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constant. To shut down the first arc, close the short-circuiting switch.
The D. C. to D. C. equipment is not a true motor generator set, but very closely resembles a balancer set. Both armatures are mounted on a common shaft, making a two-bearing outfit, the two armatures being connected in series across the line. The arc circuit is taken from a connection between the armatures and side of the line. This volt ampere characteristic is such that one lamp can be operated from this machine without the use of a steadying resistance. The machine, therefore, not only lowers the line potential to that suitable for the projection arc, but allows the arc to be operated in a highly efficient manner.
The D. C. to D. C. outfits are built for 35, 50 and 70 ampere one lamp equipments, and 35 and 50 ampere two lamp equipments. The two lamp equipments are designed with a compound field which is cut in service when two lamps are operating. The two lamps are connected in multiple across the armature of the machine ; the machine very closely resembling a constant potential generator.
With the two lamp equipments there is furnished a special panel which contains an ammeter, voltmeter, generator field rheostat and a change-over switch. The function of this changeover switch is to cut in the series field on the generator, changing its characteristic so as to allow the operation of two lamps. It is, however, necessary to use a slight amount of steadying resistance with this type of outfit when operating two lamps. This is because the outfit is practically a constant potential generator. Due to the use of the steadying resistance, the overall efficiency is lowered about 18% when operating two lamps, 70% one lamp.
Operation.
The method of operating: the D. C. to D. C. outfit is as follows :
After the set is up to speed with the change-over switch on the panel closed, the first arc is struck in the usual manner. When one arc is burning, there is no steadying resistance to be used. When ready to change over, to include the second arc, the shortcircuiting switch is opened, which automatically cuts in the series field of the generator and at the same time inserts a steadying resistance in series with the arc. The second arc is then struck. When ready to shut down the first arc, the arc switch is opened and the change-over on the panel closed.
The single arc D. C. to D. C. outfit consists of a motor generator complete with motor starter and generator field rheostat.
Constant Potential Equipment.
The motor generator sets built for this service each comprise a 75 volt compound wound commutating pole direct current