The present invention relates to an electric motor. The present invention particularly relates to synchronous electric motors and even more particularly to iron-less synchronous electric motors.

Synchronous electric motors are known in the art. An example thereof is shown in figure 1 , which illustrates a known iron-less synchronous linear motor 1 both in a perspective view (left) and an exploded view (right). This motor comprises a primary part 10 that includes a plurality of electrical coils 11, and a secondary part 20 that includes a plurality of magnets 21, such as electromagnets or permanent magnets. Typically, primary part 10 has a casted body 12 in which electrical coils 11 are mutually fixed by a solidified casting material, such as a solidified epoxy resin. Such casted body has a first surface 13 and a second surface 14 that are directed towards opposing parts of secondary part 20. Primary part 10 and secondary part 20 are configured to move relative to each other upon energizing electrical coils 11.

The maximum force that can be generated by electric motor 1 depends, inter alia, on the distance between electrical coils 11 and magnets 21. This distance is typically determined in a direction that is perpendicular to first surface 13 and/or in a direction that is perpendicular to second surface 14 and should be kept as small as possible.

The distance between electrical coils 11 and secondary part 20 is determined at least to a large extent by the thickness of the solidified casting material covering electrical coils 11. This material provides mechanical fixation of electrical coils 11 and provides electrical insulation between electrical coils 11 and secondary part 20 or other conductive structures that may touch or may get into close proximity of primary part 10.

To enable smooth motion of primary part 10 relative to secondary part 20, it is important that electrical coils 11 have a well-defined position relative to each other and relative to secondary part 20. Typically, such arrangement of electrical coils 11 is achieved using an elaborate casting process. This process may comprise two casting steps as will be discussed next referring to figures 2A-2D.

In a first casting step, shown in figure 2A, a sub-unit 100 is formed of which a cross section is shown in figure 2B. In this step, a first casting mold 200 is used that comprises a bottom part 210 and a cover part 220. Bottom part 210 can be coupled to cover part 220 to form a molding cavity. Furthermore, bottom part 210 comprises an inlet 211 and an outlet 212 for allowing a first liquid casting material, such as epoxy resin, to flow in and out of the mold cavity. As shown, bottom part 210 comprises outwardly protruding first ridges 213 and inwardly protruding first alignment pins 214. Cover part 220 comprises outwardly protruding second ridges 223 and inwardly protruding second alignment pins 224. Electrical coils 11 are mounted on bottom part 210 such that first alignment pins 214 extend in a central opening of these coils. In this manner, the mutual positioning of electrical coils 11 can be defined during the first casting step. When a liquid first casting material is inserted through inlet 211, a layer will be formed around and in between electrical coils 11. In addition, ridges will be formed in this layer as a result of first casting material flowing into first ridges 213 and second ridges 223. Although electrical coils 11 lay essentially flat on bottom part 210, a thin layer may be formed in between electrical coils 11 and bottom part 210.

Sub-unit 100 formed using first casting mold 200 and shown in figure 2B comprises three electrical coils 11 that are mutually fixated by a solidified first casting material 101. In sub-unit 100, ridges 113, 123 are formed that correspond to ridges 213, 223 of bottom part 210 and cover part 220, respectively. In addition, recesses 114 and 124 are formed that correspond to alignment pins 214, 224 of bottom part 210 and cover part 220, respectively.

Typically, primary part 10 comprises more than three coils. For example, a three phase motor comprises n x three coils, wherein n is an integer larger than or equal to one.

Figure 2C illustrates how a primary part 10 can be formed using multiple sub-units 100. To this end, a second casting mold 300 is used that comprises a bottom part 310 and a cover part 320. In figure 2C, casting mold 300 is shown in exploded view. A mold cavity can be formed by coupling cover part 320 to bottom part 310. Similar to bottom part 210, bottom part 310 comprises an inlet 311 and an outlet 312 for receiving and outputting a second liquid casting material, respectively.

Sub-units 100 are flipped before arranging them in casting mold 300. This allows alignment pins 314, which are provided in bottom part 310, to engage recesses 124 of sub-units 100 allowing these units to be mutually positioned before applying the second casting material.

During the second casting step, sub-units 100 rest on bottom part 310 using ridges 123. Furthermore, cover part 320 rests on or touches ridges 113. During casting, the liquid second casting material will fill the spaces between sub-units 100 and bottom part 310 and cover part 320.

