Improvements in Systems for Converting the Energy of Alternating Electric Currents into Mechanical Energy by Means of Induction Motors.
Year: 1898

page_text: Date of Application, 30th Aug., 1898
Complete Specification Left, 30th May, 1899-Accepted, 22nd July, 1899
PROVISIONAL DI DUITIVATION.
[Communicated from abroad by BENJAMIN GARVER LAMME, of Pittsburg, in the
County of Allegheny and State of Pennsylvania, U.S.A., Electrical
Engineer.]
Improvements in Systems for Converting the Energy of Alternating
Electric Currents into Mechanical Energy by Means of Induction
Motors.
I, REGINALD BELFIELD, of 32, Victoria Street, Westminster, in the County of
London, Electrician, do hereby declare the nature of this invention to be as
follows:-
1
My invention relates to systems for converting the energy of alternating
electric currents into mechanical energy by means of induction motors, and has
for its object to provide for securing a variable speed and a large starting torque
and also for running the motor at an approximately constant speed with less
than its rated load so as to secure a maximum efficiency with a minimum con-
sumption of current. Another object of my invention is to provide means
whereby the motor may be efficiently operated at different speeds without any
substantial variation in torque.
According to my invention I employ a motor the secondary member of which
has in some cases a comparatively high resistance winding, and whose magnetic
leakage between the primary and secondary members is small. With a motor
having these characteristics the maximum torque of the motor may be secured at
zero speed, and if the electromotive force applied to the primary of a motor of
the character last above described be decreased, the maximum torque will be
decreased in a greater degree, the law being that the maximum torque varies
as the square of the electromotive force applied. The form of the curve repre-
senting the torque for the lower primary electromotive force is similar to that
for the higher, but the drop in speed for a given torque is much greater. By
varying the electromotive force applied to such a motor, the speed may, therefore
be varied over a wide range. This variation of electromotive force may be
obtained by any suitable means, but a convenient means for this purpose consists
of transformers provided with variable ratios of transformation. This variation
in the electromotive force supplied by the transformer may be secured by vary
ing the effective lengths of either their primaries or their secondaries, or by
A. variable resistance in the primary
varying the relative position of the two.
circuit is not satisfactory for the purpose of securing variations in electro-
motive force, for the reason that as the electromotive force applied to the motor
is reduced the current for a given torque is increased, consequently the watts
supplied to the motor are approximately constant and the resistance in the circuit
would involve a very considerable loss."
If an induction motor is worked at a small part of its rated load, it takes a

page_text: large amount of current from the line in proportion to the work performed, and
if such a motor is to work for a considerable length of time under such con-
ditions, a smaller exciting current can be obtained by varying the electro-
motive force applied to the primary member of the motor; as, for example, by
varying the relation between the primary and secondary of the transformer or
transformers supplying the motors. If current indicating instruments are con-
nected in the circuit supplying the transformers, the relation between the
primary and the secondary may be adjusted until such instruments indicate the
minimum amount of current supplied. The exciting watts or volt-amperes in
the motor vary practically as the square of the voltage, while the true watts
remain substantially constant as the voltage is varied, unless the condition of
overload for some certain voltage is reached; consequently, for a light load on
the motor, the reduction of the voltage reduces the exciting watts in ratio above
stated, while the working watts remain almost constant, and the current indi-
cating instruments above referred to will indicate lower currents supplied to the
system from the mains. The means for varying the electromotive force supplied
to the primary member of the motor may obviously be made automatic, if
desired, by operating the voltage-changing means of current supplied to the
motor. When such currents arrive at either a certain minimum or a maximum
value, they cause the operation of the regulating means to vary the voltage sup-
plied to the motor.
It has been proposed to secure constant torque in that class of motors (having
high secondary resistance and small magnetic leakage) by employing a secondary
resistance and providing automatic means which shall vary the amount of resist-
ance in the secondary circuit as the speed of the motor varies. Such a method
involves the use of more or less complicated apparatus having relatively moving
parts which are liable to get out of order, and for that reason and to that extent
it is objectionable.
