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BR WR Train Lighting System


Chairman: Mr. C. T. ROBERTS, B.Sc. M.I.Mech.E.
"An Outline of the Train Lighting System of the British Railways,Western Region." by A. H. COLE, A.M.I.Mech.E., A.M.I.Struct.E. (Member).

Current for the electric lighting of a steam hauled train may be provided by a generator, a battery, or both. If a generator alone is used, it must be driven by a prime mover, the operation of which is independent of the motion of the train, such as an auxiliary steam engine. This may be conveniently supplied with steam from the locomotive, and then, of course, the train can only be lighted when the locomotive is attached. If a battery alone is used then it has to be charged at a stationary depot between journeys. The former of these methods is in use today in various parts of the world. The latter was only used in the very early days of electric train lighting.
When both generator and battery are used, the generator has to be a direct current machine, so that it may be capable of charging the battery, and it can then be driven from an axle of one of the vehicles to which it is to supply power. When the train is stationary, or moving too slowly to generate a sufficiently high voltage, current for the lamps is taken from the battery. When the speed rises the generator, or dynamo, as the direct current machine used is more usually termed, takes over and also supplies a charging current to the battery. This latter duty it also carries out when the lamps are off, and indeed it is then that the battery receives most of its charge. Usually, today, a dynamo and a battery are installed on each vehicle, so that trains may be made up in any order without affecting the lighting. This is the case in the system to be described.

Of course, the battery cannot be allowed to remain connected to the dynamo, when the train is stationary, or when it is travelling so slowly as to cause the dynamo to generate a voltage less than that of the battery. lf it were, the battery would try to drive the dynamo as a motor, with rather disastrous effects to both components. Therefore a device, known as the auto switch, is provided, which automatically connects the battery to the dynamo at the correct voltage and disconnects it when the generated voltage falls appreciably below this value. In some systems, including the one to be described, the battery not only supplies current when the dynamo is unable to do so, but also helps to keep the generated voltage within certain limits in spite of the varying train speed. These limits however are too wide for satisfactory operation of the lamps, for the voltage of the battery varies considerably according to its state of charge, and therefore a device called a regulator is supplied to keep the fluctuations of lamp voltage within very narrow limits, actually within about plus or minus 1 volt in the system to be described. In this system the regulator also controls the charging current to the battery in relation to its state of charge, so that the charging process is such as to keep,the battery in good condition and prolong its life.

A main switch has to be supplied on each vehicle in addition to the tumbler switches in the compartments, so that lights may be switched on and off according to the hour of the day and the existence of tunnels on the route, and thereby the battery may be kept as well charged as possible. ln order that the guard may control the lights on the whole train, these switches can be electrically operated and are arranged so that the operation of an auxiliary switch on any vehicle will open or close all the main switches on the train. This system is known as the "Through Control" and couplings and flexible cables between adjacent vehicles are provided for this purpose. The main switch in any given vehicle can also be manually operated, in which case only the lamps on that particular vehicle are lighted. The main switch is known as the distance switch. Through control on non-corridor stock is operated from the guard's compartment and an auxiliary switch is installed on each end of the vehicle, capable of being operated from station platforms. Being exposed to the weather, this switch is of different design from the through control switch used on corridor stock.

The principal components, therefore, of the train lighting system used on the Western Region are,

  • (l) The dynamo,
  • (2) The battery,
  • (3) The auto switch,
  • (4) The distance switch,
  • (5) The regulator,
  • (6) The through control switch and wiring.

In addition a fuse board is supplied on which are mounted the fuses controlling the various circuits.
The dynamo and battery are carried under the vehicle slung from the underframe. On corridor stock the auto switch, regulator, distance switch, through control switch and fuse board are housed in a cupboard at one end of the corridor, whilst on non-corridor stock they are in a box slung from the underframe. On brake vehicles they are now installed in the guard's compartment.

