Vacon 20 CP, Installation, Technical And Maintenance Manual

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3
vacon • 31 Mounting
The formula to calculate the maximum thermal resistance of the heatsink is as follows:
For a given ambient temperature T
amb
the cold plate temperature T
CPmax
must not exceed the
maximum allowable value (85°C). As the R
CP
is essentially fixed, this condition must be satis-
fied with a proper heatsink selection. The table below shows the typical values for R
CP
for VA-
CON
®
20 CP:
Select a heatsink with a smaller thermal resistance than R
HSmax
. The heatsink dimensions
should be closed to the cold plate dimensions.
Note: remember that the heatsink cooling capacity can be reduced over time due to dirt.
In order to choose an heatsink from a catalogue please consider that usually the reported heat
resistances are measured under free convection conditions. In this case the heatsink has to be
oversized with respect to the cold plate dimensions, otherwise an additional fan must be used
to reduce the heatsink resistance and its dimensions. Most heatsink manufacturers give cor-
rection factors according to various air flow speeds.
The designing factors which influence the thermal performances of an heatsink are as follows:
• Spreading resistance : Spreading resistance occurs when thermal energy is transferred
from a small area to a larger area in a substance with finite thermal conductivity. In a heat-
sink, this means that heat does not distribute uniformly through the heatsink base. The
spreading resistance phenomenon is shown by how the heat travels from the heat source
location and causes a large temperature gradient between the heat source and the edges
of the heatsink. This means that some fins are at a lower temperature than if the heat
source were uniform across the base of the heatsink. This non uniformity increases the
heatsink's effective thermal resistance.
•
Heatsink manufacturer dimensioning data : the heatsink thermal resistance given in a
catalogue is measured on a temperature sink to ambient differential (
Δ
T) and since Rhs
under free convection depends on
Δ
T with the power law Rth ~
Δ
T
-0,25
(with laminar flow),
Frame R
CP
MS2
R
CP
=0.091 K/W
MS3
R
CP
= 0,055 K/W
Table 12. Typical values for the equivalent thermal resistances of the cold plate.
If the heatsink height and width are much larger than the drive cold plate dimen-
sions, or if multiple drives are installed on one heatsink, it may be necessary to
apply correction factors to the thermal resistance value given in the heatsink
specification. Contact the heatsink manufacturer.
R
HSmax
T
CPmax
T
amb
–
CP
loss
------------------------------------- R
CP
– =