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BACKGROUND OF THE INVENTION
This invention refers to a reluctance electric machine with a rotor
comprising members of ferromagnetic material, inserted into a stator
having electric windings, and separated therefrom by an air gap.
The reluctance electric machines are synchronous machines comprising a
stator with a customary polyphase, distributed winding having two or more
poles, substantially similar to that of an induction machine, and a rotor
free from windings and permanent magnets, whose number of poles is equal
to the number of poles of the stator, and whose structure is anisotropic.
The anisotropy of the rotor is so carried out that the rotor shows, for
each pair of poles of the machine, a direction of minimum reluctance, the
so called direct axis, and a direction of maximum reluctance, the so
called quadrature axis, which is situated at a distance of 90 electric
degrees from the direct axis. When a magnetomotive force is generated by
the suitably fed stator windings, the rotor tends to displace its direct
axis of minimum reluctance until it is aligned with the magnetomotive
force generated by the stator, and this generates a utilizable mechanical
couple. Particularly, when the stator generates a rotating field, this
couple is suitable for keeping the rotor in synchronous rotation with
respect to the rotating field, with a phase angle, subtended between the
direct axis and the axis representing the stator field, whose amplitude
and sign depend on the value of the couple itself. The thus generated
utilizable couple depends on the degree of anisotropy of the rotor, and
therefore it is interesting to render the ratio between the rotor
permeance along the direct axes, and its permeance along the quadrature
axes, to be maximal. This may be done in various ways, namely by
constructing the rotor in massive form with salient poles, or by means of
stacks of profiled ferromagnetic sheets superimposed along the direction
of the rotation axis (transverse lamination: see for example U.S. Pat. No.
2,769,108), or even by inserting in the rotor suitably oriented
ferromagnetic sheets having one of their main dimensions parallel to the
rotation axis (axial lamination: see for example the British Pat. No.
1,114,562).
The reluctance electric machines have been operated until now by
controlling the voltage of the stator electrical feed, and starting has
been obtained by inserting in the rotor suitable members acting as a
squirrel-cage. In this way, the machine starts as an asynchronous motor by
action of the squirrel-cage members and, when the rotor attains a
rotational speed near that of the rotating field generated by the stator,
the anisotropy of the rotor produces a hooking; after that the operation
continues with a synchronous character.
Consistently with this manner of use, in developing these electric
machines, the trend has been until now to increase the permeance ratio,
mainly by increasing the rotor reluctance along the quadrature axes, by
means of slots or inclusions of non-ferromagnetic material, so oriented as
to cut the magnetic flux lines which extend along the quadrature axes,
though having care to cause the minimum possible decrease of the effective
cross section of ferromagnetic material available for the magnetic flux
lines extending along the direct axes, in order that during operation no
saturation of the ferromagnetic material of the rotor took place. Such
requirement arises from a saturation of the ferromagnetic material of the
rotor being unfavorable during the asynchronous starting phase of
operation, whereas it would be of no aid during the synchronous operation.
From such premises result some difficulties and high costs of manufacture,
a relatively low power factor, and an excessively low ratio between the
generated couple and the size of the machine. These drawbacks have limited
the propagation of the reluctance machines.
BRIEF SUMMARY OF THE INVENTION
The object of this invention is to open new application fields to
reluctance electric machines, as well as to allow realizing machines
having high characteristics and a simple and economical construction, by
starting from the idea of using such machines under electronic vectorial
control of the feed current. This kind of controlled feed is per se well
known for other types of electric machines, but until now it has not been
employed in connection with reluctance machines particularly designed for
such type of feed. A substantial consequence of the application of this
idea is that the problem of a starting asynchronous phase of operation no
longer exists and therefore, on one hand, there is no longer a need for
inserting members acting as a squirrel-cage, and on the other hand there
is no longer any need for avoiding saturation of the ferromagnetic
material of rotor.
Under such premises, the further idea on which the invention is based is
that of systematically leading the ferromagnetic material of the rotor, in
the operation conditions of whole load and at least in the regions
contiguous to the machine air gap, to the magnetic saturation conditions,
though causing no noticeable local magnetic saturation in the
ferromagnetic material forming the stator, which would lead to great
energy losses, to some waviness in the generated couple and to undesired
mechanical stresses.
