What Does the AC’s Capacitor Do? Everything You Need to Know

If you’re troubleshooting your AC, you’ll eventually come across one of its critical components—the capacitor.

For some folks, there’s a lot of mystery around the capacitor—

Why is the capacitor important?

And what does the capacitor do, anyway?

In this article, I’ll go over what the AC’s capacitor does. I’ll also provide some tips on how to tell if your capacitor is bad, and how to test your AC’s capacitor.

What is the AC’s capacitor used for?

The AC’s capacitor is used to help its compressor or fan motor turn on. Without the capacitor, the AC’s motor won’t be able to start rotating.

So how does the capacitor work, anyway? And why is it needed?

Whether it’s your AC’s blower, condenser fan, or compressor—all of these devices use electric motors to run.

One thing about electric motors is that they need a rotating magnetic field to start spinning.

The problem with single-phase electric motors is that they can’t produce a rotating magnetic field on their own—that’s where the capacitor comes in.

The AC’s capacitor creates a leading current in the motor’s start winding. The leading current creates a rotating magnetic field that makes the motor start spinning.

But what happens once the motor starts up and runs at full speed?

That’s where the run capacitor comes in.

The run capacitor maintains the rotating magnetic field in the motor while the motor runs. This improves the efficiency of the motor and helps it use less energy.

What’s the difference between a start capacitor and a run capacitor?

The AC’s start capacitor gets the motor running, while the run capacitor helps keep the motor running smoothly.

In the permanent split capacitor (PSC) motors found in most AC units, the run capacitor acts as both a start AND run capacitor.

For more information about start versus run capacitors, check out my article below:

How to tell if your AC’s capacitor is bad

One of the most common issues of an AC system is a bad capacitor.

Here are a few different signs that your AC’s capacitor might be bad:

I’ll go into detail about each of these situations below.

Your AC’s blower won’t turn on

The blower motors in most AC systems are permanent split capacitor (PSC) motors. PSC motors require a capacitor to start up and run.

If your blower motor’s capacitor is bad, then the blower won’t turn on.

Keep in mind that a blower motor that’s not turning on could be caused by a multitude of other issues. However, a bad capacitor is one of the most common issues of a blower motor that won’t turn on.

If your AC’s blower won’t turn on, check out my article below:

Your AC’s blower is making a humming noise

If your AC’s blower motor is humming, then there’s a good chance that its capacitor is bad.

So what causes the AC’s blower motor to hum, anyway?

The AC’s blower motor hums because of the magnetic field generated by the motor’s windings.

The energy from the alternating current power supplied to the motor causes the motor to vibrate at 60Hz (or 50Hz, depending on what country you’re in).

The humming noise gets really loud if the motor’s capacitor is bad because the energy that would normally go toward spinning the motor goes toward making a humming noise and producing heat instead.

If your AC’s blower motor is not turning on and making a humming noise, then you should shut off the power to your AC and change its capacitor.

If the motor is still making a humming noise with a new capacitor, then there is likely another issue with the motor that you’ll need an HVAC professional to check out.

Your AC’s condensing unit won’t turn on

Your AC’s condensing unit has two components that require capacitors:

  1. The condenser fan motor
  2. The compressor

Most AC condensing units use a dual run capacitor for their condenser fan motor and compressor.

So what’s a dual run capacitor, anyway?

A dual run capacitor consists of two run capacitors in a single component. AC manufacturers like to use dual run capacitors because they save space.

Instead of using two individual capacitors for the condenser fan and compressor, most AC condensing units use a dual run capacitor.

Dual run capacitors also save on manufacturing costs because they use fewer parts and labor when compared to two individual capacitors.

If the dual run capacitor in your AC condensing unit goes bad, then the condenser fan or compressor won’t turn on. So it’s worth changing the dual run capacitor if your AC condensing unit is not turning on.

Your AC’s condensing unit is making a humming noise

Similar to the blower motor, the compressor or condenser fan inside of the condensing unit may make a humming noise if one of their respective capacitors is bad.

So if you hear a humming or buzzing sound coming from your AC condensing unit outside, then there’s a good chance that the dual run capacitor needs to be replaced.

Your AC has a blown fuse or a tripped circuit breaker

A blown fuse or a tripped circuit breaker is a symptom of a bad capacitor in an AC.

Why does the fuse blow or the circuit breaker trip when the AC’s capacitor goes bad?

When the capacitor goes bad, the respective component in the AC does not have enough energy to start up. This causes the component (whether it be the compressor, blower, or condenser fan) to stall out and overamp.

When the component overamps, the fuse blows, or the circuit breaker trips.

So if your AC is blowing fuses or tripping breakers, the problem could be the capacitor.

But be sure to rule out the other usual suspects first—such as a dirty air filter or a dirty coil.

How to test your AC’s capacitor

If you have a multimeter with a capacitance testing function, then you can test your AC’s capacitor.

CAUTION: Capacitors contain dangerous amounts of electrical charge, so exercise caution if you decide to test your capacitor with a multimeter.

Whether you’re testing the capacitor for your blower, compressor, or condenser fan—the processes are very similar.

To test your AC’s capacitor, follow these steps:

  1. Turn off the power to your AC.
    If you’re testing the blower motor capacitor, turn off the power to your air handler or furnace.
    If you’re testing the compressor or condenser fan capacitor, turn off the power to your condensing unit.

  2. Disconnect the wires going to the capacitor. Be sure to remember which wire goes where, since you’ll need to reconnect them when you’re finished.

  3. Discharge the capacitor. This is the most dangerous part of the process. Short the capacitor terminals together with a screwdriver. See my section below on how to discharge your AC’s capacitor for more information.

  4. Connect your multimeter leads to the capacitor terminals.
    If you’re testing a condenser fan motor capacitor, connect the leads to the “C” and “FAN” terminals.
    If you’re testing a compressor motor capacitor, connect the leads to the “C” and “HERM” terminals.

  5. Turn your multimeter on and set it to the capacitance setting (μF, MFD, or capacitor symbol – see below).

  6. Take the capacitance reading with your multimeter. It will usually be measured in microfarads (μF).

  7. Check if the capacitance reading is within the capacitor’s specification. The capacitance should be within the specification of the capacitor. If the capacitance reading is not within the capacitor’s specification, then the capacitor is bad.
Capacitor Symbol on Multimeter

How to discharge your AC’s capacitor

Discharging your AC’s capacitor is important an important step if you’re going to be testing or replacing the capacitor.

Discharging a capacitor can be a dangerous process—so take care in following the steps below.

To discharge the capacitor, short its terminals together with a screwdriver. 

Be sure that the screwdriver’s handle is insulated from the metal shaft of the screwdriver. And don’t touch the metal shaft of the screwdriver while you discharge the capacitor—only touch the handle.

If you’re discharging a blower motor capacitor, short the two terminals together.

If you’re discharging a dual run capacitor, short the “C” and “FAN” terminals, and then short the “C” and “HERM” terminals.

One trick that I like to do is slide the shaft of the screwdriver along the terminals of the capacitor—this ensures that sufficient contact is made to short the capacitor.

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