The set stopping time (tStop) must be longer than the loaddependent,
free run down time of the machine. This process is
also load-dependent just as the acceleration. The thyristors of
the soft starter are also subject to the same thermal stresses
that were present during the start-up process. If, for example,
the delay time is also activated for a soft starter with 10 permissible
starts per hour, 5 starts per hour (plus 5 stops per hour)
are permitted. The stop ramp time (tStop) can be selected independently
of the start time, which is frequently required on
pumps to prevent pressure waves (water hammer). Jerky
movements during an uncontrolled run down, which, for example,
can cause higher wear on drive belts, drive chains and
bearings, can also be prevented.
The design of the switchgear and protection devices (electromechanical
components) in the main circuit of the motor feeder
is undertaken in accordance with the rated operational current
(Ie) of the motor and the utilization category AC-3 (standard IEC
60947-4-1). The design of the soft starter is undertaken in
accordance with the rated operational current (Ie) of the motor
and the utilization category AC-53a or AC-53b (standard IEC/
EN60947-4-2):
• AC-3 = squirrel-cage motors: start-up, switch off during operation.
• AC-53a = control of a squirrel-cage motor: eight-hour duty
with starting currents for start processes, manoeuvring,
operation.
• AC-53b = control of a squirrel-cage motor: intermittent operation
(intermittent operation means that the soft starter is
bypassed externally during continuous operation, e.g. by a
bypass contactor).
The in-line configuration corresponds with the motor feeder
during direct-on-line start. Only three cables lead to the motor
and are connected to U1, V1 and W1 in the terminal box. The
winding ends are switched as star or delta in accordance with
the available motor and supply voltage.