Year: 1898

page_text: Date of Application, 1st Mar., 1898
Complete Specification Left, 30th Nov., 1898-Accepted, 4th Feb., 1899
I, REGINALD BELFIELD, of 32, Victoria Street, in the City of Westminster,
Electrician, do hereby declare the nature of this invention to be as follows:
My invention relates to alternating current electric motors; and more particularly
to such motors pertaining to this general class as are operated by the inductive
action of two or more currents differing in phase and generally known as rotary
field, or Tesla, motors.
The object of my invention is to provide a form of winding for the primary
members of rotary field motors which, when supplied with multiphase currents, will
produce a rotating field that closely approximates in effect a single pole of uniform
strength mechanically rotated.
In motors of the Tesla type, as constructed prior to my invention, the resultant
poles as they revolve and draw after them the armature, vary in strength and in
distribution of lines, as will be seen from the following description and the
drawings accompanying the same. The result is that there is an uneven or non-
uniform torque productive of dead points, which is liable to produce difficulty in
starting. Furthermore, the fluctuations in the strength of the magnetic poles pro-
duce secondary currents in the coils and in the iron of the machine, and thus entail
losses of energy through heating of the coils and iron.
By the employment of my invention these difficulties are obviated and all the
advantages of uniformity of torque and of pole strength are obtained.
My invention is illustrated in the accompanying drawings, wherein Figure 1
is a diagram of an old form of Tesla four-pole motor, wherein the primary member or
field-magnet is the rotating member. Figure 2 is a similar diagram of a machine,
the primary member or field magnet of which is wound in accordance with my
invention.
In both Figure 1 and Figure 2, the core slots in the primary member of the motor
are indicated as radial and the conductors as located side by side therein for
convenience of illustration. In practice the slots and the conductors will be
located as is usual in armatures of the drum type.
Figure 3 is a diagram illustrating the relative position and direction of the
effective field producing currents in the machine represented in Figure 1, and
Figure 4 is a similar diagram illustrating the relative position and direction of
the field producing currents in the machine represented in Figure 2, one of the
phases of current being indicated by full lines and the other by broken lines.
Figure 5 is a diagram illustrating the differences of magnetic field strength
at different portions of the rotating field magnet shown in Figure 1, the surface
of said magnet being represented by a straight line and the distribution being that
DQ17

page_text: which occurs when one of the two phases of current supplied is at a maximum
and the other is at zero. Figure 6 is a diagram similar to Figure 5, the distribution
illustrated being that which occurs when the phase which was at a maximum in
Figure 5 is at zero. Figure is a diagram similar to Figures 5 and 6, the
distribution illustrated being that which occurs when the currents flowing in
the two supply circuits have the same value. Figures S, 9, and 10 are distribution
diagrams illustrating the magnetic conditions in the field magnet shown in Figure 2
at instants corresponding respectively to those of Figures 5, 6, and 7. Figures 11
to 14 inclusive show the distribution of magnetic strength in the field magnet shown
in Figure 1, one magnetic pole being shown in full lines and the other in dotted
lines, and the four figures represent the progression incident to a quarter revolution
in a four pole machine.
Figures 15 to 18 inclusive are diagrams corresponding to Figures 11 to 14,
but illustrating the condition present in the form of machine shown in Figure 2.
It has been hitherto customary in constructing distributed windings and apply-
ing the same to the primary members of rotary field motors to make each coil
embrace that part of the full circumference of the core which is obtained by
dividing the same into as many parts as there are poles. Thus, if the motor is
a four-pole motor, each of the coils will cover or embrace 90 degrees of arc, and if
a six-pole machine, each coil will embrace 60 degrees, and so forth.
This old form of winding, when of the closed coil type, is shown in Figure 1,
the core being divided, as regards the winding, into four segments, and the slots
in each segment being numbered from 1 to 8 inclusive. The leads a, b, c, d, corre-
spond to one phase of current and the arrows indicate the direction of current
flow when this phase is at a maximum and the other phase corresponding to the
leads a, b, c, d, is at zero. To avoid confusion, the arrows indicating the direction
of current flow when the second phase of current is at any other value than zero
are omitted.
