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Figure 3 Simplified diagram of a track circuit
~ A source of energy--for example, a battery.
~ A limiting resistance, called because it limits the current from the battery. * Rails and rail bonding, both offering resistance.
~ Ties and ballast, both offering a path for current leakage from rail. This path has resistance, referred to as \
~ Relay series resistance (resistance placed in series with the relay) . ~ A track relay.
The arrows show the direction of current flow. Starting from positive post of the battery, the current flows through the limiting resistance, the one rail, the relay windings, the relay series
resistance, and the other rail, then back to the negative post of the battery.2 With the relay energized, It closes a contact to light the lamps (or to control mechanism) .
As the wheels and axles of a train move onto the track circuit, they provide a path from rail to rail through which the battery current flows, thus robbing the relay of its current and causing it to open the contact.
In other words, when no train is present, the relay is energized by the current flowing from the power source through the rails. When a train is present, its axles shorten (shunt) the rails together, the current to the track relay coil drops, and the relay is de-energized. Therefore circuits through the relay contacts report whether or not the track is occupied.
Track circuits allow railway signaling systems to operate semi-automatically, by displaying signals for trains to slow down or stop when track ahead is occupied. They help prevent dispatchers and operators from causing accidents, both by informing track occupancy and by preventing signals from displaying unsafe indications. Resistance of the track circuit
The resistance of the track circuit includes: rail resistance, ballast resistance, limiting resistance. In common track circuits, the resistance of the rails may vary from 0.015 to 0.05 ohm per 1000 feet of the track, the ballast resistance may vary from one to hundreds of ohms per 1000 feet of track. For example, a 4000-foot circuit may have a total rail resistance of 0.1 ohm and a total minimum ballast
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resistance of 0.25 ohm in wet weather. In dry weather, or in zero weather, this ballast resistance may increase to hundreds of ohms.
The maximum distance which a track circuit can operate properly is dependent on several factors, principally on ballast leakage.3
Because there is a limiting resistance in the battery feeding to the track, the track voltage and, in turn, the relay voltage vary path consisted of the various conducting paths through the ballast and ties and, when track is occupied, the path also consists of the wheels and axles of the train. Thus we have a widely varying resistance across the rails and a widely varying voltage across the relay.
Figure 4 shows how relay current varies as the resistance across the rails is varied. To simplify our example, we have chosen a relay that picks up at 100 milliamperes and drops away at 50
milliamperes. By \its contacts. In signalman's language we would say this relay has 100 mills pickup and 50 mills release.
Looking at curve B in Figure 4, as we move shunt resistance across the rails from zero to 0.25
ohm, the relay current increases 100 mills, picking up this relay. In other words, this circuit will not operate unless the ballast resistance is 0.25 ohm or more. Now moving backward along curve toward zero resistance, we find the current of this relay releases below 50 mills at 0.1 ohm. This is well above the 0.06-ohm shunt required by the Federal Railroad Administration to ensure the pr0t~ sensitivity of track circuit shunting.
The picture is varied by many factors: percent release of the track relay, relay resistance chang~ with temperature, \changed with temperature, etc.
New Words and Expressions
Arrangement n．布局，安排 Principle n．法则，原则，原理
Insulated section 绝缘区段
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Relay 继电器
Resistance n． 电阻 ‘ Rail bonding 钢轨引接线 Ballast resistance 道碴电阻 Tie n. 轨枕
Leakage n． 漏，泄漏，渗漏 Positive post 正极 Negative post 负极 Contact n． 节点 Wheel and axle 轮轴 Voltage 电压 Attract 吸引
Armature n． 衔铁 exert 施力口力
Shunting sensitivity 分路灵敏度 Be fundamental to 对??很重要 Consist of 由·．．．．·组成 Refer to as 称为
In series with 与?．．·串联 Rob of 使·．．．．·失去 Be dependent On 视·．．．．·而定 Vary with 随??而变化 ·
Notes to the Text
1．The simplest form Of track circuit is an insulated section Of track with a relay On one end and with A bakery Or some other source Of energy On the other end．
译文：轨道电路最简单的形式是一段绝缘钢轨，在其一端有一个轨道继电器，在另一端有一 个蓄电池或其他类型的电源。
2．Starting from positive post Of the battery，the current flows through the limiting resistance，the one rail，the relay windings，the relay series resistance，and the other rail，then back to the negative post Of the battery。
译文：电流从电源的正极开始，流经限流电阻、其中一根钢轨、继电器线圈、继电器串联电 阻，最后经由另外一根钢轨回到电源的负极。
“Starting from positive post of the battery”为现在分词作状语，“the current”为句子的主语。 3．The maximum distance which a track circuit can operate properly is dependent On several factors， principally on ballast leakage．
译文：保证轨道电路正常工作的最大距离取决于若干因素，尤其是道碴漏泄。
“The maximum distance”为句子的主语“which a track circuit can operate properly”修饰主语。 4．In other words，this circuit will not operate unless the ballast resistance is 0．25 ohm or more．
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译文：换句话说，除非道碴电阻达到0．25欧姆或0．25欧姆以上，否则电路不会工作。 not．．．unless 除非??才 Exercises to the Text I Translate the following terms into Chinese.
