ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING MOTOR.
In a preceding chapter we have described a method by which Mr. Tesla
accomplishes the change in his type of rotating field motor from a
torque to a synchronizing motor. As will be observed, the desired end is
there reached by a change in the circuit connections at the proper
moment. We will now proceed to describe another way of bringing about
the same result. The principle involved in this method is as follows:--
If an alternating current be passed through the field coils only of a
motor having two energizing circuits of different self-induction and the
armature coils be short-circuited, the motor will have a strong torque,
but little or no tendency to synchronism with the generator; but if the
same current which energizes the field be passed also through the
armature coils the tendency to remain in synchronism is very
considerably increased. This is due to the fact that the maximum
magnetic effects produced in the field and armature more nearly
coincide. On this principle Mr. Tesla constructs a motor having
independent field circuits of different self-induction, which are joined
in derivation to a source of alternating currents. The armature is wound
with one or more coils, which are connected with the field coils through
contact rings and brushes, and around the armature coils a shunt is
arranged with means for opening or closing the same. In starting this
motor the shunt is closed around the armature coils, which will
therefore be in closed circuit. When the current is directed through the
motor, it divides between the two circuits, (it is not necessary to
consider any case where there are more than two circuits used), which,
by reason of their different self-induction, secure a difference of
phase between the two currents in the two branches, that produces a
shifting or rotation of the poles. By the alternations of current, other
currents are induced in the closed--or short-circuited--armature coils
and the motor has a strong torque. When the desired speed is reached,
the shunt around the armature-coils is opened and the current directed
through both armature and field coils. Under these conditions the motor
has a strong tendency to synchronism.
[Illustration: FIGS. 49, 50 and 51.]
In Fig. 49, A and B designate the field coils of the motor. As the
circuits including these coils are of different self-induction, this is
represented by a resistance coil R in circuit with A, and a
self-induction coil S in circuit with B. The same result may of course
be secured by the winding of the coils. C is the armature circuit, the
terminals of which are rings _a b_. Brushes _c d_ bear on these rings
and connect with the line and field circuits. D is the shunt or short
circuit around the armature. E is the switch in the shunt.
It will be observed that in such a disposition as is illustrated in
Fig. 49, the field circuits A and B being of different self-induction,
there will always be a greater lag of the current in one than the other,
and that, generally, the armature phases will not correspond with
either, but with the resultant of both. It is therefore important to
observe the proper rule in winding the armature. For instance, if the
motor have eight poles--four in each circuit--there will be four
resultant poles, and hence the armature winding should be such as to
produce four poles, in order to constitute a true synchronizing motor.
[Illustration: FIG. 52.]
The diagram, Fig. 50, differs from the previous one only in respect to
the order of connections. In the present case the armature-coil, instead
of being in series with the field-coils, is in multiple arc therewith.
The armature-winding may be similar to that of the field--that is to
say, the armature may have two or more coils wound or adapted for
different self-induction and adapted, preferably, to produce the same
difference of phase as the field-coils. On starting the motor the shunt
is closed around both coils. This is shown in Fig. 51, in which the
armature coils are F G. To indicate their different electrical
character, there are shown in circuit with them, respectively, the
resistance R' and the self-induction coil S'. The two armature coils are
in series with the field-coils and the same disposition of the shunt or
short-circuit D is used. It is of advantage in the operation of motors
of this kind to construct or wind the armature in such manner that when
short-circuited on the start it will have a tendency to reach a higher
speed than that which synchronizes with the generator. For example, a
given motor having, say, eight poles should run, with the armature coil
short-circuited, at two thousand revolutions per minute to bring it up
to synchronism. It will generally happen, however, that this speed is
not reached, owing to the fact that the armature and field currents do
not properly correspond, so that when the current is passed through the
armature (the motor not being quite up to synchronism) there is a
liability that it will not "hold on," as it is termed. It is preferable,
therefore, to so wind or construct the motor that on the start, when the
armature coils are short-circuited, the motor will tend to reach a speed
higher than the synchronous--as for instance, double the latter. In such
case the difficulty above alluded to is not felt, for the motor will
always hold up to synchronism if the synchronous speed--in the case
supposed of two thousand revolutions--is reached or passed. This may be
accomplished in various ways; but for all practical purposes the
following will suffice: On the armature are wound two sets of coils. At
the start only one of these is short-circuited, thereby producing a
number of poles on the armature, which will tend to run the speed up
above the synchronous limit. When such limit is reached or passed, the
current is directed through the other coil, which, by increasing the
number of armature poles, tends to maintain synchronism.
[Illustration: FIG. 53.]
In Fig. 52, such a disposition is shown. The motor having, say, eight
poles contains two field-circuits A and B, of different self-induction.
The armature has two coils F and G. The former is closed upon itself,
the latter connected with the field and line through contact-rings _a
b_, brushes _c d_, and a switch E. On the start the coil F alone is
active and the motor tends to run at a speed above the synchronous; but
when the coil G is connected to the circuit the number of armature poles
is increased, while the motor is made a true synchronous motor. This
disposition has the advantage that the closed armature-circuit imparts
to the motor torque when the speed falls off, but at the same time the
conditions are such that the motor comes out of synchronism more
readily. To increase the tendency to synchronism, two circuits may be
used on the armature, one of which is short-circuited on the start and
both connected with the external circuit after the synchronous speed is
reached or passed. This disposition is shown in Fig. 53. There are three
contact-rings _a b e_ and three brushes _c d f_, which connect the
armature circuits with the external circuit. On starting, the switch H
is turned to complete the connection between one binding-post P and the
field-coils. This short-circuits one of the armature-coils, as G. The
other coil F is out of circuit and open. When the motor is up to speed,
the switch H is turned back, so that the connection from binding-post P
to the field coils is through the coil G, and switch K is closed,
thereby including coil F in multiple arc with the field coils. Both
armature coils are thus active.
From the above-described instances it is evident that many other
dispositions for carrying out the invention are possible.