Hi guy's just found your site and thought I would throw in a question, sorry if it is a bit long.
I have a problem with a single phase 240V 1/3 hp motor which has a 440 uf start capacitor and an unknow sized electronic starter switch, not a centrifugal switch, the motor is used to turn the large tub of an industrial cloths dryer. The problem stated to me by the owner is that the motor would start and run for a couple of hours, then after a few loads of cloths had been dried the motor would only start if the belts were pulled. I have tested the motor twice now, the first time I tested it it worked fine, the second time I tested it it wouldn't start until I pulled on the belts, on both times the motor had not been used for a few hours and was cold. On talking to my local motor rewinders I replaced the start capacitor, this didn't work, the next thing I tried was to test the starter switch, I removed it from the case but left it connected electrical and checked the voltage through the switch contacts, the contacts seem to be open then close when power is turned on, power into the switch is 240V but leaving is only 156V, the motor worked everytime I tried it. This time the rewinder guys told me that the contacts in the starter switch may of been stuck open and freed up when I removed it from its case. The thing I don't understand is that I thought the contacts should of been closed at start and then open after a given time opening the start winding. The rewinder guy's said that the switch sould be open and close at start, and that both the start and run windings will stay in circuit. It does seem that the problem will be with this starter switch, which I will be replacing tomorrow, but how do they workand how are they connected, are they in series with the start winding along with the start capacitor.
Thanks for any help, and sorry about the length of my question.
Tony.

Hello Tony

The switch must be closed at start in order to generate the single rotating magnetic field. If the switch is open, you get two counter rotating magnetic fields and the motor will shake and vibrate but not rotate.
The switch is a centrigugal switch that only opens once the motor has attained a high speed. - perhaps 80% of full speed. As the motor slows down at switch off, the switch will close again. It does not open and close at start, it opens when the motor is near full speed, other wise it should be closed.
Have a look at our page on single phase motor for more information.
Best regards,