Chapter X

DYNAMOS AND MOTORS

To adequately treat of dynamos and motors, a good-sized book rather than this single chapter would be necessary, and only a general survey of the subject is possible. Its importance is unquestionable; indeed, the whole science of applied electricity dates from the invention of the dynamo. Without mechanical production of electricity there could be no such thing as electric traction, heat, light, power, and electro-metallurgy, since the chemical generation of electricity is far too expensive for commercial use. Surely it is a part of ordinary education nowadays to have a clear and definite idea of the principles of electrical science, and in no department of human knowledge has there been more constant and rapid advance. It is only a truism to assert that the school-boy of to-day knows a hundredfold more about electricity and its varied phenomena than did the scientists and philosophers of old--Volta and Galvani and Benjamin Franklin. Yet it was for these forerunners to open and blaze the way for others to follow. A beginning must always be made, and the Marconis and Edisons of to-day are glad to acknowledge their indebtedness to the experimenters and inventors of the past. And now to our subject.

All dynamos are constructed on practically the same principle--a field of force rapidly and continuously cutting another field of force, and so generating electric current. The common practice in all dynamos and motors is to have the armature fields revolve within, or cut the forces of the main fields of the apparatus. There are many different kinds of dynamos generating as many varieties of current--currents with high voltage and low amperage; currents with low voltage and high amperage; currents direct for lighting, heating, and power; currents alternating, for high-tension power or transmission, electro-metallurgy, and other uses. It is not the intention in this chapter to review all of these forms, nor to explain the complicated and intricate systems of winding fields and armatures for special purposes. Consequently, only a few of the simpler forms of generators and motors will be here described, leaving the more complex problems for the consideration of the advanced student. For his use a list of practical text-books is appended in a foot-note.[3]

[3] _First Principles of Electricity and Magnetism_, by C. H. W. Biggs; _The Dynamo: How Made and Used_, by S. R. Bottone; _Dynamo Electric Machinery_, by Professor S. P. Thompson; _Practical Dynamo Building for Amateurs_, by Frederick Walker.

The Uni-direction Dynamo

The uni-direction current machine is about the simplest practicable dynamo that a boy can make. It may be operated by hand, or can be run by motive power. The field is a permanent magnet similar to a horseshoe magnet. This must be made by a blacksmith, but if a large parallel magnet can be purchased at a reasonable price so much the better, as time and trouble will be saved. This magnet should measure ten inches long and four inches and a half across, with a clear space seven inches long and one inch and three-quarters wide, inside measure. The metal should be half an inch thick and one inch and a quarter wide. A blacksmith will make and temper this magnet form; then, if there is a power-station near at hand where electricity is generated for traction or lighting purposes, one of the workmen will magnetize it for you at a small cost; or it can be wound with several coils of wire, one over the other, and a current run through it. When properly magnetized it should be powerful enough to raise ten pounds of iron. This may be tested by shutting off the current and trying its lifting power. If the magnet is too weak to attract the weight the current should be turned on and another test made a few minutes later.

Before the steel is tempered there should be four holes bored in the magnet and countersunk, so that screws may be passed through it and into the wooden base below, as shown at Fig. 1. This wooden base is fourteen inches long, eight inches wide, and one inch in thickness. It may be made of pine, white-wood, birch, or any good dry wood that may be at hand. The blocks on which the magnet rests are an inch and a quarter square and seven inches long. The magnet is mounted directly in the middle of the base, an equal distance from both edges and ends, as shown in the plan drawing (Fig. 10). The blocks are attached with glue and brass screws driven up from the underside of the base.

From a brass strip three-eighths of an inch wide and one-eighth of an inch thick cut a piece six inches long, and bore holes at either end through which long, slim, oval-headed brass screws may pass. Use brass, copper, or German-silver for this bar, and not iron or steel. To the underside, and at the middle, solder or screw fast a small block of brass, through which a hole is to be bored for the spindle or shaft. This finished bar is shown in Fig. 2. When mounted over the magnet and held down with brass screws driven into the wood base, its end view will appear as shown in Fig. 3, A being the bar, B B the screws which hold it down, D the base into which they are driven, and C C the blocks under the magnet (N S). The object of this bar is to support one end of the armature shaft. From brass one-eighth of an inch thick bend and form two angles, as shown at Fig. 4. Two holes for screws are to be drilled in the part that rests on the base, and one hole, for the shaft to pass through, is bored near the top of the upright plate. The centre of this last hole must be the same height from the base as is the hole in the bar (Fig. 2) when mounted over the magnet, as shown at Fig. 3. The location of these plates is shown in the plan (Fig. 10). There is one plate at each end of the base, as indicated at B and B B, the shaft passing through the hole in the brass block at the underside of the bar (C). These angles are the end-bearings for the armature shaft, and should be accurately centred so that the armature will be properly centred between the N and S bars of the magnet.

