HK1160552B - Multiple armature linear motor/alternator having magnetic spring with no fringe fields and increased power output - Google Patents

Description

Multi-armature linear motor/generator with magnetic springs without fringing fields and increased power output

Technical Field

The present invention relates generally to electromagnetic-mechanical transducers used to drive loads or driven by prime movers, and more particularly to reciprocating linear motors and reciprocating linear generators.

Background

Reciprocating linear generators are used to generate electrical power when driven by various prime movers, including stirling engines, while linear motors are used to drive various mechanical loads when powered by an ac power source. Like the rotary electric motor, the generator and the generator, such a linear motor and a linear generator are substantially the same in that they all have the same basic elements, but differ in their connection and operation. Therefore, they are collectively called linear motor/generators.

The prior art includes substantially linear motor/generators of the type shown in figure 3, which are also described in US patent US4602174 and US patent US4623808, which are incorporated herein by reference. While the linear motor/generator can be constructed in a variety of configurations known in the art, the preferred configuration is an axisymmetric configuration in which an actuator with permanent magnets performs reciprocating motion along a reciprocating axis within an armature. Permanent magnets are mounted on the reciprocating actuator in a cylindrical configuration concentric with the axis. The main portion of the armature core and the armature coils or windings are also mounted in a concentric cylindrical configuration around the magnets and on a frame so that they remain stationary. The remainder of the core completes the flux loop formed by the core and is also mounted in a cylindrical configuration on the stationary frame. The remaining portion of the core is spaced inwardly from the main portion of the core to form a linearly aligned gap in the high reluctance magnetic flux path of the core. The linearly aligned gaps are parallel to the axis. The magnet (or ring magnet) reciprocates in the gap formed within the core. The armature may be a series of individual armatures arranged spaced around a circle of the cylindrical structure or it may be a circular armature with circular coils in circular slots. Similarly, the magnets may be discrete magnets placed side by side in a cylindrical configuration or one circular ring of magnets. As an alternative to the prior art, the coil may be wound around one leg of the core. In all of these alternative configurations, the time-varying magnetic flux in the core induces a current in the coil, which induces a magnetic flux in the core. These structural configurations are illustrated and described in the two patents mentioned above and in U.S. patent No. US5148066, which is incorporated herein by reference.

Figure 3 shows the basic elements of a prior art linear motor/generator. For an axisymmetrical linear motor/generator, fig. 3 is a cross-sectional view in a plane of the reciprocation axis 10 and in a radial direction from the shaft. The cross-sectional views of these basic elements in opposite radial directions and in the same plane are mirror images of fig. 3 and are therefore not repeated.

Referring to fig. 3, the armature 12 has an associated armature coil 14 and an associated core 16. The armature coil 14 is wound in a circular configuration concentric with the axis 10. The core 16 forms a low reluctance magnetic flux circuit consisting of a U-shaped main portion 18 and a remainder portion 20, both of which are constructed of laminations of iron or other high permeability material as is known in the art. The core loop has a pair of spaced gaps 22 and 24 parallel to the reciprocation axis 10 and separated from each other by armature winding slots 26. Each of the gaps 22 and 24 is defined by two opposing pole faces, with pole faces 28 and 30 defining gap 22 and pole faces 32 and 34 defining gap 24.

Gaps 22 and 24 are linearly aligned along a gap parallel to the axis such that field magnet 36 associated with armature 12 can reciprocate within gaps 22 and 24 in an axial direction. The field magnet 36 is mounted on a reciprocatable actuator 38, wherein the actuator 38 carries all of the magnets such that the magnets reciprocate within the gap path of the gaps 22 and 24. The field magnet 36 is polarized across the gaps 22 and 24, preferably perpendicular to the pole faces 28-34, as indicated by the arrow drawn at the center of the magnet 36. The actuator 38 is drivingly connected to a prime mover or load 40 depending on whether the linear motor/generator of fig. 3 is functioning as a linear generator or a linear motor. As the magnet 36 reciprocates to alternately enter between the gaps 22 and 24, the magnetic flux generated by the magnet 36 in the core is alternately reversed. Since the flux path through the core extends through the armature coil and varies with time, an EMF is induced in the coil and the current in the coil creates a magnetic flux in the core that applies a force to the magnet, in a manner well known to those skilled in the art.

