Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 95)

The star point contactor in a PAM circuit, because of the asymmetrical phase currents and of
the harmonic content, should have the same rating as the feeding contactor of the YY step.
The rating of contactors in phase modulation circuits is based on the specifications of the motor
manufacturer with respect to the rated operational current.
Fig. 3.7-1
Circuit diagram for motors in Dahlander or PAM circuits
For all arrangements and circuit layouts, a separate motor protective device should be provided
for the thermal overload protection of the motor in each step that is adjusted to the respective
rated operational current.
As a change of current direction occurs in Dahlander, PAM and phase modulation circuits when
the pole number is changed in a section of the windings, a de-energized interval is necessary
during switching over to prevent unacceptably high switching current surges. If the making delay
of the feeding contactors of both steps is smaller than 20 ms, electrical interlocking must be
performed with a switching interval (approx. 30 … 50 ms). When switching over between two
separate windings, the currents that are produced are only in the range of the starting currents
that the switchgear can easily cope with.
Note
Normally with multi-speed motors, a common short-circuit protection is provided for all steps,
that is rated according to the largest rated operational current. It must be checked whether this
short-circuit protection is also permissible for the selected feeding contactor of the smaller step.
Otherwise a larger contactor should be selected.
3.7.3 Rating of the starter for steps with star-delta starting
If a winding is designed for six terminals, star-delta starting can be provided in this step. Instead
of the feeding contactor, a star-delta contactor combination is required. This is rated according
to the rated operational current of the relevant step.
A reduction of the starting current can also be achieved in star-delta starting in Step l with the
Dahlander circuit
Δ/YY (Step l: Y- Δ; Step II: YY). The circuit can be realized with only four
contactors ( Fig. 3.7-2 ).
LVSAM-WP001A-EN-P - April 2009 3-19