There are basically two main control principles used by variable
speed drives to control the motor speed. Within the two main
areas there are several varieties of these basic principles.
In order to control the motor, it is necessary to
convert the a/c. mains power to d.c. This is done internally in
the drive (see also Harmonics in section called Technical
Notes). After the power signal has been converted to d.c., the
d.c. is chopped into square signals and directed to the three
power output terminals. This is done in the internal inverter
The way the d.c. signal is chopped and shaped in
the inverter is a question of how sophisticated the drive's
manufacturer has decided to re-create the a.c. current for the
motor. The more sinusoidal the current signal out of the
inverter is, the better motor performance is obtained.
The VLT 6000 HVAC drive is using a modified PWM
(Pulse-Width-Modulation) called VVCplus (Voltage Vector Control)
to control the motor current and voltage, and thereby control
the motor performance.
Please also refer to
the book "Facts worth knowing about frequency
converters", ISBN 87-87411-22-9.