WO1997003301A1

WO1997003301A1 - Magnetic bearing with reduced bias-flux-induced rotor loss

Info

Publication number: WO1997003301A1
Application number: PCT/US1996/010694
Authority: WO
Inventors: Roy S. Colby; Zbigniew Piech
Original assignee: United Technologies Corporation
Priority date: 1995-07-11
Filing date: 1996-06-21
Publication date: 1997-01-30
Also published as: TW305083B

Abstract

A magnetic bearing with reduced bias-flux-induced rotor loss includes a stator (12), a rotor (14), the stator (12) having a configuration which minimizes a gap (g1) between stator teeth (80) thereby providing a more nearly uniform spatial distribution of the bias flux around the rotor-stator gap. The reduction in rotor losses include reduction in eddy current and hysteresis losses, thereby reducing electromagnetic drag and heat dissipation in the rotor associated with such losses, especially in high speed rotor applications.

Description

Magnetic Bearing with Reduced Bias-Flux-Induced Rotor Loss

Cross References to Related Applications

Copending US Patent Application, Serial No. 08/500,718 entitled, "Magnetic Bearing with Reduced Control-Flux-Induced Rotor Loss," filed contemporaneously herewith, contains subject matter related to that disclosed herein.

Technical Field

This invention relates to magnetic bearings and more particularly to a magnetic bearing with reduced rotor loss.

Background Art

It is known in the art that magnetic bearings have a rotating member (or rotor) and a stationary member (or stator) concentrically located with respect to each other and that the magnetic bearing (and its associated control circuitry) typically controls the radial or axial distance between the rotating rotor and the stationary stator. For example, the stator may be located concentrically internal to the rotor or visa versa. As is also known, a magnetic bearing may use adjustable electro-magnetic forces generated by current flowing through coils wrapped around the stator, as controlled by a control circuit, to adjust the distance between the stator and rotor.

In particular, a "radial" magnetic bearing adjusts the radial distance between the concentrically located stator and rotor. As radial

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SUBSTTTUTE SHEET (RULE 26) forces are exerted on the rotor, the electro¬ magnetic forces must be adjusted so as to return the gap between the stator and the rotor to a substantially constant equal value around the circumference of the rotor/stator gap. Also, distance sensors are typically used to measure the distance of the gap and to provide input signals to the control circuit to adjust the electromagnetic forces by adjusting the current through the coils. Current techniques used for exerting such variable electromagnetic forces on radial magnetic bearings include energizing or pulsing one or more coils with electrical current to generate electromagetic flux to counter external radial forces as is well known in the art.

In addition to electromagnetic control flux, a magnetic bearing may also have a constant DC "bias" flux to improve bearing control (e.g., linearize the current-force relationship) , and/or reduce the electrical power requirements, as is known. The bias flux may be generated by a permanent magnet, such as that described in US Patent No. 5,111,102, entitled "Magnetic Bearing Structure", to Meeks, or by electromagnets, such as that described in US Patent No. 5,179,308, entitled "High-Speed Low-loss

Antifriction Bearing Assembly", to Malsky. However, both techniques employed by the prior art cause discontinuities or sharp changes in bias flux density in the gap as a function of angular position around the gap. Such sharp flux changes create eddy current and hysteresis losses in the rotor which cause electromagnetic drag on the rotor and cause heat to be generated in the rotor, thereby reducing efficiency. Also, the higher the speed of the rotor, the greater the losses. Thus, for high speed rotor applications, such as for an energy storage flywheel devices, the losses can be significant.

Additionally, increased heat in the rotor can demagnetize permanent magnets which may be embedded in the rotor. Furthermore, if the rotor is located in an isolated vacuum chamber, such as for an energy storage flywheel device, it may be difficult to cool the rotor. Thus, reducing such losses directly relates to a reduction in heat generated in the rotor.

Thus, it would be desirable to provide a magnetic bearing which does not exhibit such rotor losses.

Disclosure of Invention

Objects of the present invention include provision of a magnetic bearing which has reduced rotor loss.

