Thermal limiting devices which protect compressors from premature failure are generally known in the art. A variety of devices have been utilized to sense the thermal characteristics of the compressor, including multiple contacts, fuses, thermistors, and bimetallic switches.
U.S. Pat. No. 3,702,065, issued Nov. 7, 1972 in the name of Jacobs and assigned to the assignee of the subject invention, discloses an ambient temperature sensing switch and refrigerant superheat temperature responsive switch for an automobile air conditioning system having an electromagnetic clutch for transmitting engine rotation to a refrigerant compressor. The ambient temperature sensing portion of the switch is connected between the automobile battery and the coil of the clutch for energizing the coil whenever ambient temperatures are above a predetermined level. A thermistor in circuit with the refrigerant temperature responsive switch delays short circuiting of the clutch coil to prevent immediate blowing the fuse. The fuse will be permanently blown and must be replaced in order to once again energize the clutch of the compressor.
U.S. Pat. No. 4,059,366, issued Nov. 22, 1977 in the name of Gannaway discloses a thermal overload protective system for protecting the compressor. The thermal overload system eliminates the need for terminal seals typically employed because of the thermal switch positioning in the flow path of the gas. A thermistor is positioned in the body of at least one cylinder head of the compressor so as to be in good heat exchange relation with the gas being discharged but not directly exposed to the gas flow.
U.S. Pat. No. 4,704,072, issued Nov. 3, 1987 in the name of Nakajima et al, discloses a compressor with a rotation sensor disposed in the portion provided for mounting the magnetic clutch. The rotation sensor is comprised of a detectable portion corotatable with the drive shaft of the compressor and a detecting portion disposed on a cylindrical head in confronting relation to the detectable portion. The rotation sensor is disposed outside of the seal means disposed between the drive shaft and the cylinder head for providing a hermetic seal therebetween. The contact plates are made of thermally deflectable material such as shape memory alloy, a thermal metal or bimetal, so that when the ambient temperature exceeds a predetermined value, the plates expand radially outwardly from each other to thereby hold the detecting contacts out of engagement with the detectable contacts.
It is found that mixed results occur with the random placements of the limit switches. None of the prior art ensures safe operating conditions by reengaging the clutch only when the temperature decreases to the normal operating temperature.