SOLUTION

A single-phase power transformer is to be energized (switched on to the input supply) to have minimum inrush current. The switching-on Instant should be at maximum Input voltage.

When energized, a transformer draws an inrush current. Therefore, when a generator is connected to a system containing transformers, the transformers will try to draw an inrush current from the generator. Apart from other factors the magnitude of the anent also depends on the short circuit capacity and the voltage of the source. Because of this, a low-voltage generator cannot supply a large inrush current like a high-voltage generator. Therefore, a single set can energize many transformers or a whole system (on no load). A high-voltage set will supply more inrush current.

When the input voltage is maximum, the rate of change of flux is minimum; as both are 90° out of the phase-in case of sinusoidal input.

If we close the switch at the instant of zero value of supply voltage, the flux demanded by the core is maximum and inrush current is also maximum.

To have minimum inrush current in the transformer the switch-on instant should be at maximum input voltage.