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...or what type of motor is used there?

I found this type of motor - usually powered with low-voltage AC (~12V), but at times with 230V, in several appliances that require very slow rotation and sometimes a fair momentum - a color-shifting lamp, the microwave plate, an ice cream mixer...

The funny property of it is it picks the start direction at random and keeps spinning in that direction until switched off - but I never faced a situation when it would get stuck in the "unstable balance" position.

So, what is this type of motor and why does it behave that way?

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  • $\begingroup$ What do you mean by "it picks the start direction at random"? Do you mean that an individual motor will rotate one direction the first time you switch it on, then another way the next time you turn it on, and you don't understand how it chooses the direction? $\endgroup$ – Adam Miller Mar 31 '15 at 20:36
  • $\begingroup$ @AdamMiller: Yes; I stop the microwave and find the hot cup near the far end. I start it to rotate it closer to the door and half of the times it will keep rotating in the original direction, the other half it will reverse. I once tried to determine the rule, checking whether it remembers the prior direction and reversed it, but the choice between "clockwise/anticlockwise" seems to be entirely random. $\endgroup$ – SF. Mar 31 '15 at 20:44
  • $\begingroup$ I don't think this is a general truth of all microwaves. What brand/model do you have? $\endgroup$ – Chris Mueller Mar 31 '15 at 21:04
  • $\begingroup$ @ChrisMueller: Clatronic MW 721, although for the sample size of 4 different models where I paid attention to it, all 4 exhibited this behavior (but I won't find the models now). I took one apart when it broke down and the motor was a very short, wide cyllinder (about 2cm height, 5cm diameter). I found a very similar motor in a fancy "optic fibre lamp", rotating a colorful, transparent disk between the bulb and a bunch of fibres fanning out from the top, so that their tips shone with colors changing over time as various colors on the disk would filter the light. $\endgroup$ – SF. Mar 31 '15 at 22:03
  • $\begingroup$ (since the motor was buzzing in an annoying way my mother asked me to disable or remove it, so that the fibres shine just with white light, but the lamp stays quiet, that's why I took it apart.) One more thing, the motor axis is off-center from the cylinder. (I suspect there are some gears inside). $\endgroup$ – SF. Mar 31 '15 at 22:05
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The motor tends to be a cheap Synchronous AC motor. The design uses the shift in AC polarity (Going from Positive to Negative phases and back) to create a magnetic field in a coil, which interacts with a multi-poled permanent magnet. As the magnetic polarity shifts in coil, the magnet moves accordingly (opposites attract). Once it is moving, it's easier for the magnetic poles to attract. The permanent magnet is attached to a shaft, which goes through multiple gears to reduce rotations and increase the torque.

enter image description here

First, the middle of the motor is a plate. Underneath it is a coil in plastic bobbin. Now notice the hole marked 1. It has fins. Some come from the bottom of the motor housing, some from the plate that's hiding the coil. That plate will take the magnetic field from the top of the coil and pass it to the fins connected to it. The bottom housing will take the magnetic field from the bottom of the coil and pass it to the fins connected to it. These alternating fins create the Stators of the synchronous motor.

The coil and fins can be seen in this video:

https://www.youtube.com/watch?v=CzhcJDqQ_h0

There are two reasons for the motor to change direction. The first, is that the motor is cheap and nothing was added to force it to go in one direction. Typically more expensive motors, one of the gears will have a stop notch, that will prevent it from going backwards. It would stall the motor for one half of the AC phase, then continue the way it should, if it starts the wrong way.

The more relevant reasons is twofold. One, the fins that make the stators of the motor are not equal size. This is to prevent the motor from getting stuck, moving back and force from equal torque. (If you push a car one way, and then push it back the other way the same exact force and distance, the car will never move from that spot, just rock back and forth gently). Since the permanent magnet can stop between these uneven sized fins, next time it starts, it will get pulled one way or the other. And since the motor could start anywhere on the AC phase, so depending on how the magnet is facing compared to the stator magnetic field, it could be pulled one way or the other.

TLDR cheap motor with no directional stopping gear, loose tolerances, uneven fin/stator sizing, and uncertain AC Positive or Negative starting phase leads to the motor randomly starting in either direction

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Three types of motor may be used, both of which could do this. One of these (the Synchronous motor) is wat is used here and is a subset of the Brushless DC motor. (A misnomer as there is no pure DC used in the motor proper in a BLDCM).

