TESLA POLYPHASE TRANSFORMER.
Applying the polyphase principle to the construction of transformers as
well to the motors already noticed, Mr. Tesla has invented some very
interesting forms, which he considers free from the defects of earlier
and, at present, more familiar forms. In these transformers he provides
a series of inducing coils and corresponding induced coils, which are
generally wound upon a core closed upon itself, usually a ring of
laminated iron.
The two sets of coils are wound side by side or superposed or otherwise
placed in well-known ways to bring them into the most effective
relations to one another and to the core. The inducing or primary coils
wound on the core are divided into pairs or sets by the proper
electrical connections, so that while the coils of one pair or set
co-operate in fixing the magnetic poles of the core at two given
diametrically opposite points, the coils of the other pair or
set--assuming, for sake of illustration, that there are but two--tend to
fix the poles ninety degrees from such points. With this induction
device is used an alternating current generator with coils or sets of
coils to correspond with those of the converter, and the corresponding
coils of the generator and converter are then connected up in
independent circuits. It results from this that the different electrical
phases in the generator are attended by corresponding magnetic changes
in the converter; or, in other words, that as the generator coils
revolve, the points of greatest magnetic intensity in the converter will
be progressively shifted or whirled around.
Fig. 92 is a diagrammatic illustration of the converter and the
electrical connections of the same. Fig. 93 is a horizontal central
cross-section of Fig. 92. Fig. 94 is a diagram of the circuits of the
entire system, the generator being shown in section.
Mr. Tesla uses a core, A, which is closed upon itself--that is to say,
of an annular cylindrical or equivalent form--and as the efficiency of
the apparatus is largely increased by the subdivision of this core, he
makes it of thin strips, plates, or wires of soft iron electrically
insulated as far as practicable. Upon this core are wound, say, four
coils, B B B' B', used as primary coils, and for which long lengths of
comparatively fine wire are employed. Over these coils are then wound
shorter coils of coarser wire, C C C' C', to constitute the induced or
secondary coils. The construction of this or any equivalent form of
converter may be carried further, as above pointed out, by inclosing
these coils with iron--as, for example, by winding over the coils layers
of insulated iron wire.
[Illustration: FIGS. 92 and 93.]
[Illustration: FIG. 94.]
The device is provided with suitable binding posts, to which the ends of
the coils are led. The diametrically opposite coils B B and B' B' are
connected, respectively, in series, and the four terminals are connected
to the binding posts. The induced coils are connected together in any
desired manner. For example, as shown in Fig. 94, C C may be connected
in multiple arc when a quantity current is desired--as for running a
group of incandescent lamps--while C' C' may be independently connected
in series in a circuit including arc lamps or the like. The generator in
this system will be adapted to the converter in the manner illustrated.
For example, in the present case there are employed a pair of ordinary
permanent or electro-magnets, E E, between which is mounted a
cylindrical armature on a shaft, F, and wound with two coils, G G'. The
terminals of these coils are connected, respectively, to four insulated
contact or collecting rings, H H H' H', and the four line circuit wires
L connect the brushes K, bearing on these rings, to the converter in the
order shown. Noting the results of this combination, it will be observed
that at a given point of time the coil G is in its neutral position and
is generating little or no current, while the other coil, G', is in a
position where it exerts its maximum effect. Assuming coil G to be
connected in circuit with coils B B of the converter, and coil G' with
coils B' B', it is evident that the poles of the ring A will be
determined by coils B' B' alone; but as the armature of the generator
revolves, coil G develops more current and coil G' less, until G reaches
its maximum and G' its neutral position. The obvious result will be to
shift the poles of the ring A through one-quarter of its periphery. The
movement of the coils through the next quarter of a turn--during which
coil G' enters a field of opposite polarity and generates a current of
opposite direction and increasing strength, while coil G, in passing
from its maximum to its neutral position generates a current of
decreasing strength and same direction as before--causes a further
shifting of the poles through the second quarter of the ring. The second
half-revolution will obviously be a repetition of the same action. By
the shifting of the poles of the ring A, a powerful dynamic inductive
effect on the coils C C' is produced. Besides the currents generated in
the secondary coils by dynamo-magnetic induction, other currents will be
set up in the same coils in consequence of many variations in the
intensity of the poles in the ring A. This should be avoided by
maintaining the intensity of the poles constant, to accomplish which
care should be taken in designing and proportioning the generator and in
distributing the coils in the ring A, and balancing their effect. When
this is done, the currents are produced by dynamo-magnetic induction
only, the same result being obtained as though the poles were shifted by
a commutator with an infinite number of segments.
The modifications which are applicable to other forms of converter are
in many respects applicable to this, such as those pertaining more
particularly to the form of the core, the relative lengths and
resistances of the primary and secondary coils, and the arrangements for
running or operating the same.