an economic standpoint, this is an interesting connection variant
with higher motor ratings.
The overload relay used can also be included in the winding
phase of the soft starter and can also be reduced by the factor
1/3 (~ 0.58 Ie) lower rated current of the motor. If the overload
relay is installed on the mains supply incomer, it will need
to be rated just like the contactor or the switchgear on the
mains supply side to the rated operational current (Ie) of the
motor.
Frequency inverters
The frequency inverter is ultimately the best solution for continuous
and stepless starting of the three-phase asynchronous
motor. The adjustable current limitation prevents high current
peaks in the electrical mains supply and abrupt loads in the
mechanical parts of the machine and systems.
In addition to the smooth start-up, the frequency inverter also
enables stepless speed (frequency) control of the three-phase
asynchronous motor. Whereas motors connected directly to
the mains supply can only achieve the ideal operating conditions
at the steady state operation point (= rating plate specifications),
they can be utilized over the entire speed range with
frequency control, for example, from 4 V at 0.5 Hz to 400 V at
50 Hz. The constant ratio of voltage to frequency (U/f) guarantee
independent operating points with rated-load torque (MM).
When compared to the previously described solutions, the frequency
inverters appear to be the most expensive solution at
first glance. Higher acquisition costs and the necessary additional
installation measures (shielded motor cables and RFI filter
for electromagnetic compatibility, EMC) are the main reasons.
But during operation at the very latest, the soft motor
start in addition to the energy efficiency and process optimisation
shows the economic benefits. This is especially true for
pumps and fans. By the matching of rotation speed to the production
process and the compensation for external interference,
the frequency controlled drive unit guarantees a longer
service life and functional security