Workings of a microwave oven turntable motor

[Naahuel]

Hello.

This is something that's been on my mind these days and I just can't make sense of it. I took apart one of these:



Here's a datasheet: http://www.datasheet4u.com/datasheet-pdf/Hengxing/TYJ50-8A7/pdf.php?id=824711

It's a single-phase syncrhonous motor. I know the basic workings of a synchronous motor, but this one threw me off.

Here is someone's tear down:
http://rmdavidson.blogspot.com.ar/2014/09/tyc-synchronous-ac-motor-teardown.html

What I don't understand is the stator's coil winding. You can't see it in that tear down, but here's a similar motor:
http://rmdavidson.blogspot.com.ar/2014/10/suh-der-sd83-ac-synchronous-motor.html



The motor has a permanent magnet rotor and the rotor's axis is aligned with the stator's coil's axis.



¿How does that work? I've found nothing online, all the graphics and explanations I find have are in the most common configuration.

¿How does that configuration provide torque? ¿How can it start in either direction at "random"?

¿Do these small metal flaps play a role?

[elgonzo]

¿How does that configuration provide torque? ¿How can it start in either direction at "random"?

¿Do these small metal flaps play a role?

Read an overview here about why the flaps are important to form the magnetic poles of the stator.

Very briefly and simplified: The magnetic field of the coil has a magnetic pole on the top side of the coil, and an opposite magnetic pole on the bottom side of it. Metal tabs originating from the top side of the coil interleave and align with metal tabs originating from the bottom side of the coil, forming a ring of tabs. The tabs route the magnetic flux from the top and bottom of the coil and form the magnetic poles of the stator.

In your case you have a 4-pole stator. Note in your last picture that the 4 tabs of the stator are not placed symmetrically in 90 degree angle steps and are of slightly different size. This asymmetric arrangement could help controlling whether the rotor turns cw or ccw.

[amyk]

The asymmetry is just so it won't continue rocking back-and-forth (i.e. it removes the "neutral point") - it still starts in a random direction, dependent on the cycle of AC that it gets first.

[SeanB]

If you look at the AC clock motors they shade the one set of poles in the motor with a copper shorting ring, so that it will tend to start in one direction. As that is not reliable they also put a small sawtoothed gear and a pawl on a light spring, so that if it tries to start in reverse it will be locked, then the moving field will drag it around in the right direction.

The magnet is made with multiple poles on the outer face, so it will be attracted to the pressed steel pole pieces and then repelled as the AC field changes. The direction it starts is random, but the inertia of the rotor keeps it turning in the direction it is going provided that the load is not too high. If you lock the output the motor will reverse direction as soon as the torque required is greater than the torque generated by the slip, and it will then reverse sharply and run in the opposite direction.

As well you will notice that in a microwave the turntable will reverse direction every time you switch the oven off with a dish in it, from the gear train having a lot of slack taken up in the one direction it was moving, so with the new power application it tries to start in one direction or the other, but the reverse direction is easier to start in as there the gear train has slack so it can get up to speed before delivering power. Take all the slop out of the gear train and these motors will not start reliably.

[gxti]

I've used microwaves probably thousands of times but never noticed that the turntable goes in alternating or random directions. This is going to drive me nuts now.

[cosmicray]

I've used microwaves probably thousands of times but never noticed that the turntable goes in alternating or random directions. This is going to drive me nuts now.

Mine has a pseudo-random starting direction. About 80% of the time it will spin one way, and the remainder the other. The direction seems to be more predictable when I interrupt it to stir something, then resume. It usually reverses direction, but not always.

[Naahuel]

Thanks elgonzo, amyk and SeanB! You cleared a lof of doubts. The metal poles did play an important part

Mine has a pseudo-random starting direction. About 80% of the time it will spin one way, and the remainder the other. The direction seems to be more predictable when I interrupt it to stir something, then resume. It usually reverses direction, but not always.

I really don't think that's right (if it has this kind of motor). Can you really predict it's going to change direction with certainty? You would need so much information, like the position of the poles in the rotor and the polarity of the wave at the exact moment the motor starts. Right?

[cosmicray]

Thanks elgonzo, amyk and SeanB! You cleared a lof of doubts. The metal poles did play an important part

Mine has a pseudo-random starting direction. About 80% of the time it will spin one way, and the remainder the other. The direction seems to be more predictable when I interrupt it to stir something, then resume. It usually reverses direction, but not always.

I really don't think that's right (if it has this kind of motor). Can you really predict it's going to change direction with certainty? You would need so much information, like the position of the poles in the rotor and the polarity of the wave at the exact moment the motor starts. Right?

When I first turn it on, 80-90% of the time will turn CCW. When I stop it to check, after it has been turning, then restart, the direction of travel is more variable. There may be a technical explanation for this.

[meevblog]

I have a damaged one of these motors
Does anyone have any information about the winding?

It belongs to a roll laminator.

[lordvader88]

What makes them so hard to turn ? I had to use pillars just to rotate the shaft, and even then its still pretty hard to turn by hand

[fubar.gr]

What makes them so hard to turn ? I had to use pillars just to rotate the shaft, and even then its still pretty hard to turn by hand

It is because of the reduction gear mechanism. These are not meant to be rotated from the output shaft

[SeanB]

I have a damaged one of these motors
Does anyone have any information about the winding?

It belongs to a roll laminator.

$Easy, just take it apart carefully, by undoing the punched areas that hold it together, and get the coil assembly out. Then you can just rewind it with enamelled copper wire, the same 40SWG or so it was wound with, or just fix the end you damaged and tore the wire off, as you have a 50% chance the damaged end is the outer winding. Otherwise find a dead microwave and grab the motor off it as a donor coil, as it will be the same in almost all cases, though the gear ratio and possibly the output shaft might be different the coil will not be different. You can just change it, provided you did not get one of those microwave turntable motors with a 36VAC winding, which is driven from a 36VAC tapping on the fan windings. You can tell this by looking to see if the fan has 3 wires running to the coil.

[TerraHertz]

An incidental point: these motors are a good source of fine enameled wire. Since the coil former is always an open ring, and the windings are not potted or glued. The wire just spools off. And since every street-toss microwave oven has one, there are a LOT of tossed microwave ovens, and they have other useful things in them too, no one should ever have to buy this kind of wire.