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306 RICHARDSON'S BLUEBOOK OF PROJECTION
ance for a given amount of work. Instances have been found where a projectionist was attempting to operate a high intensity arc off an 80-volt generator, using a ballast resistance designed to take care of a 25-volt drop. In such a case, if a 65-volt arc is maintained (the lowest voltage practicable with a high intensity arc), there is a 25-volt resistance and only a 15-volt drop, the result being that the generator cannot force through sufficient current. The remedy is either a higher generator voltage or a ballast resistance suited to the work.
The foregoing is designed to emphasize the importance of maintaining proper relationship between generator voltage and the ballast resistance.
Parallel Generator Equalizers
(98) Between two parallel compound-wound generators it is necessary to have a special connection called an equalizer. The reason for this is easily understood. To secure constant potential performance compounding is necessary, which varies in different designs and sizes of generators. In fact it may vary somewhat between two generators of identical design. Even changes which involve alteration in operating voltage and (or) brush position may cause variations.
Presuming two generators to be set for parallel operation their compounding will be balanced by means of a proper equalizer (99) consisting of a connection of a heavy copper bar known as a bus bar between their series coils. This prevents the current in the series fields from differing to any extent, regardless of how much the variation may be in their armature currents.
(100) If, however, one of the generators is started and warmed up to normal operating temperature and the second generator, while still cold, is cut immediately into the circuit with its voltage adjusted to match that of the first machine, what happens ?
As the second generator warms up its voltage will drop, compelling the first generator to assume a portion of its load. The field regulation of the second generator