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| Fig-1 |
As previously discussed, the star delta motor controller provides a changeable motor wiring configuration which is made possible by the sequential switching of magnetic contactors each wired accordingly with a star and a delta connection directly to the motor terminals.
When the motor terminal connection is switched to the delta mode after running in the star mode, a time gap exists in between this transition period wherein high inrush transient current generated from residual voltage is developed during the changeover delay. This intense current spikes can cause extreme over current to overload the motor or damage the magnetic contactors.
To prevent such an occurrence, the best way is to follow an orderly wiring configuration most suitable to achieve a reduced current peaks during the star to delta switching transition period. The delta wiring configuration U1-V2-L1, V1-W2-L2, W1-U2-L3 in Fig-1 achieves the best desirable result of reducing high inrush current upon switching of the magnetic starters from star (wye) to delta connection.
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| Fig-2 |
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| Fig-3 |
Transposing the blue and the green arrows to form a vector diagram to plot the vector difference between them yields a third red arrow Vdiff as shown in Fig-3. Notice the length of the resulting red arrow Vdiff which shows a relatively small differential voltage. Thus providing a relatively small current spike during the transition period when the motor connection switches from star to delta configuration.
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| Fig-4 |
Moving further, if we are to consider reconfiguring the delta connection in Fig-1 above with a different phase orientation sequence such as the illustration in Fig-4. The delta wiring connection in Fig-4 is rearranged in a different manner which is not in accordance to the conforming orderly configuration previously explained. The rearranged delta connection in Fig-4 is reoriented in the order of U1-W2-L1, V1-U2-L2, W1-V2-L3.
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| Fig-5 |
The corresponding vector diagram resulting from the rearranged configuration from Fig-4 above is shown in Fig-5 which illustrates the phase relationship of voltage behavior during the star delta transition period which uses a motor wiring connection not according to a conforming orderly configuration. In Fig-5, the blue arrow VL1-L2 is the primary phase sequence selected in the example where L1 is connected to the starting end coil U1 where it meets with its finishing end coil U2 now connected to L2 instead of the previous connection to L3 (see connection difference between Fig-1 and Fig-4 above). The green arrow VL1b-N represents the transition residual voltage developed during the idle changeover duration from star to delta configuration of the motor connection.
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| Fig-6 |
Transposing the blue and the green arrows to form a vector diagram to plot the vector difference between them yields a third red arrow Vdiff as shown in Fig-6. Notice the length of the resulting red arrow Vdiff which shows a relatively longer differential voltage compared to the red arrow Vdiff from Fig-3. The red arrow in Fig-6 is definitely longer than the red arrow in Fig-3, thus Fig-3 provides a relatively smaller current spike during the transition period when the motor connection switches from star to delta configuration.
It is therefore a recommended practice in the field of industrial process automation control technology to follow a wiring configuration suitable for a reduced high changeover transient current. The best method to achieve a minimal inrush peak current during when the motor transitions from star to delta connection is the one in accordance to the conforming orderly configuration of the reference voltage's phase sequence with respect to the motor terminal connection illustrated accoridngly in Fig-1, Fig-2 and Fig-3.
