most commonly used motor in HVAC installations today is the
three-phase, asynchronous AC motor, which is both inexpensive and of
very reliable construction. Ordinary asynchronous AC motors are
designed to make the operating speed dependent on the frequency and
the voltage connected to the motor. For example, the mains supply in
most European countries is 400V 50Hz. This gives a four-poled
asynchronous AC motor an approximate speed of 1500 rpm.
Drive or static frequency converter is an electronic unit which
provides infinitely variable control of the speed of three-phase AC
motors by converting fixed mains voltage and frequency into variable
While the basic operating principle of the drive remains the same,
the product has undergone considerable development.
Frequency converters, which once featured thyristors, have
been replaced by todays microprocessor-controlled, digital units.