A rotor for a synchronous reluctance machine wherein a torque ripple behaviour of the machine is optimized by altering the geometry of insulating barriers of the rotor. A q-axis pitch angle is used as a design variable instead of setting its value equal to the rest of the rotor slot pitches or binding its value to the stator slot pitch. The resulting rotor has a non-regular slot pitch across the q-axis and substantially regular slot pitch otherwise. synchronous reluctance machines that employ rotor discs and rotor assemblies in accordance with the present invention may exhibit low torque ripple without sacrificing high torque values.
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0. 9. A synchronous reluctance machine comprising:
a rotor and a stator with 36 stator slots, a cross-section of the rotor having:
four pole sectors arranged symmetrically about a central aperture of the cross-section, each pole sector having a plurality of insulating barriers, each insulating barrier extending between two pitch points,
a plurality of q-axes, each q-axis defining a direction of maximum reluctance of the corresponding pole sector,
a perimeter defining the outer contour of the cross-section,
a plurality of reference points located on the perimeter symmetrically with regard to the q-axes, the angular intervals between the reference points defining reference angles which between two circumferentially adjacent q-axes have an equal value of αm=γ/(k−1), where γ is an angle between two pitch points that are furthest apart between two circumferentially adjacent q-axes and k is the number of pitch points between two circumferentially adjacent q-axes,
a q-axis pitch angle defined by an angular distance δ between two circumferentially adjacent pitch points on opposite sides of a q-axis, the q-axis pitch angle having a value which is equal to δ=I*αm, where I is a non-integer coefficient,
the angular positions of each pitch point and the reference point closest to that pitch point have a deviation having a value Δ
the q-axis pitch angle has a value δ 50 degrees or less.
1. A rotor for a synchronous reluctance machine comprising:
a rotor and a stator with 36 stator slots, a cross-section of the rotor comprising having:
four pole sectors arranged symmetrically about a central aperture of the cross-section, each pole sector comprising having a plurality of insulating barriers, each insulating barrier extending between two pitch points,
a plurality of q-axes, each q-axis defining a direction of maximum reluctance of the corresponding pole sector,
a perimeter defining the outer contour of the cross-section,
a plurality of reference points located on the perimeter symmetrically with regard to the q-axes, the angular intervals between the reference points defining reference angles which between two circumferentially adjacent q-axes have an equal value of αm=γ/(k−1), where γ is an angle between two pitch points that are furthest apart between two circumferentially adjacent q-axes and k is the number of pitch points between two circumferentially adjacent q-axes,
a q-axis pitch angle defined by an angular distance δ between two circumferentially adjacent pitch points on opposite sides of a q-axis, the q-axis pitch angle having a value which is different from δ=3*αm δ=I*αm, where I is a set of all integers #36# ,
the angular positions of each pitch point and the reference point closest to that pitch point have a deviation having a value Δ
the q-axis pitch angle has a value δ 50 degrees or less.
0. 10. A rotor for a synchronous reluctance machine, a cross-section of the rotor comprising:
four pole sectors arranged symmetrically about a central aperture of the cross-section, each pole sector having a plurality of insulating barriers, each insulating barrier extending between two pitch points,
a plurality of q-axes, each q-axis defining a direction of maximum reluctance of the corresponding pole sector,
a perimeter defining the outer contour of the cross-section,
a plurality of reference points located on the perimeter symmetrically with regard to the q-axes, the angular intervals between the reference points defining reference angles which between two circumferentially adjacent q-axes have an equal value of αm=γ/(k−1), where γ is an angle between two pitch points that are furthest apart between two circumferentially adjacent q-axes and k is the number of pitch points between two circumferentially adjacent q-axes,
a q-axis pitch angle defined by an angular distance δ between two circumferentially adjacent pitch points on opposite sides of a q-axis, the q-axis pitch angle having a value which is equal to δ=I*αm, where I is a non-integer coefficient,
the angular positions of each pitch point and the reference point closest to that pitch point have a deviation having a value Δ
the q-axis pitch angle has a value δ 50 degrees or less, and the rotor is surrounded by a stator having 36 stator slots.
2. The rotor synchronous reluctance machine according to
3. The rotor synchronous reluctance machine according to
4. The rotor synchronous reluctance machine according to
5. The rotor synchronous reluctance machine according to
0. 6. The synchronous reluctance machine according to claim 1, wherein the q-axis pitch angle has a value δ between 30 and 38 degrees.
