Characteristic Analysis of a Linear Induction Motor for ...
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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
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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
Characteristic Analysis of a Linear Induction Motor for 200-km/h 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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Jae-Hoon Jeong, Jae-Won Lim, Do-Young Park, Jang-Young Choi and Seok-Myeong Jang / IJR, 8(1), 15-20, 2015
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.
Characteristic Analysis of a Linear Induction Motor for 200-km/h Maglev
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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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Jae-Hoon Jeong, Jae-Won Lim, Do-Young Park, Jang-Young Choi and Seok-Myeong Jang / IJR, 8(1), 15-20, 2015
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
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