Elements of electric traction for motormen and others

Preface

"Elements of Electric Traction" is based upon a short series of lectures and practical demonstrations which have been given by the Author during the last two years to a class of motormen and others at the Leeds Institute Technical School.

During these lectures and the subsequent period the writer has had repeated inquiries for a suitable textbook, and while he has recommended from time to time books dealing with the Elements of Electrical Engineering, as well as Handbooks for Motormen, he has always found a need for a book dealing in a simple manner with the fundamental, mechanical, and electrical principles underlying electric traction. To fill this need this work has been written.

The work is intended to serve as an introduction to the more advanced works on electric traction, and to supplement the information given in the various Handbooks for Motormen and others.

To make the work more helpful to students of Electric Traction generally, a number of formulae have been inserted, and, to further illustrate the text, examples have been added to the more important chapters.

Electric Traction is a branch of Electrical Engineering of rapidly increasing importance, and the Author trusts that this volume will help those intending to follow up this career by giving them a sound knowledge of the fundamental principles of their subject.

Finally, the writer takes this opportunity of thanking the following firms for having kindly given him diagrams and particulars of their traction controllers and motors : The British Thomson-Houston Co., Ltd. ; Dick-Kerr & Co., Ltd. ; The British Westinghouse Electric vi PREFACE

and Manufacturing Co., Ltd. ; also Kaworth's Traction Patents, Ltd., and The Johnson- Lundell Electric Traction Co., Ltd., for particulars of their regenerative systems ; and the Eomapac Tramway Construction Co., Ltd., for particulars and details concerning their patent tramrail. The writer also desires to thank Professor W. W. Haldane Gee for having kindly advised on the matter at an early stage, and also for a number of suggestions.

L. W. GANT.

Leeds,

December, 1906.

Introduction

Distribution of Electrical Energy Car Equipment — Path of Current Lightning Arresters Earth Switches — Leakage Lamps Lighting of Car

PAOB

1 3 6

7 8

CHAPTER n

Magnetic and Non-magnetic Materials — Characteristic Properties of Magnets . 9

Earth's Magnetism ........... 10

Polarity — Magnetic Force — Magnetic Field — Magnetic Induction ... 11

Retentivity — Methods of Magnetizing ........ 12

Hysteresis — Lines of Force .......... 13

Electro-magnetism — Electro-magnetic Laduction — Magnetizing Force — Permeability — Magnetic Flux — Magnetization Curves — Magnetic Saturation . 14 Magnetic Screening ........... 19

CHAPTER in

Effects of a Current ........... 20

Electric Generator — Pressure — Current — Resistance ..... 21

Conductors and Non-conductors . . . . . . . . 22

Insulating Materials — Measuring Instruments ...... 23

Heating Effect — Magnetic Effect — Chemical Effect ..... 23

Ohm's Law 25

Practical Proof of Ohm's Law ......... 26

Drop — E.M.F. and P.D. of a Dynamo ........ 29

Drop in Rail Return — B.O.T. Requirements ....... 30

Electrical Terms — Positive and Negative Poles — Series — Parallel — Shunt — Short

Coupling up Batteries in Series and in Parallel ...... 32

Resistances in Series and in Parallel ........ 34

Cables — Contact Surfaces . . . . . . . , . .37

Fuses — Automatic Cut-outs .......... 33

VUl

CONTENTS

CHAPTER IV

FAQE

Fundamental Principle .......... 43

Essential Parts— Essential Feature — Electromotive Force .... 44

Simple Dynamo — Simple Alternator ........ 47

Uniform Pressure ............ 49

Dynamo Construction — Commutator— Field Magnets — Series Dynamo — Shunt Dynamo — Compound Dynamo — Separately Excited Dynamo — Armature

Core — Conductors ........... 50

Characteristic Properties of Shunt, Series, and Compound Dynamos . . 62

CHAPTER V

Fundamental Principle ........... 56

Essential Parts — Essential Feature — Magnetic Blow-out .... 57

Left-hand Rule 59

Back Electromotive Force — Motor Starting Switches— Interlocking of Tramway

Controller Handles .......... 60

CHAPTER VI

Principle of Work — Conservation of Energy ..... Power or Rate of doing Work — Horse Power — Force, Energy, and Power Electrical Energy and Power — Relation between Electrical and Mechanical Units — B.O.T. Unit — Method of measuring Electrical Power — Efficiency

Test of a Series Motor .....