The linear motor/generator configuration described above provides suitable performance for many linear motor/generator applications. However, for some applications, it may be desirable to apply a spring force to the reciprocating actuator. For example, if the linear motor/generator is driven by or is used to drive a free piston stirling cooler, a spring force applied in a direction toward centering the actuator is desirable to maintain the axial mean position of the free piston machines at a selected center position because the stirling machines have a tendency to drift their mean position away from a nominal center position. As another example, it is sometimes desirable for the actuator of a linear motor/generator and its load or prime mover to reciprocate in a resonant system, which requires a spring. As another example, if the reciprocating motion of the actuator and its load or prime mover has a vertical component, it is sometimes necessary to provide a centering spring force on the actuator to resist gravity and prevent the actuator from moving from its average position to its lowest limit of travel.

Mechanical springs can and have been used for this purpose. However, mechanical springs have some detrimental characteristics. The above-mentioned US5148066 discloses a way of introducing a magnetic spring force in a linear motor/generator. As described therein, a pair of smaller secondary magnets are placed on opposite sides of the primary magnet and are oppositely polarized to the primary magnet. These auxiliary magnets cause a centering force to be exerted on the reciprocating actuator whenever one of the secondary magnets extends outwardly from between the pole faces defining one of the two gaps. Since the centering spring force is only applied to the actuator when a secondary magnet moves out of a gap, the secondary magnets extend from the primary magnet all the way to the outer edge of their respective gap. In this way, there is no dead zone around the average center position, where no spring force is applied to the actuator to tend the actuator to return to its average position. The combination of these three magnets illustrated and described in US5148066 extends from the outer edge of one gap to the opposite outer edge of the other gap. When the actuator moves, an auxiliary magnet moves out of the gap between the pole faces, resulting in a force being applied in a direction toward centering the magnet, the magnitude of the force being proportional to the displacement of the magnet out of the gap.

However, some problems arise from the fact that: one of the auxiliary magnets is substantially always moved out of the gap so that a spring force will be exerted on the actuator without power being generated. It is an object, object and feature of the present invention to obviate these problems. The first problem is: the further the auxiliary magnets extend out of the gap into the air, the more the auxiliary magnets contribute to the generation of electrical energy in the generator or driving energy in the motor in motor applications. The reason for this is that the permeability of air is very low, and therefore the magnetic flux from the auxiliary magnet is small in the core. A second problem arises because the alternating reciprocating motion of the auxiliary magnet from within the gap to a position extending away from the gap thereof generates a time-varying magnetic fringing field outside the pole face. This alternating magnetic fringing field couples with the surrounding ferromagnetic material and induces eddy currents in the ferromagnetic material that create resistive power losses. Furthermore, the same alternating fringing fields couple to nearby conductors, which can disturb the current in these conductors.

Disclosure of Invention

The present invention is an electromagnetic reciprocating linear motor or generator having at least two armatures adjacent along a reciprocating axis. The field magnets mounted on the actuator include not only a plurality of primary field magnets each associated with one armature, but also secondary magnets interposed between the primary magnets, the secondary magnets extending in the axial direction from within the gap of one armature core to within the adjacent gap of the adjacent armature core. Each auxiliary magnet is magnetically polarized in a direction opposite to the polarization of the primary magnet. No fringing fields are generated because no magnet is moved sufficiently to extend out of the gap into the air during reciprocation; and the auxiliary magnets contribute to the power of the linear motor/generator because the auxiliary magnets are always reciprocally moved in and out in the gaps of adjacent armature cores.

Drawings

FIG. 1 is a schematic view of a preferred embodiment of the present invention in radial cross section taken in any radial direction along the axis of reciprocation.

