Improvements in or relating to Electric Motors.
Year: 1891

page_text: Date of Application, 15th July, 1891-Accepted, 17th Oct., 1891
COMPLETE SPECIFICATION.
Improvements in or relating to Electric Motors.
I, Dr. JOHANN SAHULKA, Docent of Electrotechnics in the German High
Technical School, Prague, Austria, 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:—
In the alternating current motors with rotating magnetic field which have been
constructed hitherto, the magnet is generally in the form of a ring and the
armature wound with closed coils, is in the shape of a disc or cylinder and is
adapted to turn within the ring upon a suitable shaft or spindle while the ring
remains stationary. This ring is magnetized either by two pairs or sets of coils
through which pass alternate currents of equal period but differing in phases
(system of two phases); or by three or more pairs of coils through which flow
three or more alternating currents of equal period having respectively equal
phase-differences (system of three or more phases).
In the two-phase system there are supplied to the motor either two alternate
currents independent of each other having a phase-difference of 90° in which case
four conductors are necessary; or the two alternate currents are formed from one
alternate current either by branching in two or by the use of a transformer and
not having a phase-difference of 90%. In the latter case there are only two
conductors required; while in the three-phase systems or where there are more
phases than three, three or more conductors are necessary. Some motors arranged
on the three- (or more) phase-system have a magnet of the shape of a cylinder
rotating upon a spindle which is wound with three or more sets of coils to which
alternate currents differing in phases are conducted by sliding rings; the armature
in this case is stationary in the form of a ring enclosing the magnet. In this
instance also three conductors are necessary.
The advantages of the alternate current motors with rotating magnetic field in
comparison with the other alternate-current-motors, only requiring one alternate
current and therefore not more than two conductors are as follows;- the first
alternate current motors even when charged remain in motion, the rotatory
portion need not move simultaneously with the movable poles of the magnets all
Bweeping or sliding contacts can be avoided, and in case the normal charge is
exceeded the operation of these motors is not interrupted. They are the more
efficient the more constant is the intensity of the rotary magnetic field generated
in the armature, as the fluctuations of intensity of the magnetic field result in
injurious induced currents whereby part of the electrical energy is wasted. Now
it is a fact that up to the present time it has not been possible even to approach
permanency in the intensity of turning magnetic fields.
In the two-phase motor for example, when it is fed by two separate alternate
currents having a phase-difforence of 90° the intensity of the magnetic field
fluctuates just as the sum of the absolute values of
sin. and cos. a
whereby a represents a value 2 mnt variable with t (being the value of time) and n
(the number of periods or pulsations of the two alternate currents per second).
The result obtained in this case is a relative fluctuation of the intensity of the
magnetic field within the limits of 1 and 14 say 40%. If the two alternate
currents required for the two-phase motor are derived from one single alternate
current, the difference of the phases does not amount to 90° and the fluctuations of
he intensity of the magnetic field are still more considerable. In the three-

page_text: phase-system the intensity of the magnetic field varies approximately like the
sum of the absolute values of
-sin, q, sin. (a_±. 120), sin. (ax+ 240),
so that the relative fluctuation is confined between the limits of 1-732 and 2 or 15%.
The reason of such fluctuation lies in the employment of one ring and it has not
been possible hitherto to counteract the irregularity except by intensely magnetizing
the-iron or by resorting to n system of more than three phases.
The alternate current motors with turning or rotating magnetic field constructed
according to this invention, while retaining the above mentioned advantages of
motors of this class, present two main improvements which may be described as
follows:-
1. The intensity of the rotary magnetic field is constant, and
2. Such constanoy is obtained not by increasing the number of conductors, but
by employing only one alternate current so that two electrical conductors are
sufficient.
The apparatus is represented in the accompanying drawings in which ;-
Figs. 1, 2 and 3 are respectively diagrammatic front, back and side views. The
apparatus mainly consists of two electro magnets BC, and B C the axes of the
magnets intersecting or crossing each other. Through the coils S S¹ of these
electro-magnets pass two alternate currents of equal period (T) but of different
phase. The armature in the shape of a disc or cylinder is placed between the
magnetic poles and is wound with closed coils, into which no current is conducted.
The magnets are stationary while the armature is capable of turning on its axis.
If preferred however the manner of operating may be reversed and in this case
the magnets may at the same time serve as fly-wheels; while the alternate currents
would then be led up to the motor by four rings insulated from the spindle and
fitted with brush contacts. The number of turns of the coils S S' of the electro
magnets is such that the "magneto motoric" forces are equal if the currents have
the maximum intensity. When the alternate currents passing through the coils S'S!
have a phase-difference of 4° the angle B O B between the two magnetic axes O B,
OB will equal the supplemental angle 180°-p. The magnet B C then causes
the armature to be periodically magnetized in the direction B C and the magnet B'C
causes the armature to be similarly excited in the direction B C In the
direction B.C the intensity of the magnetic field will vary according to formula
H sin. a.
Habeing the maximum intensity; whereas in the direction BC the variation of
The resulting magnetic field is a rotary one and completes.one entire.revolution
during the time T of one period of the alternate current. The resulting intensity.
of the magnetic field is to be calculated according to the law of the parallelogram
And its constant value being
H. sin. p.
This result is obtained by substituting, for one ring two magnets with crossing
axes. Each of these magnets will thus magnetize only its core and the armature 4
along the line of connection of the poles while imparting little or no magnetism
(never more than 3% of its own power) to the other magnet. It follows that the
intensities obtained in the armature are not added to each other but compounded
according to the law of the parallelogram. In the alternate current motors with
turning magnetic field hitherto, constructed in, which; one ring constitutes the 6
magnet or armature, the magnetomotoric" forces arising in the armature are
practically added together; hence the inconstancy of the rotary magnetio field.
In the motor herein described the magnetic field obtained acquires its maximum,

