8 3RW3 semiconductor motor control unit (soft starter) - Siemens ...

3RW3 semiconductor motor control unit
8.2.4 Comparison of the 3RW3 semiconductor motor control unit (soft starter)
with the 3RA star-delta combination
The comparison of soft starter and star-delta combinations shows that the
3RW3 has the following advantages (example here 22 kW):
3RW3 soft starter
Width: 55 mm
Wiring: 3 motor supply leads
Selectable startup parameters
3RA star-delta starter
Width: 165 mm
Wiring: 6 motor supply leads
None
Minimum current values at startup Fixed current ratios (I * = 1/3I d )
No dangerous switchover current peaks
Switchover current peaks when switching
from star to delta
Special variant for Dahlander motors —
Soft coasting-down function —
Table 8-4: Comparison of 3RW3/3RA
8.2.5 Notes on configuration
In order for a motor to reach its rated speed, motor torque at any given time
during startup must be greater than the torque needed by the load, since
otherwise a stable operating point would be reached before the motor
achieved its rated speed (the motor would "drag to a stop"). The difference
between motor torque and load torque is the accelerating torque that is
responsible for the increase in the speed of the drive. The lower the accelerating
torque, the longer the motor needs to run up to its operating speed.
Starting torque
Reducing the terminal voltage of a three-phase asynchronous motor
reduces the motor's starting current and the starting torque.
Current is directly proportional to voltage, whereas voltage is proportional to
the square root of motor torque.
Example:
Motor = 55 kW, rated current = 100 A, starting current = 7 x rating current,
motor torque = 355 Nm, starting torque = 2.4 x rated torque
Settings for the soft starter: starting voltage 50 % of rated voltage for motor
The reductions are thus as follows:
• The starting current is reduced to half the starting current for a direct
start: 50 % of (7 x 100 A) = 350 A
• Starting torque is reduced to 0.5 x 0.5 = 25 % of the starting torque for a
direct start: 25 % of 2.4 x 355 Nm = 213 Nm
SIRIUS System Manual
8-16 GWA 4NEB 430 0999-02c