What Drives Electric Multiple Units? Hiroshi Hata

T Technology

echnolo Railway Technology Today 4 (Edited by Kanji Wako) What Drives Electric Multiple Units? Hiroshi Hata

the overhead wire converted to DC when three phase in one step. Evolution to AC Motors the motor is powered by AC?’ It is be- Clearly, it is confusing to use AC and DC, cause current from the overhead wire is for the electrification system on one side Direct current (DC) traction motors were single phase at a constant voltage and fre- and the traction motor on the other and used for a long time after the development quency, while the motor current is three this requires a clear explanation. Viewed of electric railways. However, since the phase with voltages and frequencies that from the pantograph as the ‘gateway’ to 1980s, they have been replaced by alter- vary according to speed (explained later); the EMU, DC was used first in Japan start- nating current (AC) motors, one type of there is no power converter able to con- ing at the end of the 19th century. The which commonly used in Japan is inverter vert voltage and frequency from single to AC electrification system was developed controlled. This article discusses the ba- sic traction systems used in today’s electric multiple units (EMUs) in Japan, with emphasis on the inverter control system. Figure 1 Main Equipment on Inverter-controlled DC EMU and Circuit Diagram of Propulsion Circuit From Current Collection to Driving

Almost everybody knows that EMUs are driven by electric motors powered by

gy current collected through pantographs on the top of the car. The entire mechanism can be explained from the viewpoint of Motors Filter reactor Inverter Motors (including how current flows and drives the motors. capacitor) Figure 1 shows a concept of the main equipment installed on an inverter-con- Motor trolled EMU and a circuit diagram of the Filter reactor propulsion circuit. The current collected Motor by the pantograph is fed to a high-speed Inverter Filter circuit breaker designed to protect the pro- capacitor Motor pulsion circuit from ground faults by iso-

lating large fault currents. The current Motor then passes through a filter reactor to an inverter where it is converted to AC that drives the AC traction motors mounted on the trucks. The motors generate torque that is transmitted via gear wheels to the driving wheels. The motor speed is reduced and torque is increased by engag- Figure 2 Location of Transformer and Main Converter on AC EMU ing 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 step- down transformer to convert the high voltage from the overhead line to a level for controlling the motors, and an AC to DC rectifier. In Japan, both the rectifier and Motors Transformer Motors Main converter inverter are often housed in a single box (Rectifier + inverter) called the main converter (Fig. 2). A common question is, ‘Why is AC from

40 Japan Railway & Transport Review 17 • September 1998 Copyright © 1998 EJRCF. All rights reserved. Traditional diamond-shaped pantograph (JR East) Single-arm pantograph (JR East) in the 1950s and has been adopted on head line. It has the following character- placeable contact strip that touches the various conventional lines (Hokkaido and istics: overhead wire and is replaced when it Kyushu, etc.) as well as on shinkansen • It must remain in continuous contact becomes worn by the running abrasion. (JRTR 16, pp. 48 to 58). DC traction mo- with the overhead wire while the EMU This strip must also be able to withstand tors, which have been in use for a long is running. arcing current that sometimes occurs be- time, are being replaced by AC motors. • It must not abrade the overhead wire, tween the overhead wire and the contact and neither must it be subject to exces- strip if the two become separated during Types of Pantographs sive wear. running. The strip is fixed to the base and Principal Mechanism • It must have low aerodynamic resis- called the collector shoe, mounted on a tance. frame. The pantograph is an integral part of the Japanese railways have long used a dia- In Europe, a single-arm pantograph has EMU used to collect current from the over- mond-shaped pantograph. It has a re- been used traditionally and it is being used increasingly in Japan. A third type of pantograph is called the wing pantograph which was developed to reduce wind noise generated by high-speed trains, it is now used on some of the latest shinkansen.

From Rheostatic to Inverter Control

The propulsion system has undergone considerable change in the history of electric railways. Up until 10 years ago, the traction motor was DC. For the EMU to accelerate 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