The armature is made from soft, round iron rod one inch and a half in diameter and five inches long. A channel is cut all around it, lengthwise, five-eighths of an inch wide and half an inch deep, as shown in Fig. 5. This will have to be done at a machine-shop in a short bed-planer, since it would be a long and tedious job to cut it out with a hack-saw. The sharp corners should be rounded off from the central lug, so that they will not cut the strands of fine wire that are to be wound round it.

Two brass disks, or washers, are to be cut, one inch and a half in diameter and from one-eighth to one-quarter of an inch thick, for the armature ends. A quarter-inch hole is to be made in the centre of each for the shaft to fit in, and two smaller holes must be drilled near the edge, and opposite each other, so that machine-screws may pass through them and into holes bored and threaded in the ends of the armature, as shown at Fig. 5. These ends will appear as shown at Fig. 6, and the middle hole should be threaded so as to receive the end of a shaft. When the shaft is screwed in tight the end that passes through the brass disk must be tapped with a light hammer to rivet the end, and so insure that the shaft will not unscrew.

The shafts should be of hard brass or of steel. The one at the front should be one inch and a half in length, and that at the rear six inches long, measuring from the outer face of the brass end to the end of the shaft. From boxwood or maple turn a cylinder three-quarters of an inch in diameter and an inch long, with a quarter-inch hole through it. Over this slip a piece of three-quarter-inch brass or copper tubing that fits snugly, and at opposite sides drill holes and drive in short screws that will hold the tube fast to the hub. They must not be so long as to reach the hole through the centre. Place this hub in a vise, and with a hack-saw cut the tube across in two opposite places, so that you will have the cylinder with two half-circular shells or commutators screwed fast to it, as shown at Fig. 7. This hub will fit over the shaft at the front end of the armature, and will occupy the position shown at F in Fig. 10.

Cut two small blocks of wood for the brushes and binding-posts, and bore a hole through them, so that the foot-screw of a binding-post may pass through the block and into the post, as shown at Fig. 8. From thin spring copper cut a narrow strip and bend it over the block, catching it at the top with a screw and lapping it under the binding-post at the outside.

From boxwood or maple have a small wooden pulley turned, with a groove in it and a quarter-inch hole through the centre. This pulley should be half an inch wide and one inch and a half in diameter, as shown at Fig. 9. This is to be attached at the end of the long shaft, where it will occupy the position shown at E in Fig. 10.

All the parts are now ready for assembling except the armature, which must be wound. Before laying on the turns of wire the channel in the iron must be lined with silk, held in place with glue or shellac. A band of silk ribbon is given two turns about the centre of the iron, and the sides are so completely covered with silk that not a single strand of wire will come into direct contact with the iron. Great care must be taken, when winding on the wire, not to kink, chafe, or part the strands. The channel should be filled but not overcrowded, and when full several wraps of insulating tape should be made fast about the armature to hold the wire firmly in place and prevent it from working out at the centre when the armature is driven at high speed. The armature, when properly wound and wrapped, will appear as shown at A in Fig. 11, and it is then ready to have the ends screwed on. Several sizes of wire may be used to wind the armature, according to the current desired, but for general use it would be well to use No. 30 silk-insulated copper wire.

About four ounces should be enough for this armature, and the ends are to be passed through small holes in the brass end (B); see Fig. 11. One end must be soldered to one commutator, the other end to the other commutator. The end-piece (B) is attached to the iron armature (A) with machine-screws; then C is to be made fast in a similar manner.

When putting the parts together, it would be well to use some shellac on the wooden cylinder and driving-wheel to make them hold to the shaft.

By following the plan in Fig. 10, it will be an easy matter to put the parts together; when they are assembled the complete machine will appear as shown in the drawing (Fig. 12).

The driving-wheel should be of wood five-eighths of an inch thick and six inches in diameter, and held in the frame of wood and metal brackets by a bolt. A short handle can be arranged with which to turn the wheel, and a small leather belt will transmit the power to the small wheel on the armature shaft. As the armature is revolved the lines of force are cut and the current is carried out through the wire attached to the binding-posts on the blocks (G G).