Torque — Measurement and Calculation of Torque Motor magnetically considered ....

Examples .

67 69

70 73 75 76 81 82

CHAPTER Vn

Motion — Speed 87

Accelerating Force . . . . . . . . . . .90

Friction — Frictional Force — Resistance to Traction ..... 92

Gradients and Gravitational Force . ....... 95

CONTENTS

IX

Horse-power, and Energy expended in Traction

Kinetic Energy— Heat Energy- — Potential Energy— Draw-bar Pull

Gearing .......

Belation between Torque and Draw-bar Pull .

Adhesive Force — Wear of Bails — Romapac System Measurement of Draw-bar Pull Formulae .....

Varying Draw-bar Pull — Car on Level Car on Gradient ....

Examples .....

PAGE

CHAPTER VIII

CHARACTERISTIC PROPERTIES OF CONTINUOUS-CURRENT

MOTORS

Classification of Motors — Shunt Motor— Series Motor Compound Motors — Cumulative Compound Motor

Motor — Self-regulating Properties .

Speed and Power Regulation .....

Load .........

Differential Compound

Motor Analysis .......

Shunt Motor — Speed Variation — Torque Variation . Series Motor — Speed and Torque Variation Particulars of Test of Traction Motor Speed Regulation — Varying Field — Varying Pressure Compound Characteristics — Motor Losses Secondary Actions ......

CHAPTER IX

Draw-bar Pull .......

Type of Motor — Series Motors as applied to Traction Series Parallel Control ....

Controllers — Motor Cut-outs

Controller Operations ....

Putting into Series — Putting into Parallel Running on Series — Late Paralleling Regenerative Systems ....

Raworth's Regenerative System Johnson-Lundell Regenerative System .

CHAPTER X

General Principle .......

Kinetic Energy — Formulae . . . . .

Classification of Brakes — Mechanical and Electrical

171 173 176

CONTENTS

Board of Trade Regulations— Mechanical Brakes — Wheel-brakea

Track-brakes .....

Electrical Brakes .....

Rheostatic Brake ....

B.T.H. Magnetic Slipper-brake

Westinghouse Magnetic Brake

Application of Rheostatic or Magnetic Slipper-brake

Emergency Brake — Application of Emergency Brake

Regenerative Braking ....

Brake Combinations ....

Reversal of Motors ....

Responsive and Non-responsive Actions .

Examples ......

PAGE

CHAPTEE I INTRODUCTION

The following elements of electric traction are primarily intended to deal with the principles underlying the application of electrical power to traction, and more definitely with the electrical power as delivered to the trolley in an ordinary overhead system. In order that an idea may be formed as to the working of an electric traction > system as a whole, two diagrams are given, one of which illustrates diagrammatically the method of distributing such power, and the other illustrates also diagrammatically the parts of a car which are essential for the safe and effective utilization of the power.

If the student does not understand all the terms used at the first reading of this chapter, he is advised to repeat the reading of it as he gets familiar with the later chapters.

The source of energy employed in this country is the heat energy derived from coal, the coal being utilized for the generation of steam, engines being employed for driving large continuous-current multipolar dynamos or generators, giving the requisite electrical pressure. This pressure, according to Board of Trade rules, must not exceed 550 volts, as delivered to the trolley wire.

Distribution. — The electrical power so generated is first delivered to the main switchboard for distribution, regulation, etc. On the main switchboard will be found all the main switches, fuses, circuit breakers, instruments, etc., for the controlling and operating of the various dynamo and feeder circuits, thus each dynamo and feeder circuit is controlled and safeguarded. Each dynamo has its own main switches, automatic circuit breaker, shunt regulator, etc., with the necessary instruments such as voltmeter and ammeter for indicating the pressure and current. By means of these switches the various dynamos can be coupled up in parallel so as to feed the circuits, or be cut out as the case may require.

