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 m2 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/m2
So, Magnetic Flux Density B = u0urH
Where u0 = Absolute permeability of free space
ur = Relative permeability
Therefore, total Flux produced in Toroid, Ø = B x A
= u0urHA Wb
= u0urNIA / L
So,
Ø = NI / (L / u0urA )
= MMF / Reluctance
Thus from above expression we can say that MMF is the Ampere Turns i.e. NI and Reluctance = L / u0urA
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 / u0urA )
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 u0uris 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.