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  1. #1
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    Default Shaded Pole Motor Hums No Start

    I have a shaded pole motor on a small right angle gear reducer that would be ideal for a diamond lap.

    It has no capacitor and only hums when powered. It will start and seems to run fine by manually spinning the motor.

    Bearings are good, no excess friction.

    The leads to winding connections are clean w/o corrosion.

    Any ideas?

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    It may have a start winding engaged by a centrifugal switch just under the back cover with a burnt contact.

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    Doug you might check that the shading rings in the pole faces are in fact intact. Sounds to me like at least one, maybe more, might be open circuit. Look for the minutest crack/s. Explanation: a shaded pole motor derives its approximation of a rotating field by the shading rings delaying the flux build-up on that edge of the pole face causing a sweeping effect .. that's it. No capacitor/s, no centrifugal switch, all pole windings identical. No easy way of checking shading rings that I know of except visual, maybe assisted with a gentle pry with a sharp bladed screwdriver under the outer part of the shading ring.
    Last edited by swarfless; 03-05-2020 at 05:24 AM. Reason: Added explanation.

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    Quote Originally Posted by swarfless View Post
    No capacitor/s, no centrifugal switch, all pole windings identical.
    Yes, none of that.
    I will examine the rings closely.
    Thanks!!

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    I would put a couple of drops at each bearing.
    It may seem free, but unless there is a nice oil film on the shaft in the bearing,
    it may have trouble running.

    Bill

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    No visible cracks but corrosion on the poles. No way to really clean it all that well.


    I tore the motor and reducer apart to isolate the motor friction from the reducer friction. It is one of those integral long motor shafts that supports the worm designs.

    Anyway somewhat odd 2 wavy springs and a flat washer to control the motor shaft ball bearing end play. Seemed way tight.
    Put back in one wavy and nothing else and it starts right up, then could hear one bearing was bad.

    Since everything is torn down, new motor bearings and seals have been ordered.

    I think the friction of cold gear oil, bad bearing and tight end play was just enough to keep it from starting. It is only 1/15 hp.

    I guess when I put it all back together I will know.

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    Yep, you're right about the oil, bearing & tight end play being enough to dampen its enthusiasm. Just for the record tho', & as not to waste the revelation of last night's sleep, there IS a way to check the integrity of the shading rings (don't worry Doug, I'm sure you're onto the real problem) & that is a German Growler, or internal growler. A rewind shop would have one. With the main windings disconnected, that is the motor simply switched off, the growler would indicate a shorted turn on each pole (the shading ring) if the shading rings are OK. Another out-of-field possibility, just for the record, is that reversible shaded pole motors did exist, with TWO shading rings per pole, that were deliberately discontinuous & the bridging wires brought out to a switch which enabled selection of which set of rings would operate & thus set the direction of rotation. In such a case a starting problem is likely in that switch or wiring. Marvellous how a sleep can mend a 55 year old memory ..

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    Quote Originally Posted by swarfless View Post
    Yep, you're right about the oil, bearing & tight end play being enough to dampen its enthusiasm. Just for the record tho', & as not to waste the revelation of last night's sleep, there IS a way to check the integrity of the shading rings (don't worry Doug, I'm sure you're onto the real problem) & that is a German Growler, or internal growler. A rewind shop would have one. With the main windings disconnected, that is the motor simply switched off, the growler would indicate a shorted turn on each pole (the shading ring) if the shading rings are OK. Another out-of-field possibility, just for the record, is that reversible shaded pole motors did exist, with TWO shading rings per pole, that were deliberately discontinuous & the bridging wires brought out to a switch which enabled selection of which set of rings would operate & thus set the direction of rotation. In such a case a starting problem is likely in that switch or wiring. Marvellous how a sleep can mend a 55 year old memory ..
    In the mid 60s, when I worked on such things, Barber Coleman made small reversible shaded pole motors. Instead of a couple of turns of heavy wire, the coils were finer, probably 22 gauge or smaller and many turns. A bipolar power transistor can be made to conduct both directions of AC if biased properly and I made servos controlled that way. As the bias increased, the transistor would conduct one half cycle, which would generate some torque, and a further increase would make it conduct for the full cycle.

    US3701601A - Photometric read out and analyzing system
    - Google Patents


    See the fifth drawing

    Bill

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    Well new bearings, starts in one direction.... most of the time.
    Hums and doesn't start in the other without a manual spin.

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    Since I was going to junk it I pressed the pole assembly out. Pretty rusty in there.

    I suspect that was the problem.

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    Quote Originally Posted by 9100 View Post
    In the mid 60s, when I worked on such things, Barber Coleman made small reversible shaded pole motors. Instead of a couple of turns of heavy wire, the coils were finer, probably 22 gauge or smaller and many turns. A bipolar power transistor can be made to conduct both directions of AC if biased properly and I made servos controlled that way. As the bias increased, the transistor would conduct one half cycle, which would generate some torque, and a further increase would make it conduct for the full cycle.

    US3701601A - Photometric read out and analyzing system
    - Google Patents


    See the fifth drawing

    Bill
    now that you explain that, that explains how the servos worked on the EA-6b antenna positioning systems i worked on in the usmc in 2008. they were 2 phase 400hz induction motor with a high ratio gear reduction but i could never understand how they worked. now that you mention this they weren't properly driving the motors with 2 phase inverters they were using transistors to dump current one way but not the other, the error loop was handled by 741 opamps and other very creative circuitry involving both synchros and control transformers, but i could never understand how they actually made torque.

    --the schematics in that patent bring back a lot of memories..

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    Shaded Pole motors have very low torque, so it doesn't take much in the way of bad lube, grime, bearing friction etc. to stall them. On the plus side, they tend to last forever if you keep them clean and maintained.

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    Quote Originally Posted by Jraef View Post
    Shaded Pole motors have very low torque, so it doesn't take much in the way of bad lube, grime, bearing friction etc. to stall them. On the plus side, they tend to last forever if you keep them clean and maintained.
    This was our old dishwasher sequence timer motor. I suspect the shading ring had broken because it started randomly
    forward or reverse. Trick was to start it, and if it shut itself back off, to start it again, until the wash cycle happened.
    And, to NEVER interrupt the wash cycle once it started, because it might start backwards when you turned it back on!

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    Yep, that's one of the consequences of a broken shading coil. Random directions.

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    Quote Originally Posted by Jraef View Post
    Yep, that's one of the consequences of a broken shading coil. Random directions.
    I always joked that if the thing went backwards it would make the dishes even dirtier.


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