When power is drawn from the secondary coil of the transformer, the dynamic resistance
increases
decreases
remains unchanged
changes erratically
A.
increases
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 the 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, the power factor is no longer zero. The power factor has increased or the phase difference is no longer 90°, ie, phase difference has decreased. Thus, dynamic resistance has increased.
