How does a washing machine drain motor work?

[max.wwwang]

I have been playing motors for a while and thought I understand how most of them work. But after a closer look at the washing machine drain motor, I was confused. How does it work? It’s even simpler than a shaded pole motor. I suppose this can only be a synchronous motor but how is the rotating magnetic field created? From the photo I can only see two coils in series and a core. Not showing is the permanent magnetic rotor. There’s a hi temperature cut-off device which is only for protection.

[Yansi]

Well, from what you tell and what I see, it indeed may be a synchronous motor. Look at the impeller in the pump, if it does not have a specified direction. Must work both ways. Motors like this usually do not have a defined direction of rotation, they just start whatever side they just want to. Same as the motor in your microwave oven that spins the plate. Also synchronous and turns in random direction.

[emece67]

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[xavier60]

The ones I see have a magnetic rotor and are bidirectional.
Also, the rotor runs in water so there is no shaft seal to fail.
One problem is if you do a lot of grinding steel, the steel particles can end up in the wash and stuck to the rotor which eventually jams.

[Yansi]

Hi, look for squirrel-cage motor.

Regards.

That requires rotating magentic filed. Here you don't have any. This ain't your regular AC induction motor.

[SeanB]

Permanent magnet synchronous motor. Whichever pole on power up that results in movement will cause it to spin, and the direction is going to be somewhat random, though to ensure it always spins the manufacturers of the magnets always do not magnetise them to be perfect, the magnetic axis will be a little bit off from the actual shaft, so that, even if it is perfectly in the middle of the 2 poles, there will be a slight difference in field between the ends, so it will move a little, and thus keep on spinning.

They rely on the pump not having to start against a head of water, so that the impeller can build up momentum in the first few cycles, before it starts to get appreciable head of fluid opposing it. Also very inefficient, but for the application perfect, as it can pump small solid particles without clogging, and if it does jam, the likelyhood of the rotor turning the other way with the next cycle is high, making them somewhat self clearing. Also large clearances, so they often can pump small amounts of sand or pebbles out via the hose, though long term it will destroy the pump.

Most common cause of failure is the stainless steel shaft wearing from fine silt, causing the rotor to bind in the housing, or the actual acetal bearing on the shaft doing the same, though I have seen them lose the impeller when the half turn clutch, there to allow it to run up before engaging the impeller, snaps off from old age or too many jams.

That is why you always hear the controller on the machine do multiple starts, one or two seconds of operation, spaced one or two seconds apart, for 5 or so times, so the pump can break free if it is dry, and has been stuck from detergent residue.

[max.wwwang]

Well, from what you tell and what I see, it indeed may be a synchronous motor. Look at the impeller in the pump, if it does not have a specified direction. Must work both ways. Motors like this usually do not have a defined direction of rotation, they just start whatever side they just want to. Same as the motor in your microwave oven that spins the plate. Also synchronous and turns in random direction.

Ok. That’s it. The impeller’s direction does not matter. And it sits in water so seal is not something to worry about. Basically the coils only create an alternating magnetic field which     very much confuses the rotor with a possibility of going back and forth. This does not happen thanks to the rotor’s moment of inertia.
Another intriguing thing-I put two sets back to back making them one transformer. The coils are identical. I applied 12V AC to one side but only got 7V or so at the other. I know there will  be some loss. But this surprises me.

[Mechatrommer]

maybe its just to create magnetic pole to turn a rotor out of place and open the drain? you are missing the important part ie the rotor.

[max.wwwang]

maybe its just to create magnetic pole to turn a rotor out of place and open the drain? you are missing the important part ie the rotor.

Haha, it’s now a transformer not two motors so rotors are gone.

[jmelson]

The secret to these motors is they have a 1/4 turn slip coupling between the rotor and the impeller.  This allows the rotor magnet to vibrate back and forth and build up amplitude until it jumps into synchronous rotation.  If you notice, the pump makes a rattling sound for a second until it starts pumping.

Jon

[amyk]

Aquarium pumps have this type of motor too.

[bsfeechannel]

I have been playing motors for a while and thought I understand how most of them work. But after a closer look at the washing machine drain motor, I was confused. How does it work? It’s even simpler than a shaded pole motor. I suppose this can only be a synchronous motor but how is the rotating magnetic field created? From the photo I can only see two coils in series and a core. Not showing is the permanent magnetic rotor. There’s a hi temperature cut-off device which is only for protection.

This is a Lavet-type stepping motor. It's of the same type used in quartz clocks and watches. And, of course, it is a synchronous motor.

If you look closely, you'll notice a slight asymmetry on the magnetic poles, which is crucial to its functioning.

I did a video where I used one of these motors as an poor man's growler to test the armature of the motor of a vintage Black & Decker drill I was trying to restore (or better, resurrect).

@3:02