Derating For Running At Low Speed; Derating For Low Air Pressure - Danfoss VLT AutomationDrive FC 301 Design Manual

System Integration
•
High ambient temperature
For more information, refer to chapter 6.2.6 Derating for
Ambient Temperature.

5.1.4.1 Derating for Running at Low Speed

When a motor is connected to a frequency converter, it is
necessary to check that the cooling of the motor is
adequate.
The level of heating depends on the load on the motor, as
well as the operating speed and time.
5 5
Constant torque applications (CT mode)
A problem may occur at low RPM values in constant
torque applications. In a constant torque application, a
motor may overheat at low speeds due to less cooling air
from the motor integral fan.
Therefore, if the motor is to be run continuously at an RPM
value lower than half of the rated value, the motor must
be supplied with additional air-cooling (or a motor
designed for this type of operation may be used).
An alternative is to reduce the load level of the motor by
selecting a larger motor. However, the design of the
frequency converter puts a limit to the motor size.
Variable (Quadratic) torque applications (VT)
In VT applications such as centrifugal pumps and fans,
where the torque is proportional to the square of the
speed and the power is proportional to the cube of the
speed, there is no need for additional cooling or derating
of the motor.
46
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VLT AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW

5.1.4.2 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
current (I
Illustration 5.1.
Max.I (%)
out
at T
AMB, MAX
100%
91%
82%
Illustration 5.1 Derating of output current versus altitude at
T
AMB, MAX
m, 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 2,000 m is elaborated for an enclosure type B
with 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.
Max.I (%)
out
at T
AMB, MAX
100%
96%
92%
Illustration 5.2 Derating of output current versus altitude at
T
AMB, MAX
MG33BF02 - Rev. 2013-12-20
AMB
) should be derated in accordance with
out
1 km 2 km 3 km
for frame sizes A, B and C. At altitudes above 2,000
= 50° C. At a temperature of 45° C (T
1 km 2 km 3 km
for enclosure types D3h.
) or max. output
(K)
D T
AMB, MAX
at 100% I
out
B and C
A
enclosure
enclosure
0 K 0 K
-5 K -3.3 K
-9 K -6 K
Altitude (km)
AMB, MAX
T
AMB, MAX
at 100% I
out
D, E and F enclosures
HO NO
0 K -5 K
-3 K -8 K
-6 K -11 K
Altitude (km)