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(This may look like a repair question, but I'm actually interested in how the simplest kind of motor failed)

This is the stator of a permanent-magnet single-phase motor from a washing machine drain pump:

PMSM stator

There's two coils in series with a thermal interrupter, and a permanent-magnet rotor goes in the gap. That's all.

The rotor spins with force (I can't stop it with my fingers) but irregularly, especially in the first few seconds after applying power:

https://i.imgur.com/137FDoU.mp4

Conventional wisdom says the windings are shorted, so I measured R≈12Ω, ~6Ω each half (it's rated 85W@120V; I don't know of a rule of thumb to estimate the number of turns). In case both halves had similar partial shorts, I unwound one down to the bobbin and didn't find stuck turns or visibly bare copper. The rotor turns smoothly, snapping magnetically every 180°. Shorting the thermal interrupter doesn't change anything.

So, how else can this simple device fail to turn evenly?

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  • \$\begingroup\$ Have you checked the bearings/bushings for whether they turn smoothly? I watched your video and wondered if it is rattling and jamming up mechanically. \$\endgroup\$
    – Kevin Reid
    May 4, 2020 at 1:52
  • \$\begingroup\$ Yup, I mentioned that the rotor turns smoothly though you can feel the magnet snapping to the stator every half-turn. It's not a mechanical issue. The rattling comes from the uneven velocity. \$\endgroup\$
    – arielCo
    May 4, 2020 at 2:17
  • \$\begingroup\$ You mention that you removed one of the windings. If you didn't already, you should just confirm that the resistance is 12 Ohms when it is all stretched out. I don't know what else could be wrong. Permanent magnet overheat and de-magnetized? Did the winding core fail somehow so that it is developing eddy currents? \$\endgroup\$
    – user57037
    May 4, 2020 at 4:04
  • \$\begingroup\$ "I'm actually interested in how the simplest kind of motor failed" - it failed in the washing machine? What were the symptoms? \$\endgroup\$ May 4, 2020 at 6:24
  • \$\begingroup\$ @mkeith, I think I measured again while it was loose, but anyway when I wound it back in a slightly different way the short should've been removed, no? I'm curious about the insulation between laminations. \$\endgroup\$
    – arielCo
    May 4, 2020 at 12:51

1 Answer 1

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I've run into a similar problem, and from my research, a countertorque of precisely the correct magnitude is required for this type of motor to start successfully and reliably.

See this patent: https://patents.google.com/patent/EP1428926A2/en The patent is for a mechanism that allows this to happen. I suspect that if you pull out the rotor of your motor (it is held in magnetically but should come out with a good tug), you'll see (or feel) a similar mechanism.

You probably aren't seeing correct starting behavior since the motor is designed to be started when submerged underwater.

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  • \$\begingroup\$ You're right - I installed it and it starts fine when the tub has water. IIRC I pulled out the rotor and there's no flywheel inside. \$\endgroup\$
    – arielCo
    Sep 5, 2022 at 2:08

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