page_text: intensity H when the two alternate currents have a phase-difference of 90°.
There is no disadvantage however in obtaining an intensity.
H sin. p.
as it is sufficient to increase the size of the iron parts, to proportionately augment
the intensity of the magnetic field. Owing to the induced currents generated in
the armature and its closed coils a couple is produced in the armature which
consequently turns in the same direction as the magnetic field. The couple is also
produced if the motor be set in operation when charged. The rotation of the
armature need not be simultaneous (synchronous) with that of the magnetic field.
The speed of rotation of the armature is generally less than that of the magnetic
field and still decreases as the charge increases. The armature need, not be
laminated. Tlie closed coils of the armature may be obtained by fitting around
the armature a considerable number of thick copper bands K (Fig. 4) which on
either side of the armature are connected with a copper ring R concentric with its
axis. At the eilge the copper hands may be inserted into the iron of the armature
to diminish the space between the armature and the poles. The electro-magnets
are to be laminated to avoid injurious eddying currents.
The two alternate currents having a phase-difference of 4° necessary for feeding
both electro-magnets are obtained from one main alternate current which for this
purpose is sent through the primary coil of an alternate-current transformer and
through the coils S of the magnet B C, while the secondary coil of the trans-
former with the coils S' of the magnet B C forms a closed circuit (Fig. 2). It
is wise so to choose the coils of the transformer that the phase-difference of the
two alternate currents does not considerably exceed 90°. The two alterante
currents may also be derived from a single alternate current by branching into two.
the pair of coils S being then switched into one branch and the pair of coils $1
into the other, but in this case the time-constants of self-induction of the two
branches must differ as much as possible. The motor can also be fed with two
separate alternate currents of equal period, having a phase-difference of 90°, which
are derived from one direct current machine. In this case the magnets BC.
and B C should be arranged at right angles.
One of the magnets BC is fixed while the other B C is capable of being
adjusted about a spindle Z (Fig. 3) common to both magnets or about a pivot
screwed into the first magnet. After the magnet B₁ Chas been adjusted at the
desired angle in relation to B C it is secured in this position. When the phase-
difference of the two alternate currents is known beforehand the two magnets may
be formed in the shape of a cross so that the four arms of the cross form, two and
two, the respective angles and (180-4). By commutating one of the two
alternate currents, say the current passing through the coils S' the armature turns
in the opposite direction since the direction of rotation of the magnetic field is also
reversed. The phase-difference of the two alternate currents will then not be
'but
(180+). If therefore the position of the magnet B¹ C¹ would remain unaltered
the intensity of the rotating magnetic field would be pulsating. To restore
uniformity the magnet B' C would have to be brought to a symmetrical
position B C by which the angle B O B equals .
The details of construction may be such that the motor may be operated in
either one or the other direction without loss of energy. The two magnets BC.
and BBC need not have one common point of intersection, but may be respectively
independent, provided the magnetic axes B C, and B'C' form two diameters
intersecting the armature. The axis of rotation of the armature may pass through
the centre of one magnet (Fig. 5) or through the point of intersection of the two
magnets there being mutual compensation between the "magneto-motoric" forces
supplied to the axis by the coils of each magnet, so that the axis itself' remains
non-magnetic. Each of the two magnets B C and B₁ C₁ may be replaced if
required by a system of magnets with magnetic axes of the same direction. The
magnets of such a system may then be connected with or independent of each

