Siemens SIRIUS 3RW40, Manual

Page: 21 / 212

Product description
3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft
SIRIUS 3RW30 / 3RW40
Manual, 01/2010, 535 1995-02 DS01
21
Polarity balancing
"Polarity balancing" effectively eliminates these DC components during the ramp-up phase. It
allows the motor to be started up with a constant speed, torque, and current rise.
The acoustic quality of the startup process comes very close to that of a three-phase
controlled startup. This is made possible by the continuous dynamic alignment and balancing
of current half-waves with different polarities during the motor startup.
Figure 3-7 Current characteristic in the three phases without DC components thanks to "polarity balancing"
3.3.2 Starting current asymmetry
With two-phase control the starting current is asymmetrical for physical reasons, because the
current in the uncontrolled phase is the sum of the currents in the two controlled phases.
This asymmetry can be as much as 30 to 40% during starting (ratio of minimum current to
maximum current in all three phases).
Even though this cannot be influenced, it is not critical in most applications. It could cause an
insufficiently rated fuse to trip in the uncontrolled phase, for instance. For recommended fuse
ratings, refer to the tables in chapter Technical data [Page 121].
Figure 3-8 Starting current asymmetry
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