IMPROVEMENTS IN AND RELATING TO ELECTRICAL GENERATORS AND MOTORS
Year: 1885

page_text: I THOMAS JOHN HANDFORD of 42 Southampton Buildings in the County of
Middlesex, Patent Agent, do hereby declare the nature of this invention for
IMPROVEMENTS IN AND RELATING TO ELECTRICAL GENERATORS AND MOTORS "
a communication to me from abroad by Frank Julian Sprague of New York in
the County of New York and State of New York, United States of America,
Electrical Engineer, to be as follows:-
The object of this invention is to so construct an electro dynamic motor or
dynamo electric generator that it will be self-regulating so that the motor may be
made to run at a constant speed under varying loads in a circuit either of constant
current or constant potential; or the generator may develope a constant current or
a constant electro-motive force.
The invention was designed with especial reference to motors, and it will there-
fore be herein described more particularly in that connection, its adaptability to
dynamo-electric generators being however explained.
The motor used by my said foreign correspondent is a differential motor-that
is, one having two sets of field coils, one in series with the armature, which is
termed the "governing coil" and the other in shunt relation to the armature. The
latter coil is of higher resistance and greater number of turns than the former, it is
the principal coil and determines the polarity of the field.
My said foreign correspondent has found that a certain proportion must exist
between the main coil and the governing coil to give the best results either with
constant current or constant potential, and it is in this that the invention chiefly
consists.
There are two classes of differential motors-first, those in which the main field
coil is shunted around both the armature and the differential or series coil, or

page_text: in other words the series coil is inside the terminals of the armature and main
field shunts; and second, those in which the main field coil is shunted around the
armature alone, or the series coil is outside the terminals of the armature and main
field shunts. In either case however the differential coil evidently is in series
with the armature.
For each of these classes of motors a different proportion must exist between the
main and differential field coils; the particular proportions of each case are the
same under all circumstances and whether the machine is used with constant
current or constant potential circuit although the numerical values will depend
upon the current and potential used and the power required.
It follows evidently that a motor of either class wound according to this
invention may be used with either a constant current or constant potential circuit
and will run at a constant speed in either case.
The invention can be most readily set forth and the above statements shown to
be correct by mathematical demonstration.
The magnetic moment of a coil may be defined as the product of the amperes
flowing therein by the number of turns, and if the main and governing coils are
practically similarly situated with regard to the field magnet cores, the magnetic
field may be considered as proportional to the effective magnetic moment-that is
to say the difference of the magnetic moments of the shunt and series field, so long
as we are working on a straight or nearly straight line characteristic. This
characteristic can be determined for any particular cores in any of the well known
ways, for instance, by running the motor as a dynamo at a constant speed, passing
variable known currents through the field coils and noting the potential existing
at the free armature terminals.
For a properly constructed motor the field magnet must at no time be too highly
saturated, that is, it must be worked with a characteristic which is a straight or
very nearly a straight line. As will be pointed out later, some deviation may be
allowed but not much.
Those classes of motors having the inside series coil will be first considered,
when working with varying loads in a constant potential circuit and running at
constant speed.
Let f denote the resistance of the main or shunt field; m the number of turns
therein; the resistance of the differential or series field and n the number of
turns E the difference of potential at the shunt terminals; e the counter electro
motive force set up in the armature; and R the resistance of the armature.
;
E-e
The work done = e that is it depends upon e, a variable quantity and upon
the constants E and r.
Now e varies with the speed and field or the effective magnetic moment of the
field but our conditions are that the speed remains constant hence e must vary
with the field alone.

