Physical Basics Of The 3-Phase Asynchronous Motor And Mode Of Operation Of The Soft Starter; 3-Phase Asynchronous Motor - Siemens SIRIUS 3RW44 System Manual

Introduction
1.1 Physical Basics of the 3-phase Asynchronous Motor and Mode
of Operation of the Soft Starter

1.1.1 3-phase Asynchronous Motor

Applications of the 3-
phase asynchronous
motor
Problem
Starting current
Disadvantage
Start torque
Disadvantages
1-2
Thanks to their robust and simple design and low-maintenance operation, 3-
phase asynchronous motors are used in large numbers in commercial
applications, trade and industry.
If switched on directly, the typical current and torque behavior of the 3-phase
asynchronous motor may negatively influence the feeding supply network and
the load machine during start-up.
3-phase asynchronous motors have a high direct starting current I
Depending on the motor version, this current may be 3 times to 15 times the size
of the rated operating current. A typical value is 7 to 8 times the size of the motor
rated current.
This results in the following disadvantage
• Higher load on the electrical supply network. This means that the supply
network must be dimensioned for this higher output during motor start-up.
Motor current
I
I
Direct
on-line
starting
I
Nom
001_Stromkurven ohne Sanftstarter.wmf
Figure 1-1:Typical starting current behavior of a 3-phase asynchronous motor
The start torque and the stalling torque can usually be assumed to be between 2
and 4 times the rated torque. For the load machine, this means that the starting
and acceleration forces in relation to rated operation result in increased
mechanical load on the machine and the conveyed material.
This results in the following disadvantages
• Higher load on mechanical parts of the machine
• Higher costs because of application wear and maintenance
(starting)
Motor speed
n
Nom
n
SIRIUS 3RW44 System Manual
GWA 4NEB 535 2195-02 DS 04
.