The Project Gutenberg eBook of Hawkins Electrical Guide, Number One, by Nehemiah Hawkins.

In construction, the field coils are wound with a greater number of turns than actually required, the machine being accurately adjusted by a running load test after completion.

Ans. A rheostat is placed in shunt with the series winding so that the current passing through the winding may be regulated to control the voltage of the machine.

Fig. 195.—Compound wound dynamo, used when better automatic regulation of voltage on constant pressure circuits is desired than is possible with the shunt machine. The compound dynamo is a combination of the series and shunt types, that is, the field magnet is excited by both series and shunt windings. With a proper selection of the number of turns in the series coils, the voltage may be kept automatically constant for wide fluctuations in the load. When the machine is over compounded its characteristic is to slightly increase the voltage with increase of load, a desirable feature for long transmission lines in order to compensate for the line drop.

Ans. There are two methods of connection, being known as the short shunt and the long shunt.

Ans. In the short shunt, the ends of the shunt winding are connected directly to the brushes as in fig. 196.

Ans. In the long shunt, one end of the shunt winding is connected to one of the brushes and the other end to the terminal connecting the series winding with the external circuit as in fig. 197.

Ans. Theoretically, the long shunt is preferable as being the more efficient; however, in practice, the gain is not very appreciable and the short shunt is generally used.

Figs. 196 and 197.—Short and long shunt types of compound wound dynamos. The distinction between the two is that the ends of the short shunt connect direct with the brush terminals, while in the long shunt type, fig. 197, one end of the shunt connects with one brush terminal and the other with the terminal connecting the series winding with the external circuit. R is the shunt field rheostat for regulating the current through the shunt.

Ques. What may be said regarding the voltage in short, and long shunt machines?

Ans. In a short shunt machine, the shunt winding is subjected to a higher voltage than with a long shunt. The pressure applied through a shunt winding with a long shunt, for any particular load, is equal to the voltage at the brushes plus the drop in the series winding.

Ques. For what other service besides incandescent lighting are compound dynamos adapted?

Ans. They are employed in electric railway power stations where the load is very fluctuating.

Ans. It overloads the machine, since the excessive current flowing through the series field tends to keep the voltage at its normal value.

Unless the line be automatically opened under such a condition either by a fuse or circuit breaker, the machine and its driving engine may be damaged. To avoid this danger fuses or automatic circuit breakers are employed.

Ans. In some isolated plants, as small country residences where it is frequently necessary to have a dynamo capable of charging a storage battery during the day, and of furnishing current for lighting during a certain portion of the evening.

Under such conditions the compound machine with slight modification is used, the ordinary shunt dynamo not being capable of maintaining the necessary consistency of voltage, without attention to the shunt regulator in driving the lamps direct, the ordinary compound dynamo on the other hand, being unsatisfactory for charging storage batteries.

Ques. How is the compound dynamo modified to adapt it to the dual service of lighting and battery charging?

Ans. It is furnished with alternative compound winding, in which the series winding is provided with a switch, which may be fixed either upon the machine itself or upon the switchboard. This switch permits the series coils to be either short circuited in part or cut out of the circuit entirely while the machine is charging the storage battery, being again cut into circuit when the machine is required to furnish current for the lamps.

Fig. 198.—Separately excited dynamo. Current for field excitation is supplied by a second and smaller generator.

Separately Excited Dynamos.—In this class of machine the current required to excite the field magnets is obtained from some independent external source. Though used by Faraday, the separately excited dynamo did not come into favor until, in 1866, Wilde employed a small auxiliary magneto machine to furnish currents to excite the field magnets of a larger dynamo.

A separately excited dynamo is shown in fig. 198. This method of field excitation is seldom used except for alternators; it is, however, to be found occasionally in street railway power houses, the shunt fields of all the dynamos being separately excited by one dynamo.

In common with the magneto, the separately excited machine possesses the property that, with the exception of armature reactions, the magnetism in its field and therefore the total voltage of the machine is independent of variations in the load.

