The question is straight forward. What are the protrusions attached to the main cylinder of this electric motor?

This one has two protrusions that are the same size. Others, like the motor for my table saw have two of these protrusions, but one is larger than the other.
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This motor has only a single protrusion:
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And some motors don't appear to have any of these at all. So what gives? What function do these serve, and why does that function appear to be optional, or at least handled in different ways for different motors?


1 Answer 1


Those contain the start capacitors that are needed in single-phase electric motors to get the armature spinning when the motor is first turned on. The start capacitor is connected to the AC mains and a small set of windings that accompany the main running windings in the motor. The capacitor delays the phase of the current in the start winding so that when the field winding comes on and turns the armature a little bit, the start winding then adds a slightly delayed extra kick in the same direction- an action which gets the armature turning instead of just vibrating back and forth, which is what it would do in the absence of the start winding and start capacitor.

Once the motor comes up to full speed, a switch automatically cuts out the start circuit and the motor is left running on the run winding only.

This is a task that is handled differently in different kinds of AC motors. Three-phase AC motors do not require start caps at all, since the delayed phases are furnished by the mains. This means 3-phase motors are self-starting, as are all DC motors.

  • $\begingroup$ On that model, one side contains a start capacitor, the other is the "run" capacitor. Some motors use "start/run" capacitors, with both capacitors in one object, but that motor mounts them separately when sold as a start/run model $\endgroup$
    – david
    Aug 11, 2023 at 7:57
  • $\begingroup$ @david, does that mean there are two run windings in this type of motor? $\endgroup$ Aug 11, 2023 at 16:01
  • $\begingroup$ a run capacitor may also be used on the shading coil of a shaded-pole motor. I don't know enough about electric motors to say how common different types are. $\endgroup$
    – david
    Aug 14, 2023 at 22:37

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