Run Capacitor The capacitor on PSC motors is called a run capacitor because it is designed and used to create better running torque. However, as shown in Figure 25‑23, the run capacitor is connected in series with the start winding. The run capacitor is usually an oval-shaped, metal cylinder with push-on-type electrical connec- tions on its top. See Figure 25‑24. Capacitor sizes vary and are prescribed by the motor manufacturer to match the operating characteristics of the PSC motor windings. When capacitor replacement is necessary, always use an exact replacement. Capacitors are described in greater detail later in this chapter. Open-Type PSC Motor Open-type PSC motors, like the one shown in Figure 25‑22, are popular for operating large fans and blowers. They are also used for condenser and evaporator fans. At start-up, the motor load is very low. At full speed, the fan blades are fully loaded by the weight and volume of the air being moved. Such a large load would cause a split- phase motor to slip too much, but the PSC motor solves the slippage problem by using both the run winding and the start winding. Reversing Rotation. The rotation of a PSC motor is reversed by simply switching the two wires connected to the start winding if the motor supplies them outside the motor. See Figure 25‑25. By connecting a capacitor in series with the start winding, the magnetic field of the start winding is reduced so it does not interfere with the run winding. See Figure 25‑23. The capacitor shifts the phase of the power in the start winding. By eliminating the conflict of magnetic fields, the start winding can assist the run winding. A controlled use of the start winding eliminates the need to disconnect the start winding after start-up. As the name permanent split capacitor indicates, the capacitor is a permanent part of the circuit. Since the start winding is not disconnected, the PSC motor has no need for a centrifugal switch or relay. Run capacitor Image courtesy of Johnstone Supply Figure 25‑22. An open-type PSC motor. The run capacitor is used to create better running torque. Run capacitor Overload Start winding Run winding Goodheart-Willcox Publisher Figure 25‑23. In a PSC motor, the run capacitor is wired in series with the start winding. It shifts the phase of the electrical power, which shifts the start winding’s magnetic field. 30/5 mfd 370 V 10 mfd 370 V 15 mfd 370 V 15 mfd 370 V 5 mfd 370 V Goodheart-Willcox Publisher Figure 25‑24. Run capacitors are similar in appearance but vary in size because various PSC motors require various capacities. 498 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.