CHAPTER XXII UNIPOLAR MACHINES Homopolar or Acyclic Machines 247.. If a conductor, C, revolves around, one pole of a stationary magnet shown as NS in Fig. 215, a continuous voltage is induced in the conductor by its cutting of the lines of magnetic force of the pole, N, and this voltage can be supplied to an external cir- cuit, D, by stationary brushes, Bi and B2) bearing on the ends of the revolving conductor, C. The voltage is: e = /$ 10-8, where / is the number of revolutions per second, $ the magnetic flux of the magnet, cut by the conductor, C. N Fig. 215. — Diagrammatic illustration of unipolar machine with two high- speed collectors. Such a machine is called a unipolar machine, as the conductor during its rotation traverses the same polarity, in distinction of bipolar or multipolar machines, in which the conductor during each revolution passes two or many poles. A more correct name is homopolar machine,* signifying uniformity of polarity, or acyclic machine, signifying absence of any cyclic change: in all other electromagnetic machines, the voltage induced in a con- ductor changes cyclically, and the voltage in each turn is alter- nating, thus having a frequency, even if the terminal voltage and current at the corjimutator are continuous. 450 UNIPOLAR MACHINES 451 By bringing the conductor, C, over the end of the magnet close to the shaft, as shown in Fig. 216, the peripheral speed of motion of brush, J32, on its collector ring can be reduced. However, at least one brush, J5i, in Fig. 216, must bear on a collector ring (not shown in Figs. 215 and 216) at full conductor speed, because the total magnetic flux cut by the conductor, C, must pass through this collector ring on which Bi bears. Thus an essential char- acteristic of the unipolar machine is collection of the current from the periphery of the revolving conductor, at its maximum speed. It is the unsolved problem of satisfactory current collection from high-speed collector rings, at speeds of two or more miles per Fia. 216. — Diagrammatic illustration of unipolar machine with one high- speed collector. minute, which has stood in the way of the commercial intro- duction of unipolar machines. Electromagnetic induction is due to the relative motion of con- ductor and magnetic field, and every electromagnetic device is thus reversible with regards to stationary and rotary elements. Howeyer, the hope of eliminating high-speed collector rings in the unipolar machine, by having the conductor standstill and the magnet revolve, is a fallacy: in Figs. 215 and 216, the con- ductor, C, revolves, and the magnet, NS, and the external circuit, D, stands still. The mechanical reversal thus would be, to have the conductor, C, stand still, and the magnet, NS, and the external circuit revolve, and this would leave high-speed current collection. Whether the magnet, NS, stands still or revolves, is immaterial in any case, and the question, whether the lines of force of the magnet are stationary or revolve, if the magnet revolves around its axis, is meaningless. If, with revolving conductor, C, and stationary external circuit, D, the lines of force of the magnet are assumed as stationary, the induction is in C, and the return circuit in D; if the lines of force are assumed as revolving, the 452 ELECTRICAL APPARATl S Induction is in D, and C is the return, but the voltage in the m- ouit, CD, is the same. If, (hen, V and D both stand still, citl.ir there is no induction in either, or, assuming the lines of magnetic force lo revolve, equal and opposite voltages are induced in ( and D, and the voltage in circuit, CD, is zero just the wum. However, the question whether the lines of force of a revolving magnet rotate or not, is meaningless for this reason: the lines <>i force are a pictorial representation of the magnetic field in space. The magnetic field at any point is characterized by an intensity and a direction, and as long as intensity and direction at ;inv point arc constant or stationary, the magnetic field is constant or sta- tionary. This is the case in Figs. 215 and 210, regardJesa vht&ht i the magnet revolves around its axis or not, and the rotation o| the magnet thus has no effect whatsoever on the induction phe- nomena. The magnetic field is stationary at any point of space outside of the magnet, and it is also stationary at any point dJ space inside of the magnet, even if the niagncl revolves, and a4 the same time it is stationary also with regards to any efemetd of the revolving magnet. lising then the pictorial representation of the lines of magnetic force, we can assume these lines of force- as stationary in space, or as revolving with the rotating magnet, whatever best suits the convenience of the problem at hand: but whichever assumption we make, makes no difference on tion of the problem, if we reason correctly from the assumption. 