( 1 ) track circuits (2) an insulated section of track (3) a limiting resistance (4) ballast resistance (5) the negative post of the battery (6) in turn (7) sensitivity of track circuit shunting ( 8 ) picks up (9) drop away II Decide whether each of the following statements is true or false according to the text. ( 1 ) When the train is present, the relay is energized. ( )
(2) In Figure 4, as we move shunt resistanle across rails from zero to 0.25 ohm, the relay current increases 100 mills. ( )
(3) When the relay is energized, its contacts will be closed. ( )
(4) With the relay energized, its contacts will be opened to light the lamps or to control
mechanism. ( )
(5) The maximum distance of a track circuit is dependent on several factors, principally on ballast leakage. ( )
III Answer the following questions according to the text. ( 1 ) What does a simplest track circuit consist of ?
(2) How does the relay current vary as the resistance across the rails is varied? (3) How does the resistance of the rails may vary in common track circuits ?
(4) When we say this relay had 100 mills pickup, what does it mean? Reading material
Track Circuit Data and Calculations
In the earlier discussion of D-C track circuit, it was pointed out that track circuit and the shunting of track circuits are not as simple as might appear from schematic diagrams. A-C track circuits introduce further considerations involving frequency discrimination and phase relationship which make additional complexity. High-frequency track circuits involve circuit parameters which are
negligible in D-C or A-C track circuits but which become important at audio frequencies controlling. Nevertheless, given characteristic data on a track, it is possible to apply mathematical methods to design track circuits for that track, including series rail resistance and inductance, also shunt leakage and capacitance. Measurements made on the track, according to input impedance of the open-circuited and short-circuited track at a remote point, are helpful to obtain the necessary data.
GRS (General Railway Signal) has developed a number of computer programs used in track circuits design which are applicable to track with a variety of physical and electrical characteristics. Track circuit performance predicated by these programs has been substantiated by extensive observations on actual track circuits.
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The computer programs are used by GRS for signal system work in which GRS has
responsibility for track circuit design. The programs are also used by GRS to provide consulting service to railroad signal engineers engaged in track circuit design. Exercise to the Material Choose the best answer according to the material.
(1) The word \ \
A. However B. Because C. So D. Though (2) What are the computer programs used by GRS to A. design track circuits
B. provide consulting service to railroad workers C. measure track circuits
D. simplify track circuits
(3) The word \ A. track circuits B. circuit parameters
C. audio frequencies D. further considerations (4) Which of the following can be inferred from the passage?
A. D-C track circuits are as simple as might appear from schematic diagrams B. A-C track circuits are simpler than D-C track circuits
C. A-C track circuits involve more parameters which make for complexity
D. A-C track circuits involve some parameters which are negligible in D-C track circuits (5) What do we not need to apply mathematical methods to assist in the design of track circuits for that track?
A. Series rail resistance and inductance B. Shunt leakage
C. Capacitance track D. The relay resistance A-C Track Circuits(j交流轨道电路） What is A-C
A-C stands for \ 0rms of supplying electrical power. The other common form is \ measure of electron flow rate. Direct Current describes current that flows in one direction. Alternating Current describes current which flow direction alternates. It flows in one direction, then reverses .to the other direction.
The outlines of A-C track circuits
In a previous article, the operation of D-C track circuits was described. Those track circuits use D-C (direct current) to power track circuit relays. An A-C track circuit uses the same principle. A-C