IJR International Journal of Railway

Vol. 9, No. 2 / December 2016, pp. 41-45

Vol. 9, No. 2 / December 2016 − 41 −

The Korean Society for Railway

A Study on Improvement of Operation Efficiency of Magnetic Levitation

Train Using Linear Induction Motor

Sang Uk Park*, Chan Yong Zun*, Doh-Young Park**, Jaewon Lim** and Hyung Soo Mok†

Abstract

In this paper, a study on the efficiency improvement of the magnetic levitation train using the LIM (Linear Induction

Motor) was presented. The maglev train has the advantage of being environmentally friendly since much less noise and

dust is produced. However, due to structural limitation, compared to a rotating induction motor, linear induction motor,

the main propulsion engine of the maglev train has a relatively greater air gap and hence has the lower operation effi-

ciency. In this paper, the relationship between the operating condition of the train and the slip frequency has been inves-

tigated to find out the optimum slip frequency that might improve the efficiency of the magnetic levitation train with

linear induction motor. The slip frequency is variable during the operation by this relationship only within a range that

does not affect the levitation system of the train. After that, the comparison of the efficiency between the conventional

control method with the slip frequency fixed at 13.5[Hz] and the proposed method with the slip frequency variable from

9.5[Hz] to 6.5[Hz] has been conducted by simulation using Simplorer. Experiments of 19.5[ton] magnetic levitation

trains owned by Korea Institute of Machinery and Materials were carried out to verify the simulation results.

Keywords: Linear induction motor, Propulsion control, Slip frequency, railroad car, ATO, Auto train operation

1. Introduction

The concentration of jobs in metropolitan area and the

distribution of population to satellite cities is still on going.

Accordingly, medium-distance and short-distance trans-

portation systems became important parts of modern soci-

ety. Magnetic levitation train, which has the advantages of

low noise, less dust and environmental friendly is being

developed actively. In this paper, study deals with the

improvement of the efficiency of the magnetic levitation

train with linear induction motor.

The linear induction motor is similar in fundamental

principle but different in structure to the rotary induction

motor. Unlike conventional induction motor which has

round, cylindrical shape, the linear induction motor has a

straight arrangement where the rail takes the role of the

rotor and the train takes the role of stator. In such arrange-

ment, several problems occur which is not the case in

rotary motor structure. Linear induction motors have finite

primary and secondary length, which generates end-

effects. While the operation, normal force is created per-

pendicular to the rail and counteract the levitation force,

†

*

**

Corresponding author: Konkuk University, Korea

E-mail : hsmok@konkuk.ac.kr

Konkuk University, Korea

Korea Institute of Machinery & Materials, Korea

ⓒThe Korean Society for Railway 2016

https://doi.org/10.7782/IJR.2016.9.2.041

Fig. 1. Linear Induction motor

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Sang Uk Park, Chan Yong Zun, Doh-Young Park, Jaewon Lim and Hyung Soo Mok / IJR, 9(2), 41-45, 2016

makes levitation difficult and unstable. Thus, the slip fre-

quency, the factor that decides the normal force is fixed to

13.5[Hz] to minimize the effect of the normal force and

this further decreases efficiency.

To minimize the disadvantages of the linear induc-

tion motor, this paper proposed variable slip frequency

drive control strategy which allows the slip frequency

to vary within a certain range but doesn’t affect the

levitation control too much. There are some works

associated with improvement of efficiency of linear

induction motor for maglev train have been done by

varying slip frequency. But few of them deals with slip

frequency that is constantly changing according to

operating condition.

In this paper, with in a range that doesn’t affect the

levitation system, the optimum slip frequency at each

operating point is proposed based on rms current con-

trol. By applying ATO (Auto Train Operation) system,

the cumulative power consumptions of both fixed slip

frequency operation and variable slip frequency opera-

tion are calculated and compared to prove the efficiency

improvement.

2. Structure and Operation Characteristics of Maglev Train

2.1 Structure of maglev train

As shown in Fig 2, a magnetic levitation train using

LIM is mainly consists of a levitation system that gener-

ates levitation force to allow train to leviate and propul-

sion system which generates both propulsion force

horizontal to the rail and normal force perpendicular to

the rail at opposite direction to levitation force. The levi-

tation force must powerful enough to overcome and bal-

ance the weight of the train plus the normal force,

otherwise the levitation fails. The normal force must be

controlled with in a range that doesn’t affect the levita-

tion system.

2.2 Operating condition and slip frequency

2.2.1 Notch command

The propulsion force of the maglev train with LIM is

controlled by stator current and the current reference is

given by the notch command. As shown in Table 1 and

Table 2, the notch commands have 4 levels for accelera-

tion and 7 levels for deceleration. The notch number goes

higher with greater propulsion force or braking force. In

this manner, the operating condition of the train can be

controlled notch commands.

