Technology - Danfoss VT2800 Series Design Manual

Technology

Control principle
A frequency converter rectifies AC voltage from
the mains supply into DC voltage, following which
it changes this voltage to an AC voltage with
variable amplitude and frequency.
The motor thus recei v es a variable voltage and
frequency, which enables infinitely variable speed
control of three-phase, standard AC motors.
1. Mains voltage
1 x 220 - 240 V AC, 50 / 60 Hz
3 x 200 - 240 V AC, 50 / 60 Hz
3 x 380 - 480 V AC, 50 / 60 Hz
2. Rectifier
Three-phase rectifier bridge which rectifies AC
voltage into DC voltage.
3. Intermediate circuit
√2 x mains voltage [V].
DC voltage
4. Intermediate circuit coils
Evens out the intermediate circuit current and
limits the load on mains and components (mains
transformer, cables, fuses and contactors).
5. Intermediate circuit condenser
Evens out the intermediate circuit voltage.
6. Inverter
Converts DC voltage into a variable AC voltage
with a variable frequency.
7. Motor voltage
Variable AC voltage depending on supply voltage.
Variable frequency: 0.2 - 132 / 1 - 1000 Hz.
8. Control card
Here is the computer that controls the inverter
which generates the pulse pattern by which
the DC voltage is converted into variable AC
voltage with a variable frequency.
6
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VLT 2800 Series
VLT 2800 control principle
A frequency converter is an electronic unit which is able
to infinitely variably control the rpm of an AC motor.
The frequency converter governs the motor speed
by converting the regular voltage and frequency from
mains, e.g. 400 V / 50 Hz, into variable magnitudes.
Today the frequency converter controlled AC motor is
a natural part of all types of automated plants.
The frequency converter has an inverter control system
called VVC (Voltage Vector Control). VVC controls
an induction motor by energizing with a variable
frequency and a voltage suitable for it. If the motor
load changes, so do its energizing and speed. That
is why the motor current is measured on an ongoing
basis, and a motor model is used to calculate the
actual voltage requirement and slip of the motor.
Programmable inputs and outputs in four Setups
In the frequency converter is possible to program the
different control inputs and signal outputs and to select
four different user-defined Setups for most parameters.
It is easy for the user to program the required functions
on the control panel or via serial communication.
Mains protector
The frequency converter is protected against the
transients that occur on the mains sometimes, e.g.
if coupling with a phase compensation system, or
if fuses blow when lightning strikes.
Rated motor voltage and full torque can be maintained
down to approx. 10% undervoltage in the mains supply.
As all 400 V units in the VLT 2800 Series have
intermediate circuit coils, there is only a low amount
of harmonic mains supply interference. This gi v es
a good power factor (lower peak current), which
reduces the load on the mains installation.
Frequency converter protections
The current measurement in the intermediate circuit
constitutes perfect protection of the frequency
in case there is a short-circuit or an earth fault
on the motor connection.
Constant monitoring of the intermediate circuit
current enables switching on the motor output,
e.g. by means of a contactor.
Efficient monitoring of the mains supply means that the
unit will stop in the case of a phase drop-out. In this
way, the inverter and the condensers in the intermediate
MG.28.E9.02 - VLT is a registered Danfoss trademark