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How to Test a Run Capacitor with a Multimeter

Author: Apogeeweb
Date: 21 May 2022
motor run capacitor



Ⅰ What is the Run Capacitor?

1.1 Specifications of Run Capacitor

1.2 The Function of the Run Capacitor

Ⅱ Motor Run Capacitors

2.1 What Does A Motor Capacitor Do?

Ⅲ How Does a Run Capacitor Work?

Ⅳ How Long do Run Capacitors Last?

Ⅴ How Do You Know if Your Run Capacitor is Failing?

Ⅵ How to Test a Run Capacitor with a Multimeter

Ⅶ Start Capacitors vs Run Capacitors

7.1 Are start and run capacitors interchangeable?

Ⅷ Dual Run vs. Run Capacitors



A run capacitor is used to continuously change current or phase shift to a motor's windings to improve the torque and efficiency performance of the motor. It has a substantially lower failure rate than a start capacitor because it is designed for continuous duty.

Ⅰ What is the Run Capacitor?

A run capacitor is a compact cylindrical device that supplies and stores energy inside your air conditioner. It is capable of storing energy in the form of an electrical charge.

start run capacitor wiring diagram

  • Specificationsof Run Capacitor

Most run capacitor applications employ capacitance ratings of 2.5-100 uf (microfarads) and voltages of 370 or 440 VAC. They are also commonly rated at 50 and 60 Hz. Cases are typically round or oval, with a steel or aluminum shell and cover. Terminations are typically 14-inch push-on terminals with 2-4 terminals per connection post.

Voltage: Choose a capacitor with a voltage rating equal to or greater than the original capacitor. If you have a 370-volt capacitor, either a 370 or a 440-volt capacitor will work, albeit the 440 volt one will last longer. A run capacitor will have a designated voltage indicating the permissible peak voltage - not the working voltage.

Capacitance: Choose a capacitor with the same capacitance value (measured in MFD, uf, or microfarad) as the original capacitor. Do not stray from the original value, as it determines the motor's working characteristics.

Hz: Choose a capacitor with the same Hz rating as the original. Almost all tun capacitors will be marked 50/60.

Is your case round or oval? Although round run capacitors are by far the most prevalent, oval versions are still used in many motors. Electrically, there is no distinction. The only question here is fit. The case style is unimportant if space in the mounting box is not constrained.

Overall Size: Just like case style, overall size has no electrical significance. Choose a capacitor that will fit in the available space.

Terminal Design: Most run capacitor terminal designs contain 1-4 14" push on tabs with 3 or 4 tabs. Just make sure you have enough tabs per connection post to connect everything.

  • The Function of the Run Capacitor

Once the system is up and running, the run capacitor takes over and provides the extra power needed to run the air conditioner for extended periods. When the air conditioner is turned on, both capacitors generate and store energy for the next cycle's push. Many air conditioning and heat pump systems include a dual capacitor arrangement that connects the start and run capacitors to the compressor and fan motors.

Motor Run Capacitors

Motor run capacitors keep a charge and remain active while a motor is running. They improve the running performance of a motor by continuously providing additional torque, whereas start capacitors are removed from the circuit after the motor reaches its operational speed. When replacing a run capacitor, it's critical to match the form (round, oval, or rectangle) as well as the original capacitor's microfarad rating, voltage, and dimensions. Run capacitors are most commonly employed in medium-to-high torque applications such as air conditioners, auger drives, compressors, and conveyors.

2.1 What Does A Motor Capacitor Do?

Capacitors are used in single-phase motors to help them start and to save energy.

Motor capacitors are classified into two types:

  1. Start Capacitors
  2. Run Capacitors

A start capacitor is used to provide an extra electrical shove to start a motor. A start capacitor is only used in the motor circuit for a few seconds when it first turns on.

When the motor reaches full speed, the start capacitor disconnects and is not used again until the motor is restarted. If the start capacitor fails, the motor will not be able to start.

A run capacitor is an energy-saving device that is always present in the motor circuit.

When a run capacitor fails, the motor can experience several issues, including not starting, overheating, and vibrating.

A faulty run capacitor deprives the motor of the full voltage required to operate.A faulty run capacitor deprives the motor of the full voltage required for proper operation.

 How Does a Run Capacitor Work?

The capacitor's job is to produce 240 volts to start the compressor, fan motor, and blower motor when your thermostat signals your air conditioner to chill the house.

After the capacitor kickstarts those components, your air conditioner can start cooling outside air and blowing it throughout your home. The capacitor, like a rechargeable battery, gathers and stores energy while your air conditioner is running. When you switch on your air conditioner, the capacitor releases a new burst of energy. Your air conditioner may feature many capacitors, depending on its model.

The run capacitor, also known as a "A/C capacitor," uses the energy it has stored to spin or run the fan motor. The fan will not turn on without the run capacitor.

The start capacitor supplies the first energy required for the system to boot up. Because starting an AC system requires a lot of torque, a start capacitor will have a higher capacitance than a run capacitor. A dual capacitor supplies electricity to both the fan and the compressor, which is the large pump installed in your outdoor cabinet that circulates refrigerant between the indoor evaporator and the outdoor condensing unit.

Ⅳ How Long do Run Capacitors Last?

Most run capacitors are intended to last ten years. Having said that, failing capacitors are most likely the most prevalent cause of A/C repair. Allowing the fins of your outdoor coil to become so dusty and congested that air cannot circulate properly, causing the compressor to work harder.

 How Do You Know if Your Run Capacitor is Failing?

If you have noticed that:

Your air conditioner no longer blasts cold air; 

Your air conditioner no longer blasts cold air; 

your air conditioner makes a low humming sound that was not present previously; your energy expenses have been rising.

Your air conditioner occasionally 

....your run capacitor may be nearing the end of its useful life.

