Figs. 314 to 318.—Brush contact resistance theory of commutation, neglecting self-induction and resistance in the coils. The total current is assumed to be 40 amperes made up of 20 amperes flowing toward the brush from the coils on the right and 20 amperes from the coils on the left. During commutation, that is, the interval during which the brush contacts with any two adjacent segments of the commutator, the current is assumed to vary directly as the contact area.
Fig. 314.—Beginning of commutation; segment A is entirely under the brush, and B is at the initial point of contact. For this position the currents from both sides flow to the brush through segment A.
Fig. 315.—One-quarter period of commutation. One quarter of the brush area is in contact with B and three quarters in contact with A; hence, 10 amperes will flow through B and 30 amperes through A.
Fig. 316.—Second quarter of commutation period. The brush now contacts equally with both segments, hence 20 amperes will flow through each segment.
Fig. 317.—Third quarter of commutation period. Three quarters of the brush area is in contact with segment B and one quarter with segment A; accordingly, 30 amperes will flow through B and 10 amperes through A.
Fig. 318.—Completion of commutation. The brush is in full contact with segment B and at the point of breaking contact with A, hence the entire current from both sides or 40 amperes will flow through B.
Figs. 319 to 323.--Brush contact theory of commutation for case in which the brush covers two segments of the commutator. Fig. 319 beginning of commutation; fig. 320 one-quarter period; fig. 321, one-half period; fig. 322, three-quarter period; fig. 323 completion of commutation.
Ques. What is the effect of increasing the degree of contact of the brushes?
Ans. It lengthens the period of commutation, and permits it to start in one coil before the preceding coil has entirely passed through this stage.
Construction of Commutators.—The commutator for a closed coil armature consists of a number of segments or L-shaped bars C of drop forged hard-drawn copper assembled around a tubular iron hub as shown in figs. 324 and 325. The bars are held in position by the nuts E, and washers F, screwed on the ends of the tube D. The bars are insulated from each other and from the washers by mica as shown by the heavy lines G, and they are also insulated from the tube either by a tube of mica H, or by a sufficient air space. The ends of the sections of winding are connected to the vertical portions of the bars K, by insertion in the slots L, where they are securely held in place by means of the binding screws, which for greater security are soldered together, and may be released from the slots, whenever necessary, by the application of a hot soldering iron.
Figs. 324 and 325.—Side and end sectional views of commutator showing construction. The parts are: C, segments; D, tubular iron hub; E, end nuts; F, clamps; G, insulation; L, riser connection.
It is very important that all the parts of the commutator should be fitted together perfectly and screwed up tightly, in order to prevent looseness. Commutator segments are often made with the washers E, projecting beyond the ends, but such construction reduces the effective length of the commutator, therefore the under cut form of bar is preferable.
In the construction of commutators, the conditions of operation require that there be:
1. Adequate insulation;
It is necessary to have good insulation between each segment, and a specially good insulation between the segments and the hub or sleeve on which they are mounted; also between the segments and end clamps. The insulating material must not absorb moisture, hence asbestos, plaster, or vulcanized fibre are not used. The end insulating rings are usually built up of mica and shellac, moulded while hot under pressure to the correct shape.
Fig. 326.—Front view of Western Electric Commutator for bar wound armature. This commutator is made of hard-drawn copper and insulated throughout, ventilating spaces being provided near the shaft.
2. Rigidity against centrifugal force;
Since the segments are subject to centrifugal force, they must be securely clamped in place. Screws cannot be used, for that would destroy the insulation. They are therefore held in place by insulated clamps as shown in fig. 324. These clamps should be strong and capable of holding the segments firmly in position, for if a segment should rise out of its place through centrifugal force, it would disturb the action of the brush and cause sparking.
3. Provision for wear.
The segments should be of considerable radial depth, so that the commutator may be turned down from time to time to preserve its circular form.
