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

Building Automation Basics for the HVAC Tech w/ Phil Zito (Podcast)
In this episode Phil from Building automation monthly comes on and talks building automation and demystifies some of how it works. If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE
Read more
HVAC Contractors Guide to COVID-19 (SARS-CoV-2, Coronavirus)
UPDATED VERSION AS OF 4/1/2020 As HVAC contractors, we are tasked with keeping staff and customers safe during the outbreak, and we are relied upon to provide some guidance to customers about how to protect themselves against the virus in the air. For employers and employees looking to navigate the new FMLA/FFRCA legislation, we put […]
Read more
Air Barrier vs. Vapor Barrier vs. Insulation
Think of it like this: It's a cold, wet, windy day. You can take an umbrella to protect you from the water alone, but that won't deal with the cold (temperature) or the wind (air convection). You can add in a light windbreaker, which will help keep the wind (convection) off. But if you also […]
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