0. 7. The synchronous reluctance machine according to claim 1, wherein k=8.
0. 8. The synchronous reluctance machine according to claim 1, wherein the reference points are distributed unevenly around the perimeter.
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The present application is a continuation of pending International patent application PCT/EP2009/052930 filed on Mar. 12, 2009 which designates the United States, the content of which is incorporated herein by reference. This is a Reissue Patent Application of U.S. Pat. No. 8,638,012, issued Jan. 28, 2014.
The present invention relates to a rotor for a synchronous reluctance machine, especially the geometry of the insulating barriers of the rotor and the geometry's effect on the torque ripple behaviour of the machine. The present invention further relates to a method of designing a rotor for a synchronous reluctance machine.
Synchronous reluctance machines known in the art typically comprise stators with poly-phase windings forming a plurality of poles in a manner resembling the stator of an induction motor. The rotor assembly of the synchronous reluctance machine does normally not include electrical windings but has a number of poles in form of magnetically permeable segments. The rotor assembly is formed as an anisotropic structure where each pole of the reluctance machine has a direction of minimum reluctance, a so-called direct axis or d-axis, and a direction of maximum reluctance, a so-called quadrature axis or q-axis. When sinusoidal currents are applied to the poly-phase windings in the stator, an approximately sinusoidal magnetic flux waveform is produced in an air gap formed between the stator poles and an outer contour of the rotor assembly. The rotor will attempt to align its most magnetically permeable direction, the d-axis, to the direction of the peak flux by displacing its d-axis of minimum reluctance until alignment of the magnetic fields in the stator poles and rotor poles is obtained. The alignment process results in rotary motion of the rotor assembly at the same speed as the rotating stator magnetic field, i.e. at synchronous speed. The rotary motion of the rotor generates a torque which can be conveyed to the exterior of the reluctance machine for example by a rotor shaft bonded to the rotor assembly and extending through a central axis thereof.
According to
An important parameter of the rotor is a rotor slot pitch which can be defined as the distance between two adjacent insulating layers measured at the air gap between the rotor and the stator. Rotor assemblies have conventionally been designed with an equal rotor slot pitch i.e. all rotor slot pitches being of equal distance, and an equal stator slot pitch, in order to minimize the torque ripple and to provide a reasonable torque. Another conventional practice has been to set the rotor slot pitch equal to the stator slot pitch.
U.S. Pat. No. 5,818,140 discloses a rotor assembly for a synchronous reluctance motor, the structure of the transversely laminated rotor assembly being aimed at minimizing its torque ripple. The disclosed rotor design and the accompanying design formula require an equal rotor slot pitch and an equal stator slot pitch around the respective perimeters of these components.
U.S. Pat. No. 6,239,526 discloses a rotor assembly for a synchronous reluctance motor, wherein the insulating barriers of the rotor are slanted toward the q-axis. The rotor slot pitch has thereby been rendered non-equal across both q-axis and d-axis, the aim being to minimize the torque ripple by dimensioning the insulating barriers such that while one end faces a centre of a slot of the stator, the other end faces a centre of a tooth of the stator. This disclosure assumes that the rotor slot pitch between the q-axis and the d-axis of each pole is equal to the stator slot pitch.
However, despite the measures taken to decrease the torque ripple, rotors designed according to the conventional design principles still exhibit a high torque ripple.
One object of the invention is to provide a rotor for a synchronous reluctance machine, which rotor causes the machine to exhibit a low torque ripple. It is a further object of the invention to provide a method of designing a rotor for a synchronous reluctance machine to obtain a machine exhibiting low torque ripple.
These objects are achieved by the rotor according to claim 1.
According to a first aspect of the invention, there is provided a rotor for a synchronous reluctance machine, a cross-section of the rotor comprising: a plurality of pole sectors, each pole sector comprising a plurality of insulating barriers, each insulating barrier extending between two pitch points, a plurality of q-axes, each q-axis defining a direction of maximum reluctance of the corresponding pole sector, a perimeter defining the outer contour of the cross-section, a plurality of reference points located on the perimeter symmetrically with regard to the q-axes, the angular intervals between the reference points defining reference angles which between two neighbouring q-axes have an equal value of αm=γ/(k−1), where γ is an angle between two pitch points that are furthest apart between two neighbouring q-axes and k is the number of pitch points between two neighbouring q-axes, a q-axis pitch angle defined by an angular distance δ between two neighbouring pitch points on opposite sides of a q-axis, the q-axis pitch angle having a value which is different from δ=3*αm, the angular positions of each pitch point and the reference point closest to that pitch point have a deviation having a value Δ
By using the rotor slot pitch across the q-axis (q-axis pitch angle) as a design variable instead of setting its value equal to the rest of the rotor slot pitches or binding its value to the stator slot pitch, an increased design freedom compared to the prior art solutions is achieved which enables the optimization of the torque ripple behaviour of the machine. The present inventors have both experimentally and by finite element method (FEM) simulations demonstrated that selecting a δ value that is different from 3*αm has a significant impact on important performance measures of the rotor disc, in particular the torque ripple. Although it may be difficult to analytically derive the optimal δ value, as soon as all the design variables of a simulating model are set, the optimal q-axis pitch angle 12 can easily be found in a small number of iteration steps.
