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 twophase

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.