Linear Electric Motors for Aerospace Launch Assist

The first half of the paper is concerned with the fundamental differences between linear induction motors and their more conventional rotating counterparts. In particular, the effects of the discontinuities which are necessarily introduced either in the stator (‘short stator’ effects) or in the rotor (‘short rotor’ effects) are discussed theoretically. Such discontinuities are shown to involve transient conditions in a linear motor, even when the machine runs at constant speed. The exploitation of double-sided and tubular stators is also discussed.The second part of the paper deals with practical applications of linear induction motors. It includes a brief account of the development of a linear motor which produces a reciprocating motion of stable amplitude without the necessity for any switching device. The possible applications of this type of motor to the textile industry are examined. The use of linear motors as liquid-metal pumps is examined in the light of the earlier theory, and the final section suggests an unusual outlook on induction-motor design which was inspired by the study of linear motors.

This paper proposes a propulsion control algorithm considering End-effect of a linear induction motor (LIM) used in a magnetic levitation train. Indirect field oriented control (IFOC) technique is one of the popular control techniques widely applied to LIM drive control. The main idea of IFOC in a LIM is the decoupling of the flux and torque. Therefore, IFOC has a higher dynamic characteristic than the scalar control [1]. The LIM applied to the magnetic levitation trains should control the thrust force and also take into account the normal force for levitation stabilization. When the LIM is driven, normal force is inevitable. The normal force is a function of the speed, the slip frequency and the current of the train. In the rotary type induction motor, the normal force is canceled due to its symmetrical structure. Therefore, IM could operate in a low slip and high efficiency range. However, in the case of LIM, a high attraction force is generated in a low slip region, which cause the instability of the levitation system. Therefore, the propulsion control for magnetic levitation trains must operate in a higher slip region than the slip used in conventional IFOC. In this paper, the slip characteristics of the LIM are analyzed to improve the efficiency and safety of the magnetic levitation train by FEM analysis. A constant slip frequency propulsion control algorithm is proposed for operation in the analyzed slip region. The proposed algorithm can be controlled by separating magnetic flux and thrust from IFOC. The proposed algorithm is verified in 250 kW test bench. The results of FEM analysis according to speed and slip at constant voltage condition are shown on Fig. 1. The dotted lines of Indirect Field Oriented Control(IFOC) and Constant Slip Indirect Field Oriented Control(CSIFOC) are shown on the figure. The slip of the IFOC is low and that leads to operate near the maximum thrust and efficiency. However, the normal force of LIM is also increase, which could not be allowed in maglev application. On the other hand, when CSIFOC is performed considering normal force, the slip is higher than the conventional algorithm. Although the CSIFOC is not operated in maximum thrust slip, the normal force could be greatly reduced. Fig.2 (a) shows the CSIFOC algorithm considering the end-effect of the linear induction motor. In general IFOC, the d-axis current reference is calculated through the magnetic flux reference value, and the q-axis is calculated based on thrust. However, the CSIFOC for the LIM generates the d-axis and q-axis current references along with the slip frequency and thrust. Fig. 2 (b) shows the waveform of the experimental results based on the general IFOC and the proposed algorithm. Both IFOC and CSIFOC were tested with the same rate pattern of the same load. It is confirmed that CSIVC has larger slip than conventional IFOC and low normal force in all regions. In this paper, a constant slip frequency IFOC algorithm is proposed considering the end effect of LIM used for magnetic levitation trains. The efficiency, thrust and normal force according to the speed and slip through the FEM analysis of the LIM were analyzed and the slip condition suitable for the magnetic levitation train is proposed. In addition, a current reference generation algorithm based on the thrust and slip is proposed in the IFOC considering the end effect to operate under the proposed slip condition. In the full paper, the detailed formulas of the proposed algorithm are developed and will be verified through simulation and experimental results.

Magnetically levitated transportation systems are used in various fields, for example in the clean room, because they have unique merits such as being contactless. The usual system has the levitation system, such as electrical magnets and drive system and a linear motor, respectively. In this system we should control the levitation in considering the attractive force caused by the drive system because the linear motor used as the drive system has not only the thrust force but also the attractive force changing the condition of the electrical power source. Then in this paper, we propose a novel transportation system without the electrical magnet for the levitation system. In this system, there is only the linear induction motor as the drive system and we use the attractive force of the motor for the levitation. We propose a novel method to control the levitation and the drive simultaneously. We use a new power source combined with the high frequency component and the low frequency one. At first, we show the basic characteristics of the linear induction motor used as the drive system and explain the concept of the proposed control method. Secondly we show the characteristics of the motor powered by the combined power supply. Next we explain the control scheme. Finally we show the results of the levitation controlled by the proposed method.