Considerable current may be generated if the armature is driven at higher speed than the hand-wheel will cause it to revolve. This can be accomplished by running the belt over a larger wheel, such as the fly-wheel of a sewing-machine, or connecting it to a large pulley on a water-motor. The latter may be attached to a faucet in the wash-tub if there is pressure enough to do the work.

A Small Dynamo

All dynamos are constructed on the same general principle as that of the uni-direction machine just described; but they differ in their windings, the quantities of metal electrified, the sizes and lengths of wire wound on both armature and field, and in their shape and speeds.

In large dynamos it is impossible to employ steel magnets of the required size. In place of them soft iron cores are used and magnetized by external electric current; or the wiring is done in “series” or “shunt,” so that the fields will be self-exciting once the machine has been properly started.

The principal difference in dynamos is, perhaps, more clearly illustrated by the diagrams shown in Figs. 13, 14, 15, and 16. In Fig. 13 the arrangement of armature and field-magnet is the same as in the uni-direction machine, the field (F) being of magnetized steel, while the armature (A) is of soft iron wound with coils of fine wire, the ends of which are brought out at the commutators (C), through which the current is carried to the brushes (B and B B). If, however, the soft iron cores are used, a separate magnetizing electric current must be passed through the coils of wire wound about the field-pieces, so that they will become temporary magnets--the same as the cores of an electric bell movement, a telegraph-sounder, or the induction-coil core when a current is passed through the primary coil. The armature (A) is then driven at high speed by power, and the current is taken off for use through wires that lead from B and B B.

In all of these figures the armatures rotate, in the space between the large pole-pieces of the field-magnets, in the same direction as the hands of a clock move. In these figure drawings the field-magnets, commutators, and brushes only are shown, the armature being indicated by the circle (A).

Figure 13 represents a dynamo, the field-magnets of which are excited by a separate battery or generator. This is known as a “separately excited” machine, and is employed for various uses. The brushes (B and B B) are connected to the external circuit--that is, with the motor or other apparatus for which current is to be generated. The magnetic field in which the armature rotates will be constant if the exciting current is constant, like the magnetism in the magnet of the uni-direction current machine.

The induced electro-motive force (which depends upon the rate at which the lines of force are cut) will be constant for the given speed at which the armature rotates. This action is the same as that described for the uni-direction current machine.

Figure 14 is the diagram of a “series”-wound dynamo. The field and armature are soft gray iron, and are wound in series--that is, one end of the magnet-winding is made fast to the brush B, the other to the brush B B, and the apparatus to be operated by the current is let in between B B and the magnet, as shown by the indicated electric arc-light in the illustration. The field-magnet coils, the armature, and the external conductors are in series with each other, forming a simple circuit. When the armature is driven at high speed the field-magnets become self-exciting, with the result that current is generated. Its simple course is through B B to commutators on the hub, thence through one winding on the iron armature A, to B, through field F, and back to B B again, operating in its course any pieces of equipment designed for electric impulse, such as motors, or lamps, trolley-cars, trains, or electric machinery.

The third type, shown in Fig. 15, is known as “shunt”-winding. The field-magnet coils and the external resistance are in parallel, or shunt with each other, instead of in series. The brushes are connected with the external circuit, and also with the ends of the field-magnet coils. This is clearly shown in the drawing. The ends of the field-coils are connected with brushes B and B B, and the external circuit wires are connected also with the same brushes, and pass down to such an apparatus as a plating bath, in which the current runs through the electrode, the electrolyte, and the cathode, most of the current generated passing through the external circuit. The field-coils are of fine wire, and when the armature is rotated there will always be a current through the field-magnets, whether the external circuit is complete or not. If a break occurs in the external circuit, a more powerful current will consequently pass through the field-magnets.

In Fig. 16 a “compound”-wound dynamo is shown. It is a combination of the series and the shunt machine. The field-magnet coils are composed of two sizes of wire. There are comparatively four turns of stout wire and many turns of fine wire, the ends of both being connected, as shown in the drawing. The stout wire leads out to lamps which are arranged in series, as shown at the foot of the drawing. The current developed by this dynamo is one of “constant potential,” and is used almost exclusively for incandescent lamps, the “constant” current from the series-wound machine being used for arc-lamps, power, and other commercial purposes.

It will not be necessary to use the first or last systems, nor to experiment with the alternating current, with its phases and cycles. All that a boy wants is a good direct-current machine that will light lamps, run sewing-machines or motors, and furnish the power for long-distance wireless telegraphy and other apparatus requiring considerable current.