A segment of primary part 10, corresponding to sub-unit 100 shown in figure 2B, is shown in figure 2D. In this figure, the difference between first casting material A and second casting material B is visible. During the second casting step, second casting material B has penetrated recesses 114 and 124. In this manner, a first covering layer 15 and a second covering layer 16 of second casting material is formed between electrical coils 11 and bottom part 310 and cover part 320, respectively. The thickness of these covering layers is well defined as a result of ridges 123, 113, respectively.

In the first casting step, electrical coils 11 are mutually aligned. At that time, electrical coils 11 may not be completely covered by solidified first casting material on both sides. Put differently, the layer of solidified first casting material, and more in particular its thickness, is not well defined.

In the second casting step, ridges 123 and 121, which were made of solidified first casting material during the first casting step, are used to prevent the plurality of coils 11 from being exposed on first surface 13 and second surface 14, respectively, of the primary part 10 during the second casting step. Hence, ridges 123 and 113 act as spacers for the formation of a first and second covering layer 15, 16 of solidified second casting material in between the plurality of coils 11 and first surface 13 and second surface 14, respectively.

As bottom part 310 and cover part 320 of second mold 300 touch ridges 123 and 113, respectively, ridges 123 and 113 are exposed on first surface 13 and second surface 14, respectively, at a plurality of exposed positions P.

A drawback of primary part 10 described above is related to the thickness of the solidified second casting material in between the electrical coils 10 and first surface 13 and second surface 14. Because two casting steps are used, it is difficult to achieve small thicknesses.

A further drawback is related to the fact that several alignment steps are required of which the alignment accuracies do not correlate. For example, even though electrical coils 11 may be aligned perfectly within a single sub-unit 100, non-optimal performance of primary part 10 may still occur if sub-units 100 are not properly mutually aligned. Finally, the costs and required processing time of a two-step casting process can be detrimental to the final costs of the product.

An electric motor as defined by the preamble of claim 1 is known from US 2015/280539A1. A further electric motor is known from EP 3422537A1.

It is an object of the present invention to provide an electric motor in which the abovementioned problems do not occur or at least to a lesser extent.

This object has been achieved using an electric motor as defined in claim 1 that comprises a primary part comprising a plurality of electrical coils, and a secondary part comprising a plurality of magnets, such as electromagnets or permanent magnets. The primary part has a casted body in which the electrical coils are mutually fixed by a solidified casting material, wherein the casted body has a first surface that is directed towards the secondary part. Furthermore, the secondary part and primary part are configured to move relative to each other upon energizing the electrical coils.

The primary part comprises a first electrically insulating first spacer by which the plurality of coils were prevented from being exposed on the first surface during casting of the casted body, and which caused the formation of a first covering layer of solidified casting material in between the plurality of coils and the first surface, and wherein the first spacer is exposed on the first surface at a plurality of exposed first positions.

According to the present invention, the electric motor is characterized in that the first spacer contacts the plurality of coils. Furthermore, the first spacer defines a plurality of first paths that each start at or near a metal part of the plurality of coils, extend solely through the first spacer, and end at or near an exposed first position among the plurality of exposed first positions, wherein, for each exposed first position, each first path ending at or near that position has a shortest path length that is at least equal to two times the thickness of the first covering layer. Moreover, the casting material was applied to simultaneously fixate the mutual positions of the electrical coils and to define the first covering layer.

It should be noted that the first path may not exactly start at a metal part of an electrical coil. A very thin layer may exist between a metal part of the electrical coil and the start of the path and/or between the end of the path and the exposed first position. For example, the metal part of the electrical coil may correspond to a core of a winding that is covered by a thin insulator to isolate windings from each other. However, the combined thickness of these thin layers may not exceed 10 percent of the thickness of the first covering layer.

The Applicant has found that a single casting step for fixating the mutual positions of the electrical coils and for forming the first covering layer can be used provided that for each exposed first position, each first path ending at or near that position has a shortest path length that is at least equal to two times the thickness of the first covering layer. The Applicant found that using a single casting step may result in electrical failure. Without being bound by theory, it is assumed that when a single casting step is used, the casting material may not attach fully to the first spacer thereby leaving small air-filled channels that extend from the exposed first positions to the metal parts of the electrical coils. Electrical discharges may occur in these channels during operation, for example when the primary part touches a conductive part, such as the secondary part, at an exposed first position. By ensuring that the shortest path length is at least equal to two times the thickness of the first covering layer, sufficient isolation can be achieved. Due to the high path length, the electric field inside the first path can be kept more easily below the critical field of air.

