CHAPTER IV SMALL POWER MOTORS

In order for a motor to develop any appreciable amount of power it must be much larger than any of those which have been described in these pages so far, and must be constructed in a most painstaking manner. It will be necessary to use a great deal more iron in the field and armature and also to make the space between them as small as possible. A motor having a small separation between the field poles and the armature will develop more power than one having a greater separation.

The most efficient types of small power motors have laminated field and armature frames, that is, they are built up of a large number of thin metal punchings. The amateur experimenter who has limited facilities for carrying out his work would find it difficult to make parts of this sort to good advantage and so the motors described here have been designed with cast iron armatures and field frames.

Those who wish to secure a set of castings from their own patterns can possibly save part of the expense if they do not consider the extra labor of first making the patterns.

Two types of motors are described, one vertical and the other horizontal. Both are intended to operate on a battery current of 3-6 volts and if carefully built will deliver a surprising amount of power.

A VERTICAL POWER MOTOR

*The Field Frame* is shown in detail in Figure 60. The exact shape and dimensions are best understood by a careful examination of the drawing.

The pattern for the field may be made of the same shape and practically the same size as indicated for the finished casting because the "rapping" or jarring which the pattern will receive in the foundry in order to free it from the sand mould will enlarge the mould sufficiently in a casting of small size to make up for any shrinkage which takes place upon the cooling of the iron.

The only exception to this is in the tunnel where the armature rotates. This should measure one and three-quarter inches in diameter when finished and should be slightly smaller in the rough casting so that there is enough material to allow for truing and bringing to equal size.

*The Armature* may be of two types, three pole or six pole. The three-pole armature is the simpler, but the six-pole type is the smoother running and gives the steadier power. The details and dimensions are shown in Figures 61 and 62. One of the armatures should be selected and a pattern built.

After the patterns are finished they should be given a coat of shellac and carefully rubbed with fine sandpaper so that they are perfectly smooth. Otherwise the sand is liable to stick in moulding and produce an imperfect casting.

Castings may be obtained from any foundry which is equipped to make grey iron castings. They should be as soft as possible. The cost will depend upon the quantity which are ordered. If only one set is required, the charge will probably be based upon the time required for making the moulds but if several sets are ordered the price may be based upon the weight.

After the castings have been received from the foundry, the first operation is to carefully remove all rough spots and burrs with a file.

Those who have a lathe or large drill press can easily finish the tunnel by turning or reaming. In the absence of these facilities, hand filing can be made to suffice, if carefully done.

The holes marked "BBBB" should be drilled with a No. 29 drill and tapped 8-32. These holes must be very carefully located because they serve to fasten the bearings. Each hole should be exactly opposite the other, two and five-sixteenth inches apart and on a line passing exactly through the centre of the tunnel.

The holes, "PP" and "SS", are three-sixteenths of an inch in diameter. The former support the Binding Posts and the latter pass the screws which fasten the motor to the wooden base.

The armature, in the case of either the six or three pole type, has a three-sixteenth inch hole drilled along the axis to accommodate a steel shaft of the same diameter.

The armature casting should be accurately turned to a diameter of one and twenty-three thirty-seconds of an inch so that it will revolve in the tunnel without touching the field but still be very close to it.

Two holes bored through one of the pole pieces at right angles to the shaft with a No. 37 drill and threaded with a 6-32 tap will allow the armature to be clamped tightly to the shaft with two headless set screws.

*The Field Winding* consists of No. 16 double cotton insulated wire. Before the winding is put on, the core should be insulated with one or two layers of shellaced paper. Two circular pieces of shellaced paper should be placed against the flanges at the end of the core, so that the winding space is thoroughly insulated and there is no liability of the wire touching the iron at any point. The wire should be wound in smooth even layers. The winding space is completely filled. The outside layer may be finished by a coat of shellac.

The three-pole armature is much easier to wind than the six pole type. The wire used should be No. 24 B. & S. Gauge, double cotton covered. Before the wire is wound on, cover the winding space with shellaced paper so that the wire will not touch the iron at any point. Each coil should be wound in the same direction as the others starting at the same end and as close as possible to the inside.

The outside end of each coil should be connected to the inside of the next coil as shown in Figure 63. The diagram indicates only one layer of wire in each coil for the sake of clearness.

The winding upon the armature shown in Figure 64 is divided into six coils. Each coil consists of as many turns as possible of No. 24 B. & S. Gauge, cotton covered wire to fill the space completely and all coils are wound in the same direction. The illustrations show the various stages of the bindings with the two, four and six coils in place. The winding spaces on the armature should be carefully insulated with shellaced paper before the coils are placed in position.

After the winding has been finished the next step is to make the shaft and commutator. The shaft is a piece of three-sixteenths steel, three and one-quarter inches long. The shaft passes through the centre of the armature and is locked-in position by the two set screws.

*The Commutator* is probably one of the most difficult parts of the motor to make. It consists of three circular brass sections insulated from one another on a fibre bushing.

The fibre bushing is a hollow cylinder, five-sixteenths of an inch in diameter and seventeen thirty-seconds of an inch long. The bushing should force tightly on the shaft. The segments are make by turning a piece of three-quarter inch brass rod in a lathe until it is one-half an inch in diameter for a distance of about seven-sixteenths of an inch. A five-sixteenths inch hole should be bored through the center so that it will fit tightly upon the fibre bushing.

