4.8 • 985 Ratings
🗓️ 1 July 2025
⏱️ 18 minutes
🧾️ Download transcript
In this short podcast episode, Bryan goes through the process of testing and replacing a run capacitor.
Run capacitors provide continuous phase-shifted current to the start winding, allowing the motor to run in the right direction and with the appropriate amount of torque. These capacitors are needed in most of the single-phase PSC motors we work with. Run capacitors are in the circuit all the time and are connected in series with the start winding (compared to start capacitors, which need to be taken out of the circuit after startup). They don't actually "boost" voltage. The bigger the capacitor, the more current can move in and out of the start winding.
Run capacitors are made of foil and oil (and the manufacturing quality usually correlates to the capacitor's lifespan), and they usually fail due to overvoltage or overheating. Capacitors may also be replaced with one of a higher voltage rating but not a lower one.
Before you test a capacitor, perform a visual inspection. Bloated tops and oil leakage are signs of failure. You'll also want to measure the microfarads and make sure it's within range. An under load test occurs while the system is running (and should NOT be done on blower capacitors for safety reasons). Bench testing happens while the system is off and the capacitor is disconnected and discharged. We have calculators for testing on our app.
To test under load, measure the amperage on the start wire and multiply it by 2652. Then, divide that number by the voltage to find the capacitance in microfarads. If the capacitance is over 10% low, replace the capacitor. When replacing a capacitor, make sure the system is off, the capacitor is discharged, and that you tighten the strap and wire it up correctly. It's also a good idea to bench test the new capacitor.
"Capacitor Testing and Replacement Procedure" tech tip: https://www.hvacrschool.com/capacitor-testing-and-replacement-procedure/
"What Is Up With Blower Amps & Watts w/ Steve Rogers" podcast: https://www.hvacrschool.com/podcasts/what-is-up-with-blower-amps-watts-w-steve-rogers/
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0:00.0 | Hey, Billy Oton-Dotten, Opoeditin. This is the HVAC school podcast. I'm Brian. This is a short episode where we're going to talk about one of the simplest and most common issues that exists. And for those of you who have done this a long time, you may be like, this is boring. Why are you covering this? But what we find on the podcast and the tech tips and the videos is that a lot of times these are things you can |
0:25.8 | share with somebody who's newer. And a lot of times things that seem simple often have some steps |
0:31.5 | that get missed. So that's what we're going to talk about testing and changing a run capacitor |
0:36.7 | right after we hear from our great sponsors. |
0:40.0 | Carrier and Carrier.com. |
0:41.9 | Our friends over at UEI at UEI test.com. |
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1:14.1 | All right, so let's start, run capacitor. What does a run capacitor do? This is a big sentence. |
1:20.3 | And I will confess, I'm reading this right off of my tech tip capacitor testing and replacement procedure. |
1:25.5 | If you want to read the whole tech tip, go to HVCRSchool.com or the app and just type in |
1:30.0 | capacitor testing into search and it will pop up. |
1:33.4 | But a run capacitor provides continuous phase shifted current to the motor start winding, |
1:41.0 | otherwise known as the auxiliary winding, allowing the motor to run. |
1:46.0 | We're talking specifically about single phase PSC type of motors. |
1:50.8 | That's the type that's still the most common in HVAC for a connector fan motors, |
1:55.3 | for compressors for blowers, rarely nowadays, rarely blowers. |
1:59.6 | Most blowers now are ECM electronically commutated motors. But |
2:04.6 | run capacitors still exist. We see a lot of them. So we're not talking about three phase. We're |
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