Why transformers take high input current at start?

Answer 1

Inrush current is a result of core saturation. The magnetic flux in the core of a transformer is 90 degrees out of phase with the voltage. Ideally the transformer switching device will be closed at a peak voltage, and the transformer core will have no residual flux from the last close. If this occurs, the flux in the core in the first cycle will be no larger than it normally would be. If the switching device closes at a zero voltage, or there is some residual flux, the transformer's core will be driven into partial saturation within the first cycle, unless it has been oversized to prevent saturation.

Decreasing the applied voltage at energization will keep this from occuring. One method is to use pre-insertion resistors which cause a voltage drop to the transformer that is significant enough to prevent saturation within the first few cycles, and then they are removed. Another method is to de-engerize the transformer core, and close at voltage peaks only (controlled closing).