Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 263 The current magnetic relay (CMR) is made with a low-resistance coil and a normally open set of contacts. The coil is wired in series with the run window. The coil’s low resistance does not drop significant voltage or affect the run winding operation. The coil requires a relatively large current compared to a standard relay to pull in the relay armature and close the NO contacts. The NO con- tacts are made of one fixed contact and a moving contact that rests on the bottom of an enclosure by gravity. This construction makes installation critical. The open contact must be facing downward. The NO con- tacts are connected in series with the start winding. See Figure 13-19. When the motor is turned on, current flows through the run winding only due to the NO contacts. The run winding increases as it induces an increasing current in the rotor, yet the motor cannot yet start. The increased run winding current produces a strong magnetic field in the relay coil and closes the NO contacts. The start winding energizes, so the motor can start. As the motor reaches about 75% of rated speed, the run winding current decreases. The lower current reduces the relay coil mag- netic field strength. The force of gravity is thus greater and pulls the contact down, and it opens the start circuit. A PTC relay uses a PTC thermistor in series with a start winding, or both start capacitor and start winding, to control the current. When power is first turned on, the thermistor is at room temperature and exhibits low resis- tance. As start winding current increases, the thermistor heats up, and its resistance increases upwards of 20 kΩ. This high resistance effectively removes the winding from the circuit. The PTC relay has to be cooled down three to five minutes before the motor can be restarted. The PTC can also be used on a PSC motor as an assist start-up device. The thermistor is wired in parallel to the run capacitor, so when power is applied to the motor, cur- rent flows through the PTC and start winding for added torque. When the PTC heats up, current feeds the start winding through the run capacitor. See Figure 13-20. Goodheart-Willcox Publisher Figure 13-19. Current magnetic relay. Run winding terminal Start winding terminal Start capacitor Contacts (open) Weight Current relay Common terminal Common terminal L1 L2 L1 L2 Run winding terminal Start winding terminal Start capacitor Current relay Weight Contacts (closed) Starting Mode Start winding terminal Run winding terminal L1 L2 PTC thermistor PTC relay Run capacitor Common terminal Running Mode L1 Start winding terminal Run winding terminal L2 PTC thermistor PTC relay Run capacitor Common terminal Goodheart-Willcox Publisher Figure 13-20. A PTC used to assist the starting of a PSC motor.