Air-conditioning motors must supply a fairly strong torque force in order to start. The motor must overcome freon pressure differences in the system. A hard-start capacitor provides a few seconds of additional electrical current to help the motor start.
Single-phase alternating current (AC) motors need at least one capacitor, called the run capacitor, to operate. Designers match the motor with a relatively small-value capacitor so the motor runs efficiently. With this arrangement, the motor's starting torque may be limited. In this case, you need to add a start capacitor.
A start capacitor has a larger microfarad value than the run capacitor. This extra capacitance provides greater starting torque, but also results in some electrical loading. The start capacitor must remain in the circuit for only a few seconds, as it will eventually heat up and burn out.
The start capacitor works with a device called a potential relay. The relay disconnects the capacitor from the motor after the motor comes up to speed, preventing capacitor burnout. After that point, the motor runs solely with the run capacitor.
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