Troubleshooting stepper motor drivers

If the centrifugal switch is OPEN when at rest, that's your problem. It should be CLOSED at rest and OPEN when at 80% speed.

When you stall and try to keep using a single phase capacitor-start motor, it slows down, then the centrifugal switch re-closes, putting the start winding and cap back into the circuit. But that winding is not designed to be used under load, neither is the centrifugal switch. You burned out the switch, maybe the winding insulation too.

This sounds like a combination of inappropriate cores with inappropriate power supply.

Do some math. The stainless steel bolt magnet has 37.5 feet of wire. #30 wire has a resistance of 103 mΩ/ft. The total resistance of your coil is therefore 3.9 Ω.

Now consider what happens when you connect that to a 9 V batteries. With 9 V across the wire, it should draw 2.3 A. That's way more than one of those wimpy 9 V clip-on batteries can source. As a result, the battery is abused and the voltage collapses. It's hard to say what current you actually get. It might be as much as a amp for a short while, but then will go down as the battery rapidly dies.

Ferrite isn't a good core for making magnets that go clink. It doesn't have the permeability of iron. Stainless steel is also not very good magnetically. With a toroidal core, it will be hard to tell from the outside whether there is much of a magnetic field inside or not. That's actually one of the points of using a toroidal core. The magnetic field largely stays inside. It therefore doesn't interfere much with the outside, and is likewise less susceptible to outside magnetic fields.

So:

1. Use a iron rod or soft iron bolt as the core. Mild steel is OK, but stainless not, and ferrite definitely not. Test the core you plan to use with a permanent magnet. You want the one with the best clink factor.

2. Use a straight or U-shaped core. Definitely not a O-shaped core.

3. Use a real power supply. One of those wimpy 9 V batteries is OK for powering small transistor circuits, but not for something that takes real current like a electromagnet.

4. Consider the power dissipation. If you had actually managed to put 9 V across your 3.9 Ω piece of wire, it would have gotten hot fast. Think about it. (9 V)²/(3.9 Ω) = 21 W! That would have gotten too hot to touch in well under a minute.

   A 5 V supply would be better. (5 V)²/(3.9 Ω) = 6.5 W. That's still a lot, but you'll have more time before things get too hot.