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Last year I bought 3 motors from a junkyward, and one of them was a 1/4 hp, 4 pole single-phase induction motor made by WEG for an Electrolux washing machine.

Or at least it was supposed to be, cause most were disassembled and thrown in a pit, so I had to go around matching rotors and stators.

One was already complete, another was the correct match and this one I'm talking about I thought to be correct, turns out it wasn't.

Firstly the rotor is about 7 mm shorter than the stator (the stator is taller). The bearings aren't fully sitting in their sockets either.

But, most notably, the airgap is absurdly small. The rotor isn't scraping the stator by a miracle, cause the airgap is literally invisible. If I put the motor against a white screen or against the Sun I can't see light passing by any angle.

I've never seen such a small airgap, most are pretty visible and specially if you put against a light source.

I've run it by some minutes to see if thermal expansion would cause it to scrape, and it didn't, but it also heat a hell lot more than it was supposed. it reached 80 ºC in about 6 minutes, and most notably the rotor heated too much.

Could the reduced gap be increasing the rotor current?

The overall operating current seems to be close to normal from my measures.

It could also be that the rotor properties aren't the same of the original design, but from pictures it seem to be mostly identical and I counted the same amount of aluminum bars.

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  • \$\begingroup\$ If you make the assumption that they were in the junkyard for a good reason, it all becomes clear. \$\endgroup\$
    – Andy aka
    Sep 15 '20 at 7:35
  • \$\begingroup\$ @Andyaka They were cause the machines in which they came stopped for some reason and most people are just too lazy to save the good parts. The guys in the junkyard only save the motors in a better shelter if they are in mint condition, but the old and bad ones they just throw in a pit. Plus it's usually either the windings or the bearings that are bad, rarely both, that's why they disassemble them before throwing in the pit. And as 70-80% of them are 1/4 hp WEG motors and the same models appear numerous times, finding a match isn't difficult. \$\endgroup\$ Sep 17 '20 at 3:50
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If you have smaller air gap, it means that more magnetic lines from the stator are reaching the rotor. That should give higher induced voltage in rotor due to higher flux. However, it should not be much greater than with normal air gap. To current on rotor it would have really tiny impact.

Also, rotor current is a function of load. It may also be that you overloaded the motor, so it gets overheated. But the most probable cause is that heat from rotor has nowhere to go, and it is trapped between rotor and stator because of small gap. Rotor is not cooled fast enough.

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Reducing the air gap increases the efficiency of the motor by increasing the flux linkage between the stator and rotor, thus reducing losses. But it requires more precise bearings and bearing wear will result in earlier failure, so bearing cost goes up. This was nevertheless implemented in the 1980s as part of a government (US) mandated motor energy efficiency program run by the Department of Energy. You can read about it in some of the papers on this site: https://www.energy.gov/eere/amo/motor-systems

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