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COMING REVOLUTION IN ;SEN GER VEHICLE LIGHTING

14th December 1945
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Page 30, 14th December 1945 — COMING REVOLUTION IN ;SEN GER VEHICLE LIGHTING
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Considerable Progress in Fluorescent Lightin Ltd., in Co-operation with Siemens Death Systems Applicable ilic-service Vehicles, Made by C.A.V., and Supplies, Ltd. Details of Various !nd Trolleybuses

FLUORESCENT lighting is a form of illumination ' which is steadily gaining ground. It is now being widely used in factories and offices, and the fact that it is being employed largely in drawing offices is indicative of its advantages Hitherto, however, it has not been used in this country for the interior lighting of road vehicles, probably mainly for the reason that during the war the problem was to cut down illumination to the minimum rather than to increase it.

• That well-known company, C.A.V, Ltd., of Acton, London, W.3, has by no means been -neglecting this subject, and has devoted considerable time, energy and expense to the possibilities of applying this form of lighting to vehicles equipped with petrol or oil engines, also to trolleybuses.

We must confess to have hag some doubts as to the practicability of carrying this out, also as to the effects which could be achieyed, but during.a recent visit to the C.A.V. works, we .saw a bus body of M.C.W. construction equipped with both fluorescent tubes and tungsten-filament lamps, and we were quickly persuaded as to the advantages of the former. Compared with the bright, well-distributed, soft light from the tubes, that from the tungsten lamps was yellowish and poor. Each type was, of course, tried separately. A fluorescent limp is a low-pressure mercury vapour discharge tube in which the ultra-violet energy, which is almost invisible, causes fluorescence of a coating of special powder on the interior surface of the tube, the colour of the light emitted varying according to the type Of coating employed. •At present, the two colours available are " daylight " and "warm-white," the special tubes employed being products of Siemens Electric Lamps and Supplies, Ltd., which company is co-operating with C.A.V., Ltd., in this interesting investigation. A fluorescent tube is not inherently self-starting, but at each end is an electrode with a coated filament which per forms the initial heating, following which a moderately high voltage surge must be passed• through the tube; immediately it lights, a more normal voltage is applied, Another difficulty is that the tube has a negative resistance characteristic; in other words, as the current rises the resistance falls. Consequently, to ensure stable operation it is necessary tO have a " governor " in the form of a currentlimiting device in series with the lamp.

This entails a supply voltage somewhat higher than that for the tube, owing to the power loss occurring in the governor. In a.c. circuits, the current limiter is a choke, and on d.c. circuits a resistance. The last named may be a tungsten-filament bulb. Losses in the choke may .vary between 10 and 25 per cent., and in the resistance about 45 per cent. On the other hand, whilst the life of a fluorescent tube is about 3,000 hours, and the cost of a set will be approximately three times that for tungsten ,lamps, the light emission is five to six times greater; thus there is some scope for economy.

It is obvious that, apart from the factors to which we have already referred, the main problem is to provide voltages higher than 'those normally employed in the vehicles concerned. This involves the provision of an auxiliary device fed from the normal battery system. There are several ways of attaining this. Dealing first with the d.c. system of 60-70 volts, this employs a 15-in, lamp of 1 ins. diameter, having a rating of 14 watts and a lamp voltage of about 41. Referring to the diagram concerned, it will be seen that the battery is connected to the tube electrodes through a main switch at one side and a tungsten lamp ballast resistance at the other. The circuit' is completed through a choke and starting switch.

In a trolleybus, on the other hand, a 60-volt battery is already available to cater for manceuvrinz. Normally, this is so arranged that the two 30-volt banks are connected in series for this purpose and in parallel for lighting. By using a 60-volt generator, the latter arrangement can be obviated and the full voltage made available for the lamps. Here, again, the ballast resistance for each tube takes the form of a tungsten bulb to compensate for fluctuations in the supply voltage.

