The New C.M.B. Battery System.
Page 5
Page 6
If you've noticed an error in this article please click here to report it so we can fix it.
Messrs. J. C. Macfarlane and H. Burge (of Crompton and Co., Ltd., Chelmsford), the inventors, outlined a new system of transmission and control for battery-driven, or pe ol-electric motorbuses or other self-propelled vehicles, in a paper which Mr. Burge read at the Institution of Electrical Engineers, on last Thursday evening. The paper, for the most part, was of a very Interesting nature, although it contained a number of inaccuracies which clearly showed that the authors had been misinformed as to recent motorbus operation and running costs.
After first outlining several previous electric and petrol-electric systems, dealing with their advantages and disadvantages, the authors proceeded to enumerate a number of supposed inherent disadvantages which they alleged are peculiar to petrol systems in which a gearbox is employed. How much reliance may be placed upon the authors' observations on petrol vehicles of that class may be gauged by the following extract : "The gear is entirely at the mercy of negligent or .malicious drivers." Surely, at the hands of such a driver, any type of vehicle would suffer.
The System Briefly Described.
The weakest link in the systems of battery-propelled vehicles of the past has been the battery, and it was with the idea of removing that weak link that the inventors evolved the system dealt with in the paper. The battery has he.en improved, but, according to the inventors, the greatest improvement is that which allows of the reduction in the size of the battery by about 50 per cent, for a given service, a reduction which also allows of the lowering of the initial and upkeep charges, and an increase in the rat io of useful load to dead load upon a vehicle. The authors' methods include : (a) braking entirely by regeneration of electric current which is absorbed by the battery ;
(b) the introduction into the system of an " electric valve," or motor-booster, which automatically limits the current that can be drawn from, or returned to, the battery, thus displacing the usual series-parallel controller.
fc) the adoption of a motor with special shunt field windings which has a torque-speed characteristic similar to that of a series motor.
The system, as applied to a battery driven motorbus, is shown diagrammatically on this page. The " electric-valve." or booster, although combined in one machine, is really a motor (which is driven by current from the battery) and a dynamo, or generator, in the arma tare windings of which a secondary current is generated. In the diagram, the upper half (A) of the booster armature acts as a motor, and it is shown directly coupled to the battery (V). The lower half of the booster armature (B) acts as a generator and is coupled in series with the driving motor (C), so that the latter may receive across its terminals a current of double the voltage of the battery. The driving motor is provided with two shunt field windings, one of which (D) is connected across the battery so as to provide constant excitation, and the other (E) is connected across the terminals of the lower portion (B) of the booster armature. The latter shunt winding is arranged so as to assist the former one during the period of acceleration of the vehicle, but the two windings are in opposition to each other when running at full speed. The excitation of the generator part of the booster armature may be varied by the winding (F) which is controlled by the regulator ((i) ; when the regulator arm is moved over to the extreme right-hand position the voltage of the generator portion (13) of
the booster is added to that of the battery, but when it is moved over to the opposite extremity the booster voltage is subtracted from that of the battery. When in mid position, the regulator effects such connections as to prevent the booster's armature's being excited. An additional winding (11) is also provided upon the generator portion of the field of the booster, and this is connected in series with the driving motor (C) so as to oppose the winding (F) when the booster is acting as a generator and is delivering current to the driving motor. For example, let us assume that the battery is giving a current at 60 volts, and that the generator portion of the booster armature is wound to give a maximum of 60 volts. When B is positively excited, its voltage will be added to that of the battery, and there will thus be available across the terminals of the driving motor a current at 120 volts ; thus, it will he cdpable of rotating at full speed, and the voltage across the winding (E) will be of full negative value so that the strength of the driving motor's field is equal to D minus E. If, now, the regulator be moved over to its extreme opposite position, the booster will be developing a counter eleetro-motive-force of 60 volts or mere, and there will be no electro-motive-force across the driving motor, which will, therefore, come to rest ; when the back electro-motive-force of the motor rises above the voltage which is supplied to it the motor will return energy to the battery, the field winding (H) meanwhile tending to increase the voltage of the supply in order to avoid the passage of any excessive current to the battery.
A Few of the Special Claims Made by the Inventors. 1. The authors claim that the energy consumption will only be about one-half that which has been found necessary with previous battery buses, which claim they make on the basis of regenerative braking, supposed economical acceleration and high torque per ampere.
2. "The system of braking reduces the tendency to skid on greasy roads, and as the controller always returns to the braking position ; this action brings the vehicle to a standstill if anything happens to the driver." There does not appear to be any provision for the absolute free-wheeling of the vehicle, and, in our opinion, the tendency to sideslip under greasy conditions would surely be increased rather than diminished, because, once a sideslip has commenced, it is absolutely necessary that all braking effort should be released.
3. It is stated that the torque when regenerative braking is in operation, and the vehicle has just come to rest, is arranged " to be of such magnitude that it holds the vehicle on a comparatively steep gradient, but is not sufficient to cause the vehicle to start backwards on the level." As one speaker at the subsequent discussion remarked, if the torque were not sufficient to start the vehicle backwards on the level then the steepest gradient on which it would hold the vehicle would not be more than 1 in 50—hardly a comparatively steep gradient.
Conclusio 11.
Although the system put forward by the authors is a most ingenious and interesting one, we think that the motorbus world will remain unconvinced until such time as the system has been practically applied to a motorbus in London service, or under similar conditions. It is not sufficient to know that a bus built to meet London regulations has given such and such results on six miles of road in the vicinity of Chelmsford—the conditions are not comparable, and are certainly not sufficient to warrant the assertions made by the builders regarding running costs. They state that, for " chassis maintenance" (exclusive of battery maintenance which is allowed for at the rate of 2d. per bus-mile), this accumulator-driven bus costs 0.10d. per bus-mile as against 1.00d. for a gear-driven petrol bus. By what means is this charge reduced by 90 per cent. with the battery bus ? The most-favourable estimate of say
ing which has previously been advanced for electric vehicles is about 50 per cent. Then, again, for "washing and preparing for running" the petrol vehicle is charged 0.90d. per bus-mile as against the battery vehicle's 0.17d. It is news, indeed, to learn that a battery vehicle can be washed and cleaned for less than one-fifth the amount involved in the cleaning of a petrol bus. These two items of cost are sufficient to indicate the need for close scrutiny of others which are submitted by the authors.
The system, of course, can equally be applied to petrol-electric transmission, the battery being replaced by a petrol engine (the authors suggest an air-cooled one of 9 h.p.), a small buffer battery being provided to absorb the regenerated current and to give it up again when required, for starting purposes and during periods of maximum load.
It is interesting to recall that we reproduced a photograph and published a description of an early form of this system so long ago as our issue of the 8th April, 1909.