To begin with, it would be better to make a small dynamo and study its principles as you progress; then it will be a great deal easier to construct a larger one. It will be necessary to have the iron parts made at a blacksmith-shop, since the various cutting, threading, and tapping operations call for the use of special iron-working tools. Soft iron should be used, and if a piece of cast-iron can be procured for the lugs or magnet ends it will give better service than wrought-iron.

From three-quarter-inch round iron cut two cores, each three inches and a half long, and thread them at both ends, as shown at B B in Fig. 17. From band-iron five-eighths of an inch thick and one inch and a half wide cut a yoke (A), and bore the indicated holes two inches and three-quarters apart, centre to centre. These should be threaded so that the cores (B B) will screw into them. From a bar of iron cut off two blocks one inch and a half by one inch and a half by two inches for the lugs. Now, with a hack-saw and a half-round file, cut out one side of each lug, as shown at C. These lugs are to be bored and threaded at one end, so that they can be screwed on the lower ends of the cores (C C).

For a larger dynamo the yoke should be made six inches long, one inch thick, and two inches and a half wide. The cores should be of one-inch iron pipe. These will be hollow, as shown at B B in Fig. 18. For the ends cast-iron blocks must be made or cast from a pattern two inches and three-quarters square and four inches high, as shown at C. The yoke (A) and the lugs (C) are bored and threaded to receive the one-inch pipe, and when set up this will constitute an iron field-magnet six inches wide, two inches thick, and nine inches high. This, if properly wound, should develop a quarter of a horse-power.

The parts shown in Fig. 17, when screwed together, will give you a field-magnet two by one and a half by five and three-quarter inches high, and will appear as shown in Fig. 19, A being the yoke at the top, B B the cores, C C the lugs, and D a strip of brass screwed fast across the back of the lugs (C C), and in which a hole is bored to act as a bearing for one end of the armature shaft. Between the lugs and the strip (D) fibre washers three-eighths of an inch in thickness are placed to keep the strip away from the lugs. A hole is bored directly through the middle of each lug, from front to rear, and it is threaded at each end so that a machine-screw will fit in it. The brass strip (D) is five-eighths of an inch wide, three-sixteenths of an inch thick, and four inches long. Copper or German-silver may be used in place of brass, but iron or steel must not be employed, since these metals are susceptible to magnetism. Two holes should be made in the bottom of each lug, and threaded, so that machine-screws may be passed through a wooden base and into them in order to hold the dynamo on the base.

Figure 20 is an end view of the field-magnets showing the yoke at A, the core at B, the lug at C, and the bearing and binding-strip of yellow metal at D. Two blocks of hard-wood, an inch square and one inch and a half long, are cut and provided with holes, so that they can be fastened to the lugs C C with long, slim machine-screws, as shown at E E in Fig. 21. This is a view looking down on the magnets, blocks, and straps. These blocks are to support the brushes and terminals, and should be linked across the face with a brass strap G, so that the other end of the armature shaft may be supported. Care must be taken, when setting straps D and G, to have them line. The holes, too, must be centred, since the armature must revolve accurately within the field-lugs (C C) without touching them, and there is but one-sixteenth of an inch space between them.

From hard-wood half an inch in thickness cut a base, six by seven inches, and two strips an inch wide and five inches long. With glue and screws driven up from the underside of the strips fasten them to the base, as shown at Fig. 22. Then make the field-magnets fast to the base with long machine-screws, using washers under the heads at the underside of the base-board. The mounting should then appear as shown in Fig. 28.

From steel, half an inch in diameter, cut a shaft five inches long. Have it turned down smaller at one end for three-eighths of an inch, and at the other end for a distance of one inch and a half, as shown at Fig. 23. This is for the armature, and it should fit between D and G in Fig. 21, and should revolve easily in the holes cut to receive it in both straps, with not more than one-eighth of an inch play forward or backward. The long, projecting end should be at the rear, and should extend beyond strip D for three-quarters of an inch, so that the driving-pulley can be made fast to it.

The armature is made up of segments or laminations of soft iron and insulated copper wire. The laminated armature works much better than does the solid metal ring or lug, and a pattern may be made from a piece of tin from which all the sections can be cut. With a compass, strike a two-inch circle on a clear piece of tin; then mark it off, as shown at Fig. 24, and cut it out with shears. The hole at the centre of the pattern need not be bored, but a small pinhole should be made so that a centre-punch can be used to indicate the middle of each plate for subsequent perforation. Ordinary soft band iron may be employed for this purpose, and the sections should not be more than one-sixteenth of an inch in thickness.

