276

ELECTRICAL APPARATUS

where:

/ = frequency, in hundreds of cycles,
n = number of armature turns in series,
<i>o = maximum magnetic flux, alternating
through the armature turns, in megalines,
e = effective value of induced voltage.

$! — $2 taking the place of 2 $o, in the inductor alternator,
the equation of voltage induction thus is:

- $2

= V27T/71 J

(2)

As seen, $1 must be more than twice as large as $0, that is,
in an inductor alternator, the maximum*magnetic flux interlinked
with the armature coil must be more than twice as large as in the
standard type of alternator.
In modern machine design, with the efficient methods of cool-
ing now available, economy of materials and usually also effi-
ciency make it necessary to run the flux density up to near satura-
tion at the narrowest part of the magnetic circuit—which usually
is the armature tooth. Thus the flux, $0, is limited merely by
magnetic saturation, and in the inductor alternator, $1, would be
limited to nearly the same value as, $0, in the standard machine,
and -~ir— thus would be only about one-half or less of the
£j
permissible value of <£o. That is, the output of the inductor
alternator armature is only about one-half that of the standard
alternator armature. This is obvious, as we would double the
voltage of the inductor alternator armature, if instead of pulsat-
ing between $1 and $2 or approximately zero, we would alternate
between $1 and — $1.
On the other ban , the single field-coil construction gives a
material advantage in the material economy of the field, and
in machines having very many field poles, that is, high-frequency
alternators, the economy in the field construction overbalances
the lesser economy in the use of the armature, especially as at
high frequencies it is not feasible any more to push the alter-
nating flux, $o, up to or near saturation values. Therefore, for
high-frequency generators, the inductor alternator becomes
the economically superior types, and is preferred, and for ex-
tremely high frequencies (20,000 to 100,000 cycles) the inductor
alternator becomes the only feasible type, mechanically.
158. In the calculation of the magnetic circuit of the inductor