CHAPTER L

CONSTRUCTION OF ALTERNATORS

The construction of alternators follows much the same lines as dynamos, especially in the case of machines of the revolving armature type. Usually, however, more poles are provided than on direct current machines, in order to obtain the required frequency without being driven at excessive speed.

The essential parts of an alternator are:

1. Field magnets; 2. Armature; 3. Collector rings;

and in actual construction, in order that these necessary parts may be retained in proper co-relation, and the machine operate properly there must also be included:

4. Frame; 5. Bed plate; 6. Pulley.

~Field Magnets.~--The early forms of alternator were built with permanently magnetized steel magnets, but these were later discarded for electromagnets.

Alternators are built with three kinds of electromagnets, classed according to the manner in which they are excited, the machines being known as,

1. Self-excited; 2. Separately excited; 3. Compositely excited.

~Ques. What is a self-excited alternator?~

Ans. One in which the field magnets are excited by current from one or more of the armature coils, or from a separate winding (small in comparison with the main winding), the current being transformed into direct current by passing it through a commutator.

Fig. 1,453 shows an armature of a self-excited machine, the exciting current being generated in a separate winding and passed through a commutator.

~Ques. For what class of service are self-exciting alternators used?~

Ans. They are employed in small power plants and isolated lighting plants where inductive loads are encountered.

~Ques. What is a separately excited alternator?~

Ans. One in which the field magnets are excited from a small dynamo independently driven or driven by the alternator shaft, either direct connected or by belt as shown in fig. 1,455.

~Ques. What is a compositely excited alternator?~

Ans. A composite alternator is similar to a compound wound dynamo in that it has two field windings. In addition to the regular field coils which carry the main magnetizing current from the exciter, there is a second winding upon two or upon all of the pole pieces, carrying a rectified current from the alternator which strengthens the field to balance the losses in the machine, and also if so desired, the losses on the line as shown in fig. 1,456.

~Ques. What is a magneto?~

Ans. A special form of alternator having permanent magnets for its field, and used chiefly to furnish current for gas engine ignition and for telephone call bells.

Details of construction and operation are shown in figs. 1,458 to 1,461.

~Ques. What are the two principal types of field magnet?~

Ans. Stationary and revolving.

~Ques. What is the usual construction of stationary field magnets?~

Ans. Laminated pole pieces are used, each pole being made up of a number of steel stampings riveted together and bolted or preferably cast into the frame of the machine. The field coils are machine wound and carefully insulated. After winding they are taped to protect them from mechanical injury. Each coil is then dipped in an insulating compound and afterwards baked to render it impervious to moisture.

~Ques. Describe the construction of a revolving field.~

Ans. The entire structure or rotor consists of a shaft, hub or spider, field magnets and slip rings. The magnet poles consist of laminated iron stampings clamped in place by means of through bolts which, acting through the agency of steel end plates, force the laminated stampings into a uniform, rigid mass. This mass is magnetically subdivided into so many small parts that the heating effect of eddy currents is reduced to a minimum. The cores are mounted upon a hub or spider either by dovetail construction or by means of through bolts, according to the centrifugal force which they must withstand in operation, either method permitting the easy removal of any particular field pole if necessary. The field coils are secured upon the pole pieces either by horns in one piece with the laminations, or separate and bolted. All the coils are connected in series, cable leads connecting them to slip rings placed on the shaft.

~Ques. What are slip rings?~

Ans. Insulated rings mounted upon the alternator shaft to receive direct current for the revolving field, as distinguished from collector rings which collect the alternating currents generated in an alternator of the revolving armature type.

In construction provision is made for attaching the field winding leads. The rings are usually made of cast iron and are supported mechanically upon the shaft, but are insulated from it and from one another.

The current is introduced by means of brushes as with a commutator. Carbon brushes are generally used.

A good design of slip ring should provide for air circulation underneath and between the rings.

~Ques. What form of spider is used on large alternators?~

Ans. It is practically the same form as a fly wheel, consisting of hub, spokes, and rim to which the magnets are bolted.