page_text: r.
1
em E (R+r) en ƒ E + en e¹ f = e' m E (R + r) — e' n ƒ E + e¹
or,
77L
= R+T
That is to say the number of turns in the shunt coil must bear the same ratio
to the number in the series coil as the resistance of the shunt coil bears to the sum
of the resistances of the series coil and the armature.
This is the law of winding for a perfect machine of the kind mentioned, and so
wound it will be perfectly self-regulating for any constant potential up to the
maximum allowed by the construction of the machine and from no load up to the
maximum.
There is a feature of motors so wound which may be here noticed.
The ratio of the magnetic moments of the shunt and sories fields is
That is the ratio of the initial to the effective electro-motive force is the same as
the ratio of the moments of the shunt and series coil.
E
When e o this ratio becomes = 1. That is the moments are equal, and this
means that, in a perfect machine if both coils be closed and in their normal position,
for any potential or current, a zero field or practically so will be formed, and the
motor will either not start at all or if it does start, will run at a very great speed,
take the maximum current at any given potential and do little work or none
at all.
How to obviate the bad effects of this peculiarity and to take advantage of the
same forms a part of the invention and will be hereafter set forth.
What has already been pointed out may be again stated that the motor will
regulate itself perfectly for all potentials so long as we work with a straight line
characteristic, but it must be with a theoretical efficiency of not less than fifty per
cent, for if we go below, this, the governing coil works in the wrong direction.
Referring to the equation ===
n R+ it will be seen that m and ƒ can be increased
in the same ratio. This means that the determined constant speed of the motor
can be varied for any given potential.
172
Also m and n can be increased in the same ratio-that is if means are provided
for varying the effective magnetic moments of shunt and series coils, the motor can
be set to run at different determined speeds.
It is evident that fand r can also be varied to change the speed.
Let us now consider the same class of motors with constant speed, varying load
and constant current.
Let the resistances and turns be designated as before. Let K be the constant
current. Let E be the variable potential at the terminals of the motor and e the
variable counter electro-motive force.

page_text: R+r
We must eliminate E making it dependent upon e and the constants R, rf and K
and hence the work can be expressed in terms of R, r, f, K and e, of which e is the
only variable quantity, e depends upon speed and field, but speed is constant.
Hence our conditions require that with the same current we make e and hence, 5
the work, variable, but by changes in the field alone.
That is if the motor is at rest and any current is sent through it a zero field will
be produced. This of course follows from what has been already said about the
constant potential motor.
The potential E which will exist if e = E and no current is passing in the arma-
ture is fK and the maximum work is done when e= K. To be self regulating,
2

page_text: the motor can be worked up to this point but not beyond it for then the regulating
coil works in the wrong direction.
Motors having the series coil outside the terminals of the armature and main
field shunts will now be considered, and first when working with a constant
potential circuit.
Let f denote the resistance of the main or shunt field; m the number of turns
therein; the resistance of the differential or series field and n the number of
turns, R the resistance of the armature, E the initial potential at the terminals
of the motor, e the counter electro motive force set up in the armature, C the total
current flowing over r.
11 1

page_text: 77%
+B
or,
71
R
That is the number of turns in the shunt main field bear the same ratio to the
number of turns in the series differential field as the sum of the resistances of the
shunt field and the armature bear to the resistance of the armature.
This is the law of winding for a machine of this character and so wound the
machine will be perfectly self regulating for any constant potential and for any
Joad up to the maximum allowed.
The same peculiarity exists in these motors which has been pointed out in
connection with the first class of differentially wound motors and this will now be
described.
The ratio of the magnetic moments of the shunt and series fields is
That is to say if a motor of this character is at rest and the series coil in its
normal governing position, if circuit be closed to the motor a zero field or nearly so
will be produced for under these circumstances the magnetic moments are equal
and either the motor will not start at all or if it does start will run at a very great
speed, take the maximum current at any given potential, and do little work or
none at all.
778
Referring to the equation f+ R
=
r
it will be seen that m and ƒ + R can be
increased in the same ratio. This means that the determined constant speed of the
motor can be varied for any given potential.
Also m and n can be increased in the same ratio-that is if means are provided
for varying the effective magnetic moments of shunt and series coils the motor can
be set to run at different determined speeds.
This motor of the second class with constant speed, varying load, and constant
current will now be considered.
Let the turns resistance etcetera be designated as before. E is the variable
potential at the terminals of the shunt field, and e the corresponding counter
electro motive force.
We must eliminate E and express the work in terms of e and constants;
e depends on speed and strength of field, but since speed is constant e depends on
the field alone.

