Increasing the engine speed means that more horse power is developed for any given cylinder dimensions, while reducing the speed of the generator involves that the machine must be larger for a given output, and in the case of an alternator more poles are required to obtain a given frequency, resulting in increased cost.
The compactness of the unit as a whole, simplicity, and general advantages are usually so great as to more than offset any additional cost of the generator.
Fig. 1,388.—Crocker-Wheeler 2,000 kva. 2,400 volt coupled type alternator. The coupled type of alternator is desirable for use with steam, gas, and oil engines, and water wheels where it is inconvenient to mount the alternator on the engine shaft or to extend the engine base to accommodate a bearing. This type consists of alternator complete with shaft and bearings similar to belt type machines, but with bearings not necessary designed for the side pull of belts.
Ques. What is the difference between a direct connected and a direct coupled unit?
Ans. A direct connected unit comprises an engine and generator permanently connected; direct coupling signifies that engine and generator are each complete in itself, that is, having two bearings, and are connected by some device such as friction clutch, jaw clutch, or shaft coupling.
Revolving Armature Alternators.—This type of alternator is one which has its parts arranged in a manner similar to a dynamo, that is, the armature is mounted on a shaft so it can revolve while the field magnets are attached to a circular frame and arranged radially around the armature, as shown in fig. 1,389. It may be single or polyphase, belt driven, or direct connected.
Fig. 1,389.—Revolving armature alternator. Revolving armatures are suitable for machines generating current at comparatively low pressure, as no difficulty is experienced in collecting such current. Revolving armature alternators are also suitable for small power plants, isolated lighting plants, where medium or small size machines are required.