Thanks for the info Sean. My interpretation of the wiring diagram (see pic) was that the capacitor was a motor start cap. Then I found the description below on Hurst's website; it's in their catalog too. The motors that I have are 9 W, 300 rpm, direct drive. Hurst specifies 0.85 mfd rated for 250 V for these motors. Hurst normally supplies capacitors with their AC, but not DC, motors. For the DC motors, they give capacitance and voltage specs and simply warn against using polarized capacitors. I have not been able to determine the type of cap Hurst supplies with its AC motors. My motors are new but came w/o caps and were surplus from a company that makes check- and currency- sorting machines. I'll be using them to drive low-speed fans in insulated boxes used to age cheese and wine. Motors will be outside the boxes with the shaft extending inside with a fan blade on the end (and cage around the blade, of course).
Mike in California
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From Hurst website, 2015: Why Capacitor Start AC Synchronous Motors?
All Permanent Magnet AC Synchronous motors manufactured at Hurst are of the capacitor start variation. These motors, which are classified in the sub-fractional group, find various applications in situations requiring frequent and prolonged starting periods. As the name suggests, these motors run at synchronous speed. The speed of a single phase AC synchronous motor can be determined using the formula Synchronous speed (in RPM) = 120f/p where f is the frequency of the power supply and p is the number of poles. For the same starting torque, the capacitor motor when compared to the split-phase motor requires half of the current for starting. The auxiliary winding of the capacitor motor has twice the number of turns of the split-phase AC motor. A split-phase AC motor basically has an inductive auxiliary winding. The lesser current in the auxiliary winding of the capacitor motor results in less copper loss and subsequently less heat generated by that motor. Because of the capacitor in the winding, the capacitor motor has the advantage of a greater phase shift between the current of the stator winding and that of the auxiliary winding. Phase shift is typically about 80% compared to 20% for the split-phase motor. The increased phase shift translates into easier starting for the capacitor start motor.