CHAPTER XXII

ARMATURE CONSTRUCTION

The armature of a dynamo has been defined as: _a collection of coils of wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field._

From the mechanical point of view the armature may be said to be made up of the following parts:

1. Shaft; 2. Core; 3. Spider (in large machines); 4. Winding; 5. Commutator. (broadly speaking).

Of the two types of armature, ring and drum, the latter is almost universally used, hence the examples of construction which follow will be confined chiefly to this type.

=Shaft.=--A typical armature shaft is shown in fig. 349. It is made of steel and, except in the smaller machines, is thicker in the middle than at the ends for stiffness to withstand the strong magnetic side pull on the core when the latter is slightly, nearer one pole piece than the other.

=Ques. What is the object of providing shoulders on the shaft as in fig. 349?=

Ans. They serve to keep the armature in the proper position with respect to the bearings.

=Ques. How is the shaft proportioned?=

Ans. If it be proportioned to secure the proper stiffness, it will be found of ample size to resist the twisting strain.

The shaft is subject also to bending by the weight of the armature, by the magnetic drag on its core, and in belt driven machines, by the lateral drag of the pulley. When running, it is also subjected to bending stresses if the armature be not properly balanced. If the bearings do not give, it is evident that all such actions tend to bend the shaft at definite points.

=Core.=--In the small and medium size dynamos, the core is attached direct to the shaft. There are two kinds of core:

1. Smooth; 2. Slotted.

=Ques. What may be said of the smooth type of core?=

Ans. It has become obsolete, except in special cases, as for machines used for electrolytic work where a large current at low voltage is required.

=Ques. What is necessary with a smooth core?=

Ans. Driving horns as later described.

=Ques. What is a slotted core?=

Ans. One having a series of parallel slots, similar to the spaces between the teeth of a gear wheel, and in which the inductors are laid.

=Ques. What provision is made to avoid eddy current in cores?=

Ans. They are laminated.

=Ques. Describe this method of construction.=

Ans. The core is made of stampings of thin wrought iron or mild steel. The numerous discs stamped from the sheet metal are threaded on the shaft as in fig. 350, forming a practically solid metal mass.

=Ques. How thick are the discs?=

Ans. The thickness ranges from .014 inch to .025 inch, corresponding to 27 and 22 B and S gauge respectively, 27 gauge being mostly used.

=Ques. How are the discs held in place?=

Ans. By two end plates pressed together either by large nuts screwed directly on the shaft as in fig. 351, or by bolts passing through the core from end to end, as in fig. 352, holes being punched in the discs for the purpose.

=Ques. What precaution is taken with respect to the core bolts?=

Ans. They are insulated from the core by tubes and washers of mica or other insulating material.

=Ques. What is the construction of the core end plates, and why?=

Ans. The rims are beveled quite thin to avoid eddy currents.

=Ques. How is the core connected to the shaft?=

Ans. Since the core has the full torque exerted upon it by the drag of the inductors, it must be firmly connected to the shaft by means of a key, as shown, so that it may be positively driven.

Core discs are stamped in one piece up to about 30 inches in diameter, and for larger sizes they are built up from sections as later described.

Figs. 353 and 354 show two forms of disc stamped in one piece. The first illustrates a solid disc, and the second a _ventilated_ disc in which more or less of the metal is cut away near the center, thus providing passages for the circulation of air which carries away some of the heat generated in the armature.

=Insulation of Core Discs.=--When the discs are stamped from very thin metal, the mere existence of a film of oxide is sufficient insulation. It is usual, however, to apply a quick drying varnish that will give a hard tough coat and not soften with heat or become brittle and crumble under vibration. The varnish may be applied either by dipping or with a japanning machine; it must be very thin, and the solvent employed should be a very volatile spirit.

=Forms of Armature Teeth.=--The teeth stamped in the core discs are made in various shapes, depending largely on the method of securing the inductors in the slots against electromagnetic drag and centrifugal force. The teeth may be cut with their sides:

1. Inclined; 2. Projecting; 3. Notched.

=Ques. What may be said of teeth with inclined sides?=

Ans. A tooth of this type is shown in fig. 356, being slightly narrower at the root than at the top, the resulting slot having parallel sides.

