Product description 3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft 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.
, $ , $ , $ W V W V W V 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]. , $ , $ , $ W V W V W V Figure 3-8 Starting current asymmetry SIRIUS 3RW30 / 3RW40 Manual, 01/2010, 535 1995-02 DS01 21