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).
Technical paper on the voltage regulation of electrical systems, focusing on different generator types and their application in vehicles.
| Identifier | ExFiles\Box 31\1\ Scan173 | |
| Date | 1st June 1925 | |
| Vol. XVI June, 1925 No. 575 VOLTAGE REGULATION OF ELECTRICAL SYSTEMS current that is given by the dynamo becomes. If charging, a decrease in the generator current due to lowered speed causes a decrease in the current passing into the battery. CHANGE OF EXCITATION A change in the value of the field current has a similar effect to that of a change of speed. If we wish to charge a battery, the field current is regulated so that the generator voltage is slightly higher than that of the battery, and the current can be controlled by a field rheostat. To stop charging, the field current is weakened until the generator voltage falls to the value of the battery voltage. The battery is then floating on the line and may act as a pressure regulator. In the shunt-wound generator, we have, then, a machine whose output is affected by variations of speed and load, to say nothing of temperature, but which has some desirable characteristics. Inasmuch as present-day automotive electrical-apparatus is seldom sold on a temperature rating, this will not be considered and it will be assumed that the temperature effect on the generator is slight which is, however, far from the case in many instances. The problem presented for solution is to furnish a generator of the shunt type, which will be simple and comparatively cheap and which can be used, safely, for lighting and battery charging on vehicles. Here, the third-brush generator enters. It is believed that Edison first used the reaction brush in the 1880's and it was applied to automotive generators about 1912 by the company I represent. In the third-brush generator, one end of the shunt field is connected to an auxiliary or third brush. The third brush usually is placed behind and close to the leading main brush. The other end of the field is connected to the external circuit of the battery at a point of opposite polarity to that of the leading main brush. The field is thus in parallel with the armature and with the external circuit, as in the straight shunt-wound machine; but, by virtue of the use of the third brush, the voltage across the field is lower than the voltage of the main-brush terminal. Further, the third brush is placed so that the armature reaction, due to increases in speed and load, has a decided effect on the field voltage and, consequently, on the field current which, in turn, reacts on the terminal voltage of the generator and functions as current control. The third-brush generator is known as a “current-controlled” generator, as compared with the voltage-regulated machines. When connected to a battery, this generator will give a substantially constant charging-rate, the tendency being to maintain the rate as the battery comes up in charge. The tendency on protracted charging is to maintain a voltage higher than the battery voltage, resulting in a comparatively high charging-rate at all times. This may result in trouble of various kinds on continuous high-capacity applications. As the battery takes its charge, the voltage rises and the charging rate tends to increase, thus causing more current to flow through the generator armature. The result is less armature drop and increased terminal pressure which, in turn, strengthens the field and tends to maintain a higher charging-rate and voltage. The third-brush generator has served well as a low-cost generator for passenger cars, and has given satisfactory service. It has, however, several disadvantages, when applied to motorcoach requirements, among them being (1) Overcharging batteries on long continuous driving, with attendant troubles (2) Undercharging batteries on slow, frequent-stop service (3) Burning out lamp bulbs because of excessive voltage (4) Reducing the length of life of lamps because of excessive voltage, especially where the third brush has been advanced to boost the output, due to conditions that may arise such as low water in the battery, loose connections, poor wiring or overloading (5) Burning out field fuses and lamps if a battery connection becomes loose or disconnected (6) Necessity for using the generator with the battery, and that it will not carry the lamp load alone at the rated voltage (7) That it does not give a tapering battery charge but maintains a comparatively high, steady rate (8) Because the lamp voltage usually is higher than it should be, which results in a waste of energy (9) For the reason that energy is also wasted in maintaining a high-charging rate The third-brush-controlled generator, as such, is not well adapted to motorbus and motor rail-car requirements where heavy loads are carried. The battery must be kept fully charged on all classes of service and, at the same time, it must be protected from overcharge. Lamp voltage must be held within the limits of safe operation. VOLTAGE-REGULATED SYSTEM The voltage-regulated system consists of a shunt-wound generator, either with or without a third brush, and a regulating unit to maintain a controlled voltage under varying conditions of load, speed and temperature encountered in service. If the third-brush generator is regulated, the action of the third brush is such as to protect the generator automatically from severe overload. If the straight shunt-wound machine is regulated, some overload protection should be used, such as an overload circuit-breaker. The third-brush voltage-regulated system thus consists simply of a generator, a reverse-current relay and a voltage-regulator element. The straight shunt-regulated system uses, in addition, an overload circuit-breaker. For a large majority of installations, the former is admirably adapted but, in some cases where high capacities at low speeds are required, FIG. 2—VOLTAGE-REGULATED CHARACTERISTICS MADE AT CONSTANT SPEED FOR A LARGE BATTERY The Purpose of a Voltage-Regulated Electrical-System Is To Furnish a Regulated Potential Within the Limits Required To Give Steady Non-Flickering Non-Charging Light and To Assure Proper Battery-Charging at the Same Time. Such Systems Are Not “Constant-Potential” Systems in That the Voltage Is Not Held Absolutely at One Value But Varies Slightly within the Limits Required by the Application | ||