The set of plates* which will be used presents complete drawings for a multipole direct-current generator having six main poles and commutating poles, running at a speed of 600 revolutions per minute, and rated 250 volts 300 kilowatts. This rating must be abbreviated on the drawings, the manufacturer having a definite form which is always followed, thus: M. P. C. 6-300-600-L-250 V. M. P. means multipolar; C means commutating field; L means the form; and the figures indicate number of poles, kilowatt output, speed, and voltage, respectively. This tells all that is necessary regarding the rating of the machine. Form letters may sometimes be added to indicate some special features of design, but these are peculiar to the manufacturer.
In any electric machine, the design must be an intermingling of electrical and mechanical features. The designing engineer usually gives more of the mechanical details than in some other classes of machinery because these details affect the electrical features. Practice varies in every manufacturing plant to some extent in this respect. Thus, in one place the engineer may go so far as to determine the size of the shaft necessary, while in another the calculation of some of the electrical features may be left to the designing draftsman.
In general, the designing draftsman is supplied with complete tables of the electrical features giving all details of the various windings, the length or size of the magnetic circuits, and the material to be used for them, together with any other features of design which must be followed in order to meet the requirements. The electrical features may be given in the form of tables which may give all necessary data as to size and number of conductors in fields and armature, the size and arrangement of slots in the armature laminations, and the kind of insulation and its arrangement. From this data the draftsman must make his drawings complete in every detail, so that the machine can be built in the shops. The information contained in the drawings must be such that every workman, from the pattern maker to the assembler, can do his work without other help.
*Courtesy of the General Electric Company, Schenectady, N. Y.
Preliminary Layout Sketch.