~Armatures.~--In construction, armatures for alternators are similar to those employed on dynamos; they are in most cases simpler than direct current armatures due to the smaller number of coils, absence of commutator with its multi-connections, etc. Alternator armatures may be classified in several ways:

1. With respect to operation, as

_a._ Revolving; _b._ Stationary.

2. With respect to the core, as

_a._ Ring; _b._ Disc; _c._ Drum.

Ring and disc armatures are practically obsolete and need not be further considered. A ring armature has the inherent defect that the copper inside the ring is inactive.

Disc armatures were employed by Pacinotti in 1878, and afterwards adopted by Brush in his arc lighting dynamos.

The design failed for mechanical reasons, but electrically it is, in a sense, an improvement upon the Gramme ring, in that inductors on both sides of the ring are active, these being connected together by circumferential connectors from pole to pole, thus, corresponding to the end connectors on modern drum armatures.

3. With respect to the core surface, as

_a._ Smooth core; _b._ Slotted core.

In early dynamos the armature windings were placed upon an iron core with a smooth surface. A chief disadvantage of this arrangement is that the magnetic drag comes upon the inductors and tends to displace them around the armature. To prevent

~Armature Windings.~--In general, the schemes for armature windings for alternators are simpler than those for direct current machines, as in the majority of cases the inductors are an even multiple of the number of poles, and the groupings are usually symmetrical with respect to each pole or each pair of poles. Furthermore, as a general rule, all the inductors of any one phase are in series with one another; therefore, there is only one circuit per phase, and this is as it should be, since alternators are usually required to generate high voltages. These general principles establish the rule, that in the circuit in a single phase armature, and in the individual circuits in a polyphase armature, the winding is never re-entrant, but the circuits have definite endings and beginnings. In exceptional cases, as those of polyphase converters, re-entrant circuits are employed, and the armature windings are so constructed that a commutator can be connected to them exactly as in direct current machines. These armatures are usually of the lap wound drum type.

Alternator windings are usually described in terms of the number of slots per phase per pole. For instance, if the armature of a 20 pole three phase machine have 300 slots, it has 15 slots per pole or 5 slots per each phase per pole, and will be described as a five slot winding. Therefore, in order to trace the connections of a winding, it is necessary to consider the number of slots per pole for any one phase on one of the following assumptions: 1, that each slot holds one inductor; 2, that there is one side of a coil in each slot; and 3, that one side of a coil is subdivided so as to permit of its distribution in two or more adjacent slots.

The voltage depends upon the number of inductors in a slot, but the breadth coefficient and wave form are influenced by the number of slots per pole, and not by the number of inductors within the slots.

~Classification of Windings.~--The fact that alternators are built in so many different types, gives rise to numerous kinds of armature winding to meet the varied conditions of operation. In dividing these forms of winding into distinctive groups, they may be classified, according to several points of view, as follows:

1. With respect to the form of the armature, as:

_a._ Revolving; _b._ Stationary.

2. With respect to the mode of progression, as:

_a._ Lap winding; _b._ Wave winding.

3. With respect to the relation between number of poles and number of coils, as:

_a._ Half coil winding; _b._ Whole coil winding.

4. With respect to the number of slots, as:

_a._ Concentrated or uni-coil winding; _b._ Distributed or multi-coil winding. Partially distributed; Fully distributed.

5. With respect to the form of the inductors, as:

_a._ Wire winding; _b._ Strap winding; _c._ Bar winding.

6. With respect to the number of coils per phase per pole, as:

_a._ One slot winding; _b._ Two slot winding; etc.

7. With respect to the kind of current delivered, as:

_a_. Single phase winding; _b_. Two phase winding; _c_. Three phase winding.

8. With respect to the shape of the coil ends, as:

_a_. Single range; _b_. Two range; etc.

