Apparatus for cooling the power electronics of a refrigeration compressor drive

Apparatus for cooling the power electronics components of a variable frequency drive for the motor of a refrigerant system compressor. The components are mounted upon a heat sink and refrigerant from the system condenser is passed through the heat sink by means of a flow line and returned to the low pressure side of the system. A control valve is mounted in the flow line which throttles refrigerant passing through the line to produce cooling of the heat sink to maintain the temperature of the components within a desired range.

BACKGROUND OF THE INVENTION 
This invention relates to method and apparatus for cooling of the 
electronics of a variable frequency drive associated with a refrigerant 
compressor. 
Compressors used in many refrigeration systems generally require close 
control over the compressor motor speed in order to maintain the system 
within desired limits under varying load conditions. The compressors are 
therefore equipped with variable frequency drives (VFD) that contain power 
electronic components in the form of insulated gate bipolar transistors 
that can overheat and thereafter require cooling. The generally accepted 
procedure to provide cooling to the power electronics is to mount the 
transistors upon a heat sink and carry the heat away from the sink by 
circulating coolant in or around the heat sink. The capability of the heat 
sink and cooling system are of primary consideration in determining the 
power capacity of the VFD. 
The heat sink is usually in the form of a relatively large block of 
material having good heat transfer and thermal inertia characteristics. A 
flow passage is formed in the block and coolant is circulated through the 
passage which absorbs excess heat and carries it out of the system. 
The use of water to cool the VFD heat sink has proven to be a satisfactory 
means of cooling the VFD transistors, however, water cooling is difficult 
to control and the heat sink temperature sometimes can move out of desired 
operating range. This, in turn, can produce overheating of the VFD 
electronics and adversely effect the operation of the refrigeration 
system. In addition, the water cooling circuit requires additional water 
handling components such as pumps, heat exchangers and the like needed to 
discharge heat from the transistors into the surrounding ambient. This 
type of cooling equipment is generally complex, costly and requires a good 
deal of space to install. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to improve 
refrigeration systems. 
It is a further object of the present invention to improve the cooling of 
the power electronics of a variable frequency drive used to control a 
refrigerant compressor. 
It is a still further object of the present invention to reduce the amount 
of space required by cooling equipment for the variable frequency drive of 
a refrigeration system compressor. 
Another object of the present invention is to more reliably control the 
cooling of the power electronic components of a variable frequency drive 
of a refrigeration system compressor. 
Still another object of the present invention is to provide refrigerant 
cooling to the power electronics of the variable frequency drive of a 
refrigeration system compressor. 
These and other objects of the present invention are attained in a closed 
loop refrigeration system that includes a condenser, an evaporator, and a 
compressor connected in series by refrigerant lines and an expansion means 
in one of the refrigerant lines for throttling refrigerant moving between 
the condenser and the evaporator from a high pressure to a lower pressure. 
A variable frequency drive is associated with the compressor that contains 
heat producing power electronic components in the form of insulated gate 
bipolar transistors that require cooling. The power electronic components 
are mounted in heat transfer relation with a block of material having good 
heat transfer characteristics. The block acts as a heat sink to draw heat 
away from the power electronic components. A flow circuit is arranged to 
pass refrigerant from the system condenser to the inlet of the system 
compressor through the heat sink. An expansion valve is mounted in the 
flow circuit which controls the expansion of refrigerant moving through 
the circuit, thus providing cooling to the heat sink and the electronic 
components thereon.

