Technology

Technology

Railway Technology Today 4 (Edited by Kanji Wako)

What Drives Electric Multiple Units?
Hiroshi Hata
Evolution to AC Motors

Direct current (DC) traction motors wereused for a long time after the developmentof electric railways. However, since the1980s, they have been replaced by alternating current (AC) motors, one type of
which commonly used in Japan is inverter
controlled. This article discusses the basic traction systems used in today’s electric multiple units (EMUs) in Japan, withemphasis on the inverter control system.

the overhead wire converted to DC whenthe motor is powered by AC?’ It is because current from the overhead wire issingle phase at a constant voltage and frequency, while the motor current is threephase with voltages and frequencies thatvary according to speed (explained later);there is no power converter able to convert voltage and frequency from single to

Figure 1 Main Equipment on Inverter-controlled DC EMU andCircuit Diagram of Propulsion Circuit

From Current Collection
to Driving

Almost everybody knows that EMUs are
driven by electric motors powered bycurrent collected through pantographs on
the top of the car. The entire mechanism
can be explained from the viewpoint of
how current flows and drives the motors.
Figure 1 shows a concept of the mainequipment installed on an inverter-controlled EMU and a circuit diagram of the
propulsion circuit. The current collectedby the pantograph is fed to a high-speed
circuit breaker designed to protect the propulsion circuit from ground faults by isolating large fault currents. The current
then passes through a filter reactor to an
inverter where it is converted to AC thatdrives the AC traction motors mounted on
the trucks. The motors generate torquethat is transmitted via gear wheels to thedriving wheels. The motor speed is reduced and torque is increased by engaging the gears at ratios of 1:5 to 1:7(commuter EMUs), to power the wheels.
The above description is for a DC EMU,
but the AC EMU has an additional stepdown transformer to convert the high voltage from the overhead line to a level forcontrolling the motors, and an AC to DC
rectifier. In Japan, both the rectifier andinverter are often housed in a single box
called the main converter (Fig. 2).
A common question is, ‘Why is AC from40

three phase in one step.
Clearly, it is confusing to use AC and DC,
for the electrification system on one sideand the traction motor on the other andthis requires a clear explanation. Viewed
from the pantograph as the ‘gateway’ to
the EMU, DC was used first in Japan starting at the end of the 19th century. The
AC electrification system was developed

Japan Railway & Transport Review 17 • September 1998

MotorsFilter reactor

Inverter
Motors(including
capacitor)
Motor

Filterreactor

Motor
Inverter

Filtercapacitor

Motor

Motor

Figure 2 Location of Transformer and Main Converter on AC EMU

MotorsTransformer

MotorsMain converter(Rectifier + inverter)

Copyright © 1998 EJRCF. All rights reserved.

Traditional diamond-shaped pantographin the 1950s and has been adopted on
various conventional lines (Hokkaido andKyushu, etc.) as well as on shinkansen(JRTR 16, pp. 48 to 58). DC traction motors, which have been in use for a longtime, are being replaced by AC motors.

Types of Pantographs
and Principal Mechanism
The pantograph is an integral part of the
EMU used to collect current from the over-(JR East)

Single-arm pantographhead line. It has the following characteristics:• It must remain in continuous contact
with the overhead wire while the EMU
is running.
• It must not abrade the overhead wire,and neither must it be subject to excessive wear.
• It must have low aerodynamic resistance.
Japanese railways have long used a diamond-shaped pantograph. It has a re-(JR East)

placeable contact strip that touches theoverhead wire and is replaced when it
becomes worn by the running abrasion.
This strip must also be able to withstandarcing current that sometimes occurs between the overhead wire and the contact
strip if the two become separated duringrunning. The strip is fixed to the basecalled the collector shoe, mounted on aframe.
In Europe, a single-arm pantograph has
been used traditionally and it is being used
increasingly in Japan. A third type of pantograph is called the wing pantographwhich was developed to reduce windnoise generated by high-speed trains, it is
now used on some of the latest
shinkansen.

