There are some basic factors affecting the inductance of an inductor (iron-core). These parameters are determined by the construction of an inductor as shown in the following (if all other factors are equal):
The number of turns (N) for the coil
More turns for a coil will produce a stronger magnetic field resulting in a higher induced voltage and inductance.
The length of the core (l)
A longer core will make a loosely spaced coil and a longer distance between each turn, therefore producing a weaker magnetic field resulting in a smaller inductance.
The cross-section area of the core (A)
A larger core area requires more wire to construct a coil, and therefore it can produce a stronger magnetic field resulting in a higher inductance.
The permeability of the material of the core (μ)
A core material with higher permeability will produce a stronger magnetic field resulting in a higher inductance. (Permeability of the material of the core determines the ability of the material to produce a magnetic field. Different materials have different degrees of permeability.) Factors affecting the inductance of an inductor are illustrated in Figure:
Where inductance is symbolized by L, measured in henries (H); area of the core is symbolized by A, measured in m²; permeability is symbolized by μ; the number of turns is symbolized by N.
From the expression of the factor affecting inductance, we can see that either when the number of turns of a coil increases, or when the cross-section area of the core increases, or when core material with higher permeability is chosen, or when the length of the core is reduced, the inductance of an inductor will increase.