Bob Engelhardt wrote:
The valve did open when the motor started, but it STAYED open even after
the motor was up to speed! Yes, the switch was open. WTF??!!

When I drew a schematic to visualize the situation, the problem was
obvious. The start winding was disconnected from the line but still
connected to the solenoid. It was acting like the secondary of a
transformer and generating enough voltage to hold the solenoid open!

If anybody figured this out without having experienced it, I am
impressed! It's something that I never would have anticipated.
Of course. It is not a transformer, but a two-phase motor. You said
something in the first message about the start capacitors. SO, the
starting switch just cuts in a larger capacitor to provide proper
phase shift during starting.

But, even a real single-phase motor with a resistance start winding will
produce an induced voltage from the start winding. This is how
potential relays are used to control the starting when you can't have
sparking contacts in the motor, like a refrigeration compressor.

To be precise about the mechanism, the start winding is displaced
angularly from the run winding (usually 90 degrees) and the rotating
induced field flux from the rotor passes by the start winding and
induces a voltage in it.

Jon