Figure 8-9: Current And Torque Characteristics With Star-Delta Start - Siemens SIMATIC ET 200pro Quick Manual

Reducing the start-up current
There are various possibilities to reduce the start-up current:
• using star-delta starters
• using frequency converters
• using soft starters
Star-delta starters
After a certain switching time, the motor windings are switched from the star
circuit into the delta circuit. At star start-up, the motor current is only approx.
1/3 of the current at delta startup (also the motor torque in the star circuit is
reduced to approx. 1/3 of the delta torque).
Disadvantages:
• 6 motor feed lines are necessary
• Switchover surges occur (in the current and torque characteristics)
• It is not possible to adapt the start-up to the circumstances of the system
• Relatively costly and time-consuming assembly
• Additional time relay or PLC overhead required for contactor switchover
• High space requirement in the control cabinet
I
d
I
A
I *=
A
1/3 I d
A

Figure 8-9: Current and torque characteristics with star-delta start

8-18
Start-up current at d
Start-up current
at * / d-Start
d
=1/3 M
A
n
M
M
M d
motor at
A
M
M *
A
n
n
syn
*n
rated
SIMATIC - ET 200pro motor starter
d
*
motor at
n
*n
GWA 4NEB 950 5661-02 DS 04
M
load
n
n
syn
rated