NZ203311A - Brushless permanent magnet electrical machine - Google Patents

Description

Z 033 1 1 Priority Date(s): Complete Specification Filed: '.'<^3'. Class: . ric&>. .... t-Ro>*v>.>.|.'.3 Publication Date: .... (11. .APR .T.9S6 — P.O. Journal, No: . /9S9 PATENTS ACT. 1953 No.: Date: COMPLETE SPECIFICATION "PERMANENT MAGNET MOTORS AND GENERATORS HAVING MAXIMIZED ENERGY DENSITY AND EFFICIENCY" rgfwe, SERVO MOTOR TECHNOLOGY CORPORATION, a corporation organized and existing under the laws of the State of , of Suites 964 - 970, 200 Market Building, 200 S.W.

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Market Street, Portland, Oyogan 97201, U.S.A. hereby declare the invention for which we pray that a patent may be granted to ^Fus, and the method by which it is to be performed, to be particularly described in and by the following statement: - (followed by page -la-) 2G3311 PERMANENT MAGNET MOTORS AND GENERATORS HAVING MAXIMIZED ENERGY DENSITY AND EFFICIENCY Background of the Invention The present invention is directed to electric motors or generators, specifically those utilizing a permanent magnet as the source of magnetic flux, in which the energy density and output power density of the motor or generator are maximized with improved efficiency (i.e. the ratio of output power to input power) and improved linear speed-torque characteristics over conventional brushless permanent magnet designs at either low or high frequencies of operation.

In the text hereof, the following terms will be used from time-to-time and their respective meanings are set forth below for convenient reference: (1) Magnetic flux — a characteristic of an energy field produced by a magnetomotive force. When this state is altered in magnitude, a voltage is induced in an electric conductor linked with it. The flux is thought to be a line or lines (imaginary). (2) Magnetic flux density (B) — the magnitude of magnetic flux perpendicularly passing through a unit area. (3) Saturation magnetic flux density (Bg) — the maximum magnetic flux density that can be induced in a material. It is the measured magnetic flux density minus that of vacuum space. z. u B 0 (4) Remanent magnetic flux density (Br) — the magnetic flux density of a permanent magnet material remaining after it has been saturated and the magnetic field intensity has been subsequently reduced to zero (sometimes also called residual flux density). (5) Magnetomotive force (F) — the spacial distribution of the time derivative of charge whereby the magnetic field is manifested. (6) Magnetic field intensity (H) — a charac-10 teristic of a magnetic field related to the magnetomotive force by a line integral (i.e. magnetomotive force per unit length), sometimes referred to as coercive force. It is generated by current loops or a permanent magnet. (7) Intrinsic magnetic field intensity (Hc) — 15 the magnetic field intensity required to reduce the magnetic flux density in a permanent magnet material to zero after it has been saturated (i.e. equal to maximum coercive force. (8) Demagnetization curve — that portion of 20 the hysteresis loop of a permanent magnet material appearing in the second quadrant. It is a curve segment terminated by Br and Hc. (9) Energy product (BH) — a convenient unit in engineering whereby permanent magnets are compared; the product of the magnetic flux density and the magnetic field intensity at a point on the demagnetization curve of a material; units of energy per volume. (10) Maximum energy product (BHmax) — the product of B and H which is larger than at any other point 30 on the demagnetization curve. 20331 1 (11) Energy density (E/V) — the energy per unit volume in cgs units found by dividing the energy product by 8 TT . (12) Output power density — the output power 5 por unit volume of a motor or generator. (13) Power conversion efficiency — the ratio of output power to input power of a motor or generator. (14) Operating point — the point on the demagnetization curve of a permanent magnet at which the magnet is utilized in a magnetic circuit, determined by air gap size or other physical characteristics of the magnetic circuit in which the permanent magnet is utilized, or by other external influences such as external magnetic fields or temperature. (15) Cogging -- a characteristic of electrical machines employing toothed components such that torque is required to rotate the rotor a displacement of one tooth to the next tooth -- employed to advantage in Stepper motors. (16) Reluctance (R) — the ratio of magnetomo tive force to magnetic flux in a magnetic circuit or component thereof. The reluctance of a particular component of a circuit is proportional to its length in the direction of the flux lines and inversely proportional to its magnetic permeability and cross-sectional area. (17) Linear speed-torque characteristic — the characteristic of a motor whereby its output torque decreases substantially linearly in proportion to increases in its rotational speed. 2 0^ « « Traditional designs of electrical machines, such as motors and generators, employing permanent (magnetically "hard") magnets, whether of the Alnico, ferrite, cobalt or other types, tend to operate the magnet at an 5 operating point near the point of remanent magnetic flux density by maintaining a small enough air gap component of the magnetic circuit in which the magnet is employed to minimize the reluctance of the circuit and thereby maximize the flux density. In such rotary electrical machines 10 the permanent magnet, which serves as the source of magnetic flux in the magnetic circuit, can be located in either the rotor or stator element, the conductive field winding of the machine which interacts with the magnetic circuit to produce rotation of a motor or output electrical power 15 of a generator being mounted on the opposite element typically affixed to a core of soft magnetic material with which the permanent magnet forms the magnetic circuit. The aforementioned air gap component of the magnetic circuit normally exists between the permanent magnet and the 20 core and, if minimized, usually minimizes the reluctance of the magnetic circuit. Such minimization of the reluctance of the magnetic circuit in turn creates an operating point of the magnet which maximizes the flux in the circuit according to the following basic circuit equation: flux = magnetomotive force reluctance The air gap, due to the low magnetic permeability of air, is normally the most significant reluctance-causing element in the circuit (the magnet and core usually being of z U JO I s high magnetic permeability unless adversely affected by temperature or high-frequency operation).

Minimization of the air gap to maximize the flux in permanent magnet machines usually is accomplished by the employment of toothed core structures which provide spaces between the teeth to accommodate the winding and which, by bringing the tips of the teeth into close proximity with the permanent magnet, tend to minimize the effective air gap. On occasion, in the past, designers have abandoned the toothless core structures in permanent magnet devices for special reasons, such as to prevent cogging, to permit easy and economic installation and removal of the winding, to facilitate fabrication or to eliminate harmonics associated with field distribution through teeth so as to obtain a sinusoidal field. However, even when such teeth are omitted from the permanent magnet device, the designers nevertheless have tried to minimize the air gap as much as possible so as to maximize the flux in the magnetic circuit. One example of a previous toothless permanent magnet design is shown in Karube U.S. patent 4,130,769 which employs a toothless core structure to permit inexpensive winding installation and removal, but nevertheless stresses minimization of the air gap consistent with the smallest winding size which will satisfy the torque requirements of the motor. Also Kamerbeek et al. U.S. patent 4,135,107 utilizes a toothless core structure to eliminate harmonics in order to obtain a sinusoidal field, but likewise stresses minimization of the air gap by utilizing as large a rotor as possible in the structure. 2033 1 t Similar toothless core designs are shown in Volkerling et al. U.S. patent 2,952,788; Faulhaber U.S. patent 3,360,668; Kagami U.S. patent 4,019,075; and Karube U.S. patent 4,080,540.

