Improvements in Means for Rectifying Alternating Electric Current.
Year: 1897

page_text: Date of Application, 27th July, 1897-Accepted, 28th Aug.,
1897
COMPLETE SPECIFICATION.
Improvements in Means for Rectifying Alternating Electric Current.
We, THE BRITISH THOMPSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Elihu Thomson, of
Swampscott, County of Essex, State of Massachusetts, United States of America),
and in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
Our invention relates to improvements in rectifiers, and especially to means for
commuting alternating current waves and turning the currents in a single direction
so as to send over a line a continuous or pulsating current for feeding devices
requiring uni-directional currents for their best operation.
The object of our invention is to maintain a rectifying commutator in exact
synchronism with the pulses of alternating current to be commuted, and to do this
notwithstanding the inductive lag which may exist in the alternating current line
or which may modify from time to time the phase relation between the prime source
and the current to be commuted. In our improved apparatus the capacity for
self starting is obtained and a balance of forces brought about whereby the
commutating action is precise and not subject to fluctuation or "hunting," as it is
termed. The invention will be better understood by reference to the accompanying
figures, in which Fig. 1 is a perspective view of an apparatus embodying the
invention; and Figs. 2, 3 and 4 are diagrams of circuits explanatory thereof.
Fig. 1 shows an induction motor driven by alternating currents, a synchronizing
machine through the coils of which the alternating current impulses to be
commuted are passed, and a commutator for rectifying the pulses or giving the
pulses a single direction. On a suitable base B is mounted an ordinary induction
motor I. Generally the stationary structure is used for the support of the coils
in induction motors. The type of induction motor may be such as is adapted to be
operated by polyphase currents; or from a single alternating current; in which
latter case it is provided with means for giving a starting torque sufficient to carry
it up to synchronism. The armature of the induction motor I is geared to or carries
upon its shaft, the armature or rotating portion of a synchronizing machine S, which
is like a small alternating current generator having field poles excited by a continuous
current, and a number of armature coils corresponding to the field poles for each phase
to be commuted. The armature is laminated, as is customary in such machines. On the
same shaft is mounted a commutator K having a number of segments corresponding
to the poles in the fields of the synchronizing armature and connected as usual for
commuting the impulses passing through the armature coils. By sets of rings R, R¹,
insulated from the shaft, together with suitable brushes, bearing on the rings, an
alternating current from an outside source can be passed through the commutator
and coils of the synchronizing machine S. Binding posts P, P! are provided for
connection to the outside alternating current circuit to be commuted, when a single-
phase current is to be dealt with. With a three-phase current, three-phase

page_text: connections would be provided, and three rings, while the segments of the
commutator would be correspondingly increased. Brushes bear on the rings R, R',
one of which is connected to the binding post P, and the other to P¹. Other brushes
also bear upon the commutator K, and connections are carried from these brushes
to posts, Q, Q from which extends a circuit for arc lamps or other devices through
which the rectifying current passes. Theso brushes may be made adjustable in the
ordinary way for exact position with respect to the slots of a commutator, to avoid
spark.
In our invention the induction motor has such a number of poles as will tend to
cause the synchronizing machine S to run faster than synchronizing speed, whereby
the machino S becomes a "booster" or adds slightly to the electromotive forco
of the current passed through it. The field magnet coils of the synchronizing
machine are preferably put in some part of the circuit from the commutator brushes
to the commutator K and receive magnetism therefore as an offect of the rectified.
current. One of the insulated rings, as R, is connected directly to one terminal of
the winding on the armature of the machine S, the other terminal being connected
to a set of segments alternating in position around the commutators, while the
remaining segments are connected to the remaining ring directly as R'. If the
induction motor "I" have four poles, then the synchronizing machine S may have
six poles, or a greater number than that of the induction motor. This puts the
inductor motor under load at all times and makes it tend to drive the synchronizing
machine; while the synchronizing machine having a tendency above synchronism
is under load as a generator, the driving torque of the induction motor being
balanced, or nearly balanced, regardless of friction, by the drag of the armature of
the synchronizing machine working as a generator. By this arrangement there is
no difficulty in starting the machine by using the induction motor alone, and
afterwards closing the circuit, (when synchronism has been attained or somewhat
surpassed) through the binding posts by connection to the alternating current
source, after which the machine runs at synchronizing speed and commutes
the current passed through it to the circuit attached to the direct current
terminals Q, Q.
At
If polyphase currents are used, the connections may be such as are shown in
Fig. 2. Here the induction motor is fed by branches taken from three mains a, b, c;
three-phase transformers T transform the currents into three secondary circuits, the
terminals of which may be attached to rings R on the shaft of the apparatus.
the same time the transformer T may be (as is now well understood in the art), made
to produce inductively alternating currents of fairly constant value, so that the
currents delivered to R will differ from those coming from the mains a, b, c, in the
fact that a certain current strength tends to be maintained in the lines leading to R,
while only a constant potential is maintained on the lines a, b, c. The polyphase
induction motor "I" would, as stated before, have a less number of poles in its
winding than the synchronizer S. The passage of the current through the three coils
of the armature of S from the three rings at R would make the machine S an
alternating current three-phase generator driven by the polyphase induction motor I,
and the pulses of current, after passing the armature coils would be carried through
the commutator and rectified for feeding the field circuit and the circuit of arc
lamps L, L.
In Fig. 3 is shown diagrammatically the arrangement of connections. The armature
coils are marked at A', A2, A3, and would be disposed on the armature in the ordinary
succession of a three-phase generator. Each of these coils receive an impulse from
one of the rings R in proper sequence, and after passing these coils the current
is carried to the opposite segments of the commutator, while brushes resting on the
segments commute the currents and carry them to the field coil F and outside
circuit. The overlap of the brushes on the commutator is made suitable for proper
commutation, and the commutator shown would be one suited to a four pole structure
in the machine S, Figs. 1 or 2; that is, it would have six segments, the opposite
ones connected, and brushes placed substantially at right angles to each other.

