the shell. Connections outside the shell are made by the technician. See Figure 25‑21. One leg of the power supply (usually the white wire) is connected to the overload at Terminal 1. Terminal 3 from the overload is connected to the common terminal on the compressor. On hermetic compressors, the overload is always connected in series with the common. The other leg of the power supply (black wire) is connected to the relay and continues to the run terminal on the shell. The start winding is connected to a switch inside the relay. At this point, it is only necessary to understand the principles involved with these connections. The types of relays, their operation, and connection are explained in Chapter 26. 25.3.3 Permanent Split-Capacitor Motor The permanent split-capacitor (PSC) motor is a split-phase motor with a capacitor in series with its start winding. It uses this design change to increase running torque, Figure 25‑22. The PSC motor is used where starting load is low but running load is high. The high running load would cause too much slip in a split-phase motor. To overcome excessive slippage, a method was devised that uses the start winding to assist the run winding. Centrifugal weight Contact points Rotor shaft Movable collar Rotor speed causes collar to move and open contacts When rotor is at standstill, contact points are closed Goodheart-Willcox Publisher Figure 25‑20. A centrifugal switch opens when the motor reaches 75% of operating speed, disconnecting the start winding. As rotor shaft speed increases, the weights swing outward, sliding the movable collar along the shaft. The movement of the collar is transmitted through mechanical linkage, opening the contacts. 1 3 Start Run N L1 Black White Overload Current-type starting relay Run terminal Start terminal Common terminal Split-Phase Motor Goodheart-Willcox Publisher Figure 25‑21. Connections for the hermetic split-phase motor are made to the three insulated terminals (Common, Start, and Run) extending through the shell of the compressor. Chapter 25 Motors 497 Copyright Goodheart-Willcox Co., Inc.