592
DYNAMO
A and B differs in phase from the current in the coils C and D by a quarter of a period, or 90J; hence if the two wires b and d be replaced by the single wire bd, this third wire will serve as a common path for the currents of the two phases either outw'ards or on their return. At any instant the value of the current in the third wire must be the vector sum of the two currents in the other wires, and if the shape of the curves of instantaneous E.M.F. and current are identical, and are assumed to be sinusoidal, the effective value of the current in the third wire will be the vector sum of the effective values of the currents in the other wires ; in other words,
if the system is balanced, the effective current in the third wire is n/2, or 1*414 times the current in either of the two outer wires. Since the currents of the two phases do not reach their maximum values at the same time, the sectional area of the third wire need not be twice that of the others; in order to secure maximum efficiency by employing the same current density in all three wires, it need only be 40 per cent, greater than that of either of the outer wires. The effective voltage between the external leads may in the same way be calculated by a vector diagram, and with the above star connexion the voltage between the outer pair of wires a and c is y/2, or 1*414 times the voltage between either of the outer wires Fig. 40. and the common wire bd. Next, if the four coils are joined up into a continuous helix, just as in the winding of a continuous-current machine, four wires may be attached to equidistant points at the opposite ends of two diameters at right angles to each other (Fig. 40). Such a method is known as the mesh connexion, and gives a perfectly symmetrical four-phase system of distribution. Four collecting rings are necessary if the armature rotates, and there is no saving in copper in the transmitting lines ; but the importance of the arrangement lies in its use in connexion with rotary converters, in which it is necessary that the wunding of the armature should form a closed circuit. If e = the effective voltage of one
Fig. 41. phase A, the voltage between any pair of adjacent lines in the diagram is e, and between to and o or w and 'p is /2 • e. The current in any line is the resultant of the currents in the two phases connected to it, and its effective value is N/2 . c, where cis the current of one phase. When we pass to machines giving three phases differing by 120°, the same methods of star and mesh connexion find their analogies. If the current in coil A (Fig. 41) is flowing Threeaway from the centre, and has its maximum value, the phase currents in coils B and C are flowing towards the centre, alterand are each of half the magnitude of the current in A ; nators. the algebraic sum of the currents is therefore zero, and this will also be the case for all other instants. Hence the three coils can be united together at the centre, and three external wires are alone required. In this star or “ Y ” connexion, if e be the effective voltage of each phase, or the voltage between any one of the three collecting rings and the common connexion, the volts between any pair of transmitting lines will be E = v/3. e (Fig. 41) ; if the load be balanced, the effective current C in each of the three lines will be equal, and the total output in watts will be W = 3Ce=^^= 1*732 EC, or 1*732 times the product of the /3 . • effective voltage between the lines and the current in any single line. Next, if the three coils are closed upon themselves in a mesh or delta fashion (Fig. 42), the three transmitting wires may be connected to the junctions of the coils (by means of collecting rings if
[alternators.
the armature rotates). The voltage E between any pair of wires is evidently that generated by one phase, and the current in a line wire is the resultant of that in two adjacent phases ; or in a balanced system, if c be the current in each phase, the current in the line wire beyond a collecting ring is C = N/3. c, hence the watts are W = 3cE = = 1 *732 EC, a/3 as before. Thus any three-phase winding may be changed over from the star to the delta connexion, and will then give 1*732 times as much current, but only 1/1*732 times the voltage, so that the output remains the same. Any of the alternator windings shown in previous diagrams are equally available for polyphase machines, if the width of the coils be altered to suit the number of phases. The field-magnet systems of alternators differ from those of continuous-current multipolar machines only in the employment of a larger number of pairs of poles ;1 hence the alternator piej^ usually has somewhat more copper in its exciting coils, ’. and a slightly greater loss of watts in the excitation.
Four of the most common types of field are shown in Fig. 13, and may be compared wuth Figs. 24 and 25. In Fig. 13 i. the armature rotates and is internal to the poles, while in Fig. 13 iii. it is stationary and external to the rotating poles; and it may be said that while in continuous-current dynamos it is most usual for the armature to rotate, in alternators it is more common for the magnet to rotate. The coils of the stationary armature, which for high voltages must be heavily insulated, are then not subjected to the additional stresses due to centrifugal force; and further, the collecting rings, which must be attached to the revolving portion, need only be employed to transmit the exciting current at a low voltage. The multipolar form of field-magnet with single exciting coil, shovm in Fig. 29, has been very largely used for alternators, since its introduction by Mordey, w*ith disc armature, and by the Oerlikon Co. of Switzerland in connexion with drum armatures ; but although advantageous for particular frequencies and speeds, it requires careful designing to avoid a somew’hat large armature reaction. A further step brings us to the so-called inductor alternator, in which both the armature and the field-magnet coils arc stationary, and the E.M.F. is set up in the armature coils by causing the flux through them to be periodically varied by the rotation of iron “inductors.” It will be seen that in Fig. 29 it is not really necessary for the field-magnet coil to revolve, and by such a modification as shown in Fig. 43 it can be held stationary, while the iron mass which completes the magnetic system is alone rotated. It p-ig. 43. is very important in such a machine to keep the reluctance of the magnetic circuit constant, so as to ensure as far as possible an equal flux in the magnet for all positions of the inductor ; otherwise eddy-currents are set up in its mass. Even when the total flux alters but little, the pole-tips are usually laminated, owing to the variations in its distribution over their faces. Since an alternating current cannot be used for exciting the field-magnet, recourse must be had to some source of a direct current. This is usually obtained from a small auxiliary continuous-current dynamo, called an exciter, which may be an entirely separate machine, separately driven and used for exciting several alternators, or may be driven from the alternator itself; in the latter case the armature of the exciter is often coupled directly to the rotating shaft of the alternator, wdiile its field-magnet is attached to the bed-plate. Although separate excitation is the more usual method, the alternator can also be made self-exciting if a part or the whole of the alternating current is “rectified, 1
For experiments on high-frequency currents, Nikola Tesla constructed au alternator having 384 poles and giving a frequency of about 10,000 {Journ. Inst. Elec. Eng. vol. xxi. p. 82, 1892). The opposite extreme is found in alternators directly coupled to the Parsons steam turbine, in which, owing to its high speed of 3000 revs, per min., only four poles are required to give a frequency of 100. By a combination of a Parsons steam-turbine running at 12,000 revs, per min. with an alternator of 140 poles a frequency of 14,000 has been obtained {Engineering, Aug. 25, 1899).
