Air conditioning compressor protection device

A method of locating a limit switch (10) on a compressor (12) utilizes thermography for determining the optimum location thereof. A thermograph is produced for normal and abnormal operating modes of the compressor (12). High temperature gradients in the thermograph indicate high rates of temperature increases during compressor failure modes for selecting an optimum location for the limit switch. A bimetal switch (10) is utilized to establish a cut-out temperature for preventing operation of the compressor (12) and a cut-in temperature lower than the cut-out temperature for automatically closing to resume operation of the compressor (12) when returned to normal temperatures.

TECHNICAL FIELD 
The invention relates to thermal protection devices for compressors used in 
cooling systems, and more particularly the determination of the location 
thereof. 
BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
The invention includes a method of positioning a limit switch on a 
compressor for measuring the temperature of the compressor and for 
allowing operation of the compressor during normal operation when the 
temperature thereof is less than a cut-in temperature and for preventing 
operation of the compressor during abnormal operations when the 
temperature thereof is greater than a cut-out temperature. The method 
includes the steps of operating a compressor near failure mode, measuring 
abnormal thermodynamics of the compressor, establishing a thermal 
differential location with respect to normal operation, and positioning 
the limit switch at the thermal differential location for sensing and 
controlling abnormal operation of the compressor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The combination of a thermal limiter switch 10 and compressor 12 in an air 
conditioning unit of a vehicle is generally illustrated in FIG. 1. 
Operation of the compressor 12 is belt driven off the crank shaft through 
the compressor clutch pulley 16. The rotation of the clutch pulley 16 is 
transferred to the compressor 12 only when the compressor clutch 18 is 
engaged, which is accomplished when current flows through the compressor 
clutch coil 20. As commonly known in the art, the compressor 12 compresses 
the low pressure refrigerant vapor into a high pressure high temperature 
vapor which is thereafter transferred to the condenser. Other compressors 
are not using a clutch to engage the compressor with the drive mechanism. 
As long as electricity is used to modulate pumping capacity between 0 and 
full ouput the proposed method of use of a thermal limiter switch can be 
used to protect the compressor from failure. Electric cars will use 
electrically driven and controlled compressors. The thermal limiter use 
and its method of placement for optimum performance can be used to provide 
compressor protection from failure. 
The thermal limiter switch 10 interconnects power to the electromagnetic 
clutch 18 via the coil 20 to other controls or to the electric motor 
driving the compressor. The limiter switch 10 is be connected in either 
the supply or return power conductor 21, 23 of the compressor 12. The 
thermal limiter switch 10 has an open condition preventing current to flow 
therethrough to the clutch coil 20 and to electrical controls or to the 
electric motor driving the compressor thereby preventing operation of the 
compressor 12 and a close position allowing current to flow to the clutch 
coil 20 allowing engagement of the electromagnetic clutch 18 with the 
compressor 12 and operation of electric controls or the electric motor 
driving the compressor. The thermal limiter switch 10 senses temperature 
and moves to its open condition when the sensed temperature is greater 
than a cut-out temperature, and moves to its closed position when the 
sensed temperature decreases to less than a cut-in temperature thereby 
indicating normal operation. 
The positioning of the limiter switch 10 on the compressor 12 is important 
to provide protection in the case of overheating. Such overheating and 
therefore abnormal operation may occur due to any of the following: loss 
of charge; resistive mechanical failure, such as bad rings or improper 
tightening; and in some compressors, due to failed thermostat expansion 
valve (TXV). Each of these failures cause increased heating of the 
compressor 12 which may be sensed to indicate abnormal operation thereof. 
The limiter switch 10 is fastened to the compressor 12 at a location where 
heat build-up rate is maximum. 
Infrared thermography is used to detect failure modes before actual failure 
occurs. Thermography allows the monitoring of extremely hot areas on 
compressors 12 which occur when subjected to failure or abnormal mode 
conditions, such as loss of refrigerant. FIGS. 1-3 illustrate the 
application of the method on a compressor 12 manufactured by General 
Motors, Harrison Division, of the type HR-6. 
