Technological Challenges for a 60 m Long Prototype of Switched Reluctance Linear Electromagnetic Actuator

Abstract

In this research project a large linear electromagnetic actuator (LLEA) was designed and manufactured. The electromagnetic performance was published in previous works, but in this paper we focus on the technological challenges related to the manufacturing in particular. This LLEA was based on the magnet-free switched-reluctance principle, having six effective energised stator “teeth” and four passive mover parts (4:6 ratio). Various aspects and challenges encountered during the manufacturing, transport, and assembly are discussed. Thermal expansion of steel contributed to the decision of the modular design, with each module having 1.3 m in length, with a 2 mm longitudinal dilatation gap. The initial prototype was tested with a 10.6 m length, with plans to extend the test track to 60 m, which was fully achievable due to the modular design and required 29 tons of electrical steel to be built. The stator laminations were cut by a bespoke progressive tool with stamping, and other parts by a CO₂ laser. Mounting was based on welding (back of the stator) and clamping plates (through insulated bolts). The linear longitudinal force was on the order of 8 kN, with the main air gap of 7.5–10 mm on either side of the mover. The lateral forces could exceed 40 kN and were supported by appropriate construction steel members bolted to the concrete floor. The overall mechanical tolerances after installation remained below 0.5 mm. The technology used for constructing this prototype demonstrated the cost-effective way for a semi-industrial manufacturing scale.

1. Introduction

Electromagnetic actuators rely on magnetic forces generated by their appropriate magnetic circuits driven by electric power. As in other electromagnetic devices (e.g., rotating machines), magnetic cores are typically used for optimising the force performance and energy efficiency. There is an implicit need for a non-negligible air gap between the parts of the magnetic circuit so that the relative mechanical movement can be facilitated, and this air gap typically represents the highest magnetic reluctance in the system [1,2,3]. For this reason, the design of a magnetic circuit is usually optimised by at least minimising the effective reluctance of the system, which can be achieved by several means, not only by reduction in the air gap length. For compact or miniature actuators, the whole magnetic core or its main component (such as the stationary “stator” and the moving “mover”) can be made as separate single pieces of bulk metal or as appropriately shaped parts made of soft magnetic composites. For larger devices, in the case of laminated cores, all the laminations can have the same (or very similar) shape and can be conveniently stamped out as a single piece of sheet, which are later stacked to the required height of the stack [3,4]. Despite significant progress in recent years in manufacturing technologies, including CNC (computer numerical control) laser cutting or additive manufacturing, the stamping process is still the most cost-effective for large-volume production of parts with the same shape, as used extensively, for example, in the electric motor industry [4].

However, magnetic cores of very large devices cannot be easily stamped out as single pieces [4]. For large rotating machines made from non-oriented electrical steel (NOES) or grain-oriented electrical steel (GOES), the directly limiting factor is the available manufacturable width of the electrical steel strip, which is on the order of 1.2 m [4]. For other devices such as large linear actuators, the limitations can be different, especially if the laminations can be manufactured within the available width of the electrical steel strip. Several practical technological difficulties of manufacturing magnetic cores for one example of a large linear actuator are discussed in this paper.

In this paper, Section 2 includes an overview of the project, and Section 3 and its several sub-sections discuss the main technical and technological challenges encountered during manufacturing of the prototype modules.

2. Large Linear Electromagnetic Actuator (LLEA)

This two-part research project was conducted towards designing, building, and testing the performance of a large linear electromagnetic actuator (LLEA). The data regarding its intended use, electrical circuit design, and performance were published before by the other research partner, Zeleros (Spain) [5,6,7], so only a brief description will be given here. This communication-type article focuses solely on the manufacturing technology challenges for such large actuators, as carried out by the first research partner, Magneto (Poland), and for this reason the technical details such as performance, losses, efficiency, etc., are not discussed hereinbelow.

The LLEA was aimed at providing a more sustainable mode of transport within the port for standard shipping containers, which are large and heavy objects with a length of up to 12 m and a weight of up to 30 tons (freight forwarding). In this particular solution, the vertical weight of these shipping containers was to be supported on a railway-like wheeled system, but the total linear thrust was provided by the LLEA. The prototype was planned and designed to be up to 60 m long for the purpose of assessing its acceleration and deceleration capabilities so that a realistic practical weight-under-test could be accelerated to the required speed, as well as decelerated solely with electromagnetic means, safely over the whole length of the test track [5,6,7].