Fig. 327.—Sectional view of a General Electric commutator. The segments are of rolled or forged copper and are separated by soft mica insulating sheets. This mica must wear down evenly with the copper, hence its consistency is important. The segments are wedge shaped so that when drawn radially inward they support each other like the stones of an arch. They are drawn together by hollow cone collars which bear upon lugs projecting from the ends of the segments. These lugs are turned to form a smooth cone after the segments are assembled. The collars are insulated with mica from the segments and they are held in place by nuts upon the commutator shell or by bolts passing from end to end under the segments. The segments are also provided with lugs for connection to the windings.
Points Relating to Commutators.—1. The number of commutator segments depends on the scheme of winding and on the number of sections in the armature winding.
2. Increasing the number of bars diminishes the tendency to spark, and lessens the fluctuations of the current.
There are two practical reasons for not using a very great number of segments: it increases the cost, and in small machines the segments would be so thin that a brush of the proper thickness to collect the current would lap over, or bridge several segments.
Types of Commutator.—Commutators are made in various forms, but they may be grouped into two general types:
1. Commutators for closed coil armatures;
These consist of a large number of segments or bars, insulated from each other and varying in number according to the scheme of armature winding, and on the number of sections into which that winding is grouped.
Fig. 328.—A large current low voltage bipolar dynamo built for electrolytic work and here shown to illustrate the large size commutator and brushes necessary to collect the large current. Carbon brushes would not be suitable for this class of machine because even with copper brushes, whose conductivity is much higher than carbon, the commutator must be of considerable size to give the required brush contact area. The contrast between the axial lengths of the armature and the commutator is very marked. The rocker construction is of the ordinary type, and heavy flexible cables conduct the current from the brush holders to the fixed terminals. The machine here illustrated gives 310 amperes at 7 volts when running at a speed of 1400 R. P. M., corresponding to an output of 2.17 kilowatts.
2. Commutators for open coil armatures;
This form of commutator is used on some machines designed especially for arc lighting, such as the Brush and Thomson-Houston machines. They consist of a comparatively small number of segments each of which covers a wide angle, and are separated from each other by air gaps.
3. The segments should be of considerable depth to permit returning occasionally so that their circular form may be preserved;
4. The insulating material must be such that it will not absorb oil or moisture;
Mica is best adapted for insulation, but as there are a great many varieties, differing greatly in hardness and other equalities, it is important to select the kind that wears at the same rate as the segments. If the mica be too hard, the wearing of the segments will leave it projecting and prevent proper contact with the brushes; again, if the mica be too soft, it will result in furrows or depressions between the segments into which copper dust will collect, causing short circuits.
With respect to construction, brushes may be broadly classified as: 1, those made of metal, and; 2, those made of carbon. There are several varieties of metal brush, such as:
1. Gauze brushes;
2. Wire brushes;
3. Laminated or strip brushes.
Gauze Brushes.—These are very flexible and yielding, their use being attended with little wear of the commutator.
Ques. What is the construction of a gauze brush?
Ans. A gauze brush is made up of a sheet of copper gauze, folded several times, with the wires running in an oblique direction, so as to form a solid flat strip of from ¼ to ½ inch in thickness, increasing with the volume of the current to be collected.
Ques. What is the object of folding the gauze with the wires running in oblique directions?
Ans. It is to prevent the ends of the brush fraying or threading out, which would be the case if the gauze were folded up in any other manner.
Ques. What are the features of gauze brushes?
Ans. They make good contact, but are quite expensive. They may be set either tangentially or radially, the latter preferably, since the point of contact remains the same as the brushes wear away.
Figs. 329 to 332.—Various forms of brush. Fig. 329 gauze brush; fig. 330 laminated or strip brush; fig. 331 strip and wire brush as used on the early Edison machines; fig. 332 carbon brush. Carbon is preferred to copper for brushes on account of the reduction of sparking secured by its use.
Wire Brushes.—This class of brush, which was extensively used before the invention of the gauze brush, is made up of a bundle of brass or copper wires, laid side by side and soldered together at one end. Since wire brushes are harder than the gauze brush, they are more liable to cut or score the commutator, and are also more troublesome to trim.
