Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 83)

Faults like shown in Fig. 3.3-3 and Fig. 3.3-4 can also be avoided with the interruption-free
(closed transition) star-delta circuit (Section 3.3.4 ).
When the load torque is too high the star-connected motor only accelerates to a fraction of the
speed and “sticks” at this speed. The switching process would proceed as in
Fig. 3.3-5 and the
purpose of the star-delta start up would not be achieved.
Moreover this condition means that the contactors have to switch off a multiple of the motor
rated current. In the example in
Fig. 3.3-5 the breaking current is around 1.3 · I
emotor
. The star
contactor is selected according to I
e(Y contactor)
= 0.34 · I
emotor
(see below) and must accordingly
switch off
1.3/0.34
≈ 4 · I
e(Y contactor)
In practice this means AC-4 operation with a correspondingly reduced electrical life span. In this
case a motor for amplified star-delta starting (Section
3.3.5 ) should be used.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
I/I
e
0 20 40 60 80 100
n/n
s
[%]
0.00
0.50
1.00
1.50
2.00
2.50
T /T
e
I
Δ
I
Y
T
Δ
T
Y
Fig. 3.3-5
Switching-over at speed- that is too low
Selecting the starter components
With star-delta circuits in accordance with Fig. 3.3-6 in delta mode the circuits of main contac-
tor, delta contactor and motor protection relays are connected in series to the motor windings
(
Fig. 3.3-7 ). The devices are therefore loaded with the phase current I
p
:
I
p
= I
e/
√3 = 0.58 · I
e
I > I > I >
M
3~
U1
U2
V1
V2
W1
W2
Q1
B
A F1
Fig. 3.3-6
Starter for normal star-delta starting
LVSAM-WP001A-EN-P - April 2009 3-7