Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 87)

harmonic current. This applies for motor protective devices such as bimetal relays, whose trip
characteristic is based on the r.m.s. value of the current.
Electronic motor protective devices frequently use measuring principles that differ from the
above (for example based on the peak value of the current). In these cases, the settings
adjustment must be made on the basis of practical tests.
3.3.4 Uninterrupted star-delta starting (closed transition)
With this circuit ( Fig. 3.3-14 and Fig. 3.3-15 ) the decay of the motor speed during star-delta
switching is avoided and the subsequent current peak is kept small.
Before the star contactor is opened, a fourth (transition) contactor K4M makes the motor circuit
via resistances in delta. This means that the motor current is not interrupted during switching-
over (
Fig. 3.3-13 ) and the motor speed hardly falls. The delta contactor K2M then establishes
the final state of operation and drops out the transition contactor K4M.
As the normal star-delta circuit, this circuit is only suitable for starting with small load torques.
Fig. 3.3-13
Oscillogram of star-delta starting with closed transition
Fig. 3.3-14
Star-delta starter for closed transition
LVSAM-WP001A-EN-P - April 2009 3-11