OT-Help needed, testing HVAC capacitor

I need some help testing an HVAC dual capacitor. I’m trying to diagnose rather than shotgun parts at this thing. The compressor comes on, but the condenser cooling fan does not. Please let me know if I am testing this capacitor correctly. Can a capacitor be bad and not be leaking or bulging? Here is the capacitor, the label says it is a 50+7.5 uF capacitor:






I discharged the capacitor and took it to my workbench. I put my multi-meter on the CAP setting, and checked between “C” and “HERM” terminals and get 21.37 uF. That reading tells me that the compressor side of this capacitor is bad, correct???



Then I check between the “C” and “Fan” terminals and get .27 n F. Note that is “nF”, and not “uF”. My multi-meter is auto ranging and I cannot change the range to read “uF”. Can I move the decimal and assume that the reading is .00027 uF? That reading just does not seen right to me.




I REALLY appreciate the help with this! Did I test it correctly, and do you think this capacitor is bad?

If your can assume your meter is "GOOD", the cap is "BAD"!

.27 nF ain't gonna start a fan motor!

Agreed. I'm surprised the compressor starts.

You'll laugh, but I learned how to test a dual capacitor from a youtube video, it was simple, showed me how to use my meter to do it, vs a capacitor tester or whatever the tool/meter is for that. I determined that my dual capacitor was bad on one side, this was for an 8500 btu window a/c unit, which I needed, hot out, fan not working, not an a/c tech, low budget, youtube is a great source of information on some things. I found a close replacement for $12 at a local supply house, the factory part through sears was $40 and that was not all that bad, sure save me some time and money and I was back up and running that day.

thank you both, very much

you tube has saved me many times, Billy! Hard to find good info on there though. One guy seemed like he knew what he was talking about, but he kept calling a capacitor a "coMpacitor". Kind of hard to trust a guy that can't even call it the right name!

But, the folks here at ytmag have never let me down

That reading to me suggests the terminal has come loose from the foil inside the cap and is just hanging on by a thread. Again bad cap.

One other check would be to discharge completely the bad side and put your meter on ohms. A digital meter may not read what you want due to high input impedance, but an alalog would. What you are looking for is a short circuit. Course a short circuit should have blown the breaker.

The C is in series with the L of the fan winding to produce torque and keep the power factor within reason so that the power company gets paid for the "juice" they supply to you. If the C is inadequate, inadequate current flows through the fan and it can't run. I never did it, but one other test would be to put your voltmeter on ac volts some range above 240v and measure across the 2 cap leads and the two fan power leads. If all the voltage is across the cap that helps to assure a bad C.

If you want more: The voltage across the cap and the motor have to do with the ac reactance (ac resistance) of each. If equal, equal and in series as they are, equal voltage will be developed across each.

The reactance for a capacitor is 1/(2xpixfreqxcapacitance in uf). As the C drops as you indicated, the reactance increases. With the number you stated, all the V should be across the high value of reactance of the C, none for the motor and hence it won't turn.

Been a long time. HTH,
Mark

The HVAC distributors in this area won't sell parts to DIY customers; servicemen will charge all he thinks he can get away with.

I looked on Amazon and found the capacitor that I needed and ordered it for $24 with prepaid shipping. Changed the capacitor in 10 minutes and heat pump is working fine.

Capacitors usually fail catastrophically, either open or shorted. (Usually they first develop an internal short, which is typically then blown, resulting in an open cap.) As you said, often you can find a bad cap just by doing a visual inspection. But before you bother trying to measure the capacitance value, first check for an open or shorted cap. Capacitance measurements are meaningless if the cap is blown.

You can check the cap with any multimeter. Discharge the capacitor, then put the meter on ohms and measure the resistance. A good cap will slowly build resistance until you see an open circuit. The switch the leads. The resistance should go negative, then swing back to positive resistance and finally and open circuit again. No sense measuring capacitance if it fails this simple test.

I find this is a lot easier with an analog meter where the input resistance of the meter is in something like 20k ohms/volt. With todays digital meters with their FET front ends and all the input resistance is in the Meg ohms and when trying to do what you said with a cap of the size used here, if it works at all it takes forever.

That's one reason I still have an old analog.

Mark

Mark, the input impedance of the meter doesn't matter much when you're on the ohms scale. In that case, the meter essentially acts as a current source, although I suspect a typical digital meter works at a lower current than an analog one. But if you're impatient there's really no need to fully charge the cap to tell if it's good or not.

I don't much care to check capacitors with an analog meter because I don't like to slam the needle against the left-hand stop.

Well that they did and the joules stored in a capacitor of that size can do a good job of that. But I never bent a needle over the years. Simpson was my favorite.

Mark

Thanks for all the help. I replaced the capacitor and the ac is
working great. Of course it was 90 degrees when the air was
broke, but right now it's 55 degrees outside