The current from the machines is thus delivered to what are

B called the main bus bars ; these consist of heavy copper bars which carry the whole output to the various feeder panels, where it is taken off for the various districts. Each district, or portion of such, as the fH case may require, is fed by means of a feeder cable, each feeder being controlled by its own set of main switches, circuit breaker, fuses, ammeter, etc., fitted on the main switchboard.

INTRODUCTION 3

The feeder cables are led underground to the section feeder boxes, and from these the overhead trolley wire is fed, as indicated below.

The trolley wire is divided up — Board of Trade rule — into sections not exceeding half a mile in length, each half-mile section of the trolley wire being insulated from one another by section insulators.

The section feeder box on each route generally feeds one section of the trolley wire, and the various sections are suitably connected together by means of switches, fuses, connecting links, etc., placed in what are termed the feeder boxes. The overhead trolley wire, being so subdivided and so connected, is not liable to entire breakdown in the event of any accident to one section of the trolley wire, as that section can be cut out, and the other sections fed by suitably cross-connecting by means of the switches, etc., fitted in the feeder boxes.

In connection with the main switchboard there will also be what is generally called the Board of Trade panel, containing the various recording voltmeters, ammeters, and other instruments required for the various Board of Trade tests and records.

Fig. 1 diagrammatically illustrates the above arrangement, two dynamos, one feeder circuit, and four sections of the trolley wire, simply being shown. In this diagram the dynamos and switches, etc., are shown in diagrammatic fashion, thus showing the method of diagrammatically illustrating the various parts of an electric circuit.

The current, after passing down the trolley and through the car, returns to the generating station through the rails, cables being led from the rails at various points back to the main negative bus bar, and in this way the current is provided with a path back to the generators, a complete circuit thus being provided. As the rails are earthed, that is, connected with earth and so having the same electrical pressure, such a system is said to have an earthed return.

Car Equipment. — The essential parts of a car equipment can be classified as follows : —

(1) The necessary switches, circuit breakers, fuses, controllers, rheostats, or resistance boxes, for regulating and controlling the electrical power supplied to the motors ; also

(a) Some form of lightning arrester, for safeguarding the car

from a lightning discharge.

(b) In some cases an earth switch, whereby in an emergency

the trolley wire can be safely and effectively earthed, as may be advisable in the event of a trolley wire breaking.

(2) The necessary motors, complete with gear wheels, etc.

(3) The necessary switches, fuses, lamps, etc., for the effective lighiing 9f the car.

(4) The necessary hand brakes, sand boxes, and gear, life-guard attachments, gongs, signalling bells, destination indicators, etc.

Different engineers adopt slightly different arrangments in connection with the above switches, etc., that shown in Fig. 2 being one of many. As the different arrangements are, however, more a

INTRODUCTION 5

matter of detail, the diagram given will enable the general purpose and principle to be understood. Following the path of the current by means of the diagram, it will be seen that after being collected from the trolley wire by means of the trolley wheel, it then passes, by means of a cable connection from the trolley head, down the trolley pole and standard to a main motor switch fitted at one end of the car under the canopy, within reach of the motorman or conductor as the case may be. From there it passes to the other end of the car to an automatic circuit breaker, this being similarly situated to the main motor switch. From the automatic circuit breaker the current then passes through a fuse, this being placed outside the car at one end, and from there through the choking coil to the main trunk leads which run under the car seats. The choking coil is connected with the lightning-arrester portion of the circuit, the purpose of which will be explained later. The arrester and choking coil are also often fixed under the seats of the car.

The main trunk leads consist of two bunches of cables running from one end of the car to the other, and by means of these the controllers, motors, and rheostats are all connected ; the various cables being connected as they pass from controller to controller to the various armatures, fields, and rheostats.

