VARIABLE FREQUENCY DRIVE HEAT SINK ASSEMBLY
Cross-Reference to Related Application
[0001] Reference is made to and this application claims priority from and the
benefit of U.S. Provisional Application Serial No. 61/487,078, filed May 17, 2011, and
entitled VARIABLE FREQUENCY DRIVE HEAT SINK ASSEMBLY, which
application is incorporated herein in its entirety by reference.
Background of the Invention
[0002] This invention relates generally to the cooling of a variable frequency
drive and, more particularly, to the cooling of a variable frequency drive associated with
a refrigerant vapor compressor of a transport refrigeration unit.
[0003] Power electronic devices are commonly used for controlling and/or
manipulating the characteristics, for example the frequency and/or the voltage, of the
electric power being supplied to a variety of electrically powered devices. For example,
variable frequency drives are commonly used in connection with variable speed motors
for controlling the speed of the motor. Variable speed motors are used in connection
with compressors, water pumps, fans and other devices. For example, refrigerant vapor
compressors, such as, but not limited to scroll compressors, reciprocating compressors
and screw compressors, to enable driving the compression mechanism of the
compressor at various operating speeds. As the operating speed of the compression
mechanism is decreased, the output capacity of the compressor is decreased, and
conversely as the operating speed of the compression mechanism is increased, the
output capacity of the compressor is increased. The variable frequency drive is
operative to vary the frequency of the electric power supplied to drive motor of the
compressor, thereby varying the operating speed of the motor, and consequently the
operating speed and output capacity of the compressor.
[0004] Transport refrigeration units are commonly used in connection with
transport containers for maintaining the cargo box wherein a perishable cargo, such as
for example, but not limited to, fresh produce, is stowed at a temperature within a
specified temperature range to maintain freshness and minimize spoilage during transit.
The transport refrigeration unit includes a refrigerant vapor compressor and
condenser/gas cooler disposed externally of the cargo box and an evaporator disposed
within the enclosed space of the cargo box. The compressor, condenser/gas cooler and
evaporator are connected in a refrigerant circuit in series refrigerant flow relationship in
a refrigeration cycle. When the refrigeration unit is operating, air is drawn from within
the cargo box, passed through an evaporator in heat exchange relationship with the
refrigerant circulating through the refrigerant circuit thereby cooling the air, and the air
is supplied back to the cargo box.
[0005] To achieve precise temperature control while maintaining system
operational efficiency, it is necessary to vary the refrigeration output capacity of the
refrigeration unit in response to the refrigeration load demand. For example, during
temperature pulldown after exposure of the cargo box to ambient temperature such as
during loading of cargo into the cargo box, the compressor of the refrigeration unit is
typically operated at maximum output capacity. However, during long periods of
operation in a temperature maintenance mode following pulldown, the compressor of
the refrigeration unit is operated at low capacity, and often at near zero capacity.
[0006] One method of varying the refrigeration capacity of the refrigeration unit
is to vary the speed of the compressor using a variable frequency drive as discussed
previously to modify the frequency of the electric power being supplied to the electric
motor driving the compressor. However, employing a variable frequency drive in
controlling compressor speed in connection with a transport refrigeration unit presents a
challenge in adequately cooling the power electronics of the variable frequency drive to
maintain the reliability and the functionally of the variable frequency drive. This
challenge is even more complex for transport refrigeration applications, where
operational environments, cargo cooling demands and power electronics heat output
vary over a wide spectrum.
Summary of the Invention
[0007] In an aspect, a variable frequency drive heat sink assembly is provided
for housing the power electronics of the variable frequency drive at a temperature below
a specified threshold temperature at all ambient conditions and power consumption
levels.
[0008] In an aspect, a variable frequency drive heat sink assembly is provided
for housing the power electronics of the variable frequency drive in a sealed enclosure
to protect the power electronics from exposure to potentially corrosive ambient
conditions.
[0009] In an aspect, a variable frequency drive heat sink assembly is provided
having a housing through which a flow of cooling air is directed over and across a heat
sink structure associated with the power electronics of the variable frequency drive
isolated in an enclosed chamber within the housing.