Then cut the brass off one-half inch from the end so that it leaves a flange at one end, three-quarters of an inch in diameter. Saw it lengthwise into three equal parts and mount it upon the fibre bushing with a small strip of mica between each two sections to fill in the space made by the saw cuts. The sections are held together by a fibre ring, three quarters of an inch in diameter outside and one-half an inch in diameter inside. The ring should fit very tightly over the commutator and be forced down flush against the shoulder. After the ring is in position, file any mica which may project out of the slots down even with the surface of the segments and force the commutator onto the shaft with the shoulder against the armature. The commutator must fit very tightly so that there is not any possibility of moving it after it is in position.

The sections should bear a certain relative position to the armature windings. The diagrams in Figures 63 and 64 show the proper position for the three and six pole armature respectively.

The coils are connected to the commutator by soldering the terminals to the shoulder on each segment. This work should be very carefully done so as to insure a neat job and connection of the proper terminal to the proper section.

CONNECTIONS FOR THE THREE POLE ARMATURE

The inside terminal of coil A and the outside terminal of coil B should be connected to Section 1, the inside terminal of coil B and the outside terminal of coil C should be connected to Section 3, the inside terminal of coil C, and the outside terminal of coil A should be connected to Section 2.

CONNECTIONS FOR THE SIX-POLE ARMATURE

The inside terminal of coil A and the inside terminal of coil B should be connected to section 2, the outside terminal of coil C and the outside terminal of coil D should be connected to Section 3, the outside terminal of coil E and the outside terminal of coil F should be connected to Section 1, the outside terminal of coil A and the inside terminal of coil C should be connected to Section 1, the inside terminal of coil D and the inside terminal of coil E should be connected to Section 2, the inside terminal of coil F and the outside terminal of coil D should be connected to Section 3.

The wires leading from the coils to the commutator should be just as short as it is possible to make them and after being soldered should be bound down tightly with linen thread or string.

The bearings are both cast from brass. The details are shown in Figure 66 It will be necessary to make up wooden patterns and send them to a foundry. The location of the holes can be ascertained from the illustration.

Each of the brushes consists of a piece of strip copper, one-quarter of an inch wide and one and three-eighths inches long mounted in a brush holder made of one-quarter inch brass rod. The brush holder is one inch long and is turned down to a diameter of one-eighth of an inch at one end for a distance of nine-sixteenths of an inch and then threaded with a 6-32 die. The opposite end is slotted to receive the brush. The threaded portion of the holder is slipped through the holes, "B and B", in the bearing and prevented from making contact with the latter by a fibre bushing.

A fibre washer should also be slipped over the holder on each side of the bearing. Two hexagonal nuts are placed on the threaded stem. One serves to clamp the holder in position and the other to hold the wire used to make connection with the brush. The right hand brush should bear against the under side of the commutator and the left hand brush against the upper side.

After the armature has been assembled in the bearings and mounted on the field frame it should revolve freely without friction and without any possibility of its striking against the field poles.

The binding posts are mounted in the holes, "PP" in the lower parts of the field frame. They are insulated by two fibre or paper busings. The left hand binding post is connected to the inside terminal of the field winding. The outside terminal of the field winding is connected to the left hand binding post. The right hand binding post is connected to the right hand brush.

The base of the motor is a wooden block of suitable size.

The motor is of the series type because all the current flows through both the field and armature. A current of 2 to 6 volts will operate the motor. The pulley or gear required in order that the motor may be used as a source of power will depend upon the work for which the motor is to be employed. A small grooved pulley such as as that shown in Figure 63 may be fastened to the shaft with a set screw and will prove most useful for general purposes.

A HORIZONTAL POWER MOTOR.

The horizontal motor does not differ very materially from the vertical one just described.

The field frame is, however, made in two pieces, and the bearings are cast directly on the frame. The details and dimensions are given in Figures 68, 69 and 70.

The field winding consists of six layers of No. 18 B. & S. Gauge double cotton-covered wire wound on a spool or bobbin.

The core of the bobbin consists of a piece of five-eighths round steel or iron rod, two and seven-sixteenths inches long. Two circular fibre heads, one-eighth of an inch thick and one and one-half inches in diameter are mounted on the core one-half an inch from one end and fifteen-sixteenths of an inch apart. The ends of the core are set in the holes, "C, C," in the two parts of the field frame and held in position by two set screws threading into the holes "S" and "S."

Either the three-pole or the six-pole armature may be used. The commutator and brushes are identical with those used in the vertical type of motor.

The shaft is three-sixteenths of an inch in diameter and four inches long. The brushes are mounted upon a brush arm which is shown in detail in Figure 63. This is made of three-sixteenths inch sheet brass. The brushes must be insulated from the arm by fibre washers and bushings in the same manner as they were from the bearings on the vertical motor.

The holes in the bearings on the field frame are drilled out three-eighths of an inch in diameter and then brushed with a piece of three-eighths inch brass rod five-sixteenths of an inch in diameter having a three-sixteenths inch hole through the center.

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