In addition to these several classes of winding, there are a number of miscellaneous windings of which the following might be mentioned:

_a._ Chain or basket winding; _b._ Skew coil winding; _c._ Fed-in winding; _d._ Imbricated winding; _e._ Mummified winding; _f._ Spiral winding; _g._ Shuttle winding; _h._ Creeping winding; _i._ Turbine alternator winding.

~Ques. Define a revolving and a stationary winding.~

Ans. The words are self-defining; a winding is said to be revolving or stationary according as the armature forms the rotor or stator of the machine.

~Ques. What is the significance of the terms lap and wave as applied to alternator windings?~

Ans. They have the same meaning as they do when applied to dynamo windings.

These are described in detail in Chapter XVIII. Briefly a lap winding is one composed of lap coils; a wave winding is one which roughly resembles in its diagram, a section of waves.

~Half Coil and Whole Coil Windings.~--The distinction as to whether the adjacent sides of consecutive coils are placed together under one pole or whether they are separated a distance equal to the pole pitch, gives rise to what is known as half coil and whole coil windings.

A half coil or hemitropic winding is _one in which the coils in any phase are situated opposite every other pole_, that is, _a winding in which there is only one coil per phase_ ~per pair of poles~, as in fig. 1,488.

_A whole coil winding is one in which there is one coil per phase_ ~per pole~, as in fig. 1,489, the whole (every one) of the poles being subtended by coils.

~Concentrated or Uni-Coil Winding.~--Fig. 1,492 shows the simplest type of single phase winding. It is a one slot winding and is sometimes called "monotooth" or "uni-coil" winding. The surface of the armature is considered as divided into a series of large teeth, one tooth to each pole, and each tooth is wound with one coil, of one or more turns per pole. Since all the turns of the coil are placed in single slots, the winding is called "concentrated."

~Ques. What are the features of concentrated windings?~

Ans. Cheap construction, maximum voltage for a given number of inductors. Concentrated windings have greater armature reaction and inductance than other types hence the terminal voltage of an alternator with concentrated winding falls off more than with distributed winding when the current output is increased. An alternator, therefore, does not have as good regulation with concentrated winding as with distributed winding.

~Ques. What should be noted with respect to concentrated windings?~

Ans. A concentrated winding, though giving higher voltage than the distributed type with no load, may give a lower voltage than the latter at full load.

~Ques. What is the wave form with a concentrated winding?~

Ans. The pressure curve rises suddenly in value as the armature slots pass under the pole pieces, and falls suddenly as the armature slots recede from under the pole pieces.

~Distributed or Multi-Coil Windings.~--Instead of winding an armature so it will occupy only one slot per phase per pole, it may be spread out so as to fill _several slots per phase per pole_. This arrangement is called a distributed winding.

To illustrate, fig. 1,496 represents a coil of say fifteen turns. This could be placed on an armature just as it is, in which case only one slot would be required for each side, that is, two in all. In place of this thick coil, the wire could be divided into several coils of a lesser number of turns each, arranged as in fig. 1,497; it is then said to be _partially distributed_, or it could be arranged as in fig. 1,498, when it is said to be _fully distributed_.

A partially distributed winding, then, is one, as in fig. 1,499, in which the coil slots do not occupy all the circumference of the armature; that is, the core teeth are not continuous.

A fully distributed winding is one in which the entire surface of the core is taken up with slots, as in fig. 1,500.

~Ques. In a distributed coil what is understood by the breadth of the coil?~

Ans. The distance between the two outer sides, as B in figs. 1,497 and 1,498.

~Ques. How far is it advisable to spread distributed coils of a single phase alternator?~

Ans. There is not much advantage in reducing the interior breadth much below that of the breadth of the pole faces, nor is there much advantage in making the exterior breadth greater than the pole pitch.

Undue spreading of distributed coils lowers the value of the Kapp coefficient (later explained) by reducing the breadth coefficient and makes necessary a larger number of inductors to obtain the same voltage.

