Saturday, 21 May 2016

Losses in Electrical Machine - Core Loss and Eddy Current Loss


There are two types of Losses in an Electrical Machine. They are

  • Core Loss or Iron Loss
  • Ohmic Loss or Copper Loss


In this post we will discuss about Core Loss. Core Loss is again classified into two types:

  • Hysteresis Loss
  • Eddy Current Loss


First we will have a look at how the core of a Transformer looks like. But the Core Loss take place in any electrical machine which face changing magnetic flux.



Hysteresis Loss:

This loss is due to magnetic properties of iron part or core.

When the magnetic field strength or the current is increased the flux increase, after a point when we further increase current the flux gets saturated. When we reduce the current from saturation to zero side the flux density starts to decrease. But when the current value reaches zero the flux density should also be zero but it is not zero. For zero current there is still some flux present in the material, this is known as Residual Magnetic Flux or Remnant Magnetic Flux. Hence the amount of power is never recovered back. The power which gets trapped in the core of the material is lost in the form of heat.



Now we will consider the mathematical part of Hysteresis Loss. The Hysteris Loss in core is given as

Ph = KhfBmx

Where Kh = Constant which depends on the volume and quality of core material.

Bm = Maximum flux density in the core

f = Frequency of Supply

x = Steinmetz’s constant whose value varies from 1.5 to 2.5.

Thus we see that Core Loss depend on Voltage as well as Frequency of Supply.

Eddy Current Loss:

Eddy Current Loss takes place when a coil is wrapped around a core and alternating ac supply is applied to it. As the supply to the coil is alternating, the flux produced in the coil is also alternating.

By faradays law of electromagnetic induction, the change in flux through the core causes emf induction inside the core. Due to induction of emf eddy current starts to flow in the core. Due to this eddy current there will be an associated Ohmic loss which is called Eddy Current Loss.



Eddy current losses can be reduced by lamination in the core. Thin sheet steels must be used which are insulated from each other. Due to insulated sheets the amount of current which flows get reduced and hence the eddy current losses also reduces.

Now we will take a look at the mathematical part of Eddy Current Loss. Eddy Current Loss is given as

Pe = Kef2Bm2

Where Ke = constant whose value depends on the volume and resistivity of the core                  material.

Bm = Maximum flux density in the core

f = Frequency of Supply

It shall be noted that, from the equation of Eddy Current Loss it seems that Eddy Current Loss depends on the frequency of supply but it is not so rather it only depends on the Supply Voltage. How?

As Pe = Kef2Bm2

But we know that



So,

Bm2f2 = KE2 where K is constant

Thus Eddy Current Loss Pe = KV2

Therefore, Eddy Current Loss only depends on the applied Voltage.


Thank you!

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