The angular positions of the pitch points and the closest reference points may have a deviation having a value Δ
The q-axis pitch angles may differ from δ=3*αm by at least 1 degree, such as at least 2, 3, 5 or 10 degrees.
The rotor may comprise a radial rib across an insulating barrier for improving the mechanical strength of the rotor.
The rotor may comprise a cut-off at the rotor perimeter on a q-axis. By providing a rotor disc with an insulating barrier in form of a cut-off at the rotor perimeter the magnetic flux at this outermost radial portion is disabled.
According to a second aspect of the invention, a method of designing a rotor for a synchronous reluctance machine is provided, the method comprising: a) providing a basic geometry of the rotor, the basic geometry comprising: a plurality of pole sectors, each pole sector comprising a plurality of insulating barriers, each insulating barrier extending between two pitch points, a plurality of q-axes, each q-axis defining a direction of maximum reluctance of the corresponding pole sector, a plurality of reference points located on the perimeter symmetrically with regard to the q-axes, the angular intervals between the reference points defining reference angles which between two neighbouring q-axes have an equal value of αm=γ/(k−1), where γ is an angle between two pitch points that are furthest apart between two neighbouring q-axes and k is the number of pitch points between two neighbouring q-axes, b) providing a simulation model of the rotor, the simulation model comprising a plurality of design parameters, one of the design parameters being q-axis pitch angle defined by an angular distance δ between two neighbouring pitch points on opposite sides of a q-axis, c) executing a simulation with at least two different δ values, d) determining the q-axis pitch angle value on the basis of the simulation results.
A simulation model is necessary in order to effectively take advantage of the present invention. When a simulation model comprising a parameter δ for the q-axis pitch angle once is created, it is easy to execute simulations with different δ values in order to find an optimum value.
The simulation model may be configured to return a value corresponding to the torque ripple. δ is increased and/or decreased until at least one local minimum of the value corresponding to the torque ripple is found, and the q-axis pitch angle value is determined to correspond to the local minimum of the torque ripple. It is relatively easy to find out the behaviour of the torque ripple curve by a small number of simulations with discrete δ values, and it is logical to give the q-axis pitch angle a value δ that results to a minimum torque ripple.
The initial value of the q-axis pitch angle may be chosen to be δ=3*αm. Although the initial q-axis pitch angle value can be chosen arbitrarily, choosing δ=3*αm is a good starting point and gives an idea how the torque ripple behaviour is improved compared to the prior art solutions.
The invention will be explained in greater detail with reference to the accompanying drawings, wherein
The insulating barriers 4 are extending between two pitch points 5 at the perimeter 2 of the rotor disc 1. The positions of the pitch points 5 have to be accurately defined as they have a great significance for a resulting torque ripple. Since the insulating barriers 4 may get many different shapes and the end portions of the cut-outs often have a rounded shape, it is not always obvious which point exactly should be considered as the pitch point 5. For the purpose of disclosure, the following definitions are made for the pitch points 5: A pitch point 5 is a point on the perimeter 2 of the rotor disc 1. In cases where the insulating barrier 4 reaches an air gap 18 between the rotor disc 1 and a surrounding stator 19, the pitch point 5 should be considered to lie in the middle of the opening separating two neighbouring magnetically permeable segments 3. In cases where the insulating barriers 4 are separated from the air gap 18 by tangential ribs 16, the pitch point 5 should be considered to lie on the imaginary continuation of the middle axis of the insulating barrier 4 taking account the overall shape of all the insulating barriers 4. This is often the narrowest point of the tangential ribs 16, but it does not need to be. However, the pitch point 5 is in this case a point on the tangential rib 16, and often this coincides with the narrowest point of the tangential rib 16. In case of a cut-off type of rotor disc 1 with an insulating barrier 4 as the outermost radial portion on the q-axis 7, the outermost insulating barrier 4 is considered not to have pitch points 5.
