Controversial Trolleybus Topics
Page 53
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the designer's difficulties
Mr. A. T. Priddle Expresses Wellbalanced Views on Current Trolleybus Problems at the T .L.R.T.A. Conference at Brighton
L' weight weight and simplicity should be the keynote
.1--dof future development," remarks Mr. T. Priddle,
of the Associated Equipment Co., Ltd., in concluding a paper entitled "Some Aspects of Trolleybus Development". which was to be read yesterday at the Tramways Light Railways and Transport Association's 26th Annual Congress, now in progress at Brighton.
Among important developments, he includes the placing of the motor amidships, giving a short shaft, and behind the legal 10-in, clearance area, so that the transmission line may be straight. Light low-consumption motors may run at 2,560 r.p.m. for a road speed of 30 m.p.h., hence the greater importance of a vibrationless propeller shaft than on the internal-combustion machine. Moreover, transmission noise is more obvious on the silent trolleybus.
His next point is the exchange of positions between the motor and the resistances. In front, the latter are out of reach of mud from the front wheels, and exposed to a free flow of air. This is augmented by the exhaust from the air compressor, on the chassis the author has in mind, and he instances a case where the temperature rose from 144° C. to 182° C. by stopping this air supply.
Air Brakes Preferred.
Under the heading of braking, he points out that modern service conditions demand a retardation rate up to 16 ft. per sec. per sec. Thus, a power-assisted system is necessary. He favours compressed air, because very light manual effort is needed, a large reserve can be stored in a small reservoir, compensation is assured and only one fluid is required. A 5-cubic-ft. compressor-unit, he adds, to-day weighs 135 lb.; a few years ago, a unit of half this capacity would have been as heavy.
There are two practices, he says, in the accommodation of the electrical control equipment—chassis mounting and cab mounting. He strongly favours the former, forestalling a criticism by pointing out the ease with which it may be perfectly protected and adequately insulated.
Mr. Priddle airs some most interesting views on regenerative control, Whilst appreciating that the principle made possible by regeneration is " fundamentally right," he brings powerful arguments to bear against this system. Introducing one of his reasons by mentioning that the opportunity to use regeneration is reduced as the schedule speed and number of stops is increased, he shows that, with regenerative and rheostatic stopping, f mile between two stops can be covered in 28 secs., whilst, with air brakes, the figure is only 23f secs. Such a saving would permit a reduction of 11 per cent, in the number of vehicles necessary for operating the service.
The retardation, at 4f ft per sec per sec., created by regeneration, necessitates that slowing clown he begun at about 26 m.p.h., whilst, with friction brakeS, retarding at 12 ft. per ,sec. per sec., acceleration may be continued up to-about 28 m.p.h.
The :transmission, especially the a.xle construction, is obviously deeigned.for the maximprn torque of the traction motor. Obvionsly, it would not stand up to the greater. torque, which wpuld be nenessaty„in the reverse direction, if the retaxdation rate were to he higher than Ahe a.ceelem
tion ,,rate contend with this, :heavier ,parts would be needed wherez.ysreight,,is daiglaly .unde.F.iralak ) ,nn, • • ` nothern;bizaii, 7t;tiamca.titlior.. giles-f*-„rpre!,erti,ye exa1-,iisg.:4:i With regearative equiprnerit his Ore is 1'7 -Units -per car -mile against 1.9 units.
e describes as a "'combination of inestimable value, where hilly districts are concerned " the linking of the air-' braking system with the regenerative rheostatic • brake, facing Wear and heating of drums .being thus minimized.
Further drawbacks of regeneration include the difficulty of metering when current is both taken by the vehicle and returned to the line, and the fact that when distribution systems incorporate rectifier substations, regenerated current cannot be returned to the high-tension supply, but can be consumed only by a small number of other buses. As a result, the over-volts relay is in frequent operation and current is dissipated in resistance losses.
Discussing radio interference, he says traction motors cause little interference, the chief source arising in the contactor control circuits where the current is about 1-10th amp. Similarly, the overhead collector gear is far worse, in this respect, when picking up small currents for lighting, etc., than when collecting current for propulsive power.
Suppressing Radio Interference.
Outlining the methods of suppression, he remarks that stopper coils on the roof are necessarily heavy, that a scheme which included filters is lighter, but requires numerous small-gauge coils, and that the use of line condensers, although not yet satisfactory alone, may become, in the future, the best system of all.
High-voltage lighting is approved by the Ministry of Transport only on composite bodies. The author fails to appreciate why, as in this type of body all parts of the panelling are in electrical contact. Furthermore, it is. in any case much easier to insulate lighting circuits than power circuits. However, as a result of the regulation, 400 lb. deadweight—the equivalent of three non-paying passengers—has to be carried.
In converting tramway systems for trolleybus operation, although a 12-ton trolleybus has an energy consumption equal to that of a 20-ton tramcar, in a number of cases the existing feeders are divided evenly as positive and negative feeds, the rail return being replaced with a wire haying a conductivity of 7.2 per cent. of the rail.
Of the two main methods of controlling the lights—that in which the main load is on the battery side of the dynamo cut-out, and that in which it is on the dynamo side of the cut-out with -the emergency lights on the battery side—the former is to be preferred.
A lighting generator with its armature on the end of the traction-motor shaft has many attractions, among which are a saving of 136 lb., the elimination of a second motor, low cost, silence and simplicity.
A comparatively recent innovation is the use of a manceuvring battery. This has "very considerably improved the mobility of the trolleybus."
Facilitating Fare Collection.,
On the subject of ,types, the rear-entrance trolleybus is the most popular bnt from the point of view of the collection of fares, there is much to be said for the body :with dtiver,cOntrolled central door.
Manufacturing pro.blems:are a serious difficulty. Economical production methods are, for many reasons, practically impossible. Ameng these are the multiplicity of types, the fact that whereas internal-combustion-vehicle elesignsnate specified by Ministry of Transport regulations, specificatsions for, trolleybuses have to be submitted to the Ministry:.:Again,.differeettowns have totally different ideas on the subject. Thus it is impossible to build in quantities.
Far...4t4c, .fmtueen. Weight reduction must receive attention.
Ai6W*441`. Oompletely self.voon&
4.1.ried weighs "tbbs 15 cwt,.: The 'similar ' 'ti011eybUS;,
-dependent on .arn-otitsi4,source of 'energy scales a iffattei
Of-2 cwt. more. 4