Accumulator Propulsion for London Bus Service.
Page 9
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By Bernard Hopps.
(Continued from page 317.) In order to reduce the current output from the battery at
!rting, or on grades, series-parallel control is essential, and this calls for the use of two motors per vehicle, which, hr addition, has the advantage of doing away with the differential gear. In some cases, single motors have been used with double commutators, but such machines have a great 'many disadvantages from the standpoint of electrical design, and they are also more dill-lett]: and costly to repair. In order to provide a smooth acceleration free from jerks, the controller should have at least live forward speeds, of which three are constant running speeds, and the remaining two rkostatic points. The running speeds are obtained by grouping the armatures and lields of the two motors, and some Provision should be made in the controller to prevent drivers from running for any length of time on controller points where the starting resistance is in the main circuit, as this is obviously a wasteful method of speed control. Fig.. 2 shows the connections of a controller arranged for five forward and two revc'rsi speeds : points 2, 4 and 5 are running points; points i and 3 are rheostatic points, for the purpose of acceleration only. Point 5 provides for shuntingthe motor fields, in order to give an accelerated speed.
. -.Transmission and Control.
Adifficulty that will no doubt be experienced With the drivers will be the tendency to move over the controller points too quickly, causing a rush of current to the motors, which would be inittrious to the batteries, In order to prevent this, an electrical interlock can he 4rrr-_piged, so that, in the event of the currents exceeding a safe amount the. controller barrel is instantly locked againSt further forward Movement. A modified form of such an arrangement has already been used with success on electric-train equipments. The safe amount of momentary over-load that May be taken from the accumulators may be accepted as being 24 times the normal discharge rate. 'f:"12e problem of the transmission from the motors to the road wheels is a matter for the chassis builders to consider ; it should not be left to the manufacturer of the electric motors to provide for this. Whatever form it takes, it is a sine qua non that-it be silent, and reasonably efficient, and so arranged that the position of the motors will leave a clear space for lire batteries under the vehicle. The speed of the motors should not exceed 1,200 10 1,400r.p.m., corresponding to a vehicle speed of 12m.p.h. Higher-speed motors, although lighter, are not satisfactory for London bus service. One of the chief advantages of the accumulator bus, is the impossibility of the driver's racing his vehicle in competition with a rival bus : the constant voltage of the battery fixes the maximum speed that can be obtained on the level, and no action on the part of the driver can increase this speed. There is no doubt a section of the public which may consider the speed of the accumulator bus too slow, after the exhilarating rides they have experienced on the petrol-driven vehicles, but these people should compare the speeds they now consider slow with the speed of the old horse buses. The fact that the accumulator bus cannot be raced by a callous driver will, in a large measure, contribute to its success, .since experience has shown that, for speeds above 12m.p.h., the cost of maintenance increases approximately as the square of the speed. [We should incline to regard the increase as being proportional to the cube of higher speeds.---En.] It will, in time, be recognised by the operating companies, if they have not already clone so, that there is a maximum speed above which the maintenance cost increase's-in greater proportion than the passenger returns, and this speed will be found not far removed from a maximum of 12m.p.h. The accumulator manufacturers who urukrtake maintenance contracts will no doubt safeguard their interests by specifying a 'certain maximum speed for vehicles using their batteries, and will charge a correspondingly higher rate for vehicles geared to exceed this speed, as the deterioration of the battery will undoubtedly increase rapidly with the speed, owing to the heavier discharge rates, and greater Adbration.
-Comfortable Running at Reasonable Cost.
It is generally admitted that, so far as the running is concerned, the accumulator bus is a considerable improvement over the present type of petrol-driven vehicle. The silent and -steady way it travels, and the absence of smell and oil droppings, is attracting favourable comment from all_sides, and what is even more important, it apparently meets with the approval of the police authorities. These conditions are very allurim.Y, to the public, and will no doubt materially increase the earning .capacity of this type of bus. The cost of the current for charging the batteries can now be obtained at extremely low rates, as compared with those
obtaining a few years. ago. Electric supply companies are desirous of cultivating a day load for their stations, and will supply energy as low as o.5d. per B.T.U., during certain hours of the day, for power purposes. Compared with the constant worry and anxiety of up-keep of the present types of vehicles, the operating companies should look upon the accumulator bus with favour; especially as it opens up the possibility of converting existing old-type vehicles at a moderate cost. With this system, the chief running expenses are reduced to a contract price per car-mile, and there should be no ambiguity as to working costs.
Accumulator traction is .admirably suited to London, as the streets are practically level, and the surface for the most part paved. The buses would be more reliable and cleaner than those which use either petrol or steam, and for this reason alone would find more favour with the drivers, who would not have to be trained to the same extent as has been necessary heretofore.
The adoption of the accumulator bus abolishes the necessity of the staff of road adjusters, and 75 per cent. of the staff of skilled mechanics employed during the night to repair and adjust engine, clutches, gearboxes, etc. The electrical equipment, apart from the accumulator, requires very little attention, beyond occasional renewal of brushes and bearings. In the light of past experience with other types of vehicles, the prospect of accumulator traction for London never was so bright as it is at the moment. The accumulator manufacturers, by agreeing to enter into maintenance contracts for their batteries, have removed the difficulty which has hithertd stood in the. way of developing the System, and it now remains for the operating companies, if they would consider their best -interests, to give accumulator buses their earnest consideration, and a thorough trial.