Difference between a start capacitor and a run capacitor?

John&Joey wrote:
So does one need to discharge that capacitor after unplugging?

Generally no as this application involves 120VAC vs 1Kv+ DC which can really kick.
The proper sequence if you want to be sure.

1. Disconnect power
2. Use an insulated screwdriver to short the capacitor's terminals
3. Remove the capacitor from the unit.

If the terminals are insulated (some form of plug etc.) you are good to go without shorting.

So does one need to discharge that capacitor after unplugging?

in other words
disconect ALL 120v power aka shore or generator
climb on the roof, remove a/c cover take a look at what is there
take some pictures (of wiring) get part numbers
replace one or two caps

if the a/c works again..great ..cheap fix
if it doesn't work
start shopping for a new a/c

An RV A/C WILL have two RUN capacitors. They may be in one shell, but there are still two RUN capacitors. One for Fan, one for Compressor. In addition there MAY be a Compressor START Capacitor. Maybe even Often, or Usually, but not Always.

The confusing part is the Compressor won't START with a bad RUN capacitor. Additionally, Compressor won't start if it HAS a START capacitor but it's bad.

In a single-phase induction motor, the phase-shifting property of the start capacitor is used to create a rotating magnetic field to get the motor turning from a dead stop. Once the motor is turning it is not needed and in most motors is disconnected. The run capacitor counteracts the inductive phase shift caused by the stator winding(s), providing power factor correction. The motor will run without it, running hotter and using more power.

If the motor never starts turning, the start capacitor would be the suspect. Of course, it's impossible to see this inside the compressor, and it could be a problem other than one of the capacitors, as suggested in the first response. Could even be a locked-up compressor.

I had no idea I was contagious.

Run capacitors can be likened to a higher voltage capacitor. Much larger per uf.
Same voltage rating. Run caps endure 24/7 rain or shine.

Sorry, I only know about a Flux Capacitor! :)

well, could be the start capacitor. if it fails, a capacitor start (as opposed to split phase start) motor usually won't start, or at least has very very low starting torque. the start cap goes out of the circuit once the motor rpm goes above a certain rpm. then the motor may or may not utilize a running capacitor.

so it could be a failed start cap or a failure in the contactor for the start cap. the basic circuit is a starting winding with a capacitor in series with the starting winding and a switch usually centrifical.

If it was a run capacitor issue, normally the motor would start to turn and then either not come up to speed, or would come up to speed, or close to it and then either stall or struggle with no run winding.

the other remote possibility is that motor is designed with a starting cap that is always in the cicuit (PSC). downside is lower starting torque, but higher than split phase, but the motor can also be reversed. In this motor if the cap fails (no switch involved) then motor won't start.

the other possibility is that the motor is trying to start to soon after shutdown. In that case, the high pressure from the compressor is high enough that the motor can't start. Usually an AC system is designed so it won't try to restart for a period of time (few minutes) to let pressure drop. If that circuit fails and allows the motor to try to restart then that may be the issue.