By CARL HERING.
The occasion of the transmission of power from Lauffen to Frankfort
has brought to the notice of the profession more than ever before the
two or three phase alternating current system, described as early as
1887-88 by various electricians, among whom are Tesla, Bradley,
Haselwander and others. As to who first invented it, we have nothing
to say here, but though known for some years it has not until quite
recently been of any great importance in practice.
Within the last few years, however, Mr. M. Von Dolivo-Dobrowolsky,
electrical engineer of the Allgemeine Elektricitats Gesellschaft, of
Berlin, has occupied himself with these currents. His success with
motors run with such currents was the origin of the present great
transmission of power exhibit at Frankfort, the greatest transmission
ever attempted. His investigation in this new sphere, and his ability
to master the subject from a theoretical or mathematical standpoint,
has led him to find the objections, the theoretically best conditions,
etc. This, together with his ingenuity, has led him to devise an
entirely new and very ingenious modification, which will no doubt have
a very great effect on the development of alternating current motors.
It is doubtless well known that if, as in Fig. 1, a Gramme ring
armature is connected to leads at four points as shown and a magnet is
revolved inside of it (or if the ring is revolved in a magnetic field
and the current led off by contact rings instead of a commutator),
there will be two alternating currents generated, which will differ
from each other in their phases only. When one is at a maximum the
other is zero. When such a double current is sent into a similarly
constructed motor it will produce or generate what might be called a
rotary field, which is shown diagrammatically in the six successive
positions in Fig. 2. The winding here is slightly different, but it
amounts to the same thing as far as we are concerned at present. This
is what Mr. Dobrowolsky calls an "elementary" or "simply" rotary
current, as used in the Tesla motors. A similar system, but having
three different currents instead of two, is the one used in the
Lauffen transmission experiment referred to above.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
In investigating this subject Mr. Dobrowolsky found that the best
theoretical indications for such a system would be a large number of
circuits instead of only two or three, each differing from the next
one by only a small portion of a wave length; the larger their number
the better theoretically. The reason is that with a few currents the
resulting magnetism generated in the motor by these currents will
pulsate considerably, as shown in Fig. 3, in which the two full lines
show the currents differing by 90 degrees. The dotted line above these
shows how much the resulting magnetism will pulsate. With two such
currents this variation in magnetism will be about 40 degrees above
its lowest value. Now, such a variation in the field is undesirable,
as it produces objectionable induction effects, and it has the evil
effect of interfering with the starting of the motor loaded, besides
affecting the torque considerably if the speed should fall slightly
below that for synchronism. A perfect motor should not have these
faults, and it is designed to obviate them by striving to obtain a
revolving field in which the magnetism is as nearly constant as
possible.
[Illustration: FIG. 3.]
If there are two currents differing by 90 degrees, this variation of
the magnetism will be about 40 per cent.; with three currents
differing 60 degrees, about 14 per cent; with six currents differing
30 degrees it will be only about 4 per cent., and so on. It will be
seen, therefore, that by doubling the three-phase system the
pulsations are already very greatly reduced. But this would require
six wires, while the three-phase system requires only three wires (as
each of the three leads can readily be shown to serve as a return lead
for the other two in parallel). It is to combine the advantages of
both that he designed the following very ingenious system. By this
system he can obtain as small a difference of phase as desired,
without increasing the number of wires above three, a statement which
might at first seem paradoxical.
Before explaining this ingenious system, it might be well to call
attention to a parallel case to the above in continuous current
machines and motors. The first dynamos were constructed with two
commutator bars. They were soon found to work much better with four,
and finally still better as the number of commutator bars (or coils)
was increased, up to a practical limit. Just as the pulsations in the
continuous current dynamos were detrimental to proper working, so are
these pulsations in few-phased alternating current motors, though the
objections manifest themselves in different ways--in the continuous
current motors as sparking and in the alternating current motors as
detrimental inductive effects.
The underlying principle of this new system may be seen best in Figs.