If it were possible to construct a motor having a magnetic leakage which
would increase with an increase of current up to a certain point, and which
would have a much lower rate of increase beyond that point, it would be pos-
sible to secure a substantially constant torque for different speeds up to the point
where the change in the rate of increase in the leakage occurs. This condition,
however, cannot be readily obtained by any construction in the motor itself. I
have found, however, that it may be obtained by employing choke coils in the
external circuits. If such choke coil is so proportioned that its iron core or mag-
netic circuit saturates at some given quantity of current, the electromotive force
may rise to a certain value with the current and then remain almost constant,
or increase but slowly with increase of current. If the circuits corresponding to
the several phases of current supplying the motor. be provided with choke coils
and the point of saturation be made to correspond substantially to the point of
maximum torque, the self-induction of the motor circuits beyond this point will
not increase as rapidly as before, and by properly proportioning the choke coils,
a torque which is practically constant over a wide range of speed may be obtained.
In the accompanying drawings
Figure 1 is a diagram illustrating my invention in which manually operated
means for varying the voltage supplied to the primary member of the motor are
provided.
Figure 2 is a similar diagram illustrating automatic means for varying the
electromotive force.
Figures 3 and 4 are diagrams hereinafter explained.
Figure 5 is a diagram similar to Figure 1 illustrating my arrangements for
securing constant torque with variable speed, and
Figure 6 is a diagram similar to Figures 3 and 4 and hereinafter explained.
Like reference numerals denote the same parts throughout the drawing.
1 is the two-phase induction motor, the primary member of which is supplied

page_text: with energy through the wires 2, 3, 4, 5, from the secondaries 6 and 7 of two
transformers, the primaries 8, 9 of which are supplied from a two-phase
generator 10. The secondary coils 6 and 7 of the transformer are divided and
provided with a series of contact points 11 with which switch arms 12 connected
to the wires 2 and 4 can engage. In Figure 2 the wires 2 and 4 are connected to
the several contact points 11 by means of a contact piece 13 sliding between such
contact points, and long bars 14, 15 forming the terminals of the wires 2 and 4.
Referring now to Figure 1, the switch arms 12 are intended to be operated
manually so as to vary the number of turns of the secondary coil included in
each circuit, and thereby vary the electromotive force supplied to the primary
member of the motor 1, 16 are ordinary current indicating instruments con-
nected with the circuit supplied to the primary coils 8, 9 of the transformers.
In Figure 2 the sliding contact 13 forming the switch arm is connected to a
magnetic core 17 surrounded by a coil 18 included in circuit with the primary
member of the motor. It will, therefore, be obvious that as the load of the motor
varies the variation in the current through the coils 18 will move contacts 13 to
vary the electromotive force applied to the primary member. At starting the
exciting current will be high because the speed of rotation of the armature
requires to be accelerated as well as the load on the motor spindle to be over-
come. The high exciting current caused by means of the coil 18 operates the
contact 13 and reduces the electromotive force of the supply and the speed of the
armature goes down; directly this happens the exiting current is reduced and the
voltage of the supply increased and the speed and exciting current rise again.
Ultimately a mean position of the contact 13 will be reached at which the torque
due to the current supplied to the motor is able to overcome the load on the
motor shaft. If the motor is running with less than its normal load the excit-
ing current will automatically cause a reduction of the voltage and therefore of
the exciting watts as hereinbefore stated thereby increasing the efficiency of the
system.
