Texas Instruments Corporation

Joseph L McMorrow

Radi Pejouhy

A motor protector characterized by low cost, by improved service life and by improved cycling properties has a bimetallic element mounted in a metal housing to move a first contact along a selected axis to engage and disengage a complementary contact in response to changes in element temperature. An electrical resistance heater coil, preferably with less than one full coil convolution, is oriented and secured externally of the housing so that the axis of the magnetic field established by the heater coil is coincident with the axis of movement of the first contact. The properties of the heater are selected relative to the thermal mass of the protector so that, when the heater and the protector contacts are arranged in series with motor windings, the external heater is adapted to heat the entire thermal mass of the protector to a sufficient temperature in response to the occurrence of selected overload or fault currents in the motor windings to actuate the bimetallic element to open the winding circuit before excessive overheating of the motor windings can take place. In this arrangement, the heater orientation avoids magnetic deflection of arcs occurring during opening of the protector circuit, thereby improving service life, and the heating of the entire thermal mass of the protector in opening the protector circuit retards subsequent reclosing of the circuit, thereby improving cycling properties of the protector. Preferably, the protector is mounted in a common housing with motor starter means to utilize common terminals with the stater means. Also, where the protector is used to protect a motor in a sealed refrigeration compressor system, the protector is preferably mounted in spaced but closely adjacent relation to the compressor shell, thereby to avoid reduction in cycling time by avoiding draining of heat from the protector into the shell and thereby to avoid opening of the protector when the shell is heated during normal compressor operation while permitting heat transfer from the shell during the occurrence of a sustained fault condition in the motor to further improve protector cycling time.

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West Bridgewater, MA, United States of America

Joseph L McMorrow