Fig. 2 shows a diagram of the centering force as a function of the displacement of the actuator, applied to the present invention.

Figure 3 is a schematic view in radial section of a prior art linear motor/generator.

Fig. 4 shows a schematic diagram of a practical example of a centering force as a function of actuator displacement, applied to the present invention.

Figure 5 is a schematic view of an alternate embodiment of the present invention in radial cross section taken in any radial direction along the axis of reciprocation.

In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended to limit the invention to the specific terminology so selected, and it is to be understood that each specific terminology includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word "connected" or similar terms are often used. They are not limited to direct connection, but include connection through other circuit elements where such connection may be recognized as being equivalent by those of ordinary skill in the art.

Detailed Description

Figure 1 shows the various elements of an electromagnetic reciprocating linear motor or generator associated with an embodiment of the present invention. At least two armatures 42 and 44 are mounted adjacent to each other along a single reciprocating axis in the manner of a single armature as shown in figure 3. Each armature is similar to the single armature shown in fig. 3, but in the invention, there are two or more armatures mounted adjacent to each other. Each armature has an associated armature winding 46 and 48 and an associated core 50 and 52, respectively, forming a low reluctance magnetic flux loop. The magnetic return path of each core has a pair of spaced gaps aligned parallel to the axis and separated by an armature winding slot. Armature 42 has gaps 54 and 56 and armature 44 has gaps 58 and 60, each of which is defined by two opposing pole faces in the manner shown in fig. 3. The gaps of adjacent armatures are linearly aligned along a gap path parallel to the axis such that the field magnets can reciprocate along the gap path. The embodiment of fig. 1 also has a reciprocatable actuator that is conventionally connected to the prime mover or load in the same manner as shown in fig. 3 and reciprocates along a reciprocation axis. The actuator and its load or prime mover are not shown in fig. 1 to avoid reducing the size of fig. 1. Generally, for a linear motor/generator, the linear motor/generator of FIG. 1 has a nominal design reciprocating stroke and an average position. The stroke refers to the amplitude (length) of the reciprocating motion of the actuator, similar to the displacement from the upper dead zone center to the lower dead zone center of the piston. The average position of the actuator and thus its field magnet refers to the center between the extreme positions of the actuator's reciprocating motion.

The two armatures 42 and 44 are depicted as being adjacent, meaning that they are side-by-side and close together. Preferably, the armatures and their cores cannot touch their pole faces, but rather have a narrow space between their pole faces. Since the permeability of iron is three orders of magnitude greater than that of air, the armatures can (and preferably are) be positioned very close together without much magnetic coupling from one armature to the other. However, they may be in contact with each other, but this may result in a small degradation in performance due to magnetic coupling from one core to another. They may also be spaced further apart, but this unnecessarily extends the length of the linear motor/generator. Therefore, the preferred distance between them is an engineering compromise between minimizing the degradation of the magnetic coupling and minimizing the linear motor/generator length with compactness.

The linear motor/generator of fig. 1 also has field magnets mounted on the actuator to reciprocate within the gap path in the manner of the magnets 36 of fig. 3. The field magnet of fig. 1 includes a plurality of primary field magnets 62 and 64, each of which is associated with an armature. Each primary field magnet 62 and 64 extends in an axial direction from within one gap of its associated core to within another gap of its associated core. The primary field magnets 62 and 64 have magnetic polarization in the same direction across the gap path, as indicated by the direction of the arrows drawn at the centers of the field magnets 62 and 64.

Of paramount importance to the present invention is the location of the secondary magnet 66 between the primary magnets 62 and 64. Like the primary magnets 62 and 64, the secondary magnet 66 also extends in one axial direction and is mechanically mounted on the actuator for reciprocating movement with the primary magnet within the gap path. The secondary magnet 66 is interposed between the primary magnets 62 and 64 and extends from within the gap 56 of one core to within the adjacent gap 58 of the adjacent core. Importantly, the secondary magnet 66 is magnetically polarized in a direction opposite to the polarization of the primary magnets 62 and 64.

