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Borax may be used as a solvent for shellac gum.

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** How to Make a Small Single-Phase Induction Motor [124] By C. H. Bell

The following notes on a small single-phase induction motor, without auxiliary phase, which the writer has made, may be of interest to some of our readers, says the Model Engineer.  The problem to be solved was the construction of a motor large enough to drive a sewing machine or very light lathe, to be supplied with 110-volt alternating current from a lighting circuit, and to consume, if possible, no more current than a 16-cp. lamp.  In designing, it had to be borne in mind that, with the exception of insulated wire, no special materials could be obtained.

[Illustration:  Motor]

The principle of an induction motor is quite different from that of the commutator motor.  The winding of the armature, or “rotor,” has no connection with the outside circuit, but the current is induced in it by the action of the alternating current supplied to the winding of the field-magnet, or “stator.”  Neither commutator nor slip rings are required, and all sparking is avoided.  Unfortunately, this little machine is not self-starting, but a slight pull on the belt just as the current is turned on is all that is needed, and the motor rapidly gathers speed provided no load is put on until it is in step with the alternations of the supply.  It then runs at constant speed whether given much or little current, but stops if overloaded for more than a few seconds.

The stator has four poles and is built up of pieces of sheet iron used for stove pipes, which runs about 35 sheets to the inch.  All the pieces are alike and cut on the lines with the dimensions as shown in Fig. 1, with the dotted line, C, to be filed out after they are placed together.  Each layer of four is placed with the pointed ends of the pieces alternately to the right and left so as to break joints as shown in Fig. 2.  The laminations were carefully built up on a board into which heavy wires had been driven to keep them in place until all were in position and the whole could be clamped down.  In the middle of the pieces 1/4-in. holes, B, were then drilled and 1/4-in. bolts put in and tightened up, large holes being cut through the wood to enable this to be done.  The armature tunnel was then carefully filed out and all taken apart again so that the rough edges could be scraped off and the laminations given a thin coat of shellac varnish on one side.  After assembling a second time, the bolts were coated with shellac and put into place for good.  Holes 5-32 in. in diameter were drilled in the corners, A, and filled with rivets, also varnished before they were put in.  When put together they should make a piece 2 in. thick.