Due to the practical length limitations, the magnetic core was designed to be modular. This was mostly dictated by the practicality of several technological limitations as discussed in the following sections, such as the lamination stamping, the required assembly on-site, the transport issues, etc. This is because the final prototype was tested at the Port of Valencia in Spain, but the laminations were manufactured in Częstochowa, Poland, so up to 29 tons of magnetic core would have to be shipped between the two countries.

Because of the sheer volume of the required magnetic material, the cost of the laminations proved to be a rather important factor. For this reason, despite the fact that improved energy efficiency would be advantageous, the final choice of the material was NOES grade M400-50A (i.e., 0.5 mm thick laminations of non-oriented steel).

The first part used for the initial experimental tests had a length of 10.6 m (constructed from eight modules, each with a length of 1.3 m), as shown in Figure 1. The testing was carried out by the research partner Zeleros, who decided to begin the initial tests on a shorter part of the stator (Figure 1), with the later plans to extend the prototype to the full length (up to 60 m), so that the full expected longitudinal speed could be investigated, including acceleration and deceleration solely by electromagnetic means. The detailed electromagnetic design, efficiency estimation, power supply design and control, and performance testing were carried out as a separate part of the project (carried out independently by Zeleros), and therefore the details of their investigation are not included in this communication.

The operating principle of the LLEA followed the switched-reluctance motor approach, with the 4:6 ratio of rotor/stator teeth. The stator was excited with a high-frequency converter configured for switching three-phase winding configuration (Figure 2a) and the mover provided the passive magnetic reluctance. No permanent magnets were used in this construction.

The operating principle of this LLEA prototype is illustrated in Figure 2. One stator module (1.3 m long) comprises six “teeth” with the magnetising coils positioned on them. This configuration provides in effect a three-phase supply (phases 1-2-3), and facilitates control over the direction of movement (forward or reverse) along the main longitudinal axis. Each phase can be energised in a sequence so that the subsequent mover sub-parts can be magnetically attracted in the desired direction of movement (forward or reverse).

The mover comprises four sub-parts mechanically coupled. These are passive and are attracted to the stator teeth, according to the applied magnetising current. This ratio of 4:6 is akin to those used in synchronous switched-reluctance motors [1]. As shown in Figure 2a, the four sub-parts of the mover constitute one “module” in the same sense as the six-teeth-long stator is one module. Each coil can be energised individually, and therefore a proportionally larger actuating force could be achieved by combining several mover modules together. However, all the investigation as presented herein was limited to just one module.

The design based on the switched-reluctance actuating principle is known to be very robust and capable of providing high-force density [3,8,9,10,11,12]. Extremely large “actuators” have been built for the levitating trains (MagLev technology), but some of these may employ permanent magnets and/or superconducting coils [9,12], making such devices expensive. Switched-reluctance design is also known to generate high acoustic noise [13], which was also the case for the prototype presented in this paper. However, the discussion of the performance parameters is beyond the scope of this paper, which focuses only on the manufacturing technology and the difficulties associated with it.

The FEM simulation shown in Figure 2c is included here just as an example of the possible static forces that are achievable in such a configuration. The exact excitation conditions were not finalised yet by the research partner, as there were several other optimisation constraints that were still being investigated. However, the simulation in Figure 2c is of the magnetostatic kind, with the excitation applied as 14.6 kA turns per each single coil. The coils in the final prototype (Figure 1) were wound from flat aluminium conductors and had the overall shape as illustrated in Figure 2a.

At the beginning of the design process, the force performance of the construction was still being optimised, as is expected for such experimental prototypes. The main functional parameter was the longitudinal force, which could be tested with relative ease on a single module. This was prototyped and tested as illustrated below in Figure 4 for prototypes A and B. The static force testing revealed several shortcomings, as also described below (magnetic saturation, unsatisfactory force level unsatisfactory lamination mounting), which were rectified in the final design of prototype C, which is shown in Figure 1.

3. Manufacturing Difficulties

3.1. Thermal Expansion

A contiguous metallic object exhibits a coefficient of thermal expansion (CTE), which must be taken into account even for purely mechanical reasons. The same problem exists for ordinary railway tracks, which can be irreversibly damaged by the mechanical buckling process due to excessive heat in direct sunshine in hot summers.

In railway tracks this problem is solved in several ways, for example, by providing suitable dilatation gaps that can accommodate the maximum thermal expansion expected in its nominal operating conditions.