=Ques. What are the features of the projecting type of tooth?=

Ans. The projecting type is shown in figs. 357 and 358 in which the tops project; this gives a larger core area around the circumference of the armature which reduces the reluctance of the air gap, and provides projecting surfaces for retaining the inductors in the slots by the insertion of wedges.

=Ques. What is the object of cutting notches in teeth?=

Ans. They are provided for the insertion of retaining wedges, as in fig. 361; this results in less area at the top of the teeth.

=Ques. How should teeth be proportioned to secure most efficient operation?=

Ans. The width of the tooth should be about equal to the width of the slot minus twice the thickness of the slot insulation; that is, the cross sectional area of the teeth should be equal to that of the slots.

=Advantages and Defects of Slotted Armatures.=--The slotted armature, sometimes called the Pacinotti armature, after its inventor, has the following advantages over the smooth type:

1. The inductors are held more firmly in place to resist stresses due to electromagnetic drag and centrifugal force;

2. The inductors are protected by the teeth against mechanical injury;

3. Less reluctance of the air gap;

4. The intermittent induction due to the presence of the teeth prevents the formation of eddy currents.

5. When the teeth are saturated they oppose the shifting of the lines due to armature reaction.

The disadvantages of slotted armatures compared with the smooth type are:

1. Greater hysteresis loss, caused by denser flux in the teeth;

2. Generation of eddy currents in the polar faces when the latter are not of laminated construction;

3. Greater self-induction in the armature coils;

4. Construction more expensive;

5. Leakage of magnetic lines through core, exterior to winding.

The generation of eddy currents in the polar faces may be overcome by making the air gap at least 50 per cent. of the distance between the teeth, so that the magnetic lines can spread from the corners of the teeth, and become nearly uniformly distributed over the polar faces. Magnetic leakage through the core may be reduced by making the amount of metal above the inductors very small.

=Slotted Cores; Built Up Construction.=--In the case of large dynamos, the core discs are built up in order to reduce the cost of construction; the following parts are used:

1. Spider; 2. Core rings split into sections.

=Ques. What is the approved method of core construction in large armatures?=

Ans. The core should be of the built up construction to avoid waste of material in the stampings.

=Ques. Describe the construction of a built up core.=

Ans. Ring sections stamped, from sheet metal are fastened to a central support or _spider_, which consists of an iron hub with radiating spokes and a rim with provision for fastening the rings. The rim of the spider is provided with dovetail notches into which fit similarly shaped internal projections on the core segments. These features are shown in figs. 362 to 364. Each layer of core sections is placed on the spider so as to break joints and the core thus formed is firmly held in place by end clamps as shown. The manner of fastening the rings to the spider is an important point, for it must be done without reducing the effective cross section of the core in order not to choke the magnetic flux.

In order to secure a better fit and reduce the machine work, the spider hub in large machines is sometimes cored with enlarged section between the outer bearing surfaces, and it is not unusual to find these surfaces turned to two different sizes as in fig. 365, to admit of easier erecting.

To avoid any trouble that may arise by unequal expansion, the rim of the spider is not made continuous, but in several sections as shown in fig. 364. The rim here consists of four sections each of which has two dovetail notches. By thus dividing the rim into sections, its weight is somewhat reduced and the ventilating spaces between the sections increased.

=Ventilation.=--In the operation of a dynamo more or less heat is generated, depending on the load; hence it is desirable that provision be made to carry off some of this heat to prevent excessive rise of temperature.

=Ques. Why do armature cores heat?=

Ans. They heat from these causes: eddy currents, hysteresis, and heat generated in the inductors.

=Ques. How is adequate ventilation secured?=

Ans. The spider is constructed with as much open space as possible through which air currents may circulate. The core is divided into several sections with intervening air spaces D as shown in fig. 363, the discs being kept apart at these points by distance pieces. These openings between the discs are called _ventilating ducts_; they are usually spaced from 2 to 4 inches apart.

=Ques. What other provision is sometimes made to secure ventilation?=

Ans. In some machines a forced circulation of air is secured by means of a fan attached to one end of the armature as shown in fig. 366.