From Rheostatic
to Inverter Control

Wing pantograph on JR West Series 500 shinkansen

Copyright © 1998 EJRCF. All rights reserved.

(JR West)

The propulsion system has undergone
considerable change in the history of electric railways. Up until 10 years ago, thetraction motor was DC. For the EMU toaccelerate as it starts, the voltage applied
to the DC motor must be increased as the
EMU picks up speed. This is accomplished by inserting a variable resistor
between the pantograph and motors. The
voltage applied to the motor is controlled

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Technology

Figure 3

Principle of Rheostatic Control

PantographTraction motors

Variable
resistor

Figure 4 Chopper Control Circuit

ChopperMii(a) Chopper on

ChopperMii(b) Chopper off

M

M

M

M

M

M

M

M

by varying the resistance. This is calledrheostatic control (Fig. 3). In this system,
the resistance consumes most of the voltage supplied when the EMU is running
slowly and the motor is operating at a low
voltage, presenting various problems,such as low energy efficiency and a firerisk if the resistor overheats.
To overcome these problems, chopper
control was developed in the 1970s. As
shown in Fig. 4, instead of a resistor, aswitch is inserted between the pantographand motor and the average voltage applied to the motor is varied by changing
the switch on/off time (Fig. 5). On/off
switching must be performed several
hundred times per second, and a semiconductor thyristor is used. In fact, commercialization of the chopper control
system was enabled by development of alarge-capacity thyristor, which also triggered a revolution in EMU control—

replacement of the DC motor by the AC
motor.
The principle of the AC motor is quite
complex and is discussed below, focusing on the differences between AC andDC motors.
There are several types of AC motors, andthe cage asynchronous motor used widelyin Japan as the traction motor, is described.
Figure 6 shows a DC motor; Figure 7
shows the stator and rotor of an asynchronous motor. The DC motor has a commutator to supply current to the armaturewindings during rotation via carbon
brushes. In contrast, the asynchronous
motor uses windings on the stator side to
feed three-phase current. A major featureis the cage-like construction of the rotor(Fig. 8), which does not exchange current
with the stator.
What are the major advantages of theasynchronous motor of this type? They
come from the absence of the commutator and brushes. First, the asynchronous
motor is smaller and lighter. Second, it isfree from the danger of ground faults occurring when the commutator turns athigh speed and supplies a large current(several hundred amps) to brushes that
may cause sparking to the motor frame or
sparking between brushes. In fact, ground
faults account for the major number oftraction motor failures. Third, brushes
require regular inspection and replacement, and the commutator requires over-Figure 5 On/Off Chopper Timing Control

Voltage
1,500V

Voltage
1,500VTime
Low motor speed

42Japan Railway & Transport Review 17 • September 1998

Voltage
1,500VTime

Time
High motor speed

Copyright © 1998 EJRCF. All rights reserved.

Figure 6 DC Motor

Figure 7 AC Asynchronous Motor

Stator of asynchronous motor. The stator
is composed of a steel frame covered bycoils inside. The coils are divided into
three blocks: the U, V, and W phases,
forming a rotating field using three-phasecurrent from the inverter.

(Mitsubishi Electric Co., Ltd.)

Figure 8 Rotor Cage of AC
Asynchronous Motor

Rotor of asynchronous motor. The lackof commutator allows a larger space fortorque-generating coils.

Bar (Copper)

End ring(Mitsubishi Electric Co., Ltd.)

haul. Elimination of this work halves
motor maintenance time and costs.

Operating Principles of
Inverter Control System
To understand the inverter control system,
the following three points are important:• How is torque generated?
• How is the rotating field created by applying three-phase AC to the asynchronous motor?
• How does the inverter create threephase AC from DC with varying frequency and voltage?
These points are explained below.

