Functional Principle Of The Sirius 3Rw30 And 3Rw40 Soft Starters - Siemens SIRIUS 3RW30 Manual

Disadvantages
This results in the following disadvantages
• A higher load is placed on the machine's mechanical components
• The costs for replacing worn parts and maintaining the application are higher
Figure 3-2
Remedy
The SIRIUS 3RW30 and 3RW40 electronic soft starters allow the current and torque
characteristics during starting to be optimally adapted to the requirements of each application.
3.3

Functional principle of the SIRIUS 3RW30 and 3RW40 soft starters

The SIRIUS 3RW30 and 3RW40 soft starters have two antiparallel thyristors in two out of the
three phases. One thyristor for the positive half-wave and one for the negative half-wave is
provided in each phase (refer to Fig. "Phase angle control and schematic diagram of a two-
phase controlled soft starter with integral bypass contacts"). The current in the third,
uncontrolled phase is the sum of the currents in the controlled phases.
The rms value of the motor voltage is increased (from a settable starting voltage) to the rated
motor voltage within a definable ramp-up time by means of the phase angle control.
The motor current changes in proportion to the voltage applied to the motor. As a result, the
starting current is reduced by the factor of this voltage.
There is a quadratic relationship between the torque and the voltage applied to the motor. As
a result, the starting torque is reduced quadratically in relation to this voltage.
SIRIUS 3RW30 / 3RW40
Manual, 01/2010, 535 1995-02 DS01
3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft
Typical starting torque characteristic of a three-phase induction motor
Product description
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