It should be appreciated by the skilled person that discharges may occur despite the abovementioned path length if the thickness of the first covering layer is too small of the voltage applied to the electrical coils too high. Nevertheless, the proposed increased path length provides an advantageous trade-off between the maximum force that can be generated by the electric motor and the likelihood of electrical discharge.

The casted body may have a second surface that is opposite to the first surface, wherein the primary part may comprise a second electrically insulating spacer by which the plurality of coils were prevented from being exposed on the second surface during casting of the casted body, and which caused the formation of a second covering layer of solidified casting material in between the plurality of coils and the second surface. The second spacer may contact the plurality of coils and may be exposed on the second surface at a plurality of exposed second positions. The second spacer may define a plurality of second paths that start at or near a metal part of the plurality of coils, extend solely through the second spacer, and end at or near an exposed second position among the plurality of exposed second positions. For each exposed second position, each second path ending at or near that position may have a shortest path length that is at least equal to two times the thickness of the second covering layer. Moreover, the casting material was applied simultaneously to fixate the mutual positions of the electrical coils and to define the second covering layer.

The first spacer and/or second spacer are preferably made of a different material than the casting material. Because the path length of the spacer(s) is sufficiently large, any problems associated with non-attachment of the casting material to the spacer(s), such as electrical discharge, are mitigated. Furthermore, the first and second spacers can be made from the same material. Additionally or alternatively, the first and second spacers can be at least substantially identical.

The shortest path length of each path defined by the first or second spacer is preferably at least equal to four times the thickness of the first or second covering layer, respectively, and more preferably 8 times.

The first spacer and/or second spacer may comprise a plurality of elastic members that are interconnected to form a meshed sheet that assumes or maintains a wrinkled state when no external force is applied. A thickness of the meshed sheet when fully flattened is preferably less than the thickness of the first covering layer or second covering layer, respectively.

The elasticity of the elastic members of the first spacer was preferably sufficient, during the casting of the casted body in a casting mold, to keep the plurality of coils spaced apart from a first wall of the casting mold that has defined the first surface by more than the thickness of the meshed sheet, preferably more than 1.5 times said thickness. Additionally or alternatively, the elasticity of the elastic members of the second spacer was sufficient, during the casting of the casted body in a casting mold, to keep the plurality of coils spaced apart from a second wall of the casting mold that has defined the second surface by more than the thickness of the meshed sheet, preferably more than 1.5 times said thickness.

The abovementioned meshed sheet is typically arranged on a bottom surface of the casting mold before the coils are arranged in the casting mold. When the coils are arranged, the meshed sheet is pressed against the bottom surface of the casting mold under the weight of the electrical coils. Flowever, the pressure exerted by the coils is insufficient to cause the meshed sheet to flatten to such an extent that the coils locally touch the bottom surface via only the meshed sheet. If such flattening would occur, the situation may arise that the casting material is unable to reach every region in between the electrical coils and the bottom surface leaving air-filled pockets in the first covering layer. Such pockets would deteriorate electrical performance and could result in electrical discharge. A meshed sheet may also be arranged in between the electrical coils and the upper surface of the casting mold. In this case, the upper and lower part of the casting mold may be pressed together to reduce the space between the walls of the casting mold and the electrical coils. Also in this case, the situation should be avoided in which the meshed sheets become fully flattened.

The tendency to wrinkle can be generated by the elastic members being biased to assume a particular shape that would result, when cooperating with the other elastic members in the sheet, the sheet to wrinkle.

A minimum radius of curvature of the elastic members of the first spacer and/or second spacer is preferably larger than half the thickness of the first or second covering layer, respectively. When the radius of curvature becomes too small, the elastic members may be positioned substantially perpendicular to the first or second surface resulting in a small path length, thereby increasing the risk of electrical failure.

The meshed sheet of the first spacer and/or second spacer can be a woven structure or a non-woven structure, comprising a plurality of polymer fibers. These polymer fibers can be fibers made of one or more materials from the group consisting of olefin, acrylic, nylon, aramid and polyester fibers.

