AC repair

One of my Air Condition Unit failed last Friday. I came home and found the out side heat pump fan not running, and initially thought it was a broken fan motor.

However after some googling, I started to think it might be the starter capacitor. I checked my upstairs AC unit, it has a slightly different capacitor but both AC unit are the same model, so they should be interchangeable (the broken capacitor is 5/30 and the other one is 5/35). So I swapped the capacitors and the broken AC unit started to work.

Both me and my wife were very happy, because if we call in a service man it could be easily hundreds of dollars.

So here are some of my findings:
1. If the fan stops working, but you can start it by spin the fan (try both directions), it is most likely a capacitor problem.
2. The capacitors has a voltage. It is OK to use a capacitor with a higher voltage rate in place of a lower rated one (i.e. a 440V cap can replace a 370V cap, but not the opposite).

I am still studying the way the motor starts. According to what I could find, the capacitor is used to start the motor, but I can't understand why adding a capacitor there could help reduce the start up current. I will try to consult some of my PHD friends :-)