The diagrams, Figures 3 and 4 and 6 are curves drawn in the ordinary manner
to shew the connection between the speed and the torque of an electric motor,
speed being indicated on the line S and torque on the line T. The diagram
Figure 3 relates to a motor in which the magnetic leakage is small and the
secondary resistance is comparatively high, and it will be seen that the maximum
torque is secured at starting this being a very desirable condition in motors
The several
employed for crane and elevator work and for similar purposes.
curves indicate different voltages supplied to the primary member of the motor,
the voltage of course being constant for each curve and as the voltage decreases
the maximum starting torque also decreases. It will be seen from the general
shape of the curves that if the motor is running under full load and it is desired to
increase the torque without diminishing the speed, it is only necessary to increase
the voltage supplied to the primary member, or the torque can be maintained
constant and the speed increased by increasing said voltage.
Figure 4 shews the connection between speed and torque of a motor, the resist-
ance of the secondary circuit of which is small and the self-induction large,
the torque will be at a maximum when the motor is running at a comparatively
high speed, and will decrease with the decrease in speed from that point.
Referring now to Figure 5, the motor 1 is of the same class as that indicated in
In circuit with the
Figures 1 and 2, although drawn somewhat differently.
primary member I provide choke coils 19 so designed that their cores will
become saturated at a given strength of current, the point at which saturation will
occur being readily determined in accordance with the conditions which it is
desired to meet to fulfil in any particular case.
'
Referring now to Figure 6, the line from a to 6 is the curve connecting the
torque and current if the choke coils be omitted; this curve is similar to that
shewn in Figure 3. The curve a cc is the curve for a motor having a low secon-

page_text: dary resistance and large magnetic leakage under similar circumstances, the
curve being the same as shewn in Figure 4. When however the choke coils 19
are employed with motors of the first-mentioned class, that is having a curve
similar to Figure 3, the curve will be that represented by the line aed from
which it will be seen that the torque will remain practically constant from zero
speed to that represented by the point e, this point representing the speed at
which saturation of the cores of the choke coils 19 takes place. It will be obvious
that after this has happened the curve will be substantially the same as if the
choke coils were non-existent. It will be seen, however, that by this means a
practically constant torque is secured over a considerable range of speed.
Dated the 30th day of August 1898.
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C., Patent Agents for the Applicant.
Improvements in Systems for Converting the Energy of Alternating
Electric Currents into Mechanical Energy by Means of Induction
Motors.
I, REGINALD BELFIELD, of 32, Victoria Street, Westminster, in the County of
London, 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 systems for converting the energy of alternating
electric currents into mechanical energy by means of induction motors, and has
for its object to provide for securing a variable speed and a large starting torque
and also for running the motor at an approximately constant speed with less
than its rated load so as to secure a maximum efficiency with a minimum con-
sumption of current. Another object of my invention is to provide means
whereby the motor may be efficiently operated at different speeds without any
substantial variation in torque.
According to my invention, I employ a motor the secondary member of which
has in some cases a comparatively high resistance winding, and whose magnetic
leakage between the primary and secondary members is small. With a motor
having these characteristics the maximum torque of the motor may be secured at
zero speed, and if the electro-motive force applied to the primary of a motor of
the character last above described be decreased, the maximum torque will be
decreased in a greater degree, the law being that the maximum torque varies
as the square of the electromotive force applied. The form of the curve repre-
senting the torque for the lower primary electro-motive force is similar to that
for the higher, but the drop in speed for a given torque is much greater. By
varying the electro-motive force applied to such a motor, the speed may therefore
be varied over a wide range. This variation of electro-motive force may be
obtained by any suitable means, but a convenient means for this purpose consists
of transformers provided with variable ratios of transformation. This variation
in the electro-motive force supplied by the transformer may be secured by vary-
ing the effective lengths of either their primaries or their secondaries, or by
varying the relative position of the two. A variable resistance in the primary
circuit is not satisfactory for the purpose of securing variations in electro-
motive force, for the reason that as the electro-motive force applied to the motor
is reduced, the current for a given torque is increased, consequently the watts

page_text: supplied to the motor are approximately constant and the resistance in the circuit
would involve a very considerable loss.