The present invention also provides a method for manufacturing a primary part intended for an electric motor as defined above. According to the invention, the method comprises the steps of providing a casting mold having a first wall that is configured for defining a first surface of a casted body of the primary part, and arranging a first electrically insulating spacer inside the casting mold and on the first wall. The method further comprises arranging a plurality of electrical coils on the first spacer in close proximity to and spaced apart from the first wall, wherein the first spacer contacts the plurality of coils, and forming the casted body of the primary part in which the plurality of electrical coils is mutually fixed by providing a liquid casting material inside the casting mold and allowing the casted casting material to solidify, wherein during said casting, liquid casting material moves in between the plurality of coils and the first wall thereby surrounding the first spacer and forming a first covering layer. According to the invention, the first spacer is exposed on the first surface at a plurality of exposed first positions, wherein the first spacer defines a plurality of first paths that each start at or near a metal part of the plurality of coils, extend solely through the first spacer, and end at or near an exposed first position among the plurality of exposed first positions, wherein, for each exposed first position, each first path ending at or near that position has a shortest path length that is at least equal to two times the thickness of the first covering layer, and wherein the casting material was applied simultaneously to fixate the mutual positions of the electrical coils and to define the first covering layer.

The method may further comprise applying pressure to compress the first spacer during the casting. Applying pressure may be useful for obtaining a small thickness of the first covering layer and may reduce the variation in thickness of the first covering layer. However, the applied pressure should not be too high as this will flatten the spacer thereby decreasing the first path lengths. In such case, an air channel may be formed in between the casting material and the first spacer in which electrical discharge may occur during operation.

The casting mold may have a second wall opposite to the first wall, wherein the second wall is configured for defining a second surface of the casted body of the primary part that is opposite to the first surface. In this case, the method may further comprise the steps of arranging a second electrically insulating spacer inside the casting mold on the plurality of coils in close proximity to and spaced apart from the second wall. During the casting, casting material moves in between the plurality of coils and the second wall thereby surrounding the second spacer and forming a second covering layer. Moreover, the second spacer is exposed on the second surface at a plurality of exposed second positions, wherein the second spacer defines a plurality of second paths that each start at or near a metal part of the plurality of coils, extend solely through the second spacer, and end at or near an exposed second position among the plurality of exposed second positions. For each exposed second position, each second path ending at or near that position has a shortest path length that is at least equal to two times the thickness of the second covering layer, and wherein the casting material was applied simultaneously to fixate the mutual positions of the electrical coils and to define the second covering layer. The first and second covering layers may be substantially identical in terms of material and/or thickness.

Pressure may be applied between the first and second walls to simultaneously compress the first and second spacers during the casting. In this manner, the thickness of both the first and second covering layer can be kept as small as possible, while still enabling good tolerance values to be achieved on the layer thickness.

The casting mold may comprise a first mold part having the first wall and a second mold part having the second wall, wherein the arranging of a first electrically insulating spacer inside the casting mold comprises arranging the first spacer on the first wall of the first mold part while the first and second mold art are decoupled. The arranging of a second electrically insulating spacer inside the casting mold may then comprise arranging the second spacer on the plurality of coils and coupling the second mold part to the first mold part to allow the second spacer to become in close proximity to and spaced apart from the second wall.

It should be noted that the method described above can be used for realizing the electric motor described earlier.

Next, the present invention will be described in more detail, wherein:

Figures 1A and IB illustrate a known iron-less synchronous linear motor;

Figures 2A-2D illustrate a known method for manufacturing the primary part of the motor shown in figure 1A; Figure 3A illustrates an exploded view of a casting mold, electrical coils, and spacers for the manufacturing of a primary part of a motor in accordance with the present invention;

Figure 3B illustrates a detailed view of the spacer of figure 3B; and

Figure 3C illustrates a detailed cross-sectional view of the primary part obtained using the assembly illustrated in figure 3A.

Figure 3A illustrates an assembly for the manufacturing of a primary part of an electric motor in accordance with the present invention. This assembly comprises a first casting mold part 410 and a second casting mold part 420 that can be coupled. More in particular, mold parts 410, 420 can be coupled to each other in a manner that they are still able to translate back and forth relative to each other in a direction indicated by arrow P.