There are several reasons why minimization of the air gap and consequent maximization of the flux in the magnetic circuit have dictated the design of previous permanent magnet motors and generators. One reason is the knowledge that the output power developed by such a motor 10 or generator is proportional to the magnetic circuit flux density and, since there is usually a desire to maximize output power, there is a corresponding desire to maximize the flux density.

More important, previous teachings have emphat-15 ically encouraged designers to operate permanent magnets at high flux densities because of the danger of irreversible demagnetization of the permanent magnet- The problem of irreversible demagnetization in any permanent magnet motor or generator is a serious one because any of 20 several external influences can change the operating point of a permanent magnet during operation such that the point becomes too near the "knee" of the demagnetization curve. Thereafter, if the flux density is further decreased for any reason, and then increased, the operating point will 25 not recoil along the original demagnetization curve but rather will recoil along a minor loop of the curve to a lower flux density. If the external effects causing such shifting of the operating point continue in a. cyclic manner, further demagnetization can occur in subsequent L U J J 1 8 cyclic changes along sequentially lower minor loops which emerge one after another, until a final minor loop appears which is reversible. When this stage is reached, no further demagnetization occurs but the magnet thereafter 5 operates at a much lower flux density with a corresponding loss in energy. The external influences which can cause such demagnetization, if an initial operating point of relatively low flux density is created, are: (1) temperature changes which change the configuration of the demag-10 netization curve; (2) changes in the reluctance of the magnetic circuit, due to temperature, frequency or mechanical variables, which tend to raise the reluctance, lower the flux density and thus shift the operating point lower on the demagnetization curve; and (3) changes in external 15 reverse magnetic fields created by the winding of the device. Because of these dangers, previous teachings have uniformly recommended that the designer select an operating point which, considering the temperature levels to be expected, lies substantially above the knee of the demag-20 netization curve and therefore is at a relatively high flux density close to the remanent flux density point. As mentioned above, this is accomplished by minimizing air gap size.

Most designers have previously been aware of the 25 fact that the above-described, generally accepted practice of minimizing the air gap size and maximizing the flux density, although maximizing output power of a permanent magnet motor and preventing irreversible demagnetization, does not theoretically produce a permanent magnet motor of 2033 1 1 the highest possible output power density (i.e. does not theoretically provide a permanent magnet motor of the smallest possible volume capable of producing a given power output). This is because, although output power is 5 proportional to output torque which is in turn proportional to the flux density, output power density is inversely proportional to the dimensions of the permanent magnet (i.e. its length between poles and its area normal to the direction of flux). Such dimensions affect the overall 10 volume of the motor. Permanent magnet demagnetization curves are such that increases in flux density produce corresponding decreases in magnetic field intensity, and vice versa. Accordingly, pursuant to the aforementioned basic magnetic circuit equation, a given percent increase 15 in flux density, because of the corresponding decrease in magnetic field intensity, requires a more than proportional increase in magnet length to compensate for the reduced magnetic field intensity, so as to thereby provide sufficient magnetomotive force (the product of magnet field 20 intensity Hm and magnet length L^) to support the increased flux density. Likewise, a given percent increase in magnetic field intensity, due to the corresponding decrease in flux density, requires a more than proportional increase in magnet area to maintain a desired flux. The dispropor-25 tionality of the enlargement of the relevant magnet dimensions increases as flux density or magnetic field intensity is maximized, as the case may be. This indicates that the highest output power density will therefore correspond to an operating point on the demagnetization curve which is 203311 somewhere between maximum flux density and maximum field intensity. It has been shown mathematically that the maximum theoretical output power density is achieved by operating the magnet at a point on the demagnetization 3 curve where the product of the flux density and magnetic field intensity (i.e. the energy product of the permanent magnet) is maximized.

However the desire to maximize output power (rather than output power density) or, more commonly, the ) fear of irreversible demagnetization of the permanent magnet if operated near the operating point of theoretical maximum output power density, has dictated the contrary, and generally accepted, principle of maximizing flux density as discussed above. Accordingly, even when a design objective is to maximize the output power density of permanent magnet devices, the traditional answer, because of fear of irreversible demagnetization, is to select a permanent magnet material having a relatively high remanent flux density, such as an Alnico magnet, and minimize the air gap to maximize the flux density since this is considered to be the practical way to maximize output power density (i.e. consistent with stability of the permanent magnet). This is the opposite of selecting an operating point on the demagnetization curve at or near the lower flux density corresponding to the point of maximum energy product and theoretical output power density. 203311 It should be noted that the same design principles and problems do not necessarily apply to nonper-manent magnet devices, i.e. where the magnetomotive force of the magnetic circuit is induced during operation.

There are several reasons for this. First, there is no danger of irreversible demagnetization of nonpermanent 2 0^318 magnet devices. Second, whereas permanent magnets operate along a second-quadrant demagnetization curve where a substantially predetermined relationship exists among magnetic field intensity, flux density and reluctance of the 5 magnetic circuit, nonpermanent induction magnets operate in the first quadrant of their hysteresis curves where magnetic field intensity and flux density need not be dependent upon the reluctance of the magnetic circuit but rather can be dependent primarily upon an induced magnetic 10 field. Accordingly there are a number of examples of non-permanent magnet machines, such as those shown in Horsley U.S. patent 3,082,337, Watanabe et al. U.S. patent 3,963,950 and Belova et al. U.S. patent 4,238,702, where toothless cores and relatively high-reluctance, wide air 15 gaps have been utilized because of the greater flexibility in the design of the magnetic circuit permitted by nonpermanent magnets. However, such gap widening exacts a price with respect to efficiency because greater input power with higher resultant heat loss is required to induce the 20 same magnetic flux density in a high reluctance magnetic circuit than in a low-reluctance magnetic circuit.

There are also some types of permanent magnet devices, such as that shown in Johnson U.S. patent No. 4,151,431, to which different principles and problems 25 apply because they are without cores and/or windings.

