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 receives a variable voltage and fre-
quency, 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
DC voltage 2 x mains voltage [V].
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.
VLT 2800 control principle
A frequency converter is an electronic unit which is
able to infinitely variably control the rpm of an AC mo-
tor. 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 sys-
tem called VVC (Voltage Vector Control). VVC con-
trols 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 volt-
age 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 parame-
ters. It is easy for the user to program the required
functions on the control panel or via serial communi-
cation.
Mains protector
The frequency converter is protected against the tran-
sients that occur on the mains sometimes, e.g. if cou-
pling 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 sup-
ply.
As all 400 V units in the VLT 2800 Series have inter-
mediate circuit coils, there is only a low amount of
harmonic mains supply interference. This gives 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
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MG.27.E2.02 - VLT is a registered Danfoss trademark
