Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 91)

If the motor has reached 80 ... 95 % of its rated speed (depending on the desired reduction of
the current surge after switching-over), the star contactor K1M on the transformer is opened.
Now the transformer part-windings act as chokes. The motor voltage is only reduced by the
chokes below the supply voltage and the motor speed does not fall. The main contactor K3M
closes via auxiliary contacts of the star contactor and applies the full supply voltage to the
motor. For its part, the main contactor K3M drops out the transformer contactor K2M. The entire
procedure is thus uninterrupted.
Fig. 3.4-1
Auto-transformer starter with uninterrupted switching over (Korndörfer starting method)
3.4.2 Rating of the starter
The main contactor K3M and the motor protective device F1 are selected according to the motor
rated operational current I
e
. Transformer contactor and star contactor are only briefly closed
during starting. Their rating is determined by the required contact breaking capacity, as they
must reliably cope with any unforeseen disconnection during start up. The star contactor also
operates with every start-up during switching-over. The values of the rated operational currents
for the transformer contactor K2M, depending on the start time and starting current, are
between (0.3 … 1) · I
e
, for the star contactor between (0.45 … 0.55) · I
e
.
3.5 Starting via chokes or resistors
The series-connected chokes ( Fig. 3.5-1 ) or resistors ( Fig. 3.5-2 ) reduce the voltage at the
motor and hence also the starting current. The starting torque is reduced by around the square
of the current.
3.5.1 Starting via chokes
At rest the motor impedance is small. Most of the supply voltage drops across the series-
connected chokes. The breakaway torque of the motor is therefore strongly reduced. With
increasing speed, the voltage across the motor increases because of the fall of the current
consumption and the vectorial voltage distribution between the motor and the reactance
connected in series. Hence the motor torque also increases. After the motor start-up, the
chokes are shorted by the time-delayed main contactor K1M and the starting contactor K2M is
dropped out.
LVSAM-WP001A-EN-P - April 2009 3-15