Fig. 1,490.—Multi-polar revolving armature alternator with half coil winding, shown in radially developed diagram to clearly indicate the path of the winding. A half coil or hemitropic winding has a slightly higher reactance than a winding in which two distinct coils are used in the same slot, one going forward and the other backward. The most usual three phase windings are of the half coil type as the three sets of coils are equispaced over a pair of poles.
Fig. 1,491.—Multi-polar revolving armature alternator with whole coil winding shown in radially developed diagram to clearly indicate the path of the winding.
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.
Figs. 1,492 and 1,493.—Concentrated windings. A concentrated winding is one in which the armature has only one tooth per phase per pole, that is, the number of teeth equals the number of poles. A concentrated winding of the half coil type has only one side of a coil in each slot as in fig. 1,492. In the whole coil variety, each slot contains neighboring sides of adjacent coils, as in fig. 1,493. In construction, wedges are generally used for retaining the half coils, and with whole coils the teeth have projecting horns for this purpose.
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.
Fig. 1,494.—Laminated core with two coils in position; type of punchings used on some machines having concentrated whole coil windings. The manner of assembling the coils is shown in fig. 1,495.
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.
Fig. 1,495.—Westinghouse single phase concentrated coil armature; view showing method of placing coils. The coils are machine wound on formers and after being taped, varnished and baked, are spread out slightly so as to pass over the teeth and are then forced into place in the deep slots by means of wooden wedges, being securely held in place by retaining wedges, as shown in fig. 1,494.
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.
Figs. 1,496 to 1,498.—Alternator coils, showing difference between the concentrated, partially distributed, and fully distributed forms. Fig. 1,496 shows a concentrated coil in which all the wire is wound in one large coil; in the partially distributed type fig. 1,497, the wire of fig. 1,496, is wound in two or more coils or "sections" connected as shown, leaving some space inside not taken up by the subdivisions. In fig. 1,498 the wire of fig. 1,496 is fully distributed, being wound in a series of coils, so that all the interior space is taken up by the wire, that is to say, the spaces not occupied by the wire (the teeth when placed on the armature) are of equal size.
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.
Fig. 1,499.—Partially distributed winding. Each coil unit is here divided into two concentric coils of different dimensions and connected in series, as shown in detail in fig. 1,497. This being a "whole coil" winding the several units are so connected that the winding of adjacent units proceeds in opposite directions, that is, one coil is wound clockwise, and the next counter clockwise, etc., so that the induced currents flow in a common direction as indicated by the arrows for the position shown.
Fig. 1,500.—Fully distributed winding. In this type of winding each coil consists of so many sub-coils that the winding occupies the entire surface of the armature core; that is, there are no extensive spaces unoccupied, the spacing being uniform as shown.
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.
Fig. 1,501.—Developed diagram of single phase concentrated whole coil winding in two slot stamping for six pole alternator. If the sides of adjacent whole coils be slightly separated by placing the winding in a two slot stamping the electrical result will not differ materially from the monotooth whole coil winding, but if the winding be hemitropic, as in fig. 1,502, and has coils of two sizes as shown, it will be suitable for high voltages.
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:
| πfZN | |||
| Emax | = | (1) | |
| 108 |
in which
The maximum value of the pressure, as expressed in equation (1), occurs when θ = 90°.
Fig. 1,502.—Developed diagram of single phase partially distributed half coil winding for six pole alternator in two slot stamping, same as in fig. 1,501. In this arrangement the direction of rotation is not reversed. It is a question as to how far the coils of a single-phase armature may be spread with advantage. There is not much advantage in reducing the interior breadth of the coils below that of the pole face, nor in widening the exterior breadth beyond that of the pole pitch.
The virtual value of the volts is equal to the maximum value divided by √2, or multiplied by ½ √2, hence,
| ½ √2 × πfZN | 2.22fZN | ||||
| 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.
Fig. 1,503.—Developed diagram of single phase winding with fully distributed coils. As explained, excessive spreading lowers the value of the "Kapp" coefficient, and consequently the voltage; also the use of a larger number of inductors to obtain the same voltage results in an increase of armature self-induction. On the other hand, if the winding were concentrated in fewer slots and these slots were closer together, the result will be an increase in distorting and demagnetizing reactions of the armature. Therefore, a compromise between these two disadvantages must be made. The common practice is to wind in two or three slots per pole per phase.
Fig. 1,504.—Allis-Chalmers lap wound coils forming a three slot distributed coil unit. In construction, after the coils have been covered with insulating materials and treated with insulating compound, the parts that lie in the slots are pressed to exact size in steam-heated moulds. This runs the insulating material into all the small spaces in the coil, excluding moisture and rendering the insulation firm and solid. The ends of the coils, where they project beyond the slots, are heavily taped.