Rotor discs 1 are distinguished between two different designs depending on whether the outermost radial portion on a q-axis 7 is a magnetically permeable segment 3 or an insulating barrier 4. A rotor disc 1 with a cut-off 11 refers to a rotor type shown in
Since the invention is based on modifications in the insulating barrier geometry which may not be immediately apparent without doing some measurements, we here define some auxiliary reference points 8 which are useful for explaining the invention and for making the modifications readily measurable.
where γ is the value of the angle 13 between two pitch points 5 that are furthest apart between two neighbouring q-axes 7, and k is the number of pitch points 5 between two neighbouring q-axes 7.
In
A q-axis pitch angle 12 is defined by an angular distance between two neighbouring pitch points 5 on opposite sides of a q-axis 7, and is denoted by δ. While in a conventional rotor 30 with an equal rotor slot pitch the q-axis pitch angle 12 becomes δ=3*αm, according to the present embodiment δ can get an arbitrary value which can be smaller or larger than 3*αm. It may, however, happen that the q-axis pitch angle 12 at which the optimum torque ripple behaviour is achieved lands to a value δ=3*αm, but this is an exceptional condition and lies outside of the scope of the protection sought. According to a large number of conducted test cases the optimal δ value normally differs considerably from 3*αm. The conducted test cases are showing values for relative difference ηδ
from ηδ=−46% to ηδ=+117% (corresponding to δ≈1.6*αm and δ≈6.5*αm, respectively). The q-axis pitch angle 12 does not reach the value δ=0 degrees, since this condition would correspond to removing one of the insulating barriers 4. The maximum value of δ is in theory limited to the angular width of the pole sector 15, but in practice δ must have considerably smaller value such that the insulating barriers 4 obtain reasonable widths and the machine obtains an acceptable torque. According to conducted test cases for a four pole rotor (90 degrees pole sector 15) δ values above 50 degrees are rare.
The modification of the q-axis pitch angles 12 affects the reference angles 9, 10 since the sum of δ and the angle 13 between two outermost pitch points 5 between two neighbouring q-axes 7, denoted by γ, is a constant depending on the number of rotor poles. This can be expressed as
where p is the number of poles of the rotor disc 1. For a four pole rotor δ+γ=π/2 rad (90 degrees), as can be readily observed from
After choosing the δ value all the insulating barriers 4 are modified accordingly to render the angular positions of the pitch points 5 to correspond to the newly defined αm. The dimensioning of the rotor disc 1 according to the present invention may comprise several iteration steps where different δ values are tested in order to find a value corresponding to a satisfactory or optimal torque ripple behaviour. The testing may be done by using a FEM and a computer simulation.
The reference points 8 represent the optimal angular positions of the pitch points 5, but small deviations 20 in the angular positions of the pitch points 5 and the corresponding reference points 8 are allowed as denoted by Δ
The present invention should preferably be used in combination with an appropriate simulation model which may be configured to return a predetermined performance measure of the rotor disc 1, such as torque ripple, torque, ratio between torque and torque ripple or power factor. As can be understood from the earlier description, it is essential for the invention that one of the design variables of the simulation model is the q-axis pitch angle 12, but the model may comprise a desired number of other design variables as well. Simulation models may differ a lot depending on the general geometry of the rotor cross-section and the shape of the insulating barriers 4. It is assumed that a person skilled in the art is capable of creating a simulation model such as a magnetic representation of a rotor disc 1 in form of a FEM disc model in order to apply the present invention in an effective way. The FEM disc model can be created with a commercially available finite element program, for example Flux2D from CEDRAFT Group or Maxwell® supplied by Ansoft, by entering relevant data defining rotor disc geometry and dimensions, and magnetic and electrical properties of the rotor disc material.
Accordingly, by introducing the new design variable δ in the rotor disc model and changing its value the resulting effect on any desired performance measure of the rotor disc 1 can be determined. For example, as illustrated in
The depicted torque ripple variation in
The torque ripple curve 43 on the graph of
The invention is not limited to the embodiments described above, but the person skilled in the art may, of course, modify them in a plurality of ways within the scope of the invention as defined by the claims. Whereas in the previous disclosure reference is mostly made to rotor discs, it should be clear for the person skilled in the art that the same design principles can be applied to axially oriented laminations.
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