OPERATING CHARACTERISTICS OF A SELF

REGULATING AUTOMOBILE LIGHTING

GENERATOR

ALEXANDER S. LANGSDORF

Professor of Electrical Engineering

INTRODUCTION

In the earliest installations of electric lights on passenger trains of steam railways, current was supplied to the lamps from storage batteries on the cars, the batteries being charged from time to time at terminal stations. This system had the disadvantage that the gradual exhaustion of the battery resulted in inferior illumination towards the end of long runs. To obviate this difficulty, present practice involves the use of one or more generators connected in parallel with the batteries, the function of the generator being to keep the batteries fully charged and to carry a part of the total lamp current at times of peak load. In the "head-end" system a single generator, driven by a turbine taking steam from the locomotive, is installed at the head end of the train, sometimes in the baggage car, sometimes on top of the locomotive boiler shell. In the axle-lighting system each car is equipped with a small generator mounted under the car and driven from the car axle. In the former case the single generator, when in use, runs at constant speed and it can easily be regulated to maintain at the lamps the constant voltage essential to steady illumination. But in the latter case the problem is greatly complicated by the fact that the speed of a generator driven directly from the axle will vary through wide limits; furthermore, its operation must be independent of the direction of rotation. Generators of the ordinary types do not possess inherent operating characteristics suitable for such service, and to make a machine of ordinary type conform to the requirements, more or less elaborate regulating devices must be used. The principal requirement to be met is, in brief, that the generator shall deliver a fairly constant

current at all speeds above a moderate "pick-up" speed. If an ordinary shunt or compound generator were used, it could be made to deliver the proper current and voltage at one particular speed, but at higher speeds, without special auxiliary devices, it would tend to develop a higher voltage with a consequent large increase of battery charging current, and therefore also of voltage at the battery and lamps; which effects would be ruinous to both battery and lamps. Constant current output from the generator would mean that the battery would be neither excessively charged nor discharged, in which case the battery voltage would remain substantially constant and the battery would then act as a voltage regulator for the generator.

Of late years the increasing use of electricity on automobiles has resulted in the development of several systems of electric lighting, among them that of the Wagner Electric Manufacturing Company of St. Louis. The principal part of this particular equipment, in addition to the usual storage battery, is a special generator driven directly from the shaft of the engine. In general, the technical problems presented by such a combination are the same as those encountered in train lighting by the axle system, except that the generator is only required to operate with one direction of rotation for the reason that the maximum speed in the reverse direction is in any case below the pick-up speed. The generator of the Wagner equipment involves the utilization of armature reaction for the purpose of obtaining automatic regulation. The fundamental idea is not new, and the details of the electrical connections are disclosed in patents issued to Mr. C. Scribner in 1885 and to Mr. W. B. Sayers in 1896. Nevertheless the machine presents a number of features of considerable technical interest which, until now, have not been described, and it has been thought desirable to present them in the form of a complete analytical theory of the operation of the machine. The author is aware that the designer of the Wagner machine, Mr. Hans Weichsel, has analyzed the machine to a sufficient extent to design it on rational principles, but the treatment given below has been worked out independently and all of it is new.

DESCRIPTION AND GENERAL THEORY

As originally constructed, the machine had four poles and an armature wound with two distinct two-circuit windings, each provided with its own commutator, the two windings being connected in series. The field winding consisted of shunt and series coils connected differentially, the magnetizing effect of the shunt winding being the greater of the two. The series winding was connected in the main circuit in the usual manner,

but the shunt winding, instead of being connected across the main brushes was connected between one of the main brushes (B1, Fig. 1) and an auxiliary brush b placed midway between the main brushes and ahead of brush B1 with respect to the direction of rotation. In other words, the shunt winding tapped that part of the armature winding lying under the leading halves of the poles. The diagram of connections is shown in Fig. 1, which, for the sake of simplicity, shows the machine reduced to an equivalent two-pole model.

In the form here described, the machine was also used as a series motor for cranking the engine, current for this purpose being taken from the battery. In later models, however, the practice of combining both motor and generator functions in a single unit has been discontinued, and the generator design has been modified by the omission of the series field winding, experience having shown that its effects contributed little or nothing of value. The analytical theory presented below has been worked out on the assumption that the series winding is present, but it is interesting to note that the form of the equations indicates that this winding exerts only minor effects, thus checking with the results of tests.

Referring to Fig. 1, it will be clear that the connections will tend to make the machine regulate for constant current without regard to change of speed, provided the battery voltage remains substantially constant, as is the case when lead batteries are used. For if the machine is delivering current at some given value of speed, an increase of speed will tend to increase both the generated e.m.f. and the current; but the increased current will weaken the field and, therefore, reduce the generated e.m.f. in two ways: (1) by increasing the magnetomotive force of the decompounding series winding, and (2) by shifting the flux, because of increased cross-magnetizing action of the armature, away from the leading pole tip, thereby reducing the e.m.f. generated in the armature between brushes B1 and b and consequently weakening the shunt excitation. It is evident that this double demagnetizing effect will ultimately prevent a further increase of current, and it is actually found that beyond a certain speed the effect of rising speed is to cause the current to fall off from a maximum value. There is a third and most important effect that arises from the fact that the auxiliary brush b short-circuits an element of the armature winding that lies opposite the middle of the pole face and in which there is generated an active e.m.f.; this e.m.f. produces a considerable current in the short-circuited element, and the direction of this current is such that it sets up an additional demagnetizing