What Does an AC Capacitor Do?


Quick Answer

An air conditioning system has several capacitors that provide the power boost required to start an electric motor or compressor. With a failed capacitor, the motor starts to run but fails, and the unit makes a loud humming noise.

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Full Answer

Capacitors store electricity. They come in many shapes and sizes, but all perform the same duty. Electronics designers measure the capacity of a capacitor using the units of farads. A capacitor fails when it is no longer able to store enough energy to start the motor turning. Depending on the particular capacitor that fails, the blower motor, compressor or outdoor fan is no longer able to start. Before the unit begins cooling the home again, the defective capacitor requires replacement.

The power should be disconnected before anyone works on an air conditioner. The circuit breaker panel turns off the power to the air handler. A pull switch is usually near the outdoor unit. To disconnect the power with the pull switch, a worker pulls the connector from the poles. However, because the capacitor contains stored energy, it has the potential to cause an electric shock after power is disconnected. To eliminate this risk, air conditioning technicians use a screwdriver with an insulated handle to discharge the capacitor before working on the unit.

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