Answer Posted / kj

The star delta starter limits the starting current of the
motor by reducing the voltage across each winding. In star
the voltage is equivalent to the phase to neutral of the
power supply. In delta the phase to phase voltage is
applied across the windings.
With a DOL starter the phase to phase voltage is applied
across the windings. The starting current can be in excess
of 7 times the full load current.
The main reason for using star delta starters is that the
power supply system to the motor does not have to support
the high current needed for DOL starting. If the power
supply is not able to supply the current a voltage dip will
occur affecting all other users on the supply.

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