MG.20.B6.02 – VLT is a registered Danfoss trademark
23
VLT
®
2000 Series
Section 1 Section 2 Section 3
The Danfoss VVC principle
VLT 2000 Series frequency converters use an inverter
control system known as the Voltage Vector Control
(VVC) developed by Danfoss.
The VVC principle is superior to the traditional PWM
(Pulse Width Modulation) principle used in most
modern frequency converters in the following ways:
− Full-rated motor voltage at rated motor frequency.
− Near perfect resemblance to the sinusoidal
mains supply.
− Extremely low switching losses, resulting in high
converter efficiency.
The features are obtained through a special switching
pattern: the switching intervals are very short, which
means high switching frequency, and the six
semiconductors of the inverter section are alternately
held inactive in pairs, throughout a 60° sine period.
The current wave form of the motor current closely
resembles that obtained on mains operation. The
switching pause in 60° of the sine period also means
that full-rated motor voltage can be obtained - and
inverter switching losses are reduced by about
one third.
The figures below show the switching pattern and
maximum motor voltage according to the VVC
principle and the traditional PWM principle
respectively.
The full-rated motor voltage and the perfect current
wave form mean that the Danfoss VLT 2000 Series
allows full motor performance without any derating -
just like running the motor on the mains.
Motor voltage and simplified switching pattern according to the Danfoss VVC principle
Motor voltage and simplified switching pattern according to the traditional PWM principle
0,50
0,00
1,00
-0,50
-1.00
0 60 120 180 300 360240
U-V U-W W-U
U-V V-W W-U
0,50
0,00
1,00
-0,50
-1.00
0 60 120 180 300 360240
0,866
U-V V-W W-U
Switching pattern for phase U
Single phase voltage
Phase-phase voltage
for motor
How the VLT works
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