Summary (also see photos of fan and motor below)
I bought a capacitor for small dollars and now the fan works again, perfectly.
Update 1: Capacitor was indeed broken (see photo at end)
Belt and upper pulley fell off 40" attic fan
Attic fan motor turns when not connected to anything
Fan blades turn easily by hand
When I put the pulley and belt together the motor does not turn the fan at all.
What should I try?
Here are some more details.
My large (40" diameter) attic fan has worked flawlessly for years, on a thermostat. Recently we have not heard it.
I climbed into the low ceiling attic and found the 18" belt on the floor and the 7" top pulley on the floor as well.
The motor turned when the I manually triggered thermostat.
I put the pulley back on the shaft and tightened it in place with the set screw. Then I put the belt back on the pulleys and triggered the thermostat. I replaced the belt with my bare hands, so it cannot be very tight.
Surprise: the motor tries to turn (jerks in place) but does not turn.
The fan blade itself is easy to turn by hand.
Why did the pulley fall off and what should I try now to get it running again?
If you need any more info, tell me what you need.