Electric Motor Shudders When Stopping.
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    Default Electric Motor Shudders When Stopping.

    I have a Rockwell 12" sanding disc powered by a 1.5 HP, 230v single phase motor. When its running and you shut it off, as the RPM's wind down and it comes to the centrifugal switch threshold, you hear the switch 'click' and something happens inside the motor and it give a momentary shudder. Because the machine is housed in a floor mounted sheet metal stand, it give a noticeable 'rattle' as all the tin shakes.

    Not being very smart..is this some kind of back EMF kind of thing, not even knowing what the heck that means. I find it strange as there is no power applied when this happens, it's only coasting to a stop.

    Stuart

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    Perhaps when the switch jumps back it's not in balance anymore?

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    Cntr. switch drops in and discharges the starting cap into the start winding, not a big deal. A 25k bleed res will fix that...Phil

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    Quote Originally Posted by Phil in Montana View Post
    Cntr. switch drops in and discharges the starting cap into the start winding, not a big deal. A 25k bleed res will fix that...Phil
    I agree with Phil's post , although we have a large disc sander that does the same thing difference is ours is 3 phase 480 volt.
    So there's no capacitors involved. With ours it's a balance issue, shudders during the wind down. done it for years.

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    Quote Originally Posted by dana gear View Post
    . . .With ours it's a balance issue, shudders during the wind down. done it for years.
    I've got one that does the same. Runs pretty smoothly at speed, but at a certain point spinning down it shudders badly. It's a harmonic thing. Regarding the start cap, I don't buy it. If that were happening all similar motors would do it. Seems to me that a start cap might have enough zip to cause a single 'thunk' or something, but not a shudder. Been wrong before though!

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    The shudder phenomenon might be better described, as Gordon put it, as a 'thunk', and the 'thunk' it timed exactly with the snap of the centrifugal switch, so my thinking is that Phil might be on to something with the cap idea. There is a lot of rotating mass and a lot of tin structure so that 'thunk' shocks the whole apparatus and magnifies it.

    Everything works fine, I just didn't know why it was doing that. If I knew where to stick a bleeder resistor, I would do it just for grins to see if it indeed cured the 'thunk'.

    'Thunks' for that,

    Stuart

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    I'd be checking the bearings.

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    Sounds like Phil is on to something alright. The bleeder goes across the cap terminals. Easy peasy.

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    Quote Originally Posted by Mike C. View Post
    I'd be checking the bearings.
    Why? When bearings go bad they invariably are worse at speed, at least for noise. Bad bearings aren't going to manifest at a single brief point in spooldown.

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    Take it apart and see if something's coming apart in the centrifugal switch mechanism.

    metalmagpie

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    I completely rebuilt the entire thing so I know 'who's on first'! No bad bearings or broken switches..it acts just like Phil and Gordon noted, a thunk when the cap discharges into the start winding as the motor decreases in RPM and the switch closes again. The bleeder resistor thingy will 'speak the truth', I hope!

    Stuart

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    It will. And you don't have to be fussy about a test. Just wrap the resistor wires around the terminals below the wire connectors (assuming spade terminals) and give it a try. Don't need to bother with solder or connectors for a test.

    Oh, you might need to let it run for a minute before shutting down for a test. The higher the resistor value, the longer it will take for the cap to discharge. A fairly wide range of values can be used, mainly you need to keep the power dissipation through the resistor well below the resistor rating.

    Most of the bleeder resistors I've seen in similar applications are in the 1 to 2 watt range for fairly rapid discharge.

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    Imbalance and the critical speed happens to be at the switch speed
    At start-up it passes the critical speed much faster so it is not noticed
    Take of the disk and see if it is still the same
    Peter

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    Peter,

    From close visual observations and troubleshooting via a mechanics stethoscope it seems fairly obvious that the motor is causing the schism, not any out of balance phenomenon with the disc or associated rotating components.

    If I was talented enough I would attempt to take before and after videos just for fun and to share here...everyone likes videos, right!

    Stuart

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    Both my Delta radial arm saw and rockwell delta unisaw have a, I assume purposeful, slow down mechanism that stops them in maybe 5 seconds. Where my Baldor grinder will run for several minutes after hitting the switch

    bug or feature?

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    Way back when I worked in a machine shop making rotors for ultra centrifuges, we always had a problem with precession causing unwanted vibration, clunking etc. This would occur at acceleration and deceleration until they changed the design. They went to a drive shaft that was only 3/16" in diameter which was very strong and very flexible. The driveshaft would actually assume an "S" shape to compensate for the difference between the physical center and the center of gravity (precession). These were titanium rotors weighting over 20 pounds spinning at over 100,000 rpms with a 3/16" drive shaft. Crazy. Perhaps that is what is causing the shudder.

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    Well hell...my sander IS spinning at 100,000 RPM. That answers why the sticky-discs keep flying through the wall!

    Stuart

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    yes. That is exactly why that is happening.

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    A sad update..worse than Pakistani tie rod repair!

    I removed the motor and soldered in a 15K, 2 watt resistor and it did make a difference, but did not cure it 100%. I'll live with it as is because removing and reinstalling the motor is akin to sliding down a 50 ft razor blade into a pool of iodine.

    Stuart

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    Quote Originally Posted by atomarc View Post
    Well hell...my sander IS spinning at 100,000 RPM. That answers why the sticky-discs keep flying through the wall!

    Stuart
    ....removing and reinstalling the motor is akin to sliding down a 50 ft razor blade into a pool of iodine
    Damn... yer good!



    Sure didn't see THAT one coming! Good my cawfee cup had just gone empty, or there'd be another keyboard-laundry project.

    Resonances surely exist. USS Washington's props, Lockheed Electra's tearing their wings off, forced-draft at flank on some destroyers, the uber-liner "hot rod" United States ringing her whole hull like a bell at full-gallop off a repurposed Iowa-Class propulsion plant kept out of public eye, MANY general-aviation aircraft with high-performance props placarded to pass quickly THROUGH one of more specific RPM, never stay there for cruise, the old WTC "Twin Towers" when the wind was just-so..BUT..

    This case, the capacitor & bleeder were far the more likely fit to sort it.
    That said, I'd probably just ignore it!

    Closest substitute does save that 50 foot run, though...

    Reachin' for uber-gentle pale green Aloe Vera crotch lotion... grabbing the Wife's pale green nail polish remover - dunno.. Acetone, mostly.. with synthetic oil of wintergreen?

    Surely weren't no "figure of speech".
    "Fireball of SCREECH!" wuz more accurate!

    Damn that shit is persistent! That 'lectric motor don't know SQRT-FA about "shuddering!"

    Sure I was gonna DIE, and worried only it weren't gonna be SOON enough so the wife wouldn't twig to what a careless FOOL I wuz... and crack up laffin!

    Wicked sense of humour some wimmin' have.



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