2.2.2 Slip frequency

Fig. 3 (a) illustrated the relationship between slip fre-

quency and normal force. For the same notch command,

the normal force increases with decreasing slip frequency,

and, the normal force increases when the notch level steps

up. Fig. 3 (b) shows the relationship between slip fre-

quency and efficiency. Efficiency goes higher with lower

slip frequency under the same notch command, and goes

lower when notch command steps up.

Fig. 4 shows the actual experiment result of train a trav-

eling at 50[km/h] with slip frequency changed by notch

command based on relationship shown in Fig. 3. Under

the same operating condition(P4_B7), the cumulative

power consumption was 1,352[kWh] for 13.5[Hz] slip fre-

quency and 1,142[kWh] for 10.5[Hz] slip frequency. The

0.21[kWh] of energy saving by lowering slip frequency

implies 15.5[%] of efficiency improvement, verifies the

conclusion that efficiency can be improved by lowering

the slip frequency.Fig. 2. Structure of the linear induction motor

Table 1. Notch pattern for propulsion force

NotchPropulsion Force

[%]

Slip Frequency

[Hz]

P4 100

13.5

~ 6.5

P3 75

P2 50

P1 30

Table 2. Notch pattern for braking force

NotchBraking Force

[%]

Slip Frequency

[Hz]

B7 100

13.5

~ 6.5

B6 85

B5 72

B4 56

B3 42

B2 28

B1 15

A Study on Improvement of Operation Efficiency of Magnetic Levitation Train Using Linear Induction Motor

− 43 −

2.2.3 Speed

Fig. 5 (a) shows the relationship between the speed of

train and the normal force with slip frequency fixed at

12.5[Hz]. Normal force increases with the speed decreases

and at a certain speed, normal force increases when the

notch level steps up. Fig. 5 (b) shows the relationship

between speed and normal force under the same condition

as Figure (a), The efficiency decreases with speed

decreases.

From Figs. 3, 4 and 5, we can draw a conclusion that:

● Small slip frequency brings better efficiency● Efficiency can be changed by operating condition.

Based on these two conclusions, it is possible change the

slip frequency according to operating condition during the

operation in order to make the train running with best effi-

ciency. Note that the slip frequency must be restricted

within a range that doesn’t affect the levitation system.

3. Simulation

3.1 Simulation condition

The train in this experiment was ran by ATO (Auto

Train Operation) system. The ATO system, which is

adopted in actual operation, automatically controls the

train to accelerate, decelerate and stop between each sta-

tion, respond to continuous change of operation pattern.

Fig. 6 shows the speed pattern and notch command pat-

Fig. 3. Characteristics according to the slip frequency

Fig. 4. Comparative efficiency according to the slip frequency

Fig. 5. Characteristics according to the speed

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Sang Uk Park, Chan Yong Zun, Doh-Young Park, Jaewon Lim and Hyung Soo Mok / IJR, 9(2), 41-45, 2016

tern of actual experiment, where the blue curve represents

speed and the black curve represents the notch command

pattern.

Two simulations have been conducted to compare cumu-

lative power consumption between fixed and variable slip

frequency drive. The fixed slip frequency is 12.5[Hz],

while the variable slip frequency ranges from 6.5[Hz] to

9.5[Hz]. At the same time, the normal force is limited to

-2.5[kN], which does not affect levitation even with full

capacity.

3.2 Variable slip frequency during the

operation

Fig. 7 illustrates the simulation results. The right side of

the figure shows that when slip frequency varies between

6.5[Hz]~9.5[Hz], the normal force changes under -

2.19[kN]. It means the normal force didn’t reach the criti-

cal value where it could threat the levitation of the train.

Fig. 8 simplifies the comparison of the cumulative power

consumption. When slip frequency varies between

6.5[Hz]~9.5[Hz], power meter counts 4.25[kWh], and

when slip frequency is fixed at 12.5[Hz], it counts

5.34[kWh].

The proposed method saved 1.09[kWh] of energy,

improved 20.41[%] efficiency shows better performance

over conventional method in terms of energy efficient.

4. Conclusion

This paper proposed a new control method to improve

operation efficiency for magnetic levitation train with lin-

ear induction motor. Linear induction motor gets better

efficiency with lower slip frequency. Thus, within a cer-

tain range, changing slip frequency according to notch

command during operation can improve overall effi-

ciency. Simulation result shows that compared to 13.5[Hz]

fixed slip frequency, the proposed control method saved

1.09[kWh] of energy, and improved efficiency by

20.41[%], proves better performance over conventional

method in terms of energy efficient.

Acknowledgement

This research was supported by a grant (16RTRP-

B070556-04) from railroad technology research program

funded by Korean ministry of land, infrastructure and

transport of Korean government.

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Fig. 6. Auto Train Operation system (ATO)

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