This idea is embodied, according to the invention, in that the cross
section of the ferromagnetic material members of rotor, at least near the
air gap, is decreased by the insertion of members of non-ferromagnetic
material, in such a way that the ratio between the ferromagnetic material
cross section, and the whole cross section, does not exceed 0.6, and the
ratio between at least one linear size of the non-ferromagnetic material
members, lying in a plane tangent to the air gap, and the thickness of the
machine air gap, does not exceed 5.
Thanks to the fact that in such a unique feature the cross section of
ferromagnetic material available for the induction flux along the direct
axes does not exceed 6/10 of the material cross section present, when a
usual, non-saturating value of the induction is generated in the
ferromagnetic material of the stator, a saturation of the ferromagnetic
material of the rotor is obtained, whereby the rotor, for any further
increase of the flux, behaves as a permanent magnet rotor. On the other
hand, thanks to the fact that the size of the non-ferromagnetic material
members present within the rotor does not exceed, near the air gap and in
at least one direction lying in a plane tangent to the air gap, five times
the thickness of the air gap, no noticeable local concentration of
induction is caused in the stator poles, and therefore the stator material
is not locally led to saturation conditions.
It should be remarked that a machine according to the invention, having the
above features, would not show a favorable behavior under voltage
controlled feed, so much the less in conditions of asynchronous operation,
and for this reason it is suitable that an operation under feed with
vectorial control of the feed current, and therefore an always synchronous
operation, is provided for this machine.
Constructively, the features according to the invention may be embodied in
a rotor in different manners. A first manner, suitable for a salient pole
rotor made of massive ferromagnetic material, comprises hollowing a number
of little cavities, for example thin millings, in the rotor surfaces
facing the air gap, thus obtaining the desired decrease of the
ferromagnetic material cross section. Another manner, suitable for an
axially laminated rotor, comprises forming the laminated stacks of rotor
from ferromagnetic material sheets alternated with non-ferromagnetic
layers, wherein the thickness of the non-ferromagnetic layers is at least
2/3 of the thickness of the ferromagnetic sheets.
As already said, the saturation of at least a part of the ferromagnetic
material of the rotor is desired as a characteristic of the invention in
the conditions of whole load operation, but of course it may not be
verified in the conditions of operation with reduced load, namely when the
machine operates under a weakened magnetic field. However, even in the use
of machines according to the invention in applications which involve
working conditions even mainly with reduced magnetic flux, which do not
give rise to saturation phenomena, when the geometry shows axial
lamination, a high decrease in ferromagnetic section leads to a
significant quality factor in the coupling between the machine and the
feeding circuitry. Such a characteristic of an axial lamination with a
high decrease in ferromagnetic section is due to the high reluctance which
is obtained along the quadrature axis, and which is nearly proportional to
the decrease factor and is scarcely sensible to the saturation. On the
contrary, with a massive structure, the reluctance along the quadrature
axis is sufficiently great, for little variations of the field, only in
the presence of saturation of the ferromagnetic material of rotor, and
therefore it becomes considerably lower when the machine field is weakened
.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other characteristics of the subject of the invention will appear
more clearly from the following description of some embodiments, referred
to by way of non-limitative examples and diagrammatically shown in the
appended drawings, wherein:
FIG. 1 diagrammatically shows the cross section of a massive rotor whose
section is decreased in correspondence of the air gap;
FIG. 2 shows on a larger scale the detail II of FIG. 1;
FIG. 3 diagrammatically shows the cross section of a rotor having an axial
lamination with decreased section;
FIG. 4 shows on a larger scale the detail IV of FIG. 3;
FIGS. 5 and 6 diagrammatically show two modifications to the embodiment
according to FIG. 3;
FIG. 7 diagrammatically shows a combined embodiment, wherein the decrease
in section is carried out partially on massive parts and partially by
means of axial lamination or division;
FIG. 8 shows the cross section of a stator structure which may be used with
the rotor structure according to any of the foregoing figures; and
FIG. 9 shows the block diagram of an electronic circuit for the vectorial
control of the current feed the machine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIGS. 1 to 7 of the drawings, the structure of the stator S has not been
detailed because use may be made of any kind of stator, per se known for
the type of machines referred to or for other types of electric
synchronous or asynchronous machines. In most cases, but without
prejudice, it will be a structure with distributed polyphase windings. It
should be noted that the specific stator structure has, per se, no
connection with the application of the invention.