If an induction motor is worked at a small part of its rated load, it takes a
large amount of current from the line in proportion to the work performed, and
if such a motor is to work for a considerable length of time under such con-
ditions, a smaller exciting current can be obtained by varying the electro-
motive force applied to the primary member of the motor; as, for example, by
varying the relation between the primary and secondary of the transformer or
transformers supplying the motors. If current indicating instruments are con-
nected in the circuit supplying the transformers, the relation between the
primary and the secondary may be adjusted until such instruments indicate the
minimum amount of current supplied. The exciting watts or volt-amperes in
the motor vary practically as the square of the voltage, while the true watts
remain substantially constant as the voltage is varied, unless the condition of
overload for some certain voltage is reached; consequently, for a light load on
the motor, the reduction of the voltage reduces the exciting watts in ratio above
stated, while the working watts remain almost constant, and the current indi-
cating instruments above referred to will indicate lower currents supplied to the
system from the mains. The means for varying the electro-motive force supplied
to the primary member of the motor may obviously be made automatic, if
desired, by operating the voltage-changing means of current supplied to the
motor. When such currents arrive at either a certain minimum or a maximum
value, they cause the operation of the regulating means to vary the voltage sup-
plied to the motor.
It has been proposed to secure constant torque in that class of motors (having
high secondary resistance and small magnetic leakage) by employing a secondary
resistance and providing automatic means which shall vary the amount of resist-
ance in the secondary circuit as the speed of the motor varies. Such a method
involves the use of more or less complicated apparatus having relatively moving
parts which are liable to get out of order, and for that reason and to that extent
it is objectionable.
If it were possible to construct a motor having a magnetic leakage which
would increase with an increase of current up to a certain point, and which
would have a much lower rate of increase beyond that point, it would be pos-
sible to secure a substantially constant torque for different speeds up to the point
This condition,
where the change in the rate of increase in the leakage occurs.
however, cannot be readily obtained by any construction in the motor itself. I
have found, however, that it may be obtained by employing choke coils in the
circuits. If each choke coil is so proportioned that its iron core or magnetic
circuit saturates at some given quantity of current, the electro-motive force
may rise to a certain value with the current and then remain almost constant,
or increase but slowly with increase of current. If the circuits corresponding to
the several phases of current supplying the motor be provided with choke coils
and the point of saturation be made to correspond substantially to the point of
maximum torque, the self-induction of the motor circuits beyond the point will
not increase as rapidly as before, and by properly proportioning the choke coils,
a torque which is practically constant over a wide range of speed may be obtained.
In the drawings accompanying my Provisional Specification,
Figure 1 is a diagram illustrating my invention in which manually operated
means for varying the voltage supplied to the primary member of the motor are
provided.
Figure 2 is a similar diagram illustrating automatic means for varying the
electro-motive force.
Figures and 4 are diagrams hereinafter explained.
Figure 5 is a diagram similar to Figure 1 illustrating my arrangements for
securing constant torque with variable speed, and
Figure 6 is a diagram similar to Figures 3 and 4 and hereinafter explained.

page_text: Like reference numerals denote the same parts throughout the drawing.
1 is the two-phase induction motor, the primary member of which is supplied
with energy through the wires 2, 3, 4, 5, from the secondaries 6 and 7 of two
transformers, the primaries 8, 9 of which are supplied from a two-phase
generator 10. The secondary coils 6 and 7 of the transformer are divided and
provided with a series of contact points 11 with which switch arms 12 connected
with the wires 2 and 4 can engage. In Figure 2 the wires 2 and 4 are connected to
the several contact points 11 by means of a contact piece 13 sliding between such
contact points, and long bars 14, 15 forming the terminals of the wires 2 and 4.
Referring now to Figure 1 the switch arms 12 are intended to be operated
manually so as to vary the number of turns of the secondary coil included in
each circuit, and thereby vary the electro-motive force supplied to the primary
member of the motor 1. 16 are ordinary current indicating instruments con-
nected with the circuit supplied to the primary coils 8, 9 of the transformers.
