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.