The prototyping track was installed in the Port of Valencia (south-east cost of Spain) and would have to operate in the rather hot summers of Spain, possibly in direct sunshine, which can heat up metal structures up to 90 °C (depending on the emissivity of the surface and the cooling conditions). Therefore, the design of the magnetic core had to take into account not only the maximum operating temperature of the system but also the actual temperature on the day of installation so that the appropriate expansion in the dilatation gaps could account for not only the hot summer but also the corresponding contraction during a cold winter.

The CTE of electrical steel is on the order of 10 μm/m per °C [14]. Therefore, in the worst-case scenario, with the highest temperature of +90 °C (direct sunshine in the summer) and the lowest of −10 °C (winter), the maximum span could be 100 °C, which could mean a linear expansion of up to 10 mm for a 10 m long magnetic core. However, the stator core was planned to be installed underground, without direct sunshine reaching it. So for this reason, it was decided that from the thermal viewpoint, the best compromise of performance, manufacturability, and thermal properties was achieved when the magnetic core was to be segmented and made from 1.3 m long parts, with appropriate dilatation spacing between the segments in the longitudinal direction. Taking into account precision of cutting, mounting, vibrations, and additional movements, the lateral dilatation gap was set as 2 mm (at the day of installation).

This gap was proved to be sufficient because each module had enough room to expand with respect to its neighbour modules. As checked experimentally and by further calculations, the temperature changes between 15 °C (night in the spring) and over 50 °C (afternoon in the summer) resulted in a sufficient remaining gap even if the temperature were to rise more. Therefore, this solution was technologically viable for any given length of the stator, starting from the initial 10.6 m (housed in a white tent; see the white wall in the background of Figure 1) to the final 60 m (which possibly could be still exposed to direct sunshine), as required by the research partner.

3.2. Block Length

Precise stamping of large shapes is relatively expensive because of the size of the punch and die that would have to be employed for a direct, single-shape stamping. As an extreme example, if the thermal expansion was not a problem, then stamping magnetic core laminations that were 60 m in length would have been impossible to be made on a single flat punch/die (as originally planned for the length of the test track). However, even the much shorter 1.3 m pieces would still require large and thus very expensive tooling, which could not be easily altered if the design of the magnetic core was to be optimised. And since such optimisation was expected (and indeed happened), a different manufacturing strategy was devised.

An alternative approach used in prototyping of devices such as rotating machines is typically to employ other techniques such as laser cutting, but this is performed on a much smaller scale, so it is still commercially viable [4]. In this case the LLEA was designed in the “heavy stator” configuration, and therefore it would require up to 29 tons of laminated core to be produced for the whole planned length of the actuator. This meant that laser cutting would be uneconomical and could not be justified in this research project.

Instead, the cutting was realised with a repurposed progressive stamping machine, which required only moderate redesign and tooling costs to an already existing manufacturing process as used by the company Magneto in Poland. The punch/die system was designed to stamp one “coil slot” at a time, but this was tracked by an appropriate feeding system for the “infinitely” long electrical steel tape, which was also synchronised with a secondary punch/die tool to introduce the mounting apertures in the laminations (Figure 3).

With this system the precision and repeatability of dimensions of cutting of the slots/teeth and the mounting apertures was sufficiently good for the required magnetic circuit. This was defined directly by the dimension of the punch/die tooling, and, therefore, reproducibility at the level of 0.025 mm was possible for the coil slot itself.

However, the relative positioning for each stator tooth/slot with respect to the start/end edges of each block lamination was dictated by the accuracy of the feeding system, and it was at the compound worst-case level of 0.5 mm. The main magnetic lateral air gap on each side (between the stators and the movers) was relatively large, between 7.5 and 10 mm (various configurations and gaps were considered during trials, Figure 4), and thus the aforementioned precision of cutting was deemed to be perfectly acceptable from the engineering viewpoint. It should be noted that the dilatation gaps of 2 mm were significantly smaller than the main air gap and thus affected the reluctance to a relatively small degree, acceptable in this case.

Further improvements to the reluctance of the dilation gaps could be easily introduced (such as angled overlap, to increase the effective cross-sectional area of this air gap), but they were not implemented in this case due to other reasons such as costs.

This modular approach to manufacturing technology proved to be an optimum one in the considered process, because the first prototype was found to exhibit unsatisfactory performance from the viewpoint of achieved forces and the weight/cost of the stator. Therefore the design of the stator core had to be adjusted (various dimensions for the stator and the mover), which obviously meant changes in dimension that had to be manufactured, as well as the assembly technology. The same manufacturing tolerances were achieved in both cases (Figure 4).