Laminated or Strip Brushes.—These probably represent the simplest form of brush, but are not extensively used owing to the lack of flexibility. They consist of a number of strips of copper or brass, laid one upon the other and soldered at one end, as in fig. 330.
Fig. 333.—General Electric brush holder. The brush holder yoke consists of a cast iron ring of elliptical section, supported from the bracket of the end shield in such a manner as to facilitate the shifting of the brushes. It is provided with a suitable handle, and may be fastened in any position by means of a thumb nut on the outside of the bracket. It is so constructed that the tension on the individual brush can be adjusted without lifting the brush from the commutator and without the use of tools. The brush can be removed while the machine is running, without moving the holder on the stud and without disturbing any other brush. Removal of the brushes for inspection can be accomplished without permanent change in the adjustment of the tension of the brush holder spring. The connection between the brush and stud is made through a flexible copper connection.
Ques. What name is generally given to strip brushes?
Ans. They are commonly and erroneously called tangential brushes, but they are really beveled at the end and set inclined to the line of tangency so that the ends of all the sheets will make contact.
In the Brush and Thomson-Houston arc dynamos, in which the current is limited to ten amperes, the brushes consist of a simple strip of flexible sheet copper, the ends of which are slit in a number of places so as to insure contact at several points.
Fig. 334.—Crocker-Wheeler brush holder. The carbon brush B is firmly clamped in the "box" C by two screws which bear on a sheet of brass to protect the carbon from being broken by the ends of the screws. The box C is carried by four flexible springs S S, one at each corner and formed of hard copper leaves. These are fixed at one end to the box and at the other to the solid base which is in one piece with the spoke attached to the rocker ring. An adjusting screw passes through appropriate lugs on the box C and loosely through the head A of a fixed arm a. Between the lower surface of a and the upper lug on the box C is placed the pressure spring.
Carbon Brushes.—When metallic brushes are used upon the commutators of high tension machines, they frequently give rise to excessive sparking and also heating of the armature, the metallic dust given off appearing to lodge between the segments of the commutator, thus partially short circuiting the armature. To obviate this, carbon brushes are extensively used in such dynamos, this material being found very effectual in the prevention of sparking.
Ques. What is the usual form of carbon brushes?
Ans. They are usually in the form of oblong blocks.
Fig. 335.—Perspective views of Crocker-Wheeler brush holder. This holder is of the parallel type in which the brushes may be adjusted without affecting the lead. Each brush is held rigidly in its box and there are no sliding contacts in the path of the current. The holder is further described under fig. 334.
Ques. How are they adjusted on the commutator?
Ans. They are set "butt" end on the commutator, and fed forward as they wear away by means of a spring holder.
Ques. Why are carbon brushes so extensively used?
Ans. Because they are the only form of brush that will give good commutation with fixed lead.
Ques. What may be said of the different grades of carbon in use for brushes?
Ans. The very soft carbon leaves a layer of graphitic matter on the commutator, and at high voltages, this may cause sparking; such grade of carbon should only be used on low voltage machines.
Fig. 336.—Western Electric box type brush holder. The box which holds the brush is broached to allow the brush to slide freely, but not loosely, to and from the commutator against which it is normally held by a lever acting directly upon the brush head. This avoids the possibility of uneven bearing on the commutator, as the brushes are allowed very slight lateral or angular motion. The adjustment of a brush is also simplified after it has been removed and then replaced. Tension on the brush head is obtained by a special spring which maintains any given tension for which it may be set. An auxiliary flat steel spring on the lower side of the lever acts as a shock absorber between the lever and the brush head, absorbing all minor vibrations caused by a worn commutator. Side contact between brush and brush holder is not relied upon to carry the current, flexible copper pigtails performing this function to the exclusion of sliding contacts or tension springs, in order to reduce the brush loss. It is not necessary to take the brush rigging apart or loosen cable connections when it is desired either to remove or reverse the brushes to change the direction of armature rotation.
Ques. How are the ends of carbon brushes treated and why?
Ans. They are usually covered at their upper part with a coating of electro-deposited copper to insure good contact with the holder.
