A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.
The present section of this volume deals with polyphase currents, and
the inventions by Mr. Tesla, made known thus far, in which he has
embodied one feature or another of the broad principle of rotating field
poles or _resultant attraction_ exerted on the armature. It is needless
to remind electricians of the great interest aroused by the first
enunciation of the rotating field principle, or to dwell upon the
importance of the advance from a single alternating current, to methods
and apparatus which deal with more than one. Simply prefacing the
consideration here attempted of the subject, with the remark that in
nowise is the object of this volume of a polemic or controversial
nature, it may be pointed out that Mr. Tesla's work has not at all been
fully understood or realized up to date. To many readers, it is
believed, the analysis of what he has done in this department will be a
revelation, while it will at the same time illustrate the beautiful
flexibility and range of the principles involved. It will be seen that,
as just suggested, Mr. Tesla did not stop short at a mere rotating
field, but dealt broadly with the shifting of the resultant attraction
of the magnets. It will be seen that he went on to evolve the
"multiphase" system with many ramifications and turns; that he showed
the broad idea of motors employing currents of differing phase in the
armature with direct currents in the field; that he first described and
worked out the idea of an armature with a body of iron and coils closed
upon themselves; that he worked out both synchronizing and torque
motors; that he explained and illustrated how machines of ordinary
construction might be adapted to his system; that he employed condensers
in field and armature circuits, and went to the bottom of the
fundamental principles, testing, approving or rejecting, it would
appear, every detail that inventive ingenuity could hit upon.
Now that opinion is turning so emphatically in favor of lower
frequencies, it deserves special note that Mr. Tesla early recognized
the importance of the low frequency feature in motor work. In fact his
first motors exhibited publicly--and which, as Prof. Anthony showed in
his tests in the winter of 1887-8, were the equal of direct current
motors in efficiency, output and starting torque--were of the low
frequency type. The necessity arising, however, to utilize these motors
in connection with the existing high frequency circuits, our survey
reveals in an interesting manner Mr. Tesla's fertility of resource in
this direction. But that, after exhausting all the possibilities of this
field, Mr. Tesla returns to low frequencies, and insists on the
superiority of his polyphase system in alternating current distribution,
need not at all surprise us, in view of the strength of his convictions,
so often expressed, on this subject. This is, indeed, significant, and
may be regarded as indicative of the probable development next to be
witnessed.
Incidental reference has been made to the efficiency of rotating field
motors, a matter of much importance, though it is not the intention to
dwell upon it here. Prof. Anthony in his remarks before the American
Institute of Electrical Engineers, in May, 1888, on the two small Tesla
motors then shown, which he had tested, stated that one gave an
efficiency of about 50 per cent. and the other a little over sixty per
cent. In 1889, some tests were reported from Pittsburgh, made by Mr.
Tesla and Mr. Albert Schmid, on motors up to 10 H. P. and weighing about
850 pounds. These machines showed an efficiency of nearly 90 per cent.
With some larger motors it was then found practicable to obtain an
efficiency, with the three wire system, up to as high as 94 and 95 per
cent. These interesting figures, which, of course, might be supplemented
by others more elaborate and of later date, are cited to show that the
efficiency of the system has not had to wait until the present late day
for any demonstration of its commercial usefulness. An invention is none
the less beautiful because it may lack utility, but it must be a
pleasure to any inventor to know that the ideas he is advancing are
fraught with substantial benefits to the public.