According to the present invention, a magnetic bearing comprises a stator having a plurality of stator teeth; a rotor, concentrically located with respect to the stator, which rotates relative to the stator, there being a rotor/stator gap between the rotor and the stator; the rotor and the stator being oriented to allow a magnetic bias flux to flow between the stator and the rotor across the rotor/stator gap; and there being a tooth gap between adjacent ones of the stator teeth at the rotor/stator gap, the tooth gap being set to a minimum distance to minimize losses in the rotor caused by the bias flux.

According further to the present invention the minimum distance creates a substantially uniform spatial distribution of the bias flux around the rotor-stator airgap. This invention represents a significant improvement over the prior art by providing a magnetic bearing which has an improved stator lamination design that reduces rotor losses related to bias flux by providing a more nearly uniform spatial distribution of the bias flux around the rotor-stator airgap. This is achieved by minimizing the gap between the stator teeth (i.e., the stator slot openings) at the rotor/stator airgap. The reduction in rotor losses include reduction in eddy current and hysteresis losses, thereby reducing electromagnetic drag and heat dissipation in the rotor associated with such losses.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.

Brief Description of Drawings

Fig. 1 is a side cutaway view of a prior art magnetic bearing having four stator teeth showing bias flux paths.

Fig. 2 is a top cutaway view of the magnetic bearing of Fig. 1 showing the four tooth stator.

Fig. 3 is a graph of bias flux along a rotor/ stator gap for the four tooth stator of Figs. 1 and 2.

Fig. 4 is a top cutaway view of a prior art magnetic bearing having an eight tooth stator.

Fig. 5 is a graph of bias flux along a rotor/stator gap for the eight tooth stator of Fig. 4. Fig. 6 is a top cutaway view of a magnetic bearing having a four tooth stator configuration, in accordance with the present invention.

Fig. 7 is a top cutaway view of a magnetic bearing having an eight tooth stator configuration, in accordance with the present invention.

Fig. 8 is a top cutaway view of a magnetic bearing having a twelve tooth stator configuration, in accordance with the present invention. Fig. 9 is a top cutaway view of a magnetic bearing having a sixteen tooth stator configuration, in accordance with the present invention.

Fig. 10 is a top cutaway blown-up view of a magnetic bearing having a stator configuration in accordance with the present invention.

Fig. 11 is a graph of bias flux rotor loss and control flux leakage to other stator teeth, in accordance with the present invention.

Fig. 12 is a graph of bias flux along the rotor/stator gap of the magnetic bearing of Fig. 6, in accordance with the present invention.

Fig. 13 is a graph of bias flux along the rotor/stator gap of the magnetic bearing of Fig. 7, in accordance with the present invention. Fig. 14 is a top cutaway blown-up view of a magnetic bearing having an alternative stator configuration in accordance with the present invention.

Best Mode for Carrying out the Invention

Referring to Fig. 1, a prior art radial magnetic bearing 10, comprises a stationary member or stator 12, a rotating member or rotor 14 having an outer portion 41, and a gap 15 between the outer diameter of the stator 12 and the inner diameter of

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SUBSTTTUTE SHEET (RULE 26) the rotor 14. The stator 12 comprises a plurality of centrally located laminations 16 made of a high grade low-loss high permeability electrical steel with good high frequency response characteristics, e.g., Rotelloy 5^®, cobalt magnetic steel. The thickness of each lamination layer 16 is about 0.004" to 0.010" (0.010 to 0.0254 cm) and the total stacked thickness for all the laminations 16 is about 3 cm. Other lamination thicknesses, total lamination stack thicknesses, and materials may be used if desired. The distances for the rotor/stator airgap 15 may be from 0.005"-0.020", and can be controlled to an accuracy of, e.g., 0.0002", for some flywheel applications. Other gap sizes may be used if desired, depending on the application involved.

