Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 84)

Fig. 3.3-7
Contactor contacts and motor protection relays are connected in series to the
motor windings in delta connection
K1M Main
contactor
K2M Delta contactor
F1 Thermal relay
I
e
Rated operational current of the motor
I
p
Phase current
For normal star-delta starting, the switchgear should be rated for the following rated operational
currents:
Main contactor K1M = 0.58 · I
e
Delta contactor K2M = 0.58 · I
e
Star contactor K3M = 0.34 · I
e
Thermal relay F1 = 0.58 · I
e
Æ Motor protection in Υ- and Δ- operation, with F1 in
Pos. A ( Fig. 3.3-6 ), t
A
≤ 15 s (normal starting)
Circuit breakers Q1 = 1.00 · I
e
Æ Restricted motor protection in Υ- operation,
with Q1 in Pos. B ( Fig. 3.3-6 ), t
A
> 15 … 40 s
The selection of contactors according to these values applies for starting times of maximum
15 seconds and 12 starts per hour. With heavy-duty starting or higher frequencies of operation,
a larger contactor K3M, possibly also K1M, should be selected (see Sections
2.3.5.2 and 2.3.6 ).
Equally the electrical life span of contactors, especially of the star contactor, should be reviewed
(see Section
2.3.6.3 ). If e.g. switching-over occurs at too low a speed, the star contactor has to
break many times its rated current ( Fig. 3.3-5 ). This would strongly reduce its electrical life
span.
3.3.2 Motor connection for clockwise and counterclockwise direction of
rotation
When the delta contactor connects at adverse vectorial positions of the supply voltage and the
rotor field, transient processes could occur in the motor that could lead to larger current peaks
than at switching-on the delta-connected motor . This can result in the making capacity of the
contactors being exceeded with as a consequence welding of contacts.
The transient currents can be reduced by appropriate wiring of the main circuit ( Fig. 3.3-8 ).
Besides the load on the contactors, this also reduces the dynamic stress on the windings-heads
in the motor.
LVSAM-WP001A-EN-P - April 2009 3-8