45 °C, a derating of the rated output current is neces-
sary.
Temperature-dependent switch frequency
This function ensures the highest possible switch fre-
quency without the frequency converter becoming
thermally overloaded. The internal temperature is the
actual expression of the degree to which the switch
frequency can be based on the load, the ambient tem-
perature, the supply voltage and the cable length.
The function ensures that the frequency converter au-
tomatically adjusts the switch frequency between f
sw,
min
and f
sw, max
(parameter 411), see drawing below.
When using the LC filter the minimum switch frequen-
cy is 4.5 kHz.
Derating for air pressure
Below 1000 m derating is not necessary.
Above 1000 m the ambient temperature (T
AMB
) or max.
output current (I
MAX
) must be derated in accordance
with the diagram below:
1.
Derating of output current versus altitude at
T
AMB
= max. 45 °C.
2.
Derating of max. T
AMB
versus altitude at
100% output current.
Derating for running at low speed
When a motor is connected to a frequency converter,
it is necessary to ensure adequate cooling of the mo-
tor. At low rpm values, the motor fan is not able to
supply an adequate volume of cooling air. This prob-
lem occurs when the load torque is constant (e.g. with
a conveyor belt) across the full regulating range. The
reduced amount of ventilation determines the permis-
sible torque in continuous operation. If the motor is to
run continuously at an rpm lower than half the rated
value, extra cooling air must be supplied to the motor.
Instead of providing extra cooling, it is possible to re-
duce the motor load ratio. This can be done by select-
ing a larger motor. However, the design of the
frequency converter puts limits on the size of motors
that can be connected to the frequency converter.
Derating for long motor cables
The frequency converter has been tested using a 75
m unscreened/unarmoured cable and a 25 m
screened/armoured cable and has been designed to
work using a motor cable with a rated cross-section. If
a cable with a larger cross-section is required, it is rec-
ommended to reduce the output current by 5% for
each step the cable cross-section is increased. (In-
creased cable cross-section leads to increased ca-
pacitance to earth, and thus to an increased earth
leakage current).
Derating for high switching frequency - VLT 2800
A higher switching frequency (to be set in parameter
411,
Switching frequency
) leads to higher losses in the
electronics of the frequency converter.
VLT 2800 has a pulse pattern in which it is possible to
set the switching frequency from 3.0- 10.0/14.0 kHz.
136
MG.27.E2.02 - VLT is a registered Danfoss trademark
