Characteristic Analysis of a Linear Induction Motor for ...

IJR International Journal of Railway

Vol. 8, No. 1 / March 2015, pp. 15-20

Vol. 8, No. 1 / March 2015 − 15 −

The Korean Society for Railway

Characteristic Analysis of a Linear Induction Motor for 200-km/h Maglev

Jae-Hoon Jeong*, Jae-Won Lim**†, Do-Young Park**, Jang-Young Choi* and Seok-Myeong Jang*

Abstract

As a result of the current population concentrations in urban centers, demand for intercity transportation is increasing

rapidly. Railway transportation is becoming popular as an intercity transportation because of its timely service, travel

speeds and transport efficiency. Among the many railway systems, the innovative and environmentally friendly maglev

system has been rated very highly as the next-generation intercity railway system. Linear induction motors are widely

used for the propulsion of maglev trains because of their light weight and low construction costs. The urban maglev that

was recently completed in Incheon airport site employs a 110km/h class linear induction motor. However, this system

was designed to meet requirements for inner-city operations and is not suitable as an intercity transportation system,

which requires medium to high speeds. Therefore, this study deals with the characteristics and designs of linear induc-

tion motors used for the propulsion of maglev trains that can be used as intercity trains. Rail car specifications for high-

speed trains have been presented, and the characteristics of linear induction motors that can be used for the propulsion of

these trains have been derived using the finite element method (FEM).

Keywords: Maglev, Linear induction motor (LIM), Finite element method (FEM), Intercity railway

1. Introduction

The recent emergence of problems related to the over-

crowding of city centers and the saturation of transporta-

tion networks have increased demand for intercity

transportation means such as the Great Train Express

(GTX). Domestically, a GTX route running straight

through Gyeonggi-do and Seoul is being planned, while

projects for two other routes have already been confirmed

and are under way. Currently, intercity railway models

such as the Intercity Train Express (ITX) are being consid-

ered as candidates for rail cars to be employed in the GTX.

However, because these systems run along the rails by

way of a rotating motor that mechanically provides rotat-

ing power to wheels, they do have some disadvantages in

terms of the noise and vibration they cause during acceler-

ation or deceleration. Maglev vehicle driven by linear

induction motors, however, operate without direct physi-

cal contact with the rails and therefore do not create much

noise or vibration, do not cause any dust, and are much

quieter inside and outside the car than electric motor

trains. In addition, not only does the maglev train system

have outstanding ascending capabilities due to its friction-

less drive, it can be grounded more because it does not

require additional mechanical power transmission devices,

which is advantageous for reducing construction costs

when making underground sections by way of reducing

the cross-section of the tunnel. Also, the extensive under-

ground sections in intercity railways require high accelera-

tion–deceleration characteristics, and the maglev train

system has characteristics that meet such requirements.

Linear induction motors, which are light weight and

inexpensive to produce, are widely used for the propul-

sion of maglev trains. The maglev train that was recently

completed in Incheon airport site also used a 110 km/h

class linear induction motor. A linear induction motor has

flat planes cut out along the central axis of a rotary induc-

tion motor and rearranged in a straight line. This serves as

a power unit for generating thrust through the magnetic

interactions between the linearly moving magnetic fields

†

*

**

Corresponding author: Korea Institute of Machinery and Materials, Korea

E-mail : [email protected]

Chungnam National University, Korea

Korea Institute of Machinery and Materials, Korea

ⓒThe Korean Society for Railway 2015

http://dx.doi.org/10.7782/IJR.2015.8.1.015

− 16 −

Jae-Hoon Jeong, Jae-Won Lim, Do-Young Park, Jang-Young Choi and Seok-Myeong Jang / IJR, 8(1), 15-20, 2015

generated by currents passing through the primary coil

winding and the eddy currents induced in the secondary

aluminum conductor plate. Linear induction motors are

currently widely used in applications where linear motions

ranging from low to high speeds are required, such as

maglev trains and train propulsion motors, linear logistics

transport systems, and factory automation and office auto-

mation applications [1-3].

The specifications of high-speed maglev trains that can

be used as GTX trains, along with the characteristics and

designs of linear induction motors that will be employed in

the propulsion systems of such trains, are presented in this

paper. Considering the operation speeds for a GTX train, a

maximum speed of 200 km/h has been set as a require-

ment, and a propulsion design that can meet the required

calculated running resistance is presented.