Comparison of Copper and Carbon Brushes.—Copper brushes tend to tear and roughen the surface of the commutator, while carbon brushes tend to keep the surface smooth. Copper causes more wear of the commutator than carbon. With carbon brushes, the armature may be run in either direction. The resistance of carbon being greater than copper, there is less short circuiting caused by carbon particles than by those of copper.
Fig. 337.—Westinghouse brush holder. It is made of brass, cast in one piece, and of standard sliding type with a shunt of braided copper wire directly connected to a clamp on each brush and to the solid portion of the holder, where it is held by a screw. This shunt relieves the spring of heavy currents. The holder is so arranged as to be easily accessible for adjustment, cleaning and renewal of carbons. Proper tension is provided by spiral strap springs so mounted as to eliminate friction and give uniform pressure over a wide working range. The spring tension is readily adjusted by a simple ratchet arrangement.
Ques. What is the chief merit of carbon brushes?
Ans. They give less sparking than other types.
Ques. How has the construction of carbon brushes been varied?
Ans. Since, for minimum sparking, it is only necessary that the brush have high resistance in the region near its edge, attempts have been made to increase the conductivity of the other portions by combining with the carbon, copper sheets or wires.
Ques. What are the objections to carbon brushes?
Ans. They are easily broken and not being flexible, vibration, or any roughness of the commutator will cause bad contact.
Fig. 338.—Holzer Cabot multiple brush holder. Each brush is fastened securely to a machined surface by one or two machine screws, making a positive contact. Several strips of flexible copper of ample section to carry the current are interposed between the part of the holder carrying the brush and the portion clamping the stud, no sliding contact or spring being therefore required to carry any current. The brushes are proportioned for a carrying capacity of not more than 25 amperes per square inch of brush surface. The brush can be adjusted to any degree of tension during the operation of the machine if necessary. Each holder is insulated in such a manner that no short circuit can occur if the holder be accidentally tipped backward while the operator is changing a brush or cleaning the commutator during a run.
Ques. For what class of machine are carbon brushes specially adapted?
Ans. For machines furnishing a small current at high pressure.
When carbon brushes are used, it is desirable that the current be small, because, on account of the low conductivity of the carbon, more contact area is necessary than with copper for equal current transmission. For fixed lead and fluctuating currents, carbon brushes should be used.
Ques. For what class of machine are copper brushes especially adapted?
Ans. For machines furnishing large current at low pressure, as in fig. 328.
Size of Brushes.—The number of brush sets depends upon the number of poles of the machine, but there may be several brushes in each set. It is usual, except in the smallest machines, to place at least two brushes exactly similar side by side instead of one broad brush, thus allowing one brush to be removed for trimming or renewal while the machine is running. Moreover, better contact is secured by this sub-division, because a slight elevation in the commutator surface at one point may slightly raise one brush of a set at each revolution without much harm, while with one broad brush, the entire brush would be lifted, causing bad sparking.
Ques. What determines the number of brushes in each set?
Ans. It depends upon the current capacity, size of machine, and judgment of the designer.
Ques. What may be said with respect to the dimensions of the brushes?
Ans. No general rule can be given for breadth and thickness of brush. The contact face must clearly be wider than the thickness of the insulation between commutator segments, since the period of commutation must last an appreciable interval of time on account of self-induction.
Ques. What should be the minimum width of the brush contact face?
Ans. It may be taken as one and one-half times the thickness of the commutator segments.
Ques. How wide should a carbon brush contact be?
Ans. The brush should be thick enough to cover two and one-half commutator segments. The thickness should in no case be excessive on account of the loss due to heating, which results from the difference of potential at the forward and rear edge of the brush.
Fig. 339.—Contact angle for the different types of brush. At A is shown a brush with tangential contact, and at B, a so called tangent brush; the latter is properly called an inclined brush. Sheet copper brushes are set tangentially as at A, and gauze brushes inclined as at B. Carbon brushes are placed radially as at C when mounted in box holders, and inclined opposite to the direction of rotation when used with reaction holders.