Returning to the design of permanent magnet machines of the more normal type having cores and windings, with which the present invention is concerned, the above discussion has pointed out why designers have followed the L 0 b 3 1 1 principle that the practical way to maximize the output power density of a permanent magnet in a motor or generator is to minimize the reluctance and maximize the flux density of the magnetic circuit, primarily to avoid irre-5 versible demagnetization. This has been the conventional approach despite the knowledge that the theoretical maximum output power density does not in fact occur at a high flux density near the remanent point on the demagnetization curve, but rather occurs at a lower flux density on 10 an intermediate portion of the demagnetization curve where, although the flux density factor of the energy product is somewhat reduced, the magnetic field intensity factor is greatly increased so that the product of the two is at its maximum value. (This contrasts with the situation in the 15 first quadrant of a hysteresis curve, applicable to non-permanent magnets, where the point of theoretical maximum energy product would normally correspond to the point of maximum flux density). Although the operating point of theoretical maximum energy product and output power density 20 on the demagnetization curve of a permanent magnet has been avoided by those skilled in the art for the foregoing reasons, it would nevertheless be extremely advantageous with respect to maximizing the output power density and thus the economy of permanent magnet devices if a practical 25 approach were devised whereby the theoretical operating point of maximum energy product and output power density could in fact be utilized effectively.

However, overcoming the irreversible demagnetization problem in order to operate the permanent magnet 203311 at or near its theoretical point of maximum energy product does not solve all of the problems. There remain the problems of maximizing output power despite reduced flux density, and of doing so in such a way as to minimize the input power requirements and promote the linear speed-torque characteristic of .brushless permanent magnet devices (the latter helping to simplify the precise control thereof). Those are primarily problems of power conversion efficiency dependent on minimizing wasteful heat generation. density corresponding to .the theoretical maximum energy product and output power density of a permanent magnet could be accomplished in a number of alternative ways. Since a reduction in flux density in the magnetic circuit to an intermediate point on the demagnetization curve, considerably below the remanent point, is needed in order to achieve the operating point of theoretical maximum energy product, and since the flux density is inversely proportional to the reluctance of the magnetic circuit in accordance with the basic magnetic circuit equation set forth above, various different ways of increasing the reluctance of the circuit may be considered to achieve the desired operating point. One possible method is adjustment of the air gap between the permanent magnet and core (i.e. widening it); another ..possible method is inserting an air gap in another location in the magnetic circuit, such as inside a hollow tubular permanent magnet rotor by filling the inside with air or a nonmagnetic material; another possibility might be to reduce the permeability or Creating an operating point of reduced flux X- yj ^ * ■ cross-sectional area of the core material to thereby increase its reluctance. Too great an increase in the reluctance of the circuit by any of these means will cause operation of the permanent magnet at a flux density too far 5 below that corresponding to its maximum energy product, thereby decreasing the output power density and increasing the likelihood of irreversible demagnetization as in the case of the aforementioned Siemens motor. Moreover, only one of these alternatives will optimize power conversion 10 efficiency by minimizing wasteful heat generation. Reducing the permeability or cross-sectional area of the core material will only increase heat generation, while inserting an air gap in a location other than between the permanent magnet and core will do nothing to minimize heat 15 generation.

Another problem which has plagued permanent magnet motors and generators is the high frequency problem of excessive core loss, in the form of hysteresis and eddy current heat losses which adversely affect efficiency, 20 create exaggerated nonlinear speed-torque characteristics and limit the maximum operating frequency of the winding as well as the revolution rate of the motor or generator. Tamaru et al. U.S. patent No. 3,657,583, Wennerberg U.S. patent No. 2,885,645 and British patent No. 760,269 all 25 teach the advantage of using a magnetically soft ferrite core material to reduce the aforementioned core energy losses in high frequency applications of nonpermanent magnet devices. However, because most permanent magnets have higher remanent flux densities than the saturation flux 30 density of magnetically soft ferrite and are conventionally ZUJ.) » I operated near such remanent flux densities for the reasons described above, and because conventional design principles of all electrical machines require that the saturation flux density of the core be at least as great as the 5 flux density available to the magnetic circuit so that the available flux density can be fully utilized, it has been considered inconsistent to utilize magnetically soft ferrite or other possibly advantageous core materials in combination with permanent magnets having higher remanent 10 flux densities than the saturation flux density of the core material. Accordingly, no teachings can be found as to how to take advantage of such potentially advantageous core materials in devices employing permanent magnets having higher remanent flux densities than the saturation 15 flux density of the core material.

Accordingly, what is needed is a design approach for permanent magnet motors and generators by which the permanent magnets can be operated at or near their operating points of theoretical maximum energy product and out-20 put power density without irreversible demagnetization of the permanent magnets. Further, the manner of creating such operating points should be the one best suited to maximize the output power, power conversion efficiency and linear speed-torque characteristics of the permanent mag-'; 25 net device. Finally, the design should make the use of'.'.

W\» magnetically soft ferrite, and other advantageous low-loss • i'. core materials, compatible with the use of permanent mag-; net materials having higher remanent flux densities than the saturation flux density of the core material. r1' /. i'fe- • * r- • " \ .? 2033 1 1 Vf?'.

S: J? Summary of the Present Invention The present invention is directed to principles Cor the design and operation of permanent magnet motors and generators, primarily of the brushless rotary type, 5 but also applicable to other types such as those employing rectilinear motion, which satisfy all of the above-described competing needs in a compatible manner to maximize output power density, power conversion efficiency and linear speed-torque characteristics. The application of 10 the inventive principles yields the smallest and most efficient permanent magnet machine for a given permanent 1 «5 3 ■' 1 % magnet material and power output level or, stated another % way, yields the most powerful and efficient permanent -g magnet machine for a given external volume and given per-15 manent magnet material.

The manner chosen to set the operating point of the permanent magnet at or near the point of theoretical I ■z maximum energy product on the magnet's demagnetization | curve is to adjust (i.e. widen) the gap between the per- ■ -it manent magnet and the core and thereby increase the reluc- 'f 4 tance of the magnetic circuit sufficiently to reduce the :i? I flux density of the circuit and permanent magnet to a flux 1 density which is in the vicinity of the flux density cor- w I responding to the operating point of theoretical maximum 25 energy product. The danger.of irreversible demagnetization normally expected from this choice of operating point is avoided in several ways. One of these is the employment of permanent magnets selected from an exclusive group whose demagnetization curves are shaped such that changes 2033 1 1 in the operating point due to the external influences mentioned previously, even though the operating point is in the vicinity of the point of theoretical maximum energy product, are likely to cause only reversible, rather than 5 irreversible, demagnetization. This exclusive group of magnets includes barium or strontium ferrite permanent magnets, rare earth and other cobalt permanent magnets (such as samarium cobalt and platinum cobalt) and other permanent magnets of a type having ratios between the 10 magnitudes of their remanent magnetic flux densities and their intrinsic magnetic field intensities, respectively, of no more than about 2:1, or mixtures thereof. Not included in the group are Alnico (aluminum-nickel-cobalt) permanent magnets-15 The choice of a widened gap between the perma nent magnet and core as the means for setting the operating point of theoretical maximum energy product also aids in the prevention of irreversible demagnetization by minimizing the effect upon the operating point of changes in 20 external reverse magnetic fields created by the winding, and by minimizing temperature variation effects on the demagnetization curve and on the reluctance of the magnetic circuit due to heat generated in the winding, for reasons to be explained hereafter.

