Electrons do not travel through the capacitor. They simply accumulate on one side of the capac- itor until the current alternates. Then they rush out of the capacitor and return to where they produced by the start winding. Figure 25‑32 shows the two types of capacitors: • • Start capacitor. This is a dry-type capacitor intended for intermittent operation. It is fragile and only withstands 20 starts per hour. • • Run capacitor. This is an oil-filled capacitor intended for continuous operation. The oil serves to dissipate any heat buildup. 25.4.1 Start Capacitor A start capacitor consists of two layers of aluminum foil, called electrodes or plates, separated by an insulating layer of specially-treated paper. The foil and paper layers are about three inches wide and several feet long. The layers are tightly rolled into a cylinder and placed inside a black plastic case. See Figure 25‑33. A cardboard cover seals the top, and two push-on-type electric terminals are anchored to the cover. One aluminum layer is connected to each of the terminals. The large surface area of the aluminum layers serves as a “storage tank” for electrons. The insu- lating paper between the layers of foil prevents electrons from traveling from one foil layer to the other. When connected to an alternating current circuit, the applied voltage forces one side of the capacitor to fill with electrons while the other side discharges electrons. When current flow alternates (reverses), the empty side fills up while the full side discharges. See Figure 25‑34. Run Capacitors Start Capacitors Goodheart-Willcox Publisher Figure 25‑32. In HVACR motor control, capacitors are of two types: run and start. Run capacitors remain in the circuit as long as the motor runs. Start capacitors are energized only during motor start-up. Paper insulation Aluminum foil 189–227 mfd 240 V ac 60 Hz Electrical terminals A B Goodheart-Willcox Publisher Figure 25‑33. Construction of a capacitor. A—Long strips of aluminum foil, separated by strips of paper insulation, are wound into a tight coil. B—An electrical terminal is attached to each of the aluminum strips (electrodes or plates). S S Empty Empty Full Full Sine wave Goodheart-Willcox Publisher Figure 25‑34. Operation of a capacitor. When connected to an alternating current, the two foil electrodes alter- nately fill with electrons and discharge the electrons. Chapter 25 Motors 503 Copyright Goodheart-Willcox Co., Inc.