I believe that this type of starting method is to avoid any current inrush to the motor but I cannot find any proof, like calculations or some description about why is this so? So, just try to seek some advice over here on where can I find such informations?

 

The purpose of star delta starting is NOT to prevent inrush current to motor but prevent the motor from taking large inrush current from the supply as it affects other systems / motors in the line. Depending on the local electrical rules the size of the largest motor you can start directly on line varies. Star delta staring is used because it is the most economical and simple method of starting if DOL is not allowed.
Best regards.
Sekar

 

It will not avoid but reduce current inrush. Imagine the motor as a transformer. When the motor stands still, only the very little resistance of the rotor is present on the secondary side. This is what determines the current in the first moment. In the "Y" cuircuit, the primary voltage is reduced by 1/(sqare root(3)) and so is the current. Later when the motor begins to spin, the rotation induces a voltage in the rotor, which is directed against the original voltage. In the above model, add a voltage source connected in series with the resistance on the secondary side. In an ideal (lossless) motor, these voltages would exactly cancel out each other when the motor spins at the synchronous rpm rate.

 

Responding to Phenol's (Fri, Sep 13, 11:31 am) query:

Search List archive for R. Foster's excellent treatise on the general subject of reduced-voltage starting. About the only thing I can
contribute are very rough relative-cost factors, devoid of size, and installation, specifics:

1) Full voltage . . . . . . . . . 1 (base case)
2) Part-winding . . . . . . . . . 3x
3) Wye-delta . . . . . . . . . . . =A05x
4) Primary resistor. . . . . . . 5.5x
5) Auto-xfmr opn trans'n . . 6.0x
6) Auto-xfmr clsd trans'n . . 6.5 x
7) Soft-start electronic . . . . ??

Regards,
Phil Corso, PE
(Boca Raton, FL)