ABSTRACT

Levitation performance of a hybrid electromagnet suspension (HEMS) has been investigated experimentally and numerically. Force measurements were made on a test setup with a prototype HEMS designed at JSC “NIIEFA”. The measured data were compared with 3D simulations. The investigation enabled validation of design solutions. The prototype demonstrated good levitation characteristics and low power consumption and stray field.

АННОТАЦИЯ

Исследованы характеристики прототипа гибридного электромагнита (ГЭМ) подвеса магнитолевитационного транспортного средства, разработанного в АО «НИИЭФА». Измерения на специализированном стенде были выполнены с целью обоснования принятых конструкционных решений. Сравнение полученных данных с результатами численного моделирования подтверждает правильность выбранной концепции построения магнитной системы подвеса. Прототип предложенного авторами ГЭМ характеризуется пониженным энергопотреблением и низким уровнем полей рассеяния.

Keywords: test setup, hybrid electromagnet suspension, maglev, permanent magnet, lift force, numerical model, measurement, prototype.

Ключевые слова: стендовые испытания, гибридный электромагнит, Маглев, постоянные магниты, подъёмная сила, математическое моделирование, результаты эксперимента, натурный макет

Introduction

The advanced transport system, maglev, is based on a concept of magnetic suspension [1] that makes a vehicle levitate. A hybrid suspension utilizes a combination of strong permanent magnets (PM) and electromagnets (EM). PM provides levitation while EM is used to control the air gap and stabilise the system. The HEMS technology cures some drawback of pure EM suspension [3], primarily reducing the stray field and the ohmic power consumption. The reduced energy consumption also means better electromagnetic compatibility and environmental safety. JSC “NIIEFA” has designed, studied, and patented several HEMS configurations [4].

This study is focused on experimental investigation of HEMS functionality on a certified test bench [5] using a prototype. The measured forces were compared with simulated data. Also, a comparison is presented for characteristics of HEMS and pure EMS.

The results of the study will be used for further improvement of instrumentation and test programs to adapt them for serial product inspection.

Hems prototype

Basically, HEMS represents а steel rail and a carriage suspended underneath due to magnetic attraction provided by the hybrid magnet. PM enables the basic attraction force while EM is used for active control of the air gap between the rail and the carriage. With variations of the air gap, the force from PM will change. Typically HEMS systems have a small air gap. Fig. 1 shows schematically the hybrid suspension.

Figure 1. Hybrid suspension with V-shaped PM

The HEMS prototype employs a V-shaped PM assembly. The magnetic circuit has a bridge below PM with a gap to prevent closure of the magnetic flux. Such configuration ensures high mechanical strength and low stray field [3]. Table 1 presents a brief specification of the prototype.

Table 1.

HEMS prototype specification

The specification was chosen taking in mind the planned revitalization of the Moscow monorail transport. However, the specification is quite representative to generalize results to other applications.

Numerical analysys

To identify the HEMS design and levitation efficiency parametric studies were performed using a special computation technique. The lift force was evaluated in terms of electromotive force, or emf. 3D simulations were made with the code КОМРОТ [6].

The mathematical description of interaction between the suspended vehicle and the guideway is based on volume integration of the emf density. The force distribution is obtained using Maxwell tension tensor. The tensor components are derived in terms of the field and field strength vectors. Fig. 2 presents evaluation of the lift force at various coil currents and air gaps.

Figure 2. Calculated lift force

Levitation efficiency of the HEMS prototype was assessed in a comparison with a pure EMS utilizing the conventional U-type configuration. The decisive parameters were power consumption, losses and operation temperature at the same lift force of 4700 N and 10 mm air gap. A critical issue is the Joule losses in the EM winding that may cause overheat and deteriorate the magnet performance.

Fig. 3 shows typical field maps of the V-shaped HEMS and pure U-type EMS obtained using numerical models.

The numerical investigation has shown that the U-type EMS needs over 600W to generate the rated lift force of 4700 N. At the coil current of 9600A, the Joule losses are as high as 576 W. Assuming the ambient temperature of 300K and convection coefficient of 10 W/m²K, the maximal winding temperature is 416 K. The overheat reaches ~116⁰K. The EMS weight is about 90 kg. 

Under the same conditions, HEMS demonstrates the power consumption of 80W and the maximal winding temperature of 324 K. The Joule losses are 76 W, the coil current is 7 A. The overheat is evaluated as~12⁰K. The HEMS weight is about 50 kg.

A comparative simulation of temperature distribution is illustrated in Fig.4.

Test setup

The numerical studies were supported by measurements to validate the design and adjust numerical procedures with respect to scaling parameters [7]. This allows reliable predictions and minimizes the need for full-scale mockups.

The measurements on the HEMS prototype were aimed to laboratory verification and planning. The lift force was measured and compared with results of 3D simulations.

The test setup is shown in Fig.5. The HEMS prototype is fixed on a movable crossbeam. The crossbeam enables the vertical motion thus changing the air gap between HEMS and a steel rail mounted above on a prop. The rail is levelled horizontally with two pins. The prop is equipped with a force gauge with the measurement resolution of 10 N and range of ±100 kN. HEMS is connected to the force gage through a tightening screw.

Figure 5. Test setup in situ and schematically: 1- fixture, 2 – HEMS, 3 – movable crossbeam, 4 – steel rail, 5 - force gage prop, 6 - tightening screw, 7 - levelling pins

During force measurements the prop moved the rail up and down. The air gap between the rail and HEMS was limited with calibrated non-magnetic spacers placed at the corners of the HEMS fixture. Possible tilts led to air gap variations along the rail that resulted in nonuniform distribution of induced emf as shown in Fig.6.

Figure 6. Variations of lift force with reduced air gap:  (а) – Non-uniform force distribution due to rail tilt and bend; (b) –support reaction occurred; (c) –equalization of force distribution

With reduced air gap, emf would increase and cause strain growing in the rail. The strength measurements revealed that the specified rail thickness of 18 mm was insufficient to prevent critical deformation. Before building a pilot track, the rail thickness should be optimized by solving a coupled problem in terms of electromagnetic and mechanical loading.

Fig. 7 shows a comparison of simulated and measured magnetic forces for the HEMS prototype. A discrepancy between measurements and computations is as low as 7 %. The maximal discrepancy occurs at high currents when emf increases and the measured air gap is widened due to the rail tilt.

Fig.8 illustrates the simulated and measured field of HEMS solely. The rail is removed. The vertical field normal to the magnet poles was evaluated at a distance of 0.4 mm from the pole surface. A good match of the results is observed.

Conclusions

A prototype of HEMS for a suspended monorail design has been tested using a measurement set-up and numerical models. The study has shown low power consumption, losses, and weight as compared to conventional EMS systems. However, strength of the guideway structures should be investigated in terms of coupled electromagnetic and mechanical loading.

The results of the study will be use to validate technological maturity and put recommendations for tuning measurement and inspection procedures. Also, this forms a base to further development of main functionalities of suspended transport for practical implementation.

References:

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7. Amoskov V.M., Arslanova D.N., Belov A.V. et al. Verification of numerical model of hybrid EMS using test bench measurements at large air gap. Modern Transportation Systems and Technologies. 2022; 8(1): 28-37. doi: 10.17816/transsyst 20228128-37