An even EASIER way to check capacitors


In a previous article, we covered the standard way to check capacitors under load.

I am now going to give you an even easier test.

A properly functioning PSC (permanent split capacitor) or CSCR (capacitor start capacitor run) motor should have a power factor of very close to 1 if they have a properly sized and functional capacitor.

If you have a multimeter that can read power factor directly (like the Testo 770-3), you can measure the power factor by reading the voltage at the contactor and the amperage at the motor common (like usual). If you are at or close to 1 power factor, then your capacitor is both functional and the right size.

In the image above, I have a compressor that calls for a 35 MFD capacitor, and the capacitor is running right at 35 in the under load test as well as the bench test. That's why the power factor is right at 1. I installed a run capacitor of 10 MFD larger and smaller, and sure enough, the power factor dropped in both cases.


So, not only do I see right away that either my capacitor is failing or improperly sized, but we can also see the wattage (power) in real time.

As a side note, I was only seeing about a .3 amp difference between start and run/common together when I oversized the capacitor, so I may have missed the issue if I had just used that test alone.

Now, in order to see exactly WHAT is wrong with the capacitor, whether it is failing or improperly sized, you would need to do the under load test (start winding amps x 2652/capacitor Volts) or bench test the cap and then check against motor data plates.

But as a regular service procedure, this power factor feature is a time-saver and gives you a unique insight into the operation of the motor.

—Bryan

P.S. – If you are interested in the 770-3, use the offer code “getschooled” at TruTechtools.com for an 8% discount

Related Tech Tips

Some Belt Tensioning Thoughts
We had a really great conversation on the HVAC School Facebook Group about some belt tension best practices. It turns out that even a lot of really smart and experienced techs are not aware of all the factors related to belt tensioning. Myth #1 is that amperage is used to set belt tension. Now, don't […]
Read more
Sight Glass/ Moisture Indicator
Often in commercial HVAC and refrigeration, you will either find or install sight glass/moisture indicators. The sight glass portion is simple; it's just there to show if the liquid line has a full line of liquid or if it has bubbles, which shows it's a liquid/vapor mix. A clear sight glass on a running system […]
Read more
Motor Service Factor
“Service factor” is an interesting motor rating that you will see on many motor data tags. It simply means how much additional “work” a motor can do or how much “load” it may be placed under for short periods of time without failure or overload. For example, the FLA or full load amps of the […]
Read more

3 responses to “An even EASIER way to check capacitors”

  1. As I am not familiar with using a DMM to measure PF, would the fan motor being parallel off of the same terminals as your probes are measuring the compressor affect the reading?

    • Ryan, the test lead probes are measuring the supply voltage part of the PF equation which is the same for the compressor and fan. The amp clamp is measuring the compressor only. You can also perform the PF test on the fan by itself by simply putting the amp clamp around the fan common instead of the compressor common as shown.

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