How is torque generated?
We should start from Fleming’s left- andright-hand rules (Fig. 9). Fleming’s lefthand rule explains the force applied to a

Copyright © 1998 EJRCF. All rights reserved.

conductor in a magnetic field when current is applied to the conductor. Underthe rule, the direction of the magnetic field
is denoted by the index finger, the direction of current is denoted by the middle
finger and direction of the force applied
to the conductor is denoted by the thumb,when each is held at 90° to the other. The
strength of the force applied to the conductor is proportional to the product of
the field strength and power of the current.
Fleming’s right-hand rule describes the
direction and strength of voltage generated on the conductor moving through themagnetic field. When the same three fingers are held at 90° to each other, the index finger denotes the lines of themagnetic force, the thumb denotes the
motion of the conductor, and the middle
finger denotes the direction of the gener-

ated voltage (positive at finger tip andnegative at base). Again, the level of thevoltage is proportional to the product of
the strength of the field and the velocityof the conductor.
The torque generation mechanism in theasynchronous motor is explained below
based on Fleming’s rules.
In the asynchronous motor, a rotating
magnetic field is generated when AC isapplied to the stator windings. Figure 10
demonstrates this point by showing amagnet turning around the rotor. In relative terms, the magnet is static and therotor bars move. Based on the right-handrule, this generates voltage at both endsof each bar. Since the two ends of the bar
are connected to an end ring (conductor),a current flows through the bar, end ring,other bar, and opposite end ring. Unlike
the DC motor, this means current flows

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Technology

Figure 10 Rotating Magnetic Field

Figure 9 Fleming’s Rules
Direction offorce applied
to conductor

Conductor motion
N

Direction ofmagnetic fieldS

Direction ofmagnetic field

S

Generatedvoltage(a) Left-hand rule(b) Right-hand rulein the rotor without the need to supply itfrom the stator. Once current is flowing
through the rotor which is in a magnetic
field, based on the left-hand rule, force isapplied to the bars and rotor rotates. In
other words, the motor runs and drives
the EMU.
As the EMU accelerates, the rotor speed
eventually catches up with the rotating
speed of the magnetic field. At this point,
there is no relative motion between therotor bar and the rotating magnetic field,
so no voltage is generated and no current
flows, resulting in loss of torque andmeaning that the motor stops when theabove state is reached. To keep the motor (and EMU) running, the speed of therotating magnetic field is increased
steadily to keep it above the rotor speed,
thereby maintaining torque. As discussed
later, this is accomplished by raising thefrequency of the AC applied to the motor.
In other words, the motor requires continuous output frequency control using theinverter.

nous motor windings? To understand themechanism, the principles of electromagnetism must be explained. Based on thescrew law, the direction of the magnetic
field generated by an electric current isclockwise to the current direction.
Figure 11 shows the simplified asynchronous motor stator windings, which areconnected alternately at four points.
Figure 12 shows the change in the current flowing in the stator windings, andthe change in the magnetic field generated by the current. There are two points
that look like the north pole of a magnet
where the magnetic flux originates andtwo points that look like the south pole
where the magnetic flux ends. Furthermore, these north and south poles appear
to rotate with time. The relationship between the current waveform and the stator shown in the figure indicates that the
shorter the current cycle (or the higher thefrequency), the faster the magnetic fieldrotates. In other words, the motor torquecan be maintained at a constant level bycontrolling the frequency generated in the
inverter and applied to the motor.

How is the rotating field createdby applying three-phase AC tothe asynchronous motor?
So how is the rotating magnetic field generated by applying AC to the asynchro-