The method of determining location or placement of the limiter switch 10 on 
the compressor 12 includes an initial step of measuring and graphing the 
normal and abnormal thermal condition of heat up and cool down. A 
thermographing device, such as infrared camera equipment by Hughes, 
provides a photograph of the thermograph or a CRT screen display. The 
compressor 12 is operated under normal operating conditions with no 
failures and a normal thermograph 19 is produced by taking a picture of 
the external compressor 12 and external housing 22 thereof, as illustrated 
in FIG. 2. Thereafter, failure or abnormal operation is simulated in the 
compressor 12 by either eliminating charge, creating excessive mechanical 
friction, etc. An abnormal thermograph 21 is produced of the external 
surface of the compressor 12 as illustrated in FIG. 3. Based on the 
thermographics 19, 21, an optimum sensing spot may be identified for each 
compressor type. The abnormal and normal graphs 19, 21 are compared to one 
another to determine the location of a thermal differential. The blue area 
36 represents 200.degree.-220.degree., green 38 represents 
220.degree.-270.degree., yellow 40 represents 270.degree.-300.degree., 
orange 42 represents 300.degree.-320.degree., and red 44 represents 
320.degree.-360.degree.. This location is identified by a large 
differential, typically 100.degree. to 150.degree. gradient, i.e., the red 
area 44. The location of this differential is dependent upon the design or 
type of the compressor 12. Such dependency depends on gas passages, 
resistive mechanics, etc. It is desirable to place the limiter switch 10 
at this location on the housing of the compressor 12, as illustrated in 
FIG. 1. It has been determined by the above method that the optimum 
location of the limit switch 10 is opposite the inside gas crossover 
passage in the case of the HR-6 type Harrison Division Compressor. 
With regard to the limiter switch 10, a bimetal snap acting disc sold by 
Texas Instruments, Klixon 7AM thermal protector, is suitable. 
Alternatively, a thermistor or other type of sensor may be used. The 
limiter switch 10 allows for a differential between the cut-out 
temperature and the cut-in temperature of approximately 100.degree. F. As 
illustrated in FIG. 8, the cut-in is set at 160.degree. and the cut-out is 
set at 260.degree. . This differential allows the operator of the vehicle 
to detect the loss of performance of the air conditioning unit, yet 
adequately protect the compressor 12 by assuring sufficient cool down 
before resuming operation. The limiter switch 10 is normally closed and 
conducts current to the compressor clutch 18. If the temperature on the 
housing of the compressor 12 increases past the cut-out temperature, the 
switch 10 is opened. 
Compressor 12 operating temperature is limited to below a certain 
temperature corresponding to switch cut-out temperature setting. After the 
temperature cools to a certain temperature, corresponding to the reset 
point of the limiter switch 10, the switch 10 cuts in and compressor 12 
operation resumes. As illustrated in FIG. 8, line A represents the 
temperature of the housing 22 of the compressor 12, and line B indicates 
the ambient temperature around the compressor 12. The positive and 
negative peaks of line A represent the cut-out and cut-in temperatures as 
inhibited by the switch 10. 
The limiter switch 10 may be connected to the compressor 12 against the 
outer housing surface 22 thereof as illustrated in FIG. 1. The limiter 
switch 10 may be mechanically attached by any mechanical connector, as 
commonly known in the art, such as fasteners 25. The limiter switch 10 
must be environmentally and electrically protected. An epoxy coating 24 
may be utilized to coat the limit switch 10 to electrically insulate the 
switch 10 from the compressor housing 22. 
Alternatively, a cavity 26 may be formed in the compressor housing 22 
within which the limit switch 10 is inserted as illustrated in FIGS. 6 and 
7. An electrical shrink wrap material 28, i.e., nylon, is placed about the 
switch 10 to electrically insulate the same from the compressor 12. 
Thereafter, the switch 10 is placed within the cavity 26 and injection 
sealed therein with a rubberized material 30. 
FIG. 4 illustrates a compressor of the type R4 manufactured by General 
Motors, Harrison Division, with the limit switch 10', directly above one 
of its radial pistons 32 on the outside surface 22', of the shell of the 
compressor 12'. 
FIG. 5 illustrates a compressor of the type V5 manufactured by General 
Motor's, Harrison Division, with the limit switch 10" on the front face 
surface of the body 22" mounted opposite the thrust bearing 34. In this 
case, the limit switch 10" may also protect against overheating caused by 
the clutch slippage or impending bearing failure. 
The invention has been described in an illustrative manner, and it is to be 
understood that the terminology which has been used is intended to be in 
the nature of words of description rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is, therefore, to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.