Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 26)

U
I
p
U/ √ 3
U
I p/3
U ss
U
Starting method Direct ( Δ, delta) Y (star, wye) Soft starting
Current in the pole
conductor 100 % 33 % 33 % 57 %
Torque 100 % 33 % 11 % 33 %
Coil voltage 100 % 57 % 33 % 57 %
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
T
Δ
T
Y
T
SS
I
Δ
I
SS
I
Y
Fig. 1.7-7
Current consumption and torques with direct starting (delta-connected), star-connected starting and
starting with the aid of a soft starting device by reducing the voltage across the motor terminals.
I
Δ
Current consumption with delta-connected direct starting
T
Δ
Torque with delta-connected direct starting
I
Y
Current consumption for star-connected starting
T
Y
Torque for star-connected starting
I
SS
Current consumption when starting by means of soft starting device at torque T
Y
T
SS
Torque when starting by means soft starting device and same current consumption
as for star-connected starting
A startup in star circuit develops a higher torque with the same loading on the power supply, as
the voltage across the motor coils is reduced by the circuit change. The windings current is at
the same time the pole current, while in the case of a delta-connected motor two windings
currents add vectorial (factor 3 ) to the current of the supplying pole.
In relation to the loading of switchgear and its correct selection for starting and operation of
squirrel-cage induction motors with voltage reduction, see Sections
3.3, 3.4 and 3.9 , in which
the possible solutions are described in detail.
When using soft starting devices, it should be remembered that the line-side switchgear
operates the input of the soft starting device, i.e. usually without load. The motor current is
subject to harmonics during start-up, which can lead to undesired tripping, especially where
electronic protective relays are used.
1.7.1.4 Performance of squirrel-cage induction motors with changing fre-
quency
The basic form of the current and torque characteristic is independent of the frequency. In the
sub-synchronous speed range (n = 0 … n
s
), the voltage must be reduced in proportion to the
LVSAM-WP001A-EN-P - April 2009 1-16