In 2D and 3D finite-element simulations (FEM), the final prototype C exhibited a peak longitudinal force of 8 kN. These designs were optimised by including the active cross-sectional area of the magnetic cores, the available room for the stator windings, the longitudinal and lateral forces, and the mechanical strength, as well as the overall cost of the prototype, which required several tons of electrical steel.

3.3. Mover

Because the LLEA was of the “heavy stator” type, the amount of magnetic core required for the mover was much smaller, with the total required steel weight at the level of 100 kg (total for all four magnetic sub-parts of the mover). At any given location only four sub-movers, or four equivalent “teeth”, would be fixed to a given “trolley” (see also Figure 2). For long trolleys, multiple mover sections could be combined.

Hence, for the prototyping, it was much more cost-effective indeed to employ laser cutting, even though all the laminations of the mover had exactly the same dimensions (Figure 5). This was caused directly by the fact that laser cutting for such a small volume was less expensive than creating another punch/die tool, which would be used only for a very limited volume of steel.

The mover’s “teeth” were also made from laminated steel, but the direction of lamination was orthogonal to that of the stator. The way the forces acted (laterally and longitudinally with respect to the axis of movement) meant that the stator laminations were mostly self-supporting against these forces, which acted within the plane of these laminations. However, this was not the case for the mover, in which the main actuating force was acting perpendicularly to the plane of the lamination, and thus unwanted bending out of the sheet plane could occur. For this reason, the front and the back of each mover pack were supported by 10 mm thick solid steel whose main task was to transfer the mechanical force from the magnetic core of the mover to the trolley to be moved. These thick steel parts were also cut with the help of a CO₂ laser, as provided commercially by an outsourced industrial process.

3.4. Longitudinal Forces

The stator laminations could largely support themselves against the longitudinal forces and could withstand the lateral forces as such. But the whole modules of the stator cores had to be mounted on suitable support members, which had to exhibit sufficient rigidity against all the forces under worst-case scenario conditions (including fault currents or full mechanical/magnetic asymmetry). These support members were manufactured from cut and welded steel shapes, equipped with appropriate mechanisms for adjusting the positioning as required by thermal expansion limitations. These external stator support members are visible in Figure 1 (painted dark blue).

3.5. Lateral Forces

The lateral forces could be significantly greater, and as is well known for linear motors, these can be on the order of even up to 30× higher than their longitudinal counterpart [9], and indeed they were higher for this prototype too, albeit to a smaller degree, corresponding roughly to a factor of 5× (Figure 6). Because of the symmetry of the air gap around the mover, the lateral forces acting on the mover were compensating each other to a large degree, and under nominal operating conditions these remained much smaller than the main actuating longitudinal force. However, any asymmetry in the main air gap was introducing asymmetry to the magnetic circuit, and thus the lateral force was increasing in the direction of the smaller air gap.

The mounting members were bolted down to a concrete bed by means of suitable bolts embedded in the concrete floor (as visible in Figure 1). The mounting, especially for the lateral positioning, had to be adjusted precisely so that an appropriate working air gap was maintained during the actuation process. This was reflected by the top and bottom stator compressing plates (visible in Figure 3d and Figure 4b), which also had to be manufactured by the laser cutting process. A special bespoke precise adjusting tool was created so that repeatable settings could be attained for each module.

However, the lateral forces were acting directly on the sides of the stator, and each side could not be compensated, and thus the whole structure (supporting members, magnetic cores, windings) had to withstand the appropriate level of force. Fortunately, the force acting on the coils of the windings was mostly pressing the winding against the magnetic cores, so that little mechanical support was required for the windings themselves.

But the full force lateral force acting on the magnetic core had to be counteracted by the supporting members, bolted to the concrete floor (Figure 1). It should be noted that the stator core was mounted at some height off the floor level, and therefore there was a large torque acting on the effective arm length of the support members. It was therefore necessary to ensure that the lateral deflection was minimised to an acceptable level so that the main air gap remained within an acceptable tolerance of 7.5 mm +/- 0.5 mm (as accepted for the final prototype). The sufficient rigidity of the structure was maintained through the top/bottom clamping plates as well as through the insulated clamping bolts, as indicated in Figure 2.

