Semiconductor soft starter 3RW3 Screen motor connection lines may be necessary. Advantages: Modification and precise adjustment of the motor speed are possible. The voltage/frequency ratio stays almost constant, thus permitting high tor- ques with relatively low currents. Soft starter In the case of a soft starter, the motor voltage is boosted by an adjustable starting voltage to the rated motor voltage by phase control within a starting time. As the motor current is proportional to the motor voltage, the starting current is reduced by the factor of the set starting voltage. The following graphic shows the operating principle of the soft starter 3RW3:
U L1- L3 α α α α ϕ U L1- L3 L1 M 3 ~ L2 G1 L3 Fig. 8-5: Phase control of the line voltage by semiconductor elements in the case of the soft starter 3RW3 Example: Starting voltage 50% of the input voltage Ue => starting current equal to 50% of the motor starting current in the case of direct starting. Soft starting also reduces the motor torque. This is why a soft-started motor no longer jerks when activated. The following applies to the relationship: motor torque proportional to the square of the motor voltage . Example: Starting voltage 50% of the input voltage Ue => starting torque 25% of the starting torque in the case of direct starting. Advantages:
Low space requirement in the control cabinet No wiring (e.g. filters) necessary to keep to the RFI suppression regula- tions (Class A; in UC 24 V control voltage version also Class B) Low assembly effort No-problem commissioning Only three motor connection leads in comparison with the star-delta starter Adjustment options permit adaption to the system. 8-8 Stand 08/99