page_text: This is the same law of winding that holds when a machine of the same class is
used for constant potential; and the same remarks in regard to the zero field apply
as in the former case.
Also as in the former case, the speed for any given current can be varied by
varying the resistance and turns or the effective turns.
From the foregoing demonstrations it follows that a motor of either class
depending for its regulation on this differential winding will regulate with a
constant current only when working at less than 50 per cent armature efficiency;
and that the same machine with the same winding will regulate on a constant
potential circuit only when working at over 50 per cent armature efficiency.
As this working below 50 per cent is uneconomical and impractical, I do not
wish to use motors with a constant current circuit and therefore I do not claim the
proportions described with reference to motors so situated.
The laws above set forth are for pure electro dynamic motors; if there is any
permanent magnetism as in hard cast iron or where permanent steel magnets are
used the law of winding is modified in so far as the residual or permanent
magnetism is the equivalent of an electro magnetic moment; but in this case too
there should exist a zero field if the governing coil is normally closed when the
motor is at rest.
The fact may here be pointed out that even with a slight flattening of the field
characteristic the motor wound according to this invention may be self-regulating
because of the distortion of the lines of force by the current in the armature.
A motor constructed with different proportions than those berein set forth or
with a much curved field characteristic may regulate to a certain extent but not
with the same range that this invention obtains. With a high potential, the speed
of such a motor will at first drop as the load is increased, then come to normal,
then increase, and if the load be still further augmented, and suddenly removed it
will then stop and reverse because the governing coil becomes predominant. Or it
will drop and then increase to normal with the maximum load but if put on any
higher potential will act as in the instance first cited.

page_text: The fact herein before pointed out that in the best self regulating motor there
is a zero or very weak field when the motor is started, necessitates in both classes
of motors especially when it is desired to start at a speed not greater than the
normal, or when there is any load on the motor, in which case there is danger of
burning out, the use of devices whereby the action of the governing coil may be
modified.
This may be done by the introduction of a resistance, by shunting the coil with
a resistance, or by the variable shunting of the armature upon the main field. My
said foreign correspondent prefers, however, to use a switch to short circuit the
governing coil, or to short circuit and reverse it. If it is reversed then the first
rush of current makes a very strong field instead of reducing it to zero or nearly
50, increases the rotary effort, and prevents the burning out of the machine.
As an instance, if a constant potential motor has the series coil reversed when
the circuit is closed, if there is margin enough on the field characteristic we shall
have a field twice as strong as the strongest normal field, four times the strength
when the motor is doing its maximum work per unit of time, and a momentary
rotary effort eight times that existing when the maximum work is on.
As soon as the speed comes up, the governing coil is short circuited and then
reversed and then the motor is self regulating.
In a machine wound according to this invention, if the maximum work per turn
is exceeded when the governing coil is normal the motor will slow down and stop
with a zero field. Hence it is sometimes desirable to provide in connection with
the reversing switch for the series coil automatic devices for operating the same
controlled by the armature current so that if at any time the maximum work per
turn is accidentally exceeded the governing coil will be short circuited and reversed
and thus the rotary power and counter electro-motive force will be increased and
the motor prevented from stopping or burning out.
The proportions above described are equally applicable to the winding of the
field magnets of dynamo-electric generators, the action being simply reversed in
these machines.

