How to Check A/C Compressor Motor & Starter Capacitor

You may take for granted the air conditioning (AC) that keeps you cool, but, when it breaks, you'll surely miss it! Learning about how it works can help you fix it. If you think your air conditioning unit might be broken, the motor and starter capacitor could be a place to start looking.

Checking a Compressor Start Capacitor

A heating, ventilation and air conditioning (HVAC) unit uses motors that move to generate electrical or wind energy. Start and run capacitors store and release charge in the electrical circuits of these units. The start capacitor holds charge on its plate that cause a motor to begin moving while the run capacitor continues to keep the motor running smoothly. Single capacitors are separate from one another, and dual round capacitors are found in the same package.

An AC compressor start capacitor is designed with a built-in relay alongside the capacitor. This gives more torque, a rotational force, to the capacity and also disconnects the capacitor after the motor has already started.

Using these capacitors over and over again can increase the likelihood they'll become damaged. When this happens, the AC compressor won't function as effectively. Damaged or broken circuit elements can cause compress run capacitor failure.

Diagnosing Compressor Run Capacitor Failure

You might have compressor run capacitor problems if you have any signs or symptoms of AC compressor run capacitor failure. Use protective insulated gloves and wires when testing the inside of your AC unit for defects.

Failure of the capacitor, starting for only short periods of time or creating a humming noise can all be air compressor run capacitor symptoms. If these individual failures of capacitors pile up over time, it's possible the entire AC unit will cease to start.

Look at the capacitor and wires themselves. A bulging or leaking capacitor will likely need to be replaced. If you notice damage or other signs of stress on it, your capacitor needs to be fixed.

Checking the capacitors of your AC compressors can prevent failures from happening or make you better equipped to address them. There are a few general techniques for fixing capacitor failure.

Fixing Compressor Run Capacitor Failure

If you've studied how your AC unit works on the level of electrical circuits, you can fix a broken capacitor. This means figuring out which capacitors are start or run and how the electricity flows through the circuit or circuits of the compressor.

Make sure that you can easily attach and detach parts of the compressor with each other. If parts of the unit are screwed or bolted down, use the appropriate tools such as screwdrivers or wrenches to remove them. Use rubber-insulated tools to ensure you don't get shocked.

Turn off the AC unit, and use a meter in the circuit to make sure it's off. A voltmeter or multimeter should do just fine, and keep track of what values of voltage or current you measure to make sure the compressor displays the same or similar values after you fix it. Remove the panel that inputs the electricity into the unit. Replace the fan motor capacitor.

Check the Molecular Frequency Discriminator (MFD) of the connection to check if the signal is being sent through. Re-connect the wires from the older capacitor with the new capacitor. Check these connections as you connect them to make sure they're secure. Use your meter to check for voltage or current in the circuit.

Other Ways of Fixing Compressors

If you install a new motor in your AC unit, make sure you've installed a new fan capacitor as well. This makes sure the motor and capacitor carry the same longevity and function more effectively together.

References

About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.

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