The choice of a widened gap is further signifi cant in maximizing the output power and power conversion efficiency of the permanent magnet machine. Since, according to the present invention, gap size is determined solely by the permanent magnet's desired operating point x. U -J 1 • (rather than by other factors such as winding size as in the above-referenced Karube patent), and since the gap is an enlarged one to achieve the reduced flux density corresponding to the desired operating point, a winding of 5 greater cross-sectional area having larger individual turns and/or a greater number of turns filling the enlarged gap can be utilized. The enlarged winding provides significant benefits when employed in combination with a permanent magnet operating point at or near the point of 10 maximum energy product. For example, the ratio between ohmic heat generation in the winding and power output will be reduced by the reduced resistance of larger individual windings due to their increased cross sections, thereby aiding power conversion efficiency by minimizing heat 15 loss. Likewise, an increased number of turns in either a parallel or series configuration permits a reduction of current in each turn sufficient, when compared to the increased number of turns, to also reduce the ratio between ohmic heat generation and output power. It can be 20 seen from the ohmic equation, P = I^R, that heat dissipation varies by the square of the current and by the first power of the resistance. Thus, although the amount of resistance may be increased by adding turns to the windings, the second power reduction in current which is per-25 mitted thereby has a greater impact on the total power dissipated than the single power increase in resistance.

The minimized ohmic heat generation also minimizes the temperature fluctuation of the device, thereby minimizing the temperature-sensitive demagnetizing effects 203311 discussed above, preventing excessive heating of other nearby temperature-sensitive components, and promoting linear speed-torque characteristics of brushless devices. Also, the enlarged winding compensates for possible reductions 5 in output power which might otherwise result from the reduced flux density which corresponds to the operating point of theoretical maximum energy product.

Moreover, the enlarged gap aids in the prevention of irreversible demagnetization by minimizing the 10 effect upon the operating point of the magnet of changes in external reverse magnetic fields created by the winding. According to the aforementioned basic magnetic circuit equation, the total flux in the magnetic circuit, including not only that created by the permanent magnet 15 but also that created by the external reverse magnetic field of the winding, is as follows: flux = mmf. of magnet (HmLm) - mmf. of winding (Nl) reluctance From the equation it can be seen that, even if the mag-20 netomotive force (mmf.) of the winding (Nl) is increased by filling the enlarged gap with winding to increase the current or number of turns in order to compensate for a reduced flux density of the permanent magnet corresponding to the point of maximum energy product, the increased 25 reluctance of the magnetic circuit due to the enlargement of the air gap will normally have a reducing effect upon the level of the demagnetizing magnetic flux induced by the winding in the permanent magnet. For example, if the gap length were to be doubled so as to double the reluc- tance by removal of core teeth or, in the absenpe'-of such //'s; >9JANI9S6 ,j 2033 1 1 teeth, by decreasing the size of a permanent magnet rotor situated inside a surrounding winding, the resultant cross-sectional area of the gap which could be filled with winding would not thereby be doubled. Accordingly, 5 although the reluctance of the circuit would be doubled, the number of turns and thus the magnetomotive force and resultant demagnetizing flux induced in the permanent magnet by the winding coil would be less than doubled. Therefore the excursions about the permanent magnet's 10 operating point caused by the demagnetizing flux of the winding would actually be reduced, further enabling the operating point to be set close to the "knee" of the demagnetization curve (i.e. in the vicinity of maximum energy product) without the danger of irreversible 15 demagnetization.

The widening of the gap between the permanent magnet and core, and its beneficial utilization as described above, is made possible by the recognition of the fact that if the permanent magnet's operating point is 20 set by widening of the gap, such operating point will be substantially independent of the magnetic flux introduced by the windings.

It will be recognized that, if it is assumed that virtually all of the reluctance of the magnetic cir-25 cuit is located in the enlarged air gap, the energy generated by the permanent magnet, i.e. the product of its flux density and its magnetic field intensity, is virtually exclusively concentrated in the gap where it will beneficially interact with the winding. Thus, if the operating 2 033 1 1 point of maximum energy product is utilized, the energy density of the magnet and the energy density within the gap are both substantially maximized.

With the high-flux-density operating points 5 selected for permanent magnets under previous design theory, core materials capable of handling correspondingly high levels of magnetic flux were required. However, with the lower flux densities obtained when operating near the point of theoretical maximum energy product on the demag-10 netization curve in accordance with the present invention, core materials having much lower saturation flux densities can be used. This, then, permits the use of magnetically soft ferrite or amorphous metal core materials, with their advantageous high-frequency, low-loss characteristics, in 15 combination with permanent magnets having much higher remanent flux densities than the saturation flux density of the core material. Thus the creation of the operating point at or near the point of maximum energy product further promotes power conversion efficiency and linear 20 speed-torque characteristics in high-frequency applications by minimizing heat loss from the core, which is accomplished by rendering a high-frequency, low-loss core material of low saturation flux density compatible with permanent magnets having higher remanent flux densitites. 25 Moreover, because the reluctance of such core material does not increase with increased operating frequency to the same degree as iron and other core materials previously used with such permanent magnets, the stability of the reluctance of the magnetic circuit and thus of the 2033 1 1 operating point is, in turn, maximized thereby further reducing the likelihood of irreversible demagnetization of the permanent magnet.' A general mathematical formula has been devised by the inventor herein by which the magnetic circuit of any permanent magnet motor or generator can be designed to achieve the objective of operating the permanent magnet at or near its operating point of theoretical maximum energy product. The formula establishes a desired ratio between the total length of all gaps in a magnetic circuit and the total length of all permanent magnets in the same circuit. According to the formula, the ratio is proportional to the ratio between an imaginary intrinsic magnetic field intensity of the particular permanent magnet material employed (determined by projecting that portion of the magnet's demagnetization curve which lies above the "knee" onto the field intensity axis), and the remanent flux density of the particular permanent magnet material. The formula is as follows: kgf = uo 1 ^cpJ Lmt Br where Lg-(- is the total length of all gaps in the magnetic circuit, Lm-j- is the total length of all permanent magnets in the same circuit, uQ is the permeability of free space (1 in cgs units), HCp is the imaginary intrinsic magnetic field intensity of the magnets projected from the portion of their demagnetization curve lying above the knee, and Br is the remanent flux density of the permanent magnets. The formula is a simplified approximation because it makes 2033 1 1 certain assumptions, including the assumption that the portion of: the demagnetization curve above the knee is substantially a straight line, that there are no significant temperature variations, and that there is no fringe 5 flux. If any substantial temperature variation is expected (for example due to external conditions) it would be wise to apply the design formula to the demagnetization curve of the magnet at the lowest expected operating temperature to minimize the likelihood of thermal demagneti-10 zation.