DESCRIPTION OF THE INVENTION 
Turning initially to FIG. 1, there is illustrated schematically a 
refrigeration system, generally referenced 10, that utilizes the Carnot 
refrigeration cycle that includes a series of refrigerant lines 12 that 
operatively connects the various system components. The system further 
includes a condenser 13 that is connected to the outlet side of a 
compressor 15 by means of a refrigerant line 12. The condenser is, in 
turn, connected in series with an evaporator 17, the outlet of which is 
connected via a refrigerant line to the inlet side of the compressor to 
complete the system loop. An expansion device 20 is mounted in the 
refrigeration line between the condenser and the evaporator which expands 
high pressure refrigerant leaving the condenser to a lower temperature and 
pressure. The expansion device can be any one of many such devices, such 
as a throttling valve or capillary tube of the types that are well known 
and used in the art. 
A substance to be chilled is circulated through the evaporator in heat 
transfer relationship with the low temperature refrigerant. The 
refrigerant, as it absorbs heat in the chilling process is evaporated at a 
relatively low pressure and the refrigerant vapor is then delivered to the 
compressor inlet for recirculation through the system. 
The compressor motor is equipped with a variable frequency drive (VFD) 25 
that controls the motor speed. The drive is shown in phantom outline in 
FIG. 1. As is well known in the art, the VFD typically contains power 
electronics that require cooling in order for the drive to operate under 
optimum conditions over the operating range of the system. In practice, 
the power electronic components requiring cooling are generally insulated 
gate bipolar transistors (IGBT) that are depicted schematically at 27 in 
the drawings. As noted above, the power electronic components have 
heretofore been cooled by placing them in heat transfer relation with a 
heat sink and circulating cooling water. This type of cooling system is 
rather complex, requires a good deal of space, and is difficult to 
control. 
As illustrated in FIG. 1, the power electronic components of the VFD are 
mounted directly upon a heat sink 30 that forms part of what is herein 
referred to as the VFD evaporator 29. The heat sink is fabricated from a 
block of material that has a high coefficient of thermal conductivity such 
that the heat energy generated by the power electronic components is 
rapidly drawn away from and absorbed into the heat sink. An internal flow 
channel 32 is mounted within the block of material. The channel follows a 
tortuous path of travel through the block of material to provide for a 
maximum amount of contact area between the channel and the heat sink. In 
practice, the flow channel can be a length of copper tubing or the like 
that is embedded in the heat sink and which has an inlet at 33 and an 
outlet at 34. 
The inlet 33 to the internal flow channel is connected to the refrigerant 
outlet 35 of the system condenser by a supply line 36. The outlet of the 
flow channel, in turn, is connected to the compressor inlet by a discharge 
line 39. A control valve, generally referenced 40, is contained in the 
supply line through which refrigerant is throttled from the higher 
condenser pressure down to a lower pressure thereby providing low 
temperature refrigerant to the heat sink for cooling the power electronic 
components. 
The control valve 40 is shown in greater detail in FIG. 5. The valve 
includes a sensor probe 42 that is embedded in the heat sink as close as 
practicable to the power electronic components that will best reference 
the operating temperature. The valve may be a temperature control valve 
which responds to the temperature sensed by the probe or a temperature 
expansion valve which responds to pressure changes at the probe produced 
by temperature changes in the heat sink. In this embodiment, the valve is 
a temperature expansion valve that includes a diaphragm 43 mounted inside 
a housing 44. Based upon the temperature of the heat sink, the bulb 
pressure changes which, in turn, sets a pressure on the high side chamber 
45 of the diaphragm. The pressure on the low side chamber of the diaphragm 
46 is determined by a preset adjustable spring 47 and an equalizing port 
49 that extends between the low pressure side of the chamber and the low 
pressure side of the valve body 50. The pressure balance across the 
diaphragm of the valve locates the valve body within the valve passage and 
thus controls the amount of cooling provided to the heat sink. Preferably, 
the heat sink temperature is held within a range of between 90.degree. and 
140.degree.. 
As can be seen from the disclosure above, the heat sink with the flow 
channel passing therethrough acts as a refrigerant evaporator with regard 
to the VFD to provide closely controlled cooling to the power electronic 
components by utilizing the refrigeration cycle to remove heat from the 
VFD. As can be seen, the heat transferred to the refrigerant in the VFD 
evaporator is moved by the system compressor to the system condenser where 
it is rejected into the condenser cooling loop. 
FIG. 2 depicts a further embodiment of the invention wherein like 
components described with reference to FIG. 1 are identified with the same 
reference numbers. In this embodiment of the invention the discharge line 
39 of the VFD evaporator is connected into the system evaporator 17 and 
combined with refrigerant being processed through the evaporator. The 
valve sensor 42 is shown mounted upon the discharge line of the VFD 
evaporator rather than embedded in the heat sink. The sensor feeds back 
temperature information to the control valve 40 which, in turn, sets the 
positioning of the valve body in response to the sensed refrigerant 
temperature to hold the sink temperature within the desired operating 
range needed to cool the power electronic components. 
Turning now to FIG. 3, there is shown a still further embodiment of the 
invention where again like numbers are used to identify like previously 
identified components. In this further embodiment of the invention the 
control valve 40 is mounted in the discharge line of the VFD evaporator 29 
which, in this case, is connected directly to the compressor inlet. 
However, as noted above, the discharge line may alternatively be connected 
directly to the system. The temperature sensor 42 is embedded in the heat 
sink 30 of the VFD evaporator and provides temperature related information 
to the control valve. Typically, the temperature of the refrigerant 
leaving the system condenser is below 140.degree. F. so that the 
refrigerant shunted to the VFD evaporator is well within the desired heat 
sink temperature range required for cooling the power electronic 
components. 
FIG. 4 illustrates a still further embodiment of the invention wherein like 
numbers are again used to identify previously above-identified components. 
In this embodiment of the invention, part of the refrigerant leaving the 
system condenser is expanded into the VFD evaporator 29 through a 
temperature control valve 40. A temperature sensor 42 is again embedded in 
the heat sink 30 and provides temperature related information to a 
microprocessor 50 that is programmed to process the data and send a 
control signal to the valve. Other system related information can also be 
sent to the microprocessor which can be additionally processed to arrive 
at a desired valve setting to provide cooling to the power electronics at 
a minimum of expense to the system's overall performance. 
As evidenced from the disclosure above, the present invention is a simple 
yet effective solution to cooling the power electric components of a 
variable frequency drive for a refrigerant compressor. The present system 
eliminates the complexities of the more traditional water cooling systems, 
is easier to install, and provides for greater control over the cooling 
process. The present system, because of its efficiency, also allows for 
greater use of the power electronics having a greater capacity than those 
presently found in the prior art used in the compressor drive of a 
refrigeration system. 
While this invention has been explained with reference to the structure 
disclosed herein, it is not confined to the details set forth and this 
invention is intended to cover any modifications and changes as may come 
within the scope of the following claims: