8.6 Special Conditions
8.6.1 Purpose of Derating
Derating must be taken into account when using the frequency converter at low air pressure (heights), at low speeds, with long motor cables, cables
with a large cross section or at high ambient temperature. The required action is described in this section.
8.6.2 Derating for ambient temperature
90% frequency converter output current can be maintained up to max. 50 °C ambient temperature.
With a typical full load current of EFF 2 motors, full output shaft power can be maintained up to 50 °C.
For more specific data and/or derating information for other motors or conditions, please contact Danfoss.
8.6.3 Automatic adaptations to ensure performance
The frequency converter constantly checks for critical levels of internal temperature, load current, high voltage on the intermediate circuit and low motor
speeds. As a response to a critical level, the frequency converter can adjust the switching frequency and / or change the switching pattern in order to
ensure the performance of the frequency converter. The capability to automatically reduce the output current extends the acceptable operating conditions
even further.
8.6.4 Derating for low air pressure
The cooling capability of air is decreased at lower air pressure.
Below 1000 m altitude no derating is necessary but above 1000 m the ambient temperature (T
AMB
) or max. output current (I
out
) should be derated in
accordance with the shown diagram.
Illustration 8.2: Derating of output current versus altitude at T
AMB, MAX
for frame sizes A, B and C. At altitudes above 2 km, please contact
Danfoss regarding PELV.
An alternative is to lower the ambient temperature at high altitudes and thereby ensure 100% output current at high altitudes. As an example of how to
read the graph, the situation at 2 km is elaborated. At a temperature of 45° C (T
AMB, MAX
- 3.3 K), 91% of the rated output current is available. At a
temperature of 41.7° C, 100% of the rated output current is available.
VLT
®
HVAC Drive Design Guide 8 General Specifications and Troubleshooting
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
179
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