Because of the characteristics of the demagnetization curves of the ferrite, and rare earth and other cobalt permanent magnets to which the invention is especially applicable as discussed above, the formula will 15 usually yield an optimum result wherein Lg-j- is approximately equal to which is a much higher ratio of total gap length to total magnet length than has previously been used in permanent magnet motor and generator designs under previous design principles, which tend to maximize mag-20 netic flux density. The ratio of Lg-j- to , according to the present invention, should be within the range of 0.5 to 2.0, and preferably within the range of 0.8 to 1.2, depending upon the particular permanent magnet material.

Accordingly, it is a principal objective of the 25 present invention to devise a way to operate permanent magnet motors and generators at an operating point of the permanent magnet at or near to the operating point of theoretical maximum energy product, without thereby causing irreversible demagnetization of the permanent 2033 1 1 magnet, so as to maximize the energy density of the permanent magnet and the energy density within the air gap.

It is a further principal objective of the present invention to maximize the output power density and 5 power conversion efficiency of the permanent magnet motor or generator in a manner compatible with the aforementioned operating point of theoretical maximum energy product.

It is a further principal objective of the present invention to make the employment of permanent magnets 10 having relatively high remanent flux densities compatible with the employment of core materials having saturation flux densities substantially lower than such remanent flux densities, so as to enable the benefits of such core materials, such as magnetically soft ferrite, to be obtained 15 if needed for the particular application.

The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the 20 accompanying drawings.

Brief Description of the Drawings FIGS. 1-4 are partially sectional, simplified axial views of the interior configurations of different 25 types of prior art permanent magnet rotary motors constructed in accordance with previously accepted design principles.

FIGS. 5-8 are partially sectional, simplified axial views of the interior configurations of exemplary 2033 1 1 motors comparable to those of FIGS. 1-4 respectively, but constructed in accordance with the design principles of the present invention for purposes of comparison.

FIG. 9 is a simplified axial view of the inte-5 rior of a further type of rotary motor constructed in accordance with the principles of the present invention.

FIG. 10 depicts a demagnetization curve for a typical rare earth cobalt permanent magnet, and a comparative virgin magnetization curve for a typical magnetically-10 soft ferrite core material.

FIG. 11 is a graph showing comparative demagnetization curves for different "known permanent magnet materials.

Detailed Description of the Invention FIG. 1 is a simplified axial view of the interior of a conventional rotating brushless permanent magnet motor in which a radially symmetrical four-pole motor 10 composed of permanent magnets 12, 14, 16 and 18 bonded to 20 an iron shaft 20 is journaled in bearings (not shown) supporting the shaft 20 so as to rotate inside a concentrically wound field stator 22. As in all of FIGS. 1-9, alternating current in the winding 24 of the stator 22 is synchronized with the position of the rotor 10 by conven-25 tional means, such as Hall effect elements and appropriate circuitry, not shown for the sake of clarity of illustration but of the general type described in Karube U.S. Patent No. 4,130,769 incorporated herein by reference. The magnets 12, 14, 16 and 18 could be composed of any 2 0 3^1 i commercially available permanent magnet material.

However, for purposes of comparison it will be assumed, unless stated otherwise, that all magnets in the figures are rare earth cobalt, such as samarium cobalt, having the 5 demagnetization curve of FIG. 10. The stator 22 is manufactured by stamping toothed laminations from sheets of electrical steel or other iron alloys which are stacked employing any of a variety of well-known techniques to form a stator core 26. The winding 24 is then inserted 10 into the slots 26a, between the teeth 26b of the core, after which it is common to pot the entire structure with epoxy, or vacuum impregnate it with a varnish.

The flux path through one of four basic radially symmetrical magnetic circuits of the motor of FIG. 1 is 15 indicated by the dashed line 28. The magnetic circuit comprises that portion of the rotor 10 between the south pole of magnet 14 and the north pole of magnet 12, the core 26 and the two air gaps 30 and 32 between the magnets 12 and 14 and the teeth of the core, respectively. The 20 total reluctance of this magnetic circuit is the sum of the individual reluctances of the magnets 12 and 14, the shaft 20, gaps 30 and 32 and the core 28. However, under normal conditions the only components of any substantial reluctance are the gaps 30 and 32 which have no magnetic 25 material therein and, as seen in FIG. 1, are quite narrow so as to minimize the total reluctance of the magnetic circuit. This in turn maximizes the flux and flux density of the magnetic circuit. Because of the low reluctance and consequent high flux density of the magnetic circuit, L U J J 3 i the operating point of the magnets is approximately at the point denoted by XI in FIG. 10. It will be noted that this operating point is quite near to the remanent flux density Br of the permanent magnet, and has a relatively 5 small energy product represented by the area of the rectangle 34 determined by multiplying the magnetic flux density and magnetic field intensity at the point XI. This results in a relatively low energy density magnet.

FIG. 5 shows a machine comparable to that of 10 FIG. 1 except that it is designed in accordance with the principles of the present invention. The construction of the rotor 110 of FIG. 5 is the same as, and has the same demagnetization curve as, the rotor 10 of FIG. 1. However, the stator 122 of FIG. 5 is quite different from the 15 stator 22 of FIG. 1. Because there are no teeth present on the stator core 126, the effective lengths Lg of each of the gaps 130 and 132 are substantially larger than the effective lengths of the gaps 30 and 32 of FIG. 1. This increase in total gap length of the magnetic circuit 128 20 of FIG. 5, relative to that of FIG. 1, renders the reluctance of the magnetic circuit of FIG. 5 substantially greater than that of FIG. 1. This means that the flux density in the magnetic circuit of FIG. 5 will be substantially lower than that of FIG. 1, creating an operating 25 point on the demagnetization curve of FIG. 10 on an intermediate portion of the curve at or near the point X2 of theoretical maximum energy product represented by the area of the rectangle 36, which is substantially larger than that of rectangle 34 thereby providing a correspondingly 30 higher energy density.