page_text: In single-phase rectifying, the mains are as seen in Fig. 4, a, b and the induction
motor I is made self starting by any device, such as the well known "shaded pole";
in the figure it is shown as having two poles. The constant current transformer,
transforming from the line a, b to the secondary 8, t is shown at T; this trans-
formation may be effected in any of the ordinary ways, such as by a leakage
transformer, transformers with movable or repelled secondaries, &c., the tendency
in the current in the line s, t being to a constant value. The generator S is shown
with four poles and will therefore tend to run slower at the same rate of alternation
than the induction motor I. The synchronizing machine modifies the speed of the
combined induction motor and synchronizer. The connections from the lines s, t
conveying alternating currents are made through rings R, R', as in Fig. 1, one ring
being connected as indicated to a terminal of the armature or interior winding of
the machine S, the other terminal being connected to a pair of diametrically opposite
segments in the commutator K, while the other pair of segments are connected to
the ring R'. Brushes on the commutator K in position at right angles relatively
connect with the segments and with the circuit of lights L through the field winding
of the machine S. Fig. 4 is indeed a diagrammatic representation of the circuit
connections such as would be used in the operation of a machine like Fig. 1. It is
of course to be understood that the number of poles in each motor may be varied
widely and increased or decreased according to the speed desired, while the number
of commutator segments will follow the relation of the poles.
In all cases
however the tendency of the motor should be to carry the apparatus beyond
synchronism so far as the synchronising machine is concerned, which is accomplished
whenever the motor has such a lessened number of oles than the synchronizer as
will give a speed somewhat above synchronism when the machine is driven by tho
induction motor.
The arrangement of the machine as described in which the impulses of alternating
current pass through the synchronizer machine S in a virtually series relation
with the lights L, secures as a result that (no matter whether the phase relation of
the current in the line s, t with respect to the currents in the line a, b be constant
or variable), the commutation is made with reference to the phase in the line 8, t
alone and always in a definite way, since the motor "I" tends to drive the coils of
the synchronizer S a little beyond synchronizm. The driving torque of the motor I
is opposed by the drag or counter torque of the synchronizer S in resisting being
driven beyond synchronizm and the counter-balancing of these two forces makes
the commutation sharp.
It is of course to be understood that the excitation of the field-magnets of the
synchronizing machine S might be obtained from some other source than the current
generated or commuted, but this is a minor element of our invention, of no
particular consequence to the actions involved, it being more convenient generally
to connect the field S in series with the lights L. It will be understood that any
machine which tends to run by alternating currents, without reference to exact
synchronizm, may supplant the machine I for the purposes of our invention and may
be regarded as the induction motor. In fact any source of power tending to drive
the shaft of the synchronizing machine S somewhat above synchronizm might take
the place of the induction motor I in our rectifying apparatus, provided that the
driving machine does not actually force the machine S beyond synchronizm, but
merely tends to do so.
Having now particularly described and ascertained the nature of our said inven-
tion (as communicated to us from abroad), and in what manner the same is to be
performed, we declare that what we claim is :-
1. The improvement in the art of rectifying alternating current, which consists in
passing the current through a rectifying device and by external power imparting
to the rectifier a tendency to rotate at a speed greater than synchronizm.
2. In combination, a rectifier, means for supplying the rectifier with alternating

page_text: current, and an external source of power tending to drive the rectifier above
synchronizm.
3. In combination, alternating current mains, a rectifying device supplied with
current thereby, an electric motor upon the shaft of the rectifier, the motor arranged
to run above synchronizm for the rectifier at no load; whereby when under load the
rectifier has a tendency to rotate at a speed slightly in excess of synchronizm.
4. In combination, alternating current maius, a rectifying device supplied with
current thereby, and an alternating current motor upon the shaft of the rectifier
taking current from the mains, the number of poles in the motor being less than in
the rectifier.
Dated this 27th day of July 1897.
For the Applicants,
J. C. CHAPMAN,
70 Chancery Lane, London, W.C., Chartered Patent Agent.