User Manual

Product description
3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft starters
SIRIUS 3RW30 / 3RW40
Manual, 10/2010, 535199502000 DS 03
21
The occurrence of DC components, caused by the phase angle and the overlapping phase
currents, is a negative physical effect of two-phase control during the startup process that
can mean a louder noise is produced by the motor. The "polarity balancing" control principle
was developed and patented by SIEMENS to prevent these DC components during starting.
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Figure 3-6 Current characteristic and occurrence of DC components in the three phases without "polarity balancing"
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.
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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 127).