The Dynamo (Fig. 1).
The dynamo is suspended, pendulum fashion, from the underframe of the vehicle by means of a pivoted hanger. It is driven by a belt from a pulley on the inner axle of the adjacent bogie and the length of the belt is so adjusted that it pulls the dynamo out of the vertical through the pivot, so that the weight of the dynamo itself tends to maintain the belt tension. This tension is further controlled by a spring attached to the. pin through the suspension lug and to a point on the underframe of the vehicle.
The dynamo itself is of the four pole, wave wound type with shunt field. There are two distinct sets of field coils. One, the main field, is connected to the positive main brush and to a third brush set nominally half way between the main brushes. The other, called the auxiliary field, is connected to the main positive and negative brushes and is only energised when the lamps are on. The brushes are mounted on rockers, one for the main brushes, and, to allow for adjustment, a separate one for the third brush. When the train reverses its direction, the friction of the brushes on the commutator is sufficient to carry them through 180 electrical degrees, so that their polarity remains the same. Without this provision, upon reversal, the residual field magnetism would be in the wrong direction, and the machine would fail to excite, and even if it would excite, the polarity would be reversed and the current to the battery would flow in the wrong direction. On open circuit the voltage between the positive and third brushes should be practically half that between the main brushes, the third brush being set exactly half way between the other two. In practice, owing to small variations in construction, this is often not realised, and furthermore the voltage generated in one direction of rotation differs from that in the other direction. It is for these reasons that provision is made for adjusting the position of the third brush independently of the main brushes.

fig2 The voltage generated by a dynamo is proportional to the product of the field strength and the speed of rotation. The speed of the train lighting dynamo, excluding those systems where mechanical provision is made to keep the speed constant above a certain maximum train speed, varies with that of the train. Diagram (a), Fig. 2, shows the lines of force of the field of a two-pole machine, this type being used for purposes of illustration, although the actual machine has four poles. Diagram (b) shows the field set up by the armature ` current. It will be seen that adjacent to the top right and bottom A at left quadrants of the armature, the flux of the latter opposes that of a the field, whilst in the other two quadrants it assists it. This is the phenomenon known as armature reaction, and it increases as the armature current increases. Now in the quadrant between the positive main and the third brushes, to which the main field coils are connected, the fluxes are in opposition. Therefore an increase fig3 in armature current results in a weakening of the flux responsible for the generation of the third brush voltage and hence a decrease of the field current. This in turn results in a weakening of the main field flux which causes a decrease in the voltage across the main brushes. When the train starts, the dynamo is not connected to the battery, and its voltage rises until it reaches about 27. At this point the auto-switch closes and connects the dynamo to the battery. Now considering the circuit through the dynamo and battery, we have, by Kirchhoff's second law, (Fig. 3),
whereVD = the terminal voltage of the dynamo
EB : the battery
RB : the resistance of the battery
IB = the current through the battery.
This would be true for any type of D.C. generator. RB is extremely small but a normal dynamo would, as its speed increased, send a very large current through the battery, so raising the value of the product RBIB and enabling VD to exceed EB considerably, and incidentally damaging itself and the battery. With the third brush type of dynamo, however, the increase in IB which results from a small increase in VD, involving, as it does, a corresponding increase V in armature current, causes the flux between the main and third brushes to be weakened. Thus the voltage across the main field coils is reduced and consequently also the main field flux. A reduction in main field flux means a reduction in terminal volts and in battery current. It will therefore be seen that the value of the product RBIB is limited and the terminal voltage of the dynamo cannot materially exceed the voltage of the battery. It must not be forgotten, however, that the voltage of the battery can vary from about 22 when discharged to as much as 31.5 when on charge, the charge being nearly complete. The terminal voltage of the dynamo therefore also varies over a similar range. The regulating effect cannot be satisfactorily obtained with a pure resistance load and therefore it is essential that the battery be connected to the dynamo at all speeds in excess of the cut-in speed. Otherwise the terminal voltage would rise with speed, and eventually the field coils would burn out. To guard against failure of the auto switch, a fuse is included in the main field circuit, so that this would blow before the field windings were damaged.
When the lamps are off, the main shunt field, connected to the third brush, is alone in use, but, when lamps are switched on, a greater current output is required, and so the operation of the distance switch is arranged to bring the auxiliary field into use automatically. The output of a third brush dynamo reaches a maximum and then falls as the speed continues to rise, and the auxiliary field helps to minimise this effect.
The series 7 dynamo, which is used for ordinary passenger stock, is capable of an output of 70 amps at 27 volts, this output being achieved at a speed of 800 r.p.m. The output at 600 r.p.m. is about 30 amps., whilst the variation at a speed of 2,000 r.p.m. is not to be more than 25% of full load output. 70 amps is the maximum output, the average over the speed range being about 55 amps. In order to keep the battery in good condition and to avoid excessive sulphation and distortion of the plates, it should receive a relatively high charging current when discharged and this current should gradually decrease when charging is nearing completion and should be reduced to a very low value when the charge is complete. fig4 When, however, a battery is directly connected to a third brush dynamo its charging current rises as the fully charged condition is approached, because the voltage across the field coils rises, so increasing the output of the dynamo. This of course is the opposite of what is required. As previously explained, the terminal voltage of the dynamo and battery can vary from 22 to over 30, but it is desirable, for good illumination and long lamp life to keep the lamp voltage as nearly constant as possible. Also the number of lamps in circuit can vary from about ll to about 38, as the passengers switch them on and off, and this variation in load must not be allowed to cause excessive fluctuations in lamp voltage.
It is in order to achieve these desirable conditions that the regulator is used.

