Special Conditions; Manual Derating - Danfoss VLT AQUA Drive FC 202 Design Manual

Special Conditions

5 Special Conditions
This section provides detailed data regarding the operating
of the adjustable frequency drive in conditions that require
derating. In some conditions, derating is performed
manually. In other conditions, the adjustable frequency
drive performs a degree of automatic derating when
necessary. Derating ensures proper performance at critical
stages where the alternative can be a trip.

5.1 Manual Derating

5.1.1 When to Consider Derating
Consider derating when any of the following conditions
are present:
•
Operating above 1000 m (3,300 ft) (low air
pressure)
•
Low-speed operation
•
Long motor cables
•
Cables with a large cross-section
•
High ambient temperature
For more information, refer to chapter 5.3 Derating for
Ambient Temperature.
5.1.2 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 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 puts a limit on the motor size.
MG20N622
Design Guide
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.
5.1.3 Derating for Low Air Pressure
The cooling capability of air is decreased at a lower air
pressure.
Below 1000 m (3,300 ft) altitude, no derating is necessary.
At altitudes above 1000 m (3,300 ft), derate the maximum
output current (I
accordance with Figure 5.1. At altitudes above 2000 m
(6,600 ft), contact Danfoss regarding PELV.
Figure 5.1 Derating of Output Current versus Altitude at T
for Enclosure Sizes A, B and C.
MAX
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 2000 m (6,600 ft) is elaborated for an enclosure
size B with T
45 °C (113 °F) (T
current is available. At a temperature of 41.7 °C (107 °F),
100% of the rated output current is available.
Danfoss A/S © 09/2014 All rights reserved.
) at ambient temperature (T
out
= 50 °C (122 °F). At a temperature of
AMB, MAX
- 3.3 K), 91% of the rated output
AMB, MAX
5 5
) in
AMB
AMB,
115