The particular path the current will take will depend upon the particular positions of the main and reverse drums of the controller in use. Assuming the power handle is on the first notch in series, and the reverse drum is in the forward position, the current will pass, not directly as here indicated, but by means of the various fingers and cables through the following path, the particular path indicated being that taken in a B18 (B.T.H.) controller. After leaving the choking coil the current will pass through the blow-out coil of the controller in use to the main trolley finger, and from there will pass by means of the various fingers and cables to K2 terminal on the resistance frame. The remainder of the connections then being such that E7 is connected to Al, AAl to Fl, El to A2, AA2 to F2, and E2 to earth wire, and so to frame, wheels, and rails. The current in this way, after leaving the choking coil, passes through the blow-out coil of one controller, and after passing through a certain amount of regulating resistance E2 to R7, it then passes through the armature Al to AAl and field Fl to El of No. 1 motor, and then through the armature A2 to AA2 and field F2 to E2 of No. 2 motor, and from there to earth wire, and so to wheels and rails, in this way finally completing its circuit back to the generating station.

In some cases two circuit breakers are employed instead of one and a main switch, and sometimes the two circuit breakers are arranged in parallel; in this case, only one is employed at a time, the rear platform one being thrown off.

In other cases the fuse is dispensed with, the circuit in such instances being simply protected by means of the circuit breaker. The characteristic difference between these two methods of protecting a circuit will be dealt with later on. Where both are employed a double safeguard is obtained.

Lightning Arresters. — A lightning arrester has two main functions to fulfil — one of providing a suitable path for the lightning discharge, and the other of preventing current flowing from the trolley through that same path direct to earth. This is often carried out as follows : —

Two metal or carbon contacts are provided, one of which is connected to the trolley cable and the other to the w^heels and so to rails, namely earth ; these two contacts, however, do not quite touch, a small distance separating them ; there is thus a small air gap breaking the circuit. In consequence of this no current will flow, the pressure of 500 volts being insufficient to break down the resistance of the air gap. Very often the air gap is so arranged in traction work that at least a pressure of 2000 volts would be required to break down, or, in other words, to spark across the gap. While the trolley pressure is thus insufficient, the tremendous pressure obtained with a lightning discharge amounting to thousands of volts will be more than sufficient, and so the discharge when such takes place can bridge across the air gap, and so to earth, and in this way a path is provided. To prevent, however, the discharge also finding its way through a parallel path, namely, that of any portion of the motor circuit to earth — the discharge piercing the insulation — a choking coil is inserted, as shown in the motor circuit. This may consist of a few turns of insulated wire wound in the form of a spiral or solenoid on to a wooden core. The action of this, as well as other portions here mentioned, will be better understood after reading through the principles explained in the following chapters.

A lightning discharge is of an oscillating nature, somewhat similar to an alternating current, and the effect of such a current passing through a coil is to set up within the core a magnetic field, first in one direction, then in the other. The changing magnetic field, cutting across the turns of wire in the coil, sets up a back electromotive force similar to that obtained in a motor. The result is that of the two paths provided, the high-resistance one, due to the air gap in the lightning arrester, offers less total resistance to the discharge than that of the choking coil, which, as its name implies, tends to choke down any alternating current or discharge that tends to pass through it. In this way not only is a path provided for the lightning discharge in the arrester, but the motor circuit is also protected by means of the choking coil.

When once the resistance of the air gap has been broken down

INTRODUCTION 7

by the discharge — the arc formed across the gap consisting of metal or carbon vapour having far less resistance than the air — a small pressure, comparatively speaking, will be able to maintain the arc. The result of this would be that after a discharge had taken place the trolley pressure would maintain the arc once formed, and in this way a dead short to earth would take place. The second function of the lightning arrester is to prevent this. This is often carried out electromagnetically : the electromagnet, being excited by means of the current flowing from the trolley, operates either a magnetic blow-out, or mechanically separates the two contacts. In this way the arc which is being maintained is ruptured, current ceases to flow, and the electromagnetic portion of the apparatus then ceases to act. If the contacts have been separated they will then return to their original position, and the operations will be repeated when the next discharge takes place.