The laminations 16 are sandwiched between two permanent magnets 18,20. Each of the permanent magnets 18,20 have a toroidal (or doughnut) shape. Other shapes for the permanent magnets 18,20 may be used if desired. In the center of the stator is a hollow region 22. The region 22 may alternatively be solid if desired. In that case the region 22 would likely be made of a non-magnetic material. The south poles of the permanent magnets 18,20 are connected to one side of supporting arms 24,26, respectively, at one end of the arms 24,26 and the north poles are connected to opposite sides of the laminations 16. The other end of the arms 24,26 make up a portion of the outer diameter of the stator 12. The arms 24,26 are made of high strength high permeability low loss steel, e.g., low carbon

(e.g., 0.01-0.02% carbon) steel, such as Arnon 5^® or AISI 1018 steel. Other materials for the arms 24,26 may be used if desired. The permanent magnets 18,20 provide a constant DC magnetic field bias φ_b which provides an attraction force between the rotor 14 and stator 12 at each of the stator teeth. The bias flux path (or circuit) for the magnet 18 is indicated by a dashed line 28 which exits the north pole of the permanent magnet 18, travels along the laminations 16, crosses the rotor/stator gap 15, travels along the rotor 14, crosses the gap 15 to enter the support member 24, and returns to the south pole of the permanent magnet 18 to complete the magnetic circuit flux path for the permanent magnet 18. A symmetrical magnetic bias flux circuit exists for the permanent magnet 20, as indicated by a dashed line 30. Also, electrical wires or coils 31,32,33,34 are wrapped around the laminations 16 as indicated in Figs. 1 and 2. It is also known to provide the bias flux using coils which are either wrapped around some or all of the stator teeth or in a separate toroidally shaped electromagnet (like the permanent magnet) to generate electromagnetic bias flux. In that case, the permanent magnets 18,20 are not needed.

Other common magnetic bearing configurations are described in US Patent No. 5,111,102, entitled "Magnetic Bearing Structure", to Meeks, US Patent No. 5,179,308, entitled "High-Speed Low-loss Antifriction Bearing Assembly", to Malsky, US Patent No. 3,865,442, entitled "Magnetic Bearing", to Studer, and US Patent No. 4,387,935, entitled "Linear Magnetic Bearing", to Studer.

Referring to Fig. 2, the laminations 16 in the stator 12 have four teeth 35,36,37,38 around which the coils 31-34, respectively, are wrapped. The stator may have more or fewer teeth if desired. The coils 31-34 carry current which generate variable

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SUBSTITUTE 5HEET (RULE 26) electro-magnetic fields which combine with the bias flux to produce controlled forces to compensate for external radial forces as discussed hereinafter.

Also, the gap 15 is measured in four places at the center or near the center of the teeth 35-38 of the laminations 16. In particular, the distances measured are indicated as y_, x_α, y₂, x₂ going in a counter-clockwise direction around the stator 12. More or less locations around the gap may be measured if desired.

The electro-magnetic forces operate to counter external forces exerted on the rotor 14 or stator 12 to keep the gap 15 uniform around the circumference of stator 12. For example, in Fig. 2, if an external radial force is exerted on the rotor 14 downward in the y-direction, as indicated by a line 42, the gap distance y₂ will become larger than the gap distance y_. In that case, current will be driven through the coils 31,32 in a direction so as to create a control flux φ_c which is opposite in direction to the bias flux φ_b across the gap yi and which is in the same direction as the bias flux φ_b across the gap y₂. As a result, the total attractive forces across the gap yi are reduced and the total attractive forces across the gap y₂ are increased so as to make yi and y₂ equal. In a similar fashion, if a horizontal radial force is exerted on the rotor, as indicated by a line 43, so as to cause the gap distance x₂ to be greater than the gap distance x_lf a current is forced through the coils 33,34 so as to create a control flux φ_c which is in the same direction as the bias flux φ_b across the gap x₂ and is in the opposite direction to the bias flux φ_b across the gap x_α. This orientation of the control flux φ_c increases the attractive forces across the gap x₂ and decreases the attractive forces across the gap Xi so as to force the gap x₂ to equal the gap i. The current through the coils 31,32 that creates the control flux that adjusts the gaps y_ and y₂ is indicated by a dashed line 39 which, for the above example, is oriented downwardly along the upper stator tooth 35 and splits into two substantially equal paths along the back iron of the stator and then recombines at the lower tooth 36 of the stator 12. The flux path 39 travels across the gap y and then splits again into two substantially equal paths and travels around half the circumference of the rotor and recombines and then crosses the gap y_α to the upper tooth 35 of the stator 12. If the laminations 29 are used in the rotor 14, the control flux φ_c will remain mostly in the laminations 29. Similarly, the current in the coils 33,34 which creates the control flux φ_c that adjusts the gaps x_α and x₂ for the above example causes the flux φ_c to propagate from right to left along the tooth 37 of the stator 12 and separates into two substantially equal paths along the back iron of the stator and then recombines again on the left side tooth 38 of the stator 12 as indicated by a dashed line 40. Similar to the flux 39, the flux 40 travels across the gap x₂ and then splits into two substantially equal paths and travels around half the circumference of the rotor 14 and recombines and crosses the gap Xi to enter the stator tooth 37 on the right side. To counter external rotor forces exerted between the X and Y directions, a combination of control fluxes from both sets of coils may be used.

