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lighting service, as satisfactory installations have been made with either the lead-plate or Edison batteries. This service is not exceptionally severe, as an examination of some lead-plate batteries with a record of 500,000 car miles during a period of three years showed that the jars contained but one inch of sediment, and plates were in excellent condition. The results point to a normal life of
ten years with but one intermediate cleaning. The batteries used are very similar in general construction to those intended for use in electric vehicles, and are installed in trays for ready handling. The illustrations at Fig. 74 show the application of the Edison cell to this service and the method of grouping the cells to form a battery. The location of the battery compartment on the car and accessibility of the battery trays are clearly depicted. While the battery is an important part of a train-lighting system, the dynamo and method of regulation are also of interest. The E. S. B. system is shown in diagram form at Fig. 75, and the distinctive features making for automatic operation may be understood by a careful study of the diagram. The dynamo has a bipolar armature rotating between heavy pole shoes, each polepiece being securely attached to the core pieces. Two pairs of brushes are used, these being spaced on quarters of the commutator, or 90 degrees apart. One pair is short-circuited, while the other is coupled through the series winding to the outside circuit. These brushes are known as the "short-circuit brushes” and the "load brushes," respectively. The pole pieces are provided with the usual field coils, Fi and F2. The series winding F2 is connected between the generator terminal and the top load brush. The control field winding, F1, is connected across a Wheatstone bridge W and provides the primary field excitation. The magnetic field produced by this primary excitation passes across the armature in the direction of the arrow P, and then through pole shoes, pole necks and frame of the machine. This magnetic field is, under normal operating conditions, of very small strength, producing a low voltage between the short-circuit brushes B'. This low voltage, however, produces a sufficient flow of current through the short circuit between these brushes and through the armature winding to develop a considerable magneto-motive force. This latter magneto-motive force produces a magnetic field at right angles to the primary field, which passes through the armature and pole shoes, as shown by arrows K, but does not pass through the pole cores or frame of the machine. This secondary field flux produces the desired voltage across the load brushes B?.
One of the principal results obtained with the Rosenberg type of machine is the development of the same polarity for either direction of rotation without employing any pole changer, or any alterations whatever in the circuit connections. This is due to the fact that when the direction of rotation changes the voltage generated across the short-circuit brushes by the primary field is reversed, and the current flowing between these brushes is, therefore, reversed in direction. This reverses the direction of the secondary
field K, which produces the voltage across the load brushes B2. It will be seen, therefore, that when the direction of armature rotation is reversed the direction of the main or secondary field excitation is also reversed, producing no change in polarity. The importance of this can be realized when one takes the method of driving the dynamo armature into consideration. It is belt-connected to one of the car axles and is just as apt to be driven in one direction as the other.
The E. S. B. dynamo is controlled for constant voltage rather
than constant current, the voltage being held at approximately 3312 volts when used with 15-cell equipments. This control is effected by means of a Wheatstone bridge W connected across the machine terminals at two opposite junction points of the bridge, while the other two opposite points are connected to the field terminals. The Wheatstone bridge consists of two fixed resistances, XX, and two iron wire “ballast” resistances, YY. The iron wire “ballasť” resistances have the characteristic of increasing their resistance rapidly with very small increments of current when operating at a dull-red heat. The design of this bridge (for 15-cell equipments) is such that when the machine voltage
is 331/2 volts the resistances of X and Y are practically, equal. Under these conditions no current will flow through the field winding Fi. When the voltage is lower than 331/2, the resistance of Y is less than the resistance of X, and Y will, therefore, carry more current than X, the excess current flowing through the field winding F1. As the voltage of the machine increases, therefore, the field excitation becomes smaller and smaller, approaching zero as the voltage approaches 331/2 volts. At high speed the current in the field is actually reversed in order to partly counteract the residual magnetism in the pole necks and the frame of the machine. In series with the Wheatstone bridge is a fixed resistance R, which is normally short-circuited by a switch H, and is also short-circuited by a contact clip or by an extra blade on the lamp switch. When the switch H and the lamp switch are both open, the resistance H is no longer short-circuited. The drop in this resistance then lowers the voltage applied across the Wheatstone bridge, and in order to restore the latter to its normal balanced value, the voltage of the machine will be increased by an amount equal to the drop in the resistance R. This arrangement permits the voltage of the machine to be increased during a daylight run, when the lamps are not in use, in order to give the battery a highvoltage charge if this should ever be deemed necessary or advisable. When the lamp switch is closed the resistance R is immediately short-circuited and the voltage of the machine is restored to normal, so that the lamps are never subjected to this high voltage. On the actual switchboard, the switch H consists of two terminals connected by a brass rod, which, by sliding lengthwise, may be disconnected from one of them. While this means for increasing the voltage has been included to provide for possible contingency, it has not been found necessary to use it on any of some seventy equipments in service from two to three years.
Automatic means are provided to prevent a battery overcharge, and a “cutout” arrangement is included to prevent the battery discharging through the generator when its voltage is greater than the electromotive force of the dynamo. Means are provided to supply generator current to the battery as it is used, so the cells are always maintained in a charged condition. The features of
current control developed in car-lighting service have been adopted by automobile makers in the modern starting and lighting systems which are so popular and now considered indispensable. The experience of the various storage-battery makers in producing satisfactory batteries for this service was turned to good