1 ; 440 ELECTRICAL APPARATUS
where 1° is the power component of the current corresponding
to the direct-current output.
The voltage ratio of a converter can be varied:
(a) By the superposition of a third harmonic upon the
star voltage, or diametrical voltage, which does not appear in
If '<' the ring voltage, or voltage between the collector rings of the
h, i1 • converter.
,< I Y (b) By shifting the direction of the magnetic flux.
% -j *i i (a) can be used for'raising the direct voltage as well as for
13 I f ' lowering it, but is used almost always for the former purpose,
f I "i \ since when using this method for lowering the direct voltage
i | \ commutation is impaired.
l\\ . ' (6) can be used only for lowering the direct voltage.
•j II ! It is possible, by proportioning the relative amounts by which
f>1 / j' | the two methods contribute to the regulation of the voltage,
»' i ! to maintain a proper commutating field at the brushes for all
f I ' loads and voltages. Where, however, this is not done, the
brushes are shifted to the edge of the next field pole, and into
the fringe of its field, thus deriving the commutating field.
241. In such a variable-ratio converter let, then, t = intensity
of the third harmonic, or rather of that component of it which
is in line with the direct-current brushes, and • thus does the
voltage regulation, as fraction of the fundamental wave, t is
: chosen as positive if the third harmonic increases the maximum
of the fundamental wave (wide pole arc) and thus raises the
direct voltage, and negative when lowering the maximum of the
fundamental and therewith the direct voltage (narrow pole arc).
pi = loss of power in the converter, which is supplied by the
current (friction and core loss) as fraction of the alternating
~if ] , ' input (assumed as 4 per cent, in the numerical example).
|;| '; I n = angle of brush shift on the commutator, counted positive
'I '>: , in the direction of rotation.
, | j i 0i = angle of time lag of the alternating current (thus negative
'^ ff ' for lead).
! ra = angle of shift of the resultant field from the position at
right angles to the mechanical neutral (or middle between the
pole corners of main poles and auxiliary poles), counted positive
in the direction opposite to the direction of armature rotation,
that is, positive in that direction in which the fielcj flux has been
shifted to get good commutation, as discussed in the preceding
article.