Background: The study of the properties of linear traction drives for new types of rolling stock seems to be an urgent task. Linear motors are multifunctional and are capable of providing both traction and braking of promising high-speed rolling stock.
 Aim: Investigation of the braking characteristics of an adjustable traction linear induction motor with variable resistance by a short-circuited winding of the secondary element.
 Methods: Analytical methods have been used to obtain relations for calculating the braking forces of an adjustable linear induction motor with a short-circuited winding of the secondary element.
 Results: The obtained ratios made it possible to calculate the braking forces of linear induction motors.
 Conclusion: The results of calculating the braking forces of adjustable linear induction motors make it possible to reasonably choose the braking modes of a new type of rolling stock.

Air-suspended hybrid linear motor vehicle is driven by linear motor (LM) and levitated by air-suspension. The vehicle is driven by single-sided linear induction motor (SLIM) in flat and gentle slope sections of the guideway, and driven by linear synchronous motor (LSM) in steep slope section. To realize a contactless motion control of air-suspended hybrid LM vehicle, lateral motion must be controlled without mechanical guidance. Air-suspended hybrid LM vehicle has special LM's for lateral guidance motion control. The special LM's enables continuously lateral running along the reaction plate constructed in the lateral direction. This paper presents the lateral running control on the lateral reaction plate for air-suspended hybrid LM vehicle. Experimental results show that the vehicle can be propelled in lateral direction stably

To incorporate the merits of simple and robust movers of induction linear motors and higher power factor and high efficiency of permanent magnet linear motors, a new long primary double-sided linear flux-switching permanent magnet (DSLFSPM) motor is proposed, investigated, and quantitatively compared with linear induction motors (LIM) for the electromagnetic launch system. First, the topology, operation principle, and electromagnetic performance of the DSLFSPM motor have been investigated. Then, in order to analyze the LIMs comprehensively, three LIMs have been designed based on the same overall dimensions and other key parameters. Finally, the electromagnetic performance of DSLFSPM motor and LIMs have been compared and summarized. The results show that the DSLFSPM motor has some merits such as higher efficiency, higher power factor, more robust mover, larger thrust force, and smaller force ripple than LIM motors. In order to validate the simulation results, a small-scale prototype of DSLFSPM motor has been built and measured.

The traditional indirect drive design for machine tool axes, a rotary motor and a ball-screw, is well known, but has some important drawbacks (like limited speed, acceleration, accuracy and lifetime). The use of linear motors can overcome these problems, but it also has its inherent drawbacks. In this direct drive design, all stiffness must result from the servo controller. This makes high demands upon the controller design. This paper presents some control techniques to achieve a good tracking behaviour for both types of axis design. The linear motor ripple results in significant tracking errors for the indirect drive design. A compensation technique is proposed based on an experimentally identified ripple model. The proposed technique is tested on three experimental set-ups: a rotary motor axis with ball-screw, a linear motor axis supported by air bearings and a linear motor axis supported by rolling element slideways.

The normal force that exists between primary and secondary of a linear motor plays an important role in Magnetically Levitated (MagLev) trains. In the particular case of the superconducting magnetic levitation (SML) method, based on the diamagnetic property of high-temperature superconductors (HTS) in the proximity of rare-earth permanent magnets, this issue is even more critical, as there is no control of the levitation gap, which changes according to the load. In this article, the previous experience of traction systems of MagLev trains in commercial operation serves as a background for the study and will be reviewed. The experience of other SML projects will also be considered to reach the proposed topology, similarly to the process of genetic evolution. The proposed solution for the normal force issue consists of a linear induction motor (LIM) optimized for an urban SML vehicle. Analytical equations and simulations with the finite element method (FEM) are presented. An innovative workbench for velocity tests was developed to measure the parameters of interest. Simulation and experimental results confirm the improved performance of the proposed LIM. The final results with the short primary LIM, designed and tested, showed excellent results. The LIM can be qualified as 3 times low: low-cost solution, for low-speed SML vehicles, and with low-normal force amplitudes.

This paper compares three possible linear motor topologies for an electromagnetic launch system to assist civil aircraft take-off. Assisted launch of civil aircraft has the potential of reducing the required runway length, reducing noise and emissions near airports and improving overall aircraft efficiency through reducing engine thrust requirements. A comparison is made of practical designs of a linear induction motor, a linear permanent magnet synchronous motor and a superconducting linear synchronous motor to propel the A320-200 aircraft. The machine design requirements are established considering aerodynamic and engine performance and allow the aircraft to safely complete the take-off procedure. Analytical design of conventional synchronous and asynchronous linear motor will be compared with finite element analysis. A superconducting synchronous motor design is also considered, accounting for full system losses including the cryocooler power requirement and the mechanical & design constraints necessary for the cooler and the superconducting coils.

