The photo below shows a bench grinder. If you look carefully, there are only two wires for the main winding. There's no capacitor or inductor. I also verified this using the parts diagram.

I was under the impression that a single phase induction motor cannot start on its own without a capacitor or inductor connected to a secondary winding.Induction motor

My only guess is that this motor is a shaded pole motor, but I was under the impression that they are too under powered for this application.

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    \$\begingroup\$ Lots of 1/4-HP shaded pole motors. How many nameplate HP is this one? Looks like a shading bar visible too. \$\endgroup\$ Oct 31, 2016 at 19:42
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    \$\begingroup\$ @user148298 Sears actually once sold lathes. People don't make metal stuff at home like they used to. See the old 1960s Popular Mechanics for what an ordinary Joe sixpack could do on the weekend. \$\endgroup\$ Oct 31, 2016 at 19:57
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    \$\begingroup\$ @SpehroPefhany Don't worry, should society suddenly collapse and practical skills be in demand, we'll have no shortage of people who can switch LEDs on and off. \$\endgroup\$
    – Ian Bland
    Oct 31, 2016 at 20:22
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    \$\begingroup\$ I it must be a shaded pole motor There is no sign of an auxiliary winding and there is something that looks like a shading bar. The grinding wheel doesn't count as load nor does the angular momentum help to start. F=MA says that no matter how small the force, there will be acceleration. The momentum will determine how long it takes for the wheel to get up to speed. As long as there is enough torque to overcome bearing friction and turn the fan, it will get up to speed. The motor torque will increase as speed increases until it reaches a peak at 3/4 speed or so. \$\endgroup\$
    – user80875
    Oct 31, 2016 at 20:58
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    \$\begingroup\$ +1, but having too much angular momentum will make the rotor heat up because of high losses at low speeds and may trip a temperature fuse. \$\endgroup\$
    – Janka
    Oct 31, 2016 at 21:05

1 Answer 1


It looks more like the old shaded Pole, with low starting torque that peaks before max RPM. (No old jokes please.)

  • \$\begingroup\$ I don't see a commutator at the OP's motor. \$\endgroup\$
    – Janka
    Oct 31, 2016 at 21:11
  • \$\begingroup\$ @Janka the heavy copper tab is actually the edge of a heavy copper ring. It's off center, producing a phase-delayed field through the iron in the center of the shorting ring. That gives a weak rotary field for starting torque. Look up "shaded-pole motor," an induction motor invented in the 1930s. (It appears that the copper ring is a "U" shape, inserted into two holes, then a copper segment welded across the tips of the "U." en.wikipedia.org/wiki/Shaded-pole_motor \$\endgroup\$
    – wbeaty
    May 19, 2020 at 1:37
  • \$\begingroup\$ @wbeaty: Please see my comments on the question. \$\endgroup\$
    – Janka
    May 19, 2020 at 11:59

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