Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 90)

Fig. 3.3-17
Part-winding star-delta starting
Circuit diagram and connections of the motor windings during starting (Y) and in operation (
∆)
Ratings of the starter components
With the exception of the star contactor, contactors and motor protective devices have the same
ratings as with the “normal” star-delta circuit (see Section
3.3.1 ). The star contactor should be
selected for (0.5 ... 0.58) · I
e
because of the larger starting current.
Notes
A sufficiently long switching interval for transition from star to delta operation should be ensured,
in accordance with Section
3.3.1 . A closed transition star-delta connection in accordance with
Section 3.3.4 is possible in both cases. With very large load-torques it may even be necessary.
The transition resistor and the transition contactor should be rated as described there.
The rules in accordance with Section 3.3.2 apply for the connections.
3.4 Auto-transformer starting
3.4.1 Circuit and function
An auto-transformer starter makes it possible to start squirrel-cage induction motors with
reduced starting current, as the voltage across the motor is reduced during starting. In contrast
to the star-delta connection, only three motor leads and terminals are required.
On starting, the motor is connected to the tappings of the auto transformer; transformer
contactor K2M and star contactor K1M are closed. The motor starts at the voltage reduced by
the transformer, with a correspondingly smaller current.
By this means the feeding current in comparison to direct starting would be reduced by the
square of the transformer voltage ratio; nevertheless, it is in most cases noticeably higher, as it
also covers the relatively high transformer losses. Depending on the tapping and starting current
ratio of the motor, the starting current lies at (1 … 5) · I
e
. In contrast, the motor torque falls with
the square of the voltage across the windings. Auto-transformers usually have three available
taps in each phase (for example 80 %, 65 %, 50 %), so that the motor starting characteristic can
be adjusted to the load conditions.
LVSAM-WP001A-EN-P - April 2009 3-14