Dynamo
DYNAMO-ELECTRIC MACHINE, or GENERATOR, a machine for transforming mechanical into electrical energy, and depending for its operation on the electro-motive force developed in any conductor moved trans- DYNAMO versely through the lines of force in a magnetic field. The manner in which the energy transformation is effected distinguishes the dynamo from the old frictional electric machine, and determines its general plan of construction. Any dynamo must consist of at least two parts, the field-magnets which create the magnetic field, and the armature which comprises the conducting system which moves relative to the field.
To these may be added the commutator, a device necessary to secure uniform direction in the case of direct current machines. Dynamos for direct current are designed invariably with moving armature and fixed fields. In the case of machines of comparatively small capacity, the field magnet may be bipolar, i. e., having one pair of poles, as in the horseshoe magnet. In fact, permanent steel magnets of this form constitute the fields of the little machines known as magneto-electric machines, or magnetos, such as are used in automobiles, in telephony, etc. Bipolar machines may be either of the overtype or undertype pattern, according as the space between the poles is arranged above or below the yoke, which corresponds to the bend of the horseshoe. The former arrangement is practicable only with smaller sizes, as it necessitates longer pedestals for supporting the bearings of the armature shaft, leading to excessive vibration. The undertype has the disadvantage that the field magnets must be supported clear of the iron bed-plate by brackets of non-magnetic material, such as brass or gun-metal, in order to prevent passage of the field through the bed-plate in preference to the armature.
In any dynamo, the current passing through the moving armature causes a distortion and weakening of the magnetic field. This effect becomes very apparent in bipolar machines at high loads, and the serious disadvantage may be largely avoided by increasing the number of pairs of poles in the fieldmagnet. Multipolar machines for continuous current may have as many as twelve pairs of poles. This form of construction has, moreover, the advantage of enabling material to be more economically arranged, thus securing relatively lighter weight; and, in addition, the speed of rotation may be reduced in inverse proportion to the number of pairs of poles for a given E.M.F. developed. These considerations have established the practice of constructing all machines for more than 150 kilowatt output of the multipolar type, and machines of considerably smaller capacity than this are regularly built with three or four pairs of poles.
DYNAMO In all cases the poles of the fieldmagnet are fashioned so as to embrace as large a portion of the armature cir- cumference, with as small an air-gap, as may be practicable. To effect this, as large a portion of the armature circumference, with as sma small an air-gap, as may be practicable. To effect this, soft-iron cheeks or pole-pieces are commonly fixed to the shanks which carry the magnetizing coils. Cross-magnetization of the poles due to armature reaction is often prevented by a deep narrow slot across the curved face of each pole-piece, paralleling the direction of the lines of force in the field.
There are several methods of arranging for the magnetization of the fields, which it will be convenient to defer until there has been given some description of the armature and its construction. what The principle underlying this may the armature and its construction. best be understood by considering what takes place when a single rectangular The principle underlying this may by considering wh takes place when a single rectangular frame or loop conductor is revolved about the longer diameter in a magnetic field of parallel lines of force. We may imagine the axis or shaft, about which the rectangular frame is rotated, to cross the field at right angles, so that the two long sides of the rectangle parallel to the axis are continually cutting lines of force as the frame revolves. The two short sides do not cross the lines of force at all, but simply slide through them, and have, therefore, no E.M.F. actually induced in them. They serve only to complete the electrical system, so enabling any E.M.F. induced in the active (long) sides to produce a current in the system. The conductor must be imagined as insulated from the shaft.
At the moment when the frame is at right angles to the direction of the field, no lines of force are being cut, and the E.M.F. induced in both active sides, and consequently the current in the system is zero.
As, during the course of a quarter revolution (90°) the plane of the frame becomes parallel to the direction of the field, more and more lines of consequently the current in the system is zero. As, during the course of a ually increasing magnitude is induced in both long sides of the frame.
As the motions of these two active sides during this, and each subsequent 90° of revolution, are in opposite directions with regard to the field, the abthe frame becomes parallel to the direc- tion of the field, more and more lines of force are cut, and an E.M.F. of contin- ually increasing magnitude is induced in one side will be opposite to that induced in the other. However, it will be seen that both induced E.M.F.'s are in the same cyclic direction round the closed frame. The resultant sum of these components will give, therefore, a total E.M.F. and current for this and each 90° of revolution, in a definite direction, depending upon the direction of rotation relative to the polarity of the DYNAMO The maximum value of the induced arcs of revolution which complete the entire revolution of 360°, the actual di- rection of rotation of the frame of course remains the same. However, the relative position of the active sides E.M.F. and current is attained when the plane of the frame lies parallel to the while remaining in the same direction round the frame until the second 90° is completed, when the total E.M.F. and current again become zero.
During the next two successive 90° arcs of revolution which complete the entire revolution of 360°, the actual direction of rotation of the frame of same.
However, course remains the the resultant induced E.M.F. and curthe relative position of the active sides is now reversed, as is also the direction of rotation of each relative to the field.
In consequence of this the direction of The changes in magnitude half of the revolution will be opposite rent in the frame during the second to that during the first half revolution considered. Th from zero to maximum and back to zero take place as before.
The effect of continuous rotation of the frame is therefore to create surges of current in alternate directions for each revolution. The complete change from zero to the maximum in one direction, back to zero, and again through a maximum in the opposite direction, back number of cycles per second. frame conductor to be modified, by leav- Suppose the form of the rectangular ing one short side open at the middle or periodicity, and is measured by the number of cycles per second.
