WO2023209727A1

WO2023209727A1 - "linear induction motor with reduced end-effects"

Info

Publication number: WO2023209727A1
Application number: PCT/IN2023/050383
Authority: WO
Inventors: Satyanarayanan R. CHAKRAVARTHY; Nikhil Yelamarthy; Parth Shah; Shivaprasad U HULYAL; Kishan Thakkar
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2022-04-27
Filing date: 2023-04-20
Publication date: 2023-11-02

Abstract

The present invention relates to a linear induction motor (LIM) for efficiently mitigating end-effects and improving thrust characteristics. The LIM comprises a primary core (102) having a plurality of teeth (104) disposed in spaced relationship along a longitudinal direction of the primary core (102). At least one tooth (106) of the plurality of teeth (104) comprises a plurality of protrusions (108) stepped in a curved profile having a convex shape. At least an outer protrusion (110) of the plurality of protrusions (108) is formed as a cubic spline using a set of two piecewise cubic functions by interpolation on three distinct points.

Description

LINEAR INDUCTION MOTOR WITH REDUCED END-EFFECTS

FIELD OF INVENTION

[0001] The present invention relates to an electromagnetic device, and more particularly to a linear induction motor for efficiently reducing end-effects and improving thrust characteristics.

BACKGROUND OF THE INVENTION

[0002] The subject matter discussed in the background section should not be assumed to be prior art merely because of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may correspond to implementations of the claimed technology.

[0003] Electromagnetic devices are devices that contain electromagnets. An induction motor, also commonly referred to as rotary induction motor, is an electromagnetic device in which electric current in a rotor needed to produce torque is obtained by electromagnetic induction from a magnetic field of a stator winding. The induction motor can therefore be made without electrical connections to the rotor. Three-phase induction motors are widely used as industrial drives because they are self-starting, reliable, and economical. Singlephase induction motors are used extensively for smaller loads.

[0004] Linear induction motors (LIM) work on the same general principles as rotary induction motors, and are designed to produce linear thrust. Uses of LIMs include magnetic levitation, linear propulsion, linear actuators, and liquid metal pumping, to name a few. LIMs also provide contactless propulsion and eliminate friction between metal-to-metal surfaces. A typical LIM is a rolled-out version of the rotary induction motor. LIMs are becoming more popular given their direct use of electric power to produce motion and, more interestingly, the absence of rotating or moving parts within the motor as for the more widely used rotary motors. Given their shape and the linear thrust they naturally produce, LIMs are commonly used in electric trains, conveyor belts, cranes and are a promising option for the Hyperloop, a high-speed sustainable alternative to commercial flights

[0005] LIMs typically include a primary core, which is a linearly extending piece of magnetic material. The primary core is often made of magnetic steel with coils or windings installed therein. A current is injected into the coils to produce a magnetic field passing through each of the coils. The primary core is designed to move linearly above another linearly extending piece of electrically conductive material, commonly referred to as the secondary core. If the primary core is fixed, the secondary core may move along the length of the primary core. The electromagnetic interaction between the coils on the primary core and the induced current in the secondary core propel the primary core forward along the secondary core. However, unlike rotary induction motors, LIMs suffer from end-effects, which causes motor de-fluxation and dragging force resulting from finite length of such motors. This phenomenon of end-effects also causes reduction in thrust force generated by the LIM.

[0006] Efforts have been made in the past to mitigate end-effects in LIMs by adding a component for balancing the end-effects using a controller, as proposed in H. O, S. L and Lei Zhang, “Adaptive Twisting Controller for Linear Induction Motor Considering Dynamic End Effects” , IEEE, 2018. Another literature art, i.e., Y. D. A. Boduroglu, R. Lyra and M. Aydin, “Influence of Auxiliary Teeth on Performance of a Permanent Magnet Linear Motor”, XIII International Conference on Electrical Machines (ICEM), 2018, propose deploying auxiliary teeth in the primary core to mitigate the end-effects. However, despite such efforts thrust force generated by LIMs is still not well managed, and encounters several disturbances. On the other hand, there was a study conducted on tooth modification in the primary core, which was described in Syafitri, Ainil, MK Iwa Garniwa, and Ridwan Gunawan. “Investigation of Reducing End Detent Force by Modification Design of End Teeth for Linear Induction Motor with Ansys” 2020 IEEE International Conference on Sustainable Engineering and Creative Computing {ICSECC). IEEE, 2020. However, the aforesaid study is limited to two-dimensional (2D) simulation. Also, the amount of increase in the thrust force was not mentioned in the above studies, and the effect of teeth modification on phase imbalance has not been explored. [0007] There is therefore a need in the art for techniques capable of increasing performance of the LIM while efficiently reducing the end-effects in a simple and reliable manner, without the requirement of complex mechanisms and components.

