R. G. Griswold

Although the majority of dynamos and motors of very recent design are of circular yoke type with radial poles, the Manchester type of field magnet has many points of merit, particularly in machines designed for amateur construction. Although there is a little more actual machine work to be done on this type of machine, it is of a character that is more easily performed than that necessary on the circular yoke type, and for this reason, more than any other, this type of machine has become such a favorite among amateurs.

Fig. 1 is a partly sectional side elevation of the field magnet frame together with the base and journal pedestals, showing the relative positions and dimensions. In Fig. 2 is shown a transverse sectional view of the field-magnet pole-pieces, mounted on the magnet cores. These magnet cores are turned from a 1 in. bar of soft wrought iron to the dimensions given, and the ends which fit into the holes in the field-magnets should be a very snug fit therein. The magnet cores are provided with flanges of 1/8 in. fibre forced on, and it is well to provide a small shoulder 1/8 in. deep by 1/32 less in diameter than the core, against which this washer may be forced.

Fig. 3 is a sectional drawing of the assembled armature, commutator and shaft with pulley. The armature core is of the usual laminated type, the disks being 2 1/2 in. in diameter and of the shape shown in Fig. 4. The coil holes in the armature discs are 3/8 in. in diameter and have a 3/16 in. slot cut through to the periphery to facilitate the winding. The central hole is 9/16 in. in diameter and is not provided with a key slot, as it is intended that these disks should be forced on the shaft, which is made a tight fit in the hole. With every one of the disks thus pinching the shaft, there is little danger of the torque twisting the armature on its shaft.

The disks are clamped between two flanged collars, the flanged portion being 1 5/8 in. in diameter and being well rounded as shown. The shaft is provided with threads as shown, onto which these collars are screwed, hard up against the disks, firmly pressing them together. If greater security is desired a 1/8 in. hole may be drilled through from end to end every 120° and bolts passed through these holes provided with nuts let into counterbores, but this is hardly necessary on so small a machine. The flange on the outer end of one of the collars is provided to prevent any oil finding its way to the armature by creeping over from the bearing. The magnetic portion of the armature is 2 1/4 in. long. The pulley, owing to its small diameter, is cast with a solid central web instead of spokes.

The commutator is of a design familiar to those acquainted with dynamo construction, the segments being bevelled at each end to fit under the bevelled groove in the collars. The clamping collar at the front end of the commutator fits smoothly over the core, but is not threaded; it is forced into place by an auxiliary nut on the core back of it. A layer of fibre or mica is laid between the segments and collars, that between the bevelled ends and the collars being in the form of a washer. Fig 5. is a front view of the commutator which is 1 1/2 in. in diameter. These lugs are 5/16 in thick axially.

The brush holder, Fig. 7, is of the adjustable type and carries two brushes of either woven copper wire or carbon, which act in a radial direction against the commutator. They may be easily shifted while the machine is in motion and clamped in position by the small handle which terminates in a 3/16 in. screw. The brush clamps need no special description as the detail drawing gives all dimensions. The actuating springs are made of No. 26 spring brass wire (B. & S.) and inserted behind the brushes, being held in place by the split pins shown. These springs feed the brushes to the commutator as they wear.

The journals shown in Fig, 7, and also in Fig 1., are of the self oiling type, having oil rings 1 in. in diameter outside and 13/16 in. in diameter inside, by 1/4 in. wide. These rings dip into the oil well below the shaft and keep the bearing well oiled by revolving with the shaft. The brass bushings are held in place by two 8-32 machine screws put through from the top.

This machine can be built in either of two windings ; one for 14 volts and 3 amperes, and the other for 32 volts and 1 1/2 amperes. The 14 volt winding consists of 12 coils of No. 20 double cotton-covered magnet wire, each coil containing 22 turns of wire. The armature winding for 32 volts consists of 12 coils of No. 28 wire, each coil containing 54 turns of wire.

Each field coil contains 56 layers of No. 25 double cotton-covered wire, each layer containing 92 turns, so that there are 2,392 turns of wire in each coil. The two field coils are connected in multiple when used in connection with the 14 volt armature winding, and in series when used with the 32 volt winding, the fields being connected in shunt with the armature. The speed of the machine for 14 volts is 2,400 revolutions per minute ; for 32 volts it can be run slightly slower than this.

The machine work on this dynamo is of the simplest possible consistent with practical results, and all of the fitting except the brush holder is plain lathe work.