Electric Motor Shudders When Stopping. - Page 2
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  1. #21
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    Thanks for the update, Stuart.

  2. #22
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    Thanks to Stuart for starting this post and thanks to Phil for saying just to put a resistor across the start capacitor and thanks to Bill for agreeing resistor would be more likely fix.

    My problem was my lathe 5 hp motor 240v, single phase, 1420 RPM doing for past 20 years as above, a serious shuddering sound after motor has been switched off deaccelerating, I did change the motor bearings years ago. I dismantled the clutch and found nothing wrong.

    So I have been trying to get some of the sponge out of my Landcruiser brakes (I wish there was a Viagra pill you could just put in the brakes) but anyway when I saw this, on just putting a resistor across the start capacitors. I thought this sounds an easier job,so 18k ohm 10 watt and now that shudder noise has all but gone. So I had one win today.

    And Bill, you left out warmthrough and getting through Critical Speed in large steam turbines.

    Jim

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    Quote Originally Posted by Jim Kennedy View Post
    And Bill, you left out warmthrough and getting through Critical Speed in large steam turbines.
    That would have been my late uncle Ray. Victory ships (Murmansk run) and no other War Two and more. Stint ashore on a Westinghouse steam power plant, nuke fueled, not bunker C. Then back to sea in "steam round and round".

    Uncle Ed was Liberties, only. North Atlantic. Three sunk out from under his Engineering spaces arse. Injured, twice, but the world's most experienced crews at abandoning torpedoed ships got each other off without getting their feet wet all three times, soon had NEW hulls to suck more of those expensive torpedos and U-Boat krews right TF OUT of Weird-Adolphs tiny economy: A tanker as had the good manners to sink without burning, a Liberty full of explosives that was polite enough to sink without blowing up, and a general cargo loadout Liberty - foodstuffs, mostly - that was valiant enough to take its sweet old time going under and cost Weird-Adolph the price of an extra torpedo. Meanwhile, the crew knew she was only pretending to be "slightly wounded", was already a goner, were beating oars, opposite side "gerada AUS!" for the "bone collector' as if they were late for brothel passes!



    Ed spent his whole career afloat, even if now and then in a lifeboat, otherwise on "steam up and down".

    G'Dad did both - flush-deck "turbojet" four-stackers USN War ONE, reached the rank of "Commander", Merchant Marine, War Two. He did "all of the above" including forced draft heavy oil/Diesel. HIS Dad (and Uncle) had both been Master Machinists, Pennsy Railroad, so he had sort of a head-start on things mechanical.

    Somewhere along the line, Doenitz, Raeder et al finally twigged to what it meant for a piss-ant economy to go head to head with:

    - America's steel mills building ships and tanks faster than they could be sunk or shot-up.

    - An Aluminium industry providing for aircraft faster than they could be shot down until they literally "paved" the skies over Europe, wall to wall.

    Elderly Burgomeister of a small town just surrendered looked up at exactly that, entire sky full of daylight bombers, horizon to horizon, East to West, North to South, said to his "new Boss", a US Army Lootenant:

    "Don't they EVER STOP?"

    "No, old man. They are just as crowded the whole way back to the factory!"

    With Ford ALONE cranking a finished bomber out of ONE plant at Willow Run every 59 wall-clock minutes? It wasn't entirely an exaggeration, either.

    Annnnnnd Perfidious Albion and her economic tricks! London's clever Banksters. Bomber Command having done sort of a "hostile corporate takeover" (if ever was!) of large sectors of the German Economy, ruining it according to financing schemes of the Old Lady of Threadneedle Street and the vultures of Kornhill - no longer under Weird-Adolph's fiscal control!



    As with sex, I prefer recips, and straight-shunt DC, not AC nor AC/DC "universal wound", thanks!

    Simpler. Less confusion.

    No arguing all night over who has the right to do what, with which, and to whom.

    More time for happy smiles...


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  5. #24
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    The concept here is called a "bleeder resistor" for a capacitor start motor. When you start the motor, the cap is put in the circuit in series with the Start winding briefly to cause the phase shift and make the motor rotate. Once the motor gets to speed, the centrifugal switch opens the circuit to the capacitor and start winding because they are no longer needed. But in that process, the capacitor is left with a charge that takes a long time to dissipate via the series resistance of the capacitor itself. If the amount of time that the motor is running is insufficient for that to happen (could be 10s of minutes), then when you turn it off, the centrifugal switch re-closes and puts that capacitor back into the winding circuit, where it discharges as a DC pulse into the start winding. That DC pulse, though short and relatively weak, acts just like a DC injection brake, setting up a stationary magnetic field that tries to make the rotor follow it, creating that "thunk" and accompanying shudder.

    In most motors / loads, this happens so fast that you don't notice it. But in the sanding disc, the inertia of the load stretches out the time that the entire process takes, so the thunk becomes evident. Discharge resistors are therefor uncommon in every day capacitor start motors, but are recommended for larger ones and for machines with inertial loads. You want one that is about 15kohms and, to make sure it lasts, rated for 20W. Less is OK, it just may not last.

    Sorry for not seeing this earlier so you could have had that info before embarking on putting one it.


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