For more complete information, however, an example of a stator structure S
which may be used with the machine of the invention is shown in FIG. 8.
The stator S comprises a stack of ferromagnetic sheets M forming a
magnetic yoke, assembled by means of bolts O. Each ferromagnetic sheet M
forms a number of teeth N and between each pair of adjacent teeth a slot,
wherein wires forming a polyphase distributed electric winding W are
inserted.
The FIGS. 1 and 3 diagrammatically show that the stator S is connected to a
feed circuit C, interposed between the stator S and a feed line R. The
feed circuit C should be intended as an electronic circuit for the
vectorial control (in amplitude and phase) of the current feeding the
stator S. The structure of circuit C is no longer detailed in the figures
because such control circuits are per se known in the application to other
types of electric machines, and in general any circuit of this kind may be
used according to the idea of the invention, which under this point of
view is only characterized by the peculiar application according to which
a circuit C for vectorial control of the feeding current, per se known, is
applied to control the current feeding the stator S, even per se known, of
a reluctance electric machine particularly designed for such feeding.
For more complete information, however, an example of a suitable feed
circuit C is shown by the block diagram of FIG. 9. Circuit C includes a
rectifier circuit RC fed by the alternate current line R and feeding an
inverter circuit IC which in turn feeds the stator S of the electric
machine. The rectifier circuit RC may include a step-up chopper for power
regeneration on line R or dissipation on a braking resistance. The
inverter circuit IC may include transistors and operate on all four
quadrants. The rotor U of the machine is mechanically connected to an
angular transducer ET, for example a resolver, and the signal produced by
the transducer ET is sent to a demodulator DM whose output signal,
representing the actual rotor position, is sent to transformer circuits T1
and T2 and to a circuit AS calculating the actual angular speed of rotor.
The transformer circuit T1 also receives from current transducers C1, C2
two signals representing the actual currents feeding the stator S, and
therefore the stator coordinates, and it operates by transforming the
current values from stator coordinates into rotating coordinates, which
are sent to proportional integrative regulators I1, I2. The proportional
integrative regulator I1 also receives the output of a function generator
FG, controlled by the output of the calculating circuit AS, which
generates a signal depending on the machine speed. The proportional
integrative regulator I2 also receives the output of another proportional
integrative regulator I3 which, in turn, is controlled by the speed signal
from circuit AS and by the signal of a circuit SE for the entry of the
desired speed value. Finally, the output signals from both proportional
integrative regulators I1 and I2, along with the output of demodulator DM,
are sent to the transformer circuit T2, which operates by transforming the
entered voltages from rotating coordinates into stator coordinates, and
controls therewith the inverter circuit IC feeding the stator S of the
machine.
In the drawing figures, T indicates the air gap existing between the inner
surface of stator S and the skirt surface of rotor U; the thickness of
this air gap T is shown as t in FIGS. 2 and 4.
With reference to FIGS. 1 and 2, rotor U is shown in a shape comprising two
butterfly-like salient poles P1 and P2, diametrically opposite with
respect to a shaft A which carries the rotor. The feature given by the
invention to this rotor resides in that the poles P1 and P2 have in their
surfaces facing the air gap T a number of little cavities, such as
millings or bores F, which are better shown in FIG. 2, and form
non-ferromagnetic material members which decrease the cross section of
poles P1 and P2 available for passage of the magnetic flux. The cavities F
occupy in their entirety at least 40% of the cross section of the poles in
which they are hollowed. A size of cavities F which extend in a plane
tangent to the air gap T (and more precisely their width, if the cavities
are millings, or their diameter d, if circular bores are concerned) does
not exceed five times the thickness t of air gap T.