Contact Angle of Brush.—This may be defined as the angle which the brushes make with the commutating plane as shown in fig. 339. The several kinds of brush, together with the varied conditions of operation require different contact angles ranging from zero to 90°.
Thus in the figure, a copper strip brush may lie at 90° or tangentially as at A.
Wire or gauze brushes should make a more or less acute angle as at B, in order to present the end and not the side of the brush to the commutator.
Carbon brushes may be placed end on or radially as at C, which is the position almost universally used in the case of traction or other reversing motors.
Sometimes the carbon brush is inclined as at D, in order that the revolving commutator may tend to push the brush against its supports and thus ensure better contact.
Brush Contact.—The relation between contact pressure, contact resistance, and friction of brushes varies greatly for different kinds of brush. Copper brushes will carry from 150 to 200 amperes per square inch of contact surface; and carbon brushes from 40 to 70 amperes per square inch. The usual contact pressure is 1.25 to 1.5 pounds per square inch for copper brushes, and 1.5 to 2 pounds per square inch for carbon brushes. The rim velocities of commutators vary from 1,500 to 2,500 feet per minute, the velocity usually increasing with the size of the machine.
Fig. 340.—Bissell double brush holder. Flexible cables carry the current between the brushes and holders. This holder works equally well for forward or reverse rotation. Two or more holders are used on each stud except for the two smallest frames. The construction permits of adjustment or renewal of brush while the machine is in operation. Sufficient contact area of brush is provided to permit running on one carbon at ordinary loads in case the other become worn or inoperative.
Ques. What is the drop in voltage at the brushes?
Ans. For carbon brushes it is about 0.8 to 1.0 volt at each contact, or 1.6 to 2.0 volts for the two, positive and negative, contacts of a machine.
This value is not materially affected by placing a number of brushes in parallel or by using several sets, as in the case of multipolar machines, as such arrangement merely reduces the current density, and since the contact resistance varies in the inverse ratio, their product remains nearly constant.
Ques. What may be said of the friction of the brushes?
Ans. The coefficient of friction of brushes is about .2 to .25 for copper and .3 for carbon.
Fig. 341.—Western Electric brush gear. The brush holders carry carbon brushes and are so designed that the brushes may be firmly clamped in position and also be capable of independent adjustment. Any brush can be removed while the machine is in operation without disturbing the others and without moving the holder on the stud.
Ques. How many watts are lost at the brushes?
Ans. The watt loss is equal to 1.6 to 2. volts for carbon multiplied by the total current carried.
The watt lost on account of friction may be calculated by the formula: ((.3 x 746)/33000) × (P x S) = watts lost by carbon friction, in which P is the total pressure in pounds on the commutator, and S, the rim velocity of the commutator in feet per minute.
The losses due to contact resistance and brush friction are very liable to be greatly increased above the values that may be obtained by the preceding methods, if the commutator and brushes are dirty and rough, or not in good condition.
Figs. 342 to 345.—Various types of brush holder. Fig. 342, arm or lever type; fig. 343, spring arm type; fig. 344, box type; fig. 345, reaction type.
Brush Holders.—These are devices employed to hold the brushes against the commutator with the proper pressure. They differ considerably in various types of machine, hence, no general rules can be given with respect to their construction or use, but any brush holder must fulfill the following requirements:
1. It must hold the brush securely and at the same time feed it forward as it wears away so as to maintain a proper contact;
2. It must hold the brush at the proper contact angle;
3. It must be capable of being raised from the commutator, and held out of contact by some form of catch;
4. It must be so constructed that the brush can be easily removed for cleaning or renewal;
5. The spring pressure must be adjustable;
6. The brush holders themselves must be carried on a rocker arm, or rocker ring.
It is desirable that brush holders be capable of individual adjustment, so that each may be set at its own point of minimum sparking. A few forms of brush holder are illustrated in figs. 342 to 345.
The various kinds of brush holder may be divided into four types:
1. Arm or lever type;
2. Spring arm type;
3. Box type;
4. Reaction type.
In the arm or lever type the brush is firmly attached to the extremity of a rigid arm capable of movement about the brush spindle, except in so far as it is restrained by a spring as in fig. 342.
