Capacitors (See Fig. 65) increase the power factor and efficiency of electric motors. Voltage is accumulated on one pole and when charged to capacity, it is released to the other. This is done in rapid succession. Start capacitors provide extra voltage to enable the motor to start easier while drawing less amperage. Run capacitors provide more energy to the motor run winding to help it run easier. Capacitor capacitance is measured in Micro-Farads.


Discharge capacitors by shorting the terminals with an insulated screwdriver described in Testing Capacitors. Very often a compressor may not start or short-cycle due to a bad capacitor. Some capacitors are used to help motors start. They are called start capacitors. They are wired in series with the compressor start winding and the start relay. (See Figs. 43 & 44). The run capacitor is in the starting circuit too. It helps the motor run smoother and more efficiently (See Fig. 45). To accurately check a capacitor, use a capacitor tester like the one shown in Figure 66. Follow the instructions furnished with the instrument. Use the same micro-farad rated capacitor to replace one with another. Never replace a capacitor with a lower rated one. In the event an exact duplicate cannot be obtained, one with up to a 10% higher rating is permissible.

As an example, suppose a 100 mfd capacitor cannot be found to replace a faulty one. Any capacitor rated 101 to 110 mfd may work, but never use one rated under 100 mfd. All capacitors are labeled with their voltage and mfd rating.

The liquid used in some capacitors is bipheny dielectric fluid, which is dangerous. Do not attempt to open the shell or burn it.

In many air conditioners, two capacitors are incorporated into one. These capacitors are seen with three terminals. One marked HERM (hermetic compressor motor), another terminal marked C (common terminal), and one marked FAN. The common terminal is connected to both the fan motor wire and the compressor motor. The fan terminal is connected to the second fan motor wire, and the HERM terminal is connected to the second compressor wire (see fig. 65). There is a red dot (or sometimes other symbols to indicate the grounded terminal) on the capacitor. It indicates the pole to which the hot wire is to be connected. In the event of a short, the current is drained off harmlessly protecting personnel and equipment.

Make it a habit to make a pencil sketch or tag the wires with a piece of tape before removing a capacitor. By doing this, there will be no doubt about the proper connections when installing a new capacitor.

Leave a Reply

Your email address will not be published. Required fields are marked *