Fig. 199.—Diagram showing principle of Dobrowolski three wire dynamo. This type of machine is shown in more detail in fig. 795 on page 708.

Dobrowolski Three Wire Dynamo.—This type of dynamo was designed to operate a three wire system of distribution without a balancer. The armature is provided with insulated slip rings connected to suitable points in the armature winding and (by means of brushes) with choking coils meeting at a common point, to which the neutral wire of the system is connected, the main terminals being connected with the outside wires.

The machine is capable of feeding unbalanced loads without serious disturbance of the pressure on either side of the system.

The principle of the Dobrowolski three wire dynamo is illustrated in fig. 199. The armature A is tapped at two points, B and B′, and connected to slip rings C C′. A compensator or reactance coil D, between the two halves of which there is minimum magnetic leakage, is connected to C and C′ by brushes, and has its middle point tapped and connected to the neutral wire E.

Fig. 200.—Armature of Westinghouse three wire dynamo. Collector rings are mounted at one end of the armature as shown, and the leads to them with the armature winding are similar to those employed on the alternating current side of a rotary converter armature. The connections from the armature to collector rings may be either single phase, two phase, or three phase. The two phase connection with four collector rings and two balance coils is used in the Westinghouse three wire dynamo.

It is clear, from the symmetry of the arrangement, that the center point of the coil must always be approximately midway in pressure between that of the brushes, and hence any unbalanced current will return into the armature, dividing equally between the two halves of the coil.

The arrangement forms a cheap and effective substitute for a balancer set, but lacks the adjustable properties of the latter.

There are various modifications of the arrangement. Thus more than two slip rings may be used. The compensator windings, however, should always be arranged so that the magnetizing effect of the neutral current is self-neutralized in the windings, as otherwise saturation occurs causing a very heavy alternating magnetizing component.

CHAPTER XVI

FIELD MAGNETS

Ans. The yoke serves to connect the two “limbs,” that is, the cores and pole pieces, and thus provide a continuous metallic circuit up to the faces of the pole pieces.

Fig. 201.—Salient pole, bipolar field magnet with single coil wound around the yoke.

Ans. The cores, which are usually of circular form, carry the coils of insulated wire used to excite the magnets.

Classes of Field Magnet.—Although numerous forms of field magnet have been devised, they can be classed into two groups according to the type of pole, as:

The distinction between these two types of pole is shown in figs. 201 to 203. By inspection of the figures, it will be seen that the term salient applies to poles produced when the pole pieces form the ends of the magnet, as distinguished from consequent poles, or those formed by coils wound on a continuous metal ring or equivalent.

In the salient pole bipolar magnet, the winding may be either upon the limbs, M M fig. 202, or upon the yoke, Y as shown in fig. 201. The magnetic circuit of salient and consequent poles is indicated in the figures by the dotted lines.

Fig. 202.—Salient pole, bipolar field magnet with two coils wound around the cores.

Fig. 203.—Consequent pole, bipolar field magnet with two coils on the cores. This is known as the “Manchester” type in which the cores are connected at the ends by two yokes—so named from its original place of manufacture at Manchester, England.

Multi-Polar Field Magnets.—In the multi-polar machine, the subdivision of the magnetic flux reduces the amount of material of both magnet and armature. Moreover, there is less heating on account of the greater capability of dissipating the heat, offered by the increased area of surface per unit of volume in each magnet pole and winding.

Fig. 204.—Modern dynamo with four consequent pole field magnets. In this construction the ring shaped yoke also serves as a frame; the circular form of yoke gives the least chance for magnetic leakage.

There may be four, six, eight, or more poles, arranged in alternate order around the armature. Fig. 204 shows a four pole field magnet having a common yoke or iron ring, with four pole pieces projecting inwardly, and over which the exciting coils are slipped.

In the larger machines the yoke is made in two parts bolted together as shown in fig. 206, so that the upper portion may be lifted off for examination of the armature.

Ques. Can the number of poles in a multi-polar machine be advantageously increased to 16, 32, or more?