The Battery (Fig. 4).
The battery of the series 7 equipment consists of 12 lead acid cells connected in series. Each cell comprises 8 positive and 9 negative plates supported on teak rests in a glass box. The teak rests provide space in the bottom where sediment can collect without interfering with the plates. A teak lid is provided and the positive and negative lugs pass through rubber gaskets in this lid. The positive plates are of the Plante type and the negatives are of box construction. Wood separators are used between the plates and the lids, rests and separ- ators are wax impregnated.
The battery is carried in two timber boxes slung from the underframe of the vehicle, each box containing six cells. The capacity of the battery is 240 ampere hours at a 9-hour rate of discharge.

The Auto-Switch (Fig. 5).

The auto-switch is mounted on a mild steel base. A U-shaped mild steel magnet frame carries a soft iron core on which are the series coil consisting of relatively few turns of insulated copper strip and the shunt coil consisting of many turns of enamelled wire. A pivoted armature is arranged to be attracted by the core and magnet frame when the shunt coil is energised, and the movement of the armature closes two contacts. One of these actually consists of two brushes, and when these impinge on their fixed contact they connect the dynamo positive to the battery positive. The other brush impinges on a separate contact and provides a negative feed to a component of the regulator called the voltage balance coil, this feed being for use when the lamps are off and the battery is on charge.
The shunt coil is permanently connected across the positive and negative of the dynamo and that is why it consists of a large number of turns of fine wire, so that it only takes a small current. As soon as the dynamo generates 27 volts, the pull of the shunt coil is sufficient to attract the armature and close the contacts. Almost the whole output of the dynamo then flows through the series coil, which is the reason it consists of a small number of turns of large section strip so that it can carry a heavy current, and the pull of this coil then assists that of the shunt coil and improves the contact pressure.
When the train slows down, the dynamo voltage falls below that of the battery and there is a small reverse flow of current through the series coil, which sufficiently weakens the pull of the shunt coil to cause the switch to open, disconnecting the dynamo from the battery.fig6

The Distance Switch (Fig. 6).
Like the auto-switch, the distance switch is mounted on a mild steel base. Each of two channel section magnet frames has a core carrying a coil, the left hand being the pull-on coil and the right hand the pull-off coil. A pivoted armature on the "on" side carries three contact brushes. One of these is in the lamp circuit, one provides the positive feed to the voltage balance coil of the regulator when the lamps are on, and the third provides the negative feed to this coil, likewise when the lamps are on. This armature is attracted when its coil is energised by operation of a through control switch anywhere on the train, or it can be pushed home by hand, a knob labelled "on" being fitted for this purpose. When contact, is made a spring controlled trigger engages with a pin on the armature and holds the switch in the "on" position. The pull-off coil is equipped with a similar pivoted armature, but this one has no contacts. I Instead, its function is to press against a lever attached to the trigger and so release the "on" armature. This armature also can be pushed in by hand and is provided with a knob labelled "off."

The Regulator (Fig. 7).
fig7 The regulator consists of a stout rectangular cast iron frame on which are mounted, concentrically, three resistances known respectively as the lamp resistance, the progressive resistance and the shunt resistance. They consist of short coils of Eureka wire, those for the progressive resistance being wound on small bobbins. In front is a circular plate of insulating material called the rheostat base. The resistances are fixed to the base with brass studs, the heads of which form contacts for the contact arm. This contact arm, a brass casting, carries six spring loaded carbon brushes which bear on the resistance studs and on two contact rings mounted on the rheostat base. At each end of each row of contact studs is a brass end plate. The contact arm is carried on, but insulated from, a steel spindle which is driven through double worm reduction gearing by a small series wound reversing motor, mounted on top of the frame. The motor has two sets of field coils, one being energised for one direction of rotation and the other being energised for the other direction. The motor exerts sufficient torque to operate the contact arm over the full range of battery voltage.
Also mounted on top of the frame are two motor operating relays, one for each direction and the voltage balance. The voltage balance consists of a pivoted arm at each end of which is an adjustable contact point arranged over a cup containing a small quantity of mercury. One end of the arm is attached to a tension spring and the other is attached to the plunger of a solenoid, the coil of which is called the voltage balance coil. When the voltage across this coil is 22, the pull of the plunger balances the pull of the spring, and both points are held clear of the mercury in the cups. When the voltage drops below 22, the spring pulls its end of the arm downwards, the point dips into the mercury and completes the circuit to the coil of one of the motor operating relays. The switch closes and the motor starts, revolving the arm until the voltage across the balance coil is again 22. When the voltage rises above 22 the solenoid pull exceeds that of the spring and the other relay is energised so that the motor starts in the opposite direction. When the arm revolves in a clockwise direction it inserts some of the shunt resistance and some of the lamp resistance or the progressive resistance, according to whether the lamps are on or not. This point will be explained more fully later. When the arm revolves in an anti-clockwise direction it cuts resistance out of circuit. The shunt resistance is in series with the main field coils of the dynamo.
At each end of the travel of the contact arm is a simple self-resetting limit switch in the circuit of the appropriate field coil of the motor. When the arm reaches the limit of its travel it makes contact with the arm of the limit switch, opens the switch, and the motor stops. This of course does not prevent the other coil from being energised when there is need for the arm to move in the opposite direction, and when this happens, the limit switch closes again.