44

Japan Railway & Transport Review 17 • September 1998

N

Direction ofcurrent

How does the inverter convert
DC to three-phase AC withvarying frequency and voltage?
The mechanism for converting DC to AC
using an inverter is explained below.
As shown in Fig. 13, when switches S1and S4 are ON, and S2 and S3 are OFF,+E (V) is applied to the motor windings.
On the other hand, when S1 and S4 are
OFF and S2 and S3 are ON, –E (V) is applied. No voltage is applied when all theswitches are off. Each switch can beturned on or off at any timing. For example, on/off switching at the timing
shown in Fig. 13 (a), produces a voltage
waveform that can be called AC. The
average voltages are shown by the dottedline. The voltage is decreased by controlling the on/off switching as shown in(b), and the frequency is increased bycontrolling the on/off switching as shownin (c). For simplicity, the figure illustrates
a single phase, but a three-phase circuit
is actually used. The switches have
evolved first from a thyristor to a smaller
and lighter gate turn-off thyristor (GTO)
that can be turned off without external
voltage, and then to the insulated gate
bipolar transistor (IGBT) capable of highspeed switching. In Japan, this inverter
control system is often called VVVF

Copyright © 1998 EJRCF. All rights reserved.

Figure 13 Inverter Operating Principle (Single Phase)
InverterFigure 11 Simplified Stator
Winding

U

S1

S3

U

S2

S4

V

V
S1, S4 ON
All OFFW: Average
voltage

S2, S3 ON
All OFF(a) Voltage between U-V(b) Voltage between U-V(at falling voltage)UV(c) Voltage between U-V(at rising frequency)

W
Winding forward
Winding backward

Figure 12 Generation of Rotating Field by Flowing Three-phase Current

Current

(1)

(2)

(3)

(4)

iu

(5)

(6)

iv

iw

Time
o

S

N

N
N

S

N

SS
N
S

(1)

W

Copyright © 1998 EJRCF. All rights reserved.

N

S
N

N
N

(2)

N

S

S

U V

S

N

S

(3)

Small current winding forward
Big current winding forward

S
S

N

(4)

(5)

(6)

Small current winding backward
Big current winding backward

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Technology

Figure 14 Synchronous Motor Structure

N

S

Rotor
direction

Field
rotation
directionS

control, an abbreviation of variable voltage, variable frequency, suggesting theability to adjust the three-phase AC voltage and frequency supplied from the inverter to the motor according to thechanging train speed.

Synchronousand Asynchronous Motors
Another type of AC motor, called the synchronous motor, is in use, especially onmost French TGVs including Thalys, andSpanish AVEs. Figure 14 shows its struc-

Gate turn-off thyristor (GTO)

46

N

ture. Like the asynchronous motor, thesynchronous motor also has three-phase
AC windings, but the rotor is energizedby DC from an external source to create
magnets. Three-phase current on the stator windings produces a rotating field. The
rotor becomes an electric magnet due tothe DC. Attraction and repulsion between
the field and rotor magnet turn the rotor.
Since the rotor speed is the same as thatof the rotating magnetic field, the motor
is synchronous. Unlike the asynchronous
motor, the synchronous motor requires aslip ring to supply current to the rotor.

(Mitsubishi Electric Co., Ltd.)

Japan Railway & Transport Review 17 • September 1998

Brushes are used for this purpose and require some maintenance, although it is not
as frequent nor as costly as that required
for DC motors. Why is the synchronous
motor used by TGVs and other rollingstock? The answer is in the level of technology available when the TGV-A wasdesigned. Since a large-capacity GTO had
not been developed, supplying voltage toturn off the thyristor was a major problem. However, the rotor in a synchronous
motor is a magnet so it produces the required voltage between the terminals of
the stator windings. In fact, the voltage is
generated at the exact timing to turn off
the thyristor, eliminating the need for aturn-off circuit and resulting in reduced
inverter weight. Conversely, the asynchronous motor does not use the above
mechanism and the inverter (at that time)needed turn-off circuits, which added
weight. In France, the TGV axle weight
was limited to 17 tonnes, and the synchronous motor was the only available choicemeeting this requirement.

Concentrated andDistributed Traction Systems
Japanese shinkansen use electrical multiple units, sometimes called a distributed
traction system. On the other hand, highspeed railways in Europe mostly use theconcentrated traction system (similar to

Insulated gate bipolar transistor (IGBT)(Mitsubishi Electric Co., Ltd.)