The actual motor type is a synchronous motor, identified correctly by jpa. The synchronous motor is a special-case of the BLDCM (Brushless DC motor) that I describe below. In the general case a BLDCM generates an AC field from a DC source - either a fixed frTequency field that the rotor follows at fixed speed, OR from a variable frequency a source whose frequency is based on the current rotor speed and applied in such a way that the rotor "chases" the field which is derived from its own motion. (Phase lead/lad allows speed change - another subject). In the synchronous motor seen here there is a coil with winding axis vertical when motor sits flat on a surface. The coil connects to (in this case low voltage AC via a transfomer from) the AC mains so alternatively produces N-S or S-N magnetisation along its axis. Poles are created by adding plates with multiple radial tabs - each tab is a pole. As the coil changes NS, SN, NS the alternative tabs are all N or all S and as the field changes the NSNSNS... patterm moves in steps around the circumference. The rotor has N and S permanent magnet poles. These initially aligh in opposite phase wrt the stator poles and when these reverse polarity the rotor is attracted AND repulse to a position one tab away. However, if fully symmetrical, a N pole on the rotor could be attracted to the S to its "left" or the S to it's right. Ince rotating it will have a preference for the pole in its direction of motion but, as startup, could go either way. And does.

Stator pole polaritiesreverses succesively

NSNSNS ...
SNSNSN ...
NSNSNS ...

Rotor follows stator changes

(1) From here

   NS     <- rotor in position 3-4
 SNSNSNSN <- Stator

(2a) To here is valid

  NS      <- rotor moves left to position 2-3
 NSNSNSN  <- Stator changes polarity from (1) 

(2b) But, so is:

    NS     -> rotor moves right to position 4-5  
 NSNSNSNSN <- Stator changes polarity from (1)

In this case there is no DC - the field is suppluied from AC mains and the rotor "chases" the rotating AC field.


Motor types:

(1) Most usual in the past - Traditionally a "shaded pole" motor may be used where a "bodge" is used to distort the magnetic field from a field winding in such a way that a rotating magnetic "vector" is produced that the rotor follows. A magnetic shunt is produced with a turn of conductor at the airgap in the steel core that the field coil is wound on. When power is first applied the rotor position relative to the airgap will cause it to be jerked in one or other direction and once motion has started the rotating field that results reinforces that motion.

Shaded pole motors are simple, cheap, and have been around for almost ever.

Excellent laymans introduction to shaded pole motors - You tube video. 8 minutes.

Shaded Pole Motors - Wikipedia

enter image description here

(2) A brushless DC motor (BLDCM) may be used.

The synchronous motor described above is a special case simple subset of a BLDCM. In both cases a permanent magnet rotor follows a rotating AC field. In a 'true' BLDCM the fe\ield is usually generated electronically by switching DC. In these simple synchronous motors the rotating field is supplied from the AC mains via a transformer.

Motors that need a clean fast start use magnetic sensors that give absolute feedback on direction and speed. Motors that must rotate the right way (eg disc drive motor) may use sensorless systems that derive back EMF voltages from the motor windings BUT circuitry is included to check rotation and adjust the powering if direction starts wrongly. Systems that do not care about direction and that want lowest cost just use a sensorless system and accepts what comes.


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  • $\begingroup$ Most likely (1) unless someone hid a rectifier circuit inside the casing - these were AC motors (making it especially surprising; most AC devices are for mains voltage, and if it's 12V it's DC. In this case it was 12V AC (as written on the label on the motor, along with RPM speed of some.... 5?). $\endgroup$ – SF. Apr 1 '15 at 5:20
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    $\begingroup$ ...I've checked the links you provided and it seems a shaded pole motor is reversible only by mechanical modification (flipping the stator). Normally, if you apply AC it will always start in the same direction - so unless that's some obscure variant, that's not it. $\endgroup$ – SF. Apr 1 '15 at 16:58
  • $\begingroup$ I had a (very) old clock like this. There was a little knob in the back who's sole purpose was to spin it the right way after it was plugged in if it started the wrong way. You could reach behind it when nobody was looking and spin it the other way, and the second hand would start moving backwards at an otherwise perfect speed. $\endgroup$ – uhoh Oct 31 '16 at 6:29
  • $\begingroup$ Unrelated to this type of motor, but I had a battery wall clock that was standard EXCEPT the mechanism ran anti-clockwise. The time could be read easily enough once you realised what had been done, but it was otherwise completely confounding. $\endgroup$ – Russell McMahon Oct 31 '16 at 10:07
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It is a synchronous AC motor. It will spin at a precise rate relative to the AC frequency (50 Hz or 60 Hz). This is useful to keep the spin rate constant under varying loads, such as in a microwave oven.

From above Wikipedia link:

A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements.

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  • $\begingroup$ It seems you correctly identified the type, it looks exactly like the second image on the linked page (mine just didn't have the nice writing "synchronous motor") - now if someone could explain what exactly goes on inside so that the direction is not defined... but I'll do it in a separate question. $\endgroup$ – SF. May 7 '15 at 7:49
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Had a similar problem with my electrolux microwave turntable rotating when door was opened and stopped when closed. Also while rotating you could force it into the opposite direction. After checking the 3 safety microswitches which were found ok. Have noticed that mains polarity, live and neutral swapped influences this. The mains sockets i have are European so the plug can be inserted any way around, not like US or UK type. This has supprised me that some kitchen equipment can be polarity sensitive.

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