Troubleshooting single-phase compressors requires a technician to have a proficient understanding of capacitors. The run capacitor is one of two types of capacitors that could be found on single-phase compressors.

The run capacitor is used to improve the running efficiency of a compressor’s motor. The run capacitor is placed in series with the start winding of the compressor and will remain in the circuit as the motor operates. As current flows through the run capacitor and the start winding, it causes a phase shift of the motor’s current, thus improving the power factor of the motor. Since the run capacitor remains in the circuit, it needs to be able to dissipate the heat generated by being in continuous operation. This is done by its design. The run capacitor is oil-filled to allow the capacitor to dissipate the heat generated.

Defective run capacitors can and will lead to problems with the operation of single-phase compressors. A defective run capacitor can cause the compressor to draw higher than normal amperage and possibly trip on its motor overload.

Replacement Concerns

When replacing a run capacitor, there are two important specifications that must be determined: its microfarad (MFD) rating and its rated operating voltage. It is always best to use a replacement run capacitor which exactly matches the MFD and voltage rating of the original run capacitor. However, if this is not possible, a replacement run with a MFD rating of plus or minus 10 percent of the original MFD rating can be safely used. If the original voltage rating cannot be matched exactly, then a capacitor with a higher rated voltage can safely be substituted. A capacitor with a lower voltage rating can never be used.

When replacing capacitors, if a single capacitor cannot be found to replace the original, multiple capacitors can be wired either in series or parallel to replace the original. To determine the capacitance of capacitors wired in series, use Formula 1.

The rated voltage of capacitors wired in series is the sum of the individual operating voltages. The voltage applied to each individual capacitor is inversely proportional to the applied voltage. For this reason it is not recommended to connect capacitors of unequal values in series.

Capacitors can also be wired in parallel. To determine the total capacitance of capacitors wired in parallel, use Formula 2.

The total rated voltage will be that of the lowest-rated voltage of an individual capacitor.

Capacitor Failure

A run capacitor will normally fail open. This could be tested with an ohmmeter or a capacitor tester. Using an ohmmeter, an infinite resistance across the terminals of the capacitor will indicate an open failure. Normally a visual inspection of a capacitor will indicate this defect. The top portion of a defective run capacitor normally bulges out, indicating a problem with the capacitor. The plates of a capacitor can also short together. A zero resistance across the terminals of a capacitor will normally indicate a shorted capacitor. Some older capacitors had the potential to short to the metal casing of the capacitor. If these capacitors were incorrectly installed, they would have the potential to allow continuous current flow through the start winding of the compressor even though the compressor had stopped.

This situation would eventually cause the start winding to fail. Older capacitors had an identification mark to make sure the technician wired the capacitor correctly to prevent this type of failure. The marked terminal needed to be wired to the run terminal of the compressor. New capacitors do not have this identification, since this potential does not exist with newer capacitors. A technician should look for this identification terminal when working with older capacitors.

Many single-phase compressors use a run capacitor to improve the running efficiency of the compressor. Understanding how to troubleshoot and identify a defective run capacitor is essential when working with these compressors. Correct troubleshooting can prevent a good compressor from being replaced when the problem was simply a defective run capacitor. Always examine the run capacitor before replacing or condemning a failed compressor.

Marchese is owner of Coldtronics in Pittsburgh. He can be reached at 412-734-443,, or

Publication date: 02/03/2003