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:
- What is the black Bakelite item?
- What circuit tests are appropriate to diagnose and repair?
Best Answer
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.