OBJECT OF THE INVENTION

[0008] The object of the present invention is to provide a linear induction motor (LIM) that efficiently mitigates end-effects characteristic while improving linear thrust generated by the LIM.

[0009] Another object of the present invention is to provide an LIM capable of preventing sudden drop of magnetic field generated by a primary core thereof.

SUMMARY OF THE INVENTION

[0010] The summary is provided to introduce aspects related to a linear induction motor (LIM), and the aspects are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter, nor is it intended for use in determining or limiting the scope of the claimed subject matter.

[0011] An aspect of the present invention relates to the LIM including a primary core having a plurality of teeth disposed in spaced relationship along a longitudinal direction of the primary core. At least one tooth of the plurality of teeth comprises a plurality of protrusions stepped in a curved profile having a convex shape. At least an outer protrusion of the plurality of protrusions is formed as a cubic spline.

[0012] According to an embodiment of the present invention, the at least one tooth is positioned at an end portion of the primary core.

[0013] According to an embodiment of the present invention, the LIM includes a plurality of coils, at least one coil being wound to each tooth of the plurality of teeth. The primary core generates a linearly moving magnetic field in the longitudinal direction when the plurality of coils are excited by a three-phase Alternating (AC) Current. [0014] According to an embodiment of the present invention, the LIM also includes a secondary core formed of an electrically conductive metal plate.

[0015] According to an embodiment of the present invention, eddy currents are induced in the secondary core when the secondary core is positioned in vicinity of the plurality of coils, and wherein a magnetic flux of the primary core and the induced eddy currents of the secondary core interact to impart a linear thrust to the secondary core when the primary core is fixed. The linear thrust may be imparted to the primary core by fixing/restricting motion of the secondary core.

[0016] According to an embodiment of the present invention, at least one coil of the plurality of coils is accommodated on the plurality of protrusions stepped in the curved profile.

[0017] According to an embodiment of the present invention, the plurality of protrusions are stepped in the curved profile having the convex shape to protrude the magnetic field outwards and reduce sudden drop in the magnetic flux at the end portion of the primary core.

[0018] According to an embodiment of the present invention, the outer protrusion is formed as the cubic spline using a set of two piecewise cubic functions by interpolation on three distinct points.

[0019] According to an embodiment of the present invention, the LIM is double-sided linear induction motor.

[0020] Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example, the principles of the invention.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0021] The accompanying drawings constitute a part of the description and are used to provide a further understanding of the present invention. Such accompanying drawings illustrate the embodiments of the present invention used to describe the principles of the present invention. The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this invention are not necessarily to the same embodiment, and they mean at least one. In the drawings:

[0022] Fig. 1 illustrates a perspective view of a primary core of a linear induction motor (LIM), in accordance with an embodiment of the present invention;

[0023] Fig. 2 illustrates an exemplary representation of a plurality of teeth of the primary core arranged in a stepped manner in a curved profile having a convex shape, in accordance with an embodiment of the present invention;

[0024] Fig. 3 illustrates a graphical representation of magnetic flux density across length of a conventional LIM in accordance with prior art;

[0025] Fig. 4 illustrates a graphical representation of the magnetic flux density across the length with the LIM, in accordance with an embodiment of the present invention;

[0026] Fig. 5 illustrates a graphical representation of linear thrust force a of the primary core generated by the LIM of the present invention compared with the conventional LIM, in accordance with an embodiment of the present invention; and

[0027] Fig. 6 illustrates a perspective view of a testing device for testing linear thrust characteristics of the LIM, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