The presence of cavities F has a twofold effect. Above all, in accordance
with the cavities, the cross section of ferromagnetic material is reduced
with respect to the corresponding cross section of the facing stator S,
and therefore the density of magnetic induction flux lines which extend in
the ferromagnetic material of rotor in the concerned region is much higher
than the density of magnetic induction flux lines which extend in the
facing stator. In order to obtain good performance and reduced heating,
the magnetic induction generated within the stator should be such as to
give rise to no saturation; for example, the induction may correspond to
0.7 times the saturation value of the material forming the stator. In the
conditions referred to, the ferromagnetic material of the rotor which
faces the stator is led to saturation, so that the rotor behaves to the
flux variations as if their poles were formed by permanent magnets. On the
other hand, the saturation of the ferromagnetic material of the rotor does
not result in any drawbacks. This is due to the fact that there is a
vectorial control (in amplitude and phase) of the current feeding the
stator S; thanks to such a vectorial control the rotor is always excited
in synchronous mode, so that the magnetic induction does not vary therein;
it is only in the presence of variations of the magnetic induction that
the saturation is detrimental. A saturation, even if only local, of the
ferromagnetic material of the stator, on the contrary, would be
deterimental because of the alternate variation of the magnetic induction
in the stator. However, the above stated saturation of the ferromagnetic
material of the rotor does not correspond, even locally, to any saturation
of the ferromagnetic material of the stator. This is due to the fact that
the dimension d of the cavities F does not exceed five times the thickness
t of the air gap T. Thus the magnetic flux lines coming out with a
practically uniform density from the stator S may deviate through the air
gap T in order to arrive at the ferromagnetic parts of rotor R which are
closer to the stator points from which the magnetic flux lines come out,
without unfavorably affecting the uniform magnetization of the stator.
This is not the case if the dimension d exceeds five times the thickness t
of the air gap T: in such a case the magnetic flux lines within the stator
are not more uniformly distributed and local saturation takes place.
In the saturation conditions, the second useful effect of the presence of
cavities F is that they pose a high reluctance to the eddy magnetic
circuits, such as that shown by broken line a, which aim to rise due to
the distribution of the winding of the stator S. This allows a better
utilization of the magnetic flux generated, and increases the power factor
of the machine.
The embodiment according to FIGS. 1 and 2 is particularly economical;
however, it does not allow realization of machines having a high ratio
between generated couple and size of the machine. The embodiment according
to FIGS. 3 and 4 is more costly but it allows realization of machines
having high performance.
In this case the rotor has four poles, and it has a shaft A which projects
to form a cross support with four arms A', between which axial laminations
are located. The shaft A with the cross support A' may be made of
conventional steel since, contrary to other known machines of this kind,
there is no need for it not being ferromagnetic; on the contrary, its
ferromagnetic character is useful, because the arms A' of the cross
support are oriented along the two direct axes of the rotor and, when
ferromagnetic, they contribute to the permeance along these directions. In
the four quadrants defined by cross support A' there are inserted four
curved axial laminations, whose general structure is per se well known for
this type of machine, and which comprise a number of ferromagnetic sheets
L alternate with non-ferromagnetic intercalary layers I. The feature
according to the invention resides, in this case, in that the
non-ferromagnetic intercalary layers I, instead of being limited to a
minimal thickness as in the known structures, have a substantial
thickness, at least 2/3 of the thickness of the ferromagnetic sheets L; at
the same time, the thickness of any non-ferromagnetic intercalary layer I
does not exceed five times the thickness t of the machine air gap T.
The intercalary layers I may be realized in various manners. An embodiment
comprises stacking ferromagnetic sheets L alternating with intercalary
sheets I of a non ferromagnetic metal, for example aluminium, or with
intercalary non-metallic sheets I formed for example by a suitable
synthetic material. Another embodiment of the intercalary layers I
comprises stacking the ferromagnetic sheets L in preestablished positions
and then injecting in the intercalary spaces a non-ferromagnetic material,
which may be for example aluminium or suitable synthetic material.
The non-ferromagnetic intercalary layers I cause a substantial decrease in
the cross section of the ferromagnetic material of the rotor, in the sense
wanted by the invention. Therefore, in this embodiment of the invention,
like in the former one, to a non-saturating induction generated in the
stator S corresponds in the rotor an induction capable of saturating the
ferromagnetic sheets L. Moreover, due to the substantial thickness of the
non-ferromagnetic intercalary layers I, the permeance along the quadrature
axes is much more reduced than it can be in the known embodiments. The
consequence is that, under some other conditions, a larger mechanical
couple is generated. In this case too, the thickness of each
non-ferromagnetic intercalary layer I not exceeding 5 times the thickness
t of the machine air gap T allows the induction flux lines, which come out
from stator S with practically uniform density, to deviate through the air
gap in order to arrive to the more closer of the ferromagnetic sheets L,
without giving rise to any noticeable local induction concentration in
stator S. Contrary to the former embodiment, the presence of the
non-ferromagnetic intercalary layers I in those regions of the rotor which
face the air gap T opposes a high reluctance to the eddy magnetic circuits
on the quadrature axis, even when there is no saturation, and therefore
also when the machine is operating under conditions of weakened field.