A choke is connected in series with the switch for the filaments. When the switch closes, the filaments are heated, and directly it is opened a powerful voltage surge takes place across the tube, causing the discharge to commence. Normally, this switch is of the thermal type and automatic in operation.

When only a 24-volt battery is employed, a motor generator booster provides the difference between the required voltage of 60-70 and the 24 volts provided by the battery.

With the first of these trolleybus systems the efficiency is about 55 per cent., and there" are -no other supply losses, but with the motor-generator this has a probable efficiency of 40-45 per cent., whilst the lamp circuit efficiency will remain somewhere in the regionof 55 per cent.

The Motor-alternator system is also being considered in conjunction with 18-in. tubes rated at 15 watts. This requires a circuit voltage of about 110. The frequency chosen is ,400 cycles per second,for the reason that this is Standard in aircraft electrical equipment practice, it avoids stroboscopic effects, simplifies. starting and permits reduction in the size of the control gear, such as chokes.

As regards the lamp circuit, a choke is. connected in series with the lamp, and a condenser between its electrodes. • Choke -and condenser are arranged to be in resonance before the discharge commences, and the voltage acro§s the condenser is insufficient to start this before the filaments of the electrodes are correctly heated. Immediately discharge commences, however, the circuit is no longer in resonance.

The motor alternator is of the conventional type, arranged to run at a speed of 3,000 r.p.m., and fed at 24 volts from the vehicle battery. In this case, the lamp circuit will have an efficiency Of some 75 per cent., whilst the overall efficiency of the motor-alternator set will be about 50 per cent. The weight of the latter is about 100 lb., to meet the needs of the installation on a double deck bus. Approximately 2 lb. per lamp must be allowed for the choke and condenser.

One further system which presents distinct advantages, but which, at the moment, is only iri the experimental stage, employs a vibrator unit with double contacts in conjunction with a split-primary transformer. Using a 24-volt battery, this will supply a.c. at about 110 volts. An a.c. voltage is induced in the secondary winding, and this is transformed according to the number of turns in the latter. The efficiency of d.c. and a.c. is about 70 percent. In this case the choke is used in series with the tube, and a condenser across it. Owing, however, to the lower frequency of operation of the vibrator, which is some 100 cycles per second, a resonance starting current is not used, and a starting switch of the thermal type is necessary.

This type of vibrator units closely resembles that employed on car radio sets, on which it is necessary considerably to boost the voltage obtainable from the normal battery. In such sets it has proved most reliable. We have had one in operation for many years without any serious trouble arising.

The methods of lighting the tubes take some time to describe, but, in practice, there is remarkably slight lag. It is a matter of only seconds. We noted that, occasionally, after a start there was a slight flicker, but switching on and off rapidly cured this immediately. Fluorescent tubes are not so bright as to be hurtful to the eyes, and the light appears to " permeate " the whole body of the vehicle—in other words, the distribution is much more even than is that of ordinary bulbs, in which the effect seems to be far more localized. Reading should be rendered much easier.

With tungsten-filament bulbs the output is some 10-12 lumens per watt, but with fluorescent tubes it is raised to 25-30 lumens. One of the other objects to be achieved is, of course, to endeavour not to take more power out , of the battery 'than is the case with the ordinary form of lighting.

The first practical start with this new development is likely to be with the motor-alternator, but it is realized that there must be further developments from this for the sake of economy and efficiency—in other words, more light for a given supply of battery current. As it was put to us rather humorously by one of the C.A.V. experts, it is a problem of marrying the power of the vehicle to this form of lighting.

In the demonstration set which we inspected there was a tube at each side for each main panel of the roof, and whilst this looks welt by adding to the symmetry, it might be possible to use fewer of these lamps and still obtain better lighting than with the normal number of tungsten bulbs. It must, of course, be remembered that the voltage of a tube is the function of its length and diameter. At present it seems that the 15-in, tube is about the lowest limit to which the length can satisfactorily be taken.

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