[0029] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0030] The present invention provide a linear induction motor (LIM) capable of being used for applications such as magnetic levitation, linear propulsion, linear actuators, and liquid metal pumping, etc. The LIM is configured with an optimized teeth design for reducing end-effect characteristics and preventing abrupt drop in magnetic field generated thereof. The LIM includes a primary core having a plurality of teeth arranged in a spaced relationship with each other, and a secondary core formed of an electrically conductive metal plate. The primary core is made of a magnetic material. The secondary core is typically made of two layers, one made of aluminium at the top and another made of magnetic iron at the bottom. A plurality of coils are wound on the teeth of the primary core. The primary core generates a linearly moving magnetic field in its longitudinal direction when the plurality of coils are excited by a three-phase Alternating (AC) current. To this effect, eddy currents are induced in the secondary core when the secondary core is positioned in vicinity of the plurality of coils. The electromagnetic interaction between the coils on the primary core and the induced eddy currents in the secondary core generate a linear thrust which propels the secondary core along a length-wise direction of the primary core when the position of primary core is fixed. The LIM of the present invention improves linear thrust characteristics while mitigating the end-effects.

[0031] Fig. 1 illustrates a perspective view of the primary core 102 of the LIM. The teeth 104 of the primary core 102 are arranged in longitudinal direction of the primary core 102, and are spaced apart from each other such that gaps are formed therebetween. Design of at least one tooth 106 located at an end portion of the primary core 102 is optimized to efficiently reduce the end-effects experienced due to finite length of the primary core 102, and increase the linear thrust generated by the LIM. [0032] Referring now to Fig. 2, where a schematic representation of the teeth 104 disposed on the primary core 102 is shown. The tooth (also referred to as “end-tooth” hereinafter) 106 located at the end portion of the primary core 102 includes of a plurality of protrusions 108 arranged in a stepped manner so that height a protrusion is larger than the height an adjacent protrusion. The protrusions 108 are formed in a curved profile having a convex shape to allow the magnetic field generated in the primary core 102 to protrude outwards, and make a drop in the magnetic field less abrupt. At least an outer protrusion 110 of the protrusions 108 is formed as using a cubic spline function, i.e., a set of two piece- wise cubic functions formed by interpolation on three points.

[0033] The shape of the end tooth 106 is optimized in such a way that change in magnetic flux generated when the coils are excited in three-phase AC current at the end portion of the primary core 102 is less abrupt. The coils are usually inclined (i.e., they have a pitch) and are wound to the teeth 104 of the primary core 102. Flux lines are known to take the path of least reluctance, and such path can be changed by optimizing the shape of the end tooth 106. Upon trying different shapes for the end tooth 106, it was observed, that flux drop was most gradual when the end tooth 106 having the protrusions 108 was curved in a convex shape. A step-like structure of the protrusions 108 was also incorporated on the curvature in order to hold the coils. Specifically, at least one coil is accommodated on the protrusions 108 arranged in the stepped manner in the curved profile of the end tooth 106. With the convex curvature the end tooth 106, the magnetic field automatically protrudes outwards, and the drop in the magnetic field occurs more gradually. Phase Imbalance in electrical wires is another consequence of end-effects which leads to negative sequence currents formed in the motor. Since the optimized design of the end tooth 106 efficiently mitigates the end-effects, the phase imbalance also reduces substantially.

[0034] During testing, parameters of the LIM were optimized by performing simulations in COMSOL Multiphysics. Figs. 3 and 4 show plots obtained from COMSOL Multiphysics in order to compare behaviour of magnetic flux density values along a length of the motor in a conventional LIM and in the LIM of the present invention having the convex curvature of the end tooth 106, keeping all other geometric parameters same. It was observed that the drop in the magnetic flux density is more gradual in the LIM of the present invention, as shown in Fig. 4. In the LIM of the present invention, the protrusions 108 are stepped in the curved profile of the end tooth 106 having the convex shape to protrude the magnetic field outwards and reduce sudden drop in the magnetic flux at the end portion of the primary core 102.

[0035] Fig. 5 shows a plot depicting an increase in linear thrust force generated by the LIM of the present invention when compared with the conventional LIM operating at the same velocity. An increase of 8-10% in the average thrust force was witnessed when the LIM of the present invention was compared to the conventional LIM, keeping all the other parameters constant throughout the testing.

