Control principle
A frequency converter rectifies AC voltage from mains
into DC voltage, after which this DC voltage is conver-
ted into a AC current with a variable amplitude and
frequency.
The motor is thus supplied with variable voltage and
frequency, which enables infinitely variable speed
control of three-phased, standard AC motors.
1. Mains voltage
3 x 200 - 240 V AC, 50 / 60 Hz.
3 x 380 - 500 V AC, 50 / 60 Hz.
3 x 525 - 600 V AC, 50 / 60 Hz.
3 X 525 - 690 V AC, 50 / 60 Hz.
2. Rectifier
A three-phase rectifier bridge that rectifies AC current
into DC current.
3. Intermediate circuit
DC voltage = 1.35 x mains voltage [V].
4. Intermediate circuit coils
Smooth the intermediate circuit current and limit the
load on mains and components (mains transformer,
wires, fuses and contactors).
5. Intermediate circuit capacitors
Smooth the intermediate circuit voltage.
6. Inverter
Converts DC voltage into variable AC voltage with a
variable frequency.
7. Motor voltage
Variable AC voltage, 0-100% of mains supply voltage.
Variable frequency: 0.5-132/0.5-1000 Hz.
8. Control card
This is where to find the computer that controls the in-
verter which generates the pulse pattern by which the
DC voltage is converted into variable AC voltage with
a variable frequency.
VVC
plus
control principle
The frequency converter features an inverter control
SYSTEM called VVC
plus
, which is a further develop-
ment of the Voltage Vector Control (VVC) known i.e.
from Danfoss VLT 3000 Series.
VVC
plus
controls an induction motor by energizing it
with a variable frequency and a voltage that matches
it. If the motor load is changed, the magnetisation of
the motor changes too, and so does its speed. Con-
sequently, the motor current is measured continuously
and the actual voltage requirement and slip of the mo-
tor are calculated from a motor model. Motor frequen-
cy and voltage are adjusted to ensure that the motor
operating point remains optimum under varying con-
ditions.
The development of the VVC
plus
principle is the result
of a wish to maintain robust, sensorless regulation that
is tolerant to different motor characteristics without
motor derating being required.
First and foremost, the current measurement and the
motor model have been improved. The current is split
into magnetising and torque-generating parts and pro-
vides for much better and quicker estimation of the
actual motor loads. It is now possible to compensate
for rapid load changes. Full torque as well as extremely
accurate speed control can now be obtained even at
low speeds or even at standstill.
In a "special motor mode", permanent magnet syn-
chronous motors and/or parallel motors can be used.
Good torque control properties, smooth transitions to
and from current limit operation and robust pull-out
torque protection are ensured.
VLT
®
5000 Design Guide
8
MG.52.B2.02 - VLT
®
is a registered Danfoss trademark