Fig. 343 shows a brush holder of the spring arm type. The brush is firmly attached to the extremity of a spring arm, the other end of which is secured to the brush spindle, and when once adjusted is not capable of movement about the brush spindle.
In the box type of brush holder as illustrated in fig. 344, the brush is free to move up and down in the brush box, so far as it is not restrained by a spring rigidly secured to the arm which carries the brush box at its extremity.
Fig. 346.—Fort Wayne type MPL dynamo; view showing details of armature, commutator and brush rigging of large machine. The laminations of the armature core are punched from thin sheet steel, annealed and japanned. Spacing ribs are built into the core at proper intervals forming air passages for ventilation. In addition, there are recesses in the inside of the flanges which permit the passage of air from the interior around the ends of the core to the openings in the end flanges. The armature coils are constructed of round or bar copper on standard forms. The coils are laid in slots in the surface of the core. The commutator is constructed of bars of hard-drawn copper of uniform size and shape, supported and clamped at either end between beveled rings and securely seated on the commutator drum. The drum is connected by radial arms to the commutator sleeve which is mounted and keyed on the armature hub extension.
Fig. 345 shows the reaction type of brush holder, in which the movement of the brush is constrained in one direction by the surface of a part rigidly secured to the brush spindle, and is further constrained by a spring controlled arm, the pressure of which is capable of ready adjustment.
Among the special forms of brush holder may be mentioned
1. Scissor type of brush holder, used for slip rings, and consisting of two arms pivoted together like a pair of scissors. The lower ends of the arms carry the brushes, suitably mounted, and the upper ends are drawn together by a spring, which thus exerts pressure on the brushes.
2. Clock spring type of brush holder in which the necessary contact pressure is applied to the brush by means of a clock spring, which, with the aid of a ratchet may be wound up and adjusted to any desired pressure.
Fig. 347.—Western Electric brush holder. This holder consists of a rugged iron casting, elliptical in section, and supported from the commutator end bearing bracket in such a manner as to provide for the shifting of the brushes. A handle attached to the yoke aids in this shifting and a thumb nut on the outside holds the whole brush gear in the desired position. The brush is fed through an accurately broached slot by a spring which maintains uniform pressure against the commutator throughout the wearing length of the brush. The long lever arm of the spring is sufficiently flexible to take up any minor vibrations of the brush. The tension of the brush may be adjusted without lifting it from the commutator or disturbing any of the other holders. The brush, may be removed for inspection by throwing the spring out of notch. The brush is connected to the holder by flexible copper pig tails of ample current carrying capacity.
Ques. How are brush holders carried?
Ans. They are carried by a rocker arm for bipolar, and by a rocker ring for multipolar machines, which is mounted upon one of the main bearings, or upon a support specially provided for it, being pivoted to revolve from the same center as the shaft, to permit shifting the brushes.
Ques. Mention one trouble sometimes encountered with brush holders.
Ans. There is sometimes trouble resulting from the current passing through the spring which heats it and destroys its elasticity.
Fig. 348.—Western Electric parallel spring brush holder as used on the larger machines.
Ques. How may this be avoided?
Ans. By insulating one end of the spring, and carrying the entire current directly from the brush itself to the main conductors by a flexible copper strip or cable firmly connected to both.
Ques. What may be said with respect to brush construction on machines for electrolytic work?
Ans. The collection of large currents at low voltage, generated by comparatively small machines, requires careful design of brushes and brush holders. The commutator is longer than the commutators on machines of equal capacity at higher voltages, and as a rule the commutator segments are thicker and fewer in number. Each brush set is made up of numerous narrow brushes rather than two abnormally wide ones.
An example of brush and brush gear designed to meet such conditions is shown in fig. 328.
In large machines for electrolytic work, it is not unusual to find the current divided between two wide commutators, one at each end of the armature, thus giving a longer axial bearing surface for the brushes without inconveniently lengthening the pins upon which the separate brushes are threaded.
