Ques. What is the disadvantage of gear driven exciters?
Ans. The space occupied by the gear.
Fig. 1,434.—Diagram showing a Westinghouse 50 kva., 2,400 volt, three phase, 60 cycle revolving field separately excited alternator direct connected to a steam engine. The exciter is belted to the alternator shaft, the driving pulley being located outside the main bearing. The small pulley on the exciter gives an indication of its high speed as compared with that of the alternator.
In the case of a chain drive very little space is required, but for belts, the drive generally used, there must be considerable distance between centers for satisfactory transmission.
Slow Speed Alternators.—By slow speed is here understood relatively slow speed, such as the usual speeds of reciprocating engines. A slow speed alternator is one designed to run at a speed slow enough that it may be direct connected to an engine. Such alternators are of the revolving field type and a little consideration will show that they must have a multiplicity of field magnets to attain the required frequency.
In order that there be room for the magnets, the machine evidently must be of large size, especially for high frequency.
Fig. 1,435.—Crocker-Wheeler 350 kva., slow speed alternator direct connected to a Corliss engine. In front is seen a belted exciter driven from a pulley on the main shaft between the alternator and the large band wheel. The latter serves to give the additional fly wheel effect needed for close speed regulation.
EXAMPLE.—How many field magnets are required on a two phase alternator direct connected to an engine running 240 revolutions per minute, for a frequency of 60?