When power is drawn from the secondary coil of the transformer, the...

When the secondary coil circuit is open, the magnetic flux in the core is produced by the primary current only. When the secondary circuit is closed, the currents in the secondary coil also produce magnetic flux in the core but in opposite direction. This decreases the core flux and hence reduces the back emf produced in the primary coil. The source emf is now in excess of the back emf, more current is drawn in the primary coil. Hence, power factor is no longer zero. The power factor has increased or the phase difference is no longer $90^{\circ}$, ie, phase difference has decreased. Thus, dynamic resistance has increased.