220V AC synchronous motor : rotation direction ?

Discussion in 'General Electronics Chat' started by hobby16, Sep 5, 2010.

  1. hobby16

    Thread Starter Active Member

    Aug 30, 2010
    30
    4
    Hi all,
    I have a 220V AC geared synchronous motor like the ones used in microwave ovens : http://cgi.ebay.fr/Moteur-synchrone...ge_ElectroniqueComposants&hash=item56381ac12d

    The seller's spec says its rotation direction can be set depending on the half wave sign of the AC voltage at switch-on. I've browsed a lot of documentations on this type of motor but can't find if it's true.
    I want to know if with a triac controlled by an ATTiny45 (that I already use to slow the speed of my laminator), I can reliably control the rotation direction since with a uP, I can easily control the sign of the voltage. It will be used to raise and lower a small flag.
    If anyone with some experience on this kind a particular use of such motor can share his knowledge, I'd be interested to know.
    Many thanks in advance!
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    If it's like many of the motors I've seen driving microwave turntables the startup direction is totally random which would mesh with the seller's description as you never know where the household current sine wave is going to be when you turn the thing on.

    Always wondered how they did that on those turntables and this certainly might explain it.
     
    John Berry likes this.
  3. eblc1388

    Senior Member

    Nov 28, 2008
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    I'm skeptical about this claim. But it could have some truth in it.

    Controlling the rotation of a two wired synchronous motor is certainly possible as I have found out on my old washer timer.

    It moves the cams without fail back to fro, alternating with a few seconds in between. It is controlled by a MCU and the element to turn ON the power to the motor is a TRIAC. When I power the motor directly using external power, it moves both ways, randomly.

    So I come to the conclusion that the technique to dictate the rotation of the motor at starting is somehow torque related. When the motor starts, it will move to the direction that has the least amount of torque.

    Regarding marshallf3 comment of why turn table motor always moves in the same direction, it can be done via(I guess):

    1. a synchronous motor with a shaded pole construction. The motor starts as a shaded pole motor and becomes a synchronous motor after it has speed up.

    2. a bearing or sleeve that allows rotation in one direction only.
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Most micrcrowave turntables alternate direction at startup, it's a random pattern as to which way they will turn as if they too rely on the luck of the draw as to where the sine wave is when power hits them.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    What I'm saying is polarity of the AC supply *ALONE* might not be the sole factor in determining which way a synchronous motor would start rotating.

    The point being we have no idea about the orientation of the rotor before starting. See the image below.

    Power is applied simultaneously to both motors. Assume the rotor rotates clockwise in the left figure. Would the rotor in the right rotates in the same direction as the one in the left?

    [​IMG]
     
  6. hobby16

    Thread Starter Active Member

    Aug 30, 2010
    30
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    Correct, theorically, the right motor with a + start halfwave would rotate in the same direction as the left one with a - start halfwave. But in practice, there is always some mechanical tweak to ensure the motor would rotate from zero speed instead of vibrate, so who knows...
    The problem in my mechanical setup is the initial torque is systematically higher in one direction (raising of the flag) than the other (lowering of the flag) so I need to know if the rotation direction setting method stated by the vendor is reliable (if needed, I can even program the uP to send several same polarity halfwaves at startup).

    Well, I guess the only way to know for sure is get the uP running and try.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
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    Yes you do that and don't forget to tell us the test results.

    I always wonder is there a pure electrical method to control the direction of a synchronous motor since my washer MCU does it so well every time.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Must not be a very large flag, motors of that type usually aren't very powerful unless geared down quite a bit.
     
  9. DonQ

    Active Member

    May 6, 2009
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    Too much over-thinking the problem...

    If it is truly synchronous, then it undoubtedly has a permanent magnet rotating part (or an externally DC excited part with the same effect, but not in cheap motors).

    In the extreme case, the motor would have stopped with the poles of the rotor exactly lined up with the poles of the motor. When the AC is applied, the pole with go either N-S or S-N, depending on the polarity, regardless of the phase. Phase will only control the magnitude.

    If the magnets are in line, the poles will attract and not move, since they are already in line.

