Page:Popular Science Monthly Volume 92.djvu/488

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��ropuiar Science Alonihly

��Making Small Generators from Telephone Magnetos

THE amateur electrician may con- struct very sturdy little direct cur- rent generators of either series, shunt or compound type from the parts available in an old telephone magneto which may be procured for a few cents from nearly any telephone exchange, especially in the rural districts. It does not matter whether the magneto is in a workable condition or not, providing the parts are all intact.

The first operation must be that of properly reconstructing the armature in order to make it adaptable for the generation of direct current in place of the alternating current, which telephone magnetos produce. First re- move the fine wire which is wound up- on the armature and replace it by ■winding each pole of the armature full of No. 22 single cot- ton covered wire. It is very necessary that both poles be wound in the same direction as shown course, understood

���A finished generator from parts of an old

��in Fig. 1. It is of by amateur elec- tricians that the smaller the wire con- tained on the armature of the dynamo the higher the voltage generated will be, with a corresponding decrease in the am- perage. Telephone magnetos are de- signed to generate a potential of several hundred volts in order to overcome the high resistance of the line. This high voltage and low current value is not suitable for practical purposes, especially in the amateur's workshop. Hence, the necessity of changing the small wire on the armature to that of a larger size.

As the armature is a two-pole affair, the commutator will need but two segments. The commutator will be of the disk type as it is very simple to make and possesses certain advantages over the drum type which make it more adaptable for this purpose. Owing to the hollow shaft of the armature, which is used to bring the leads to the cummutator, it will be found rather difficult to fit a drum commutator

��to it. The details of the small disk com- mutator are shown in Fig. 2. The copper segments are fastened to the fiber disks by means of small brass brads used as rivets. The brads should be filed perfectly flush with the surface of the copper segments after being hammered into place. The hole in the center of the fiber disks should be a trifle smaller than the shaft so it can be forced on the shaft and made to remain rigidly in olace. After the two leads from the winding are brought through the hollow shaft and soldered to the segments, the armature of the machine is completed. The construction of the armature is just the same whether the gene- rator is of the shunt, series or com- pound type.

Only one magnet of the magneto is used in the construction of these machines. If a more powerful generator be desired, two magnets may be used. In fact, the three mag- nets may be used together without any winding at all by merely taking the current off the commutator with two small brushes. The writer would advise, however, that either one or two magnets be used with a field winding. Aside from giving the mechanic a better understanding in dynamo construction, the types with field winding possess cer- tain advantages over those without it. If the mechanic wishes a series wound ma- chine, the field coils should be wound with No. 24 single cotton covered copper wire. A little more than ]i lb. will be needed. Paper is first wound around the magnet to insulate it from the wire. It is not neces- sary to wind the wire on carefully or to make bobbin heads to hold it in place. It is essential, however, to have approxi- mately the same amount of wire on each pole. It is also necessary to wind each field coil in the same direction. After the field coils are wound, they are given a coat of shellac and covered with friction tape, leaving the leads protruding for con- nections. It is also desirable to shellac the tape after it is wound on to make the winding as nearly moisture proof as possible.

��which can be made telephone magneto

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