IMPROVEMENTS IN ELECTRIC MOTORS
Year: 1884

page_text: I SILVANUS PHILLIPS THOMPSON, Doctor of Science, Professor of Physics in
University College, Bristol do hereby declare the nature of my invention for
IMPROVEMENTS IN ELECTRIC MOTORS, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
The Improvement which is the subject of this my invention is for the purpose of
obtaining in electric motors an approximately uniform speed of running under
varying loads, by simpler means than those hitherto adopted. My Improvement
consists in placing in certain positions with respect to the brushes which supply
the current to the armature, a certain combination of electromagnets and pole-
pieces as hereinafter described. The Invention applies to all leading types of
electric motors and is not confined to any one pattern of armature or design of field
magnets. In electric motors several devices for the purpose of automatic govern-
ment of the speed are known. Some of these devices consist in the application of
centrifugal or dynamometrical governors to vary the resistances or connexions of
the circuit or of some part thereof, or to vary the lead of the brushes according to
the speed or load. Another class of devices consists in arrangements for either
combining together two machines on one spindle or for arranging a differential
winding of the field-magnets so that one part of the coils should be in series with
the armature in the main circuit while another part of the coils is arranged as a
shunt or branch to the main circuit; the two parts of the coils being so connected
that the currents traverse them in opposite directions giving a differential effect;
the magnetism evoked by the current in one part opposing that evoked by the
current in the other. Some of these devices and others more or less closely related
to them, are described by me in my Specifications of Letters Patent Numbers 5122
of the year 1882 and 1639 of the year 1883 and some are described in the
Specification of Letters Patent Number 2830 of the year 1882.
My present Invention is wholly different from the devices already described and
known. I wind the field magnets with two independent sets of coils, not wound
on the same core, not wound differentially, that is to say not wound so as to oppose
one another's magnetising action, but on the contrary wound so as to assist one
another in producing the same polarity at their common pole-piece or at adjacent
pole-pieces. One of these coils, (or, in the case of machines with more than two
coils, one set of these coils), is wound with stout wire united in series with the

page_text: armature; and the other coil or set of coils is wound with finer wire and con-
nected in some cases as a shunt to the armature part of the machine only and in
other cases as a shunt to the armature and series coils together. As an example I
will describe how my invention may be applied in practice to a common Gramme
machine. In such a machine the armature usually revolves between two iron
cheeks or pole pieces. The pole piece on one side is made a south pole, that on the
other side a north pole, by the device of attaching each to the end of an electro-
magnet the coil of which is traversed by the current in such a direction as to
produce a south and a north pole respectively. In applying my invention to such
a case I should attach each iron cheek or pole-piece to the ends of two electro-
magnets, and wind one of the two with series coils, the other with shunt coils, both
sets of coils being arranged, however, so as to produce the same polarity at the
pole piece on the same side. They must on no account be so wound or connected
that the current in the coils which are in the same circuit as the armature
diminishes the magnetism produced in the magnet pole piece at the same side of
the armature by the shunt coils. Further it is essential to my invention that the
poles of these two mutually assisting coils or electromagnets which meet with their
poles side by side at a common pole piece or at adjacent pole-pieces, should be so
fixed that they point toward different sectors of the armature, the pole of one coil
or electromagnet being somewhat in advance of the other. It is a common practice
to make each of the two cheeks or pole-pieces extend round nearly half the circum-
ference of the armature on their respective sides. In such cases it is advantageous
to place the series coils near or at one end of the pole-piece, and the shunt coil near
or at the other, so that while both coils assist to magnetise the same pole piece.
with the same polarity, they act on different parts of the pole-piece. It may be
convenient to regard such a pole-piece as a double pole. Though double poles are
already known in the construction of dynamo machines their use in electric motors
in connexion with shunt and series coils acting at different parts of the pole-piece
as now described is new. It is also-new to use such double poles in the particular
arrangement now described, in which there is purposely given a lead to one of the
poles in advance of the other. It is evident that if the two electromagnets are of
unequal power the two parts or ends of the pole-piece to which they are severally
attached will not be equally magnetised, and consequently any change in the
.relative strength of the currents in the shunt and series coils respectively which
may occur during the running of the machine will virtually alter the position of
the effective pole in the pole-piece; for the effective pole, if the poles are of unequal
strength, will lie nearer to the stronger of the two separate poles. Now it is
known that it is possible to regulate the speed of an electric motor by the
mechanical device of shifting the brushes so that these have a greater or a lesser
lead at the commutator with respect to the line joining the poles of the field magnets.
Several such mechanical devices for altering the lead are described in the several
Specifications bereinbefore enumerated. But in my Invention the lead is virtually
altered without any such mechanical device. The brushes remain in the same
position during the running of the machine; but, since the position of the effective
pole in the pole-pieces varies with the relative strength of the current in the two
coils or sets of coils, the effective lead will be varied according to the speed of the
motor just as truly as if the brushes were moved forward or backward by some
mechanical governor. Further, if the motor is being run from an electric supply
with the standard condition of a constant electromotive force the brushes should be
so set that the part of the pole-piece magnetised by the shunt coil shall be in
advance of that part magnetised by the series coil on that side, so that any increase
in the speed of the motor, resulting as such increase will in a diminution of the
current in the series coil, shall decrease the lead of the polarity of the field magnet,
that is to say shall diminish the value of the angle between the diametral line in
which the brushes lie and the diametral line in which the effective poles are
situated. If the shunt coil is set in advance of the series coil at one pole piece, for
example at that pole piece which becomes a North pole, the shunt coil on the other

page_text: side at the pole piece which becomes a South pole must also be set in advance
of the series coil at that side. In some cases it is found advisable to divide the
pole piece into two portions set side by side but not connected by iron. In some
cases also it is advantageous to replace one set of coils or electromagnets by either
a permanent magnet or an electromagnet or coil deriving its current from a separate
source or supply, the shunt coil being so replaced when the distribution is with
constant or nearly constant potential, the series coil being so replaced when the
distribution is with constant or nearly constant current. When these arrangements
are adopted it is possible in adjusting the finished machine to find by trial a
position for the brushes such that when once set there, the motor if supplied with
electricity under standard conditions shall run at a constant or nearly constant
speed even though the load vary within certain limits. Having now particularly
described and ascertained the nature of my said Invention and in what manner
the same is to be performed I declare that I do not claim as new the use in an
electric motor of a combined winding of shunt and series coils wound on one core,
or even when wound on different cores if they are wound so that the magnetism
due to one set of coils opposes that due to the other set, but I declare that what I
claim is
First; In an electric motor the device of varying the effective lead substantially
as set forth by applying at the same side of the armature, but at different angles
with respect to the brushes of the machine, the pole of a series electromagnet and
the pole of a shunt electromagnet, such pairs of poles being of the same name; that
is to say both being north poles on the one side of the armature, and both being
south poles on the other side of the armature, such pairs of poles being united
to a common pole-piece on that side, or to a pair of adjacent pole-pieces on that
side, substantially as set forth.
Second: In an electric motor the combination (a) of a permanent magnet with a
series electromagnet, or (b) of a separately excited magnet with a series electro-
magnet, or (c) of a permanent magnet with a shunt electromagnet, or (d) of a
separately excited magnet with a shunt electromagnet, such pairs of magnets or
electromagnets being arranged in positions substantially as described in the case of
the combination of a series electromagnet with a shunt electromagnet in the
preceding claim, and for the same purpose of varying the effective lead substantially
as set forth.
Dated this fourth day of January 1884.