The increase in the number of inductors causes more armature self-induction. From this point of view, it would be preferable to concentrate the winding in fewer slots that were closer together. This, however, would accentuate the distorting and demagnetizing reactions of the armature. Accordingly, between these two disadvantages a compromise is made, as to the extent of distributing the coils and spacing of the teeth, the proportions assigned being those which experience shows best suited to the conditions of operation for which the machine is designed.

~The Kapp Coefficient.~--A volt or unit of electric pressure is defined as the pressure induced by the cutting of 100,000,000 or 10⁸ lines of force per second. In the operation of an alternator the maximum pressure generated may be expressed by the following equation:

π_f_ZN Eₘₐₓ = ------ (1) 10⁸

in which

_f_ = frequency; Z = number of inductors in series in any one magnetic circuit; N = magnetic flux, or total number of magnetic lines in one pole or in one magnetic circuit.

The maximum value of the pressure, as expressed in equation (1), occurs when θ = 90°.

The virtual value of the volts is equal to the maximum value divided by √2̅, or multiplied by ½ √2̅, hence,

½ √2̅ × π_f_ZN 2.22_f_ZN Eᵥᵢᵣₜ = -------------- = --------- (2) 10⁸ 10⁸

This is usually taken as the fundamental equation in designing alternators. It is, however, deduced on the assumptions that the distribution of the magnetic flux follows a sine law, and that the whole of the loops of active inductors in the armature circuit acts simultaneously, that is to say, the winding is concentrated.

In practice, the coils are often more or less distributed, that is, they do not always subtend an exact pole pitch; moreover, the flux distribution, which depends on the shaping and breadth of the poles, is often quite different from a sine distribution. Hence, the coefficient 2.22 in equation (2) is often departed from, and in the general case equation (2) may be written

~_kf_ZN Eᵥᵢᵣₜ = ------- (3) 10⁸~

where _k_ is a number which may have different values, according to the construction of the alternator. This number _k_ is called the _Kapp coefficient_ because its significance was first pointed out by Prof. Gisbert Kapp.

The value of _k_ is further influenced by a "breadth coefficient" or "winding factor."

The effect of breadth in distributed windings is illustrated in figs. 1,506 to 1,508.

~Wire, Strap, and Bar Windings.~--In the construction of alternators, the windings may be of either wire, strap, or bar, according to which is best suited for the conditions to be met.

~Ques. What conditions principally govern the type of inductor?~

Ans. It depends chiefly upon the current to be carried and the space in which the inductor is to be placed.

~Ques. What kind of inductors are used on machines intended for high voltage and moderate current?~

Ans. The winding is composed of what is called _magnet wire_, with double or triple cotton insulation.

~Ques. Where considerable cross section is required how is a wire inductor arranged?~

Ans. In order that the coil may be flexible several small wires in multiple are used instead of a single large wire.

~Ques. How is the insulation arranged on inductors of this kind?~

Ans. Bare wire is used for the wires in parallel, insulation being wrapped around them as in fig. 1,510.

This construction reduces the space occupied by the wires, and the insulation serves to hold them in place.

~Ques. What precaution is taken in insulating a wire wound coil containing a large number of turns?~

Ans. On account of the considerable difference of pressure between layers, it is necessary to insulate each layer of turns as well as the outside of the coil, as shown in fig. 1,513.

~Ques. Do distributed coils require insulation between the separate layers?~

Ans. Since they are subdivided into several coils insulation between layers is usually not necessary.

~Ques. How is a coil covered?~

Ans. It is wound with a more or less heavy wrapping of tape depending upon the voltage.

Linen tape of good quality, treated with linseed oil, forms a desirable covering. Where extra high insulation is required the tape may be interleaved with sheet mica.

~Ques. Is the insulation placed around the coils all that is necessary?~

Ans. The slots into which the coils are placed, are also insulated.

~Ques. How are bar windings sometimes arranged?~

Ans. In two layers, as in fig. 1,523.

~Single and Multi-Slot Windings.~--These classifications correspond to _concentrated_ and _distributed windings_, previously described. In usual modern practice, only two-thirds of the total number of slots (assuming the spacing to be uniform)

Fig. 1,525 shows the pressure plotted out as vector quantities, and the table which follows gives the relative effectiveness of windings with various numbers of slots wound in series.

