Scroll compressor protector

The motor protector for a motor is located so as to be responsive to the discharge temperature of a compressor. The protector is thereby responsive to motor current and compressor discharge temperature. Additionally, by conduction, the protector is responsive to the motor temperature. For scroll compressors, specifically, this provides protection from excess heating of the scroll wraps without requiring an additional sensor.

BACKGROUND OF THE INVENTION 
In hermetic compressors used in refrigeration and air conditioning 
applications, the motors of the compressors are provided with overload 
protection. Typically, the motor protection is in the form of a bimetal 
switch or contact that opens an electrical circuit responsive to motor 
current and/or motor winding temperature. Except for isothermal processes, 
gases undergoing compression are heated. In scroll compressors 
specifically, the compression process produces a thermal gradient which 
results in a differential thermal expansion of the scroll wraps. Various 
schemes have been employed to overcome the effects of the differential 
expansion, particularly the axial expansion of the wrap, as evidenced by 
U.S. Pat. Nos. 4,457,674 and 4,472,120. Thus, when a scroll compressor is 
operating at design conditions, there will be contact/sealing between the 
tips of the scroll wraps and their facing plates for most, if not all, of 
the wrap length. Any non-contacting areas would be in the outer/low 
pressure region of the wraps where leakage would not be a significant 
problem. If, however, there should be a further heating of the scroll 
wraps beyond the design operating temperature such as the result of a 
blocked fan or fan failure, thermal expansion will be greatest and produce 
the highest loads on the inner wraps. This can result in localized failure 
of the wrap(s) and galling before the motor gets hot enough to trip the 
thermal protector. Even if this may result in severe localized wear 
resulting in leakage and poor performance at design operating conditions. 
SUMMARY OF THE INVENTION 
The protector for the motor of a hermetic scroll compressor is located at 
or near the discharge port of the scroll elements. The thermal protector 
is then responsive to motor overcurrent in the conventional manner, is 
responsive to motor overheating through conduction via the motor leads, 
and is, additionally, responsive to compressor discharge temperature. 
Because the protector is placed directly in the discharge gas stream 
immediately adjacent to the discharge port of the scroll elements, it can 
react more quickly then it could if attached in the conventional manner 
directly to the motor windings and far from the discharge gas stream. 
It is an object of this invention to improve reliability of scroll 
compressors. 
It is another object of this invention to eliminate the need for an 
additional sensing means to protect the scroll elements from overheating. 
These objects, and others as will become apparent hereinafter, are 
accomplished by the present invention. 
Basically a line break protector is placed in the discharge gas stream 
close to the discharge port in a scroll compressor rather than being 
directly attached to the motor windings. The protector senses both the 
motor current and motor temperature (via conduction) as is conventional 
and, additionally, senses the discharge gas temperature near the discharge 
port with rise rates similar to those obtained in the scroll vanes, 
thereby eliminating the use of additional sensors to properly protect a 
scroll compressor from overtemperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1 and 2, the numeral 10 generally designates a low side hermetic 
scroll compressor. A low side compressor is one in which all or most of 
the interior of the shell 11 is at suction pressure. A top cap 12 is 
secured to shell 11 in a fluid tight relationship. Separator plate 16 is 
secured to shell 11 and discharge tube 18 so as to coact with top cap 12 
to define discharge chamber 22 and to separate discharge chamber 22, which 
is at discharge pressure, from the rest of the interior of shell 11, which 
is at suction pressure, during operation. Discharge tube 18 is also 
connected to outlet 15 in fixed scroll 14. 
As is conventional, electric power is supplied to compressor 10 through 
shell 11 via hermetic terminal 24 which is wired to motor 26. Normally, 
protector 40 is located on the windings of motor 26 so as to be responsive 
to the temperature of the motor as well as the current. However, according 
to the teachings of the present invention, protector 40 is relocated from 
the motor windings to discharge chamber 22. This requires a second 
hermetic terminal 25 to be located in separator plate 16. Protector 40 is 
located in discharge chamber 22 near the outlet of discharge tube 18. As a 
result, gas passing into discharge chamber 22 passes over protector 40 
whereby protector 40 will be maintained at a temperature close to that of 
the gas passing from outlet 15 and the temperature of orbiting scroll 13 
and fixed scroll 14. 
