Troubleshooting Reasons for Failing Start Capacitors

Many single-phase compressors require a start capacitor to assist in starting the motor. These capacitors will occasionally fail, causing a compressor to fail to start. Overheating is a primary cause of a failed start capacitor.

Start capacitors are not designed to dissipate the heat associated with continuous operation; they are designed to stay in the circuit only momentarily while the motor is starting. If a start capacitor stays in the circuit too long, it will overheat and fail. If a failed start capacitor is discovered while troubleshooting, the start relay should be inspected as the possible cause. The contact points of the relay could be damaged, causing it to stick closed when starting and keeping the start capacitor in the circuit too long.

To eliminate this potential problem, there should be a bleed resistor across the terminals of the start capacitor. The damaged contacts could be caused by the high voltage stored in a start capacitor being discharged across the contacts of a start relay. The bleed resistor will quickly dissipate this energy to prevent any damage to the start relay’s contacts. Many new OEM start capacitors will come with this bleed resistor already installed. However, general replacement start capacitors may not come with one. Before installing a replacement start capacitor, verify there is a bleed resistor across its terminals. If the replacement does not have one, a 15,000-18,000 2W resistor should be placed across its terminals.

Another possible cause of a failed capacitor is a motor that is short cycling. The start capacitor may not have sufficient time to cool down after each start, and this can lead to the capacitor overheating. It is generally recommended that a start capacitor be limited to a maximum of 20 starts per hour.

A start capacitor will normally fail open, and this can be tested with an ohmmeter or a capacitor tester. Normally, a visual inspection of a capacitor will indicate if it is defective. The membrane on top of a defective capacitor will normally blow out when the capacitor fails. However, a failed start capacitor may not always have a blown membrane, so always check the capacitor with your test meter.

When replacing a start capacitor, there are two important specifications that must be determined. First, its microfarad (MFD) rating, and second, its rated operating voltage must be verified.

The MFD rating of the replacement capacitor should be equal to its original. If the exact rating cannot be found, a capacitor with a rating up to 20 percent greater can be used. A lower MFD rating should not be used. The voltage rating on the capacitor should also be matched to the original. If the original voltage rating cannot be found, a capacitor with a higher rated voltage can be substituted. Do not use a capacitor with a lower rated voltage.

Also, it is a good practice to replace the start relay when changing out a start capacitor. The damage to its contact may not always be easily identified, so it is best to simply replace the relay at the same time as a precautionary measure.

Joe Marchese is an author, instructor and HVACR service contractor. He can be reached at [email protected].