Wing pantograph on JR West Series 500 shinkansen (JR West) voltage applied to the motor is controlled

replacement of the DC motor by the AC motor. Figure 3 Principle of Rheostatic Control The principle of the AC motor is quite complex and is discussed below, focus- ing on the differences between AC and Pantograph Traction motors DC motors. M M M M There are several types of AC motors, and the cage asynchronous motor used widely in Japan as the traction motor, is described. Variable M M M M resistor 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 armature by varying the resistance. This is called windings during rotation via carbon rheostatic control (Fig. 3). In this system, brushes. In contrast, the asynchronous the resistance consumes most of the volt- motor uses windings on the stator side to Figure 4 Chopper Control Circuit age supplied when the EMU is running feed three-phase current. A major feature slowly and the motor is operating at a low is the cage-like construction of the rotor voltage, presenting various problems, (Fig. 8), which does not exchange current Chopper such as low energy efficiency and a fire with the stator. M risk if the resistor overheats. What are the major advantages of the i i To overcome these problems, chopper asynchronous motor of this type? They control was developed in the 1970s. As come from the absence of the commuta- shown in Fig. 4, instead of a resistor, a tor and brushes. First, the asynchronous (a) Chopper on switch is inserted between the pantograph motor is smaller and lighter. Second, it is and motor and the average voltage ap- free from the danger of ground faults oc- Chopper plied to the motor is varied by changing curring when the commutator turns at M the switch on/off time (Fig. 5). On/off high speed and supplies a large current i switching must be performed several (several hundred amps) to brushes that i hundred times per second, and a semi- may cause sparking to the motor frame or conductor thyristor is used. In fact, com- sparking between brushes. In fact, ground (b) Chopper off mercialization of the chopper control faults account for the major number of system was enabled by development of a traction motor failures. Third, brushes large-capacity thyristor, which also trig- require regular inspection and replace- gered a revolution in EMU control— ment, and the commutator requires over-

Figure 5 On/Off Chopper Timing Control

Voltage Voltage Voltage 1,500V 1,500V 1,500V

Time Time Time Low motor speed High motor speed

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

(Mitsubishi Electric Co., Ltd.)

Figure 8 Rotor Cage of AC Rotor of asynchronous motor. The lack Asynchronous Motor of commutator allows a larger space for torque-generating coils. Bar (Copper)

End ring

(Mitsubishi Electric Co., Ltd.)

haul. Elimination of this work halves conductor in a magnetic field when cur- ated voltage (positive at finger tip and motor maintenance time and costs. rent is applied to the conductor. Under negative at base). Again, the level of the the rule, the direction of the magnetic field voltage is proportional to the product of Operating Principles of is denoted by the index finger, the direc- the strength of the field and the velocity Inverter Control System tion of current is denoted by the middle of the conductor. finger and direction of the force applied The torque generation mechanism in the To understand the inverter control system, to the conductor is denoted by the thumb, asynchronous motor is explained below the following three points are important: when each is held at 90° to the other. The based on Fleming’s rules. • How is torque generated? strength of the force applied to the con- In the asynchronous motor, a rotating • How is the rotating field created by ap- ductor is proportional to the product of magnetic field is generated when AC is plying three-phase AC to the asynchro- the field strength and power of the cur- applied to the stator windings. Figure 10 nous motor? rent. demonstrates this point by showing a • How does the inverter create three- Fleming’s right-hand rule describes the magnet turning around the rotor. In rela- phase AC from DC with varying fre- direction and strength of voltage gener- tive terms, the magnet is static and the quency and voltage? ated on the conductor moving through the rotor bars move. Based on the right-hand These points are explained below. magnetic field. When the same three fin- rule, this generates voltage at both ends gers are held at 90° to each other, the in- of each bar. Since the two ends of the bar How is torque generated? dex finger denotes the lines of the are connected to an end ring (conductor), We should start from Fleming’s left- and magnetic force, the thumb denotes the a current flows through the bar, end ring, right-hand rules (Fig. 9). Fleming’s left- motion of the conductor, and the middle other bar, and opposite end ring. Unlike hand rule explains the force applied to a finger denotes the direction of the gener- the DC motor, this means current flows

Figure 9 Fleming’s Rules Figure 10 Rotating Magnetic Field

Direction of Conductor motion force applied to conductor N Direction of Direction of magnetic field magnetic field

