Why won't my motor spin?

[Robert Reed]

Well the mystery is solved and I thought I should share this with you before I close out this post. I did some experimenting with the one shot timing which slowed down rotation speed. At 50 RPM the motor worked perfect just as the circuit was designed to do. Decreasing the one shot timing and conversely increasing RPM, the motor continued to perform OK up to about 500 RPM. Beyond that point it began to perform erratic and finally stall and hum. What I had suspected in my last post was the reason and it all comes down to mass ,inertia and momentum. When the rotor is attracted to the next energized pole we have motor action, however when it reaches that pole it stops and will sit there until that pole is de-energized. When that pole is de-energized and the succeeding pole is "lit" up, the rotor once again goes into motor action by that attraction and then continues that GO/STOP action continuously. This happens at any speed of rotation but at slower speeds and longer pulse lengths the rotor has ample time to overcome inertia and still make to the next pole while it is still energized. At the initial speed I was trying to achieve (3000 RPM) the time to overcome inertia from a near standstill was longer than the pulse width of energizing the next pole in line, so by the time the rotor started to move, that following pole was de-energized and the drive pulse was already moving to the following and further away pole. Now the rotor just picks some point in rotation and won't move - just slightly rock a little. A smaller lighter motor with less inertia would probably spin faster to a point. A heavier motor with more inertia would never spin fast at all. What is happening here is when the rotor that's chasing a revolving magnetic field finally catches up with it, it will just sit there until that field is removed, which is why I reasoned that the pole that it is attracted to should be de-energized just before the rotor reaches it and the following pole lit up at the same time. That way the "chase" never ends as its momentum will swing it past that pole while the next pole draws it towards it. As I had mentioned I have no way of doing this externally. I did locate a small fan type motor that will match right up to the inlet port in the equipment's housing. If it lives up to its specs as to CFM air flow I will leave it in permanently. At that time I will disassemble the old motor and see if its even repairable, but the way its put together I am pretty sure I will ruin it in the process.

[Lenp]

This qualifies as an Eureka moment!
Thanks for the follow through. Too many people wander off and we never know the results.
Len

[jwax]

Indeed, thanks Robert! That could be why they don't use a simple pulse generator to power a brushless motor!
The Hall effect device allows the motor to "come up to speed".
Appreciate the education!