Since iron is an electrical conductor, the rotation of an iron core within a magnetic field generates an EMF in the iron core itself, which causes current to flow in the core. The DC excitation flux is shown in Figure 1 as the green magnetic field, while the blue dotted lines represent the current that flows due to the applied field flux and the rotation of the iron rotor.
Figure 1 Eddy Currents in a Rotor
Although the voltage is low in value, it is applied across the very low resistance of the iron core and, consequently, heavy currents will flow. These currents, known as eddy currents, cause heat in the iron core, wasting power.
No useful work is done by the eddy currents and the heat produced is a source of power loss measured in watts.
As the iron rotor of the generator rotates, the iron core passes under the influence of the north and south poles of the magnetic field alternatively. Consequently, there will be continuous and rapid alternation of flux within the core, giving rise to the effect known as hysteresis, which has been discussed in the chapter on magnetism.
Since hysteresis leads to energy loss in the form of heat generated within the core material, this effect results in another energy loss known as hysteresis loss.
Eddy-current losses and hysteresis losses both occur within the iron core of a generator and are referred to jointly as ‘iron losses’.
Reducing Iron Losses
Eddy-current losses may be greatly reduced by using iron cores which are made of laminated sheet iron. That is, the cores are built up from many thin sheets of iron which have an insulating oxide on the surface. The oxide is used deliberately to provide a relatively high resistance between the laminations, to break the core up into many thin conductors. This has two effects:
|1.||The generated voltage within the core is broken up into much smaller voltages in each lamination.|
|2.||These smaller voltages are applied across paths of very thin sections and therefore much higher resistance.|
As the number of laminations increases, the value of EMF across each path decreases, while the resistance of each path increases. The overall current flow will therefore reduce rapidly as the number of laminations used in a core of given length is increased.
Hysteresis losses can be kept within reasonable limits by using special steels. In particular, silicon-alloy steel is well suited to alternating magnetic fields by exhibiting very low losses.