Technical Background Information - Siemens SIRIUS 3RT Manual

6.13 Starting three-phase motors with reduced starting current peaks (contactor assembly for star-delta (wye-
delta) start)
6.13.2

Technical background information

Starting current ratio
Star-delta (wye-delta) startup can only be used if the motor normally operates in a delta
connection or starts softly, or if the load torque is low and does not rise sharply during star
startup. In the star (wye) stage motors can be subjected to around 50% (class CL16) or
30% (CL10) of their rated torque. The starting torque falls to about 1/3 of the relevant value
during direct switch-on.
The starting current is approximately 2 to 2.7 times the rated motor current.
Switching over
The switchover from star (wye) to delta cannot be carried out until the motor has been fully
accelerated to the rated speed. The necessary changeover delay and interlock are
integrated in the contactor assembly; drives which require this switchover to be performed
earlier are not suitable for star-delta (wye-delta) start.
Reducing the switchover current peak by means of preferred wiring
During star-delta (wye-delta) switching of three-phase motors, the motor may be subjected to
compensation processes, which would result in higher current peaks than would be the case
if the stationary motor were connected directly in the delta circuit.
The worst-case scenario would lead to the following problems, which can be minimized by
using preferred wiring:
● Tripping of short-circuit protection devices
● Welding or substantial contact erosion of the delta contactor
● High dynamic load on the motor
Using a favorable method of connection for the main circuit will reduce the equalizing
currents and current peaks which occur when switching over from a star to a delta circuit.
SIRIUS - SIRIUS 3RT Contactors/Contactor assemblies
Manual, 09/2017, A5E03656507120A/RS-AE/006
Configuration
153