From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' describing the B.R.C. Dynauto, a car lighting system with several interesting features.
| Identifier | ExFiles\Box 61\2\ scan0075 | |
| Date | 25th April 1914 | |
| THE AUTOCAR, April 25th, 1914. 781 The B.R.C. Dynauto. A Car Lighting System with Several Interesting Features. THE lighting dynamo which forms the subject of this article is a beautifully built machine embodying a very ingenious principle, and marketed in this country by Messrs. Fenestre, Cadisch and Co., of Great Portland Street, London. The B.R.C. is what is known as a constant voltage dynamo, but this description must not be read too literally. What is meant is that, at all dynamo speeds above a certain minimum limit, the voltage remains constant for any given load. It does not necessarily follow that the voltage given by the dynamo is the same when the lamps are switched off as it is when they are switched on, or the same when the accumulator is charged as when it is discharged. To understand the system it will be best first of all to consider the simplified wiring diagram (fig. 1). In this diagram, F C represents the coils which are in fact round the field magnets of the dynamo. These coils are, of course, wired in shunt with the main circuit, and it will be noted that the field coils in this instance are in series with the resistance coil R C. Further to the left will be seen the shunt coil wound round the iron core of the automatic cut-out which also carries, as usual, a short series coil S C, which, when the contacts C are closed, forms a part of the completed main circuit. At the other end of the cut-out core is a trembling blade T B carrying on its back a tungsten or iridium contact point which normally makes contact with a contact screw C S, and thus provides a passage of low resistance for the current that passes through the field coils F C. This passage is alternative to the one provided permanently through the resistance coil R C. Now when the dynamo begins to speed up, the first thing that happens is that the core of the cut-out becomes sufficiently strongly magnetised to attract the cut-out lever C L and so to bring together the contacts which complete the main circuit. Current now flows round the series coil S C to the battery B and the lamps L. When the dynamo speed further increases, a rate of revolution is presently reached at which the current flowing through the shunt coil S H is sufficiently strong to attract the trembler blade T B, so breaking contact at C S. When this happens, the current through the field magnet coils is prevented from flowing by the easiest possible path, and has to pass entirely through the resistance coil R C. This, of course, reduces the strength of the field current which in turn tends to reduce the output of the dynamo. The output being slightly reduced, the current flowing through S H is no longer strong enough to hold up the trembler blade T B, which moves back again making contact at C S and allowing the full current to flow by the easiest route. This cycle of operations goes on repeating itself very rapidly so long as the speed of the dynamo is above the limit at which movement of the trembler blade T B begins. Thus, however much the dynamo speed is increased, the output is regulated by means of the resistance coil R C. It will be noted that the current flowing through the coil S H has nothing to do with the current flowing through the main circuit, but is dependent entirely upon the voltage across the terminals of the dynamo. The control of output for any given load is, as a matter of fact, a control of voltage and not of current. Now if the dynamo control had no connection at all with the current flowing through the main circuit, there would be certain disadvantages in the system. Among others, it would be really impossible to charge the accumulators up to the fullest extent, and at the same time to provide that, when the accumulators are more or less discharged, current shall not be put through them at too great a rate. This difficulty is met by the presence of the coil S.C. When a strong current begins to flow—as, for example, when the batteries are in a discharged condition—this coil is more effective in assisting the coil S H than it would be were the batteries nearly fully charged. The two coils working together set the vibrator in motion sooner than the coil S H would have done if acting alone, and so, by reducing the voltage permitted across the field coils, reduce the strength of the current in the main circuit and prevent too strong a current from being pushed through the exhausted accumulators. The presence of the series coil S C must also mean a slight variation in the voltage at the dynamo terminals according to the number of lamps that are burning. Assuming for the moment that the battery is disconnected, any increase in the number of lamps tends somewhat to reduce the voltage at their terminals. The field magnets are cylindrical and carry two pole pieces so shaped as to enable the machine to run as silently as possible. Fig. 1.—The wiring diagram of the B.R.C. lighting system. B, battery C, main circuit contacts C L, cut-out lever C S, contact screw D, dynamo F C, field coils L, lamps R C, resistance coil S C, series coil of cut-out and regulator S H, shunt coil of cut-out and regulator T B, trembler blade of voltage regulator Fig. 2.—The arrangement of the B.R.C. dynamo brush gear. B, brush C, commutator F, flexible connections to terminals G, brush guide or holder R, rod which can be drawn back to facilitate removal of the brush T S, spring holding brush in position on commutator terminal Fig. 3.—Diagram of the automatic cut-out on the B.R.C. dynamo. C P, contact piece C S, contact surface I C, iron core S S, spiral spring | ||