Capacitor Testing and Replacement - Short #247 · HVAC School - For Techs, By Techs summary

Key takeaways

Understanding the distinction between run capacitors and start capacitors is crucial for HVAC technicians.
Capacitor failure can lead to various operational problems in HVAC motors, often exacerbated by environmental conditions.
Proper testing and safety protocols are essential when dealing with run capacitors.
Ensure that any replaced capacitor matches the required specifications for optimal motor performance.
Surge protection should be considered in areas with high environmental risk to protect HVAC equipment.

Summary

Understanding Run Capacitors

The episode begins with a detailed explanation of what a run capacitor is and its function in HVAC systems, specifically in single-phase PSC type motors. Brian explains that the run capacitor is essential for providing continuous phase-shifted current to the motor start winding, allowing it to operate efficiently. He distinguishes between run capacitors and start capacitors, emphasizing that run capacitors are always in the circuit while start capacitors are temporary.

Causes and Symptoms of Run Capacitor Failure

Listeners learn about the common causes of run capacitor failure, including overvoltage and overheating. Brian points out that symptoms of a failing run capacitor can manifest in various motor behaviors, such as motors failing to start, running in the wrong direction, or exhibiting poor torque. He highlights how environmental factors, especially in hot regions, contribute to capacitor deterioration.

Testing a Run Capacitor

The podcast delves into the process of testing a run capacitor, including visual inspections for physical damage, and performing under-load or bench tests. Brian instructs listeners on how to measure amperage and voltage to determine capacitance and what actions to take if the capacitance is found to be outside of acceptable ranges. He also discusses the importance of using proper safety measures and personal protective equipment (PPE) during testing.

Replacing a Failed Capacitor

Brian outlines the steps necessary for replacing a failed run capacitor, stressing the importance of safety, including discharging the capacitor before handling it. He provides tips on selecting the appropriate capacitor and confirms that the new capacitor's microfarad rating matches that required by the motor. The necessity of ensuring proper connections and securing the capacitor during installation is emphasized.

The Importance of Surge Protection

Finally, Brian touches on the importance of surge protection, especially in areas prone to voltage spikes due to environmental factors. He encourages technicians to discuss surge protection options with clients as part of routine maintenance and emphasizes that surge protection devices need checking for functionality after installation.

Show notes

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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Capacitor Testing and Replacement - Short #247