The coils 31,32 may be connected together in series and driven by a voltage applied across them to create the control flux φ_c in the y-direction. Similarly, the coils 33,34 may be connected together in series and driven by a voltage applied across them to create the control flux φ_c in the x- direction. Alternatively, the coils 31-34 may be driven individually. Further, alternatively, the bias flux φ_b may be oriented in the opposite direction, i.e., radially inward, if desired. In that case the direction of the control flux φ_c would be reversed for the above examples. Referring to Figs. 2 and 3, if the magnetic bias flux φ_b (or flux distribution or flux density) in the rotor/stator airgap 15 is viewed from a point 50 on the rotor 14, as the rotor rotates clockwise 360 degrees around the stator 12, the bias flux φ_b exhibits a series of large changes from a value of φbi to close to zero. As used herein, the term flux φ is used to represent flux, flux distribution, or flux density (φ/Area) , as the context requires. In particular, when the point 50 is across from the stator tooth 35 a first pulse 100 having a magnitude of φ_bα is seen by the rotor. When the point 50 is between the teeth 35,37, the rotor sees a low bias flux value (e.g., close to zero), as indicated by a region 102. When the point 50 is across from the stator tooth 37, a second pulse 104 having a magnitude of φ_bι is seen by the rotor. Similarly, when the point 50 is between the teeth 37,36, the rotor sees a low bias flux value as indicated by a

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SUBSTTTUTE SHEET (RULE 26) region 106. When the point 50 is across from the stator tooth 36, a third pulse 108 having a magnitude of φ_bι is seen by the rotor. Similarly, when the point 50 is between the teeth 36,38, the rotor sees a low bias flux value as indicated by a region 110. When the point 50 is across from the stator tooth 38, a fourth pulse 112 having a magnitude of φ_bι is seen by the rotor. Lastly, when the point 50 is between the teeth 38,35, the rotor sees a low bias flux value as indicated by a region 114. It should be understood that instead of a square wave as shown in Fig. 3, the corners of the waveform may be rounded.

Thus, for a single rotation of the rotor 14, a given point (such as the point 50) on the rotor sees a number (e.g., 8 in the previous example) of large abrupt changes in the bias flux. These flux changes cause eddy current and hysteresis losses in the rotor which induce heat and torque (electro-magnetic drag) on the rotor as discussed hereinbefore in the Background Art section. The faster the rotor rotates the greater these losses become. For speeds above 10,000 rpm, the losses can be significant. Also, the sharper the spatial change in bias flux, the more high frequencies are generated in the rotor which further contributes to such losses.

