Siemens SIRIUS 3RW40, Manual

Page: 20 / 212

Product description
3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft starters
SIRIUS 3RW30 / 3RW40
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Manual, 01/2010, 535 1995-02 DS01
Figure 3-5 Phase angle control and schematic diagram of a two-phase controlled soft starter with integral bypass
contacts
3.3.1 Method of operation of a two-phase controlled soft starter
A special method of operation is used for the SIRIUS 3RW30 and 3RW40 two-phase
controlled soft starters based on SIEMENS' patented "polarity balancing" control principle.
Two-phase control
The SIRIUS 3RW30 and 3RW40 soft starters are two-phase controlled soft starters, in other
words they are designed with two antiparallel thyristors in each of phases L1 and L3. Phase 2
is an uncontrolled phase, which is merely guided through the starter by a copper connection.
In a two-phase controlled soft starter, the current that results from the superimposition of the
two controlled phases flows in the uncontrolled phase. The main advantages of two-phase
control include the more compact size compared to a three-phase version and the lower
hardware costs.
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.
Figure 3-6 Current characteristic and occurrence of DC components in the three phases without "polarity
balancing"
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