Because of the small gaps 30 and 32 in the prior art embodiment of FIG. 1, the ratio of total gap length to total magnet length in the magnetic circuit represented by the dashed line 28 is only about 1:4. In contrast, the ratio of the total gap length (2Lg) to the total magnet length (2Lm) of the magnetic circuit represented by the dashed line 128 in FIG. 5 is on the order of 1:1. This is in keeping with the general mathematical formula set forth previously for establishing the ratio between total gap length and total magnet length in the circuit. According to the formula, the ratio is proportional to the ratio between an imaginary intrinsic magnetic field intensity of the particular permanent magnet material determined by projecting that portion of the magnet's demagnetization curve which lies above the "knee" onto the field intensity axis (i.e. HCp in FIG. 10), and the remanent flux density of the magnet material (Br in FIG. 10). (A projected imaginary intrinsic magnetic field intensity HCp, rather than the actual intrinsic field intensity Hc, is used in the formula because it, in combination with 3r, more accurately represents the slope of that portion of the demagnetization curve above the "knee" where an operating point can usefully be established without the likelihood of demagnetization according to the present invention.) Since, according to the particular demagnetization curve of FIG. 10, HCp is 8,000 oersteds while Br is 8,000 gauss, the ratio of total gap length to total magnet length is 1:1 according to the formula meaning that, for the magnetic circuit 128 of FIG. 5, 2Lg should equal 2Lm and therefore Lg should equal Lm. 2 033 1 1 In FIG. 5, the removal of the teeth from the stator core 126 and the resultant widening of the gap relative to magnet length have made room for an enlarged winding 124 which substantially fills the gap for optimum 5 benefit. As mentioned previously, the enlarged winding, because it is placed in the enlarged gap where the great majority of the energy of the permanent magnets is applied, maximizes output power while its demagnetizing flux effects are reduced, and minimizes the ratio between 10 ohmic heat generation in the winding and power output.

This maximizes power conversion efficiency, minimizes temperature fluctuation and thus temperature-sensitive demagnetizing effects, promotes linear speed-torque characteristics of the device and prevents excessive heating of 15 other nearby temperature-sensitive components.

If the motor of FIG. 5 is intended for relatively high-speed, high-frequency operation, the stator core 126 may be advantageously constructed of magnetically soft ferrite, amorphous metal or other high-frequency, 20 low-loss core material. In particular, the application of any magnetically soft ferrite featuring a spinel crystal structure, conforming in most cases to the formula XFe204, where X may be manganese, zinc, cobalt, nickel or other metallic ion, or any mixture thereof, is possible. At 25 high frequency operation, the core loss for such a material would typically be three to four orders of magnitude less than the best electrical iron, such as that from which the stator core 26 of FIG. 1 is constructed. As can be seen from FIG. 10, the saturation flux density Bg of 1 1 the magnetically soft ferrite is incompatible with the narrow gap structure of FIG. 1 because the flux density of the operating point XI of the magnet is higher than the saturation flux density of the ferrite core material and 5 thus cannot be maintained unless a core material, such as iron, of at least an equally high saturation flux density is employed. In contrast, the operating point X2, created in accordance with the principles of the present invention by the employment of a much greater ratio between gap 10 length and magnet length, renders the ferrite core material compatible with the permanent magnet because the operating point X2 is at a reduced flux density lower than the saturation flux density of the core material, and the attainment of the operating point X2 will not therefore be hind-15 ered by the ferrite core material as would be the case with the operating point XI. The use of such a core material enables the attainment of very high rates of revolution and frequency of operation without excessive core losses in the form of heat, which further maximizes power 20 output, power conversion efficiency and linear speed-torque characteristics while minimizing temperature-sensitive demagnetizing effects and adverse high-temperature effects on other nearby temperature-sensitive components.

FIG. 11 shows a number of demagnetization curves 25 of typical permanent magnet"materials available on the market which, except for the Alnico material, would be suitable for use in the present invention. For each curve a dashed line is shown projecting that portion of the curve lying above the knee onto the magnetic field intensity 203311 axis to show how the value of Hcp would be arrived at in each case for application of the formula for determining the ratio of total gap length to total magnet length according to the present invention- Such ratio will 5 usually be in the area of unity, and should be within the range of 0.5 to 2.0, and preferably within the range of 0.8 to 1.2, depending upon the particular permanent magnet material. unsuited for the application of the present invention, due to the extremely high ratio (approximately 20:1 Gs/Oe) between the magnitudes of its remanent flux density Br and its intrinsic field intensity Hc, respectively, creating such a steep demagnetization curve as to tolerate very little 15 variation in flux density without demagnetization if the initial operating point is not close to the remanent point-In general, the permanent magnet materials suitable for application of the present invention will have much lower ratios between the magnitudes of their remanent flux den-20 sities Br and their intrinsic field intensities Hc, respectively, such ratios being no higher than about 2:1. Gs/Oe shown in Karube U.S. patent 4,130,769. This motor features a four-pole rotor 38 upon which the magnets are not seg-25 mented but rather magnetized into an isotropic magnet. The motor features a toothless stator core 40 separated by a gap 42 of length Lg from the rotor 38, with a winding 44 interposed in the gap 42. One of the four radially symmetrical magnetic circuits of the motor of FIG. 2 is- -.-inline a ted . 1 - N 'r'%.

FIG. 11 also shows why an Alnico magnet is FIG. 2 illustrates the permanent magnet motor / - 9 JAN 1986 203311 by the dashed line 46 and comprises a magnet of length Ljj,, two gaps of length Lg and a portion of the core 40. As pointed out earlier, this motor employs a toothless core structure to permit inexpensive winding installation and 5 removal, but stresses minimization of the gap consistent with the smallest winding size which will satisfy the torque requirements of the motor. Accordingly, although the ratio between the total gap length 2Lg of the magnetic circuit 46 and the magnet length L is illustrated as approximately 1:3, it is in reality much less in accordance with the teachings of the patent. Therefore the operating point of the permanent magnet is not much further from the remanent point than the operating point of the motor of FIG. 1.

