Motor Run/Start Capacitor Checks and Replacement

Run and start circuits on single-phase compressor motors use capacitors which affect the wattage, amperage draw, torque, speed, and efficiency o f the motor. Figure SP-9-10 shows typical run and start capacitors.

Run capacitors are connected in the motor circuit at all times and are therefore referred to as continuous-duty capacitors. Older run capacitors are usually larger in physical size but have lower capacitance ratings than start capacitors. Newer ones may be smaller in physical size and encased in hard plastic shells.

Because run capacitors are in the circuit at all times, they typically are filled with a dielectric fluid that dissipates heat. A shorted capacitor may give a visual indication of its failure. For example, the pop-out hole at the top o f a start capacitor can bulge or blow out. A run capacitor may bulge or leak. If a capacitor is found to be defective, it should always be replaced with one specified by the manufacturer.

Testing of capacitors to determine if they are good or bad is commonly done by making resistance checks using a VOM/DMM. This method is described in the detailed procedure given at the end of this section. A capacitor analyzer should be used when accuracy is required in checking the electrical condition of the capacitor, especially when it is necessary to measure the actual capacitance (MFD) value of a capacitor. Note that some DM Ms also have a capability to measure the MFD value of capacitors.

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