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  1. #1
    Milacron's Avatar
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    Default Indramat DC axis motor kaput - Should I open her up ?

    1988 vintage Indramat MDC 10.20F to be precise...typical on Maho mills from the mid to late 1980's. This motor hasn't turned in nearly a decade so beyond me what the conditions were that cause it's demise....but knowing where it was positioned I suspect coolant ingress and internal corrosion issues.

    I noticed when I turn the shaft by hand one can feel a slight "bump" sensation every quarter turn...barely noticable but seems not quite right. When I hook the motor to 30 VDC it will turn 1/4 revolution (if the shaft happened to be in the right spot) and stop. I can "help" it by turning by hand, but it still stops every 1/4 turn and the "bump" feeling is way more noticable under power.

    I can tell you what it's not.... it's probably not bearings, it's definitely not related to brushes on armature or tachogenerator (had them all out and still felt the same).

    Literature on the motor says other than tachogen maintenance, these motors "must not be dismantled" as this may lead to their partial demagetization. True ? If so, any way to avoid demagetization if one is careful ? Regardless, what the heck might be wrong with this thing ?

  2. #2
    machtool is online now Titanium
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    Sounds to me like one of the magnetic segments in the case may have come unstuck. Iíve seen that numerous timeís before, they behave like you describe. My local Servo motor rewind guy has good success re-gluing them back in, if they havenít shattered. They are very brittle.

    Itís a bitch when only part of it has come unstuck. I destroyed a fanuc yellow cap just a few weeks ago, one end of the magnet was loose, and it was rubbing on the armature. Tore it apart, and had to try and pry the rest of it off. I snapped the segment.

    That demagnetizing thing has some merit. But Iíve only ever known of it happening when the magnetic armature is left open for a long time with nothing inserted into the field. I always insert a dummy shaft back into housing, nothing lavish, just some stock that fits. Iíve even stuffed it full with a clean load binder chain. Works for me.

    Given its cactus, I donít see you have much to loose.

    Regards Phil.

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    Mark McGrath is offline Diamond
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    Is the "bump sensation" not the poles passing the magnets.I`ve felt that on quite a few dc servos.I think the de-magnetising is also overstated.I`ve had rotors out for a few hours with no ill effects.If I`m waiting on bearings or something I just stick the rotor back in the hole.
    Obviously something wrong with it if you can turn it by hand and it won`t spin on 30 volts.What resistance do you have across the leads?

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    Milacron's Avatar
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    Quote Originally Posted by machtool View Post
    That demagnetizing thing has some merit. But Iíve only ever known of it happening when the magnetic armature is left open for a long time with nothing inserted into the field. I always insert a dummy shaft back into housing, nothing lavish, just some stock that fits. Iíve even stuffed it full with a clean load binder chain. Works for me.
    I'm a bit mystified as to the "why" demagetization might occur without a dummy shaft. Is it due to the opposing magnets somehow ? Does anyone here have a drawing that shows what the inside of this motor is likely to look like ?

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    Sounds like 'cogging'. That's when the armature has gotten magnetized and you feel it when it's poles pass the poles of the field magnets. I had that happen on a big Gettys servo, we had a severe surface finish problem because it would make a divot in the profile every time the VMC table X axis hit that point. Gettys demagnetized the armature and serviced the whole servo, problem fixed. Their explaanation was that a severe overload, ie a crash, could cause the magnetization of the armature.

    I believe you can take the end bell off the servo and have a looksee at the commutator and brushes without causing a problem. Sometimes the end bell is adjustable for brush timing, mark the position before loosening it up.

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    Milacron's Avatar
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    Quote Originally Posted by Mudflap View Post
    Sounds like 'cogging'. That's when the armature has gotten magnetized and you feel it when it's poles pass the poles of the field magnets. I had that happen on a big Gettys servo, we had a severe surface finish problem because it would make a divot in the profile every time the VMC table X axis hit that point. Gettys demagnetized the armature and serviced the whole servo, problem fixed. Their explaanation was that a severe overload, ie a crash, could cause the magnetization of the armature.
    Interesting theory as this motor was rotating a rotary table where the morons pinned the table top to the table body. So anytime I (or anyone since they pinned it 9 years ago...I only know this from a pencil scribbling in the manual... "B" axis no longer used, 1999) tried to jog the B axis (not knowing about the secret "pinning" of the table) the motor would try to turn for a second but not succeed and then error. So I wonder if multiple attempts like that with the motor in the exact same position might have magnetized the armature ?

    If that turns out to be the situation I wonder if there is an easy way for me to demagnetize it "in house" ? Would this work ? http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INLMK3 Or this ? (with mystery ring ID) http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INSRHI

    I believe you can take the end bell off the servo and have a looksee at the commutator and brushes without causing a problem. Sometimes the end bell is adjustable for brush timing, mark the position before loosening it up.
    Been there done that...note my previous mention of removing all the brushes already.

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    3t3d is offline Titanium
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    Sometimes you see a "Flux Keeper" on a horseshoe magnet.
    It gives the magnetic lines a return path from one pole to another.
    This keeps the flux lines concnetrated, and thus stronger. Removing the keeper allows the flux lines to spread out and weaken. That is for "classroom" Horsehoe magnets.

    Some types of magents are more easily magnetized, but at the same time can "hold" more magnetism. Such as the ones in your motor.
    Some stepper motors are instant junk if you take the rotor out of the stator. When you take away the flux path provided by the assembled motor, the magnets lose to much of their magnetism. Without the flux path, the magnetic field "spreads out" and the field gets weaker as a result. And, those types of magnets can partially demagnetize.