page_text: Motors.
I THOMAS JOHN HANDFORD of 42 Southampton Buildings in the County of
Middlesex Patent Agent, 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:-
The object of this invention is to so construct an electro-dynamic motor or
dynamo-electric generator that it will be self-regulating so that the motor may be
made to run at a constant speed under varying loads in a circuit either of constant
current or constant potential; or the generator may develope a constant current or
a constant electro-motive force.
The invention was designed with especial reference to motors, and it will
therefore be herein described more particularly in that connection, its adaptability
to dynamo-electric generators being, however, explained.
The motor used by my said foreign correspondent is a differential motor-that is,
one having two sets of field coils, one in series with the armature, which is termed
the "governing coil," and the other in shunt relation to the armature. The latter
coil is of higher resistance and greater number of turns than the former; it is the
principal coil and determines the polarity of the field.
My said foreign correspondent has found that a certain proportion must exist
between the main coil and the governing coil to give the best results either with
constant current or constant potential, and it is in this that the invention chiefly
consists.
There are two classes of differential motors-first, those in which the main field
coil is shunted around both the armature and the differential or series coil, or in
other words the series coil is inside the terminals of the armature and main field
shunts; and second, those in which the main field coil is shunted around the
armature alone, or the series coil is outside the terminals of the armature and main
field shunts. In either case, however, the differential coil evidently is in series
with the armature.
For each of these classes of motors a different proportion must exist between the
main and differential field coils; the particular proportions of each case are the
same under all circumstances, and whether the machine is used with constant
current or constant potential circuit although the numerical values will depend
upon the current and potential used and the power required. It follows, evidently,
that a motor of either class wound according to this invention may be used with
either a constant current or constant potential circuit and will run at a constant
speed in either case.
The invention can be most readily set forth and the above statements shown to
be correct by mathematical demonstration.
The magnetic moment of a coil may be defined as the product of the ampères
flowing therein by the number of turns, and if the main and governing coils are
practically similarly situated with regard to the field magnet cores, the magnetic
field may be considered as proportioned to the effective magnetic moment-that
is to the difference of the magnetic incments of the shunt and series field coils so
long as we are working on a straight or nearly straight line characteristic. This
characteristic can be determined for any particular cores in any of the well known

page_text: ways, for instance, by running the motor as a dynamo at a constant speed, passing
variable known currents through the field coils and noting the potential existing at
the free armature-terminals.
For a properly constructed motor the field magnet must at no time be too highly
saturated, that is, it must be worked with a characteristic which is a straight or
very nearly a straight line. As will be pointed out later, some deviation may be
allowed, but not much.
Those classes of motors having the inside series coil will be first considered,
when working with varying loads in a constant potential circuit and running at
constant speed.
Let f denote the resistance of the main or shunt field coils; m the number of
turns therein; the resistance of the differential or series field coils and n the
number of turns; E, the difference of potential at the shunt terminals; e the
counter electro-motive force set up in the armature; and R the resistance of
the armature.
E e
; that is it depends upon e, a variable quantity and
T
The work done = e
upon the constants E and r.
Now e varies with the speed and field, or the effective magnetic moment of the
field, but our conditions are that the speed remains constant hence e must vary
with the field alone
That is to say the number of turns in the shunt coil must bear the same ratio to
the number in the series coil as the resistance of the shunt coil bears to the sum of
the resistances of the series coil and the armature.
This is the law of winding for a perfect machine of the kind mentioned and so
wound it will be perfectly self-regulating for any constant potential up to the
maximum allowed by the construction of the machine, and from no load up to
the maximum.
There is a feature of motors so wound which may be here noticed.
4

page_text: That is the ratio of the initial to the effective electro-motive force is the same as
the ratio of the moments of the shunt and series coil.
E
When e o this ratio becomes 1. That is the moments are equal and this
means that, in a perfect machine if both coils be closed and in their normal
position, for any potential or current, a zero field or practically so will be formed,
and the motor will either not start at all or if it does start will run at a very great
speed, take the maximum current at any given potential and do little work or none
at all.
How to obviate the bad effects of this peculiarity and to take advantage of the
same forms a part of the invention and will be hereinafter set forth.
What has already been pointed out may be again stated that the motor will
regulate itself perfectly for all potentials so long as we work with a straight line
characteristic, but it must be with a theoretical efficiency of not less than fifty
per cent, for if we go below this, the governing coil works in the wrong direction.
772 S
Referring to the equation
it will be seen that m and ƒ can be increased
in the same ratio. This means that the determined constant speed of the motor
can be varied for any given potential.
"1 R+r'
Also, m and n can be increased in the same ratio-that is, if means are provided
for varying the effective magnetic moments of shunt and series coils the motor can
be set to run at different determined speeds.
It is evident that f and can be also varied to change the speed.
Let us now consider the same class of motors with constant speed, varying load
and constant current.
Let the resistances and turns be designated as before. Let K be the constant
current. Let E be the variable potential at the terminals of the motor and e the
variable counter electro-motive force.
e (E-e)
The work done = R+*
We must eliminate E making it dependent upon e and the constants R, r, f.
and K and hence the work can be expressed in terms of R, rf K and e, of which e
is the only variable quantity, e depends upon speed and field, but speed is constant.
Hence our conditions require that with the same current we make e and hence the
work, variable, but by changes in the field alone.
T

