5 Special Conditions
5.1 Derating
This section provides detailed data regarding the operation
of the frequency converter in conditions that require
derating. In some conditions, derating is performed
manually. In other conditions, the frequency converter
performs a degree of automatic derating when necessary.
Derating ensures proper performance at critical stages
where the alternative can be a trip.
5.2 Manual Derating
5.2.1 When to Consider Derating
Consider derating when any of the following conditions
are present:
•
Operating above 1000 m (low air pressure).
•
Low-speed operation.
•
Long motor cables.
•
Cables with a large cross-section.
•
High ambient temperature.
For more information, refer to
.
5.2.2 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.
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 has to run continuously at an RPM
value lower than half the rated value, supply the motor
with extra air cooling. Alternatively, use a motor designed
for this type of operation.
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 on 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, extra cooling, or derating of the motor is not
required.
5.2.3 Derating for Low Air Pressure
The cooling capability of air is decreased at lower air
pressure.
Below 1000 m altitude, no derating is necessary. At
altitudes above 1000 m, derate the maximum output
current (I
out
) at ambient temperature (T
AMB
) in accordance
with
. At altitudes above 2000 m, contact
Danfoss regarding PELV.
Max.I
out
(%)
at T
AMB, MAX
A
Altitude (km)
enclosure
B and C
enclosure
T
at 100% I
out
D
100%
91%
82%
0 K
-5 K
-9 K
1 km
2 km
3 km
0 K
-3.3 K
-6 K
130BA418.11
AMB, MAX
(K)
Illustration 5.1 Derating of Output Current versus Altitude at
T
AMB, MAX
for Enclosure Sizes A, B, and C.
An alternative is to lower the ambient temperature at high
altitudes to ensure 100% output current at high altitudes.
As an example of how to read
, the situation
at 2000 m is elaborated for an enclosure size B with T
AMB,
MAX
=50
°
C. 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.
5.3 Derating for Long Motor Cables or
Cables with Larger Cross-section
NOTICE
Applicable for frequency converters up to 90 kW only.
The maximum cable length for this frequency converter
is 300 m unscreened and 150 m screened cable.
The frequency converter has been designed to work
using a motor cable with a rated cross-section. If a cable
with a larger cross-section is used, reduce the output
current by 5% for every step the cross-section is
increased.
Increased cable cross-section leads to increased capacity
to ground, and thus an increased ground leakage
current.
Special Conditions
Design Guide
MG16G202
Danfoss A/S © 08/2015 All rights reserved.
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Содержание VLT FC 103
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