The figures in the last column of the table show that a large increase in the weight of active material is required if the inductors in a single phase machine are to be distributed over more than two-thirds the pole pitch. Again, if much less than two-thirds of the surface be wound, it is more difficult to provide a sine wave of pressure.

~TABLE OF RELATIVE EFFECTIVENESS OF WINDINGS~

---------------------------------------------------------------- | | | | | |Slots wound | Pressure across | Winding | Quantity of | | in series | coils | coefficient | copper to produce | | | | | same pressure | +------------+-----------------+-------------+-------------------+ | | | | | | 1 | 1 | 1 | 1 | | 2 | 1.93 | .97 | 1.03 | | 3 | 2.73 | .91 | 1.10 | | 4 | 3.34 | .84 | 1.19 | | 5 | 3.72 | .74 | 1.35 | | 6 | 3.86 | .64 | 1.56 | +------------+-----------------+-------------+-------------------+

~Ques. What other advantage besides obtaining a sine wave is secured by distributing a coil?~

Ans. There is less heating because of the better ventilation.

~Single Phase Windings.~--There are various kinds of single phase winding, such as, concentrated, distributed, hemitropic, etc. Fig. 1,527 shows the simple type of single phase winding. It is a "one slot" winding, that is, concentrated coils are used.

The armature has the same number of teeth as there are poles, the concentrated coils being arranged as shown. In designing such a winding, the machine, for example, may be required to generate, say, 3,000 volts, frequency 45, revolutions 900 per minute.

These conditions require 720 inductors in series in the armature circuit, and as the armature is divided into six slots corresponding to the six poles, there will be 120 inductors per slot, and the coil surrounding each of the six teeth on the surface of the armature will consist of 60 turns. The connections must be such as to give alternate clockwise and counter-clockwise winding proceeding around the armature.

~Ques. In what other way could the inductors be arranged in concentrated coils?~

Ans. They could be grouped in three coils of 120 turns each, as shown in fig. 1,528.

When thus grouped the arrangement is called a hemitropic winding, as previously explained.

~Ques. What is the advantage, if any, of a half coil winding?~

Ans. In single phase machines a half coil winding is equivalent, electrically, to a monotooth winding, and, therefore, is not of any particular advantage; but in three phase machines, it has a decided advantage, as in such, a concentrated winding yields a higher pressure than a distributed winding.

~Two Phase Armature Windings.~--This type of winding can be made from any single phase winding by providing another set of slots displaced along the surface of the armature to the extent of one-half the pole pitch, placing therein a duplicate winding.

For instance: If the six pole monotooth, single phase winding, shown in fig. 1,527, be thus duplicated, the result will be the one slot two-phase winding shown in fig. 1,529, which will have twelve slots, and will require four slip rings, or two rings for each phase.

By connecting up the two windings in series, the machines could be used as a single phase, with an increase of voltage in the ratio of 1.41 to 1.

~Ques. How must the coils be constructed for two phase windings?~

Ans. They must be made of two different shapes, one bent up out of the way of the other, as in fig. 1,534.

There are numerous kinds of two phase windings; the coils may be concentrated or distributed, half coil or whole coil, etc. Fig. 1,530 shows a two phase winding with four slots per pole, and fig. 1,533 one with six slots per pole.

~Three Phase Armature Windings.~--On the same general principle applicable to two phase windings, a three phase winding can be made from any single phase winding, by placing three identical single phase windings spaced out successively along the surface of the armature at intervals _equal to one-third and two-thirds, respectively, of the double pole pitch_, the unit in terms of which the spacing is expressed, being that pitch, which corresponds to one whole period.

Each of the three individual windings must be concentrated into narrow belts so as to leave sufficient space for the other windings between them. This limits the breadth or space occupied by the winding of any one phase to one-third of the pole pitch.