[0010] A heat sink assembly is disclosed for cooling a power electronics
module. The heat sink assembly includes a housing and a heat sink structure. The
housing defines an interior chamber for enclosing the power electronics module and
also defines a cooling air flow channel exterior to the interior chamber. The heat sink
structure is disposed in conductive heat transfer relationship with the interior chamber
and has a heat transfer surface positioned within the exterior cooling air flow channel.
A fan is disposed in operative association with the housing for passing a flow of cooling
air through the exterior channel across and over the heat transfer surface. The heat
transfer surface of the heat sink structure is thus disposed in convective heat transfer
relationship with the flow of cooling air whereby heat is removed from the interior
chamber through the external heat transfer surface while isolating the power electronics
module from the flow of cooling air.
[0011] The external heat transfer surface may include a plurality of external heat
transfer fins extending outwardly from the housing into the exterior flow channel. The
plurality of external heat transfer fins may extend outwardly from a base portion of the
heat sink structure disposed in conductive heat transfer relationship with said interior
chamber to a tip portion. The plurality of external heat transfer fins may be disposed in
spaced relationship thereby defining a plurality of flow subchannels within the flow
channel. In an embodiment, the plurality of external heat transfer fins may have an
arcuate contour in longitudinal expanse. To facilitate drainage of condensate from the
heat transfer fins, a plurality of condensate drain troughs may be formed in the plurality
of external heat transfer fins. The plurality of external heat transfer fins may be formed
integral with the housing.
[0012] In an embodiment, the heat sink assembly constitutes a variable
frequency drive heat sink assembly that includes a variable frequency drive module, a
housing that defines an interior chamber for enclosing the variable frequency drive and
also defines a flow channel exterior to the interior housing, a heat sink structure
disposed in conductive heat transfer relationship with the interior chamber and having a
heat transfer surface positioned within the exterior flow channel, and a fan in operative
association with the housing for passing a flow of cooling through the exterior flow
channel across and over the heat transfer surface of the heat sink structure. In an
embodiment, the variable frequency drive heat sink assembly may be mounted to a
support plate on a transport refrigeration unit with the tips of the external heat transfer
fins in juxtaposition to the support plate. When mounted on a transport refrigeration
system, the variable frequency drive heat sink module may be positioned in the path of
air flow being drawn through the transport refrigeration unit by a condenser/gas cooler
fan.
[0013] A method is disclosed for cooling a power electronics module. The
method includes the steps of: providing a housing defining an interior chamber for
enclosing the power electronics module and defining a flow channel exterior to said
interior chamber, providing a heat sink structure having an external heat transfer
surface, disposing the external heat transfer surface of the heat sink structure in
conductive heat exchange relationship with the interior chamber, and passing a cooling
air flow through the flow channel across and over the external heat transfer surface
thereby removing heat from the interior chamber through the external heat transfer
surface while isolating the power electronics module from the flow of cooling air. The
method may further include the step of providing the external heat transfer surface with
a plurality of external heat transfer fins extending into the flow channel. The method
may include the further step of passing the cooling air flow through the flow channel at
an air flow velocity in the range of 4 to 20 millimeters per second per Watt of heat
release by the power electronics module.
Brief Description of the Drawings
[0014] For a further understanding of the disclosure, reference will be made to
the following detailed description which is to be read in connection with the
accompanying drawing, where:
[0015] FIG. 1 is a perspective view of a refrigerated transport container,
equipped with a refrigeration unit, with a portion of the side wall and ceiling removed;
[0016] FIG. 2 is a perspective view of the front of the refrigeration unit mounted
to the forward wall of the container of FIG. 1;
[0017] FIG. 3 is a perspective view of an exemplary embodiment of the variable
frequency drive heat sink assembly disclosed herein mounted to a support plate of the
transport refrigeration unit of FIG. 2;
[0018] FIG. 4 is a perspective view from the underside of the variable frequency
drive heat sink assembly of FIG 2 removed from the transport refrigeration unit;
[0019] FIG. 5 is a plan view of the underside of the variable frequency drive
heat sink assembly of FIG. 4; and
[0020] FIG. 6 is a cross-section elevation view of the heat sink structure of the
variable frequency drive heat sink assembly of FIG. 5 taken generally along line 6-6.
