Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 82)

Switching-over itself is usually automatic (rarely manual) and performed by a timing relay set to
the required operating period of the star contactor. Between switching off of the star contactor
and the making of the delta contactor there must be a sufficient time interval to be certain that
the breaking arc in the star contactor is quenched before the delta contactor is switched on. If
switching-over is too rapid, the breaking arc causes a short-circuit and the short-circuit protec-
tion disconnects the circuit (see
Fig. 3.3-3 ).
On the other hand, when the switching interval is too long, the motor speed falls during the de-
energized interval, depending on inertial mass and load, so strongly that the in-rush current in
the delta connection is very high, defeating the purpose of the star-delta start up (
Fig. 3.3-4 ).
A sufficiently long switching interval between breaking of the star contactor and making of the
delta contactor is achieved in small contactors with short pull-in and dropout times by electronic
timing relays with a switching-over delay of approx. 50 ms. Larger contactors have an inherent
switching delay of > 25 ms. In this case, timing relays without additional switching delay may be
used. The switching interval then is of the optimum length. To avoid phase short-circuits, the
star and delta contactors are additionally mechanically interlocked.
If the delta contactor is switched via an auxiliary contactor (e.g. at low control voltages), no
switching-over delay is required on the time relay. A switching interval of adequate length
results from the sum of the making delay times of the auxiliary and delta contactor.
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. 3.3-3
A switching interval that is too short results in a short-circuit via the switching arc – the short-circuit
protection is activated and breaks the circuit
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. 3.3-4
With switching intervals that are too long, the speed falls again behind
Æ direct starting in delta connec-
tion
LVSAM-WP001A-EN-P - April 2009 3-6