What is Core Loss, Hysteresis loss, Eddy current loss

Core losses

Core loss is also known as Iron Loss. And Core losses occur in the Armature of a d.c. machine and are due to the Rotation of Armature in the Magnetic field of the poles.

They are of two types

(i)  Hysteresis loss

(ii) Eddy current loss

1. Hysteresis loss

The d.c. Machine’s Armature experiences Hysteresis loss because any particular part of the armature experiences magnetic field reversals as it passes beneath Successive poles. An Armature rotating in a two-pole machine is Depicted in Figure 1.36. Take a look at a little bit of the Armature, ab. Magnetic lines travel from a to b when the piece ab is under an N-pole. The same piece of iron is placed under an S-pole half an Evolution later, and Magnetic lines pass from b to a, Reversing the iron’s magnetism. Hysteresis loss is the term for the power required to Continuously reverse the Molecular magnets in the armature core. The Steinmetz formula provides it. This

formula is Hysteresis loss, Ph=B16maxfV watts

where Bmax = Maximum flux density in armature f = Frequency of Magnetic Reversals

V = Volume of Armature in m3

h = Steinmetz Hysteresis co-efficient

In order to reduce this loss in a d.c. machine, armature core is made of such materials which have a low value of Steinmetz Hysteresis Co-efficient e.g., silicon steel.

2. Eddy current loss

There are voltages induced in the armature core in addition to the voltages induced in the armature conductors. As seen in Figure, these voltages cause the armature core to experience circulating currents (1.37). Eddy currents are what these are, and the power loss brought on by their flow is referred to as eddy current loss. Eddy current loss manifests as heat, raising the machine’s temperature and decreasing efficiency. Due to the continuous solid iron core’s large cross-sectional area, the resistance to the eddy current path will be minimal. As a result, there will be a significant eddy current and eddy current loss. Making the core resistance as high as is practical will help to reduce the size of the eddy current.

The construction of the core from thin, rounded iron sheets known as laminations can significantly increase the core resistance. Using a varnish coating, the laminations are separated from one another. Little current flows from one lamination to the next because the insulating coating has a high resistance. Furthermore, due to the extreme thinness of each lamination, there is a significant amount of resistance to current flowing through a lamination’s width. Thus, by laminating a core, more core resistance is created, which reduces eddy current and, in turn, eddy current loss.

Eddy current loss, Pe = KeB2maxf2t2V watts

where

Ke = Constant

Bmax = Maximum flux density in Wb/m2

f = Frequency of magnetic reversals in Hz

t = Thickness of lamination in m

V = Volume of core in m3

It may be noted that eddy current loss depends upon the square of lamination thickness. For this reason, lamination thickness should be kept as small as possible.

3. Mechanical losses

These losses are due to friction and windage.

(i)             friction loss e.g., bearing friction, brush friction etc.

(ii)          windage loss i.e., air friction of rotating armature.

These losses depend upon the speed of the machine. But for a given speed, they are practically constant.

Note. Iron losses and mechanical losses together are called stray losses

Eddy current

An emf will be induced in the conductors when the rotating armature with the conductors cuts the magnetic lines. Since metal is a conductor and the armature is made of metal, eddy current will circulate because of the induced emf in that metal. The effects that are produced by this current can be used. Focault current is another name for this current. If the resistance of the path is increased by laminating the cores, eddy current always tends to flow at a right angle to the direction of the flux. The eddy current loss varies as the square of the laminations’ thickness, which allows for a reduction in power loss.