Non-ferrous and metal scrap separation nowadays is becoming more and more actual problem. There are several traditional methods of separating ferrous and non-ferrous metal waste. To automate this process, it is necessary to use mechanized systems based on linear induction motors. This paper is presented to the study of such type object. In this paper several field mathematical models have been developed, which are implemented by the finite element method in Comsol Multiphysics software package. The object of the study is a linear three-phase induction motor which is constructed for the non-ferrous metal scrap separation. With the help of the developed mathematical model the basic parameters and characteristics of a linear motor are calculated and the optimal limits of its operation are determined and the minimum configuration of the scrap is required for its reliable separation. A series of calculations was performed to determine the optimal operating modes of the linear separator and to formulate recommendations for the practical implementation of the experimental one. In this paper relation between electrical conductivity and force was calculated. It was performed due to optimal practical recommendations, which are used in real life operation.

The single-sided stator and the double-sided stator configurations of the infinitely long linear induction motor are analyzed from electromagnetic field theory, and the traction force and the normal force equations are derived using the Maxwell magnetic stress tensor. The force normal to the air-gap plane, which has not been quantitatively evaluated or understood before, is found to be highly repulsive for slips greater than St = 1/Rm (Rm magnetic Reynolds number). The engineering use of this large repulsive force to provide frictionless contact-free means for levitation, suspension, and mechanical stabilization of high-speed transport at speeds of 300 mi/h is considered. A physical explanation is given for the existence of the normal force components. The equivalent circuit is derived for the linear induction motor so that the forces and the design parameters obtained from field theory can be understood in terms of traditional induction motor theory. The performance characteristics are investigated, and it is found that the two key design parameters are 1) the magnetic Reynolds number Rm and 2) the air-gap-wavelength ratio ßg. The single-sided stator configuration is considered for levitation, and the criterion for a self-levitating vehicle is given. In the double-sided stator configuration, the net normal force is a restoring force which is considered for keeping the vehicle aligned on track against lateral perturbations. In this case, the equivalent electromagnetic spring constant Ck and the natural frequency of oscillation fm are derived and their performance characteristics investigated.

An equivalent circuit model in the rotor flux oriented frame is developed for the vector control of a linear induction motor (LIM) by adding the so called to the rotary induction motor (RIM) model. We describe the effects of the end effect by introducing speed dependent scale factors to the magnetizing inductance and series resistance in the d-axis equivalent circuit. Torque and flux equations are correspondingly modified. With the vector control scheme based on the proposed equivalent circuit, one can resolve the thrust attenuation problem in a high speed. We have demonstrated through experiments the improvements achieved by the proposed scheme.

Considering the characteristics of linear induction motor that the primary leakage inductance and the secondary leakage inductance are not equal, the off-line parameter identification method for linear induction motor based on PWM inverter is proposed in this paper. Under the condition that the motor is stationary, single-phase DC voltage, single-phase high-frequency AC voltage, and single-phase low-frequency AC voltage are sequentially supplied to the motor to obtain primary resistance, mutual inductance, secondary leakage inductance, primary leakage inductance, and secondary resistance of the linear induction motor successively. Then, the offline parameter identification model of linear induction motor is established to carry out simulation experiments and verify the accuracy of the method. Finally, the proposed identification method is compared with the traditional no-load and locked-rotor experiment methods. The results show that the proposed identification method has higher precision for it considers the problem that the primary and secondary leakage inductance of linear induction motor are not equal.

In material handling applications where high precision, very long tracks and closed paths are required, short primary linear drives are proposed. The short primary type uses an active vehicle (moving winding) on a passive track (stationary magnets or induction rail). To implement this topology, this paper presents a combined operation of a short permanent magnet linear synchronous motor (PMLSM) and a linear induction motor (LIM). The costs can be reduced considerably by using a simple induction rail, instead of permanent magnets on the track. The control unit and converters are mounted on board of the active vehicle. A contactless energy transmission provides the average power and an ultracapacitor bank provides the peak power demanded by the active vehicle via a bidirectional DC-DC converter. The field oriented control, the drive control, the machine model and the control structure are based on field oriented control (FOC) using the rotating AB-reference frame. The transition control strategy is described and experimental results demonstrate the transition between sections with permanent magnets and induction rail.

A 45-m-long high-temperature superconducting (HTS) Maglev ring test line, named "Super-Maglev," has been successfully developed in Chengdu, China, in February 2013, 12 years after the birth of the first man-loading HTS Maglev test vehicle. The Maglev vehicle (2.2 m in length, 1.1 m in width) is designed for one passenger with a levitation height of 10–20 mm; the permanent-magnet guideway (PMG) (45 m in length, 0.77 m of track gauge) is a racetrack shape with a curve radius of 6 m; the driving is accomplished by a linear induction motor with a maximum running speed of 50 km/h. The linear motor is composed of four submotors installed at one straight section in the middle of the double PMGs, and the total length is 3 m. This second-generation HTS Maglev vehicle system is highlighted by the cost-performance and the wireless multiparameter onboard monitoring function. The current same-level load capability has been achieved over a small-section low-cost PMG whose cross-sectional area is only 3000 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . On the vehicle, parameters of levitation weight, levitation height, running speed, acceleration, lateral offset, online position, and total running distance of the vehicle are real-time monitored and displayed on the onboard tablet computer. The system component and test data are reported in detail in this paper.