SSE JE Electrical Engineering 28 Oct 2020 Shift 1 Papers

Flux density is defined as flux per unit area. i.e. ${\mathrm{B}}_{\mathrm{m}}=\frac{\mathrm{\varphi }}{\mathrm{A}}$
Where $\mathrm{\varphi }=$ flux per pole
$\mathrm{A}=$ area of core
Size of the machine is directly proportional to area
i.e. size of machine ∝ area of machine $\propto \frac{1}{{\mathrm{B}}_{\mathrm{m}}}$
Hence with the increase in magnetic flux density, the size of the machine decreases.
As the machine size decreases, p.u. armature resistance increases and p.u. synchronous reactance decreases.