In contrast, a motor comparable to the motor of FIG. 2, but designed in accordance with the principles of 15 the present invention, is shown in FIG. 6. The motor of FIG. 6 has the same outside dimensions as that of FIG. 2, but has a substantially greater energy density due to the creation of an operating point X2 (FIG. 10) which is in the vicinity of the theoretical maximum energy product of 20 its permanent magnet rotor 138. The operating point of theoretical maximum energy product is created by increasing the ratio between the total gap length 2Lg and the magnet length of the magnetic circuit 146 such that they are approximately equal. This requires enlargement 25 of the gap 142 between the £otor 138 and the core 140, a corresponding reduction in the diameter of the permanent magnet rotor 138, and filling of the enlarged gap 142 with an enlarged winding 144 in accordance with the principles of the present invention. 203311 FIG. 3 illustrates the motor shown in Kamerbeek J et al. U.S. patent 4,135,107 featuring a two-pole permanent I magnet rotor 48, with a steel shaft 49 through its center, I separated by a gap 50 from a stator core 52 about which is f ■j wound a relatively flat winding 54. One of the motor's I I• ' * ' t two diametrically opposed magnetic circuits is shown by ■ , ■ % the dashed line 56 and includes a total gap length of I I 2Lg and a total magnet length of 2Lm. Like the motor of I FIG. 1, the ratio between the total gap length of the I. & magnetic circuit and the total magnet length is in reality much less than illus- I trated. Like the motor of FIG. 2, a toothless gap was used f in the motor of FIG. 3 for purposes other than maximizing j the energy product of the permanent magnet rotor 48, in ] that the toothless core was utilized to eliminate har- i r monies to obtain a sinusoidal field. However, minimiza- i i { tion of the air gap was stressed by utilizing as large a ! rotor as possible in the structure. f | FIG. 7 shows a motor comparable to that of FIG. 3 f i designed in accordance with the principles of the present || % H invention. As in the motor of FIG. 6, the diameter of the | i , | permanent magnet rotor 148 is reduced considerably to en- § I. able a much larger gap 150 between the rotor 148 and the i 1 ■ stator core 152. The external windings of the motor of I FIG. 3 are dispensed with in FIG. 7 because they are out- f a ' .1 side of the magnetic circuit, and therefore do not con- f tribute to output power. In accordance with the princi- 1 pies of the present invention, the two gap lengths Lg of FIG. 7 are approximately equal to the two magnet lengths Lfn, and the enlarged gap 150 is substantially fi^^""^i;th << o\ an enlarged winding 154. // 33_ |2 ~9JAN1936.d \v 0 £ i 203311 FIG. 4 depicts in simplified form the major elements of ahypothetical . type of permanent magnet motor not constructed with a nonmagnetic zinc core 62 inside the tubular permanent magnet and a steel shaft 63 at its center. A winding 64 is affixed to the stator core 58 within the gap 66 separating the rotor 60 from the stator core 58. In one of 10 the motor's two diametrically opposed magnetic circuits, indicated by the dashed line 68, there are not only two gaps Lg between the rotor 60 and the stator core 58 but also, since the zinc 62 is nonmagnetic, two very large effective gaps LgZ inside the tubular permanent magnet 60 15 itself, increasing the reluctance of the circuit greatly. The total length of magnets in the magnetic circuit are the two distances as shown in FIG. 4. Here there exists a ratio, between total gap length 2Lg+2LgZ to total magnet length 2Lm of the magnetic circuit 68, which is substan-20 tially the reverse of the relationship of the motors of FIGS. 1, 2 and 3 in that, in FIG. 4, the total gap length is approximately three times the total magnet length.

This causes an operating point at a flux density far below that of the operating point of theoretical maximum energy 25 product and, in fact, can lead to extensive demagnetization.

If a motor comparable to that of FIG. 4 were to be designed with a magnetic circuit more in accordance with the principles of the present invention, it would in accordance with the present invention. > This motor has a toothless stator core 58 surrounding a cylin- drical two-pole tubular permanent magnet rotor 60 203311 look substantially like that shown in FIG. 8 where the tubular permanent magnet rotor 160 is substantially thicker than that of FIG. 4, yielding a larger total magnet length 2Lm. The zinc core 62 would be substantially smaller pro-5 ducing substantially smaller gap lengths LgZ, with the gap 166 between the rotor 160 and the stator core 158 being approximately the same length Lg as in FIG. 4 such that 2Lg+2LgZ would approximately equal 21^ in the magnetic circuit 168. Also, a suitable ferrite or rare earth co-10 bait permanent magnet would be used.

However, more preferably, the motor according to the present invention would not be designed with a zinc core such as 162 at all. Rather, it would be designed more like the motor of FIG. 7 with the entire gap of the magnetic cir-15 cuit filled substantially with winding so that none of the gap is wasted, as in the case of the zinc core. designed in accordance with the present invention. In this case a permanent magnet rotor 170, comprising a magnetic steel outer casing 172 upon which are mounted a pair of arcuate permanent magnets 174, rotates about an inner stator 176 having a core ring 178 enclosed by a winding 180. One of the two diametrically opposed magnetic circuits of the motor is indicated by the dashed line 182.

The magnetic circuit bypasses the center portion 183 of the core ring 178 and winding 180, and it is therefore unnecessary that this center portion contain magnetic material. The two magnet lengths of the circuit are indicated as Lm and the two gap lengths are indicated as Lg.

FIG. 9 shows a still further type of motor -35 / u o J > • In accordance with the general design formula of the present invention, 2Lm is approximately equal to 2Lg to produce the desired operating point at or near the point of theoretical maximum energy product.

In all of the foregoing machines, the winding is fixedly mounted on, and insulated from, the stator core and may advantageously be potted in a suitable material such as epoxy to hold its shape and maintain the necessary mechanical clearance between the winding and the rotor. 10 It will be understood that in any of these machines the role of the rotor and stator can be reversed and that either the rotor or stator can act as the rotating element. Likewise, the permanent magnets can be placed either on the internal or external member, and likewise 15 with respect to the winding and core. Typical variations of these electrical machines include permanent magnet DC motors, tachometers, alternators, generators and stepper motors.

The terms and expressions which have been 20 employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that 25 the scope of the invention is defined and limited only by the claims which follow.

I % I 203311

Claims (26)

WHAT WE CLAIM IS:

1. A method of making an electrical machine comprising the steps of mounting a permanent magnet and a core in predetermined relationship to each other separated from each other by a gap of predetermined size, interposing an electrically conductive winding within said gap between said magnet and said core and permitting relative motion between said magnet and said winding/ said magnet, said core and said gap forming a magnetic circuit in which said magnet is operated at a predetermined magnetic flux density and a predetermined magnetic field intensity determined by said size of said"gap, and further wherein the product of said predetermined flux density and said predetermined field intensity corresponds to an energy product, said magnet having a predetermined remanent flux density and having a predetermined maximum energy product which occurs at a flux density lower than said remanent flux density, characterized by selecting said size of said gap to obtain a corresponding predetermined flux density and field intensity which substantially maximizes said energy product, so that said magnet is maintained in the vicinity of said maximum energy product during said relative motion, selecting the material of said permanent magnet such that the ratio between the magnitude of the remanent magnetic flux density and the magnitude of the intrinsic magnetic field intensity thereof is no more than about 2:1 Gs/Oe, and filling said gap substantially completely with said winding. .'rfX t ' ■ ■ <? . j A N1986 r>i ] ': X X v A-

2. The method of claim 1 wherein said magnet/ said core and said gap form a magnetic circuit in which a predetermined magnetic flux density and a predetermined magnetic field intensity are induced within said gap/ the predetermined flux density and predetermined field intensity within said gap being determined by said size of said gap and corresponding to an energy density within said -gap# further Including selecting said size of said gap so. that the value of the predetermined flux density within aaid gap is substantially less than said remanent flux density/ and so that during said relative motion said energy density within said gap is substantially maximized.

3. The method of claim 1 wherein said magnetic circuit has one or more gaps operatively interposed therein and one or more mutually cooperating permanent magnets operatively interposed therein/ said magnetic circuit having a total gap length composed of the total length of said gap or gaps and a total magnet length composed of the total length of said magnet or magnets.

