Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 25)

breakers with high magnetic trip (c.b.’s for transformer protection) may be required for avoiding
nuisance tripping due to high switching transients.
Above factors should particularly be considered in retrofit applications when replacing old
standard motors with new high efficiency motors.
Also, if softstarters are used, the start current may be higher for a given start torque, so a check
should be made to ensure the equipment is rated accordingly. The available start torque may
also need to be evaluated to ensure matching to the load characteristics.
1.7.1.3 Influence of the voltage across the windings
In order to reduce the high current consumption of squirrel-cage induction motors on starting
and the associated, often disruptive, power supply loading and to reduce the high starting
torque when driving sensitive machines, a wide variety of methods is available that is based on
the reduction of the voltage applied across the motor windings. A reduced voltage across a
motor winding results in a proportional reduction in the current flowing through the winding and
consequently in a reduction of the torque developed, by the square of the reduction in the
voltage applied. ½ voltage for example thus means ¼ torque.
A reduction of the voltage across the motor windings can in principle be achieved in one of two
ways:
ƒ Reduction of the voltage on the motor while leaving the internal connections of the individual
windings unchanged (normally connected in delta). Example: electronic soft starter devices.
ƒ Rearranging the connections of the motor windings so that the voltage on the windings is
reduced. Example: Star-delta circuit.
The ratio of the available motor torque to the motor current flowing is different in above cases.
This is illustrated with the example of the conventional star-delta circuit compared with the
electronic soft starter device (
Fig. 1.7-7 ):
LVSAM-WP001A-EN-P - April 2009 1-15