page_text: other. (Figs. 6 and 7). The connection of the coils may be either in parallel
or in series. Such a system when a periodical alternate current is caused to
circulate round it, causes the armature to be periodically magnetized in the
direction of the line of symmetry or main magnetic axis so that it may be employed
instead of one single magnet especially in motors of the larger type. Fig. 8 shows
the course taken by the lines of magnetic force in the armature when the magnetic
field is not directed towards the line of junction of two opposite groups of poles
but occupies an intermediate position. The phase-difference of the two alternate
currents being predetermined, the two systems of magnets may be connected so as
to form one individual star, the arms of one system receiving one of the alternate
currents and those of the other system the second alternate current. Branches
may also be formed of the magnets in both directions and two armatures may be
operated simultaneously (Fig. 9); or conversely one armature may be inserted'
between two series of magnets.
+
If desired each electro-magnet B C and B₁ C' may be constructed in the shape
of a ring (Fig. 10) but in that case only one alternate current should be allowed to
circulate in each ring, so that euch ring has a magnetic axis of certain direction.
The two annular magnets are then arranged parallel to each other and surround
the cylindrical armature; their magnetic axes cross. The same applies to the
arrangement of two systems of annular magnets. The armature in the latter case
should be laminated and can also consist of a hollow cylinder. The rotating
magnetic field will be permanent only owing to the fact that two rings are used as
field magnets through each of which passes one alternate current, instead of one
ring receiving two alternate currents and that the magnetic axes of the two rings.
comprise a given angle (180—ø); inasmuch as the two magnetic fields thus created
in the armature are compounded according to the law of the parallelogram and not
by addition.
At the point of intersection of the two electro-magnets B C and B'C' (Figa. 1, 2
and 3) a second armature may if desired be arranged so that each of the two
magnets BC and B' C is divided into two halves (Fig. 11). In this manner are
obtained four magnets (or systems of magnets) B C, B'C', having two armatures
between their poles; each of the opposite pairs of magnets (or systems of magnets)
having one of the alternate currents circulating round it, so that the armatures
are again excited along the two diameters B C and B'C', which diameters are
situated at an angle of (100p) to each other.
For measuring the energy consumed by the motor any suitable electrical meter
may be employed. Into one of the conductors say L, (Fig. 2), a variable resistance
sliould moreover be inserted as otherwise where there is a given difference of tension
between the conductors L L the intensity of the current in the motor would
increase to an excessive degree if the motor is not sufficiently charged. The best
course is to have this resistance regulated automatically by the motor itself so that
as the speed of rotation of the armature increases more resistance is inserted.
It should further be observed that in the motor herein described the angular
velocity of the rotating magnetic field is not constant as the said field passes
through the different arcs between the magnets viz. and (180) in the same
space of time
(F). The consequence is that the couple produced in the armature
during a period. (T) is not constant but oscillates between two extreme limits.
Let T second; the couple will then undergo 20 oscillations in one second, but
these will exert no influence whatever. It is an advantage of the motor herein
described that only the couple can thus fluctuate and not the intensity of the
magnetic field; for fluctuations of the couple do not involve any loss of electrical
energy and owing to their extreme frequency are not noticeable; whereas an equal
number of oscillations in the intensity of the magnetic field would just as often
cause injurious induced currents attended by loss of energy. Another advantage
not to be overlooked lies in the fact that where electric energy is to be conveyed
:

page_text: to long distances two conductors are sufficient; in the economical respect this
circumstance is of great importance. Even one conductor may be sufficient where
there is good connection with the earth.
Having now particularly described and ascertained the nature of the said
invention, and in what manner the same is to be performed, I declare that what I
claim is:-
1. To producing a rotating magnetic field of constant intensity in an alternate
current motor the arrangement of two electro-magnets or systems of electro-
magnets either permanently fixed or adjustable to each other the main magnetic
axes of which intersect in combination with an iron armature placed between the
pole of the magnets and provided with closed coils the position of the electro-
magnets or systems of electro-magnets being such that the angle between their
main magnetic axes equals the supplemental angle of the phase-difference of the
two alternate currents, which excite the electro-magnets substantially as described.
2. The modification mainly consisting of four magnets or systems of magnets
between the poles of which are placed two armatures; each of the opposite pairs
of magnets having one of the alternate currents flowing round it and the armature
being magnetized periodically along two diameters substantially as described and
illustrated in Figure 11 of the accompanying drawings.
3. The feeding of this alternate current motor by means of one alternate current
which by means of a transformer or by suitable branching may be converted into
two alternate currents of different phase but equal period substantially as
described.
4. The feeding of this motor with two independent alternate currents of equal
period having a phase-difference of 90° substantially as described.
Dated this 15th day of July 1891.
W. P. THOMPSON & BOULT,
Agents for the Applicant.