=Insulation of Core.=--Before the winding is assembled on the core, the latter should be thoroughly insulated. Japan or enamel insulation is not sufficient because it is liable to have bubbles or minute holes in it, or be pierced by particles of metal or by the rough edges of the core discs. Two or more layers of strong paper, fibre, canvas or mica, should be applied to the core before placing the inductors in position. The ends of the core should be insulated with thicker material, since the strain upon it is greater, especially at the edges.

=Armature Windings.=--The subject of windings has been fully treated from the theoretical point of view in chapter XVIII. It remains then to explain the different methods employed in the shop and the mechanical devices used to construct the scheme of winding adopted.

=Ques. What is the construction of the inductors?=

Ans. They are made of copper; the ordinary form consists of simple copper wire, insulated with a double or triple covering of cotton, and in some cases copper bars are used for large current machines.

=Ques. What is the objection to copper bars?=

Ans. They are liable to have eddy currents set up in them as illustrated in fig. 291.

=Ques. What may be said with respect to the sizes of wire used for inductors?=

Ans. Wire larger than about number 8 B and S gauge (.1285 inch diameter) is not easily handled, hence for large inductors, two or more wires may be wound together in parallel.

According to the mechanical features and manner of assembling on the core, drum windings may be divided into several classes, as follows:

1. Hand winding; 2. Evolute or butterfly winding; 3. Barrel winding; 4. Bastard winding; 5. =Former= winding.

=Hand Winding.=--The first windings were put on by hand and proved objectionable on account of the clumsy overlapping of the wires at the ends of the armature, which stops ventilation and hinders repairs, while the outer layers overlying those first wound, bring into close proximity inductors of widely varying voltage. The method is still used in special cases and for small machines. Such a winding has rarely, if ever, been made with one continuous wire.

=Evolute or Butterfly Winding.=--This mode of winding, was introduced by Siemens for electroplating dynamos to overcome the objections to hand winding. It takes its name from the method of uniting the inductors by means of spiral end connectors as shown in fig. 374, also in figs. 369 and 370, which show more modern forms.

=Ques. What are evolute connectors?=

Ans. The fork shaped strips used to connect bars at different positions on the armature, as shown in fig. 369.

In large machines, especially where the teeth are wide, these connections may be straight, but in small cc machines they must be curved in the manner shown in the upper part of the figure, as the room available may diminish by as much as half, as the lowest point is reached, and the room occupied by the strip is the width of a horizontal section at various points. This width, in the case of the straight connections, is constant.

In place of the wooden block, used in early machines, for fastening the middle part of the connectors, they may be anchored to an insulated clamping device built up like a commutator and for that reason called a _false commutator_.

=Ques. How are the inductors arranged in evolute winding?=

Ans. In fig. 373, it will be seen that the ends of the evolute connectors lie in two planes, hence the inductors must project to different distances beyond the core. Accordingly, one long and one short bar may be conveniently placed in each slot, side by side. In large machines, especially where the teeth are wide, the connectors may be straight as in fig, 370. Evolute connectors may be used for either lap or wave windings.

=Barrel Winding.=--This is a form of drum winding _in which the inductors are arranged in two layers and carried out obliquely on an extension of the cylindrical surface of the drum to meet and connect with radial risers_.

Barrel winding has been very widely adopted. Although it involves an increased length of armature, this gives additional cooling surface and provides for good ventilation.

In barrel winding, the coil ends must of necessity be arranged in two layers but the method may be used for either one or two coils per slot, the difference in arrangement for these two cases being shown in figs. 375 and 376. In the single layer barrel winding, fig. 375, each slot is occupied by but one side of one coil. In the double layer barrel winding, fig. 376, the opposite sides of two separate coils occupy space in the same slot. The coils, on emerging from the slots bend in opposite directions, and if one side of a coil occupy the bottom portion of a slot, its other side usually occupies the top portion of a slot distant from the first slot by the polar pitch.