It will take some time to cut out the required number of pieces for this small armature. When they are all ready they should be slipped over the shaft, and if they have been properly matched and cut, they should appear as a solid body, one inch and a half long.

Arrange these laminations on the armature shaft so that when the shaft is in position the mass of iron will be within the lugs of the field-magnets. The holes through the iron plate should be so snug as to call for some driving to put them in place. Each disk of iron should be given a coat of shellac to insulate it, and between each piece there should be a thin cardboard or stout paper separator to keep the disks apart. These paper washers should be dipped in hot paraffine, or thick shellac may be used to obtain a good sticking effect and so solidify the laminations into a compact mass. When this operation is completed the armature core should appear as shown in Fig. 25.

From maple, or other hard-wood with a close grain, make a cylinder three-quarters of an inch long and one inch in diameter to fit the shaft. Over this drive a piece of copper or brass tubing, and at four equal distances, near the rear or inner edge, make holes and drive small, round-headed screws into the wood. Then, with a hack-saw, cut the tube into four equal parts between the screws. This is the commutator. In order to hold the quarter circular plates fast to the cylinder, remove one screw at a time, and place thick shellac on the cylinder. Then press the plate firmly into place and reset the screw. Repeat this with the other three, and the armature will be ready for the winding.

The voltage and amperage of a dynamo is reckoned by its windings, the size of wire, the number of turns, and the direction. This is a matter of figuring, and need not now concern the young electrician, since it is a technical and theoretical subject that may be studied later on in more advanced text-books.

For this dynamo use No. 22 cotton-insulated copper wire for the armature, and No. 16 double cotton-insulated copper wire for the field. The armature, when properly wound and ready for assembling with the brushes and wiring, will appear as shown in Fig. 26.

A small driving-wheel two inches in diameter and half an inch thick must now be turned from brass and provided with a V-shaped groove on its face. The hub, at one side, is fitted with a set-screw, so that it can be bound tightly on the shaft. This pulley is made fast to the shaft at the rear of the dynamo, and on the opposite end to where the commutator hub is attached.

A diagram of the wiring is shown in Fig. 29, and in Fig. 30 the mode of attaching the ends of the coil wires to the commutators is indicated. Two complete coils of wire must be made about each channel of the armature, as illustrated on the drum of Fig. 30. These are separated by a strip of cardboard dipped in paraffine and placed at the centre of a channel while the winding is going on. In some armatures the coils are laid one over the other; but with this construction, and in the case of a short-circuit, a broken wire, or a burn-out, it is impossible to reach the under coil without removing the good one.

Begin by attaching one end of the fine insulated wire to commutator No. 1; then half fill the channel, winding the wire about the armature, as indicated in Fig. 30. When the required number of turns has been made, carry the end around the screw in commutator No. 2, baring the wire to insure perfect contact when caught under the screw-head. From No. 2 carry the wire around through the channel at right angles to the first one, and after half filling it bring the end out to commutator No. 3. Carry the wire in again and fill up the other half of the first channel, and bring the end out to commutator No. 4. Fill up the remaining half of the second channel; then attach the final end to commutator No. 1, and the armature winding will be complete without having once broken the strand of wire.

To keep the coils of wire in place, and to prevent them from flying out, under the centrifugal force of high speed, it would be well to bind the middle of the armature with wires or adhesive tape.

After driving down the small screws over the leading-in and leading-out wires the armature will be ready to mount in the bearings. As the blocks that support the brushes and binding-posts partly close the opening to the cavity at the front, the armature will have to be inserted from the back into the strip (G) in Fig. 21. Then the back strip (D) is screwed in place. The armature, when properly mounted, should revolve freely and easily within the field-lugs without friction, and the lugs must by no means touch the armature. From thin spring-copper brushes may be cut and mounted on the block under the binding-posts, so that one will rest on top of the commutators while the other presses up against the underside. The wiring is then to be placed on the field-magnets. This is carried out as described for the electric magnets on pages 54-58 of chapter iv., each core receiving five or seven layers, or as much as it will hold without overlapping the lug or yoke. The ends of the wires are connected as shown at Fig. 14 or Fig. 15, the ends being carried down through the base and up again in the right location to meet the foot of a binding-post. The complete dynamo will appear as shown in Fig. 28.

Before the dynamo is started for the first time it would be well to run a strong current through the field coils. The residual magnetism retained by the cores and iron parts will then be ready for the next impulse when the dynamo is started again. Larger dynamos may be made of this type. With an armature, the core of which is four inches in diameter and six inches long, having eight instead of four channels, and placed within a field of proportionate size, the dynamo will develop one horse-power.