248. As in the unipolar machine each conductor (cot ing to a half turn of the bipolar or multipolar machine) requires a separate high-speed collector ring, many attempts have been made fund arc still Ix'ing made) to design a coil-wound unipolar machine, that is, a machine connecting a number of peripheral conductors in series, without going through collector rings. This is an impossibility, and unipolar induction, that is, continues induction of a unidirectional voltage, is possible | in mi open conductor, but not in a coil or turn, as the voltage electro magnetically induced in a coil or turn must alwi alternating voltage, The fundamental law of elect romagnetic induction i- indnced voltage is proportional to the rate of cutting of the con- ductor through the lines of force of the magnetic field. Applying this to a closed circuit or turn; every line of magnetic fun,- bbJ by a turn must either go limn il utside to the inside, or from the inside to the outside of the turn. This mean- : UNIPOLAR MACHINES 453 induced in a turn is proportional (or equal, in absolute units) to the rate of change of the number of lines of magnetic force en- closed by the turn, and a decrease of the lines of force enclosed by the turn, induces a voltage opposite to that induced by an increase. As the number of lines of force enclosed by a turn can not perpetually increase (or decrease), it follows, that a voltage can not be induced perpetually in the same direction in a turn. Every increase of lines of force enclosed by the turn, inducing B Fig. 217. — Mechanical an- alogy of bipolar induction. Fig. 218. — Mechanical analogy of unipolar induction. a voltage in it, must sometime later be followed by an equal decrease of the lines of force enclosed by the turn, which induces an equal voltage in opposite direction. Thus, averaged over a sufficiently long time, the total voltage induced in a turn must always be zero, that is, the voltage, if periodical, must be alter- nating, regardless how the electromagnetic induction takes place, whether the turn is stationary or moving, as a part of a machine, transformer, reactor or any other electromagnetic induction device. Thus continuous-voltage induction in a closed turn is impossible, and the coil-wound unipolar machine thus a fallacy. Continuous induction in the unipolar machine is pos- sible only because the circuit is not a closed one, but consists of a conductor or half turn, sliding over the other half turn. Mechan- ically the relation can be illustrated by Figs. 217 and 218. If in Fig* 217 the carriage, C, moves along the straight track of finite length — a closed turn of finite area — the area, A, in front of C decreases, that B behind the carriage, C, increases, but this decrease and increase can not go on indefinitely, but at some time C reaches the end of the track, A has decreased to zero, B is a 454 ELECTRICAL A PPA MA TVS Fig. 219. — Drum type of unipolar machine with sta- tionary magnet core, section. maximum, and any further change can only be an increase ol I and decrease of H, by a motion of (' in opposite direction, repre- senting induction of a reverse voltage. On the endless circular D track, Fig. 218, however, the carriage, C, can continuously move in the same direction, continuously reduce the area, A, in front and increase that of H behind C, corresponding to con- tinuous induction in the same direc- tion, in the unipolar machine. 249. In the industrial design of a unipolar machine, naturally a closed magnetic circuit would be used, and the form, Fig. 216, would be exe- cuted as shown in length section in Fig. 219. N is the same pole as in Fig. 216, but the magnetic return circuit is shown by S, concentrically surrounding N. C is the cylindrical con- ductor, revolving in the cylindrical gap be- tween N and 8. B, and B% are the two sets of brushes bearing on the collector rings at the end of the conductor, C, and F is the field exciting winding. The construction, Fig. 219, has the me- chanical disadvantage of a relatively light structure, (", revolving at high speed between two stationary structures, N and S. As it is immaterial whether the magnet is stationary Of revolving, usually the inner core, iV, is re- volved with the conductor, as shown in Figs. 221 and 222. This shortens the gap between N and S, but introduces an aux- iliary gap, G. Fig. 221 has the disadvantage of a magnetic end thrust, and thus the con- struction, Fig. 222, is generally used, or its duplication, shown in Fig. 223. The disk type of unipolar machine, shown in section in Fig. 220, has been frequently proposed in fon&0 times, but is economically inferior to the construction of Figs. 221, 222 and 223. The limitation of the unipolar machine is the high collector speed. In Fig. 220, the average conductor speed is less than the collector speed, and the latter thus relatively UNIPOLAR MACHINES 455 higher than in Figs. 221 to 223, where it equals the conductor speed. Higher voltages then can be given by a single conductor, are Fig. 221. — Drum type of unipolar Fio. 222.— Drum type o/ unipolar machine with revolving magnet core machine with revolving magnet core and auxiliary end gap, section. and auxiliary cylinder gap, section. derived in the unipolar machine by connecting a number of con- ductors in series. In this case, every series conductor obviously • ;^-.,^,:,&L, q * n ro Fio. 223. — Double drum type of unipolar machine, section, requires a separate pair of collector rings. This is shown in Figs. 224 and 225, the cross-section and length section of the rotor of Fig. 224. — Multi- Fia. 225.— Mult i-«inductor unipolar machine, conductor unipolar length section. machine, ■ a four-circuit unipolar. As seen in Fig. 224, the cylindrical con- ductor is slotted into eight sections, and diametrically opposite 456 ELECT H1C Al. APPARATUS sections, 1 and 1', 2 and 2', 3 and 3', 4 and 1', are connected in multiple (to equalize the flux distribution) between four pairs of collector rings, shown in Fig. 225 as 1 and 1|, 2 anil 2j, 3 and 3i, 4 and 4i. The latter are connected- in series. This machine. Figs. 224 and 225, thus could also be used as a three-wire or five-wire machine, or as a direct-current converter, bj nut intermediary connections, from the collector rings 2, 3. 4. 