In order to maximize the effect of the invention, it is preferred to design the length of the magnets in the axial direction such that they have the required relationship with the nominal design stroke and with the distance to the gap edge. The distance from each axially opposed edge of the respective primary magnet to the nearest outer edge of the gap associated therewith is preferably slightly less than half of the nominal design stroke when the actuator is in its average position. This ensures that the primary magnets do not reciprocate out of their associated gaps as the actuator reciprocates within the limits of its design stroke. This relationship may prevent fringing fields at the outer edges of the gap and limit the magnetic flux from the primary magnet to the core, where the magnetic flux may couple to the armature coil.

It is also desirable that the axial length of each auxiliary magnet is greater than the nominal design stroke. This relationship ensures that the edges of the auxiliary magnets do not move out of a gap and cause fringing fields at the edges of the gap. This relationship may also ensure that the spring force generated from the auxiliary magnet is maintained in a linear relationship with the displacement of the actuator. If an edge of the auxiliary magnet moves outside a gap between the pole faces, the spring force can decrease significantly and non-linearly.

Although not preferred as described above, the cores of adjacent armatures may be separated by a significant distance. In order to prevent the inner edges of the auxiliary magnets from moving outside a gap, the axial length of each auxiliary magnet should be slightly greater than the sum of the nominal design stroke and the distance by which the cores are spaced, i.e., the sum of the distance between the cores and their gaps.

In order to maximise the change in magnetic flux in the cores over time, it is preferred that each primary magnet extends in the axial direction from substantially in the middle of one gap of its associated core to in the middle of another gap of its associated core when the actuator is at its average position. In order to maximize the magnitude of the centering force exerted by the auxiliary magnets, it is preferred that each auxiliary magnet extends substantially in one axial direction to the primary magnet between which it is interposed.

These relationships can be best achieved by making the length of the pole faces defining the gap slightly greater than the design nominal stroke. In general, it is most preferred to lengthen the pole face by 10%.

As is well known in the engineering arts and those skilled in the art, non-adherence to these preferred relationships can result in degradation or degradation of performance. Slight deviations will have only a slight effect, while significant deviations will have a significant effect.

FIG. 2 is a diagram showing centering magnetic spring force applied to an actuator in an embodiment of the invention. The spring force is zero when the magnet and actuator are centered on their average position. Movement in either reciprocating direction will generate a centering spring force proportional to the displacement of the actuator and the magnet it carries.

FIG. 4 is a graph for a representative displacement X_PSimilar pattern of spring force of the linear motor/generator. Only half of the figure is drawn, since the other half has the same value, but in the opposite direction (always towards centering the magnet).

The application of the linear motor/generator of the present invention is not limited to two armatures with one auxiliary magnet as shown in fig. 1. For example, fig. 5 shows four armatures 70, 72, 74 and 76 stacked along a reciprocation axis 78. Each of these armatures has an associated primary magnet 80, 82, 84 and 86, respectively, having the characteristics described above. Secondary magnets 90, 92 and 94 are interposed between primary magnets 80, 82, 84 and 86. The present invention may be embodied as two or more adjacent armatures arranged in the above-described structure. In each case, the number of secondary magnets is one less than the number of primary magnets. The present invention has no end magnets that reciprocate out of the gap into the air to create undesirable fringing fields at the ends of the adjacent armature sets because the secondary magnets will only be between the primary magnets.

An important feature of the invention is that the auxiliary magnet (or magnets in the case of three or more armatures) not only provides a spring force that centers the magnet and actuator towards, but also helps to generate power in the generator or to provide power to the motor. As known to those skilled in the art, the EMF generated at a coil is proportional to the rate of change of the magnetic flux linked to the coil. Since the magnetic polarity of the auxiliary magnet is opposite to that of the main magnet, the auxiliary magnet always causes a change in magnetic flux in the same direction as the adjacent main magnet. For example, when the secondary magnet moves into a gap, the adjacent primary magnet moves out of the gap. Thus, the direction of the magnetic flux change from the exiting primary magnet is the same as the direction of the magnetic flux change generated by the entering secondary magnet. The magnetic flux drops continuously in the direction of the primary magnet and increases continuously in the opposite direction of the secondary magnet, all at the same time.

