3RW3 semiconductor motor control unit
SIRIUS System Manual
8-6
GWA 4NEB 430 0999-02b
8.2.1 Physical principles
Starting current
Three-phase current asynchronous motors have a high inrush current I
(star-
ting)
. This inrush current can be between three and fifteen times as high as
the rated operational current, depending on the type of motor. A figure bet-
ween seven and eight times the rated operational current can be postulated
as typical.
Figure 8-1: Typical current and torque curve of a three-phase asynchronous motor
Important
This starting current must be taken into consideration in the design of the
supply network, among other things by adapting the supply (high heat deve-
lopment) and the fusing (inadvertent tripping of the fuses).
Reducing the starting
current
There are various ways of reducing the starting current:
• By star-delta starter
• By frequency converter
• By soft starter
I
starting
I
0 (no-load)
I
I
rated
n
rated
n
syn
n
M
n
M
starting
M
saddle
M
motor
M
cusp
M
motor
M
rated
n
rated
n
syn
M
accel
M
load
M
load
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