 How to Test a Run Capacitor with a Multimeter

1. Using The Capacitance Function on a Multimeter

Turn on the meter

Select the capacitance function on the dial. (See note below.) In this example, we're using a Klein multimeter, and we need to press the select button till we see capacitance mode.

Capacitance Setting on the Multimeter

2. Testing the Condenser Fan Section of the Capacitor

Put one multimeter probe on C (common)

Put the other probe on FAN.

Reading the Capacitance for the Fan Motor Section of the Capacitor

3. After a few seconds, the capacitance reading should appear on the monitor.

Microfarads should be within 10% of the labeled specification in a good reading.

4. Testing the Compressor Fan Section of the Capacitor

Put one multimeter probe on C (common)

Place the second probe on HERM. (HERM is an abbreviation for hermetic, which refers to a hermetically sealed compressor.)

Reading the Capacitance for the Compressor Section of the Capacitor

Wait a few seconds and you should see the capacitance reading on the display. A good reading will have the microfarads within 10% of the labeled specification.

5. Using The Resistance Function on a Multimeter

A capacitor can also be examined by measuring resistance, which is best done using an analog meter. Digital meters often do not show the ohms jump that signals a good capacitor.

Switch on the meter.

Set the meter to Ohms. (Resembles the omega sign)

6. Get a Quick Ohm Reading Between the Terminals

Connect one probe to C and the other to FAN. The ohm reading on the needle should leap and then return to infinity ohms.

Reverse the probes and check the multimeter needle for the same behavior.

Do the same with C and HERM.

7. Get an Ohm Reading Between the Terminals and the Casing of the run Capacitor

Place one probe on C and the other on the capacitor's outside metal shell. If you receive a reading that indicates continuity, the capacitor is faulty.

Rep this procedure for the FAN and HERM terminals.

Testing for Shorts from the Terminals to the Casing on a Capacitor

8. Test the Run Capacitor

Dual capacitors contain three terminals: one common (C), one fan (F), and one hermetic (Herm) connector for the hermetically-sealed compressor. The line-voltage wires connect to two more terminals on the capacitor's other end.

Put one lead on the Common (C) terminal and the other on one of the other two terminals with a digital multimeter set to "Capacitance." The meter should display a number rather than "OL," which denotes a short.

Here's a video that explains how to test and replace a capacitor in further detail.

Before disconnecting the wires, take a short digital photo of them so you know where to reinstall them.

 Start Capacitors vs Run Capacitors

The starting capacitor's purpose is to lag the current in a gallery winding during the motor's starting operation, and it is detached from the circuit when the router reaches its predetermined speed.

The run capacitor can be used as a start capacitor, but the start capacitor cannot be used as a run capacitor. A high capacitance value is necessary to show the run capacitor array (two or more capacitors are connected in cascade) can be connected to start the motor or create strong torque across the motor.

Both the start capacitor and the run capacitor are motor capacitors that serve separate functions in motor operation. Because the construction of both capacitors is the same, let's compare Start Capacitor versus Run Capacitor.

Start Capacitor

Run Capacitor

They are used for starting the motor. 

They are used for providing continuous power to the motor. 

Small duty cycle.

Long duty cycle.

Higher Capacitance rating (70-120 micro Farad)

Lower Capacitance rating (7-70 micro Farad)

Remain connected to the circuit for short period of time.

Remain connected to the circuit for long period of time.

The start capacitor is used during the startup phase of the motor and gets disconnected from the circuit when the motor reaches its predetermined speed, which is 75% of the maximum speed of the motor. 

Run Capacitor stays directly linked with the auxiliary coil even after the starting capacitor is disconnected (or deactivated) from the circuitry. The run capacitor remains constantly charged while providing a controlled contiguous (or continuous) power to the motor. 

7.1 Are start and run capacitors interchangeable?

A run capacitor could be utilized as a start capacitor in uncommon circumstances, although the values available for them are substantially lower than the values typically available for dedicated start capacitors. The capacitance and voltage ratings would have to match the specifications of the original start capacitor. A start capacitor can never be used as a run capacitor since it cannot handle continuous current (only a couple of seconds).

Learn the distinctions between the start and run capacitors by watching the video instruction below.

 Dual Run vs. Run Capacitors

The only advantage of the twin-run capacitor design is that it comes in a tiny container with only three connections. Aside from that, there is no distinction between run and dual run capacitors. If there is enough room for mounting, replacing your original dual-run capacitor with two independent run capacitors is okay. They typically have "C" connectors for "common," "H" or "Herm" connections for "Hermetic Compressor," and "F" connections for "Fan." They will also have two distinct capacitor ratings for each part.


1. Wll a capacitor leak?

Electrolytic capacitors can rupture and spill fluid if they are damaged or subjected to high loads over extended periods.

2. Do capacitors boost voltage?

No. While they can be employed in circuits that increase voltage, a capacitor does not increase the voltage on its own.

3. Can you replace a dual run capacitor with two single run capacitors?

A dual run capacitor is just two single runs housed in one housing to save space. If you have enough space and can't find a twin-run cap replacement, you can use two separate ones.

4. Does the style of casing on the outside of an AC capacitor matter?

The design does not affect performance as long as the capacitor is of high quality and properly placed. However, round shapes are more frequent than oval styles.

5. Do all HVAC systems need a capacitor to work?

While all single-phase motors have a start capacitor, certain electric motors are built without one.

6. Why do start capacitors have higher ratings than run capacitors if a run capacitor is used continuously?

A start capacitor requires a lot of energy to provide adequate torque to start motors in an alternating current system.

7. What happens if the wrong size run capacitor is used?

It will cause several issues and put unnecessary strain on the motor, such as overheating or a decrease in speed.

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