In this application the term armature is applied for convenience of description
to that portion of the machine wherein the currents are induced, and the term
field-magnet, to the part which receives current from the line, and while the
armature is here shown as stationary and the field-magnet as rotatable, it is well
understood in the art that this relation is reversible.
In each of Figures 5 to 10, the surface of the drum field magnet is represented
by a straight line y, y. The vertical lines e, f, g, h, in these figures indicate, at
their intersection with the line y, y, the corresponding points in Figures 1, 2, 3,
and 4. In Figure 5, for example, the shaded wave indicates the various strengths
of magnetic field at successive points around the field-magnet when the current of
the phase corresponding to leads a, b, c, d, is positive and at a maximum, and
the current of the phase corresponding to leads a¹, b, c, d is at zero. It will be
seen that the greatest field strength is found only at a point, and that the wave is
sharp. Of course, this may not be the exact form of wave in every respect in
any particular machine, but the general principle is illustrated correctly in this
figure.
Figure 6 shows the distribution of magnetism when the current corresponding to
leads a¹, b, c, d, is at a maximum, and that corresponding to leads a, b, c, d, is at
zero. The form of wave is seen to be the same.
In Figure 7 the dotted lines show the two components which produce the wave
shown in shaded form, which latter illustrates the virtual wave of distribution of
field strength at the moment when the two currents are equal.
It will be seen that this wave has a flat top and that its height is equal to that
of the two components. Since the two waves will be equal 45 degree in time from
their maximum values, it may be demonstrated mathematically that the height of
the two component waves shewn in dotted lines in Figure 7 is about three-quarters
of that of the simple waves in Figure 5; that is, it is equal to the maximum value
multiplied by sine 45 degrees.
Another method of representation of the magnetic conditions illustrated in

page_text: Figures 5, 6, and 7 is found in Figures 11 to 14, the conditions of Figure 5 being
indicated in Figure 11, and those of Figure 6 in Figure 13. The curves in this
case are seen to come to a point, while in Figures 12 to 14, which correspond to
Figure 7, the tops of the curves are shewn flattened so that over a considerable
portion of the surface of the field magnet the same field strength exists. The
different diameters of outside dotted circles in Figures 11 and 14, on the one
hand, and in Figures 12 and 14 on the other, indicate the different extremes of
field strength present in the two cases. Thus it is seen that not only do the
strengths of the poles vary as they rotate in the old form of motor, but there is a
sudden change in the extent of surface over which the maximum reaches. Both
of these fluctuations are productive of secondary or induced currents which are
more or less deleterious. Now, if instead of the distribution of windings found
in Figure 1, we use a form of winding in which each coil embraces an arc which is
either greater or less than that resulting from dividing the entire surface by the
number of poles, the magnetic effects of the various coils will overlap, so to speak,
even as the coils themselves overlap, and there will be an interference of inducing
efforts which will result in a spreading of the maximum over an increased area and
in cutting down the amount of this maximum. I have found that by constructing
the coils so that each embraces an arc that is from three-fourths to eight-tenths the
arc resulting from dividing the circumference by the number of poles, or by making
the first-named arc greater than the second in about the same degree, I can produce
very nearly the equivalent of a rotating pole of constant strength without variation
in the space over which the maximum extends.
My invention is not limited, however, to any specific degree of variation from the
old form of winding. A four-pole machine having my new form of winding is
illustrated in Figures 2 and 4 wherein it is seen that each coil embraces about
three-fourths of 90 degrees.
Upon comparing Figures 8 to 10 with the corresponding Figures 5 to 7, it will
be seen that when either of the exciting currents in my improved winding is at a
maximum, the magnetic waves of distribution are flat at the top and their height
is only about three-fourths of that of the sharp-pointed waves indicated in said
Figures 5 to 7. This relation of the two forms of distribution waves will be more
readily understood by reference to Figures 1, 2, 3, and 4. In the construction
indicated in Figures 1 and 2, the conductors in the several slots are so located
that the several magnetic effects of their currents are arranged in four groups,
there being eight slots in each group. The magnetic effects in each group thus
combine to form a magnetic pole which gradually increases in strength or value
from the points h and f, to the points g and e.