The members B, which are located in the central part of the curved
laminations L,I, define the interpolar spaces of the rotor, and they
should be made of a non-ferromagnetic material; for the purposes of the
invention it is immaterial whether these members are conductive or
insulating. Members B may actively participate to the assembly of the
rotor by keeping fixed to the cross support A' the laminations L,I. This
may be made, in a manner per se well known (see for example the British
Pat. No. 1,114,562), by means of bolts extending from the members B
towards the axis of the rotor, and/or by means of flanges or end rings
arranged to retain the members B. It is of advantage that, as it appears
from FIG. 3, the arms A' of the cross support and, correspondingly, the
laminations L,I are substantially curved like a quarter of a circle in
their central portions and they are substantially straight in the lateral
portions, which are nearly perpendicular to one another. Such a
conformation confers a larger thickness to the non-ferromagnetic members B
and allows a higher ratio between the permeance along the direct axes and
the permeance along the quadrature axes, with the same length of the polar
arc. It also ensures an increase in strength of the members B, which is
useful when these members contribute to the assembly of the rotor
structure.
As a consequence of the high rate of decrease in ferromagnetic section, in
a rotor with axial lamination it is found of advantage, with respect to
all various characteristics of the machine, to choose a large angular
extension of the rotor poles. Therefore the angular extension of each
pole, expressed in terms of electric angle, is preferably chosen of at
least 2 radians.
A structure conceptually corresponding to the described one may be realized
for machines having more than four poles, namely having 2n poles, where n
is greater than 2, by providing onto the shaft A a number of 2n support
arms A' and shaping the laminations along a curve which extends for 1/n
angle of 180.degree. and from both ends of which extend straight sections
substantially parallel to the contiguous direct axes.
However, it is not needed that the laminations have curved portions. On the
contrary, due to manufacturing reasons it may be preferable that the
laminations be given a polygonal shape, for example as shown in FIG. 5;
the structure shown there may be considered as operationally equivalent to
that according to FIG. 3.
A constructive simplification may be realized by giving the laminations a
dihedral form, as shown in FIG. 6. In this case the support arms A' of the
shaft A form a stellar figure in their whole. Of course, also from the
structures according to FIGS. 5 and 6 may be derived corresponding rotor
structures for machines having more than four poles; thus, the structure
according to FIG. 6 is assumed to be particularly suitable for machines
having a high number of poles.
Finally, FIG. 7 shows how the features shown by FIG. 1 for a massive rotor
may be combined with the features shown by FIGS. 3 to 6 for a rotor with
axial lamination. Rotor U of FIG. 7 has two poles, and it shows a pair of
massive salient poles P1 and P2 which have cavities F facing the air gap
T, similarly to that shown in FIG. 1. Two laminations L,I, which in this
case are planar, are applied at both sides of the salient poles P1 and P2,
and they are retained by non-ferromagnetic members B which define the
interpolar spaces. It should be understood that from the structure shown
by FIGS. 1 and 7 for a rotor U having two poles, it is possible to derive
corresponding structures for machines having more than two poles.
As it may be understood from the above, the association of two ideas
forming the basis of the invention: that of using for a reluctance machine
a vectorial control of the feed current, and that of decreasing in a
substantial manner the cross section of the ferromagnetic material
included within the rotor, in the stated conditions, leads to the
possibility of realizing sturdy machines having a simple and economical
construction, as well as high characteristics, which may be employed in
applications for which the reluctance machines could not be suitable until
now. In particular, useful applications of such machines may be foreseen
as servomotors, as motors for machine tools, as driving motors for
electrical vehicles and so on, to replace direct-current motors, brushless
motors and other electric machines, with the particular advantage of
having a rotor which is free from both windings and permanent magnets and
is not subject to heating.
The embodiments of the machines according to the invention may
substantially vary though respecting the stated conditions, and this
allows realizing machines specifically suitable for different applications
.
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