have or can find to anyone; lurker, poster, or regular. The gadflies like the Bodger with their "convince me" atitude don't bother me.

    If there are a few I can help I take my reward in abstract satisfaction. Once in a while I get an eMail or PM from soneone I never heard thanking me for something I wrote that benefitted them. For me, that's the frosting on the cake.
    Last edited by Forrest Addy; 04-19-2011 at 03:41 PM.

  6. #36
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    Quote Originally Posted by Forrest Addy
    And for the record, I write to convey information I may have or can find to anyone; lurker, poster, or regular. The gadflies like the Bodger with their "convince me" atitude don't bother me.

    If there are a few I can help I take my reward in abstract satisfaction. Once in a while I get an eMail or PM from soneone I never heard thanking me for something I wrote that benefitted them. For me, that's the frosting on the cake.
    Which is exactly why I bother also...... but I have limits. One explanation, and some "clarification"....... if the doubters and "head gamers" persist, too bad, I am not playing that "game" longer than that.

  7. #37
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    They say one picture is worth a thousand words so two are probably even better. I stayed up till 3:00AM doing these so do look at them.

    They show the relationship between the current in the motor windings and the torque being produced in both single phase and three phase motors. I do not claim absolute accuracy here, just an attempt to show the general relationships. The point is, the torque in a single phase motor goes to essentially zero two times in every cycle of the AC current in it's windings.



    This is essentially a 100% variation in torque that occurs 120 times per second. I can not offer direct proof that this will be transmitted by a belt drive to the spindle, but it would certainly be transmitted via a gear drive.

    On the other hand, because a three phase motor uses three phases that are out of phase with each other and all three of these phases contribute to the total torque, the sum of the three is always a positive number, never zero. In fact, the variation in torque is closer to 12 to 15 percent. Far, far lower than the 100 percent variation of the single phase motor.



    The mass of the rotationg load (spindle, chuck, and work piece) will certainly have a flywheel effect, but this will be the case in both single and three phase motors. So the difference in the motor's torque will still be a factor.

    My drawings show simplified motors and as the notes on them state, real world motors generally have more poles on the rotor or stator or on both. But this does not in any manner negate the conclusion. More poles would simply produce motor that rotate slower. The sequence between adjacent poles would be essentially the same and the resulting torque would vary in much the same manner. The motor's sped will drop more due to the load when the torque drops to zero than when it only drops by 12 to 15 percent.

    As for the vibration of the motor being transfered to the lathe via the mounts, that could also be a factor. However, a three phase motor can vibrate just as much as a single phase one and with a well balanced motor it may not be as much of a factor as the varying torque. I suspect that the variation in the torque is a much larger factor in this situation.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  8. #38
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    Those a good diagrams Paul and they certainly demonstrate the variation in torque inherent in a single phase motor however they only show what is happening in the magnetic fields and do not show how much these variations are smoothed by the inertia of the spinning rotor.

  9. #39
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    Quote Originally Posted by The Artful Bodger
    Those a good diagrams Paul and they certainly demonstrate the variation in torque inherent in a single phase motor however they only show what is happening in the magnetic fields and do not show how much these variations are smoothed by the inertia of the spinning rotor.
    No, they don't. But that would be a similar factor for both types of motors. So the single vs. three phase torque difference remains even with that smoothing and any other factors.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  10. #40
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    Actually, Paul's diagrams don't show what is happening in a single phase motor. A single phase motor is modeled as having two rotating fields in opposite directions. The first is the forward field and it exhibits a phase lag to the input AC. The second field counter-rotates with an additional phase lag to the first. The difference between the two rotating fields is the slip angle and is responsible for the generation of torque. Neither field is synchronous to the input AC and two of the three are always out of phase at any moment in time. Because of this the actual magnetic fields are never all zero at any moment in time. To further mix it up most modern motors have skewed rotors to reduce torque ripple. On top of that many machines have capacitor start-capacitor run motors which generate a large phase difference in the induced field and consequently have very little torque ripple even under heavy load.
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