Magnetic Circuit is defined
as the path followed by a Magnetic Flux. Magnetic Flux always has a tendency to
follow low Reluctance path. The laws of Magnetic Circuit are quite similar to
the Electric Circuit but the terms used in Magnetic Circuit analysis is
different.

Let us consider a Toroid
having a magnetic path of length L meter. Mind that length of Magnetic path is
equal to the periphery of the Toroid. Also, assume that the Cross Sectional area
of the Toroid is A m

^{2}and N number of turn is wound on the Toroid carrying current I as shown in figure below.
As we know that,

Magnetic Field Strength H
inside Toroid = NI / L Wb/m

^{2}^{}

So, Magnetic Flux Density B
= u

_{0}u_{r}H
Where u

_{0}= Absolute permeability of free space
u

_{r}= Relative permeability
Therefore, total Flux
produced in Toroid, Ø = B x A

= u

_{0}u_{r}HA Wb
= u

_{0}u_{r}NIA / L
So,

Ø = NI / (L / u

_{0}u_{r}A )
= MMF / Reluctance

*Thus from above expression we can say that MMF is the Ampere Turns i.e. NI and Reluctance = L / u*_{0}u_{r}A

*It shall be noted at this point that, Flux Ø is analogous to the electric current, MMF analogous to Voltage and Reluctance to the Resistance.*

Thus like in Electric
Circuit,

Current = Voltage /
Resistance

Likewise in a Magnetic
Circuit,

Flux = MMF / Reluctance

Also as Reluctance = (L / u

_{0}u_{r}A )
It can be seen that
Reluctance is directly proportional to Length of magnetic circuit but inversely
proportional to the cross sectional area of the magnetic circuit. Mind that
this is similar to the Resistance. Reluctance opposes the flow of flux in the
magnetic circuit.

**The term u**

_{0}u_{r}is called Reluctivity which is similar to the Resistivity in electric circuit.**Like conductance in electric circuit, there is one term called Permeance in magnetic circuit which is defined as Reciprocal of Reluctance.**

These are very basics of
magnetic Circuit but very important for understanding whole Electrical Machine.
So one should have crystal clear concept of magnetic circuit.

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