250, As each conductor of the unipolar machine requires a separate pair of collector rings, with a reasonably moderate number of collector rings, unipolar machines of medium capacity are suited for low voltages only, such as for electrolytic machines, and have been built for this purpose to a limited extent, but in general it has been found more economical by series connection of the electrolytic cells to permit the use of higher voltages, and then employ standard machines. For commercial voltages, 250 or f>00, to keep the number of collector rings reasonably moderate, unipolar machines m|iun very large magnetic fluxes — that is, large units of capadl j and very high peripheral speeds. The latter requirement made tin- machine type unsuitable during the days of theslow-spe connected steam engine, but when the high-speed steam turbttM arrived, the study of the design of high-powered steam-turbine- driven unipolars was undertaken, and a number of such machines built and installed. In the huge turbo-alternators of today, the largest lo— i- the core loss: hysteresis and eddies in the iron, which often is K than all the other losses together. Theoretically, the Uni point machine has no core loss, as the magnetic flux does not change anywhere, and solid steel thus is used throughout — and has to be used, due to the shape of the magnetic circuit. However. with the enormous magnetic fluxes of these maclunes, in suinl iron, the least variation of the magnetic circuit, such as caused by small unequalities of the air gap, by the reaction of the :ir ma- ture currents, etc., causes enormous core losses, mostly addfaa, and while theoretically the unipolar has no cove loss, designing experience has shown, that it is a very difficult problem to keep the core loss in such machines down to reasonable values. Fur- thermore, in and at the collector rings, the magnetic n the armature currents is alternating or pulsating. Thus in Vfgt. 224 and 225, the point of entrance of the current from the arma- ture conductors into the collector rings revolves with the rotation UNIPOLAR MACHINES 457 of the machine, anil from this point flows through the collector ring, distributing between the next brushes. While this circular flow of current in the collector ring represents effectively a frac- tion of a turn only, with thousands of amperes of current it represents thousands of ampere-turns m.m.f., causing high losses, which in spite of careful distribution of the brushes to equalize the current flow in the collector rings, can not be entirely eliminated. 251. The unipolar machine is not free of armature reaction, as often believed. The current in all the armature conductors (Fig. 224) flows in the same direction, and thereby produces a circular magnetization in the magnetic return circuit, S, shown by the arrow in Fig. 224. While the armature conductor mag- netically represents one turn only, in the large machines it repre- t sents many thousand ampere-turns. As an instance, assume a peripheral speed of a steam-turbine-driven unipolar machine, of 12,000 ft. per minute, at 1800 revolutions per minute. This gives an armature circumference of 80 in. At }'i in. thickness of the conductor, and 2500 amp. per square inch, this gives 100,000 ampere- turns m.m.f. of armature reaction, which probablyis sufficient to magnetic- ally saturate the iron in the pole faces, in the direction of the arrow in Fig. 224. At the greatly lowered permeability at saturation, with constant field excita- tion the voltage of the machine greatly drops, or, to maintain constant voltage, f,0 220. — Multi-con- a considerable increase of field excita- ductor unipolar machine .... -it wit" compensating pole turn under load is required. Large fftce winding, erow-wection. unipolar machines thus are liable to give poor voltage regulation and to require high compounding. To overcome the circular armature reaction, a counter m.m.f. may be arranged in the pole faces, by returning the current of each collector ring 1,, 2i, 3i, 4,, of Fig. 225, to the collector rings on the other end of the machine, 2, 3, 4 in Kg. 225, not through an external circuit, but through conductors imbedded in the pole face, as shown in Fig. 226 as 1', 2', 3', 4'. The most serious problem of the unipolar machine, however, is that of the high-speed collector rings, and this has not yet been solved. Collecting very large currents by numerous collector 458 ELECTRICAL APPARATUS rings at Bpeeda of 10,000 bo 15,000 ft. per minute, leads to high losses and correspondingly low machine efficiency, high tempero- ture rise, and rapid wear of the brushes and collector rings, and this has probably been the main cause of abandoning the develop- ment of the unipolar machine for steam-turbine drive. A contributing cause was that, when the unipolar steam-tur- bine generator was being developed, the days of the huge direct- current generator were over, and its place had been taken by turbo-alternator and converter, and the unipolar machine offered no advantage in reliability, or efficiency, but the disadvantage of lesser flexibility, as it requires a greater concentration of direct- current generation in one place, than usually needed. 262. The unipolar machine may be used :i^ motor as well as generator, and has found some application as motor meter. The general principle of a unipolar meter may be illustrated by Fig. 227. The meter shaft, A , with counter, F, is pivoted at P, anil carries the brake disk and conductor, a copper or aluminum disk. D, be- tween the two poles, N and S, of a circular magnet. The shaft, .4, dips into a mercury cup, C, which is insulated and contains tbc one terminal, while tiie other terminal goes to a circular mercury trough, 67. An iron pin, B, projects from the disk, D, into this mercury trough and completes the circuit.