Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 92)

Fig. 3.5-1
Motor starting via series-connected chokes
3.5.2 Starting via resistors
The basic circuit diagram is the same as described in Section 3.5.1 , only that the chokes are
replaced by lower-cost resistors.
Fig. 3.5-2
Motor starting via series-connected resistors
With this method, the starting current can only be slightly reduced, as the motor torque falls with
the square of the voltage and the voltage across the motor, other than with starting via chokes,
only increases slightly with increasing speed. It is more advantageous to reduce the series-
resistance during starting in steps. This reduces the voltage across the resistor and increases
that across the motor. The expenditure on hardware is thereby significantly larger.
A simpler solution are enclosed electrolytic resistors with a negative temperature coefficient.
Their ohmic resistance decreases automatically during starting because of heating by the
starting current.
3.6 Stator resistance soft starting
3.6.1 Circuit and function
This method is used with relatively small induction motors with squirrel-cage rotors to achieve a
soft starting effect. The starting torque is reduced because an ohmic resistance is connected in
the supply line of one phase (
Fig. 3.6-1 ). This means that the motor is asymmetrically supplied,
resulting in a more gentle, surge-free motor start-up. The motor current is not reduced in the two
phases without series resistance. Modern solutions make use of controlled power semiconduc-
tors instead of resistors.
LVSAM-WP001A-EN-P - April 2009 3-16