One method of carrying this out is shown in the diagram. A high non-inductive resistance — no back electromotive force is set up in a non-inductive resistance — is inserted in series with the air gap, and a portion of it is placed in parallel with the coil for operating the electromagnet. The lightning discharge takes the path through the non-inductive resistance to earth in preference to that through the coil, as this would have a choking effect. The current from the trolley, however, when such passes, being continuous in direction, prefers the path of least resistance to a continuous current, namely, that through the coil of the electromagnet. In this way, as soon as current begins to flow from the trolley through the arrester, the electromagnet automatically operates so as to break the circuit and prevent current from the trolley passing through the arrester to earth.

Earth Switch. — An emergency-switching arrangement is sometimes fitted to cars, so that in the event of a broken trolley wire the motorman can connect the trolley wire — the portion that is not down — direct to earth ; this causes a dead short which will blow the circuit breaker out at the power station, thus removing a source of danger from the streets, namely, a live trolley wire. This switch is entirely an emergency one, and is operated by the motorman breaking a glass, the breaking of the glass releasing a catch which holds the earth switch in its off position.

Leak on Trolley Standard. — As a danger exists of the passengers getting a shock from the trolley standard in the event of the insulation of the cable or trolley lead breaking down, the trolley standard in this way becoming alive, the following method of detection for such is sometimes adopted : —

Two lamps (tinted red) are coupled in series, one terminal is connected to the iron standard, and the other remaining terminal to earth. In the event of the insulation breaking down and the standard becoming " alive," the lamps light up, and warning is thus given of the state of affairs, and the conductor can then warn passengers to leave the top of the car.

Lighting. — One advantage of electric traction is that the car can be lighted electrically, and so much more effectively. For this purpose incandescent lamps are employed, these generally being 100-volt lamps, and arranged five in series. With 500-volts pressure this means that each lamp has 100 volts across its terminals. The lighting cables are attached to the trolley side of the main motor switch, and in this way, by switching on the lights, the motorman can test whether the section is " alive " or " dead ; " that is, whether pressure is available for use or not. Each set of live lamps will be protected by a fuse and controlled by a suitable switch, the complete circuit also being protected by a main fuse. The lighting generally consists of interior lights, fore and aft signal, destination, dash, canopy, and roof lights.

In an ordinary double-decker without roof these may amount to some seventeen lights, fifteen being in use at one time, one canopy and one dash-light only being lit at a time. This means three circuits, each circuit being composed of five lamps in series, one circuit being so arranged by means of switches that one canopy and one dash-light are always cut out.

There are various arrangements of switches and lights adopted ; the one mentioned above will, however, indicate the general lines upon which these may be arranged.

Chapter II Principles Of Magnetism

It has been found by experiment that only certain materials can be magnetized to an extent suitable for practical engineering purposes, and materials so magnetized are found to possess certain characteristic properties quite distinct and apart from any other properties they may possess.

The word '' magnet " is supposed to have been derived from the place Magnesia, where it is supposed iron ores possessing similar properties were first discovered.

The materials which can be magnetized are iron and steel and their alloys, though there are, it should be said, other materials which can be magnetized, such as nickel and cobalt, but these only to a very slight extent.

The other materials in use in engineering work, such as copper, brass, gun-metal, zinc, and lead, can be treated as practically non-magnetic or non-magnetizable. It should, however, be added that certain alloys of iron can be made, such as a special mixture of manganese steel, which are only magnetizable to a very slight extent.

At the outset it must be said there is nothing of an outward indication to show whether a body is magnetized or not, but if a straight bar of iron or steel be taken and magnetized in some way, that is, if an ordinary simple bar magnet be taken, it will be found on experimenting to possess the following magnetic properties : —

That one end of the bar magnet will repel one end of another bar magnet, and attract the opposite end of the same magnet. Thus there is a mutual action either of attraction or repulsion set up between two magnetized bars, and depending upon this the following properties will also be found to exist on further experimenting —

(a) That of setting or tending to set always in a certain direction, if carefully and delicately suspended, so as to be free to twist in any given direction.

(b) That of attracting unmagnetized pieces of iron or steel, namely, magnetizable bodies ; strictly speaking, this is not quite true, as will lo ELEMENTS OF ELECTRIC TRACTION

be seen later, the unmagnetized pieces of iron or steel really becoming ^magnetized by induction, so the real action is between two magnets, as stated above.