Detailed Description of the Invention
[0021] Referring initially to FIG. 1 of the drawing, there is depicted an
exemplary embodiment of a refrigerated cargo container, generally referenced 10, for
ship board transport or intermodal transit by ship, rail or road. The cargo container 10
has a box-like structure formed of a forward or front wall 12, a back or rear wall 14, a
pair of opposed sidewalls 13 and 15, a ceiling 16 and a floor 18. The box-like structure
defines a cargo space, referred to herein as cargo box 11, in which the bins, cartons or
pallets of cargo 100 being transported are stacked on the floor 18. The rear wall 14 is
provided with one or more doors (not shown) through which access to the cargo box
may be provided for loading the cargo into the container 10. When the doors are
closed, a substantially air-tight, sealed cargo space is established within the container 10
which prevents inside air from escaping the cargo box 11.
[0022] A transport refrigeration unit 20 is mounted to a wall of the container 10.
Generally, the transport refrigeration unit 20 is received in an opening in the forward
wall 12 of the container 10 and mounted around its perimeter to the forward wall 12 of
the container 10, for example as depicted in FIG. 1, for conditioning the air within the
refrigerated chamber, i.e. the cargo box 11 of the container 10. Referring now to FIG. 2
also, the transport refrigeration unit 20 includes a compressor 22 with an associated
compressor drive motor, a condenser/gas cooler module (not shown) isolated from the
cargo box 11, and an evaporator module including evaporator fan and motor assemblies
operatively associated with the cargo box 11 defined within the container 10. The
transport refrigeration unit may incorporate various additional components, including
but not limited to, a filter-dryer, an expansion device, an intercooler, a receiver, an
economizer, a flash tank and various control valves.
[0023] The condenser/gas cooler module includes a refrigerant heat rejection
heat exchanger (not shown) mounted in the forward section of the refrigeration unit 20
external to the cargo box 11 and positioned generally behind the condense/gas cooler
fan 24. The condenser/gas cooler fan 24 draws ambient outdoor air through an opening
25 at the lower front of the refrigeration unit 20, thence passes that air through the
condenser/gas cooler heat exchanger behind the front panel 2 1 and discharges that air
back into the outdoor environment. The evaporator fan and motor assemblies 26 draw
return air from the cargo box 11, pass the return air and any fresh outdoor air that may
be admitted and mixed therewith an evaporator heat exchanger (not shown) for box
environment conditioning, and deliver that conditioned air as supply air back into the
cargo box 11 of the container 10.
[0024] A variable frequency drive heat sink assembly 30 is mounted to the
structure of the transport refrigeration unit 20, for example to a support plate 28 behind
the front panel 21. The variable frequency drive heat sink assembly 30 is positioned
relative to the condenser/gas cooler fan 24 such that a portion of the ambient outdoor air
drawn into the unit 20 by the condenser/gas cooler fan 24 passes over the exterior of the
variable frequency drive heat sink assembly 30. Although described herein as a
variable frequency drive heat sink assembly mounted on a transport refrigeration unit, it
is to be understood that the heat sink assembly disclosed herein may be adapted for
cooling other power electronics modules in other applications. It is to be understood
that application of variable speed drives is not limited to the refrigeration system
compressor. For instance, one or more fans or pumps may be driven at a variety of
speeds by means of a variable speed drive shared with the compressor or a separate
variable speed drive.
[0025] Referring now to FIGs. 3-6, there is depicted an exemplary embodiment
of the variable frequency drive heat sink assembly 30 disclosed herein. The variable
frequency drive heat sink assembly 30 includes a housing 32 defining an interior
chamber 34 for enclosing a variable frequency drive module 36 and also defining an
exterior flow channel 38. The housing 32 further includes a cooling air inlet opening 40
at a first end of the housing 32 and a cooling air outlet opening 42 at a second end of the
housing 32 longitudinally opposite the first end of the housing 32. The housing 32 may
have a cover 35 that forms a part of the housing 32 covering the chamber 34 and is
releasably secured to the housing 32. The cover 35 may be removed to provide access
to the chamber 34 for installing, removing or servicing the variable frequency drive
module 36. When the cover 35 is secured to the housing 32, for example by screws,
releasable fasteners or the like, an air tight enclosure is provided for protecting the
variable frequency drive module 36 and its components from exposure to the cooling air
flow.