Ans. A large number of poles is not advisable except in very large machines, since it involves an increase in the expense of machine work, fittings, etc., somewhat out of proportion to the reduction in cost of material and increase in efficiency.

There are a number of considerations which govern the selection of the materials to be used in a particular machine, such as initial cost, weight, efficiency, etc.

Figs. 205 and 206.—Solid and split construction of yoke for multi-polar dynamos. In the latter type the yoke is in two halves joined along a horizontal diameter; while the upper half may be conveniently removed to give access to the armature, it has the disadvantage of the joint, which, no matter how well made, will add to the reluctance of the magnetic circuit. The figures also illustrate the circular and segmental forms of yoke construction.

Ques. In the construction of field magnets, what governs the choice of materials?

Ans. For cores, wrought iron is most desirable, as requiring the smallest amount of material for a given flux. There is a saving in copper due to using wrought iron for the core since, on account of its small size, the length of each turn of the magnetizing coil is reduced. For heavy yokes, where lightness is not essential, but very often the reverse, cast iron is used, as its cross section can be made larger than that of the cores, this increase in area serving to give strength and rigidity to the machine. Cast steel occupies a place intermediate between cast iron and wrought iron both in cost and magnetic properties.

Figs. 207 to 209.—Various sections of cast iron yoke. In form, these yokes may be either circular or segmental as shown in figs. 205 and 206.

Figs. 210 to 212.—Various sections of cast steel yoke. The ribs shown in figs. 210 and 211 are provided to secure stiffness.

Ans. To permit access to the armature, where the construction does not admit of removal of the latter from the side.

Ans. Across its horizontal diameter into an upper and lower half, as shown in fig. 206, the lower half being seated on, or more frequently cast in one piece with the bed plate.

Ans. The joints introduced, even if carefully faced and well bolted together, add a little reluctance to the magnetic circuit.

Figs. 213 to 215.—Some methods of attaching detachable cores. The core seat is machined to receive the core, it being necessary to secure good contact in order to avoid a large increase in the reluctance of the magnetic circuit.

Ques. How does this affect the poles adjacent to the points, and what provision is made?

Ans. It weakens them, and in order to overcome this, the coils of these poles are given a few extra turns.

Ans. By enlarging the area of contact; the flange for the bolts furnishes the necessary increase.

Ans. Since considerable cross sectional area of core is required, the problem confronting the designer is to design the core by judicious selection of material and shape, that the required number of turns in the magnetizing coil is obtained with the shortest length of wire.

Figs. 216 to 221.—Comparison of field magnet core sections. The shorter the perimeter or outside boundary of the core for a given cross sectional area, the less will be the amount of copper required for the magnetizing coils. All the above sections are of equal area, and the figures marked on each represent relative values for the perimeters, the circle for convenience being taken at 100.

Ques. What is the principal objection to the use of cast iron for core construction?

Ans. Since its sectional area must be considerably more than wrought iron, a much greater quantity of copper is required for the magnetizing coils.

Copper is expensive, while cast iron cores are less expensive than equivalent ones of wrought iron; in this connection, it is interesting to observe how different designers aim at true economy in construction.

Steel is sometimes used in place of wrought iron, and though less efficient magnetically, it can be cast into the desired shape, thus avoiding the somewhat expensive processes of forging and machining, which are necessary in the case of wrought iron.

Ques. What form of core requires the least amount of copper for the magnetizing coils, and why?

Ans. The cylindrical core, because it has the shortest periphery or boundary for a given area enclosed.

Figs. 222 to 225.—Several forms of pole piece. Where the extremities project as in figs. 222 and 223, they are called horns. The object of these is to reduce the reluctance of the air gap. The width of “fringe” of the magnetic field is influenced by the shape of the pole piece; the margin of fringe should be such that the flux density will vary from zero to a high value where the inductors enter.

Figs. 216 to 221, show a series of cross sections, all of the same area. The number marked on each section indicates the length of the boundary line, that of the circle being taken for convenience as 100.