Fig. 1505.—Allis-Chalmers armature construction; view showing section of frame and two layer winding.
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
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.
Figs. 1,506 and 1,507.—Effect of breadth of coils in distributed windings. In the section of the alternator shown in fig. 1,506 the directions of the pressures induced as the armature rotates clockwise are represented by dots for those which act towards the reader, and by crosses for those which act from the reader (the dots and crosses representing respectively the heads and tails of arrows). Since the field is not uniform but maximum at the center and gradually weakening towards the extremities, it is obvious that the maximum pressure is induced in any inductor as it passes the center of the pole, this variation being indicated by the heavier dots and crosses toward the center. Now if a number of these inductors be connected up to form a distributed coil as in fig. 1,507, the pressures induced in each will be added, but all the maximum pressure will not be induced in all at the same time, hence the total pressure induced in the distributed coil is less than it would be if the coil were concentrated as in fig. 1,509.
Fig. 1,508.—Diagram of distributed coil whose inner breadth is less than the breadth of the pole face, showing the disadvantage of such arrangement. The pressures induced in the inner windings of such a coil are opposing each other at the instant depicted, that is, while the inductors are under the pole face, such action of course being objectionable.
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.
Fig. 1,509.—Simple form of alternator coil, consisting of numerous turns of insulated wire wound around a form, then covered with a tape winding, varnished and baked.
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.
Figs. 1,510 and 1,511.—Multi-wire inductors. When the cross section of inductor necessary to carry the current is large, the use of a single wire would present difficulties in winding on account of its stiffness. Accordingly two or more smaller wires are used in parallel to secure the required cross section. Bare wire is used and the several sections encased in insulation as shown, the combination being more flexible than an equivalent single wire.
Fig. 1,512.—Two coil slot for whole coil winding. The slot has two recesses A and B for the reception of separate coils. In assembling the winding, the inner wedge is first placed in position and then the slot line with the insulating material. This usually consists of alternate layers of mica and pressboard. The coils composed of several turns of wire or copper strip are wound in place, and after covering with a layer of insulation, the outer wedge is pushed in place to retain the inductors in position.
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.
Fig. 1,513.—Method of winding a coil containing a large number of turns, when there is considerable difference of pressure between the layers. In such cases to guard against short circuits or breakdown of the insulation, each layer of turns is insulated from the next layer by the insulating strips A, B, C, in addition to the regular insulation around each wire. After the coil is made up it is wound with insulating tape, varnished and baked.
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.
Figs. 1,514 and 1,515.—Single and double layer multi-wire inductors and methods of placing them on the core. Here the term layer means unit, in fact each unit is made up of several "layers" of wires. In fig. 1,514, where so many wires are bunched together in one unit, each layer of turns is separated from those adjacent by insulating strips on account of the considerable difference of pressure between layers. This insulation is not necessary in fig. 1,515 where there are two units or so called layers. In both cases the inductors are held in place by wedges driven into dovetail grooves.
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.
Fig. 1,516.—Copper strap or ribbon with insulation. These are generally from 1/32 to 1/16 inch thick with rounded edges as shown to avoid cutting the insulation.
Fig. 1,517.—Bar inductor. Its shape enables putting the maximum cross section of copper into the slot and is used to advantage on machines which generate large currents.
Fig. 1,518.—Style of armature core stamping used with bar wound machines. This construction, since there are no indentations in the teeth for wedges, makes it necessary to provide bands to hold the bars in place.
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)
Figs. 1,519 and 1,520.—Bent bar inductor and method of connection with soldered joint. Fig. 1,519 shows one bar and shape of bent ends. The portion from C to D is placed in the slot; B to C and D to E, bent or connector sections; A to B and E to F, ends bent parallel to slot for soldering. Fig. 1,520 shows two bar inductors connected.
Figs. 1,521 and 1,522.—Method of avoiding a soldered joint at one end of a bar inductor by using a bar of twice the length shown in fig. 1,519, and bending it into a long U form, as in fig. 1,521, after which it is spread out forming two inductors, as in fig. 1,522.
of a single phase armature are wound with coils. The reason for this may be explained by aid of fig. 1,524, which shows an armature with six slots per pole, four of which are wound. Owing to the different positions of, say, coils A and B, there will be a difference in phase between the pressure generated in them and consequently the resultant pressure of the two coils joined in series will be less than the sum of the pressure in each coil.
Fig. 1,523.—Arrangement in slot of two layer bar winding. With bar inductors, as must be evident from the illustration, the maximum cross section of copper can be placed in a slot of given dimension, hence a bar winding is used to advantage for alternators designed to carry a large current. Bar inductors, on account of the shape of their ends, must be placed in the slots from the top, because the bent ends do not admit of pushing them in. Straight slots are therefore necessary, the inductors being held in place by wooden strips and tie bands as shown.
