Capacitor Testing and Replacement Procedure


❶ Understand What a Run Capacitor Does

The run capacitor provides continuous phase-shifted current to the motor start winding, allowing the motor to run:

  1. With the design efficiency
  2. In the right direction
  3. With the appropriate torque
  4. With near “unity” power factor (power factor near 1.0)

If the run capacitor is failed, the motor often won't run in the case of high torque motors like a compressor, or in the case of fans, they may run backward or slowly or with high amperage or overheat.


❷ Understand Why a Run Capacitor Fails

A run capacitor fails due to:

  1. Poor manufacturing quality
  2. Overtemperature (they got hot)
  3. Over-voltage (the voltage was too high)

Many will say a failed motor “takes out” the capacitor. In actuality, a failed or weak capacitor can take out a motor.

❸ Visually Inspect

If a capacitor shows physical damage, such as the top bubbling or oil leakage, it should be replaced. Normal rust is not a reason to replace a capacitor. Note the microfarad (MFD or µF) rating listed on the capacitor. The voltage rating is also worth noting; you may use a HIGHER voltage-rated capacitor but not lower.


❹ Decide if under load or bench testing is best

If the system is currently running, then an under load test may be best. Do not do an under load test on blower capacitors due to the risk of the meter leads wrapping around a spinning blower wheel. If the system is NOT running, then a bench test will be the best bet.


❺ Bench or Under Load Test?

If possible, choose under load because it can be done in real load conditions on a running system. Choose bench for simplicity or if the system is not running.

Under Load Test—Go to step ❻

Bench Test—Go to step ❼


❻ Test Under Load

To test under load, you need to take measurements with the system running. Wear proper PPE and only do so when safe. You need to have an accurate multimeter that can measure voltage and amperage reliably. Often, under load measurements may come out high if the amp clamp picks up interference from other circuits. Measure the amperage on the start wire with the wire centered in the clamp and multiply by 2652. Now, measure the voltage across the capacitor and divide the amperage x 2652 by that voltage to find the capacitance in MFD. If the under load MFD is less than 10% low, we suggest replacement. If it is over the rating, it is often a measurement error.

If a capacitor measures weak via under load test, go ahead and perform the bench test.


❼ Bench Test

Bench testing is simply removing both leads from the run capacitor after safely disconnecting power and discharging the capacitor. You then place a meter designed to test capacitance across the terminals and note the reading. Be careful not to touch the meter probes as you attempt to get a good, solid connection to the metal connection spades on the capacitor. If the measurement is more than 10%, we suggest a replacement.


❽ Inspect the dual run capacitor and note where each wire is connected

Taking a picture is one of the easiest ways to remember before removing the wires.


❾ Remove disconnect

It is imperative that the disconnect is removed or the unit is off and without potential. Test using a meter that is pre-tested to a known voltage source and check L1 to L2, L1 to ground, and L-2 ground to ensure no voltage is present.


❿ Discharge capacitor

Before testing, touching, or removing the capacitor, you need to discharge it. Do this using a high-resistance resistor bridging HERM and Fan terminals across to C or across a single capacitor.

NOTE: It is actually very rare for a run capacitor to contain a charge on a normally running system because it bleeds off through the compressor windings UNLESS one of the windings is open. For this reason, many techs opt to use a screwdriver to discharge, which is a controversial but common practice. (HERE is how we discharge capacitors.)


⓫ Disconnect wires and remove used dual run capacitor

To remove, disconnect the wires on the top of the capacitor and remove the strap holding the capacitor in place.


⓬ Replace capacitor with same MFD rating

Apples to apples: You must use the same MFD rating capacitor during this process. This will be located on the box and also on the side of the capacitor. Mount the capacitor upright with terminals pointed up. TEST THE NEW CAPACITOR VIA BENCH TEST BEFORE INSTALLING.


⓭ Re-sizing of strap

Sometimes, the new capacitor might be larger or smaller. At this time, use the metal strapping and create a new strap for the capacitor. Cut it to the correct size and use self-tapping screws to attach it to the correct area. Always check before using self-tappers to confirm that you are not in any danger of puncturing the coil.


⓮ Attach the wires

At this time, reattach the wires onto the top of the dual run capacitor. Ensure that the common, the HERM (compressor), and the FAN  are connected to the capacitor. You should always double-check to make sure all wires are in their proper places. Ensure the terminals fit very snugly; tighten them by squeezing them with needle-nose pliers before installing to make sure they are very tight.


⓯ Test system

Reconnect the disconnect or flip the breaker back on. Check to make sure that all aspects of the system are running and the compressor and fan are running at the proper amperage. A good additional practice is to use a power meter and test motor power factor to ensure it is near unity.


⓰ Clean up and reattach panel on the condenser

Make sure you clean up the area and reattach the panel to the condenser.


Mark Troy
Mark Troy
7/15/20 at 07:36 AM

Good morning,
Thank you for doing what you do, you are doing a great service for our industry. One question about today’s article. It says: “If the under load MFD is less than 10% low we suggest replacement.” Do you mean if the under load MFD is more than 10% low?
Thanks again

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