S S Direction of Generated

current voltage N (a) Left-hand rule (b) Right-hand rule

in the rotor without the need to supply it nous motor windings? To understand the How does the inverter convert from the stator. Once current is flowing mechanism, the principles of electromag- DC to three-phase AC with through the rotor which is in a magnetic netism must be explained. Based on the varying frequency and voltage? field, based on the left-hand rule, force is screw law, the direction of the magnetic The mechanism for converting DC to AC applied to the bars and rotor rotates. In field generated by an electric current is using an inverter is explained below. other words, the motor runs and drives clockwise to the current direction. As shown in Fig. 13, when switches S1 the EMU. Figure 11 shows the simplified asynchro- and S4 are ON, and S2 and S3 are OFF, As the EMU accelerates, the rotor speed nous motor stator windings, which are +E (V) is applied to the motor windings. eventually catches up with the rotating connected alternately at four points. On the other hand, when S1 and S4 are speed of the magnetic field. At this point, Figure 12 shows the change in the cur- OFF and S2 and S3 are ON, –E (V) is ap- there is no relative motion between the rent flowing in the stator windings, and plied. No voltage is applied when all the rotor bar and the rotating magnetic field, the change in the magnetic field gener- switches are off. Each switch can be so no voltage is generated and no current ated by the current. There are two points turned on or off at any timing. For ex- flows, resulting in loss of torque and that look like the north pole of a magnet ample, on/off switching at the timing meaning that the motor stops when the where the magnetic flux originates and shown in Fig. 13 (a), produces a voltage above state is reached. To keep the mo- two points that look like the south pole waveform that can be called AC. The tor (and EMU) running, the speed of the where the magnetic flux ends. Further- average voltages are shown by the dotted rotating magnetic field is increased more, these north and south poles appear line. The voltage is decreased by con- steadily to keep it above the rotor speed, to rotate with time. The relationship be- trolling the on/off switching as shown in thereby maintaining torque. As discussed tween the current waveform and the sta- (b), and the frequency is increased by later, this is accomplished by raising the tor shown in the figure indicates that the controlling the on/off switching as shown frequency of the AC applied to the motor. shorter the current cycle (or the higher the in (c). For simplicity, the figure illustrates In other words, the motor requires con- frequency), the faster the magnetic field a single phase, but a three-phase circuit tinuous output frequency control using the rotates. In other words, the motor torque is actually used. The switches have inverter. can be maintained at a constant level by evolved first from a thyristor to a smaller controlling the frequency generated in the and lighter gate turn-off thyristor (GTO) How is the rotating field created inverter and applied to the motor. that can be turned off without external by applying three-phase AC to voltage, and then to the insulated gate the asynchronous motor? bipolar transistor (IGBT) capable of high- So how is the rotating magnetic field gen- speed switching. In Japan, this inverter erated by applying AC to the asynchro- control system is often called VVVF

Inverter Figure 11 Simplified Stator Winding S1 S3 U

V U S2 S4

V S1, S4 ON All OFF : Average S2, S3 ON voltage All OFF W (a) Voltage between U-V

(b) Voltage between U-V (at falling voltage)

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

Current (1) (2) (3) (4) (5) (6)

iu iv iw

Time o

S N N S N S S N N S S N

N S S N S N N S S N N S (1) (2) (3) (4) (5) (6)

Small current winding forward Small current winding backward U V W Big current winding forward Big current winding backward

Brushes are used for this purpose and require some maintenance, although it is not Figure 14 Synchronous Motor Structure as frequent nor as costly as that required for DC motors. Why is the synchronous motor used by TGVs and other rolling stock? The answer is in the level of tech- Rotor nology available when the TGV-A was direction N S designed. Since a large-capacity GTO had not been developed, supplying voltage to turn off the thyristor was a major prob- Field lem. However, the rotor in a synchronous rotation direction motor is a magnet so it produces the required voltage between the terminals of S N the stator windings. In fact, the voltage is generated at the exact timing to turn off the thyristor, eliminating the need for a turn-off circuit and resulting in reduced inverter weight. Conversely, the asynchronous motor does not use the above mechanism and the inverter (at that time) control, an abbreviation of variable volt- ture. Like the asynchronous motor, the needed turn-off circuits, which added age, variable frequency, suggesting the synchronous motor also has three-phase weight. In France, the TGV axle weight ability to adjust the three-phase AC volt- AC windings, but the rotor is energized was limited to 17 tonnes, and the synchro- age and frequency supplied from the in- by DC from an external source to create nous motor was the only available choice verter to the motor according to the magnets. Three-phase current on the sta- meeting this requirement. changing train speed. tor windings produces a rotating field. The rotor becomes an electric magnet due to Concentrated and Synchronous the DC. Attraction and repulsion between Distributed Traction Systems and Asynchronous Motors the field and rotor magnet turn the rotor. Since the rotor speed is the same as that Japanese shinkansen use electrical mul- Another type of AC motor, called the syn- of the rotating magnetic field, the motor tiple units, sometimes called a distributed chronous motor, is in use, especially on is synchronous. Unlike the asynchronous traction system. On the other hand, high- most French TGVs including Thalys, and motor, the synchronous motor requires a speed railways in Europe mostly use the Spanish AVEs. Figure 14 shows its struc- slip ring to supply current to the rotor. concentrated traction system (similar to

Gate turn-off thyristor (GTO) (Mitsubishi Electric Co., Ltd.) Insulated gate bipolar transistor (IGBT) (Mitsubishi Electric Co., Ltd.)

