Electric Motor Capacitor Checks and Replacement

Capacitor Checks and Replacement Start and run circuits on single-phase motors use capacitors. Capacitors affect the wattage, amperage draw, torque, speed, efficiency, and power factor o f a motor. Figure SP-10-6 shows a typical run capacitor used with a PSC motor.

Ran capacitors are connected in the PSC 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 are typically filled with a dielectric fluid that acts to dissipate heat. If a capacitor is found to be defective, it should always be replaced with one specified by the manufacturer.

A run capacitor may be bulged and/or leaking, giving a visual indication of its failure. 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 DMMs also have a capability to measure the actual capacitance MFD value of capacitors.

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