Control of AC Synchronous Motor Question

Discussion in 'General Electronics Chat' started by David Babbitt, Oct 18, 2010.

  1. David Babbitt

    Thread Starter Member

    Oct 8, 2010
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    I have a 2 wire AC synchronous motor, low wattage, that runs the ice tray for a portable ice machine. It should run alternatively between CW and CCW rotation through its gear train to the tray that contains water. It should never skip a beat changing direction each time it is to run though it does pause at the end of each stroke for making or harvesting ice.

    How does the manufacturer maintain control of this 2 wire motor? It missed reversing one time and broke the drive end of the tray. I fixed the end but don't want it to break again. Limiting microswitches are at each end of the stroke which tell the ice maker's microprocessor to stop the motor.

    I found 12 poles in this clock motor drive. It is OK for random directions of rotation in, say, a microwave oven turntable, but it dare not make a mistake in the ice maker tray else things break. The motor may also be a stepper motor but it runs off of 120V 60Hz power. Thanks for your help and insight.
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I'm of opinion that the fault had occurred *BEFORE* the motor missed the correct rotation.

    When it last stopped before the event of wrong rotation, there wasn't sufficient mechanical force built up acting to oppose the travel motion. This could be caused by a limit switch detecting end of travel too early.

    Or the repeated travel operation had cracked the end stop in the first place ending up with less or no opposing mechanical torque.

    As you have mentioned in another thread the drive system has a 100:1 gearbox, the motor will thus rotate in either direction if there is no "desired" direction with least amount of torque.
     
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  3. David Babbitt

    Thread Starter Member

    Oct 8, 2010
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    I've also noted that upon disassembly of the motor, the 12 metal pole bars that split with 6 coming from one side and 6 from the other, the bars are not perpendicular to the coil ends but are cut at a slant. King of like Z shaped. Have others noticed that, too?

    I agree with the comment above that there is, by design in these Chinese ice makers costing around $200 each, very little counter torque presented to the motor when it stops in the "fill" position as compared to the "dump ice" or harvest position. I'll post a photo of the ice tray, motor drive end, later. Yet, another brand (Magic Chef as compared to Danby) follows the same design and works fine in our office. I pulled its motor off the Magic Chef and operated on 120 VAC 60 Hz. It would run CW or CCW, unpredictably, off the machine. But place it back on to the icemaker, works fine and predictably.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,777
    932
    The 'slant' you see, is there to reduce the 'cogging' effect. which is the magnetic attraction trying to hold the rotor in position over the stator poles.
     
  5. David Babbitt

    Thread Starter Member

    Oct 8, 2010
    12
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    Here are the photos I promised of the ice maker tray and motor. Photos were taken of ice tray for determining that 8 oz.in. is the external torque on motor during fill or harvest. Note the repaired drive end of ice tray
    using split wooden dowel. Note gears and slanted pole pieces of stator.

    By comparing gear train diameters, the calculated torque at output shaft is 143 that of the drive motor. So the weight of the tilted or whatever ice tray at 8 oz.in. of torque produces only 0.054 oz.in. of torque to try to force the motor to spin in a preferred direction. I contend it isn't enough to force the motor to drive either CW or CCW. Yet it happens correctly on the ice machine (Magic Chef) that is still running. I still don't know what preferencially turns the motor back and forth.
     
  6. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
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    First time I've ever seen a peanut used that way.
     
  7. David Babbitt

    Thread Starter Member

    Oct 8, 2010
    12
    0
    In addition to my photo and Word document, attached, I would like to conclude my situation.

    1) The motor has a stalling/reversing torque value of 192 oz.in. which the motor drive end, ice tray, and "back stops" should be capable of handling.
    2) The preferred motor drive design is for a D shaped motor shaft fitting a D shaped ice tray end. Contrast that to the pin drive I'm working with that provides point loading to the slots in the plastic tray, leading to cracks.
    3) Timing of the motor/tray with the two microswitches is critical. The arc the motor/tray take should be centered between both switches with switches closing before the backstops are reached, yet not moving so far as to damage the microswitches.
    4) Then for the purist, the wrench/wooden stick adds to the true reversing torque to achieve 192 oz.in. The wrench/wooden stick on their own created 3oz. x 12" = 36 oz.in. of torque. Subtract that from 192 oz.in. for the true reversing torque of the motor.

    As for the peanut used to help keep the motor plate in place, will feed it to the squirrels or let the cat play with it.
     
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