How to check a capacitor...

[mrcaptainbob]

The furnace blower was working fine a few days ago. Tried to start the motor today and it just hummed. Checked the wiring, all is good. I suspect the capacitor went bad. How can I check that it is bad? The numbers on it are: 85E96, Aerovox Z50P3710E, 1OuF 330-370 VAC 60HZ, PROTECTED A10000 AFC, 80-197 ENVIROMET, 8634.
And, where can I get info on how to read the values on something like this? I'm sure some are part numbers, but how to tell what the necessary stuff is?

[HayZee518]

all you need is the working voltage and the microfarad value. the rest don't matter.
you test it with a screwdriver. you short across one terminal to the other. if you get a good spark, the cap is ok. no spark, no good.

[mrcaptainbob]

Capacitor....

So, I check it with voltage applied to it? Do I connect it to the motor, try to start it, it doesn't. Then unplug the unit and THEN check it for the spark? ....

[HayZee518]

NO! try to start the motor. shut off power to the unit then short across the cap. never test a cap with power applied!

[Speedbump]

If you have an analog ohm meter you can test it by first shorting it with the screw driver, put the meter on the X1K scale. Touch one lead to the first terminal, then while holding it in place, touch the other terminal with the second lead. If the meter deflects to the right and settles back to the left slowly, it's probably good. Reverse the leads and try it again. If it deflects and drifts back slowly again, it's good. If the meter goes to the right and stays or doesn't move at all, the cap is bad.

[HayZee518]

Essentially Speedbump is correct however the needle deflection will work on ALL ranges.
You must use an analog multimeter because it puts the 1 1/2 volt battery into the resistance check function. with the probes across the cap, the cap will charge, then immediately discharge through its resistance, hence the upswing of the needle then downswing of the needle back to zero. reverse the leads and the same thing should happen. a bad capacitor will deflect the needle upscale and stay there or decay very slowly.

[Speedbump]

It has always been my experience that on the lower scales with low UF capacitors the needle will deflect very little, giving a bad or confusing test. The higher the Microfarad, the further the needle will deflect. With a run cap, which a ten UF is, on the X100 or X10 scale, the needle may not move at all. With a shorted cap, the needle will deflect and stay there. With an open cap, the needle won't move at all.

[mrcaptainbob]

Capacitor....

Great info. Thanks, guys! I checked it a different way. I bypassed the whole furnace electrical and wired the motor direct. It started and ran fine! So....something in the stove circuit is messing up. Thanks for the help, though.

[mrcaptainbob]

Close call...

What a close call that was! You see, the capacitor and blower motor are on a waste oil heater I'm installing in my barn. It is an old unit that came from a tool shop. Along with doing the normal cleaning, replacing and just general trying to learn this thing, I also made sure the wiring was 'proper'. Although I plugged in the unit to verify it did work, and the blower ran fine, I still wanted to go over everything. Found the ground wire from the plug cord just dangling and connected that to the unit. After dinkin' around with other stuff and satisfied with it all I found the blower would stall and the circuit breaker would pop. Finally traced it to the lead for the blower that ran through the 1/2" conduit. That lead is 'stuck' in the conduit! All the others came out. Needless to say, it's all replaced with fresh, good wire. And the blower works as it should....including having the unit grounded! A shorted, non-grounded unit. In a tool shop. What were they thinkin'?? Sure am glad for two things...the capacitor is good...and I found the short!

[HayZee518]

I kinda though about that as an after thought. I had a guy here locally that had an oil fired boiler with a standard plenum temperature switch. plenum temp goes up, contacts close, voila fan starts. but the fan wouldn't run. what is was, was, a piece of conduit ran inside the heat exchanger next to the blower housing to the motor. all the wires melted together and blew the contactor apart. I changed the position of the conduit outside of the plenum, ran new wires, changed the contactor and everything works.

[mrcaptainbob]

The fooler....

Yes.....and all along I was checking continuity. Well, of coarse they all did check. From now on, one of my 'continuity' checks will be to verify hot (and neutral) to ground.