Eaton Corporation

William A Autio

Edward A Mallonen

Improved response in an ambient temperature compensated thermal overload relay is obtained by providing a bimetal element (2) wherein the main bimetal section (4) is coupled through a reversed compensating bimetal section (6) to a heat sink mounting base (8) either directly or through a heat transfer control member (18) and placing the heater (10) that is energized by the overload current only adjacent the main bimetal section (4). With this arrangement, the main bimetal section (4) bends in the switch tripping direction and the compensating bimetal section (6) bends in the subtractive direction in response to temperature change therein to reduce the net deflection for ambient temperature compensation under normal operating conditions and gradual small overload conditions. The heater (10) provides a temperature differential between the main (4) and reverse (6) bimetal sections although the reverse bimetal section (6) is subject to some conduction temperature rise above ambient temperature due to controlled thermal conduction from the main bimetal section (4) thereto and therethrough to the heat sink (8). But under fast high overload conditions, the reverse bimetal section (6) has insufficient time to incur a substantial amount of the conduction temperature rise to afford quicker tripping of the switch (16) for improved response. The ratio of the lengths of the main and reverse bimetal sections (4,6) is set or selected for the desired compensation.

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Menomonee Falls, WI, United States of America

William A Autio