[0026] The variable frequency drive heat sink assembly 30 further includes a
cooling air fan 44 for passing cooling air through the exterior flow channel 38. The
cooling air fan 44 may be mounted in the cooling air inlet opening 40 or in the cooling
air outlet opening 42. In either arrangement, the cooling air fan 44 is operative to draw
ambient air from the flow of ambient air that may be at least partially drawn into the
transport refrigeration unit 20 by the condenser/gas cooler fan 24. In the depicted
embodiment, the cooling air fan 44 is mounted in the cooling air inlet opening 40 and is
operative to draw ambient air into and through the inlet duct 45 to and through the
exterior flow channel 38 to exit through the cooling air outlet opening 42 at the
longitudinally opposite end of the flow channel 38. The cooling air fan 44 can itself be
a variable speed fan driven by the variable frequency drive module 36. The speed of
the cooling air fan 44 may be changed in response to the measurement of the
temperature of the power electronics of the variable frequency drive module 36 and
comparison to a threshold temperature. In an embodiment, the condenser/gas cooler fan
24 can provide the cooling air flow through the flow channel 38 for cooling the variable
frequency drive module 36.
[0027] The variable frequency drive heat sink assembly 30 further includes a
heat sink structure defining a heat transfer surface disposed within the exterior flow
channel 38 and exteriorly of the chamber 34 enclosing the variable frequency drive
module 36. When the cooling air fan 44 is in operation, the cooling air fan 44 passes
ambient air through the exterior flow channel 38 across and over the heat transfer
surface of the heat sink structure 46 for cooling the power electronics of the variable
frequency drive module 36, such as for example, but not limited to, an insulated-gate
bipolar transistor (IGBT) or other power semiconductor devices and capacitors. In this
manner, the power electronics of the variable frequency drive module 36 may be
effectively cooled without being in direct contact with the ambient air thereby avoiding
potential corrosion and erosion of the power electronics attendant with direct contact of
the power electronics with moist high chlorine content sea air or land air in high
humidity conditions.
[0028] The heat transfer surface of the heat sink structure may include a
plurality of heat transfer fins 48 on the exterior of the housing 32 extending outwardly
from the base 50 of the housing 32 into the flow channel 38. The plurality of heat
transfer fins 48 may be arrayed in laterally spaced relationship and extend generally
longitudinally along the flow channel 38 thereby dividing the flow channel into a
plurality of subchannels 52 between the various sets of neighboring heat transfer fins
40.
[0029] In an embodiment, each heat transfer fin 48 may extend outwardly to the
same extent as the longitudinally extending upper side wall 54 and lower side wall 56 of
the housing 32, which define the flow channel 36 therebetween, extend outwardly from
base 50 of the housing 32. So constructed, when the variable frequency drive heat sink
assembly 30 is mounted to the support plate 28 on the transport refrigeration unit 20, the
tip portions of the respective heat transfer fins 48 and the tip portions of the upper and
lower side walls 54, 56 of the housing 32 will all contact the surface of the support plate
28.
[0030] In the depicted embodiment, the heat transfer fins 48 are arcuate in their
longitudinal extent as best seen in FIGs. 4 and 5. The upwardly convex in a vertical
plane contour of the arcuate heat transfer fins 48 facilitates drainage of condensate from
the surface of the heat transfer fins 48. In an embodiment, the arcuate fins 48 have a
nominal curvature radius and a channel length, wherein a ratio of the nominal curvature
radius to the channel length has a valve in the range of 0.5 to 3.0, and more narrowly, in
the range of 0.8 to 1.5. Condensate may be formed on the surface of the heat transfer
fins 48 due to condensation of moisture in the ambient cooling air flowing through the
flow channel 38. Additionally, rather than extending outwardly the same extent as the
side walls 54, 56 of the housing 32, the heat transfer fins 48 may be foreshortened
relative to the upper and lower side walls 54, 56 so as to provide a gap between the tip
portions of the heat transfer fins 48 and the support plate 28 when mounted thereto
sufficient to allow condensate to drain off the tip portions of the heat transfer fins 48.
To further facilitate the drainage of condensate from the surface of the heat transfer fins
48, the plurality of heat transfer fins 48 may be provided with troughs 58 for collecting
condensate and draining the collected condensate to the outboard tip portion of the heat
transfer fins 48. The troughs 58 may be formed integrally with the heat transfer fins 48.
