Siemens SIMATIC ET 200pro - Figure 8-9: Current and Torque Characteristics with Star-Delta Start

SIMATIC - ET 200pro motor starter
8-18 GWA 4NEB 950 5661-02 DS 04
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
Figure 8-9: Current and torque characteristics with star-delta start
I
I
A
d
I
A
*=
Start-up current at d
Start-up current
at
* / d-Start
n
n
n
syn
*n
rated
M
M
A
d
M
motor at
d
M
motor at
*
M
A
*
=1/3 M
A
d
M
load
n
n
n
syn
*n
rated
1/3 I
A
d