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.
Fig. 1,524.—Diagram of single phase multi-coil or distributed winding to show characteristic differences in action and construction from single coil or concentrated winding.
Fig. 1,525.—Vector diagram of pressures induced in the single phase multi-coil or distributed winding shown in diagram in fig. 1,524.
| Slots wound in series | Pressure across coils | Winding coefficient | Quantity of 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.
Fig. 1,526.—Developed diagram of a single phase monotooth or one slot bar winding; it is suitable only for operation at low voltage.
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.
Fig. 1,527.—Diagram of six pole single phase revolving armature alternator, with monotooth or concentrated whole coil winding. For 3,000 volts at 900 revolutions per minute, 120 inductors are required. And in the case of a concentrated or monotooth winding they may be arranged in "whole coils" as above or in "half coils" (hemitropic) as in fig. 1,528.
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.
Fig. 1,528.—Diagram of six pole single phase alternator with concentrated half coil or hemitropic winding of same capacity as in fig. 1,527. There are an equal number of inductors, but in this case arranged in three instead of six coils. In this winding the direction of winding is alternately reversed so that the induced pressures do not oppose one another.
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.
Fig. 1,529.—Two phase concentrated whole coil winding. In this style winding the total number of slots is twice the number of poles, or one slot per pole per phase. It comprises two windings identical with fig. 1,527, being spaced 90 polar degrees as shown. The two circuits are independent, the windings terminating at the four collector rings.
Fig. 1,530.—Two phase winding in two slots per pole per phase. This stamping distributes the coils of each phase into two sections, as A and B. The coils are of the "whole" type and with six poles the total number of slots is 4 × 6 = 24, uniformly spaced as shown.
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.
Figs. 1,531 and 1,532.—Developed diagram of the single phase monotooth windings shown in figs. 1,527 and 1,528.
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.
Fig. 1,533.—Two phase winding in three slots per pole per phase. The coils of each phase are of the partially distributed type, each coil being made up of three sections as shown. The direction of winding is alternately reversed.
Fig. 1,534.—Section of two phase winding showing shaping of the coil ends. Every other coil is flat, while the alternates have their ends bent down as shown. With respect to the shaping of the coil ends, it is called a two range winding.
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.
Fig. 1,535.—Section of Triumph armature showing method of arranging the three phase winding.
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.
Fig. 1,536.—Three phase winding with distributed coils—wound in four slots per pole per phase; diagram showing placement of the coils.
Fig. 1,537.—Treatment of coil ends in two phase, two range windings. In this arrangement straight out (B) and bent up (A) coils are used which are placed on the armature as is clearly shown in the illustration.
Fig. 1,538.—Three phase, 10 pole, 30 slot winding in two ranges. In this winding perfect symmetry occurs after every four poles. Accordingly in the case of an odd number of pairs of pole, one of the coils must necessarily be askew going from the inner to the outer range as at M.
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.
Fig. 1,539.—Three phase 10 pole 30 slot winding in three ranges. The coils of each phase are alike, those of the A phase being all in the straight out range, those in the B phase, in a bent up range, and those in the C phase in a bent down range. This arrangement has the disadvantage, that by reason of the third range, the field magnet cannot be withdrawn. This treatment of the coil ends is more clearly shown in fig. 1,540.
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.
Figs. 1,540 and 1,541.—Treatment of coil ends in three phase, three range windings. Fig. 1,540, inadmissible arrangement in which the field magnet cannot be withdrawn; fig. 1,541, admissible arrangement in which the armature segments can be divided. This enables the top half of armature to be removed by disconnection without unwinding any coil.
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.
Fig. 1,542.—Three phase winding with half coils. The advantage of employing half coils is that the ends may be arranged in two ranges as shown. There is one slot per phase per pole, that is, total number of slots = 3 × number of poles.
Fig. 1,543.—Three phase winding with whole coils. Two sides of adjacent coils come in one slot. Number of coils per phase = number of poles per phase. Total number of slots = 3 multiplied by number of poles per phase. Whole coils require the ends arranged in three ranges as indicated. The coils are concentrated.
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.
Fig. 1,544.—Diagram of two phase star grouping.
Ques. What does this arrangement give?
Ans. It is practically equivalent to a four phase system.
Fig. 1,545.—Diagram of two phase mesh grouping.
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.
Fig. 1,546.—Diagram of three phase star grouping, commonly called Y grouping owing to its resemblance of the letter Y. The current in each main is obviously equal to the current in each phase winding, but the terminal pressure is the vector sum of the pressures in the component phase windings, that is, √3 multiplied by the pressure in one phase.
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.
Fig. 1,547.—Radial diagram of three phase, one slot winding with Y connection.
Fig. 1,548.—Radial diagram of three phase one slot winding with delta connection.
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.