Motor protector

Abstract

A motor protector for a split-phase motor having a starting switch, the arm of which is connected to the run winding and which moves from a first position in engagement with a first contact to energize the start winding into a second position in engagement with a second contact thereby to deenergize said start winding upon the motor attaining a predetermined starting speed. The protector includes a thermostatic element to be positioned in heat-exchange relationship with the motor windings and which is heated by the flow of electrical current therethrough. It has a first curvature position at ambient temperatures and moves to a second curvature position at an elevated predetermined temperature. The protector further includes a switch having contacts which are opened in response to movement of the thermostatic element into its second curvature position. The thermostatic element is serially connected with the contacts of the latter switch in a circuit between one terminal of an a.c. power source and the switch arm, and the thermostatic element is shunt-connected across said switch arm and the aforesaid second contact. While the motor is operating below its starting speed, the thermostatic element is responsive to motor overcurrent conditions that cause self-heating of the element to the elevated predetermined temperature thereby to open the contacts of the switch and deenergize the motor windings. While the motor is operating at speeds in excess of its starting speed the motor current is shunted around the thermostatic element and is responsive to its temperature rising above the pedetermined elevated temperature substantially as a sole function of the motor winding temperature thereby to move into its second position and open the switch contacts to deenergize the motor windings.