426

ELECTRICAL APPARATUS

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Such a variable-ratio converter, then, comprises an armature
A} Fig. 201, with the brushes, B, B', in fixed position and field
poles, P, P', separated by interpolar spaces, C, C', of such width as
required for commutation. Each field pole consists of two parts,
P and PI, usually of different relative size, separated by a narrow
space, DD'y and provided with independent windings. By vary-
ing, then, the relative excitation of the two polar sections, P and
Pi, an effective shift of the resultant field flux and a corresponding
change of the direct voltage is produced.

As this method of voltage variation does not depend upon the
wave shape, by the design of the field pole faces and the pitch
of the armature winding the alternating voltage wave can be
made as near a sine wave as desired. Usually not much atten-
tion is paid hereto, as experience shows that the usual distributed
winding of the commutating machine gives a sufficiently close
approach to sine shape.

Armature Reaction and Commutation

232. With the brushes in quadrature position to the resultant
magnetic flux, and at normal voltage ratio, the direct-current
generator armature reaction of the converter equals the syn-
chronous-motor armature reaction of the power component of
the alternating current, and at unity power-factor the converter
thus has no resultant armature reaction, while with a lagging
or leading current it has the magnetizing or demagnetizing re-
action of the wattless component of the current.

If by a shift of the resultant flux from quadrature position
with the brushes, by angle, r, the direct voltage is reduced by
factor cos T, the direct current and therewith the direct-current

armature reaction are increased, by factor,-----, as by the law

COS T

of conservation of energy the direct-current output must equal
the alternating-current input (neglecting losses). The direct-
current armature reaction, $F, therefore ceases to be equal to the
armature, reaction of the alternating energy current, 9% but is

greater by factor,------:

cos T

COS T

The alternating-current armature reaction, JFo, at no phase dis-
placement, is in quadrature position with the magnetic flux.

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