AC Operation of an iron core

Hence, if a wattmeter were used to measure the power delivered over a full period of the waveform (which will also be the average power delivered over a long period of time), the answer should be zero.  In practice, there will be a finite energy loss recorded, which is found to be much larger when the coil is wound on a conductive former than when it is air cored.

The energy loss is attributed to:
 
 
1) 'Copper losses', i.e. loss due to current flowing through the resistance of the windings, (P=iR).This power loss occurs whether the
core is air cored or not.

2) 'iron losses'. These arise due to two causes:
 
a)  the hysteresis loss due to the energy dissipated in continually reversing the magnetisation of the core. The energy taken per cycle is the area of the hysteresis B/H curve.

b) Eddy current losses due to  induced currents circulating within the conductive core of the coil. These are induced by the changing current in the coil and circulate in such a way as to try to oppose the change in the coil current. They are minimised by laminating the core to increase the path resistance for the circulating current and hence reduce its value. Since Power = i.R, reducing the current reduces the power even though the resistance is increased.

A transformer is produced by winding a second coil onto the former.

Note the polarity of the currents. The current in the primary coil creates a flux in the core with a direction according to Maxwell's RH corkscrew rule. The current induced into the secondary coil is in such a direction as to try to oppose the original flux change in the core.