A Split-ring Dynamo

Another type of dynamo is shown in Fig. 31. This is composed of a wrought or cast iron split-ring wound for the field, an armature made up of laminations, and the necessary brushes, posts, commutators, and wire.

Have a blacksmith shape an open ring of iron, in the form of a C, three-eighths of an inch thick and four inches wide. The opening should be three inches wide, as shown in Fig. 32. This ring should measure five inches on its outside diameter, and the ends are to be bored and threaded to receive machine-screws. Two lugs are to be made from wrought-iron to fit on these ends. These should be four inches long, an inch and a half high, and three-quarters of an inch thick at top and bottom. They should be hollowed out at the middle, so that an armature two inches in diameter will have one-eighth of an inch play all around when arranged to revolve within them. Holes are made through the lugs to receive machine-screws, which are driven into the holes in the ends of the iron (C). Wrought-iron L pieces are made one inch and a half high and an inch across the bottom, and with machine-screws they are made fast to the backs of the lugs to act as feet on which the field-magnet may rest, as shown in Fig. 33. Across the back of the lugs, and set away from them by fibre washers, a strap of brass is made fast. This measures three-quarters of an inch wide and a quarter of an inch thick, and at the middle of it a three-eighth-inch hole is bored to receive the rear end of the armature shaft. This is shown in Fig. 34, which is a front view of the field, or C, iron, the lugs (L L) and feet (F F), the armature bearing (S), and the base (B), of three-quarter-inch hard-wood. The field-magnet is bolted to the base with lag-screws, so that it will be held securely in place.

The laminations for the armature core are two inches in diameter, and are cut from soft iron one-sixteenth of an inch thick. They have eight channels, as shown in Fig. 35, and the tubing on the commutator hub is divided into four parts so that the terminals from each coil can be brought to a commutator, as described for Fig. 30. In the eight-channel armature, however, there is but one coil of wire in each channel.

In Fig. 36 a plan of the armature is shown, S representing the shaft, B B the bearings, L the laminations, C the commutators and hub, P the driving-pulley, and N N the nuts that hold the laminations together and lock them to the shaft. The shaft is half an inch in diameter, the laminations four inches thick, and the commutator barrel one inch in diameter and three-quarters of an inch long. The shaft is turned down from the middle to where P and C are attached; then at the front end it is made smaller, where it passes through the front bearing.

With the detailed description already given for the construction of the small dynamo, it should be an easy matter to carry out the work on this one, and a quarter horse-power generator should be the result. The field-magnet is wound with five or seven layers of No. 16 double cotton-insulated wire, and the armature with No. 22 silk or cotton-covered wire. The connections may be made for either the series or the shunt windings shown in Figs. 14 and 15. Another type of field is shown in Fig. 37, where two plates of iron are screwed to one core, and the lugs are, in turn, made fast to the inner sides of the plates within which the armature revolves. The “Manchester” type is shown in Fig. 38, where two cores, constructed by a top and bottom yoke, are excited by the coils, and the lugs are arranged between the cores, so that the armature revolves within them.

A Small Motor

The shapes, types, powers, and forms of motors are as varied and different as those of dynamos, each inventor designing a different type and claiming superiority. The one common principle, however, is the same--that of an armature revolving within a field, and lines of force cutting lines of force. A motor is the reverse of a dynamo. Instead of generating current to develop power or light, a current must be run through a motor to obtain power.

Motors are divided into two classes: the D C, or direct current, and the A C, or alternating current. For the amateur the direct-current motor will meet every requirement, and since the battery, or dynamo current, that may be available to run a motor, is in all probability a direct one, it will be necessary to construct a motor that is adapted to this source of power and for the present avoid the complications of the alternating current both in generation and in use.

The direct-current motor is an electrical machine driven by direct current, the latter being generated in any desired way. This current is forced through the machine by electro-motive force, or voltage; the higher the pressure, or voltage, the more efficient the machine. Be careful lest too much current (amperage) is allowed to flow, for the heat developed thereby will burn out the wiring.

Motors are so constructed that when a current is passed through the field and armature coils the armature is rotated. The speed of the armature is regulated by the amount of amperage and voltage that passes through the series of magnets, and this rotating power is called the torque.

Torque is a twisting or turning force, and when a pulley is made fast to the armature shaft, and belted to connect with machinery, this torque, or force, is employed for work.