46 Japan Railway & Transport Review 17 • September 1998 Copyright © 1998 EJRCF. All rights reserved. conventional loco-hauled system), where and Europe—the braking frequency as a system to minimize the required adhesion the train consists of one or two power cars result of distances between stations. On coefficient, to increase passenger capacity and several passenger cars. For example, the 515-km Tokaido Shinkansen connect- and, perhaps more importantly, to lower the Series E2 shinkansen has six powered ing Tokyo and Osaka, originally there the maximum axle load. passenger cars and two non-powered pas- were ten intermediate stations separated senger cars (trailers), while the TGV- by an average of 47 km (currently there Réseau has one power car at each end of are 14 intermediate stations). On the other Future Challenges eight passenger cars. The TGV-Réseau hand, on the 389-km TGV Southeast Line power cars carry no passengers. between Paris and Lyon, there are only What are the major challenges facing fu- The relative merits of the two systems are two intermediate stations where most ture propulsion systems? compared below: TGVs do not stop. Electric brakes offer a First, development of more compact Concentrated traction system large advantage when braking is frequent. lighter on-board equipment is an issue to • Relatively small amount of electrical Overall, European railways selected the pursue. Although the shift from DC to AC equipment per train resulting in lower concentrated traction system based on the motors has permitted major progress in manufacturing and maintenance costs initial cost and ride comfort, while the these fields, there is still plenty of room • Lower noise and vibration in passen- distributed traction system was selected for improvement. Second, energy effi- ger cars due to physical isolation of pas- in Japan in consideration of the impact ciency is an important challenge. Recent senger cars from power units on track and maintenance cost. concern about environmental issues is Distributed traction system With the recent technological advances, prompting railways to step up energy sav- • Electrical braking on multiple axles re- electrical maintenance requirements have ing efforts, although they already offer a sulting in: (a) less maintenance require- declined, a factor favouring the distributed clear advantage over other transport ments for pneumatic brake parts such traction system. Also, stricter environmen- modes. In particular, railways are ex- as brake shoes, (b) good braking per- tal regulation, including public demand pected to develop hardware and control- formance for a variety of performance for reduced wayside vibration, serves as lers to improve inverter and motor conditions, (c) better energy efficiency a tail wind for the distributed traction sys- operating efficiency, while fully leverag- through use of regenerative braking tem. ing regenerative braking. • Reduced maximum axle load, resulting However, there are already high-speed On the negative side, inverter control pro- in: (a) simplified tracks and structures European trains using the distributed trac- duces high-harmonic noise affecting sig- which reduce maintenance cost, (b) tion system—the Italian ETR450, 460 and nalling and communication systems. lowered maximum ground vibration 470 , and the future German ICE3. Italy Research is required to solve this prob- • Low adhesion coefficient required for selected the distributed system to imple- lem. Some other technical hurdles, in- driving axles ment pendulum EMUs (pendulum locomo- cluding noise reduction and optimized • Lower performance loss if propulsion tives are not technically feasible at present). control for wheel slip remain to be unit fails in powered car The German ICE3 chose the distributed cleared. I • High seating capacity The final choice depends upon which fac- tors are emphasized. In Europe, high- speed trains have traditionally been Kanji Wako hauled by locomotives which have been Mr Kanji Wako is Director in Charge of Research and Development at RTRI. He joined JNR in 1961 after graduating in superceded by the TGV and ICE. In Ja- engineering from Tohoku University. He is the supervising editor for this series on Railway Technology Today. pan, the distributed traction system was selected to reduce maximum axle load thereby minimizing impact on track and Hiroshi Hata other structures and reducing construction Mr Hata has been Senior Engineer in the Vehicle Technology Development Division of the Railway costs, as well as to reduce mechanical Technical Research Institute since 1987. He is engaged in R&D of EMU electric components. He braking requirements by using electric joined JNR in 1979 after graduating from the Tokyo Institute of Technology. braking on all axles. A second reason reflects a major differ- ence in railway conditions between Japan