The Commutator of a dynamo, of which previous mention has been made, is a very important part of the machine. Its function is to convey in part the successive pulsations of the currents generated by the coils of the armature to the brushes so that the current in the line shall be practically a continuous one. To make the line current as uniform as possible, it is necessary to have the number of coils in the armature, and consequently the number of segments in the commutator, as great in number as is mechanically possible, when cost and facility of construction are considered. As the mechanical and electrical difficulties encountered are considerable, they determine to quite an extent the design of the armature and commutator.
Each segment must be insulated from those adjacent to it and all of them must be securely held in position on the shaft and yet insulated from it. This involves careful work in the making and assembling of the parts. The material used for the segments is generally hard copper of as great a
The important point to guard against is not to have so much carbon in the space between the electrodes that it will not allow a free movement of the particles.
Next the cover bearing the diaphragm should be laid on top of the bridge, the carbon on the diaphragm coming centrally over the carbon on the bridge. By replacing the screws in the same holes in the bridge posts the exact position would be obtained and the instrument will be completed.
The connections are taken, one wire being placed between the two nuts, A, B, the top nut being screwed tightly down upon same. The other connection is obtained by putting a washer under one of the screws holding the diaphragm, and bending a wire between this washer and the diaphragm, then screwing the screw tight. If the box and mouth piece is now given a good coat of shellac, we can now put it away and construct the rest of our apparatus.
purity aud uniformity of texture as can be obtained. The latter feature is a necessary one, to ensure its wearing evenly. A cross section, as illustrated in Fig. 35, shows each segment to be of a wedge shaped form. Mica, because of its excellent insulating properties and power to resist great heat and pressure, is almost entirely used as the insulating material. Between each segment and also at the surfaces of the ends where they bear on the wedge shaped clamps holding them in position on the shaft, are placed strips of mica free from cracks or ridges. This is shown in Fig. 35.
The Brushes are in contact with the commutator and convey the current to the line. The materials usually employed are woven copper gauze or carbon sticks. The former consists of several layers of fine copper gauze rolled into a suitably shaped bundle and then firmly cempressed and stitched to retain the shape. Excellent contact is secured with this kind of brush, as it is somewhat flexible and has numerous points bearing upon the commutator. It is made of softer copper than the commutator, so that the wear between the two parts will be on the part of the machine most convenient and inexpensive to renew.
The carbon brush is, however, the kind most generally used, having several advantages over the copper ones. It wears a better surface on the commutator, the dust is less injurious to other parts of the machine, being less liable to cause a short circuit; no injury results should the armature be run backwards for any reason. It also causes less sparking, resulting in less heating and therefore less wearing of the segments and insulation. The size of the brushes is determined by the maximum current to be carried with a liberal addition for safety, as with brushes too small, the current generated would develop excessive heat in the armature with very injurious results.
The Brush Holders are so constructed and attached to the machine that the position of the brushes and the point of contact with the commutator may be altered as occasion requires. Adjustable springs are attached to the brush holders, the pressure of which give the brushes a firm contact with the commutator. These springs are attached in such a way that little or no current passes through them.
The Field Magnets are made of wrought iron, cast steel and cast iron, the desirability of these metals being in the order named. Wrought iron has the highest permeability and is used when a small cross section is required of the field cores. In designing a dynamo it is customary to allow about 90,000 lines of magnetic flux per square inch for wrought iron. Certain grades of soft steel, having a very low percentage of carbon, are almost universally used in dynamo construction owing to its lower cost. It has a permeability almost equal to wrought iron and about 80,000 lines are usually allowed per square inch. Cast iron is the least adapted for dynamo parts, but where size is not important, is on account of its low cost used for yokes, bases and some other parts. About 45,000 lines per square inch are usually allowed in designing.
The Field Coils are usually wound on separate forms and when complete, placed on the core. If a metal form is used, such as brass or tin, it is necessary to insulate the coil from the form. In any case the coil must be insulated from the core. Firm paper is usually used for such insulating, being shellaced to keep it from being affected by moisture. The ends of the coil wires, called "leads," are carefully insulated, so that a short circuit will not be formed with any part of the machine which they touch. In large machines with series winding the necessary size of the wire would be so large as to make the work of winding a difficult one, so ribbon copper is used or several smaller wires are used, these being connected in parallel, and serving to secure about the same results as would the larger wire. In shunt windings the shunt wire is wound over the series winding, usually on a separate form to facilitate ease in construction and repair, though it may be wound on one form.