Copyright © 1998 EJRCF. All rights reserved.

conventional loco-hauled system), where
the train consists of one or two power cars
and several passenger cars. For example,
the Series E2 shinkansen has six powered
passenger cars and two non-powered passenger cars (trailers), while the TGVRéseau has one power car at each end ofeight passenger cars. The TGV-Réseau
power cars carry no passengers.
The relative merits of the two systems arecompared below:
Concentrated traction system• Relatively small amount of electricalequipment per train resulting in lower
manufacturing and maintenance costs• Lower noise and vibration in passenger cars due to physical isolation of passenger cars from power units
Distributed traction system• Electrical braking on multiple axles resulting in: (a) less maintenance requirements for pneumatic brake parts suchas brake shoes, (b) good braking performance for a variety of performance
conditions, (c) better energy efficiency
through use of regenerative braking• Reduced maximum axle load, resultingin: (a) simplified tracks and structures
which reduce maintenance cost, (b)lowered maximum ground vibration• Low adhesion coefficient required for
driving axles• Lower performance loss if propulsion
unit fails in powered car• High seating capacity
The final choice depends upon which factors are emphasized. In Europe, highspeed trains have traditionally beenhauled by locomotives which have beensuperceded by the TGV and ICE. In Japan, the distributed traction system wasselected to reduce maximum axle load
thereby minimizing impact on track andother structures and reducing construction
costs, as well as to reduce mechanicalbraking requirements by using electricbraking on all axles.
A second reason reflects a major difference in railway conditions between Japan

Copyright © 1998 EJRCF. All rights reserved.

and Europe—the braking frequency as aresult of distances between stations. On
the 515-km Tokaido Shinkansen connecting Tokyo and Osaka, originally there
were ten intermediate stations separated
by an average of 47 km (currently there
are 14 intermediate stations). On the other
hand, on the 389-km TGV Southeast Linebetween Paris and Lyon, there are only
two intermediate stations where most
TGVs do not stop. Electric brakes offer alarge advantage when braking is frequent.
Overall, European railways selected theconcentrated traction system based on theinitial cost and ride comfort, while the
distributed traction system was selected
in Japan in consideration of the impact
on track and maintenance cost.
With the recent technological advances,electrical maintenance requirements havedeclined, a factor favouring the distributed
traction system. Also, stricter environmental regulation, including public demand
for reduced wayside vibration, serves asa tail wind for the distributed traction system.
However, there are already high-speed
European trains using the distributed traction system—the Italian ETR450, 460 and
470 , and the future German ICE3. Italy
selected the distributed system to implement pendulum EMUs (pendulum locomotives are not technically feasible at present).
The German ICE3 chose the distributed

system to minimize the required adhesion
coefficient, to increase passenger capacity
and, perhaps more importantly, to lower
the maximum axle load.

Future Challenges
What are the major challenges facing future propulsion systems?
First, development of more compact
lighter on-board equipment is an issue topursue. Although the shift from DC to AC
motors has permitted major progress inthese fields, there is still plenty of room
for improvement. Second, energy efficiency is an important challenge. Recent
concern about environmental issues is
prompting railways to step up energy saving efforts, although they already offer aclear advantage over other transportmodes. In particular, railways are expected to develop hardware and controllers to improve inverter and motor
operating efficiency, while fully leveraging regenerative braking.
On the negative side, inverter control produces high-harmonic noise affecting signalling and communication systems.
Research is required to solve this problem. Some other technical hurdles, including noise reduction and optimized
control for wheel slip remain to becleared.
■Kanji Wako
Mr Kanji Wako is Director in Charge of Research and Development at RTRI. He joined JNR in 1961 after graduating inengineering from Tohoku University. He is the supervising editor for this series on Railway Technology Today.

Hiroshi Hata
Mr Hata has been Senior Engineer in the Vehicle Technology Development Division of the Railway
Technical Research Institute since 1987. He is engaged in R&D of EMU electric components. He
joined JNR in 1979 after graduating from the Tokyo Institute of Technology.

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