[0031] The housing 32 may be formed of aluminum, aluminum alloy or other
material having a relatively high thermal conductivity. The housing 32 may be formed
by extrusion or by casting. In the depicted embodiment, the housing 32 comprises a
housing cast from aluminum alloy with the heat transfer fins 48 formed integrally with
the housing 32 during the casting process. Additionally, in the depicted embodiment,
the housing 32 is cast so as to provide capacitor wells 60 which extend outwardly into
the flow channel 38 and open to the chamber 34 for receiving capacitors that constitute
components of the variable frequency drive module 36.
[0032] In the depicted embodiment, the heat transfer fins 48 are formed with an
arcuate contour, convex upwardly, in the longitudinal direction which facilitates
condensate draining. It is to be understood that in other embodiments, the heat transfer
fins 40 may be flat plate fins or wave-like fins extending longitudinally in parallel
spaced relationship. Additionally, the heat transfer fins 48 may be of uniform thickness
from base to tip or tapered inwardly from base to tip. In an uniform thickness
embodiment for example, the heat transfer fins may have a thickness in the range of
from 3 to 4 millimeters (0.12 to 0.157 inches) and spaced side to side at a spacing in the
range of 10 to 11 millimeters (0.39 to 0.43 inches). In a tapered fin embodiment, which
facilitates casting of the housing with the heat transfer fins 48 formed integral with the
housing 32, the heat transfer fins 48 may, for example, have a thickness at the fin base
in the range of 3 to 4 millimeters (0.12 to 0.157 inches) and inwardly sloping sides
having a slope greater than 1 degree and less than 1.5 degree.
[0033] According to the method disclosed herein for cooling a power electronics
module, heat may be removed from the interior chamber 34 of the housing 32 through
the external heat transfer surface 48 while isolating the power electronics module, such
as, but not limited to a variable frequency drive 36, from the flow of cooling air. The
method includes the steps of: providing a housing 32 defining an interior chamber 34
for enclosing the power electronics module and defining a flow channel exterior 38 to
the interior chamber 34; providing a heat sink structure having an external heat transfer
surface, including the external heat transfer fins 48; disposing the external heat transfer
surface of the heat sink structure in conductive heat exchange relationship with the
interior chamber 34; and passing a cooling air flow through the flow channel across and
over the external heat transfer surface thereby removing heat from the interior chamber
through the external heat transfer surface while isolating the power electronics module
from the flow of cooling air. To achieve sufficient convective heat transfer to ensure
cooling of the power electronics of the variable frequency drive 36 or other power
electronics module to a temperature below a threshold temperature of 85°C (185°F)in
accord with the method disclosed herein, the cooling air flow may be passed through
the flow channel at an air flow velocity in the range of 4 to 20 millimeters per second
per Watt of heat release by the power electronics module.
[0034] The terminology used herein is for the purpose of description, not
limitation. Specific structural and functional details disclosed herein are not to be
interpreted as limiting, but merely as basis for teaching one skilled in the art to employ
the present invention. Those skilled in the art will also recognize the equivalents that
may be substituted for elements described with reference to the exemplary
embodiments disclosed herein without departing from the scope of the present
invention.
[0035] While the present invention has been particularly shown and described
with reference to the exemplary embodiments as illustrated in the drawing, it will be
recognized by those skilled in the art that various modifications may be made without
departing from the spirit and scope of the invention. Therefore, it is intended that the
present disclosure not be limited to the particular embodiment(s) disclosed as, but that
the disclosure will include all embodiments falling within the scope of the appended
claims.
We Claim:
1. A heat sink assembly for cooling a power electronics module
comprising:
a housing defining an interior chamber for enclosing the power electronics
module and defining a flow channel exterior to said interior chamber; and
a heat sink structure disposed in conductive heat transfer relationship with
said interior chamber and having a heat transfer surface positioned within the
exterior flow channel.
2. The heat sink assembly as recited in claim 1 wherein said housing
further comprises:
a cooling air inlet opening and a cooling air outlet opening in flow
communication with the exterior flow channel; and
a fan disposed in operative association with the housing for passing a flow of
cooling air through the exterior channel across and over the heat transfer surface.
