Diagnosing AC Condenser Capacitor Failure

[russ_watters]

So the starter capacitor in my AC condenser exploded a couple of days ago. I'm headed out to get a new one (found someone open!) and I'm just concerned that the failure could be just a symptom of a bigger problem like a bad compressor or fan motor (its a dual capacity capacitor that serves both). I bought a new fan motor accidentally(?) because at first I thought that was the problem. It was dark out, the fan was hot and humming but not spinning and I didn't open the control panel to find the blown cap.

Here's the thing: when the cap exploded, I heard it and went outside. The fan was spinning slowly, but laboring and the compressor was running. It took a few minutes before the fan stopped spinning and I'm a little concerned that leaving it on for 15 minutes or so could have damaged the other components.

I have a service appointment for Tues, but with some parts in hand I may be able to fix it myself. What I don't want is to blow another cap because the fan motor really was the problem when I have a spare fan motor sitting in my basement. On the other hand, if I install the new fan motor unnecessarily, I won't get my $100 back. Maybe I'll buy two caps...

Suggestions?

 

[Dmytry]

I would compare old motor versus new using multimeter. I.e. winding resistance, winding to case resistance must be infinite, the motor must be possible to turn manually, etc.
Especially, check the winding that is powered through the capacitor.

It is really hard to give the advice without knowing the circuit. Especially as you say that capacitor serves both fan and compressor motors, which implies some really strange circuitry.

 

[TurtleMeister]

It is really hard to give the advice without knowing the circuit. Especially as you say that capacitor serves both fan and compressor motors, which implies some really strange circuitry.

A dual run capacitor is just two capacitors in one case.

The fan motor has a thermal overload located in the main winding of the motor. If the overload is working properly it will shut off current to the motor before damage is done. Odds are the motor itself is ok - if it was the capacitor that initially failed. According to Wikipedia, run capacitors now have a limited shelf life due to the EPA requirement that PCBs not be used in their construction.

 

[Chunkysalsa]

My dad runs an AC company and a blown capacitor is one of the most common problems.

 

[russ_watters]

Thanks guys. The cap was cheap so I bought 2.

 

[Dmytry]

A dual run capacitor is just two capacitors in one case.

Ahh. I was thinking it actually shared capacitors between motors

The fan motor has a thermal overload located in the main winding of the motor. If the overload is working properly it will shut off current to the motor before damage is done. Odds are the motor itself is ok - if it was the capacitor that initially failed. According to Wikipedia, run capacitors now have a limited shelf life due to the EPA requirement that PCBs not be used in their construction.

Yep... well since he got a new motor anyway, he could check old motor against new motor with multimeter to make sure it's ok... won't hurt to make sure.

I think you won't kill the capacitor... the worst that can happen to capacitor is that the winding powered through that capacitor is shorted out, which could overheat capacitor eventually.

What is the capacitance?

 

[russ_watters]

What is the capacitance?

35/5 uf. Here's a pic of the wiring diagram for those interested.

It's in and seems to be working. The service contractor wanted $108 just to diagnose it. The capacitor cost $9.50 (and I got a spare). Thanks for the help, guys.