page_text: That is if the motor is at rest and any current is sent through it a zero field will
be produced. This of course follows from what has been already said about the
constant potential motor.
The potential E which will exist if e= E and no current is passing in the
armature is fK and the maximum work is done when e =
To be self
regulating, the motor can be worked up to this point but not beyond it for then
the regulating coil works in the wrong direction.
Motors having the series coil outside the terminals of the armature and main
field shunts will now be considered, and first, when working with a constant
potential circuit.
Let denote the resistance of the main or shunt field coils; m the number of
turns therein; r the resistance of the differential or series field coils; and n the
number of turns; R the resistance of the armature; E the initial potential at the
terminals of the motor; e the counter electro-motive force set up in the armature;
C the total current flowing over r.
terminals will be Er C.
Then the potential existing at the shunt

page_text: But since C can be expressed in terms of e and constants, the work can be also
expressed in terms of e and constants.
m
E C
= shunt moment
n C series moment
E-C
m
-
n C
=
effective moment
Cancelling we have mf R (e- e¹) = n ƒ³ (e — e¹) + n R f (e — e¹)
122
or, ==+R
- К
That is, the number of turns in the shunt main field bear the same ratio to the
number of turns in the series differential field, as the sum of the resistances of the
shunt field and the armature bear to the resistance of the armature.
This is the law of winding for a machine of this character, and so wound the
machine will be perfectly self regulating for any constant potential and for any
load up to the maximum allowed.
The same peculiarity exists in these motors which has been pointed out in
connection with the first class of differentially wound motors and this will now be
described.
The ratio of the magnetic moments of the shunt and series fields is,-

page_text: That is to say if a motor of this character is at rest and the series coil in its
normal governing position, if circuit be closed to the motor, a zero field or nearly
so will be produced for under these circumstances the magnetic moments are equal
and either the motor will not start at all or if it does start will run at a very
great speed, take the maximum current at any given potential, and do little work
or none at all.
-
+ R
r
Referring to the equation
it will be seen that m and f + R can be
increased in the same ratio. This means that the determined constant speed of the
motor can be varied for any given potential. Also m and n can be increased in
the same ratio that is, if means are provided for varying the effective magnetic
moments of shunt and series coils, the motor can be set to run at. different
determined speeds.
This notor of the second class with constant speed, varying load, and constant
current, will now be considered.
Let the turns, resistance, etcetera be designated as before. E is the variable
potential at the terminals of the shunt field, and c the corresponding counter
electro-motor force.
We must eliminate E and express the work in terms of e and constants;
e depends on speed and strength of field, but since speed is constaut e depends on
the field alone.
or, em RK+me e ¹ - f n Ke-R n Ke=e¹m R K + me e ¹ - f n K e¹ - R n K e¹
This is the same law of winding that holds when a machine of the same class is
used for constant potential; and the same remarks in regard to the zero field apply
as in the former case.
Also as in the former case, the speed for any given current can be varied by
varying the resistance and turns or the effective turns.
From the foregoing demonstrations it follows that a motor of either class
depending for its regulation on this differential winding will regulate with a con-
stant current only when working at less than 50 per cent armature efficiency;
and that the same machine with the same winding will regulate on a constant
potential circuit only when working at over 50 per cent armature efficiency.
As this working below 50 per cent is uneconomical and impractical, I do not
wish to use motors with a constant current circuit, and therefore I do not claim
the proportions described with reference to motors so situated.

