VLT
®
2800 Series
All about VLT
2800
■
Temperature-dependent switch frequency
This function ensures the highest possible switch
frequency 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
temperature, the supply voltage and the cable length.
The function ensures that the frequency converter
automatically 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
frequency 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
motor. At low rpm values, the motor fan is not able
to supply an adequate volume of cooling air. This
problem 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
permissible 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 reduce the motor load ratio. This can be done by
selecting 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 recommended to reduce the output current
by 5% for each step the cable cross-section is
increased. (Increased cable cross-section leads
to increased capacitance 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.
The frequency converter will automatically derate
the rated output current I
VLT,N,
when the switching
frequency exceeds 4.5 kHz.
MG.28.E9.02 - VLT is a registered Danfoss trademark
133