fig8 At the bottom of the rheostat base are five terminals numbered from left to right 6, 8, 7, 5 and 8a (Fig. 8). 6 is connected internally to the left hand end plate of the lamp resistance and externally to the positive of the battery. 8 is connected internally to the lamp resistance via the inner contact ring and the contact arm, and externally to the lamps via the distance switch. The two lamp resistance terminals are therefore 6 and 8. 7 is connected internally to the right hand end plate of the shunt resistance and externally to the main shunt coil of the dynamo. 5 is connected internally to the outer contact ring and externally to the positive of the dynamo. The two terminals for shunt resistance are therefore 5 and 7. 8a is connected internally to the left hand end plate of the progressive resistance and externally to the positive end of the voltage balance coil. The contact arm connects the progressive resistance to the outer contact ring and so via 5 to the positive of the dynamo. The progressive resistance is thus in series with the voltage balance coil.

fig9 The Function of the Regulator.
In order to understand the functioning of the regulator it is helpful to consider the following three conditions: (Fig. 9).

  • (l) Dynamo connected, lamps off, battery on charge.
  • (2) Dynamo not connected, battery supplying lamps.
  • (3) Dynamo connected, lamps on, battery on charge.

Condition (1). Under this condition the whole of the output of the dynamo is employed in charging the battery. As soon as the dynamo voltage reaches a value of 27 volts, the current through the shunt coil of the auto-switch causes the latter to close, connecting the dynamo to the battery via the series coil. The full output current of the dynamo then flows through the series coil and as previously explained, strengthens the action of the shunt coil ensuring firm pressure between the contacts. When the distance switch is open, that is when the lamps are off, the auxiliary contact 4 is open and the positive feed to the voltage balance coil is via the progressive resistance. That is to say the voltage balance coil and the progressive resistance are in series and are together in parallel with the battery, as shown in 9a.

Assuming that the battery is in a discharged condition, minimum resistance will be required in series with the voltage balance coil so that the voltage across the latter may be at the requisite value of 22, enabling it to balance the pull of the spring and maintain equilibrium. The regulator contact arm will therefore be well over to the left and there will be no resistance in series with the shunt held. The dynamo output and the battery charging current will be high.
As charging proceeds, the battery voltage rises, the dynamo voltage rises with it, and consequently the voltage across the voltage balance coil increases. The pull of the latter overcomes the spring, contact is made with the right hand cup, and the left hand motor operating relay closes. The left hand field coil of the reversing motor is energised and the motor drives the arm in a clockwise direction, inserting progressive resistance in series with the voltage balance coil, until the pull of the latter is just sufficient once more to balance the pull of the spring and hold both contacts clear of the cups so stopping the motor. Owing to the length of the end plate of the shunt resistance the movement of the arm has not as yet inserted any resistance in series with the field. The rising battery voltage causes the voltage across the voltage balance coil to tend constantly to rise with the further insertion of progressive resistance as above described. Eventually the contact arm brush controlling the shunt resistance leaves the long end plate and begins to insert resistance in the shunt field circuit. This causes the excitation and hence the output of the dynamo to fall and the battery charging current gradually decreases giving a tapering charge characteristic. Eventually the brush controlling the progressive resistance makes contact with the right hand end plate from which there is a back connection to the third stud. The effect of this is to cut out the greater part of the progressive resistance so that the voltage across the voltage balance coil rises, the arm travels right over, inserting all the shunt resistance, until it strikes the right hand limit switch, cuts out the motor, and remains in that position. With all the shunt resistance inserted, the dynamo output is greatly reduced, and by this time the battery is fully charged. The charge then continues at a rate sufficiently low to prevent damage to the plates. (Fig. 10). fig10 In the event of the battery voltage exceeding that of the dynamo, as will happen if the train slows down, the current in the series coil of the auto-switch is momentarily reversed and causes the auto-switch to open, disconnecting the battery from the dynamo.