[0036] In an implementation, a plurality of primary cores 102 of the LIM may be installed under floor of a railway wagon, and a long secondary core may be used a fixed railway track. The primary core 102 has the optimized end tooth 106. The coils wound on the teeth 104 of the primary core 102 including the end tooth 106. The primary core 102 generates a linearly moving magnetic field when the coils are excited by a three-phase AC current. Eddy currents are induced in the secondary core when the secondary core is positioned in vicinity of the coils. The electromagnetic interaction between the coils on the primary core 102 and the induced eddy currents in the secondary core generate a linear thrust which propels the primary core 102 along a length-wise direction of the secondary core. To this effect, the railway wagon moves linearly along the secondary core. Positioning of the primary core 102 and the secondary core may also be interchanged to obtain desired motion of the railway wagon. The primary cores 102 may be fixedly positioned as the railway track, and the secondary core may be installed under floor of the railway wagon. The electromagnetic interaction between the coils on the primary core 102 and the induced eddy currents in the secondary core generate the linear thrust which propels the secondary core along a length-wise direction of the primary core 102. Also, with the optimized design of the end tooth 106, the magnetic field generated by the primary core 102 protrudes outwards, and the drop in the magnetic field occurs more gradually. The LIM of the present invention may be a double-sided linear induction motor. Each side of such LIM may have a size of 79.2 cm in length, 6.7 cm in breadth, and 5.7 cm in height. Moreover, each side of such LIM may weigh approximately 6 kg. [0037] Fig. 6 illustrates a perspective view of a testing device 600 for testing linear thrust characteristics of the LIM. The testing device 600 includes an aluminium wheel 602 which is free to rotate around its axis. The LIM 604 having the optimized design of the end tooth 106 was fixed on either side of the wheel 602 and was used to accelerate the wheel 602. By measuring angular acceleration of the rotating wheel 602, the linear thrust provided to the wheel 602 by the LIM 604 was measured. It was observed that the LIM 604 having the optimized design of the end tooth 106 generated an increased linear thrust compared to a conventional LIM. An increment of 8-10% in the average thrust force was witnessed when the LIM 604 of the present invention was compared to the conventional LIM using the testing device 600, keeping all the other parameters constant throughout the testing.

[0038] The LIM of the present invention is of simple configuration which eliminates the requirement for complicated equipment to reduce the end-effects and increase thrust force generated therefrom. The LIM also mitigates phase imbalance in electrical wires that may occur due to the end-effects, which leads to negative sequence currents formed in the conventional LIM, thereby improving motor performance.

[0039] In view of the present disclosure, which describes the present invention, all changes, modifications and, variations within the meaning and range of equivalency are considered within the scope of the invention. It is to be understood that the aspects and embodiment of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiment may be combined together to form a further embodiment of the disclosure.

Claims

CLAIMS:

1. A linear induction motor (LIM), comprising: a primary core (102) having a plurality of teeth (104) disposed in spaced relationship along a longitudinal direction of the primary core (102), wherein at least one tooth (106) of the plurality of teeth (104) comprises a plurality of protrusions (108) stepped in a curved profile having a convex shape, and at least an outer protrusion (110) of the plurality of protrusions (108) is formed as a cubic spline.

2. The LIM as claimed in claim 1, wherein the at least one tooth (106) is positioned at an end portion of the primary core (102).

3. The LIM as claimed in claim 1, further comprising a plurality of coils wound to the plurality of teeth (104).

4. The LIM as claimed in claim 3, wherein the primary core (102) generates a linearly moving magnetic field in the longitudinal direction when the plurality of coils are excited by a three-phase Alternating (AC) Current.

5. The LIM as claimed in claim 1, further comprising a secondary core formed of an electrically conductive metal plate.

6. The LIM as claimed in claim 5, wherein eddy currents are induced in the secondary core when the secondary core is positioned in vicinity of the plurality of coils, and wherein a magnetic flux of the primary core (102) and the induced eddy currents of the secondary core interact to generate a linear thrust.

7. The LIM as claimed in claim 3, wherein at least one coil of the plurality of coils is accommodated on the plurality of protrusions (108) stepped in the curved profile.

8. The LIM as claimed in claim 7, wherein the plurality of protrusions (108) are stepped in the curved profile having the convex shape to protrude the magnetic field outwards and reduce sudden drop in the magnetic flux at the end portion of the primary core (102).

9. The LIM as claimed in claim 1, wherein the outer protrusion (110) is formed as the cubic spline using a set of two piecewise cubic functions by interpolation on three distinct points.

10. The LIM as claimed in claim 1, wherein the LIM is double-sided linear induction motor.