User Manual

Product description
3.2 Basic physical principles of a three-phase induction motor
SIRIUS 3RW30 / 3RW40
16 Manual, 10/2010, 535199502000 DS 03
3.2 Basic physical principles of a three-phase induction motor
SIRIUS soft starters are used to reduce the current and torque of a three-phase induction
motor during the startup process.
3.2.1 Three-phase induction motor
Fields of application
Three-phase induction motors are used in a wide range of applications in commerce,
industry, and trade owing to their simple, robust design and their minimal maintenance.
Problem
If a three-phase induction motor is started directly, its typical current and torque
characteristics can cause disturbances in the supply system and the load machine.
Starting current
Three-phase induction motors have a high direct starting current I
starting
. Depending on the
motor type, this current can be between three and fifteen times as high as the rated
operational current. Seven or eight times the motor's rated current can be assumed as a
typical value.
Disadvantage
This results in the following disadvantage:
● Higher load on the electrical supply system. The supply system must therefore be
dimensioned for this higher power during the motor startup.
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Figure 3-1 Typical starting current characteristic of a three-phase induction motor