These fundamental experiments will now be dealt with a little more in detail, and from these will be obtained some knowledge of the characteristic properties of magnets generally.

If a simple bar magnet be dipped into a mass of iron or steel filings, it will be found that the filings are attracted and cling to the magnet chiefly in two clusters, the clusters being at opposite ends, thus giving a rough indication that the magnetism is chiefly concentrated in two places, or poles, as they are called, one being called the north, and one the south pole, the reason for which will be seen later.

If a piece of copper had been dipped into copper or brass filings, no such action would have taken place, and if an unmagnetized bar of iron or steel had been dipped into unmagnetized iron or steel filings, again no such action would have taken place; also if a magnetized bar of iron or steel had been dipped into copper or brass filings, no attraction whatever of the filings would have occurred. The action only takes place between any material that has been magnetized and materials which can be magnetized. This principle is employed in the magnetic separation of iron and brass filings ; it also explains the reason copper oil-cans are often employed in connection with dynamos, as the ordinary tinned iron might be attracted, and so damage some portion of the machine.

The reason for calling one end of the magnet a north pole and the other a south, is because, if a magnet be suspended as indicated above, it will be found to always set in one direction, namely, approximately due north and south ; that is, the end which is called the north pole is the end that always tends to point towards the north pole of the earth. If now a simple bar magnet be suspended, it will set as indicated approximately due north and south. On bringing near to the north end the corresponding end of another magnet, it will be found that the north pole of the one repels the north pole of the other, and that the north pole of the one attracts the south pole of the other, this being one of the fundamental facts in magnetism that like poles repel, and unlike attract. Thus there is a force of attraction between a north and a south pole, but a force of repulsion between two north poles or two south poles.

Earth's Magnetism. — It has been found that the earth itself is a magnet possessing a magetic north and south pole, and it may just be mentioned that the magnetic north pole is not the geographical north pole, consequently in navigation an allowance has to be made for this fact.

It is the action of the earth's magnetism acting upon the

PRINCIPLES OF MAGNETISM ii

maguetized needle, or bar of iron, which causes it to twist, and so set in a given direction. As it often leads to confusion, it should be noted that if that magnetic pole of the earth which lies approximately toward the actual north pole is a magnetic north pole, then the pole of the compass needle which is attracted in that direction must, since opposite poles attract, be a magnetic south pole. This difficulty is got over by calling the end of the compass needle which points northward, the north-seeking end or pole, or simply the north pole or end.

The action of the earth's magnetism upon a magnetic bar is simply that of twisting it in a certain direction ; it will not tend to move it bodily, as can be tested by floating a compass needle in a bath of water, the reason for this being that while one of the magnetic poles of the earth will attract one end of the compass needle, that same magnetic pole will also repel the other pole of the small magnet with practically the same force, the length of a small compass needle being exceedingly small compared with the distance of the earth's magnetic poles.

Polarity. — The north and south poles of a magnet cannot be separated. If a simple bar magnet be broken in two it will be found that each half contains a north and a south pole ; therefore wherever there is a north pole in that same bar there must also be a south pole. Sometimes it is useful to imagine such a thing as a free north pole, that is, a magnet possessing only a north pole, but such a thing actually is an impossibility. It is possible, however, to have magnetization without having poles ; thus, if an ordinary straight bar be taken and magnetized, and then bent into a circle so that the north and south poles touch one another, the effect of these two poles when close together will be to simply neutralize one another's action on any external piece of iron or steel, yet the iron ring will be in a state of magnetization.

Magnetic Force. — The force with which a magnet attracts or repels another magnet or any piece of iron or steel is called the magnetic force. This force will depend not only on the degree of magnetization, but also upon the distance the two magnets, or the magnet and the piece of iron, are apart ; the nearer they are the much greater will the force of attraction or repulsion be.

Magnetic Field. — The space all round a magnet pervaded by the magnetic force is called the magnetic field of that magnet. A magnetic field may be due either to a magnet or to a number of magnets, or some equivalent of a magnet in the shape of a coil of wire through which a current of electricity is flowing.