Ans. These are the end portions of the field magnets, joined to, or cast together with the core and placed adjacent to the armature.

The faces of the pole pieces are of circular shape, thus forming the sides of the so-called armature chamber within which the armature rotates.

Fig. 226.—Unsymmetrical pole piece introduced by Gravier to concentrate the magnetic field. When the dynamo is working at small loads, the flux in the gap is nearly uniform, but at heavy loads, the distortion due to the armature current forces the flux forward and saturates the forward horn, thus preventing much change in its flux density, on account of the saturation, and the diminishing area. Lundell combined the unsymmetrical and slotted forms of pole piece as shown in fig. 237.

Ans. In order to reduce the reluctance of the air gap between the face and the armature, thus enabling fewer magnetizing coils to be used.

Fig. 227.—Pole piece with oblique slots; a modification of Lundell’s form of pole piece as suggested by Thompson. In operation, the neck of the casting becomes saturated and offers considerable reluctance, which tends to prevent distortion of the magnetic field.

It is important that the field should be magnetically rigid, that is, not easily distorted. This stiffness of field can be partially secured by judicious shaping of the pole pieces. A few forms of pole piece are shown in figs. 222 to 231.

If the projecting tips of the pole pieces, or horns as they are called, be widely separated, as in fig. 222, they are not always good, even though thin. It is better that they should be extended as in fig. 223 so that they may be saturated by the leakage field or else cut off as in fig. 224.

An extreme design, suggested by Dobrowolski, as shown in fig. 225, surrounds the armature with iron.

Fig. 228.—Non-concentric pole faces; one method of securing suitable magnetic “fringe” with fair magnetic rigidity of field.

Figs. 229 to 231.—Various shapes of pole piece for securing a gradual entrance of the armature inductors into the magnetic field.

Another scheme, proposed by Gravier, employed the unsymmetrical form shown in fig. 226. In this pole piece the forward horn is elongated. The action due to this arrangement is such that when the machine is working at small loads, the field in the gap is nearly uniform, but at heavy loads with distorting reactions which have a tendency to drive the flux into the forward horn, the small section of the latter causes it to become saturated, thus reducing the distortion to a minimum.

Eddy Currents; Laminated Fields.—The field magnet cores and pole pieces, as well as the armature of a dynamo are subject to eddy currents, that is, induced electric currents occurring when a solid metallic mass is rotated in a magnetic field. These currents consume a large amount of energy and often occasion harmful rise in temperature. This loss may be almost entirely avoided by laminating the pole piece, or both pole piece and core; in the latter case, both form one part without any joint.

Fig. 232.—Illustrating the alteration of magnetic field due to movement of mass of iron in the armature. If the masses of iron in the armature are so disposed that as it rotates, the distribution of the lines of force in the narrow field between the armature and the pole piece is being continually altered, then, even though the total amount of magnetism of the field magnet remain unchanged, eddy currents will be set up in the pole piece and will heat it. This is shown in the above figures, which represent the effect of a projecting tooth, such as that of a Pacinotti ring, in changing the distribution of magnetism in the pole piece.

Fig. 233.—Eddy currents induced in pole pieces by movement of masses of iron. These diagrams, which correspond to those of fig. 232, show the eddy currents grouped in pairs of vortices. The strongest current flows between the vortices and is situated just below the projecting tooth, where the magnetism is most intense; it moves onward following the tooth. At C is shown what occurs during the final retreat of the tooth from the pole piece. These eddy currents penetrate into the interior of the iron, although to no great depth. Clearly the greatest amount of such eddy currents will be generated at that part of the pole piece where the magnetic perturbations are greatest and most sudden. A glance at the figures shows that this should be at the forward horn of the pole piece. However, when a dynamo, with horned pole pieces, has been running for some time as a motor the forward horns are cool and the hindward horns hot.