    If the magnets are opposed, they will repel. Which way will it rotate? Toss a coin.

    If the poles start off not exactly aligned, they will begin rotation depending on the side of 'center' they began from. They may actually go through a couple of false starts before the motor gets in step with the pole changes. Generally too fast for the eye to see, but they will jerk back and forth when the motor is not yet yanked up to speed when the pole alternate back to the other polarity.

    So it has more to do with where it stopped from the previous use, than the phase of the applied AC. Even controlling it's stop position, and the starting AC phase, would wind up with some starts in each direction because of the erratic starting.

    The shaded pole is used in induction motors (never seen it on synchronous) to guarantee a starting direction in this much less efficient motor type. It is possible to switch on and off opposing 'shades' to allow 2-way starting, but generally not worth the effort.
     
  10. hobby16

    Thread Starter Active Member

    Aug 30, 2010
    30
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    I'll do, next WK.

    Not sure for you but my washer and my dryer are all with asynchronous motors: the rotations directions are alternated by applying phase or neutral to the dephasing capacitor. I know it because I've repaired both of them several times and have downloaded their service manuals :D
     
  11. hobby16

    Thread Starter Active Member

    Aug 30, 2010
    30
    4
    It's like a citrus juicer motor : the rotation direction is random. But the vendor has confirmed that if you can control the sign of the half wave voltage at startup, you can control the direction. With a simple switch, I have no way to know if it's a dubious claim or not.
    But as I said, I can program the uP to control the sign, the number of halfwave at startup and even their phase delay. Do you think there is a way to reliably control such motors' rotation direction by software (e.g. 2 + halfwave followed by 1 - halfwave for one direction then, another sequencing for the other direction) ?
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You would also have no way of knowing exactly what position it stopped in the last time it was switched off.

    Unless you found some sort of incredible deal I'd keep looking, that or I'd have a sensor that immediately detected the startup rotation and tried to reverse it in extremely short order.
     
  13. eblc1388

    Senior Member

    Nov 28, 2008
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    I'm referring to the mechanical wash program control timer, which is driven by a small two-wire synchronous motor, with motor size similar to what you have just bought from eBay.

    It drives a series of cams and control the direction of the AC universal main motor.

    The MCU operates this synchronous motor to rotate in alternate direction smoothly and perfectly every time.
     
  14. David Babbitt

    Member

    Oct 8, 2010
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    If you read my new thread today, Oct. 18, 2010, regarding the AC synchronous motor driving the "tray" in a portable ice machine via a high torque gear train, my question is the same as those asking above. If it is externally torque based on which way it will turn the next time, it will need lots of that torque. If 100 inch-pounds of torque can be applied from outside the motor, then about 1 inch-pound of torque is applied to the motor shaft for a 100:1 gear reduction.

    Those are crude numbers. I know that if I try to grab the shaft and force the motor to start in the direction of my applied force, the motor can actually buck my applied force, so again, I don't know the answer. In my case, it must be predictable for CW or CCW direction and I still don't have that answer.
     
  15. eblc1388

    Senior Member

    Nov 28, 2008
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    Under the control of a microcontroller via a triac, a fraction of the full voltage can be applied to motor during starting.

    This will produce much less torque and encourage the motor to run in the direction of least torque as it basically don't have enough torque to overcome the resistance in the undesired direction.

    Then the voltage is increased to full value for normal running.
     
  16. David Babbitt

    Member

    Oct 8, 2010
    12
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    The control of the motors in portable ice machines (my situation) isn't via triac but 5 VDC relay, with contacts connecting 120 VAC to the motor. The microprocessor has no clue as to 120V 60Hz timing or phase angle. It just picks up the relay.
     
  17. ariemeir

    Member

    Jun 24, 2011
    68
    1
    Hobby16,

    Could you please share with me how do you slow down your synchronous ac motor with a uController ?

    I am guessing that I try to do the same thing as you, program my laminator to go back and forth in slow speed to avoid putting the pcb back in multiple times.

    If so, please advise - how did you end up doing that : both the speed and the direction of rotation ?

    Thank you.
    Arie
     
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