False Negatives on One Capacitor Test

I have spent the last few days checking run capacitors on various systems with several different meters, and this is what I found:


#1 – Comparing Start wire amps against Run + Common under the clamp together is meaningless as a practical test.

I used this test on 3 different systems with 3 different meters and came to the same conclusion. Whether the capacitor was way too large, way too small, or the right size, the test made no repeatable difference in the reading no matter how we read it.

Even if this is a valid test (which I cannot confirm at this time), the difference is within the uncertainty tolerance of the meter, so it's not useful for field testing.

#2 – The under load test does work (if your meter works)

Reading the amps at the herm (compressor start wire) terminal, multiplying by 2652, and dividing by the start voltage (herm to c) on the capacitor does work consistently on the compressor and the fan motor. However, some meters are less accurate at lower amperage readings, so that may make a slight difference.

#3 – Power Factor works as a test, but it's a small change

I tested several systems with the Testo 770-3 in power factor mode by installing too large and too small capacitors. The power factor did decrease in all cases when the incorrect size was installed, but in some cases, the difference was very slight (from 1 to .99 with a 15 mfd too small run capacitor in one case). That means that while it is a valid and useful test, it may not be sensitive enough to verify that a capacitor is slightly outside of allowable specs.

—Bryan

Related Tech Tips

Old Books
I've always liked old books. Think about an old printing press somewhere in Chicago or Boston or Scranton, Pennsylvania. Imagine workers with their hands covered in ink up to their elbows, setting type while giant machines of iron, steel, and brass stamp out a book page by page. Then those pages go on to be […]
Read more
Static Pressure and Combustion Test Ports and Plugs
My good friend and contributor to HVAC School, Neil Comparetto, made this video showing how he creates access ports for static pressure and gas combustion analysis. As techs, we find ourselves in the tough position of needing to drill access holes to take measurements, but the drilling and sealing of the holes can sometimes create […]
Read more
How to Keep From Getting Bored at Work
Almost nothing is as de-motivational and soul-sucking as boredom and complacency, and there is often no other place that we experience as much monotony as our work. Here is my top list of four ways to keep from dying of boredom in business. 1. See the Art in Your Work Many of us may be […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from