Abstract
This chapter discusses the nature of the main design problem associated with direct current (DC) machines and oscillogram of DC machine armature-coil current, and it gives a schematic representation of a DC machine, coil circuit during commutation and typical current-reversal curves and brush curves; it shows a simplified diagram of a coil in a slot just before commutation commences. Armature reaction is the term used to describe the effects of the armature magnetomotive force (MMF) on the air-gap field. The axis of the MMF is the same as the brush axis. The chapter demonstrates in quadrature with the main poles on what is sometimes called the geometric neutral axis, or the quadrature axis. To take advantage of the inherent DC machine's flexibility of performance, the total main field MMF is often made up from two or three sources, each having a particular modifying effect on its characteristics. Thus, there may be several field windings. The range of total field MMF required varies with the machine size from less than 1000 At to more than 10000 At per pole.
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