    The other danger...
    It certainly is possible that locking the table might have driven the motor into an overcurrent condition. Again, too much coil current can demagnetize the magnets.
    They are powerful, but also fragile to unusal magnetic levels.
    They might need to be RE-magnetized. Again, without the proper procedures, you can Demagnetize the magnets with impoper dis-assembly.

    Thus many motor list a MAX current level. It is at this level that the coils can overpower the permanent magnets and damage them. That MAX level if applied for a few seconds might not hurt the coils, but might damage the magents.
    Flame away.

  8. #8
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    Mud:

    Did this just go down recently?

    Are youi the one that was having "catch" marks on your Gettys in a thread last spring?



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    Quote Originally Posted by Ox View Post
    Mud:

    Did this just go down recently?

    Are youi the one that was having "catch" marks on your Gettys in a thread last spring?



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    No, was 10-12 years ago. I still don't have the surface finish I want, still looking at drive tuning, resolvers, VOC gain, tachs, etc, etc, on 2 machines.

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    Quote Originally Posted by Milacron View Post
    Been there done that...note my previous mention of removing all the brushes already.

    I didn't miss that. I concluded "Should I open her up?" in your title meant you wanted to know if it was safe to do so. I concluded that you removed the brushes without disassembling it any further.

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    Quote Originally Posted by Mudflap View Post
    I didn't miss that. I concluded "Should I open her up?" in your title meant you wanted to know if it was safe to do so.
    The armature brushes are accessed directly outside the motor casing and the tachogen brushes are accessed by removing the end bell. I didn't preceive removing the end bell as "opening her up" any more than I would consider examining the inside of a persons mouth "opening her up" in comparison to open heart surgery...which is what is needed in this case. By "opening her up" I meant "all the way"...guts exposed, the whole deal

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    Oh I see. I can take all that off mine with the end bell still intact. The end bell is the last thing to come off before the armature comes out. Looks a lot like this -


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    Milacron's Avatar
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    Quote Originally Posted by Mudflap View Post
    Oh I see. I can take all that off mine with the end bell still intact. The end bell is the last thing to come off before the armature comes out. Looks a lot like this -
    Serious looking motor... looks like it has it's own squirrel cage fan and filter ??

  14. #14
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    BTW

    Per the mags neding "something" in there....

    My Snap-On mag screwdriver set has on the label that you are to always store driver with a tip in it "to retain magnetism".


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    Milacron's Avatar
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    I took the motor apart this morning. The rotor is not magnetic at all and the permanent magnet segments don't seem to be dislodged. I say "seem" because they are arranged in 4 poles, with 2 magnets in each pole, butting end to end. And 1 of the segments is just ever so slightly higher than the other where they butt together...just a few thousandths difference. If I press on that area it seems solid...so I'm thinking that's just the way it was attached originally....but not sure.

    With magnet housing seperated, on the workbench, I can pass a 1.25" steel rod down the middle, the magentism seems to be equal on each pole...I can imagine that maybe one pole is weaker...but if so, very little difference. I wonder how precisely the magnetism strength has to equal on each pole ?

    It was a bit dirty in there, so cleaned it up, put it back together and it acted exactly the same as before. So, getting real curious what these motors are "supposed" to "feel" like, I remove the (identical) X axis motor from the machine, and it feels smooth as silk...no "bump" feeling whatsoever.

    So, that bump feeling almost certainly is a clue to the problem, which means it has to be either unequal magnet segment strength, the slighly higher magnet segment... or bad bearings. I said earlier the problem wasn't the bearings...but now I wonder. I can't feel any axial play in either bearing but the shaft end bearing sounds a little suspect and I can pivot the tachogenerator end bell more than I might expect if the bearing was as new.

    But now I can't figure out how to remove tacho armature so I can get to that bearing...

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    Milacron's Avatar
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    FWIW, I found some literature that explained how to remove the tacho armature....so it's off and both bearings out. Be a few days before I get new bearings.... I will be a little surprised if that is the root of the problem, as the bearings just don't seem that worn... but sure seems most likely culprit at the moment

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    Photo showing internal segments. Note there is some pitting on one of the segments. This is not the segment I preceived to be possibly raised a few thou but rather one that seems level with the other in line.

    Below that is photo of the armature puller I had to make to get it off.





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    metlmunchr is offline Diamond
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    Is that evidence of some slight dragging on the bottom magnet or just discoloration?

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    Just a followup that new bearings didn't help at all. So I began to suspect a magnet segment that was just barely loose...not loose enough to show up with visual or feel inspection of the open housing, but loose enough to be attracted to and scrape the rotor when the rotor is in place.

    Alas, this is not the case either. I drilled an inspection hole in the end plate and passed an aluminum shim between rotor and magnet segments as I rotated the housing by hand, so as to check gap between rotor and magnent of every segment. I even did this with 30 VDC connected as well. The shim passed all the way end to end 360 degrees without obstruction..proof of no mechanical drag.

    So, you tell me...profound mystery.... could it be wire or insulation broke in the winding of the rotor somehow causes the rotor to be attracted to the magnet segments differently from a not broken rotor, such that a working motor feels smooth to turn by hand but this one doesn't...due to "gaps" or whatever in the rotor windings that shouldn't be there ?

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    Quote Originally Posted by Milacron View Post
    The rotor is not magnetic at all
    How did you determine that?

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