In the form illustrated in Figures 2 and 4, the two currents in each of the
slots 1 and 8 oppose each other, and hence there is nothing added to the magnetic
field by virtue of these currents. It will be readily seen, therefore, that the
number and location of these conductors and slots with reference to the other
conductors and slots of the respective groups is such that the height of the dis-
tribution wave will be substantially three-fourths of the height which would be
secured if the magnetic effect of the currents in the coils 1 and 8 were not
neutralised. Of course the number of slots and conductors in each group might be
made such as to make the ratio between the flattened wave and the sharp-pointed
wave different from that which obtains in the construction illustrated. It will be
understood also that the magnetism due to the other phase of current will be
shifted as indicated in Figures 6 and 9, and that the interfering, non-effective
currents of that phase will be in the conductors located in slots 4 and 5. The
resultant wave shown in Figure 10 and corresponding to that shown in Figure 7
approximates very closely in form to that of the wave shown in Figures 8 and 9,
it being higher than either of its components. Comparing these figures with
Figures 15 to 18, as Figures 5 to 7 were compared with Figures 11 to 14, it will
be seen that in each successive position of the wave of magnetic distribution, its

page_text: shape remains the same or approximately so, and that in all there is substantially
the same height representing the maximum.
It will, of course, be understood that my above described invention is applicable
to machines of any desired number of poles driven by currents of any desired
number of phases, and I therefore do not intend to limit the scope of my invention,
in these respects to what is specifically illustrated and described.
Dated this 1st day of March 1898
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C.,
Patent Agents for the Applicant.
I, REGINALD BELFIELD, of 32, Victoria Street, in the City of Westminster,
Electrician, do hereby declare the nature of this invention and in what manner the
same is to be performed to be particularly described and ascertained in and by the
following statement:
-
My invention relates to alternating current electric motors, and more particularly
to such motors pertaining to this general class as are operated by the inductive
action of two or more currents differing in phase and generally known as rotary
field, or Tesla, motors.
The object of my invention is to provide a form of winding for the primary
members of rotary field motors which, when supplied with multiphase currents, will
produce a rotating field that closely approximates in effect a single pole of uniform
strength mechanically rotated.
In motors of the Tesla type, as constructed prior to my invention, the resultant
poles as they revolve and draw after them the armature, vary in strength and in
distribution of lines, as will be seen from the following description and the
drawings accompanying the same.
The result is that there is an uneven or non-
uniform torque which is liable to produce difficulty in starting. Furthermore, the
fluctuations in the strength of the magnetic poles produce secondary currents in
the coils and in the iron of the machine, and thus entail losses of energy through
heating of the coils and iron.
By the employment of my invention these difficulties are obviated and all the
advantages of uniformity of torque and of pole strength are obtained.
My invention is illustrated in the drawings filed with my Provisional Specifica-
tion, wherein Figure 1 is a diagram of an old form of Tesla four-pole motor, wherein
the primary member or field-magnet is the rotating member. Figure 2 is a similar
diagram of a machine, the primary member or field magnet of which is wound in
accordance with my invention.
In both Figure 1 and Figure 2 the core slots in the primary member of the motor
are indicated as radial and the conductors as located side by side therein for
convenience of illustration. In practice the slots and the conductors will be
located as is usual in armatures of the drum type.
Figure 3 is a diagram illustrating the relative position and direction of the
effective fields producing currents in the machine represented in Figure 1, and
Figure 4 is a similar diagram illustrating the relative position and direction of
the field producing currents in the machine represented in Figure 2, one of the
phases of current being indicated by full lines and the other by broken lines.
