Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 88)

Fig. 3.3-15
The four switching steps of the closed transition star-delta – circuit
A
Starting in star – connection
B Switching-over: Star and transition contactors are closed
C Switching-over: Delta circuit via transition contactor and resistors
D Operation in normal delta circuit
Rating of starters
Main contactor K1M 0.58 · I
e
Delta contactor K2M 0.58 · I
e
Star contactor K3M 0.58 · I
e
Transition contactor K4M 0.27 · I
e
(typical value, varies with R1)
Overload relay F1 0.58 · I
e
Transition resistor R1 (0.35...0.4) · U
e
/I
e
The factor should be selected from the stated range so that a
standard resistance value results.
Unlike in the normal star-delta circuit, the star contactor in the starter for closed transition has
the same rating as the main and delta contactor. This is for two reasons:
- The K3M star contactor must break the star current of the motor and of the transition
resistances. A current of approx. 1.5 · I
e
flows in the transition resistors. Therefore a corre-
spondingly higher contact rating is required.
- The closed transition star-delta circuit is often used in plants with higher frequencies of
operation, in which also a longer electrical life span is required.
The resistors are only loaded for a maximum of 0.1 seconds (short-time duty). However in most
cases only the continuous load capacity of the resistors is known. For wired ceramic-tube
resistors the continuous load capacity P
R
required for selection can be calculated by help of the
following approximation formulas:
P
R
≈ U
e
2 /(1200 · R) [W] for max. 12 operations/h
P
R
≈ U
e
2 /(500 · R) [W] for max. 30 operations/h
Notes
In a star-delta circuit with closed transition, no excessive switching current surge can be
produced. With large inertial masses, it should also be ensured that the motor is correctly wired
for clockwise or counterclockwise rotation (see Section
3.3.2 ), to prevent damage by torque
surges.
3.3.5 Amplified star-delta starting
With a large load torque, an adequate speed is not achieved in the normal star connection
because of the reduction of the starting torque of the motor (see
Fig. 3.3-5 ). A larger motor
torque can be achieved with amplified star-delta starting. That being said, the starting current
also increases with the motor torque (see
Tab. 3.1-1 ).
Two starting methods are possible:
- Mixed star-delta starting
- Part winding star-delta starting
For both methods, motors with suitable windings tappings are required.
LVSAM-WP001A-EN-P - April 2009 3-12