Variable thermal model overload in electrical switching apparatus

摘要

A motor starter for an electrical circuit includes separable contacts for switching an electrical current flowing between a power source and a motor; an operating mechanism responsive to a trip signal for opening and closing the separable contacts; a separable contact state detection circuit providing an operating signal having two states corresponding to the open and closed positions of the separable contacts; a current transformer sensing the electrical current flowing between the power source and the motor and providing a sensed current value; and an overload relay generating the trip signal as an I2t function of the sensed current value and the two states of the operating signal. The overload relay employs a pair of thermal time constants which closely model the motor under both natural convection and forced air cooling conditions.