Chapter: Design of Electrical Machines : DC Machines
Real & Apparent flux densities
Real & Apparent flux densities
Although a detailed description of
the design of a DC machine is beyond the scope of this material, some design
principles are still worth mentioning. The methods presented previously are
applicable to the design of a DC machine with certain adjustments. One of the
most important special features of a DC machine is the armature reaction and,
in particular, its compensation.
According to the IEC, the armature
reaction is the current linkage set up by the currents in the armature winding
or, in a wider sense, the resulting change in the air-gap flux. Since the
brushes are on the quadrature axis, the armature current produces the armature
reaction also in the quadrature direction; that is, transversal to the
field-winding-generated flux. Figure depicts the armature reaction in the air
gap of a non compensated DC machine.
Resulting air-gap flux density as a
sum of the field winding flux density and the armature reaction. As a result of
the armature reaction, the flux densities at the quadrature axes are not zero.
This is harmful for the commutation of the machine.
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