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Garage Door Opener Reversable AC Motor Problem

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Hello! I have a garage door opener that I would like to repair. I am posting here because it seems to have a Synchronous-Reversible AC Motor. It runs ok but usually will not start in either direction. What got me to post this is there is a black capacitor, with two poles, that when idle both have 120VAC on the terminals, When running there is 117 VAC on one terminal and almost 0 VAC on the other. One person told me it was just a start capacitor. To my thinking if it was taken out of the circuit the voltage to it would be switched off and both poles would be dead or the switch would be after the cap and both would still be hot. Ok what am I missing here ?? Oh and if you turn it to close the contacts close and it runs in the closed direction until it hits the ens and stops and then immediately runs the other direction until it stops. So it seems the directional control is lost ... I replaced the cap a while back ... I got as close as I could find on size ...
Any help here would be appreciated! Thanks and God bless! Joe
P. S. if there is 120 VAC on the hot terminal should I not be able to hook up a incandescent drop light to the terminal loosing power and ground and be able to see if the cap will conduct?
 
The capacitor is a run capacitor, it produces a phase shift in one of the coils so the motor starts. It's called a run capacitor because it's connected to the motor continuously, a start capacitor is only connected until the motor reaches full speed.

It's true that capacitors hold there charge but most capacitors like this are fitted with bleeder resistors and it should discharge fairly quickly through the motor even if there isn't one.

EDIT:
Yes, if the capacitor has a hole in it, it means it's broken and needs replacing.
 
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I am going to replace the cap ... just so I understand ... If there is 115VAC on one pole in this type of circuit it should be on the other pole too ... current flows through the cap all the time the motor is running ... True ... and thank you !!!
 
I'm not sure what you're asking.

You need a capacitor rated to at least 120VAC with the same value; note the latter is very important, if it's not within 20% of the original value, it won't work.
 
I have located one that is 370VAC .... 20% thank you!
While the motor is running I should read 120VAC on both poles ... now one side goes open under load ...
Current flows through this cap to supply current to its windings ...
 
As long as it's the same µF value then it'll be fine.
 
I have seen the voltage on run capacitors as low as 80V. Since the motor gets a lot of power directly to one winding, the other winding may not need full power. It depends on the motor.

[edit] You have crossposted this question in another thread on this site. Did you not like the answers from us? Do you expect different answers in the other thread? [/edit]
 
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Well before I registered with the site I posted on another thread. After that I decided to register and thy to start a new thread. I am sorry if it has been a problem. Please accept my appology.
Thank y'all for all the help!
Joe
 
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