4. The method of claim 3/ further including selecting the size of said gap so that the ratio of said total gap length to said total magnet length i3 within the range of 0.5 to 2.

5. The method of claim 3/ further including selecting the size of said gap so that the ratio of said total gap length to said total magnet length is within the range of 0.8 to 1.2. Iz -9 JAN 1986 m} V\ _ // -38 203311

6. The method of claim 3, further including selecting the aize of said gap so that the ratio of said total gap length to said total magnet length is substantially unity.

7. The method of claim 1 or 2 wherein said magnetic circuit has one or more gaps operatively interposed therein and one or more mutually cooperating permanent magneto operatively interposed therein, said magne-1 tic circuit having a total gap length composed of the total length of said gap or gaps and a total magnet length composed of the total length of said magnet or magnets, said magnet having a demagnetization curve characterized by a remanent magnetic flux density, by an intrinsic magnetic field intensity, and by a knee which is positioned between the remanent flux density and the intrinsic field intensity, further including the step of selecting the size of said gap and said magnet according to the formula Lgt a "d Hcpj *-TT\t **r wherein Lgt •= Total Gap Length; L^t- *= Total Magnet Length; u0 «=» Permeability Of Free Space; Br -• Remanent Flux Density; and Hcp •= Imaginary Intrinsic Magnetic Field Intensity and further wherein Hcp ia determined by 1 inearly extending the portion of the demagnetization curve which lies between the knee and Br onto the magnetic field intensity axis of the demagnetization curve. £ N r ft *< ■" \ f 1 = -9 \ / 203311

8. The method of any one of claims 3 to 5, further including filling each gap, the length of which is included in said total gap length/ substantially completely with said winding.

9. The method of claim 1 wherein said core is composed of a magnetically soft material having a predetermined saturation magnetic flux density less than said remanent magnetic flux density of said permanent magnet/ further including selecting the size of said gap to limit the magnetic flux density of said permanent magnet to a flux density less than said saturation magnetic flux density of said magnetically soft material.

10. The method of claim 9 wherein said magnetically soft material is magnetically soft ferrite.

11. The method of claim 9 wherein said magnetically soft material is amorphous metal.

12. The method of any one of claims 1/ 2 or 4, further including mounting said permanent magnet and said winding for rotation relative to each other. 203311

13. An electrical machine comprising a permanent magnet having a predetermined remanent flux density/ a core separated from said permanent magnet by a gap of predetermined size/ an electrically conductive winding interposed within said gap between said permanent magnet and said core and means for permitting relative motion between said permanent magnet and said winding, said magnet/ said core and said gap forming a magnetic circuit in which ' i said magnet is operated at a predetermined magnetic flux density and a predetermined magnetic field intensity determined by said size of said gap/ and further wherein the product of said predetermined flux density and said predetermined field intensity corresponds to an energy product/ said magnet having a predetermined maximum energy product which occurs at a flux density lower than said remanent flux density/ characterized in that the size of said gap is sufficient to obtain a corresponding predetermined flux density and field intensity which substantially maximizes said energy product/ so that said magnet is maintained in the vicinity of said maximum energy product during said relative motion, said permanent magnet having a ratio between the magnitude of the remanent magnetic flux density and the magnitude^ of the intrinsic magnetic field intensity thereof of no more than about 2:1 Gs/Oe, said gap being filled substantially completely with said winding . 203311

14, The electrical machine of claim 13 wherein said magnet/ said core and said gap form a magnetic circuit in which a predetermined magnetic flux density and a predetermined magnetic field intensity are induced within said gap, the predetermined flux density and predetermined field intensity within said gap being determined by said size of said gap and corresponding to an energy density . within said gap/ the size of said gap being sufficient so that the value of the predetermined flux density within said gap is substantially less than said remanent flux density, and so that during said relative motion said energy density within said gap is substantially maximized.

15. The electrical machine of claim 13 wherein said magnetic circuit has one or more gaps operatively interposed therein and one or more mutually cooperating • permanent magnets operatively interposed therein, said magnetic circuit having a total gap length composed of the total length of said gap or gaps and a total magnet length composed of the total length of said magnet or magnets. the ratio length is the ratio length is

16. The electrical machine of claim 15 wherein of said total gap length to said total magnet within the range of 0.5 to 2.

17. The electrical machine of claim^S wherein of said total gap length to said total magnet within the range of 0.8 to 1.2. 203311

18. The electrical machine of claim 15 wherein the ratio of said total gap length to said total magnet length is substantially unity.

The electrical machine of claim 13 or 14 wherein said magnetic circuit has one or more gaps operatively interposed therein and one or more mutually cooperating permanent magnets operatively interposed therein, said magnetic circuit having a total gap length composed of the total length of said gap or gaps and a total magnet length composed of the total length of said magnet or magnets, said magnet having a demagnetization curve characterized by a remanent magnetic flux density, by an intrinsic magnetic field intensity, and by a knee which is positioned between the remanent flux density and the intrinsic field intensity, the size of said gap and said magnet being selected according to the formula Lg t » uol ^cpl ^rnt Br wherein Lgt - Total Gap Length; Lmt ™ Total Magnet Length; uQ = Permeability of Free Space; Br » Remanent Flux Density; and HCp "= Imaginary Intrinsic Magnetic Field Intensity ' t and further wherein Hcp is determined by 1 inearly.ie^tending the portion of the demagnetization curve which lies between the knee and Br onto the magnetic field intensity axis of the demagnetization curve. 20331J. .

20. The electrical machine of any one of claims 15 to;17wherein each gap, the length of which is included in said total gap length/ is filled substantially completely with said winding.

21. The electrical machine of claim 13 wherein said core is composed of a magnetically soft material • . having a predetermined saturation magnetic flux density less than said remanent magnetic flux density of said permanent magnet and wherein the si.ze of said gap is sufficient to limit the magnetic flux density of said permanent magnet to a flux density less than said saturation magnetic flux density of said ma'gnetically soft material.

22. The electrical machine of claim 21 wherein said magnetically soft material is magnetically soft ferrite. <

23, The electrical machine of claim 21 wherein said magnetically soft material is amorphous metal.

24. The electrical machine of any one of clains 13, 14^0r 16 wherein said permanent magnet and said winding are mounted for rotation relative to each other.

25. A method of making an electrical machine substantially as hereinbefore described with reference to figures 5 to 11 of the accompanying drawings.

26. An electrical machine substantially as hereinbefore described with reference to figures 5 to 11 of the accompanying drawings. ff " ky .toS7thcir authorised Agents, in ol\ A- J- r'ARK & SON. II - 9 JAN 1986 n,<l ./ f]