Let the open Suppose the form of the rectangular frame conductor to be modified, by leavon the shaft so as to be insulated from ing one short side open at the middle where the shaft crosses. have a very elementary form of alterends be led out along the shaft, and by rotating the frame and using each electrically connected to one of two and by conducting rings mounted side by side it and from each other. We should then nating current dynamo, or alternator, collecting brushes pressing on the rings, could lead an alternating current away to an external circuit. split into two Instead of two collecting rings, we can arrange one ring split we can arrange to reverse the segments to an external circuit. can arrange one ring split into two halves, each segment insulated from the other and from the shaft. By connect- Instead of two collecting rings, we other and from the shaft. By connecting the open ends of our rectangular conductor to these two segments, and by we can arrange to reverse the segments under each brush simultaneously with the reversal in the direction of current in the conductor. By this means we obtain an elementary direct current dynamo, the split ring constituting the simplest form of commutator.
The practical construction of an arprinciple. To build up the induced mature is based upon the foregoing DYNAMO E.M.F., insulated conductors are wound in coils about an internal coil of soft iron, which serves to concentrate the lines of force within the coils.
To avoid the loss of power and the heating effect due to eddy-currents induced by the field in the core itself, this must be laminated, or built up on the shaft of the stampings, insulated from one another by shellac. In large machines, the sections are often pierced in addition, so as to form channels, and by means of distance pieces left on the shaft at intervals when assembling, ventilation and dissipation of internal heat are secured.
The simplest form of armature so constructed is the shuttle or Siemens' armature, consisting of a simple coil of many turns. This form has its practical limitations, the principle being the fluctuation of voltage during each revolution when used in a direct-current machine, and the tendency to self-induction which increases very rapidly with the number of turns, when used in an alternator, thus limiting the voltage capacity of the machine. Both these defects are practically obviated in the drum armature. The core of this is cylindrical, being built of thin insulated discs of charcoal-iron keyed to the shaft. Slots are cut at intervals along the curved face parallel to the axis of the drum, in which are arranged the insulated conductors. This construction allows of a very small air-gap between armature and pole-pieces, which has the effect of reducing the number of conductors necessary for a given capacity. For connecting together the straight segments in the slots, special connecting pieces of thin sheet copper, semi-circular in form, and suitably insulated, are laid side by side round the shaft, the connecting lugs at their ends forming thus a spiral at each end of the commutator. These are necessary to economize space at the ends of the drum, and also to enable individual segments to be conveniently withdrawn, if necessary, for replacement. Each coil is arranged diametrically to the drum, and the whole number is arranged in one series round the armature.
The commutator for such an armature consists of a cylindrical ring, built up of bars of hard-drawn copper insulated from one another by means of mica spacing pieces, and carried on a cast iron sleeve in such a way as to be mechanically stable and insulated from the shaft. The section and length of the bars is determined by the capacity of the machine, and the maximum current density that is found practicable in leading off the current to the brushes.
DYNAMO The commutator must have as many segments as there are coils in the armature, and connections to the segments are made in turn at successive junction points of continuous coils in the series.
Brushes are of laminated copper, copper gauze embedded in carbon, or carbon alone. They are carried on rocker arms which allow of some adjustment in position round the commutator circle, this being necessary to avoid sparking at the brushes when the load is varied. The holders for the brushes are designed also so that the brushes may be fed radially to the commutator as they wear away, and individual units withdrawn for replacement. are In the Gramme, or ring armature, the lamine are ring-shaped, and supported on a framework of brass or gun-metal, keyed to the shaft. Round the hollow cylinder thus formed, the coils wound, the return winding being passed through the inside of the cylinder, all coils being in series and connected to the commutator segments as in the drum armature. This form of armature is suitable only for small size machines, being weak mechanically. It is, moreover, harder to wind, and carries a much larger proportion of inactive conductor than in the drum pattern, since only those segments outside the cylinder contribute to the total E.M.F.
The same general principles apply on the whole as to direct-current machines.
The manner of connecting contiguous coils on the armature is different, and of course no commutator is required, its place being taken by collecting rings.
The absence of the commutator simplifies matters considerably, and in particular enables high voltages to be developed, a matter of considerable economic importance in connection with power transmission over long distances. The necessity of having alternating current of minimum periodicity corresponding to about 60 cycles per second for lighting purposes leads to the adoption of multipolar fields, thus avoiding high rotating speeds. It is quite practicable, and indeed advantageous, to reverse the relative position of armature and field magnets, having the former stationary and the latter revolving, and most modern alternating machines are built in this way.
Regarding the magnetization of the field magnets, alternators are dependent for this purpose on direct-current machines; in some cases these exciters are built on the shaft of the alternator, but it is more convenient to have them independent. Direct - current machines may also be classified as separately excited, in cases where the magnetizing DYNAMOMETER coils of the fields derive their current from a source external to the machine. Another method is to lead the current in the external circuit round the field magnets. Such an arrangement constitutes a series dynamo, and in such the building up of the field and the induced E.M.F. depend on the residual magnetism in the field magnets. Series machines are not practicable except when only small deviations from constant load occur. A shunt-round machine is one in which the field magets are wound with coils arranged as a high resistance shunt in parallel with the external cir- cuit. Machines of this class are much less affected than the series machines, by variations in the external circuit.
By combining both the series and shunt winding on the one machine there is obtained the compound-wound machine, which is practically sel self-regulating as regards voltage, with all changes in the external load.