~Ques. How are three phase coil ends treated?~

Ans. They may be arranged in two ranges, as in fig. 1,538, or in three ranges, as in fig. 1,539.

~Ques. What kind of coil must be used for three phase windings in order that the ends may be arranged in only two ranges?~

Ans. Hemitropic or half coils; that is, the number of coil per phase must be equal to one-half the number of pole.

~Grouping of Phases.~--In the preceding diagrams, the general arrangement of the coils on the armature surface are shown for the numerous classes of winding. In polyphase alternators the separate windings of the various phases may be grouped in two ways:

1. Star connection;

2. Mesh connection.

~Ques. Describe the two phase star connection.~

Ans. In this method of grouping, the middle points of each of the two phases are united to a common junction M, and the four ends are brought out to four terminals _a_, _a'_, _b_, _b'_, as shown in fig. 1,544, or in the case of revolving armatures, to four slip rings.

~Ques. What does this arrangement give?~

Ans. It is practically equivalent to a four phase system.

~Ques. How is the two phase mesh connection arranged?~

Ans. In this style of grouping, the two phases are divided into two parts, and the four parts are connected up in cyclic order, the end of one to the beginning of the next, so as to form a square, the four corners of which are connected to the four terminals _a_, _b_, _a'_, _b'_, as shown in fig. 1,545, or in the case of revolving armatures, to four slip rings.

~Ques. Describe a three phase star connection?~

Ans. In three phase star grouping, one end of each of the three circuits is brought to a common junction M, usually insulated, and the three other ends are connected to three terminals _a_, _b_, _c_, as shown in fig. 1,546, or in the case of revolving armatures to three slip rings.

~Ques. What other name is given to this connection, and why?~

Ans. It is commonly called a ~Y~ connection or grouping owing to the resemblance of its diagrammatic representation to the letter ~Y~.

~Ques. How is a three phase mesh connection arranged?~

Ans. The three circuits are connected up together in the form of a triangle, the three corners are connected to the three terminals, _a_, _b_, _c_, as shown in fig. 1,549, or in the case of revolving armatures to three slip rings.

~Ques. What other name is given to this style of connection, and why?~

Ans. It is commonly called a _delta_ grouping on account of the resemblance of its diagrammatic representation to the Greek letter Δ.

In polyphase working, it is evident that by the use of four equal independent windings on the armature, connected to eight terminals or slip rings, a two phase alternator can be built to supply currents of equal voltage to four independent circuits. Likewise, by the use of three equal independent windings, connected to six terminals or slip rings, a three phase alternator can be made to supply three independent circuits.

This is not the usual method employed in either case, however, as the star grouping or mesh grouping methods of connection not only gives the same results, but also, in star grouping, a greater plurality of voltages for the same machine, and a higher voltage between its main terminals.

Radial diagrams of the arrangement and connections of ~Y~ grouping of lap windings and wave windings for three phase alternators are shown by figs. 1,551 and 1,552.

~Ques. In three phase star grouping, what is the point where the phases join, called?~

Ans. The star point.

~Ques. In a three phase star connected alternator what is the voltage between any two collector rings?~

Ans. _It is equal to the voltage generated per phase multiplied by √3̅ or 1.732._

~Ques. In a three phase star connected alternator what is the value of the current in each line?~

Ans. The same as the current in each phase winding.

~Ques. What is the value of the total output in watts of a star connected alternator?~

Ans. It is equal to the sum of the outputs of each of the three phases. When working on a non-inductive load, the total output of a star connected alternator is equal to √3̅ multiplied by the product of the line current and line voltage.

~Ques. What is the value of the line voltage in a three phase delta connected alternator?~

Ans. It is equal to the voltage generated in each phase.

~Ques. What is the value of the line current in a three phase delta connected alternator?~

Ans. It is equal to the current in each phase multiplied by √3̅.

~Ques. What is the total output of a three phase delta connected alternator working on a non-inductive load?~

Ans. The total watts is equal to √3̅ multiplied by the product of the line current and the line voltage.