The centering spring force in embodiments of the present invention is not as strong as the centering spring force of the linear motor/generator in US5148066 (where the centering magnets reciprocate out of the gap and into the air). However, the present invention provides more generator or motor power, since the auxiliary magnets of the present invention make a significant and substantial contribution to power generation. The present invention is particularly applicable to larger, high power motor/generators where a large amount of power is required. The use of two smaller armatures allows the construction of an engine/generator much smaller than if a single large armature were used. The present invention therefore also provides an opportunity to design a linear motor/generator with an improved aspect ratio, i.e. an aspect ratio which makes the motor/generator less objectionable and allows the pressure vessel of the prime mover of the stirling engine to be of a smaller diameter which reduces the pressure on the pressure vessel of the stirling engine.

The detailed description set forth above in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. The above description, in conjunction with the illustrated embodiments, describes designs, functions, apparatus, and methods that implement the present invention. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be resorted to without departing from the scope of the invention or the following claims.

Claims (9)

1. An electromagnetic reciprocating linear motor or generator including a reciprocatable actuator having a reciprocation axis, a nominal design stroke and an average position, the linear motor or generator comprising:

(a) at least two armatures adjacent along the axis, each armature having an associated armature winding and an associated core to form a low reluctance magnetic flux loop, each core loop having a pair of spaced gaps aligned parallel to the axis and separated by one armature winding slot, each gap being defined by two opposed pole faces, the gaps of the armatures being linearly aligned along a gap path parallel to the axis; and

(b) a field magnet mounted on the actuator for reciprocating movement in the gap path, comprising:

(i) a plurality of primary field magnets, each primary field magnet associated with an armature, each primary field magnet extending in an axial direction from within one gap of its associated core to within another gap of its associated core, the primary field magnets having magnetic polarization in the same direction across the gap path;

(ii) at least one auxiliary magnet extending in an axial direction and mechanically mounted for reciprocal movement with the primary field magnets within the gap path, the auxiliary magnet being interposed between the primary field magnets and extending in the axial direction from within a gap of one core to within an adjacent gap of an adjacent core, each auxiliary magnet being magnetically polarized in a direction opposite to the polarization of the primary field magnets.

2. A linear generator or motor as claimed in claim 1, wherein the distance from each axially opposed edge of each primary field magnet to the nearest edge of its associated gap is less than half the nominal design stroke when the actuator is at its average position, such that the primary field magnets do not reciprocate out of their associated gaps.

3. A linear electric generator or motor as claimed in claim 2, characterized in that the axial length of each auxiliary magnet is greater than said nominal design stroke.

4. A linear generator or motor according to claim 3, characterised in that the cores of adjacent armatures are separated, the axial length of each auxiliary magnet being greater than the sum of the nominal design stroke and the distance by which the cores are separated.

5. A linear electrical generator or motor as claimed in claim 4, wherein each primary field magnet extends in the axial direction from substantially in the middle of one gap of its associated core to substantially in the middle of another gap of its associated core when the actuator is at its average position, and each secondary magnet extends in the axial direction substantially to the primary field magnet between which the secondary magnet is interposed.

6. A linear electric generator or motor as claimed in claim 5, characterized in that the number of said auxiliary magnets is one less than the number of said primary field magnets.

7. A linear electric generator or motor as claimed in claim 6, characterized by having more than two primary field magnets and armatures stacked along said axis.

8. A linear electric generator or motor as claimed in claim 1, characterized in that the number of said auxiliary magnets is one less than the number of said primary field magnets.

9. A linear electrical generator or motor according to claim 1, wherein each primary field magnet extends in the axial direction from substantially midway in one gap of its associated core to substantially midway in another gap of its associated core when the actuator is at its average position, and each secondary magnet extends in the axial direction substantially to the primary field magnet between which the secondary magnet is interposed.