I'm refurbishing an old Black & Decker 6" bench grinder. It's got a power cord, an On/Off switch, and an A/C motor. The motor doesn't energize, even with a manual start, and I'd like to check the circuit.

The motor has three wires, and there's a gizmo I don't recognize. It could be a thermal overload protector, or it could be a starter circuit, or something else I haven't thought of.

I'm familiar with A/C motors having multiple windings for speed settings (common on fans) but this grinder doesn't have a speed control.

My questions are:

  1. What is the black Bakelite item?
  2. What circuit tests are appropriate to diagnose and repair?

mystery component in motor circuit

  • 1
    I'm going to guess it's some kind of old capacitor, but not sure enough for more than a guess.
    – JPhi1618
    Jan 15 '20 at 16:12
  • Any writings on it when u woul take it out. Can u make a close-up
    – Ride Sun
    Jan 15 '20 at 16:20
  • Is there a safety interlock button or equivalent in this tool - and if so is it possibly connected to this? then it would be a relay. Jan 15 '20 at 17:59
  • My guess is if you stop the grinder wheel from turning, that device will pop like a circuit breaker and shut it down. Maybe that little red recessed button will pop up so you can reset it. Garbage disposals have similar protection. Jan 15 '20 at 18:17
  • No safety interlock evident on the grinder. And the little red dot is solid - it is not actually a reset button, although it looks like it in the photo. When I can get back to it, I'll remove the bakelite box to look for markings on the top.
    – jbbenni
    Jan 16 '20 at 1:09

Single phase bench grinders typically use Capacitor Start motors, which would typically use a centrifugal switch inside of it to take the starting capacitor off line once it is at speed. But because of what a grinder DOES, those centrifugal switches are prone to failure when they get loaded up with conductive dust. So some grinder manufacturers use what's called a "potential relay" as the way to detect when it is OK to take the capacitor off-line, utilizing the back EMF of the motor to know when it is close to full speed. If that potential relay fails open, your grinder will not start. Usually they are larger than what that appears, but it's hard to judge the scale of your picture other than using the butt splices as a reference point. If the motor just sort of hums and/or shakes, that's the likely culprit. If the motor does absolutely nothing, as in no power getting to the windings at all, then that would most likely be an open thermal switch. But a thermal switch would usually only have 2 wires, not three. Potential Relays have three.

  • 1
    This is plausible and consistent with the third wire to the motor. I will remove the component to look for markings. I’ll accept this answer once I can verify it. The project is remote and it may take some weeks before I’m back at it. Thanks!
    – jbbenni
    Jan 15 '20 at 23:19
  • The motor doesn’t hum or show any sign of life, as if the circuit is open. I’ll put a meter on it to get more info.
    – jbbenni
    Jan 15 '20 at 23:21

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