The speed of an armature when at full work is usually from twelve hundred to two thousand revolutions a minute. As few machines are designed to work at that velocity, a system of speeding down with back gears, or counter-shafts and pulleys, is employed. The motor itself cannot be slowed down without losing power. The efficiency of motors is due to the centrifugal motion of the mass of iron and wire in the armature and the momentum it develops when spurred on by the magnetism of the field-magnets acting upon certain electrified sections of the armature. The armature of a working motor is usually of such high resistance that the current employed to run it would heat and burn out the wires if the full force of the current was permitted to flow through it for any length of time. As the armature rotates it has counter electro-motive force impressed upon it. This acts like resistance, and reduces the current passing through. The higher the speed the less current it takes, so that after a motor has attained its highest, or normal speed, it is using less than half the current required to start it.

Reduction of current in the armature reduces torque, so that the turning force of the armature is reduced as its speed of rotation increases. On the other hand, the momentum, or “throw,” produces power at high speed, together with an actual saving of current. An armature revolving at sixteen hundred revolutions, and giving half a horse-power on a current of five amperes, is more economical than one making three to five hundred revolutions, and giving half a horse-power on a current of fifteen to twenty amperes. Thus, a slowly turning armature takes more current and exerts higher torque than a rapidly rotating one.

To protect the fine wire on the armature from burning, in high-voltage machines a starting-box, or rheostat, is employed. The motor begins working on a reduced current, and as it picks up speed more current is let in, and so on until the full force of the current is flowing through the motor. It is then turning fast enough to protect itself through the counter electro-motive force. This can be understood better after some practical experience has been had in the construction and running of motors. Of the various forms of motors but three will be illustrated and described; but the boy with ideas can readily design and construct other types as he comes to need them.

The Flat-bed Motor

The simplest of all motors is the flat-bed type, illustrated in Fig. 39. This is composed of a magnet on a shaft revolving before a fixed magnet attached to the upright board of the base. Where space is no object, this motor will develop considerable power from a number of dry-cells or a storage-battery. Now, in the section relating to dynamos, four different systems of wiring were shown. In motors of the direct-current type but one system will be described--that of the series-winding, illustrated in Fig. 40. The current, entering at A, passes to the brush (B), thence through the commutator (C) and the armature coils. It runs on through the brush (B B), the field-coils (F), and out at D. This is the same course the current takes in the series-wound dynamo illustrated in Fig. 14, page 241, and with such a dynamo current could be generated to run any series-wound, direct-current motor.

From hard-wood half an inch thick cut a base-piece six inches and a half long by three inches and a half wide. Arrange this base on cross-strips three-quarters of an inch wide and half an inch thick, making the union with glue and screws driven up from the underside. To one end of this base attach an upright or back two inches and three-quarters high, and allow the lower edge to extend down to the bottom of the cross-strip, as shown at the left of Fig. 39. Make this fast to the end of the base and side of the cross-strip with glue and screws; then give the wood a coat of stain and shellac to properly finish it.

Now have a blacksmith make two [U] pieces of soft iron for the field and armature cores, as shown in Fig. 41. These are of quarter-inch iron one inch and a half in width. They are one inch and three-quarters across and the same in length. One of them should have a half-inch hole bored in the end (at the middle), and above and below it smaller holes for round-headed screws to pass through. By means of these screws the [U] is held to the wooden back. The other [U] is to have a three-eighth-inch hole bored in it so that it will fit on the armature shaft. Wind the [U] irons with six layers of No. 20 cotton-insulated wire, having first covered the bare iron with several wraps of paper. Use thick shellac freely after each layer is on, so that the turns of wire will be well insulated and bound to each other. Follow the wiring diagram shown in Fig. 40 when winding these cores, and when the field is ready, make it fast to the back with three-quarter-inch round-headed brass screws.

Directly in the middle of the hole through the field iron bore a quarter-inch hole for the armature shaft to pass through; then make an [L] piece, of brass, two inches high, three-quarters of an inch wide, and with the foot an inch long, as shown at Fig. 42. Two holes are made in the foot through which screws will pass into the base, and near the top a quarter-inch hole is to be bored, the centre of which is to line with that through the back, at the middle of the field core. The shaft is made from steel three-eighths of an inch in diameter and six inches and a half long. One inch from one end the shaft should be turned down to a quarter of an inch in diameter, and one inch and a quarter from the other end it must be reduced to a similar size. The short end mounts in the back and the long one receives the pulley, after the latter passes through the [L] bearing. A piece of three-eighth-inch brass tubing an inch long is slipped over the shaft two inches from the pulley end and secured with a flush set-screw. This tubing is then threaded and provided with two nuts, one at either end, so that when the armature [U] is slipped on the collar the nuts can be tightened and made to hold the magnet securely on the shaft. This shaft is clearly shown in the sectional drawings Fig. 43.

