Disadvantages of high specific loadings in DC machine

High specific loadings reduce the size and cost of a dc machine (since $D^2L = P_a/(\pi^2B_{av}\,ac\,n\times10^{-3})$), but they have the following disadvantages.

Higher specific magnetic loading ($B_{av}$)

• Higher iron loss: core loss rises roughly with $B^2$, so efficiency falls and temperature rises.
• Teeth saturation: flux density in the teeth may exceed about 2.2 Wb/m², needing a very large field mmf.
• More field copper: more ampere-turns are needed for the gap and teeth, so field copper loss, field coil size and pole size increase.
• Larger poles and yoke: the flux per pole is larger, so the poles and yoke must be bigger, and the overall diameter may increase.

Higher specific electric loading ($ac$)

• Higher copper loss and lower efficiency.
• Higher temperature rise: more heat per unit surface of armature; may need forced cooling.
• Stronger armature reaction: armature AT per pole $= ac\,\pi D/2P$ increases, causing more field distortion, loss of flux and need of compensating winding.
• Poorer commutation: reactance voltage increases, causing sparking; interpoles may be needed.
• Less room for insulation: risky in high-voltage machines.