Figure 5 is a diagram illustrating the distribution of magnetic field strength
at different portions of the rotating field magnet shown in Figure 1, the surface

page_text: of said magnet being represented by a straight line and the distribution being that
which occurs when one of the two phases of current supplied is at a maximum
and the other is at zero. Figure 6 is a diagram similar to Figure 5, the distribution
illustrated being that which occurs when the phase which was at a maximum in
Figure 5 is at zero. Figure 7 is a diagram similar to Figures 5 and 6, the
distribution illustrated being that which occurs when the currents flowing in
the two supply circuits have the same value. Figures 8, 9 and 10 are distribution
diagrams illustrating the magnetic conditions in the field magnet shown in Figure 2
at instants corresponding respectively to those of Figures 5, 6, and 7. Figures 11
to 14 inclusive show the distribution of magnetic strength in the field magnet shown
in Figure 1, one magnetic pole being shown in full lines and the other in dotted
lines, and the four figures represent the progression incident to a quarter revolution
in a four pole machine.
Figures 15 to 18 inclusive are diagrams corresponding to Figures 11 to 14,.
but illustrating the condition present in the form of machine shown in Figure 2.
It has been hitherto customary in constructing distributed windings and apply-
ing the same to the primary members of rotary field motors to make each coil
embrace that part of the full circumference of the core which is obtained by
dividing the same into as many parts as there are poles. Thus, if the motor is
a four-pole motor, each of the coils will cover or embrace 90 degrees of arc, and if
a six-pole machine, each coil will embrace 60 degrees, and so forth.
This old form of winding, when of the closed coil type, is shown in Figure 1,
the core being divided, as regards the winding, into four segments, and the slots
in each segment being numbered from 1 to 8 inclusive. The leads a, b, c, d, corre-.
spond to one phase of current and the arrows indicate the direction of current
flow when this phase is at a maximum and the other phase corresponding to the
leads a, b, c, d, is at zero. To avoid confusion the arrows indicating the direction
of current flow when the second phase of current is at any other value than zero
are omitted.
In this application the term armature is applied for convenience of description
to that portion of the machine wherein the currents are induced, and the term
field-magnet, to the part which receives current from the line, and while the
armature is here shown as stationary and the field-magnet as rotatable, it is well
understood in the art that this relation is reversible.
In each of Figures 5 to 10, the surface of the drum field magnet is represented
by a straight line y, y. The vertical lines e, f, g, h, in these figures indicate, at
their intersection with the line y, y, the corresponding points in Figures 1, 2, 3,
and 4. In Figure 5, for example, the shaded wave indicates the various strengths
of magnetic field at successive points around the field-magnet when the current of
the phase corresponding to leads a, b, c, d, is positive and at a maximum, and
It will be
the current of the phase corresponding to leads a, b, c, d is at zero.
seen that the greatest field strength is found only at a point, and that the wave is
sharp. Of course, this may not be the exact form of wave in every respect in
any particular machine, but the general principle is illustrated correctly in this
figure.
Figure 6 shows the distribution of magnetism when the current corresponding to
leads a, b, c, d, is at a maximum, and that corresponding to leads a, b, c, d, is a
The form of wave is seen to be the same.
zero.
In Figure 7 the dotted lines show the two components which produce the wave
shown in shaded form, which latter illustrates the virtual wave of distribution of
field strength at the moment when the two currents are equal.
It will be seen that this wave has a flat top and that its height is equal to that
of the two components. Since the two waves will be equal 45 degrees in time from
their maximum values, it may be demonstrated mathematically that the height of
the two component waves shewn in dotted lines in Figure 7 is about three-quarters
of that of the simple waves in Figure 5; that is, it is equal to the maximum value
multiplied by sine 45 degrees.

