Operating Mode Of The Sirius 3Rw44 Electronic Soft Starter - Siemens SIRIUS 3RW44 User Manual

Introduction

1.1.2 Operating Mode of the SIRIUS 3RW44 Electronic Soft Starter

Example
1-4
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.
SIEMENS motor 1LG4253AA (55 kW)
Rated data at 400 V:
P :
e
I :
e
I :
Direct on line starting
M :
e
n :
e
M :
Direct on line starting
Set start voltage:
=> I 1/2 of the direct on line start switch-on current (approx. 350 A)
Start
=> M 1/4 of the direct on line start torque (approx. 175 Nm)
Start
The following graphs illustrate the behavior of the starting current and torque of
a 3-phase asynchronous motor in combination with a soft starter:
Motor current
I
I
Direct
on-line
starting
I
Soft starter
I
Nom
Figure 1-3: Reduced current behavior of the 3-phase asynchronous motor during start-up
with the SIRIUS 3RW44 soft starter
55 kW
100 A
approx. 700 A
355 Nm ;
Example: M = 9.55 x 55 kW x
e
-1
1480 min
approx. 700 Nm
50 % (1/2 line voltage)
n
Nom
1000
-1
1480 min
Motor speed
n
SIRIUS 3RW44 manual
GWA 4NEB 535 2195-02 DS 06