3.6. Lamination Clamping—Back

At the back of the stator core there was a practically negligible magnetic flux leakage; therefore, the laminations could be held together by a number of suitable means. The practical method that was utilised in this case was welding (the positions of the vertical weld lines are indicated in Figure 2a and visible in Figure 3d), because there was no danger of introducing unwanted eddy currents and thus additional power loss. Such methods are also used in other electromagnetic devices such as rotating machines, where the weld lines can be placed on the back side of the stator core [4].

3.7. Lamination Clamping—Front

Welding is a very useful mechanical mounting process, but it could not have been used from the tooth-side of the laminations because, by definition, the main flux was penetrating those areas, and the short-circuited turn would prove disastrous to the operation of the device and the power loss. A few approaches were considered during the design process for the side of the stator teeth, including providing appropriate notches or mounting holes with isolated bolts. However, in all such cases the effective cross-sectional area of the teeth would have been reduced, which would negatively impact the operating parameters such as the magnetic saturation and thus the achievable actuating force. This would also complicate slightly the stamping tool, but it could still be manufactured with relatively little difficulty.

One of the considered and prototyped solutions is shown in Figure 4b. The stator laminations were packed and clamped by the 10 mm plates from top and bottom (visible in Figure 3d and Figure 4b). The individual laminations were held together in the pack by means of adhesive forces, by impregnating the core (from the laminations edges) with a low-viscosity resin. However, this approach proved to be completely unsuitable, because due to the resulting static forces as well as vibrations due to dynamic forces, some individual stator laminations could be pulled out from under the clamping plates. This core was used for quantifying the static forces achievable in such a design, but even for this testing the laminations had to be held in place by additional holding strips/belts, which are visible on the left-hand side of the picture in Figure 4b. So ultimately the practicality of this solution was disproved.

The actual solution used in the prototype was to rely partially on the weld lines at the back, the top-bottom clamping plates, and on the excitation coils themselves, which were wrapped around the teeth and provided sufficient operating strength for keeping the laminations together.

3.8. Mounting Apertures

The laminations could be kept together at the back by weld lines, but a similar method could not be used to provide the required strength for the lateral forces, especially in view of the large vibrations that could be expected with fast transit of the moving trolley, with the forces for one module exceeding 8 kN for the longitudinal force and even more for the lateral (depending on the air gap). Instead, it was decided to use additional mounting apertures, positioned towards the back of the stator core (20 mm from the back edge, as visible in Figure 3c,d). These apertures were used for mounting with M16 bolts (16 mm diameter), but through isolating tubes so that eddy currents would not be generated due to short-circuited turns, neither through the remainder of the metallic structure (clamping plates and the supporting members) nor between the laminations themselves (due to the welds present at the back).

4. Summary

The modularity of such a long stator core was necessitated strongly by the thermal expansion coefficient of electrical steel used for this project. The length of the stator blocks was set at 1.3 m, which resulted in the best compromise between the overall cost of the solution, precision of manufacturability, transportation challenges, tolerances of installation, and also flexibility of prototyping, with the test track starting at 10.6 m but also able to be expanded to 60 m (and beyond if necessary). The semi-industrial manufacturing scale posed different challenges than either small-scale, small-volume prototyping or mass-market manufacturing. The target application for this prototype was for investigating electrification of freight forwarding of shipping containers within a seaport.

As demonstrated in this project, the modular approach proved itself in this case because of the flexibility for providing a shorter test track for the initial testing and a much longer one for further performance tests without the need for significant changes in the manufacturing method. The achieved tolerances were commensurate with the employed technological processes and were perfectly sufficient for the purposes of this very large linear electromagnetic actuator.

Despite relatively large dimensional changes (as compared to other tolerances), the thermal expansion coefficient was taken into account by designing in and providing sufficient dilatation gaps in the modular stator core assembly. The modular approach provided the required flexibility of the redesign and facilitated relatively low-cost changes in the manufacturing tools required for stamping and cutting the electrical steel laminations for the more challenging parts of the stator cores. The mover parts were manufactured using a different process of laser cutting because the total volume of electrical steel was much smaller. The large longitudinal and lateral forces had to be supported by the main steel construction members, but they also had to be borne and maintained through the lamination clamping and bolting solutions. Glueing the laminations proved to be insufficient during early experimental trials, and a more reliable solution had to be employed, such as welding (stator) or insulated bolting (stator and mover).

The length of the test track was designed and could be extended to 60 m and beyond, pending further force performance experiments and requirements of the project. The remaining economical and thus practical challenge for devices of this type is the high cost of the stator, due to the large volume of relatively expensive electrical steel laminations.