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* Candy — Constant Current 217
dicated by Fig. 1, which is not sufficiently high to damage the shunt field windings and, therefore, eliminates the use of a relay to limit the open circuit voltage.
The current values delivered by the machine can be varied from 60 amperes to 75 amperes b}^ changing the setting of the generator field rheostat. The position of the brushes and commutating poles remains fixed at all times so that perfect commutation at all loads is assured.
The onh' control equipment required for such an installation is a starter for the motor; a small panel mounting an ammeter, voltmeter and field rheostat for the generator, and two single pole enclosed knife switches, one to be • mounted on each projection machine for short circuiting the arc. A complete schematic diagram including emergency service is shown b} Fig. 3.
For installations where no more than two projection arcs of the same current value are to be operated at one time, this eciuipment is indisputably the most satisfactory to use and the least expensive to install and operate.
It is not practical, however, to design generators of this t^'pe for the operation of more than two arcs in series on account of the high operating voltage and abnormally high open circuit voltage involved .
A great many theatres especially those on the west coast rerequire several projection and other arcs in operation at one time, such as a motion picture projector, a stereopticon and a spot or flood light. For such an installation the constant potential parallel arc machine is best suited.
It is not necessary to go into a detailed description of this equipment as it was described fully by the authoi in Vol. lY of the S. M. P. E. proceedings. The schematic diagram of connections is shown b}' Fig. 4 which is sufficient to show wherein it differs from the constant current equipment. When the constant potential equipment is used each arc has a ballast or stabilizing resistance in its circuit and the various arc circuits are connected in parallel to the generator. Therefore, when such an equipment is used the only limitation to the number of arcs which can be operated at one time is the ampere capacity of the generator.
Suninianj
To summarize briefly, we find that where not more than two projection arcs of the same current value are to be operated at one time, the constant current series arc equipment is best suited on account of its simplicity, good operating characteristics and improved operating efficiency. When more than two arcs are to be operated at one time at the same or different current values, the constant potential multiple arc equipment is less expensive and more satisfactory relative to gencn-al opei-ating conditions.
General Engineering Dept.
Westinghouse Electric and ^Ifg. Co.