At the left side the shaft (S) passes into the wood back through the quarter-inch hole. At the outside a brass plate with a quarter-inch hole is screwed fast and acts as a bearing. The shaft does not touch the field-magnet (F M), because the hole is large enough for the quarter-inch shaft to clear it. A fibre washer (F W) is placed on the shaft before it is slipped through the back. This prevents the shaft from playing too much, and deadens any sound of “jumping” while rotating.

At the middle the shaft (S) passes through the brass collar on which the threads are cut. A M represents the armature magnet, and W W the washers and nuts employed to bind it in place. At the right, S again represents the shaft, B the bearing, C the commutator hub, and P the pulley, while R is the small block under the hub to which the brushes and binding-posts are attached.

From the descriptions already given of dynamos, and with these figure drawings as a guide, it should be an easy matter to assemble this motor.

The ends of the field and armature magnets should be separated an eighth of an inch. The hub for the commutators is three-quarters of an inch long and three-quarters of an inch in diameter. The commutators are made as described for the uni-direction current machine, care being taken to keep the holding screws from touching the shaft. A three-quarter-inch cube of wood is mounted on the base, under the commutator hub, and to this the brushes and binding-posts are made fast, as shown in Fig. 39. Unless the armature happens to be in a certain position this motor is not self-starting, but a twist on the pulley, as the current is turned on, will give it the necessary start. Its speed will then depend on the amount of current forced through the coils.

Another Simple Motor

Another type of motor is shown in Fig. 44, where one field-winding magnetizes both the core and the lugs. The frame of this motor is made up of two plates of soft iron a quarter of an inch thick, six inches long, and two inches and a half wide. Each plate is bent at one end so as to form a foot three-quarters of an inch long, and a half-inch hole is drilled one inch and a quarter up from the bottom, at the middle of each plate. Through this hole pass the machine-screws which hold the iron core in place between the side-plates. The core is made of three-quarter-inch round iron two inches and three-quarters long, and drilled and threaded at each end to receive the binding machine-screws.

Two lugs are cut from iron, and hollowed at one side so that an armature two inches in diameter will rotate within them when made fast to the side-plates. The lugs are two inches and a half long, an inch wide, and two inches and a half high.

From iron five-eighths of an inch wide and one-eighth of an inch thick make two side-strips with [L] ends. These are four inches long, and are provided with two holes so that the machine-screws which hold the lugs to the inside plates will also hold these strips in place, at the outside, as shown in Fig. 45. At the rear these strips extend half an inch beyond the frame. Across the back a brass strip of the same size as the iron strips is arranged. It is held at the ends by screws, or small bolts, made fast to the [L] ends of the side-strips. Directly in the middle of the back-strip a hole is made for the armature shaft, and beyond it the pulley is keyed or screwed fast to the shaft.

At the front a similar strip is made and attached. This latter has a small hole in the middle of it to serve as a bearing for the forward end of the shaft. Across the top of the motor a brass strip or band is made fast with machine-screws; and at the angles formed by the front ends of the side-strips and the front cross-strips hard-wood blocks are attached. To these the brushes and binding-posts are made fast, so that one brush at the top of the left-hand block rests on the top of the commutator. The one at the underside of the opposite block must rest on the underside of the commutator.

The armature core is made up of laminations as described for the dynamo armatures. In a really efficient motor the armature should have eight or more channels.

The other parts of the motor may be assembled and wired as described on the preceding pages. The armature should be wound with No. 20 or 22 insulated copper wire, and the field with No. 16 or 18. For high voltage, however, the armature should be wound with finer wire and a rheostat used to start it.

A Third Type of Motor

The third type is but a duplicate of the series-wound dynamo, the general plan of which is shown in Fig. 40.

This motor can be made any size, but as its dimensions are increased the weight of the field-magnets and armature must be proportionately enlarged. For an efficient and powerful motor, the field should stand ten inches high and six inches broad. The iron cores are five inches long and one inch and a half in diameter. These should be made by a blacksmith and bolted together. The armature is three inches in diameter and four inches long, and should develop two-thirds of a horse-power when sufficient current is running through the coils to drive it at sixteen hundred revolutions.

The wiring is carried out as shown in Fig. 40, and the armature hung and wound as suggested for the dynamo shown in Fig. 28, page 246.