A small motor can be constructed by any one having ordinary skill in the use of tools, and having access to a screw-cutting lathe with a swing of nine inches or more, by following the instructions given here.

The motor is of the type known as a "creeping field" induction motor, and is designed to run on a 100- to 120-volt, 60-cycle, single-phase alternating-current circuit, such as is now in widespread use for the lighting of dwellings. Being a four-pole motor, it will run at a speed of something like 1,600 revolutions per minute, and will, if well made, deliver about 1/8 horse-power. This is sufficient to drive either a 16-inch brass fan, a small lathe, a 50-watt dynamo for generating direct current for charging storage batteries, or, in fact, almost any kind of work that can be done by one-man power. It should be noted, however, that a creeping field motor is adapted to run in one direction only; so that when set up for driving a screw-cutting lathe the motor should be belted to a light countershaft having two belts, as is done when steam power is used. For most other cases where reversing has to be done it is sufficient to merely turn the motor around and put the pulley on the other end of its shaft.

A small alternating-current motor is much easier to build than a direct-current motor, for the reason that the armature, or "rotor" as it is called in an A.C. machine, requires no such delicate parts as insulated wire coils, commutator, and brush-rigging. The field magnet, or "stator," offsets much of this advantage, however, as it is impossible to use an iron or steel casting for this part, since the entire magnetic circuit must be built up of thin plates of sheet steel. If a solid casting were used the alternating current would set up wasteful or eddy currents within it, and the motor would be burned up by the energy thus converted into heat. In factories where small motors of this kind are made, the thin sheets for the stator and rotor are punched out by machines built for the purpose. For the amateur, however, the only successful way in which so many irregular-shaped pieces of metal can be made all alike is to first bolt the required number of steel sheets on the face-plate of a lathe, and then bore out the inside and turn off the outside to the required dimensions. This will leave a heavy ring made up of the sheets, on the inside of which, in the case of the stator, the four pole-pieces can be readily formed by drilling and sawing away. When finally completed, the stator and rotor core plates should have the form and dimensions as shown in Fig. 215.

Fig. 215 - Details of stator and rotor core plates.

For the stator core about 25 pounds of thin sheet steel are required, cut 7 inches square. This is sold at hardware stores under the somewhat misleading trade names of "Russia iron."

"sheet" or "stovepipe iron," and "roofing tin." The most desirable thickness is about 15/1,000 or 1/64 of an inch, but anything thicker than 25/1,000 will answer the purpose. If roofing tin is selected, the cheaper grades are the most desirable, and better if somewhat rusty. When tightly compressed, the bundle of sheets should measure 1 7/8 inches in thickness or a trifle over.

If the lathe is large enough to swing a piece 11 inches in diameter, the bundle of sheets may be mounted on the face-plate without further trimming; but if a 9- or 10-inch lathe is used, then an inch or so must be clipped from the four corners of each of the sheets. The ease with which the work of boring and turning can be done depends very much on how firmly the sheets are bolted to the face-plate; and if the following method is adopted, themiass will behave almost as if it were a solid block.