~Ques. What are the features of the star connection?~

Ans. It gives a higher line voltage than the delta connection for the same pressure generated per phase, hence it is suited for machines of high voltage and moderate current.

The delta connection gives a lower line voltage than the star[5] connection for the pressure generated per phase, and cuts down the current in the inductors; since the inductors, on this account, may be reduced in size, the delta connection is adapted to machines of large current output.

[5] NOTE.--In the star connected armature the proper ends to connect to the common terminal or star point are determined as follows: Assume that the inductor opposite the middle of a pole is carrying the maximum current, and mark its direction by an arrow. Then the current in the inductors on either side of and adjacent to it will be in the same direction. As the maximum current must be _coming from_ the common terminal, the end toward which the arrow points must be connected to one of the rings, while the other end is connected to the common terminal. The current in the two adjacent inductors evidently must be flowing into the common terminal, hence the ends toward which the arrows point must be connected to the common terminal, while their other ends are connected to the remaining two rings.

~Ques. How is the path and value of currents in a delta connected armature determined?~

Ans. Starting with the inductors of one phase opposite the middle of the poles, assume the maximum current to be induced at this moment; then but one-half of the same value of current will be induced at the same moment in the other two phases, and its path and value will best be shown by aid of fig. 1,560, in which X may be taken as the middle collector ring, and the maximum current to be flowing from X toward Z. It will be seen that no current is coming in through the line Y, but part of the current at Z will have been induced in the branches _b_ and _c_.

~Ques. Since most three phase windings can be connected either Y or delta, what should be noted as to the effects produced?~

Ans. With the same winding, the delta connection will stand 1.732 as much current as the ~Y~ connection, but will give only 1 ÷ 1.732 or .577 as much voltage.

~Chain or Basket Winding.~--One disadvantage in ordinary two-range windings is that two or three separate shapes of coil are required. The cost of making, winding, and supplying spares would be less if one shape of coil could be made to do for all phases. One way of accomplishing this is by the method of chain winding, in which the two sides of each coil are made of different lengths, as shown in fig. 1,563, and bent so that they can lie behind one another.

In the case of open slots the coils may be former wound and afterwards wedged into their places.

In chain winding the adjacent coils link one another as in a chain (hence, the name); the winding is similar to a skew coil winding. This plan of winding is supposed to have some advantage in keeping coils of different phases further separated than the two range plan.

~Skew Coil Winding.~--In this type of winding the object is to shape the coils so that all may be of one pattern. This is accomplished by making the ends skew shape as shown in figs. 1,566 to 1,568.

~Fed-in Winding.~--This name is given to a type of winding possible with open or only partially closed slots, in which coils previously formed are introduced, only a few inductors at a time if necessary. They are inserted into the slots from the top, the slot being provided with a lining of horn fibre or other suitable material, which is finally closed over and secured in place by means of a wedge, or by some other suitable means. An example of a fed-in winding is shown in figs. 1,566 and 1,568.

~Imbricated Winding.~--This is a species of spiral coil winding in which the end connections are built up one above the other, either in a radial, or in a horizontal direction.

The winding is used especially on the armatures of turbine alternators and dynamos.

~Spiral Winding.~--This is a winding in which "spiral" coils, as shown in fig. 1,569, are used. The spiral form of coil is very extensively used for armature windings of alternators.

~Mummified Winding.~--The word _mummified_ as applied to a winding is used to express the treatment the coils of the winding receive in the making; that is, when a winding, after being covered with tape or other absorbent material, is saturated in an insulating compound and baked until the whole is solidified, it is said to be mummified.

~Shuttle Winding.~--This type of winding consists of a single coil having a large number of turns, wound in two slots spaced 180° apart. It was originally used on Siemens' armature and is now used on magnetos, as shown in figs. 1,459 to 1,461.

~Creeping Winding.~--Another species of winding, known as a creeping winding is applicable to particular cases.

If three adjacent coils, each having a pitch of 120 electrical degrees, be set side by side, they will occupy the same breadth as 4 poles, and, by repetition, will serve for any machine having a multiple of 4 poles, but cannot be used for machines with 6, 10 or 14 poles. Fig. 1,571 shows this example.