Condition (2). Under this condition the essential connections are as shown in 9b. Current flows from the positive of the battery via the end plate or the studs of the lamp resistance to the lamps and back to the negative of the battery. The voltage balance coil is in parallel with the lamps but the progressive resistance is not now in circuit since it can only receive a positive feed from the dynamo, which is not connected. The contact 4 on the distance switch now provides the negative feed to the voltage balance coil, in place of the contact 4 on the auto-switch. The voltage of the battery can vary from about 22 to nearly 30 according to the state of charge. The working voltage of the lamps is 22. Therefore whenever the battery voltage exceeds 22 it is necessary to insert resistance in series with the lamps to absorb the difference. This is the lamp resistance. Now the voltage balance coil is in parallel with the lamps and therefore the voltage across it is the same as that across the lamps, and as previously explained whenever this voltage varies from 22 the voltage balance operates to drive the contact arm in the appropriate direction to re-establish the value at 22. Under the conditions now being considered this results in the addition or subtraction of lamp resistance. As current is taken from the battery, it is gradually discharged and its voltage falls, the voltage across the voltage balance coil and the lamps falls . below 22 and the contact arm moves anti-clockwise to subtract lamp resistance to re-establish the correct lamp voltage.
Now there are some 38 lamps in a coach, most of which can be switched on and off by the passengers. Suppose that only a few of the lamps are on and suficient lamp resistance is in circuit to maintain the lamp voltage at 22. The voltage drop in the lamp resistance, equal to the difference between 22 and the battery voltage, is equal to IR, where R is the value of the lamp resistance in circuit and I is the value of the current in it. If now more lamps are switched on, the current in the circuit and therefore through the lamp resistance increases. Consequently the value of IR, the drop in the lamp resistance increases, so that the voltage across the voltage balance coil and the lamps falls below 22. The balance then operates to move the contact arm anti-clockwise to subtract resistance until the lamp voltage is again 22. If lamps are switched off, the reverse takes place.

Condition (3).
The lamps will be fed from the battery as just described if the train is stationary, or is moving at a speed less than that at which the auto-switch cuts in. lf the speed of the train increases so that the dynamo generates a voltage of 27 the current in the shunt coil of the auto-switch is sufficient to close the auto-switch as previously described, and then the conditions shown in 9c obtain. Current now flows through the battery in the reverse direction, from the dynamo, and the battery is on charge. Its voltage begins to rise and tends to increase the voltage across the lamps and the voltage balance coil, and the voltage balance acts to insert more lamp resistance and at the same time more shunt resistance, so that the charging current to the battery is controlled in a similar manner to that already described for the condition "Lamps off, battery on charge". It will be noted however that the control is now exercised _ by the lamp resistance instead of the progressive resistance.* Switching individual lamps on and off produces the same results as described for the condition "Dynamo not connected, battery supplying lamps". If more lamps are switched on lamp resistance is cut out and also of course shunt resistance, but while this increases the output of the dynamo, it does not materially affect the charging current, because the extra output is required for the extra lamps. Similarly when lamps are switched off the dynamo output is reduced and this compensates for the lower lamp consumption, and prevents the charging current from increasing.
When the train slows down again the auto-switch will disconnect the dynamo as previously described, and the regulator will subtract lamp resistance to compensate for the fact that the battery voltage is lower on discharge than on charge.

It is hoped that the foregoing has given a clear impression of the principles of the train lighting system at present in use on British Railways, Western Region. The description has been confined to the equipment of ordinary passenger stock. Another paper would probably be needed to describe the equipment in more detail and also the additional items used on various other types of vehicles. Fans, refrigerators, electric food warmers and fluorescent lamps are all operated from what is basically the same power plant, though various modifications are made to cope with the differing requirements. Probably the most fundamental alterations and additions are made for the fluorescent lamps.
At the present time it is difficult to say how long the system will remain in use but, developed in close co-operation with Great Western engineers, it has for many years efficiently performed its functions. Finally, it may be said that although the various systems in use differ widely in detail, an understanding of the basic principles of the Western Region system will be very helpful in understanding other systems.

* In point of fact, the progressive resistance is now in parallel with the lamp resistance, as shown dotted in the diagram, but since its resistance is so much greater than that of the lamp resistance its effect is small.