Today’s teardown subject is a motor that once spun the turntable inside a microwave. Emily Velasco salvaged it from a broken microwave and reused it in a kinetic sculpture named Dark Star.
Sadly, Dark Star met an unfortunate end when it fell off the wall. Among the debris was this motor, its output shaft now severed. I asked for the damaged motor so I could see what’s inside.
Information was stamped into the front and back of this motor. I read the following information on the front:
SYNCHRONOUS MOTOR TYJ50-8A19 Heng Xing RoHS CW/CCW E199324 100/120V ~50/60Hz 4W4/4.8 R.P.M.
And on the back:
Since “TYJ50-8A19” was stamped on both sides of the motor, I used that for my online search. Multiple Amazon vendors offered to sell very similar but not identical motors(*) and there were eBay and AliExpress vendors as well. (Some even have metal output shafts, which might have survived the fall.) Most of the listings described them as microwave oven turntable motors, some listings even had explicit model numbers of microwave ovens that used this style of motor. I guess “TYJ50-8A19” was the model number used by a specific manufacturer, but it has since been copied by others and became a generic designation. (For another example, see 28BYJ-48 unipolar stepper motor.*)
Front face of this motor was held in by the outer casing pressed inwards at four locations. Bending those tabs out of the way freed the face, showing this geartrain. The lubricant in this gearbox seem to get darker and more viscous (thicker) as we go from motor rotor towards output shaft. I can’t tell if this is because multiple different lubricants were used, or if this is the same lubricant responding to different stresses in use.
Flipping over the broken output shaft, I see broken plastic on the back side as well. Given how low the costs are for these motors, I doubt I could find replacement gears. The strength and precision required to replace this gear is beyond what my hobbyist FDM 3D printer is capable of, so I can’t make my own. If I had a resin printer I could emulate the shape, but I’m not sure if hobbyist level resins are strong enough. Another concern is the lubricant, which might damage certain resins.
There were six moving parts in this motor: the output gear/shaft, the rotor with a ring of permanent magnet mounted to a blue plastic hub, and four gears in between them.
A metal plate in the middle of the motor held four metal pins acting as axles for each intermediate gear. The axle for the rotor is a similar metal pin mounted to the outer shell. The output shaft which I had expected to receive the most stress does not have a metal axle shaft, a curious design decision that probably contributed to this motor’s demise.
Below that plate is… a single coil? This was unexpected. From the Wikipedia article for synchronous motor, I had expected to see multiple coils. Most of my teardown experience to date have been with DC motors, so I didn’t know quite what to expect with this AC motor. Given its price I knew it had to be simple to manufacture, but I hadn’t known they could be quite this simple in construction.
If it is indeed a single coil connected directly to the single phase of household 60Hz 120V AC, it would generate a magnetic field that flips polarity at 60Hz. In theory I understand that’s enough to get a rotor turning, but with a single phase there’d be no control over which way the rotor decides to start turning. This fits with the “CW/CCW” stamped on this motor, and ideal for the microwave turntable use case where we don’t really care which direction it spins.
But to make it work, what kind of magnetic field does this rotor’s permanent magnet need? In my mental model, aligning the magnet’s north/south poles to the rotor axis wouldn’t impart a rotational force as the electromagnetic field oscillates, neither would aligning the poles perpendicular to the axis. Now I’m curious and I want to visualize this particular magnet. I could buy a sheet of magnetic viewing film (*), or some ferrofluid(*), or some iron powder/filings (*). Or perhaps I could make my own metal filings? That will be a project for another day. Right now, I want to build on this experience and take apart another appliance motor.
(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.