Referring to Figs. 4 and 5, the prior art stator 12 of Fig. 2 with eight (8) teeth 80 is shown. In that case, the bias flux φ_b in the airgap 15 as seen by the rotor at the point 50 exhibits 8 pulses 120, as the point 50 passes by each tooth 80. Similarly, when the point 50 is between the teeth 80, the bias flux φ_b will be a low value (e.g., close to zero), as indicated by the regions 122. Referring now to Figs. 6,7,8, and 9, the present invention employs a stator tooth design which minimizes the abrupt changes in bias flux φ_b seen by the rotor. In particular, the stator tooth design for 4,8,12, and 16 teeth is shown in Figs. 6,7,8, and 9, respectively. The bias flux φ is directed radially outward from each stator tooth 80 across the gap 15 as discussed hereinbefore with Figs. 1,2. However, as discussed hereinbefore, the direction of the bias flux φ_b may be reversed if desired.

Referring to Fig. 10, more specifically, we have found that if a tooth gap gl (or stator slot opening) between adjacent stator teeth 80 at the rotor/stator airgap 15 is minimized, the bias flux- induced rotor losses are greatly reduced. In particular, a region 84 between stator teeth 80 (called the stator "slot") holds coil windings 86 which generate electromagnetic control and/or bias flux (as discussed hereinbefore) . The stator teeth 80 have a head region 82 which extends circum¬ ferentially into the slot 84 to set the tooth gap gl at a predetermined minimum amount. Also, each tooth 80 has a width wl that radially monotonically increases outward from the center of the stator. If the rotor were located within the stator, the teeth would be radially monotonically increases inward from the outer diameter of the stator. Other shapes of the stator teeth 80 may be used if desired, provided the gap gl is minimized to reduce bias- flux-induced rotor loss. For example, the teeth may have a substantially constant tooth width wl up to the head region which sets the tooth gap gl such as that shown in the dashed lines 83. Also, more or fewer stator teeth may be used if desired.

The tooth gap gl distance should be set to a value which minimizes bias flux losses in the rotor while not adversely affecting the ability of the control flux φ_c to control the radial position of the rotor 14 for a given application. Referring to Fig. 11, in particular, as the gap gl decreases, the rotor loss caused by bias flux decreases, as indicated by a curve 200. However, conversely, as the gap gl decreases, the control flux leakage to adjacent stator teeth increases, as indicated by a curve 202. As the control flux leakage increases, the ability to control the rotor correspondingly decreases. Thus, the precise value for the gap gl must be determined based on the maximum amount of control flux leakage allowable for a given application. Ideally, the gap gl should be made as small as possible to minimize rotor loss. Referring to Figs. 12 and 13, for the 4 and 8 tooth stator configurations of Figs. 6 and 7 of the present invention, respectively, the resultant bias flux changes seen by the rotor are greatly reduced over that of the prior art. In particular, viewing the gap 15 starting from a point 52 (Figs. 6,7) for 360 degrees, the bias flux is substantially spatially constant around the rotor/stator airgap except for minor perturbations 210 due to any remaining tooth gap gl. As a result, there is a corresponding reduction in bias-flux-induced rotor loss. A similar improved result over the prior art occurs for the 12 and 16 tooth stators of Figs. 8 and 9, respectively.

The rotor losses for the 16 tooth stator winding configuration of Fig. 9, using a tooth gap gl of 0.658 cm, a stator tooth width Wl of 1.317 cm at the gap 15, a rotor thickness of about 2 cm, a stator outer diameter of 10.16 cm, a nominal rotor/stator airgap 15 of 0.05 cm, and at three different rotor speeds, are shown in Table 1. These losses are compared to the prior art design described hereinbefore with Figs. 1-4, having a tooth gap gl of 4.78 cm, a stator tooth width Wl of 3.22 cm at the gap 15, and the remaining dimensions are substantially the same as that described hereinbefore for Fig. 9. Also, the materials used for this comparison of the invention to the prior art are the same as that described hereinbefore for the prior art with Figs. 1,2. The reduction in losses of the present invention (Invtn) over the prior art (P/A) is greater than 20:1, as indicated in Table 1.