Fig. 234.—Fort Wayne laminated pole piece before being cast welded into frame. In the faces of solid pole pieces there exist minute electric currents called eddy currents which cause heating of the iron and increase the energy required to maintain a magnetic circuit in much the same manner as does reluctance. This loss is reduced by dividing the magnetic circuit in the line of flux into numerous parallel paths separated by some material of relatively high resistance. In construction, the above core and pole piece is made up of sheets of annealed steel of two different widths assembled together to form proper size and shape. The minute spacing between these laminations and the slight oxidizing on each surface is sufficient to reduce considerably the eddy currents. By cast welding the pole piece into the frame, a low reluctance is secured.

Ans. One built up of layers of iron sheets, stamped from sheet metal and insulated, as shown in fig. 234.

Ans. It is a most approved method, and one frequently employed in the construction of cores and pole pieces.

Fig. 234 shows a combined core and pole piece made entirely of sheet iron punchings assembled and riveted together, and fig. 235, a core to be used with separate pole piece. It should be noted that in both cases there is a longitudinal slot extending from the end into the core. This was first suggested by Lundell, the object being to prevent, as far as possible, the distortion of the magnetic field due to armature reaction especially on heavy overloads.

Fig. 235.—Fort Wayne laminated core without pole piece, as used on large dynamos. It is constructed of punchings from sheet iron, and riveted under pressure. The alternate end projections and grooved base insure good mechanical union of metal in cast welding to magnet frame. Reluctance between core and yoke is reduced to a minimum by cast welding. The core is slotted parallel with the shaft to prevent, as far as possible, the distortion of the magnetic field, especially on heavy overloads.

Ques. What mode of construction is adopted to reduce the reluctance of the magnetic circuit when laminated poles are used?

Fig. 236.—Fort Wayne one piece frame with cast welded combined cores and pole pieces. In any electrical apparatus a magnetic circuit of low reluctance requires less energy to maintain a given flux than one having a comparatively high reluctance. To reduce this to a minimum the pole pieces and cores are combined into one part and then cast welded into the yoke or frame. Thus the continuity of the magnetic circuit is practically unbroken save for the air gap.

The frame end of the core as shown in the illustrations has irregularities in the heights of the different sheets, as well as grooved undercut surfaces, in order to enable the molten metal of the frame to key well into the laminations of the core, making a good joint, both mechanically and electrically. By this construction, the continuity of the magnetic circuit is practically unbroken save for the air gap between the pole piece and armature.

Fig. 236 shows a one piece frame of a six pole dynamo having cast welded into it, combined cores and pole pieces.

Ans. It necessitates a nearly square or rectangular section, which requires more copper for the winding than the cylindrical form.

Fig. 237.—Lundell type of combined core and pole piece; a combination of Gravier’s unsymmetrical horns and longitudinal slot designed to prevent distortion of field.

The Magnetizing Coils.—The object of the magnetizing coils, is to provide, under the various conditions of operation, the number of ampere turns of excitation required to give the proper flux through the armature to produce the desired electromotive force.

With respect to the manner in which magnetizing coils are wound they are said to be:

Ans. The spool is made in various ways, sometimes entirely of brass, or of sheet iron with brass flanges, or of very thin cast iron. Some builders use sheet metal with a flange of hardwood, such as teak. If a spool be simply put upon a lathe to be wound, the inner end of the wire, which must be properly secured, should be brought out in such a way that it cannot possibly make a short circuit with any of the wires in the upper layers. To avoid this difficulty, the wire is sometimes wound on the spool in two separate halves, the two inner ends of which are united, so that both the working ends of the coil come to the outside as shown in fig. 238.

Fig. 238.—Method of winding magnet spool so that the two ends of the coil will come to the outside. This method has also been used for induction coils, where it is desirable to keep the ends of the wire away from the core and primary coil.

Ans. Former wound coils are wound upon a block of wood having temporary flanges to hold the wire together during the winding. Such coils have pieces of strong tape inserted between the layers and lapped at intervals over the windings to bind them together. Coils are usually soaked with insulating varnish and stove dried.