In the same way 9 coils, each of 160 electrical degrees, will occupy the same angular breadth as 8 poles.

Further, 9 coils of 200 electrical degrees will occupy the same angular breadth as 10 poles.

Now of these 9 coils, any three contiguous ones are nearly in phase, if wound alternately clockwise and counter-clockwise.

For the 8 pole machine, the phase difference between adjacent coils is 20 degrees.

For the 10 pole machine, the phase difference is also 20 degrees.

The cosine of 20 degrees is .9397, consequently, if 3 adjacent coils be united in series, their joint pressure will be 2.897 multiplied by that of the middle one of the three.

The 9 coils may therefore be joined up in three groups of 3 adjacent coils, for the three phases.

By repetition, the same grouping will suit for any machine having a multiple of 8 or of 10 poles. These two cases are illustrated in figs. 1,572 and 1,573. In the figures, the coils are represented as occupying two slots each, but they might be further distributed.

~Turbine Alternator Winding.~--For the reason that steam turbines run at so much higher speed than steam engines, the construction of armatures and windings for alternators intended to be direct connected to turbines must be quite different from those driven by steam engines. Accordingly, in order that the frequency be not too high, turbine driven alternators must have very few poles--usually two or four, but rarely six.

The following table will show the relation between the revolutions and frequencies for the numbers of poles just designated.

~TABLE OF FREQUENCY AND REVOLUTIONS~

-------------------------------------- | | | | | REVOLUTIONS | | Frequency +--------------------------+ | | | | | | | 2 pole | 4 pole | 6 pole | +-----------+--------+--------+--------| | | | | | | 25 | 1,500 | 750 | 500 | | 60 | 3,600 | 1,800 | 1,200 | | 100 | 6,000 | 3,000 | 2,000 | +-----------+--------+--------+--------|

From the table, it is evident that a large number of poles is not permissible, considering the high speed at which the turbine must be run.

~Ques. How is the high voltage obtained with so few poles?~

Ans. There must be either numerous inductors per slot or numerous slots per pole.

~Ques. What form of armature is generally used?~

Ans. A stationary armature.

~Ques. What difficulty is experienced with revolving armatures?~

Ans. The centrifugal force being considerable on account of the high speed, requires specially strong construction to resist it, consequently closed or nearly closed slots must be used.

~Ques. How is the design of the rotor modified so as to reduce the centrifugal force?~

Ans. It is made long and of small diameter.

Some examples of revolving fields are shown in figs. 1,579 to 1,584. Figs. 1,577 and 1,578 show some construction details of a stationary armature of turbine alternator.

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~ELECTRICAL GUIDE, NO. 1~

Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.

~ELECTRICAL GUIDE, NO. 2~

The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.

~ELECTRICAL GUIDE, NO. 3~

Electrical Instruments, Testing, Practical Management of Dynamos and Motors.

~ELECTRICAL GUIDE, NO. 4~

Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.

~ELECTRICAL GUIDE, NO. 5~

Principles of Alternating Currents and Alternators.

~ELECTRICAL GUIDE, NO. 6~

Alternating Current Motors, Transformers, Converters, Rectifiers.

~ELECTRICAL GUIDE, NO. 7~

Alternating Current Systems, Circuit Breakers, Measuring Instruments.

~ELECTRICAL GUIDE, NO. 8~

Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.

~ELECTRICAL GUIDE, NO. 9~

Telephone, Telegraph, Wireless, Bells, Lighting, Railways.

~ELECTRICAL GUIDE, NO. 10~

Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.

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TRANSCRIBER'S NOTES

Silently corrected simple spelling, grammar, and typographical errors.

Retained anachronistic and non-standard spellings as printed.

Enclosed italics markup in _underscores_.

Enclosed bold markup in ~tildes~.

End of Project Gutenberg's Hawkins Electrical Guide v. 5 (of 10), by Hawkins