TABLE 1

Rotor Speed— > 17,000 rpm 27,670 rpm 35,000 rpm

Losses ( ) -U P/A Invtn P/A Invtn P/A Invtn

Eddy current 26 0.5 39 1 60 2

Hysteresis 16 1 37 2 63 3.5

Total Loss: 41 1.5 76 3 103 5.5

Improvement: >20:1 >20:1 >20:1 >20:1 >20:1 >20:1

Referring to Fig. 10, alternatively, instead of or in addition to minimizing the gap gl, the gap gl may be completely closed (i.e., zero gap) with a bridge 90 made of a material having low magnetic permeability, e.g., plastic-iron-powder composite material having a relative permeability value lower than iron (e.g.,50π x 10^""7 - 500π x IO^"7 Henry/meter) but greater than air (4π x 10^"7 H/m) , such as five times higher than air (i.e., 20π x IO^"7 H/m) . Other values may be used if desired. This allows for less of a decrease in the bias flux as the rotor passes the gap gl because the material is more permeable than air but less than that of the stator material. The bridge 90 may be made of the same material as that of the stator if desired, provided the increase in the control flux leakage, discussed hereinbefore, is acceptable.

Referring to Fig. 14, alternatively, instead of bridging the gap gl with a smooth surface as in Fig. 10, the bridge 90 may have a notch 92 in the outer surface. In that case, the same material as that of the stator may be used and the reduced thickness of the bridge 90 reduces the amount of control flux leakage. Alternatively, the notch 92 may be on the inner surface of the bridge 90 if desired. It should be understood that the bridge may likely be installed after the coils are wound around the stator. It should be understood that the orientation of the stator and rotor are not critical to the operation of the present invention (i.e., the rotor may be located concentrically inside the stator) . Also, both members of the bearing may rotate in the same or opposite directions relative to each other. Also, other configurations and orientations of the permanent magnets and laminations than that discussed herein may be used provided a bias flux between the stator teeth and the rotor exists across the gap. Further, as discussed hereinbefore, the bias flux may be oriented radially outward or inward across the rotor/stator gap, and any means for generating the bias flux is usable with the present invention.

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SUBSTITUTE 5HEET (RULE 26)

Claims

Claims We claim: 1. A magnetic bearing, comprising: a stator having a plurality of stator teeth; a rotor, concentrically located with respect to said stator, which rotates relative to said stator, there being a rotor/stator gap between said rotor and said stator; said rotor and said stator being oriented to allow a magnetic bias flux to flow between said stator and said rotor across said rotor/stator gap; and there being a tooth gap between adjacent ones of said stator teeth at said rotor/stator gap, said tooth gap being set to a minimum distance to minimize losses in said rotor caused by said bias flux.

2. The bearing of claim 1 wherein said stator teeth have a monotonically radially increasing width.

3. The bearing of claim 1 wherein the number of said plurality of stator teeth comprises: 8, 12, or 16 stator teeth.

4. The bearing of claim 1 wherein said minimum distance creates a substantially uniform spatial distribution of said bias flux around said rotor/stator gap.

5. The bearing of claim 1 wherein said bias flux is generated by a permanent magnet.

6. A magnetic bearing, comprising: stator means; rotor means, concentrically located with respect to said stator means, which rotates relative to said stator means, there being a rotor/stator gap between said rotor means and said stator means stator, bias flux means for generating a magnetic bias flux between said rotor means and said stator means across said rotor/stator gap; and said stator means for minimizing losses in said rotor caused by said bias flux.

7. The bearing of claim 6 wherein said stator means comprises a plurality of stator teeth disposed on said stator means and there being a tooth gap between adjacent ones of said stator teeth at said rotor/stator gap, said tooth gap being set to a minimum distance to minimize losses in said rotor caused by said bias flux.

8. The bearing of claim 7 wherein said stator teeth have a monotonically radially increasing width.

9. The bearing of claim 7 wherein the number of said plurality of stator teeth comprises: 8, 12, or 16 stator teeth.

10. The bearing of claim 7 wherein said minimum distance creates a substantially uniform spatial distribution of said bias flux around said rotor/stator gap.

11. The bearing of claim 6 wherein said bias flux means comprises a permanent magnet.