Multipolar Brush Gear.—The brush gear which includes the holders and carrier arm or ring, becomes more complicated as the number of poles and magnitude of the current is increased.
In the early days of multipolar machines, schemes of armature winding were devised such that all the necessary cross connections were made inside the machine, and the number of brush holders reduced to two and placed at an angular distance apart depending upon the number of poles. Such windings, though possible, are not used much, chiefly on account of their complexity, which not only increases the danger of error in construction, but also makes repairs costly. In modern multipolar machines, such complicated windings are avoided, and the several sets of brushes are connected together in two groups, positive and negative. These connections are carefully designed as part of the brush gear.
Ques. How are the brushes held in large multipolar dynamos?
Ans. They are held at the proper points of commutation by arms offset from a cast iron rocker ring, which is itself supported by brackets projecting from the magnet yoke as shown in fig. 346.
Ques. What provision is made for shifting the ring to adjust the lead?
Ans. The ring is rotated by means of a worm gear and hand wheel.
The armature of a dynamo has been defined as: a collection of coils of wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field.
From the mechanical point of view the armature may be said to be made up of the following parts:
1. Shaft;
2. Core;
3. Spider
(in large machines);
4. Winding;
5. Commutator
(broadly speaking).
Of the two types of armature, ring and drum, the latter is almost universally used, hence the examples of construction which follow will be confined chiefly to this type.
Shaft.—A typical armature shaft is shown in fig. 349. It is made of steel and, except in the smaller machines, is thicker in the middle than at the ends for stiffness to withstand the strong magnetic side pull on the core when the latter is slightly, nearer one pole piece than the other.
Ques. What is the object of providing shoulders on the shaft as in fig. 349?
Ans. They serve to keep the armature in the proper position with respect to the bearings.
Ques. How is the shaft proportioned?
Ans. If it be proportioned to secure the proper stiffness, it will be found of ample size to resist the twisting strain.
Fig. 349.—Typical shaft for an armature. The illustration shows the keyways for pulley armature and commutator. In the smaller sizes, there is usually a flange at A, and threads at B and C for retaining nuts.
The shaft is subject also to bending by the weight of the armature, by the magnetic drag on its core, and in belt driven machines, by the lateral drag of the pulley. When running, it is also subjected to bending stresses if the armature be not properly balanced. If the bearings do not give, it is evident that all such actions tend to bend the shaft at definite points.
Core.—In the small and medium size dynamos, the core is attached direct to the shaft. There are two kinds of core:
1. Smooth;
2. Slotted.
Ques. What may be said of the smooth type of core?
Ans. It has become obsolete, except in special cases, as for machines used for electrolytic work where a large current at low voltage is required.
Ques. What is necessary with a smooth core?
Ans. Driving horns as later described.
Fig. 350.—Laminated smooth core armature partly assembled. It consists of numerous discs of thin sheet iron threaded on the shaft and pressed together by end plates. The object of this construction is to prevent eddy currents.
Ques. What is a slotted core?
Ans. One having a series of parallel slots, similar to the spaces between the teeth of a gear wheel, and in which the inductors are laid.
Ques. What provision is made to avoid eddy current in cores?
Ans. They are laminated.
Ques. Describe this method of construction.
Ans. The core is made of stampings of thin wrought iron or mild steel. The numerous discs stamped from the sheet metal are threaded on the shaft as in fig. 350, forming a practically solid metal mass.
Fig. 351.—Sectional view of laminated smooth core armature showing end plates, flange and retaining nut. A key is provided to prevent rotation of the core with respect to the shaft.
Ques. How thick are the discs?
Ans. The thickness ranges from .014 inch to .025 inch, corresponding to 27 and 22 B and S gauge respectively, 27 gauge being mostly used.
Ques. How are the discs held in place?
Ans. By two end plates pressed together either by large nuts screwed directly on the shaft as in fig. 351, or by bolts passing through the core from end to end, as in fig. 352, holes being punched in the discs for the purpose.
Ques. What precaution is taken with respect to the core bolts?
Ans. They are insulated from the core by tubes and washers of mica or other insulating material.