CHAPTER XXI
REGULATING POLE CONVERTERS
230. With a sine wave of alternating voltage, and the com-
mutator brushes set at the magnetic neutral, that is, at right
angles to the resultant magnetic flux, the direct voltage of a syn-
chronous converter is constant at constant impressed alternating
voltage. It equals the maximum value of the alternating voltage
between two diametrically opposite points of the commutator,
or "diametrical voltage," and the diametrical voltage is twice
the voltage between alternating lead and neutral, or star or Y
voltage of the polyphase system.
A change of the direct voltage, at constant impressed alter-
nating voltage (or inversely), can be produced:
Either by changing the position angle between the commuta-
tor brushes and the resultant magnetic flux, so that the direct
voltage between the brushes is not the maximum diametrical
alternating voltage but only a part thereof.
Or by changing the maximum diametrical alternating voltage,
at .constant effective impressed voltage, by wave-shape distortion
by the superposition of higher harmonics.
In the former case, only a reduction of the direct voltage be-
low the normal value can be produced, while in the latter case
an increase as well as a reduction can be produced, an increase
if the higher harmonics are in phase, and a reduction if the higher
harmonics are in opposition to the fundamental wave of the dia-
metrical or Y voltage.
A. Variable Ratio by a Change of the Position Angle between
Commutator Brushes and Resultant Magnetic Flux
231. Let, in the commutating machine shown diagrammatic-
ally in Fig. 195, the potential difference, or alternating voltage
between one point, a, of the armature winding and the neutral, 0
(that is, the Y voltage, or half the diametrical voltage) be repre-
sented by the sine wave, Fig. 197. This potential difference is
a maximum, e, when a stands at the magnetic neutral, at A or B.