2. Design and Analysis of LIM

2.1 Rail Car Specifications

2.1.1 Concept of High-Speed Maglev

A lightweight metal structure with electromagnetic sus-

pension is used for maglev trains. A train set is composed

of 6 vehicles, where 1 vehicle has 6 bogies and 12 linear

induction motors. The configuration of these trains is

shown in Fig. 1.

2.1.2 Traction Force Calculation of High-Speed Maglev

The traction force of a train must be calculated with

comprehensive considerations of tolerance, load at full

capacity, travel acceleration/deceleration, and running

resistance. The running resistance of a maglev train can

be divided into components such as those caused by pan-

tographs, magnetic resistance caused by levitation

devices, and aerodynamic drag. Each type of running

resistance was theoretically and experimentally presented

and verified through the urban maglev project. The run-

ning resistance of a maglev train R can be divided into

components from pantographs Rc, magnetic resistance

from levitation devices Rm, aerodynamic drag Ra and,

finally, a component from tunnels Rt and can be calcu-

lated using Equation (1).

(1)

Magnetic resistance caused by the electromagnetic com-

ponents is the running resistance due to the magnetic flux

that flows between the levitation magnets and the rails

when the levitation magnets are in operation and can be

expressed as two equations relative to 20 km/h, as shown

in Equation (2), where W is weight and V is speed.

for

for (2)

The running resistance caused by the pantographs can be

assumed to be a constant value that is expressed in Equa-

tion (3), where Nc is the number of pantographs.

(3)

Although aerodynamic drag should be set according to

the shape of the frontal part of the train, it was calculated

using Equation (4), where N is the number of cars based

on the constant value used for existing maglev trains.

(4)

The resistance caused by the gradient is related to the

weight of the car and is determined by the gradient angle.

It can be calculated from Equation (5) as follows:

(5)

In general, the acceleration of intercity rail systems is

relative to 2 km/h/s. Considering that the weakening field

characteristics of a linear induction motor are not superior

to those of a rotary induction motor, it is proper to select

the rated speed of 90 km/h as the base speed when the

highest operation speed is 200 km/h. To calculate the trac-

tion force in the constant torque region, each train set

under a load of 249 t is required to have an acceleration of

2 km/h/s up to a speed of 90 km/h. According to Equation

(1), the train resistance for one set is calculated as 8.4 kN,

and the required traction force is calculated as 146.7 kN,

considering the load, acceleration, and rated speed. As

there are a total of 48 linear induction motors mounted on

R Rc Rm Ra Rt+ + +=

Rm 3.54 W V××= 0 V 20 km h⁄[ ]≤ ≤

Rm 18.22 0.074 V×+( ) W×= V 20 km h⁄[ ]≥

Rc 41.68 Nc+=

Ra 1.652 0.552 N×+( ) V2

×=

Rg N M g× θ( )sin××=

Table 1. Specs of Linear induction motor

Parameter Value Parameter Value

Poles 12 Base Velocity 90 (km/h)

Slots 77 Pole pitch 192.6 (mm)

Turn/slot 3 Stack Length 220 (mm)

Fig. 1 System diagram of high speed MAGLEV


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each set, each motor should be designed to produce an

output thrust of approximately 3.1 kN or higher based on

the calculated traction force per motor so that the average

acceleration exceeds 2 km/h/s up to 90 km/h[4].

2.2 Characteristics of Linear Induction Motor

2.2.1 Design Model of Linear Induction Motor

The initial concept of 200km/h class maglev train con-

sisted of 6 vehicles for 1 set with 1 vehicle composed of 6

bogies. The car length for 1 vehicle is 20 m, and because it

is composed of 6 bogies, it was decided that each linear

induction motor should have a maximum length of 2.5 m.

Fig. 2 shows a plane view of the initial design of a linear

induction motor. Detailed specifications for the 2.5 m, 12

pole linear induction motor are listed in Table 1. The

motor is composed of 12 poles with 77 slots and a base

velocity of 90 km/h (25 m/s). Because the concept was for

a maglev train, an air gap of 12 mm was adopted for the

levitated state, and the length between poles, which can

have an effect on the operation frequency, was determined

to be 192.6 mm [5].