3. The heat sink assembly as recited in claim 2 wherein the fan is
disposed in the cooling air inlet opening of said housing.
4. The heat sink assembly as recited in claim 2 wherein the fan is
disposed in the cooling air outlet opening of said housing.
5. The heat sink assembly as recited in claim 2 wherein the fan is a
variable speed fan.
6. The heat sink assembly as recited in claim 1 wherein the heat transfer
surface comprises a plurality of external heat transfer fins extending outwardly from
said housing into the flow channel of the housing.
7. The heat sink assembly as recited in claim 6 wherein the plurality of
external fins comprises a plurality of external heat transfer fins that extend
outwardly from a base portion of said heat sink structure disposed in conductive heat
transfer relationship with said interior chamber to a tip portion terminating adjacent
a bounding wall of said housing.
8. The heat sink assembly as recited in claim 7 wherein the plurality of
external heat transfer fins are disposed in spaced relationship thereby defining a
plurality of flow subchannels within the flow channel across the heat transfer
surface.
9. The heat sink assembly as recited in claim 6 wherein the plurality of
external heat transfer fins are formed integral with said housing.
10. A variable frequency drive heat sink assembly comprising:
a variable frequency drive module;
a housing defining an interior chamber for enclosing the variable frequency
drive and defining a flow channel exterior to said interior housing,
a heat sink structure disposed in conductive heat transfer relationship with
said interior chamber and having a heat transfer surface positioned within the
exterior flow channel; and
a fan in operative association with said housing for passing a flow of cooling
air through the exterior flow channel across and over the heat transfer surface of the
heat sink structure.
11. The variable frequency drive heat sink assembly as recited in claim
10 wherein the fan is disposed in the cooling air inlet opening of said housing.
12. The variable frequency drive heat sink assembly as recited in claim
10 wherein the fan is a variable speed fan.
13. The variable frequency drive heat sink assembly as recited in claim
10 wherein the heat transfer surface comprises a plurality of external heat transfer
fins that extend outwardly from a portion of said heat sink structure disposed in
conductive heat transfer relationship with said interior chamber.
14. The variable frequency drive heat sink assembly as recited in claim
13 wherein the plurality of external heat transfer fins are disposed in spaced
relationship thereby defining a plurality of flow subchannels within the flow
channel.
15. The variable frequency drive heat sink assembly as recited in claim
14 wherein the plurality of external heat transfer fins have an arcuate contour in
longitudinal expanse.
16. The variable frequency drive heat sink assembly as recited in claim
15 further comprising a plurality of condensate drain troughs formed in said
plurality of external heat transfer fins.
17. The variable frequency drive heat sink assembly as recited in claim
10 wherein the plurality of external heat transfer fins are formed integral with said
housing.
18. The variable frequency drive heat sink assembly as recited in claim
10 wherein the variable frequency drive heat sink assembly is used in connection
with a transport refrigeration unit.
19. The variable frequency drive heat sink assembly as recited in claim
10 wherein the variable frequency drive heat sink assembly is used in connection
with a transport refrigerant unit charged with carbon dioxide refrigerant.
20. The variable frequency drive heat sink assembly as recited in claim
10 wherein the variable frequency drive heat sink assembly is operatively associated
with at least one of a compressor, a condenser/gas cooler fan, an evaporator fan, a
water pump, and a variable frequency drive cooling fan of a transport refrigeration
unit.
21. A method for cooling a power electronics module comprising the
steps of:
providing a housing defining an interior chamber for enclosing the power
electronics module and defining a flow channel exterior to said interior chamber;
providing a heat sink structure having an external heat transfer surface;
disposing the external heat transfer surface of the heat sink structure in
conductive heat exchange relationship with the interior chamber; and
passing a cooling air flow through the flow channel across and over the
external heat transfer surface thereby removing heat from the interior chamber
through the external heat transfer surface while isolating the power electronics
module from the flow of cooling air.
22. The method as recited in claim 2 1 further comprising the step of
providing the external heat transfer surface with a plurality of external heat transfer
fins extending into the flow channel.
23. The method as recited in claim 2 1 further comprising the step of
passing the cooling air flow through the flow channel at an air flow velocity in the
range of 4 to 20 millimeters per second per Watt of heat release by the power
electronics module.