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

System Integration
•
Even devices with high protection ratings must
adhere to the specified ambient temperature
ranges.
•
Additional air conditioning of the cabinet or
installation site may be required.

5.1.4 Manual Derating

Consider derating when any of the following conditions
5 5
are present.
•
Operating above 3,300 ft [1,000 m] (low air
pressure)
•
Low speed operation
•
Long motor cables
•
Cables with a large cross-section
•
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 an adjustable frequency
drive, it is necessary to make sure 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.
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
adjustable frequency drive limits 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.
50
Design Guide

5.1.4.2 Derating for Low Air Pressure

The cooling capability of air is decreased at a lower air
pressure.
Below 1,000 m altitude, no derating is necessary but
above 1,000 m the ambient temperature (TAMB) or max.
output current (Iout) should be derated in accordance with
Figure 5.1.
Figure 5.1 Derating of output current versus altitude at
T
AMB, MAX
6,600 ft [2,000 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 6,600 ft [2,000 m] is elaborated for an
enclosure type B with T
temperature of 113°F [45°C] (T
rated output current is available. At a temperature of 107°F
[41.7°C], 100% of the rated output current is available.
Max.I (%)
out
at T
AMB, MAX
100%
96%
92%
Figure 5.2 Derating of output current versus altitude at
T
AMB, MAX
Rev. 2014-04-04 All rights reserved.
for frame sizes A, B and C. At altitudes above
= 122°F [50°C]. At a
AMB, MAX
- 3.3 K), 91% of the
AMB, MAX
1 km 2 km 3 km
for enclosure types D3h.
T
AMB, MAX
at 100% I
out
D, E and F enclosures
NO
HO
0 K -5 K
-3 K -8 K
-6 K -11 K
Altitude (km)
MG33BF22