page_text: The laws above set forth are for pure electro dynamic motors; if there is any
permanent magnetism as in hard cast iron or where permanent steel magnets are
used the law of winding is modified in so far as the residual or permanent
magnetism is the equivalent of an electro-magnetic moment; but in this case too
there should exist a zero field if the governing coil is normally closed when the
motor is at rest.
The fact may here be pointed out, that even with a slight flattening of the field
characteristic the motor wound according to this invention may be self regulating
because of the distortion of the lines of force by the current in the armature.
A motor constructed with different proportions than those herein set forth or
with a much curved field characteristic may regulate to a certain extent but not
with the same range that this invention obtains. With a high potential the speed
of such a motor will at first drop as the load is increased, then come to normal,
then increase, and if the load be still further augmented and suddenly removed it
will then stop and reverse because the governing coil becomes predominant. Or it
will drop and then increase to normal with the maximum load, but if put on any
higher potential will act as in the instance first cited.
The fact hereinbefore pointed out, that in the best self-regulating motor there is
a zero or very weak field when the motor is started necessitates in both classes of
motors especially when it. is desired to start at a speed not greater than the normal,
or when there is any load on the motor, in which case there is danger of burning
out, the use of devices whereby the action of the governing coil may be modified.
This may be done by the introduction of a resistance, by shunting the coil with
a resistance or by the variable shunting of the armature upon the main field. My
said foreign correspondent prefers however to use a switch to short-circuit the
governing coil or to short-circuit and reverse it. If it is reversed then the first
rush of current makes a very strong field instead of reducing it to zero or nearly
so, increases the rotary effort, and prevents the burning out of the machine.
As an instance, if a constant potential motor has the series coil reversed when
the circuit is closed, if there is margin enough on the field characteristic we shall
have a field twice as strong as the strongest normal field, four times the strength
when the motor its doing its maximum work per unit of time, and a momentary
rotary effort eight times that existing when the maximum work is on.
the speed comes up, the governing coil is short circuited and then reversed, and
then the motor is self regulating.
As soon as
In a machine wound according to this invention, if the maximum work per
turn is exceeded when the governing coil is normal, the motor will slow down and
stop with a zero field. Hence it is sometimes desirable to provide in connection
with the reversing switch for the series coil, automatic devices for operating the
same, controlled by the armature current, so that if at any time the maximum
work per turn is accidentally exceeded the governing coil will be short-circuited
and reversed and thus the rotary power and counter electro-motive force will be
increased and the motor prevented from stopping or burning out.
The proportions above described are equally applicable to the winding of the
field magnets of dynamo-electric generators, the action being simply reversed in
these machines.
The invention is illustrated in the accompanying sheet of drawings in which the
same letters of reference indicate corresponding parts in all the figures.
Figure 1 is a view partly in diagram of an electro-dynamic motor with inside
series coil, embodying the said invention.
Figures 2, 3 and 4 are diagrams illustrating the use of the switch for the series
coil in a motor of this class.
Figure 5 illustrates an automatically operating switch for the same motor.
Figures 6 and 7 illustrate the variable shunting of the armature upon the main
field coils in the same motor.
Figure 8, is a view partly in diagram of a motor with outsi le series coil
embodying the invention.