In operation, motor 26 drives orbiting scroll 13 through crankshaft 28 and 
orbiting scroll 13 is held to an orbiting motion by Oldham coupling 30. 
Orbiting scroll 13 coacts with fixed scroll 14 to compress the gas and in 
compressing the gas the gas is heated. The hottest gas is at the center of 
the fixed and orbiting scrolls and the greatest thermal expansion takes 
place there. As noted above, at design operating conditions the wraps of 
each scroll are in engagement with the plate portion of the facing scroll. 
Thus, any further temperature rise across the scrolls will reduce the area 
of contact by the wrap tips localizing the wear and stress at the 
innermost portions of the wrap which are already exposed to the highest 
pressures. The gas acted on by the scrolls 13 and 14 serially passes 
through outlet 15 and discharge tube 18 into discharge chamber 22 and in 
so doing passes over protector 40 which is thereby subjected to the 
highest temperatures encountered by scrolls 13 and 14. The hot, high 
pressure gas delivered to discharge chamber 22 is supplied to the 
refrigeration system (not illustrated) via discharge line 23. 
Referring now to FIG. 2, it will be noted that for a single phase wiring 
configuration, motor 26 has two main windings 26-1 and 26-2 which are 
located in parallel with each other and in series, via terminal 25, with 
main winding heater 40-1 which is located in protector 40. Motor 26 also 
has start windings 26-3 and 26-4 in series, via terminal 25, with start 
winding heater 40-2 which is located in protector 40. Bimetal switch 40-3 
is located in protector 40 in proximity to heaters 40-1 and 40-2 and is 
connected directly to the common lead. Thus, overcurrent to windings 26-1, 
26-2, 26-3 and/or 26-4 will cause heaters 40-1 and/or 40-2 to heat bimetal 
switch 40-3 sufficiently to cause it to open thereby breaking the 
electrical circuit and thereby stopping motor 26 and compressor 10. 
Because protector 40 is in heat transfer relationship with the gas passing 
from discharge tube 18, bimetal switch 40-3 is also responsive to the 
temperature of the discharge gas, which reflects the highest temperature 
of the scrolls, and will open responsive to an excessive discharge 
temperature. Protector 40 would also be responsive to the temperature of 
windings 26-1 and 26-2 through conduction via the leads to hermetic 
terminal 25 and due to the heating of suction gas which cools the motor 
windings 26-1 and 26-2 and also contacts separator plate 16. 
In FIG. 3, the parts have been labeled 100 higher than similar structure in 
FIG. 2. The low side hermetic scroll compressor 110 has a motor 126 which 
is in a three phase configuration as is protector 140 and they are 
connected via terminal 125. Main windings 126-1, 126-2 and 126-3 are 
respectively located in the three power lines. Protector 140 includes main 
winding heaters 140-1, 140-2 and 140-3 and associated ganged bimetalic 
switches 140-4, 140-5 and 140-6, respectively. As in the case of the FIG. 
2 embodiment, overcurrent in one or more of the windings 126-1, 126-2 
and/or 126-3 will cause the corresponding heater 140-1, 140-2 and/or 140-3 
to heat and thereby open all of the ganged bimetal switches 140-4, 140-5 
and 140-6 breaking the circuit and stopping motor 126. As in the case of 
protector 40, the protector 140 is located so as to be responsive to the 
temperature of the gas passing through discharge tube 18 as well as being 
responsive to the temperature of windings 126-1, 126-2, and 126-3 via 
thermal conduction. 
Although preferred embodiments of the present invention have been 
illustrated and described, other changes will occur to those skilled in 
the art. It is therefore intended that the scope of the present invention 
is to be limited only by the scope of the appended claims.