The assembly further comprises a first spacer 431 and a second spacer 432 that is substantially identical to first spacer 431. These spacers, of which one is illustrated in more detail in figure 3B, comprise a meshed sheet for example made of polymer fibers such as fibers made of one or more materials from the group consisting of olefin, acrylic, nylon, aramid and polyester fibers. The meshed sheets can be a woven or non-woven sheet. For example, the meshed sheets can for example be in the form of a tulle made of nylon, polyester or other polymeric or natural material.

Spacers 431, 432 have a tendency to wrinkle due to the elasticity of the fibers. More in particular, when no external force is applied, spacers 431 , 432 will assume or maintain a wrinkled state such as shown in figure 3B. The wrinkles will not completely disappear when subjected to the weight of electrical coils 11.

Mold part 410 comprises an inlet 411 for receiving liquid molding compound and an outlet 412 for outputting liquid molding compound. Furthermore, mold part 410 comprises pins 414 or other alignment structures for aligning electrical coils 11 relative to mold part 410.

To manufacture a primary part, spacer 431 is arranged inside mold part 410. Due to its limited thickness, pins 414 will protrude through spacer 431. Thereafter, electrical coils 11 are arranged inside mold part 410 using pins 414 that will engage an inner space 11A of coils 11. Spacer 432 will then be arranged on top of coils 11 and mold part 420 will be coupled to mold part 410.

Prior to injecting molding compound through inlet 411, compressive pressure will be applied to mold parts 410, 420 for bringing them closer together in the direction of arrow P. When spacers 431 , 432 are sufficiently compressed, but not too much, liquid molding compound is fed to inlet 411. This compound will penetrate spacers 431, 432 to fill the spaces inside the meshed structure thereof and will also penetrate the space between the coils. After solidification, the casted primary part can be removed from mold parts 410, 420. Figure 3C illustrates a detailed cross sectional view of the primary part obtained using the assembly of figure 3A, wherein the vertical scale is enlarged relative to the horizontal scale for illustrative purposes. As shown, electrical coils 11 are embedded in a layer 451 of solidified casting material.

In figure 3C, an exposed position A for spacer 431 is indicated as well as an exposed position B for spacer 432. At these positions, spacers 431, 432 are exposed to the outside as they are not or hardly covered by molding material. For spacer 431, a shortest path A-A’ is indicated, wherein A’ indicates the position where spacer 431 touches a metal part of electrical coil 11.

During casting, spacer 431 may not or not fully adhere to the casting material so that a very narrow air channel may be formed in between spacer 431 and the casting material. During operation, the electric field in this channel, for example caused by a potential difference between electrical coils 11 and a nearby conductive part of secondary part 20, may become too high resulting in electrical breakdown.

According to the invention, the abovementioned risk of electrical breakdown can be mitigated by ensuring that any possible air channel between spacers 431, 432 and the casting material is sufficiently long. As illustrated in figure 3C, this is achieved by the elastic character of spacers 431 , 432, wherein these spacers have a tendency to wrinkle when no external force is applied.

In figure 3C, a polymer fiber of spacers 431, 432 is indicated that displays a wavy form. Due to this form, the thickness of covering layer 450, 452 is higher than the thickness of spacer 431, 432 and more in particular larger than the thickness of the polymer fiber. It is noted that fibers having a circular cross section may display an improved behavior compared to fibers having a more square-like cross section due to the increased length of the path around their outer surface.

As illustrated, the thickness of spacers 431 , 432, when fully flattened, would be less than the thickness of covering layer 450 or covering layer 452, respectively. The fully flattened state could have been achieved if excessive pressure was applied to mold parts 410, 420 during the casting of the primary part. In such case, the path length would substantially equal the thickness of covering layer 450, 452 as a single small fiber segment of spacer 431 , 432 would touch electrical coil 11 while at the same time being exposed on an outside surface 450A, 452A of covering layer 450 or covering layer 452, respectively. However, by using less pressure during casting, spacers 431, 432 are able to maintain their wrinkled state thereby resulting in the desired increase in path length A-A’.

In the abovementioned description, the invention has been explained using detailed embodiments thereof. However, the invention is not limited to these embodiments. Various modifications can be made without departing from the scope of the invention, which is defined by the appended claims and their equivalents.