Further advantages of the frequency inverters include the
higher speed stability with fluctuations in the load (speed fluctuations
less than about one percent) and the option for a direct
change in the direction of rotation. As the rotating field in the
frequency inverter is generated electronically, a simple control
command is all that is required to change the phase sequence
and the direction of motor rotation. The electronic motor protection
(I2t control) integrated into frequency inverters also
assures safe operation without the need for additional safety
measures (overload relays). Depending on the method of implementation,
parameterised temperature models in the frequency
inverter provide a higher level of motor heat protection. Socalled
full motor protection is also possible in conjunction with
thermistors. Overload and underload detection also enhance
the operational safety of the drive unit.
The frequency inverter operates as a power converter in the
main circuit of a motor feeder. Separated from the power of the
DC link, the power converter draws active power via the rectifier
from the mains supply and supplies the connected motor
with active and reactive power via the inverter. The reactive
power required for motor operation is provided by the capacitors
in the DC link. As far as the electrical supply is concerned,
the frequency-controlled drive behaves virtually like a resistive
load (cos ~ 1).
The power conversion and the associated current types must
be considered in the design of the switchgear and protective
devices on the motor feeder. For this purpose, the electromechanical
components (e.g. fuses, line reactors, mains contactors)
on the mains supply side of the frequency inverter are
dimensioned in accordance with the input current (active current)
and the utilization category AC-1 (standard IEC60947-4-1).
The components on the frequency inverter output (e.g. motor
reactors, sinewave filters, motor cables) are dimensioned in
accordance with the rated operational current of the connected
motor and the utilization category AC-3.
During motor operation, the frequency inverters differ through
the method of operation of the inverter that can be adjusted by
the user. In addition to the standard U/f control with a linear or