Notes On A Unipolar Dynamo

Wednesday, September 2, 1891
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S6Pt-2»1891.] THE ELECTRICAL ENGINEER. 259 ing eddy current and on the speed of rotation ; these ele- ments, namely, determine the retardation of this current and its position relative to the field, For a certain speed there would be a maximum energizing action ; then at higher speeds, it would gradually fall off to zero and finally reverse, that is, the resultant eddy current effect would be to weaken the field. The reaction would be best demon- strated experimentally by arranging the fields N s, N’ s, freely movable on an axis concentric with the shaft of the disc. If the latter were rotated as before in the direction of the arrow D the field would be dragged in the same direction with a torque, which, up to a certain point, " 6- L' l 0 § is A D In \

260 THE ELECTRICAL ENGINEER [Sept 2,1891 that instead of taking tho current from two such discs by sliding contacts, as usual, a flexible conducting belt may be employed to advantage. The discs are in such case provided with large flanges, affording a very great contact surface. The belt should be made to bear on the flanges with spring pressure to take up the expansion. Several machines with belt contact were constructed by the writer two years ago and worked satisfactorily, but for want of time the work in that direction has been temporarily suspended. A number of features pointed out above have also been used by the writer in connection with some types of alter- nating current motors.

258 THE ELECTRIC NOTES ON A UNIPOLAR DYNAMO. nr ¢%#_ ,/agar, I'r is characteristic of fundamental discoveries, of great achievements of intellect, that they retain aii undiminished power upon the imagination of the thinker. The Inem- orable experiment of Faraday with a disc rotating between the two poles of a magnet, which has borne such magnifi- cent fruit, has long passed into everyday experience; yet there are certain features about this embryo of the present dynamos and motors which even to-day appear to us striking, and are worthy of the most careful study, Consider, for instance, the case of a disc of iron or other metal revolving between the two opposite poles of a mag- net, and the polar surfaces completely covering both sides of the disc, and assume the current to be taken off or con- veyed to thc same by contacts uniformly from all points of the periphery of the disc. 'Fake first the case of a motor. In all ordinary motors the operation is dependent upon seine shifting or change of thc resultant of the magnetic attrac- tion exerted upon thc armature, this process being effected either by some mechanical contrivance on thc motor or by the action of currents of the proper character. \Ve may explain the operation of such a motor just as we can that of a water-wheel. But in the above example of the disc surrounded completely by the polar s\n'faces, there is no shifting of the magnetic action, no change what. ever, as far as we know, and yet rotation ensues. Here, then, ordinary considerations do not apply ; wc can not even give a superficial explanation, as in ordi- nary `niotors, and the operation will be clear to us only when we shall have recognized the very nature of the forces concerned and fathonn-d the mystery of thc invisible con- necting niechanisni. Considered as a dynamo machine, the disc is an equally interesting object of study. In addition to its peculiarity of giving currents of one direction without the employment of commutating devices, such a machine differs from ordinary dynanios in that there is no reaction between armature and field. The armature current tends to set up a magnet- ization at right angles to that of the field current, but sincc the current is taken off uniformly from all points of the periphery, and since, to be exact, the external circuit may also be arranged perfectly symmetrical to the field magnet, no reaction can occur. 'l‘his, however, is true only as long as the magnets are weakly energized, for when the magnets are more or less saturated, both niagnctizations at right angles seemingly interfere with each other. For the above reason alone it would appear that the out- put of such a machine should, for the same weight, be much greater than that of any other machine in which the armature current tends to deniagnetize the field. The extraordinary output of the Forbes unipolar dynamo and the experience of the writer confirm this view. Again, the facility with which such a machine may be made to excite itself is striking, but this may he due- besides to the absence of armature reaction-to the perfect smoothness of the current and non-existence of self- induction. If the poles do not cover the disc completely on both sides, then, of course, unless the disc be properly subdivided, the machine will be very inefiicient. Again, in this case there are certain points worthy of notice. If the disc be rotated and thc field current interrupted, the current through the armature will continue to fiow and the field magnets will lose their strength comparatively slowly. The reason of this will at once appear when wc consider the direction of the currents set up in the disc. Referring to diagram in Fig. 1, fl represents the disc with the sliding contacts B B’ on the shaft and periphery. Ar. ENGINEER. [sspr.2,mi. N and s represent the two poles of a magnet. If the pole N be above, as indicated in the diagram, the disc being sup- posed to be in the plane of the paper, and rotating in the direction of the arrow n, the current set up in the disc will flow from the centre to the periphery, as indicated by the arrow A. Since the magnetic action is more or less con- Ened to the space between the poles N s, the other portions of the disc may bo considered inactive. 'l‘l\c current set up will therefore not wholly pass through the external circuit if, but will close through the disc itself, and gener- ally, if the disposition bein any way similar to the one illustrated, by far the greater portion of the current generated will not appear externally, as the circuit if is practically short-circuited by the inactive portions of the disc. The direction of the resulting currents in the latter may be assumed to be as indicated by the dotted lines and arrows m and IL ; and the direction of the energizing field current being indicated by the arrows a b c d, an inspec- tion of the figure shows that one of the two branches ofthe eddy current, that is, A B' m n, will tend to demagnetize the Held, while the other branch, that is, A ii' u n, will have the opposite effect. Therefore the branch A n m n that is, the one which is approaching the field, will repel the lines of the same, while branch A B’ n B, that is, the one leaving the field, will gather thc lines of force upon itself. ///7/Y / //I /ll I