Semiconductor soft starter 3RW3 8.2.4 Comparison: soft starter 3RW3 with star-delta combination 3RA The comparison between soft starters and star-delta combinations shows the following advantages in the case of the 3RW3 (example here: 22 kW):: Tabelle 8-4: Comparison: 3RW3/3RA 8.2.5 Configuration notes For a motor to be able to reach its rated speed at all, the motor's torque must be greater at any moment of starting that the torque demanded of the load as otherwise a stable operating point will set in even before the motor's rated speed is reached.
The difference between the motor torque and the load torque is the acceleration torque, which is responsible for the speed increase of the drive. The lower this acceleration torque is, the greater is the motor's starting time. Starting torque By reducing the terminal voltage of a three-phase asynchronous motor, a reduction in the starting current and torque is achieved. The current depends directly on the voltage, while a square relationship pre- vails between the voltage and the motor torque. Example: Motor = 55 kW, rated current = 100 A, starting current = 7 x rated current, motor torque = 355 Nm, starting torque = 2,4 x rated torque Settings on the soft starter:
starting voltage 50% of the motor's rated volt- age This results in the following reductions: The starting current is reduced to half the starting current for direct start- ing: 50% of (7 x 100 A) = 350 A The starting torque is reduced to 0.5 x 0.5 = 25 % of the starting torque in the case of direct starting: 25% of 2.4 x 355 Nm = 213 Nm Note Owing to the square relationship between the starting voltage and the torque, the starting voltage must therefore not be set too low. This must above all be observed in the event of a distinct pull-up torque, the lowest motor torque that occurs during starting to the rated speed. Rev. 08/99 8-15 Soft starter 3RW3 Star-delta starter 3RA Overall width:
55 mm Overall width: 165 mm Wiring: 3 motor connection leads Wiring: 6 motor connection leads Optional starting parameters None Minimum current values during starting Fixed current conditions (I* = 1/3Id) No dangerous changeover current spikes Current spikes when changing over from star to delta Special variant for Dahlander motors ----- Soft coasting function -----