page_text: Another method of representation of the magnetic conditions illustrated in
Figures 5, 6, and 7 is found in Figures 11 to 14, the conditions of Figure 5 being
indicated in Figure 11, and those of Figure 6 in Figure 13. The curves in this
case are seen to come to a point, while in Figures 12 and 14, which correspond to
Figure 7, the tops of the curves are shewn Hattened so that over a considerable
portion of the surface of the field magnet the same field strength exists. The
different diameters of outside dotted circles in Figures 11 and 13 on the one
hand, and in Figures 12 and 14 on the other, indicate the different extremes of
field strength present in the two cases. Thus it is seen that not only do the
strengths of the poles vary as they rotate in the old form of motor, but there is a
sudden change in the extent of surface over which the maximum reaches. Both
of these fluctuations are productive of secondary or induced currents which are
more or less deleterious. Now, if instead of the distribution of windings found
in Figure 1, we use a form of winding in which each coil embraces an arc which is
either greater or less than that resulting from dividing the entire surface by the
number of poles, the magnetic effects of the various coils will overlap, so to speak,
even as the coils themselves overlap, and there will be an interference of inducing
efforts which will result in a spreading of the maximum over an increased area and
in cutting down the amount of this maximum. I have found that by constructing
the coils so that each embraces an arc that is from three-fourths to eight-tenths the
arc resulting from dividing the circumference by the number of poles, or by making
the first-named are greater than the second in about the same degree, I can produce
very nearly the equivalent of a rotating pole of constant strength without variation
in the space over which the maximum extends.
My invention is not limited, however, to any specific degree of variation from the
old form of winding. A four-pole machine having my new form of winding is
illustrated in Figures 2 and 4 wherein it is seen that each coil embraces about
three-fourths of 90 degrees.
Upon comparing Figures 8 to 10 with the corresponding Figures 5 to 7, it will
be seen that when either of the exciting currents in my improved winding is at a
maximum, the magnetic waves of distribution are flat at the top and their height
is only about three-fourths of that of the sharp-pointed waves indicated in said
Figures 5 to 7. This relation of the two forms of distribution waves will be morė
readily understood by reference to Figures 1, 2, 3 and 4. In the construction
indicated in Figures 1 and 3, the conductors in the several slots are so located:
that the several magnetic effects of their currents are arranged in four groups,
there being eight slots in each group. The magnetic effects in each group thus
combine to form a magnetic pole which gradually increases in strength or value
from the points h and f, to the points g and e.
In the form illustrated in Figures 2 and 4, the two currents in each of the
slots 1 and 8 oppose each other, and hence there is nothing added to the magnetic
field by virtue of these currents. It will be readily seen, therefore, that the
number and location of these conductors and slots with reference to the other
conductors and slots of the respective groups is such that the height of the
wave will be substantially three-fourths of the height which would be
secured if the magnetic effect of the currents in the coils 1 and 8 were not
neutralised. Of course the number of slots and conductors in each group might be
made such as to make the ratio between the flattened wave and the sharp-pointed
wave different from that which obtains in the construction illustrated. It will be
understood also that the magnetism due to the other phase of current will be
shifted as indicated in Figures 6 and 9, and that the interfering, non-effective
currents of that phase will be in the conductors located in slots 4 and 5. The
resultant wave shown in Figure 10 and corresponding to that shown in Figure 7
approximates very closely in form to that of the wave shown in Figures 8 and 9,
it being higher than either of its components. Comparing these figures with
Figures 15 to 18, as Figures 5 to 7 were compared with Figures 11 to 14, it will
be seen that in each successive position of the wave of magnetic distribution, its

page_text: shape remains the same or approximately so, and that in all there is substantially
the same height representing the maximum."
It will, of course, be understood that my above described invention is applicable
to machines of any desired number of poles driven by currents of any desired
number of phases, and I therefore do not intend to limit the scope of my invention
in these respects to what is specifically illustrated and described.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, as communicated to me
by my foreign correspondent, I declare that what I claim is:-
1. A rotary field motor having a primary member provided with a winding each
coil of which embraces a circumferential core space the width of which is either
materially greater or materially less than that obtained by dividing the whole
circumference by the number of poles, substantially as described for the purpose
specified.
2. A rotary field motor the primary member of which is wound substantially as
described with reference to Figure 2 of the accompanying drawings.
ليفة
1
1
1000
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C.,
Patent Agents for the Applicant.
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C.,
Patent Agents for the Applicant.