Ans. Several methods of bringing out the ends of coils are shown in figs. 238 to 241. In fig. 239 copper strip, laid in behind an end sheet of insulating material, makes connection to the inner end, as shown in the right side of the figure, while another strip, shown on the left side similarly inlaid, serves as a mechanical and electrical attachment for the outer end of the winding.

Fig. 239.—Core and edge strip winding for shunt field coils of large multi-polar dynamo. The winding consists of a copper strap S carefully insulated and placed edgewise on the core C in a single layer of winding. With this arrangement, the space occupied by insulation is reduced to a minimum, and, although the cooling surface is small, each turn of the winding has one edge on the outer surface, being ample for adequate cooling.

Two other methods are shown in figs. 240 and 241. A simple device for securing the outer end is to fashion a terminal piece so that it can be laid upon the winding, the last three or four turns of which are wound over its base, and after winding, are bared at the place and securely soldered.

Ans. The spools upon which the coils are wound are usually insulated with several layers of paper preparations; a thickness of one-tenth of an inch made up of several superposed layers is generally sufficient. Varnished canvas is useful as an underlay, and vulcanized fibre for lining the flanges. It is important to protect the joint between the cylindrical part and the flanges. A core paper may be laid upon every four layers of winding. Between series and shunt coils, in compound wound machines there should be an insulation as efficient as that on the cores. When the winding is completed, two layers of pressed board or equivalent are laid over and bound with an external winding of hard rope or tape. This protective external lagging covering the outer surface of the completed coils is not altogether a benefit for it tends to prevent dissipation of heat.

Fig. 240.—One mode of bringing out the coil ends, in which copper strip is laid in behind an end sheet of insulating material.

Fig. 241.—Another mode of bringing out the coil ends. A narrow insulated strip of thin copper G, leading to terminal H, is connected with the end e of the coil before winding.

Figs. 242 and 243.—Square and hexagonal order of “bedding.” The term bedding is an expression used to indicate the relation between the cross sectional area of the winding when wound square, as in fig. 242, and where wound in some other way, as in fig. 243. In the square order of bedding, the degree of bedding equals zero.

Fig. 244.—Method of securing coils in position when the pole pieces are simply extensions of the core without enlargement.

Ans. Where the pole pieces are simply extensions of the cores without enlargement, the coils can be slipped over the ends, but some kind of clamping device is necessary to hold them in place, as for instance, the method shown in fig. 244.

In case the pole piece be made larger than the core and separate therefrom, it is put into position after the coils are in place, thus serving the double purpose of pole piece and clamp.

Ans. Coils are generally united in series so that the same magnetizing current may flow through all of them. The coils should be so connected that they produce alternate north and south poles.

If all the coils be similarly wound with respect to the terminals, and similarly placed; that is, so placed that the winding, considered from the coil terminal nearest the pole face, starts in all the coils in the same direction, then the connections will come at the north end and at the south end of the spools.

Fig. 245.—Western Electric set of former wound field coils for four pole dynamo. These coils are wound around a former or template, and are then slipped over the cores before the latter are bolted to the yokes or frame.

Heating.—The heat generated in the magnetizing coils is dissipated in three ways; by:

In the first instance, it passes through the copper and the insulation, either to the external surface, whence it passes off by radiation and convection into the air, or to the magnet core and yoke, which in turn conduct it away. In large multi-polar machines the masses of metal in the pole cores and frame are more efficient in dissipating heat than the external surface of the coil.

Fig. 246.—Fort Wayne compound wound rectangular ventilated spool field coil. The series and shunt coils are wound side by side, ventilating passages being provided lengthwise through each coil and between the shunt and series coils as shown.

Ventilation.—Sometimes provision is made for ventilation of the field magnet coils as shown in fig. 246. Here the series and shunt coils are wound side by side, ample ventilation being provided lengthwise through and between the coils.

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ELECTRICAL GUIDE, NO. 1

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

ELECTRICAL GUIDE, NO. 2

CHAPTER XVI FIELD MAGNETS

CHAPTER XVI

FIELD MAGNETS