The rectified voltage generated in the armature has already been discussed before for
an elementary single-coil armature. The effect of distributing the winding in several slots is
shown in figure. In which each of the rectified sine wave is the voltage generated in one of
the coils, commutation taking place at the moment when the coil sides are in the neutral
zone. The generated voltage as observed from the brushes and is the sum of the rectified
voltages of all the coils in series between brushes and is shown by the rippling line labeled
in figure. With a dozen or so commutator segments per pole, the ripple becomes very small
and the average generated voltage observed from the brushes equals the sum of the average
values of the rectified coil voltages. The rectified voltage between brushes, Known also as
the speed voltage, is
(1-4)
where is the design constant. The rectified voltage of a distributed winding has the same
average value as that of a concentrated coil. The difference is that the ripple is greatly
reduced.
From the above equations, with all variable expressed in SI units,
(1-5)
This equation simply says that the instantaneous power associated with the speed
voltage equals the instantaneous mechanical power with the magnetic torque. The direction
of power flow being determined by whether the machine is acting as a motor or generator.
The direct-axis air-gap flux is produced by the combined m.m.f. of the field windings.
The flux-m.m.f. Characteristic being the magnetization curve for the particular iron
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