=Bastard Winding.=--In this type of winding, the end connectors project from the inductors in straight lines parallel to the shaft and then are bent inward. It has the effect of being somewhat shorter than the barrel winding. In order to secure better ventilation, it is usual to combine a bastard winding at the rear end of the armature with a barrel winding at the commutator end. This class of winding is used only with bar armatures.

=Former Winding.=--This relates to a method of winding coils, and not to any particular type; that is, mechanical winding as distinguished from hand winding. While hand winding is necessary for ring armatures, a drum armature is wound better and more easily by the aid of machinery.

=Ques. What is a "former" coil?=

Ans. A former coil, as its name suggests, is one that is wound complete upon a former before being placed upon the armature.

=Ques. What is the advantage of this method of winding coils?=

Ans. By the use of formers much time is saved, thus reducing the cost, and also by their use all the coils are symmetrical which improves the appearance of the finished winding.

=Ques. How is the required shape of the template or former for winding the coils determined?=

Ans. By winding one coil on the armature in order to ascertain its dimensions and shape; it is then removed from the armature and used as a pattern in constructing the former.

=Types of Former Coil.=--Of the numerous shapes of former coil, mention should be made of:

1. Evolute coils; 2. Straight out coil

=Ques. Describe the evolute type of former coil.=

Ans. The evolute coil is wound around eight pins inserted in a board as shown in fig. 381. The required number of turns are taken around these pins and their ends G and H left projecting. The coil thus formed is now covered with tape and after removal from the board, is put into a clamp at C and F, and opened up as shown in fig. 382, which is the form required for insertion in the proper slots of the armature.

=Ques. What is the peculiarity of the evolute coil?=

Ans. The two sides of the evolute coil have unequal dimensions. The part marked AB, in fig. 381 which is an upper layer inductor is longer than the part DE, which constitutes a lower layer inductor. The portions DC and EF act as parts of an inner layer of evolutes, and the portions AF and BC as parts of an outer layer of evolutes. These features are shown in fig. 382.

=Ques. How are evolute coils placed on the core?=

Ans. They are placed in position as shown in figs. 372 and 373, continuing around the core until all the slots are filled. To complete the operation it is necessary to raise some of the first laid coils and insert the last ones below them. The winding is thus completed and is symmetrical.

=Ques. Describe the method of winding the "straight out" type of former coil.=

Ans. The straight out coil may be wound on a former such as shown in fig. 384. This consists of a board having four upright pins, A, B, D, E, properly spaced and two horizontal pins C, F, attached to extensions at each end of the board. A coil of the required number of turns is wound around these pins and then opened out as in fig. 385. After varnishing and baking it is ready to be placed on the armature.

=Ques. For what class of winding are straight out former coils suitable?=

Ans. For barrel winding.

=Ques. How are straight out coils placed on the core?=

Ans. In the same manner as described for evolute coils; when in position straight out coils appear as in fig. 372.

=Ques. What is the approved method of putting tape on a coil?=

Ans. Considerable time is saved by the use of a machine designed for the purpose, such as shown in fig. 387.

The construction of these machines is such that a roll of tape placed on a split metal ring is revolved around the coil to be taped, the coil being gradually moved until it is entirely covered.

=Coil Retaining Devices.=--In the operation of a dynamo there are two forces which tend to throw the inductors out of position:

1. Armature drag; 2. Centrifugal force.

Both of these forces are present with smooth core armatures, but only centrifugal force with slotted armatures. The devices used to hold the inductors in position against these forces are:

1. Driving horns; 2. Binding ribbons; 3. Retaining wedges.

=Ques. What are driving horns?=

Ans. They are simply pins or strips projecting from the surface of a smooth core as shown in fig. 251.

=Ques. What other kinds of retainer are used on smooth core armatures?=

Ans. They require several binding ribbons or brass bands placed around the winding to prevent the inductors being thrown off the core by centrifugal force.

=Ques. With slotted armatures what provision must be made for retaining the inductors in position?=

Ans. Retaining wedges must be inserted into the notches or between the projecting tops of the teeth.

=Ques. How are the wedges made?=

Ans. They are usually made of well baked hard wood, such as hornbeam, or hard white vulcanized fibre. Sometimes a springy strip of German silver is used.