In Figure 2 the sliding contact 13 forming the switch arm is connected to
magnetic core 17 surrounded by a coil 18 included in circuit with the primary
member of the motor. It will, therefore, be obvious that as the load of the motor
varies the variation in the current through the coil 18 will move contacts 13 to
vary the electro-motive force applied to the primary member. At starting the
exciting current will be high because the speed of rotation of the armature
requires to be accelerated as well as the load on the motor spindle to be over-
come. The high exciting current caused by means of the coil 18 operates the
contact 13 and reduces the electro-motive force of the supply and the speed of the
armature goes-down; directly this happens the exciting current is reduced and the
voltage of the supply increased and the speed and exciting current rise again.
Ultimately a mean position of the contact 13 will be reached at which the torque
due to the current supplied to the motor is able to overcome the load on the
motor shaft. If the motor is running with less than its normal load the excit-
ing current will automatically cause a reduction of the voltage and therefore of
the exciting watts as hereinbefore stated, thereby increasing the efficiency of the
system.
The diagram Figures 3 and 4 and 6 are curves drawn in the ordinary manner
to shew the connection between the speed and the torque of an electric motor,
speed being indicated on the line S and torque on the line T.
Figure 3 relates to a motor in which the magnetic leakage is small and the
The diagram
secondary resistance is comparatively high, and it will be seen that the maximum
torque is secured at starting, this being a very desirable condition in motors
employed for crane and elevator work and for similar purposes. The several
curves indicate different voltages supplied to the primary member of the motor,
the voltage of course being constant for each curve and as the voltage decreases
the maximum starting torque also decreases. It will be seen from the general
shape of the curves that if the motor is running under full load and it is desired to
increase the torque without diminishing the speed, it is only necessary to increase
the voltage supplied to the primary member, or the torque can be maintained
constant and the speed increased by increasing said voltage.
Figure 4 shews the connection between speed and torque of a motor, the resist-
ance of the secondary circuit of which is small and the self-induction large;
the torque will be at a maximum when the motor is running at a comparatively
high speed, and will decrease with the decrease in speed from that point.
Referring now to Figure 5, the motor 1 is of the same class as that indicated in
Figures 1 and 2, although drawn somewhat differently. In circuit with the
primary member
I provide choke coils 19 so designed that their cores will
become saturated at a given strength of current, the point at which saturation will
occur being readily determined in accordance with the conditions which it is
desired to meet or fulfil in any particular case.
Referring now to Figure 6, the line from a to b is the curve connecting the
torque and current if the choke coils be omitted; this curve is similar to that

page_text: shewn in Figure 3. The curve a e c is the curve for a motor having a low secon-
dary resistance and large magnetic leakage under similar circumstances, the
curve being the same as shewn in Figure 4. When however the choke coils 19
are employed with motors of the first-mentioned class, that is having a curve
similar to Figure 3, the curve will be that represented by the line aed from
which it will be seen that the torque will remain practically constant from zero
speed to that represented by the point e, this point representing the speed at.
which saturation of the cores of the choke coils 19 takes place. It will be obvious
that after this has happened the curve will be substantially the same as if the
choke coils were non-existent. It will be seen, however, that by this means a
practically constant torque is secured over a considerable range of speed.
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. The combination with an induction motor of means whereby the electro-
motive force supplied to the primary member is automatically varied dependent
on the current flowing in said member, for the purpose specified.
2. The combination with an induction motor having a small degree of magnetic
leakage and a relatively high resistance in the secondary member, of a reactive
or choke coil inserted in each of the circuits of the primary member, the cores
of said coils becoming saturated by a pre-determined amount of current, for the
purpose specified.
3. The various systems of operating induction motors substantially as described.
Dated the 30th day of May 1899.
W. P. THOMPSON & Co.,
Of London, Liverpool, Manchester and Birmingham.