page_text: Figure 9, a diagram illustrating an automatic arrangement for operating the
series coil switch of this motor in a constant potential circuit.
Figure 10 a diagram illustrating the variable shunting of the armature upon the
main field in this class of motors; and
Figure 11 is a diagram of an automatic device for the series coil switch adapted
for a constant current circuit.
The motors shown in figures 1 and 8 are those in which field magnet cores
extend in different directions from the same pole-pieces. It is however evident
that this invention is applicable as well to any other known arrangement of field
magnets.
Referring first to figures 1 to 7 inclusive A is the armature, and B, C, D, E are
the magnet cores. The coarse coil a is in series with the armature, and the fine
coil is shunted around both the armature and the series coil. These coils are to
be wound according to the law of proportions hereinbefore set forth for motors of
this class.
It is of course understood, however, that such proportions are not illustrated
in the drawing which is intended only to show the general arrangement of the
circuits.
F is the reversing switch for the series coil. It has three positions. In figure 2,
the coil is directly in circuit and opposes the main coil. In figure 3 the coil is
short-circuited or cut out of circuit. In figure 4 current is reversed through the
coil and it therefore acts to assist the main coil. When the motor is started the
coil is reversed as before explained, and as soon as the proper speed is reached, it
is short-circuited and thrown back to its normal condition.
The automatic device for the switch shown in figure 5, is a solenoid G placed in
the armature circuit and having its movable spring-retracted core c connected with
switch F. If the maximum load or work per turn on the motor is exceeded so
that too much current passes in the armature and series coil, such excessive current
causes the solenoid to move the switch and first short-circuit and then, if necessary,
reverse the series coil.
At a normal current, that is, with any load up to the maximum allowed, the
spring throws the core back and maintains the direct circuit as shewn in the
figure. The switch is also provided with the handle e for working it by hand in
starting the motor.
Figures 6 and 7 illustrate the gradual throwing of the armature into circuit by
shunting it around more or less of the main field coils. It will be readily under-
stood that such coils are divided into sections and a suitable commutator is
provided to shift the terminals of the armature circuit to the different sections.
In figure 6 there is no current in the armature while in figure 7, the armature
being shunted around more of the field coils receives a certain amount of current.
Referring now to figures 8 to 11 inclusive; A is the armature and B, C, D, E,
are the field magnet cores. The coarse differential coil a is in series with the
armature and the finer main coil b in a shunt around the armature alone. These
coils are to be wound according to the proportions herein before specified. It is of
course understood, however, that such proportions are not in reality illustrated in
the drawing, which is intended only to show the general arrangement of the
circuits.
F is the reversing switch for the series coil. Its operation is the same as is set
forth with reference to the preceding figures.
The automatic device for the switch shown in figure 9 is a solenoid G placed in
the armature circuit and having its movable spring-retracted core c connected with
switch F. If the minimum load or work per turn on the motor is exceeded so that
too much current passes in the armature and series coil, such excessive current
causes the solenoid to move the switch and first short circuit and then reverse the
series coil. The switch is also provided with the handle e for working it by hand
in starting the machine.

page_text: At a normal current-that is with any load up to the maximum, the spring
keeps the switch in its normal position with the current direct in the series coil.
In the arrangement shown in figure 11, the solenoid G is placed in the shunt
field circuit, this being for a constant current circuit.. An increase of load and
armature current diminishes the current in the shunt and weakens the solenoid, so
that the spring moves the core to reverse the current in the series coil.
Figure 10, illustrates the gradual throwing into circuit of the armature. The
terminals p p are movable upon the sections of the main field, a suitable commutator
being employed.
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 what
I claim is
1st A differential electro dynamic motor working on a circuit of constant potential,
or a dynamo-electric generator, each having its series field coil inside the terminals
of the armature and main field shunts, and its shunt and series coils wound in the
following proportions, viz: so that the number of turns in the shunt coil bears the
same relation to the number in the series coil as the resistance of the shunt coil
bears to the sum of the resistances of the series coil and the armature.
2nd A differential electro dynamic motor working on a circuit of constant
potential, or a dynamo-electric generator, each having its series field coil outside
the terminals of the armature and main field shunts, and its shunt and series coils
wound in the following proportions, viz: so that the number of turns in the shunt
coil bears the same relation to the number in the series coil as the sum of the
resistances of the shunt coil and the armature bears to the resistance of the
armature.
3rd A differential motor of either of the two classes described having its shunt
and series coils so proportioned that their magnetizing effects are equal (or nearly
so) and opposed if circuit is closed to both sets of coils when the motor is at rest.
4th A differential motor having its shunt and series coils so proportioned that
their magnetizing effects are equal (or nearly so) and opposed if circuit is closed to
both sets of coils when the motor is at rest, in combination with means substantially
as hereinbefore described whereby the magnetizing effect of the said series coil may
be modified for the purposes set forth.