DC
machines
103
Changes in characteristics due to Armature reaction causes the magnetic field in a d.c. machine to be
armature reaction twisted round which concentrates the flux into one side of each pole
piece so that this side may be driven into saturation as a result. Since
the top limit to flux density in iron is in the region of 1.6 to 1.8 tesla
this may result in a reduction in total flux.
Example
10.
The
normal flux density of
the poles
in a d.c. gen-
erator on no load is 1.1 T. Each pole has a cross-sectional area
of 0.05 m
2
. The effect of armature reaction at a particular load
is to concentrate all the flux into one half of the pole face area.
The saturation flux density is 1.8 T. Calculate the new value of
flux per pole and the per unit reduction in flux at this load.
Total flux per pole =