Thermally responsive switch

A thermally responsive switch which is used to cut off AC current flowing through a compressor motor, the thermally responsive switch comprising:

• a hermetically sealed container including a metal housing and a header plate hermetically secured to an open end of the housing;
• at least one conductive terminal pin inserted through a through hole formed through the header plate and hermetically fixed in the through hole by an electrically insulating filler;
• a fixed contact fixed to the terminal pin in the container;
• a thermally responsive plate having one of two ends conductively connected and fixed to an inner surface of the container and formed into a dish shape by drawing so as to reverse a direction of curvature at a predetermined temperature;
• and
• at least one movable contact secured to the other end of the thermally responsive plate and constituting at least one pair of switching contacts together with the fixed contact;
• wherein each of the fixed contact and the movable contact comprises a silver-tin oxide system contact,
• wherein the container is filled with a gas containing helium ranging from 50% to 95% so that an internal pressure of the container ranges from 0.35 atmospheres to 0.7 atmospheres at room temperature,
• wherein an intercontact distance between the contacts ranges from 0.7 mm to 1.5 mm,
• wherein each contact has a minimum diameter ranging from 3 mm to 5 mm, and
• wherein a switching operation ranges from 19,000 times to 24,000 times when a durability test in which an energized state and a substantially two-minute de-energized state are repeated alternately is conducted under following conditions:
• (a) a power supply of 240 V 50 Hz is applied to a locked electric motor so that a locked-rotor current of 52 A is caused to flow into the motor,
• (b) the container is filled with 90%-helium and 10%-dried air,
• (c) each of the movable and fixed contacts contains 11.7 weight percentage of metal oxide and has a three layer structure including an intermediate layer comprising copper and a lower layer comprising iron, is formed into a disc shape with a diameter of 4 mm and a thickness of 0.9 mm and has a contact surface formed into a spherical shape with a radius of 8 mm and an intercontact distance of 1 mm in an open state, and
• (d) the thermally responsive plate is set to reverse its direction of curvature in a contact opening direction at 160° C. and in a contact closing direction at 90° C.