2.2.2 Inverter System

Fig. 3. shows the operation system for the linear induc-

tion motors of a 1 vehicle, 6 bogie maglev train. The wir-

ing voltage for driving the linear induction motor is 1500

Vdc, and assuming that a three phase inverter is being used

to drive it, the 2P6S system and the 2P3S system can use

123 Vmax and 245 Vmax for a power source, respectively.

The input voltage necessary to drive a linear induction

motor is very closely related to the induced electromotive

force within the device. The induced electromotive force

increases with increasing speed, and when it becomes

larger than the input voltage, the current moves in the

opposite direction and the motor acts as a generator. In

motor-driven systems, compared with the induced electro-

motive force that increases proportionally with increasing

speed, there is a limit to increasing the applied voltage.

Therefore, the 2P3S system where the voltage utilization

can be increased is more suitable for a 200-km/h-class

high-speed maglev propulsion system [6].

2.2.3 Rated Characteristic Analysis for the Linear Induc-

tion Motor Using the 2D Finite Element Method

This paper employed a numerical analysis technique

called the finite element method (FEM) to analyze the

electromagnetic fields generated by the linear induction

motor. There are two ways to analyze linear induction

motor models using the FEM: first, the method ana-

lyzes a linear model identical to the real one, and sec-

ond, considering time and stability, the method analyzes

a very large approximated arc model. The advantage of

the first method is that the analysis motor can be pre-

cisely modeled. The disadvantage is the limitation of the

analysis because the moving direction of a motor can-

not be infinite. In addition, this method is time-consum-

ing because it is necessary to design very long tracks for

linear induction motor travel. The advantage of the sec-

ond method is that the analysis is stable and can be done

in a short time. However, the configuration of the motor

becomes distorted because it is being attached to a arc

model. Therefore, the rated thrust and the thrust and lev-

itation according to slip were analyzed using the linear

model, and the power curve of the motor was analyzed

using the arc model because it requires a great deal of

analysis [6,8].

Fig. 4 shows the results of the thrust force of the linear

induction motor. The input voltage was set at 245 Vmax, the

coil fill factor was set at 0.6 and the base speed was set at

90 km/h. The input frequency for the rated analysis was

selected considering the slip frequency in relation to the

pole pitch and linear motion speed. In the present analy-

sis, a slip frequency of 12.5 Hz was chosen. From the

rated analysis, the rated thrust was 4.17 kN, which was

30% higher than the required thrust, and the current den-

Fig. 2 Schematic of high speed MAGLEV

Fig. 3 6 Inverter configuration of six-bogie system

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sity was 4.95 A/mm2. These results validated those of our

initial design.

Fig. 5 shows the analysis results of the thrust and normal

force according to the slip frequency. Because a maglev

train is a system that travels after it uses magnetic force to

levitate above the rails, the amount of normal force gener-

ated by a linear induction motor is a very important param-

eter in terms of train levitation stability. In addition, both

analytical values display non-linear characteristics, and the

determination of the appropriate location becomes critical

in designing and operating linear induction motors. In the

present paper, considering the normal forces that affect lev-

itation, we set 12.5 Hz as the rated slip frequency, even

though the thrust was not at its highest value.

2.2.4 Analysis of Linear Induction Motor Characteris-

tics in Accordance with Length

The maximum allowable length of a linear induction

motor in comparison with the bogie for current maglev

trains is 2.5 m. The motor initially designed in this paper

was also 2.5 m, which meets the requirement for the maxi-

mum allowable length. However, the dimensions of the

motor can change depending on how the vehicle was man-

ufactured. In preparation for such situations, an FEM anal-

ysis was performed to determine how a reduction in length

would affect the motor characteristics. Linear induction

motors were analyzed for 2 to 2.5 m in intervals of 0.1 m.

The results of the analyses are shown in Figs. 6 and 7.

Fig. 6 shows the results for the rated analysis of linear

induction motors for varying lengths under the same oper-

ating conditions of fixed input voltage and frequency.

The thrust and normal force decreased with a change of

0.5 m in length. The normal force, which affects levita-

tion, decreased with increasing lengths of the linear induc-

tion motor. Fig. 7 shows the results of the rated analysis

Fig. 4. Steady state characterization of LIM

Fig. 5. Comparison of thrust and normal force by slip

frequency.

Fig. 6. The results for the rated analysis for varying lengths

under the same operating conditions.

Fig. 7. The results for the rated analysis for varying lengths

under the same slip conditions.


− 19 −

for varying lengths under the same slip conditions. The

normal force decreased compared with the result in the

previous analysis because the slip frequency was changed

to maintain the same slip conditions. By comparing these

results, it is possible to find a design point that would sat-

isfy the current density conditions and required thrust.

