Siemens SIMATIC ET 200pro Quick Manual page 128

8.3.4 Notes on configuration (project planning)
So that a motor can reach its rated speed in the first place, the motor torque
greater than the torque required by the load all through powering up, as other-
wise a stable operating point sets in even before the rated speed of the motor is
reached ("the motor gets stuck"). The difference between the motor torque and
load torque is the acceleration torque, which is responsible for the speed
increase of the drive. The lower this acceleration torque, the longer the power-
ing up time of the motor.
Start-up torque
The reduction of the terminal voltage of a three-phase induction motor achieves
a reduction in the start-up current and start-up torque of the motor.
The current is directly related to the voltage, whereas there is a square relation
between voltage and motor torque.
Example:
Motor = 5.5 kW, rated current = 11.4 A, start-up current = 6.3 x rated current,
motor torque = 36 Nm, start-up torque = 2.4 x rated torque
Settings at the soft starter: start voltage 50% of rated motor voltage
This results in the following reductions:
• The start-up current is reduced to half of the start-up current for direct start:
50% of (6.3 x 11.4 A) = 36 A
• The start-up torque is reduced to 0.5 x 0.5 = 25% of the start-up torque for
direct start: 25% of 2.4 x 36 Nm = 21.6 Nm
Note
Due to the square relationship between the start voltage and torque, the start
voltage must not be set to low. This is to be borne in mind above all with distinc-
tive pull-up torque, the lowest motor torque that occurs during powering up until
rated speed is reached.
8-36
SIMATIC - ET 200pro motor starter
GWA 4NEB 950 5661-02 DS 04