Answer Posted / sashi kanth

COnsider a 3 phase induction motor is switched on by the
means of starter in delta configuration winding of stator.
This is usually done to reduce the time the machine takes
to run at rated speed as the voltage across each phase is
not V/1.732 = 230 V but is 415 V. During normal operation,
winding has to be converted to star configuration. This
transition can be done either by disconnecting the prior
configuration for some instant of time and connecting the
other configuration. As the machine is disconnected from
the excitation for some instant of time it is called open
transition. Closed transition of stator configurations is
done without losing the continuity of the excitation.
Refer to Control of electric drives by Bhattacharya and it
will be more clear.

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