Siemens SIRIUS 3RW44 - Operating Mode of the SIRIUS 3 RW44 Electronic Soft Starter

Introduction
SIRIUS 3RW44 manual
1-4 GWA 4NEB 535 2195-02 DS 06
1.1.2 Operating Mode of the SIRIUS 3RW44 Electronic Soft Starter
The 3RW44 soft starter has two antiparallel thyristors in each of the phases.
There is one thyristor for the positive and one thyristor for the negative half
wave.
Using phase angle control, the r.m.s. value of the motor voltage is increased
from a definable start voltage or start torque to the motor rated voltage within a
selectable starting time using various control methods.
The motor current acts proportional to the voltage applied to the motor. The
starting current is thus reduced by the factor of the voltage that is applied to the
motor.
The torque behaves quadratically in relation to the voltage applied to the motor.
The starting torque is thus reduced quadratically in relation to the voltage
applied to the motor.
Example
The following graphs illustrate the behavior of the starting current and torque of
a 3-phase asynchronous motor in combination with a soft starter:
Figure 1-3: Reduced current behavior of the 3-phase asynchronous motor during start-up
with the SIRIUS 3RW44 soft starter
SIEMENS motor 1LG4253AA (55 kW)
Rated data at 400 V:
P
e
:55kW
I
e
: 100 A
I
Direct on line starting
: approx. 700 A
M
e
: 355 Nm ;
Example: M
e
= 9.55 x 55 kW x
n
e
: 1480 min
-1
M
Direct on line starting
: approx. 700 Nm
Set start voltage: 50 % (1/2 line voltage)
=> I
Start
1/2 of the direct on line start switch-on current (approx. 350 A)
=> M
Start
1/4 of the direct on line start torque (approx. 175 Nm)
1000
1480 min
-1
004_Stromkurven mit Sanftstarter.wmf
Motor current
I
I
Direct
I
Soft starter
I
Nom
n
Nom
Motor speed
n
on-line
starting