2.2.5 Thrust and Efficiency Derivation in Accordance

with Slip

In linear induction motors, the mover is always slower

than the moving magnetic field; the difference in the speed

is called slip. Slip is a very important factor in induction

motors, as it determines all characteristics such as effi-

ciency, torque, power factor, and current. Therefore, analyz-

ing slip characteristics is a very important step in

understanding the capacity of a linear induction motor

[9][10]. Fig. 8 shows the thrust and efficiency curves for the

designed linear induction motor according to slip. Analysis

was performed using two-dimensional FEM. The rated fre-

quency for the motor designed in this paper is 77 Hz; if the

pole pitch is considered to be 0.193 m, the slip frequency is

12.5 Hz. If this is calculated in terms of slip, the slip for the

present design point is 0.16. Comparing this with the graph

shown in Fig. 8, it can be seen that the design was made at a

point indicating the highest efficiency.

2.2.6 Power Curve Derivation for Linear Induction

Motor

Fig. 9 and 10 show the analysis results for the deriva-

tion of the power curves of linear induction motors.

According to the graph in Fig. 9, which shows the analy-

sis results for a constant thrust region, it is possible to

operate at a higher required thrust value from 3.1 to 3.4

kN for base speeds up to 90 km/h.

The graph in Fig. 10 represents the analysis results for

the characteristics region and shows the thrusts generated

when accelerating from a base speed of 90 km/h to a max-

imum speed of 200 km/h. The propulsion capacity for the

full speed range of the designed linear induction motor can

be understood by referring to the graph.

3. Conclusion

This paper investigates the design of linear induction

motors for powering a 200km/h class maglev train. The

running resistance was determined based on mechanical

specifications, and a drivable linear induction motor meet-

ing this resistance was designed. Characteristics analyses

considering slip frequency as well as analysis of the rated

thrust characteristics were performed to confirm the rated

operating characteristics. In addition, the ranges of the

rated thrust and characteristics were analyzed to verify the

feasibility of the design.

Fig. 8 Thrust and efficiency curves according to slip. Fig. 9. Characteristic analysis results of thrust-velocity curve

at constant thrust.

Fig. 10. Characteristic analysis results of thrust-velocity curve

at constant power.

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Acknowledgement

This research was supported by a grant(14RTRP-

A069839-02) from Railroad Technology Research Pro-

gram funded by Korean Ministry of Land, Infrastructure

and Transport of Korean government

References

1. Boldea, I. (1987). “Linear Electric Motors,” Englewood

Cliffs, NJ, USA: Prectice-Hall.

2. Boldea, I. (2001). “Linear Motion Electromagnetic Devices,”

New York, NY, USA: Taylor & Francis.

3. Gieras, J. F. (1986). “Performance calculation for single-

sided linear induction motors with a double-layer reaction

rail under constant current excitation,” IEEE Trans. Magn.,

Vol. M-22, No. 1, Jan., pp. 54-62.

4. Im, D. H. (1993). “Design of single-sided linear induction

motor using the finite element method and SUMT,” IEEE

Trans. Magn., Vol. 29, No. 2, Mar., pp.1762-1766.

5. Nonaka, S. (1982). “Approximate equations for calculation

of characteristics of single-sided linear induction motors,”

Trans. of IEEJ, Vol. 102-3, pp. 565-573.

6. Lee, B. H. (2011). “Optimum shape design of single-sided

linear induction motors using response surface methodology

and finite-element method,” IEEE Trans. Magn., Vol. 47,

No. 10, Oct., pp. 3657-3660.

7. Shiri, A. (2012). “Design optimization and analysis of sin-

gle-sided linear induction motor, considering all phenom-

ena,” IEEE Trans. Energy Convers., Vol. 27, No. 2, Sep., pp.

516-525.

8. Hamzehbahmani, H. (2010). “Modeling and simulating of

single side short secondary linear induction motors for

improved motor performance,” Eur. Trans. Electr. Power,

Vol. 22, Sep., pp. 747-757.

9. Woronowicz, K. (2004). “Optimized thrust control of linear

induction motors by a compensation approach,” Int. J. Appl.

Electromang. Mech., Vol. 19, No. 1-4, Apr., pp.533-536.

10. Lu, J. (2011). “Research on end effect of linear induction

machine for high-speed industrial transportation,” IEEE

Trans. Plasma Sci., Vol. 39, No. 1, Jan., pp. 116-120.

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