4, 5, 6, 7 and 8. In Fig. 4 are shown two currents, I_{1} and I_{2},
which differ from each other by an angle, D. Suppose these two
currents to be any neighboring currents in a simple rotary current
system. Now, if these two currents be united into one, as shown in the
lower part of the figure, the resulting current, I, will be about as
shown by the dotted line; that is, it will lie between the other two
and at its maximum point, and for a difference of phases equal to 90
degrees it will be about 1.4 times as great as the maximum of either
of the others; the important feature is that the phase of this current
is midway between that of the other two. Fig. 5 shows the winding of a
cylinder armature and Fig. 7 that, of a Gramme armature for a simple
three-phase current with three leads, with which system we assume that
the reader is familiar.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
The two figures, 4 and 5 (or 7), correspond with each other in so far
as the currents in the three leads, shown in heavy lines, have a phase
between those of the two which compose them. Referring now to Fig. 6
(or 8), which is precisely like Fig. 5 (or 7), except that it has an
additional winding shown in heavy lines, it will be seen that each of
the three leads, shown in heavy lines, is wound around the armature
before leaving it, forming an additional coil lying _between_ the two
coils with which it is in series. The phase of the heavy line currents
was shown in Fig. 4 to lie between the other two. Therefore, in the
armature in Fig. 6 (or 8) there will be six phases, while in Fig. 5
there are only three, the number of leads (three) remaining the same
as before. This is the fundamental principle of this ingenious
invention. To have six phases in Fig. 5 would require six leads, but
in Fig. 6 precisely the same result is obtained with only three leads.
In the same way the three leads in Fig. 6 might again be combined and
passed around the armature again, and so on forming still more phases,
without increasing the number of leads. Figs. 7 and 8 compound with 5
and 6 and show the same system for a Gramme ring instead of a cylinder
armature.
As was stated in the early part of this description, the main object
in a rotary current motor is to have a magnetic field which is as
nearly constant in intensity as possible, and which changes only its
position, that is, its axis. But in Fig. 4 it was shown that the
current I (in dotted lines) is greater than the others (about as 1.4
to 1 for a phase difference of 90 degrees). If therefore the coils in
Fig. 6 or 8 were all alike, the magnetism generated by the heavy line
coils would be greater than that generated by the others, and would
therefore produce very undesirable pulsations in the magnetic fields;
but as the magnetism depends on the ampere turns, it is necessary
merely to have correspondingly fewer turns on these coils, as compared
with the others. This is shown diagrammatically in Figs. 6 and 8, in
which the heavy line coils have less windings than the others. In
practice it is not always possible to obtain the exact ratio of 1 to
1.4, for instance, but even if this ratio is obtained only
approximately, it nevertheless reduces the pulsations very materially
below what they would be with half the number of phases. It is
therefore not necessary in practice to have more than an approximation
to the exact conditions.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
Fig. 9 shows a multiple phase armature having double the number of
phases as Fig. 1, and would according to the old system, therefore,
require eight leads. Fig. 10 shows the new system with the same number
of phases as in Fig. 9, but requiring only four leads instead of
eight. Figs. 11 and 12 correspond with Figs. 7 and 8 and show the
windings for a multipolar motor in the two systems.
[Illustration: FIG. 13.]
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
These figures show how a motor may be wound so as to be a multiple
phase motor, although the current entering the motor is a simple,
elementary three or two phase current, which can be transformed by
means of a simple three or two phase current transformer, before
entering the motor, such transformers as are used at present in the
Lauffen-Frankfort transmission. But the same principle as that for the
motor may also be applied to transformers themselves, as shown in
Figs. 13 and 14. Fig. 13 shows a set of transformers which are fed by
a simple three-phase current shown in heavy lines, and which gives in
its secondary circuit a multiple phase rotary current. The connections
for the primary circuit of a transformer with six coils are shown
diagrammatically in Fig. 15, the numbers 1 to 6 representing the
succession of the phases. Fig. 14 shows a transformer for a two-phase
current with four leads, transforming into a multiple phase current of
16 leads. The transformer in this figure is a single "interlocked"
transformer in which the fields are magnetically connected and not
independent of each other as in Fig. 13. This has advantages in the
regulation of currents, which do not exist in Fig. 13, but which need
not be entered into here. The transformers used in the Lauffen-Frankfort
transmission are similar, magnetically, to Fig. 14, only that they are
for a simple three-phase current in both primary and secondary
circuits. Attention is also called to the difference in the
connections of secondary circuits in Figs. 13 and 14; in the former
they are connected in a closed circuit similarly to an ordinary closed
circuit armature, while in Fig. 14 they are independent as far as the
currents themselves are concerned, though magnetically their cores are
connected. It is not the intention to enter into a discussion of the
relative values of these various connections, but merely to draw
attention to the wide range of the number of combinations which this
system admits of.--_Electrical World_.
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