Air gap flux distribution in direct-current machines

The armature of a direct-current machine under load reacts on the main field, the effect being commonly known as armature reaction. This reaction always has a tendency to distort the main field and may increase or decrease the total flux per pole depending on the brush position and the relative values of armature and field strength. The distribution of the flux, then, along the air gap of a direct current machine is different, when the armature conductors carry current of appreciable strength, from what it is when the machine is running light. The extent of this change in air gap flux distribution determines largely the success of the design, so that it is highly important to be able to determine if possible what the flux distribution will be under load; in fact, no scheme of design can be regarded as complete unless it enables the designer to predetermine with reasonable accuracy the full load as well as the no-load characteristics of a given machine.