8 starting wound-rotor motors, Starting wound-rotor motors -20 – Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual
Page 96
The contactors are rated according to the rated operational currents I
eI
(Step l) or I
eII
(Step lI).
For the contactors K3 and K4, the higher value applies (
).
Contactor Function
Load
K1
K2
Feeding contactor
Feeding contactor
Step I
Step II
(Y-
Δ)
(YY)
I
eI
I
eII
K3 Delta
contactor
and
1
st
star contactor
Step I
Step II
(Y-
Δ)
(YY)
0.58 · I
eI
and
0.5 · I
eII
K4 Star
contactor
and
2
nd
star contactor
Step I
Step II
(Y-
Δ)
(YY)
Ca. 0.33 · I
eI
and
0.5 · I
eII
Tab. 3.7-3
Rating of starters for steps with star-delta starting
Fig. 3.7-2
2-step star-delta starter for motors in Dahlander circuit (with nine terminals), star-delta starting in Step I
3.8 Starting wound-rotor motors
With slip-ring motors (wound-rotor motors) the starting current can be limited to (1,1 ... 2,8) · I
e
with high load torque and at extended starting times. This means that heavy-duty starting is also
possible with supplies with poor loading capacities. See also Section
Slip-ring motors have rotor windings, whose three ends extend over the slip rings. When
resistors are connected in the rotor circuit, the starting current and hence the torque are affected
(
). The starting resistors in each rotor phase are shorted stepwise by contactors during
start-up.
(
). In automatic starting arrangements, the contactors of the individual starting steps
are controlled by adjustable time relays. In so-called Combi-motors, the rotor resistances are
switched dependent of speed by centrifugal switches.
LVSAM-WP001A-EN-P - April 2009
3-20