I had a service call to a building with a pneumatic thermostat system for the heat. The call came in because the fuse protecting the compressor motor had blown on several occasions over the last week. When I arrived the fused disconnect was in the off position and when I turned it on the motor would not turn and sounded as if it were seized. I immediately shut it off before the fuse blew. When I then turned it back on the motor started and everything worked fine. I repeated this several more times and again it worked just fine.

I am no expert on motors but my theory was that perhaps the motor had a bad start capacitor. Since the capacitor was only 10 bucks I decided to change it. So far it is working fine, but I'm wondering if anyone with more experience working with motors has any different opinions as to what might be the problem.

Thanks in advance

Use cap test on multimeter. It should have mf range printed on outside of cap. If connections are tight and right and ampacity and voltage readings are good, and cap. reading is bad, it is a bad cap. The motor may also read very amps beyond just inrush current at startup. Hope that helps.

IIRC there are few ways to test the capaitor before you do that make sure you " short " it out first before you touch the leads otherwise they will really kick whopper to ya.

once you discharged the capaitor and unhooked then take the ohmmeter it should start at infently then it will go down near zero the excat number wil varies a bit depending on the size of capaitor.

Merci,Marc

I think Marc meant to say that the needle or display will deflect or display towards zero and then go back towards infinity, settling on a value that would represent leakage resistance.
Congrats Marcerator!
Joe

Thanks Joe for get the correct word and yes that is correct.

Again keep in your mind each capaitor will have diffrent readings so expect little diffrent values even thru both look the same size and same rating.

Merci,Marc

If this is an intermittent problem I would think "start relay" before I jumped on the capacitor. If it was capacitor related I would be looking for a bad spade connector. That is the kind of thing that gets better if you poke at it or unplug and replug it.

Electrolytic capacitors have a fairly short lifespan, though, as little as 5 years in many applications. They gererally work or they don't, though, and aren't intermittant. So, preventative replacement of the cap isn't necessarily a bad thing, but the cause of the intermittant start is likely elsewhere.

One easy way to tell if a cap is going bad is if it's bulging or leaking electrolyte.

Electrolytic equals DC device.

Typical motor capacitors are MetalPaper or alike and lose permanently a little bit of their capacitance. But usually isolation remains constant because of a "self-healing"(direct German English translation) mechanism until end of life.

I have found that intermittant problems with capacitors have two causes, the first is most likely and both have been mentioned.

1-

Sorry, I have no idea what just happened while I was typing, but suddenly my screen was jumping around with my unfinished reply being sent. Oh wait, tab after the 1- will do it. Here is what I was looking at...

1- A bad or higher resistant connection which is usually an oxidized spade conector.

2- A failing, as in the capacitance is dropping below the needed starting level or begining to short within, capacitor.

If it continues to operate then you have corrected the problem. If it happens again, take it to a motor repair shop. If it is the centrifugal switch you can't fix it yourself.
You did not mention what size motor it is. If it's small, a new motor may be cheaper than a motor shop. If it's a special motor (non-Nema) then you need the motor shop or manufacturer.

Electrolytic were actually invented for use as a motor starting cap, and most work just fine at 60Hz.

Steve,

as far as I could "google" on that issue you're right for the 19th century. But I have never seen an electrolytic capacitor on an AC motor in my last 52 years, at least on this side of the pond.

POLARIZED Electrolytic Caps equal DC usage.

NON-POLARIZED Electrolytic Caps may be used on AC Circuits.

Start & Run Caps were typically Non-Polarized Electrolytics, as were Power Factor Correction Caps.
Now I believe there are options for "OK Caps" (Non-Polarized Electrolytics), and "Better Caps" (Polypropylene, etc.).

The last Start Capacitor I replaced - apx. 1998, was a Non-Polarized Electrolytic (the replacement item).

HID Ballast Caps were mostly all "Wet" Non-Polarized Electrolytics (pre-year 2000), but now they almost universally use "Dry Film" Caps, which I would imagine are something in the Mylar realm.

"Cheap-O" Crossover Caps are Non-Polarized Electrolytics.
Better quality X-Over Caps are Non_Polarized Polypropylene, and Mylar.

Scott

Okay, it looks like that there really is a significant difference in use, probably not due to different frequency but to single phase voltage of 230V.

I have learned something new to me.

A non-polarized cap is usually 2 polarized caps back to back.