From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Page from The Electrical Review discussing single-phase repulsion motors and a report on lightning and transmission lines.
| Identifier | ExFiles\Box 61\2\ scan0048 | |
| Date | 27th September 1929 | |
| 518 THE ELECTRICAL REVIEW. SEPTEMBER 27, 1929. fact that any rotor coil short-circuited during commutation by the brushes acts in exactly the same manner as the short-circuited secondary of a transformer, thus giving rise to a large local current in the coil, which has to be commutated in addition to the current normally carried by the coil.⁸ It follows that unless suitable precautions are taken for nullifying or reducing this short-circuit current, destructive arcing is likely to be experienced at the brushes. This condition imposes restrictions on the design which are difficult to overcome. One of the first suggestions for improving commutation was made by B. G.{Mr Griffiths - Chief Accountant / Mr Gnapp} Lamme,⁹ who put forward a method employing commutator leads of high resistance, which were intended to reduce the value of the short-circuit current in rotor coils undergoing commutation. This arrangement proved satisfactory from the electrical point of view, but mechanical difficulties were experienced due to the excessive vibration of the traction motors with which the arrangement was first tried. There is also the difficulty of accommodating the resistance connections in the rotor, in addition to the normal rotor windings, a difficulty which renders manufacturing costs very high. A further objection to the local commutation currents, in the case of repulsion motors, refers to their effect on the motor torque; during commutation, the magnetic axis of the rotor coil is at right-angles to the brush axis. In accordance with the dynamical laws of self-induction, it follows that a torque is exerted on the coil, tending to place its magnetic axis in quadrature with the field. This torque is in opposition to that due to the remaining portions of the rotor winding, and the result is a reduction of the total rotor torque. The methods now employed for improving commutation make use of auxiliary windings on the stator, arranged in a similar manner to the commutating winding on d.c. machines. These windings are so distributed and connected that an auxiliary flux is produced which reacts upon the flux producing the short-circuit current in commutated rotor coils. By such means it is possible to obtain practically sparkless commutation at any required speed. Difficulties are, however, experienced in the case of variable-speed machines, such as are used for traction work and in cases where large fluctuations of load take place, on account of the variation of the commutating pole flux and the short-circuit current with varying load conditions. Perfect commutation can only be obtained by the complete elimination of the short-circuit current under the brushes and the reversal of the load current in the rotor coils during commutation. For the first condition an auxiliary flux is required in quadrature with the primary flux, and for the second condition is required a further auxiliary flux, in phase with the primary flux; thus the resultant commutating flux is out of phase with the primary e.m.f. The various possible ways of exciting the two auxiliary fluxes has resulted in the development of the large number of types of motor at present available. An arrangement making use of a non-inductive resistance shunting the interpole windings has been suggested by Latour and Dr. Behn-Eschenberg for producing the conditions normally provided by the two compensating fluxes. The arrangement employing an interpole on the stator fed from the rotor for the neutralisation of the transformer e.m.f., and a further interpole connected across the field coil for producing the current-reversing e.m.f., is due to Arnold. There are at present some twenty different types of single-phase commutator motor in existence, all of which have particular characteristics recommending them to certain classes of duty. In the repulsion motor class there are two different types :— (1) The plain repulsion motor. (2) The compensated repulsion motor. During starting, and at low speeds, there is little difference between the commutation of the plain series motor and the repulsion type. The latter is, however, much better in this respect at speeds approaching synchronism, and for this reason has found a much wider sphere of use. It is possible to obtain a limited amount of speed variation on the plain repulsion motor by shifting the brush position, but speed control of the series motor is rendered impossible by the bad commutation. The compensated repulsion motor in its best known form is due to Latour, Wightman, Winter, and Eichberg. It is provided with an additional set of commutator brushes, which are fed from the secondary of a transformer whose primary is in series with the stator winding. In this way the e.m.f. of self-inductance of the motor field is reduced from the value corresponding to primary frequency, as in the case of the plain repulsion motor, to a value corresponding to slip-frequency; it follows that at hypersynchronous speed this e.m.f. is negative, and it is possible to obtain unity power-factor or even to draw leading currents from the line with this type of machine.¹⁰ Conclusion. The statement made at the commencement of this article concerning the number of single-phase supply systems at present in existence is probably at variance with the opinion of many; we do not wish to enter into a discussion on this point. It will, no doubt, be agreed that there is a sufficient number of such systems available to lend importance to a discussion of single-phase motors in their practical application for commercial purposes. There is, however, a much wider scope for the single-phase motor in connection with traction work and other heavy-duty applications, where the saving of supply lines is an important feature, and where simplicity in motors and control gear is of importance. It is hoped that the foregoing discussion of the general aspect of the problem will make clear the fact that there are few instances of power application which are unsuited to single-phase working. Lightning and Transmission Lines. During the course of a paper recently read at a meeting held at Toulouse on "Lightning and Electric Transmission Lines" by Mr. C. Dauzere, director-general of the Globe Physical Institute and Observatory, on the summit of the Pic du Midi Mountain, France, the author stated that he had made experiments to discover the cause of storms and lightning, and had found that certain places, depending on the geological formation of the ground and where negative ions were produced in abundance, were dangerous. He urged that in locating electric power transmission lines regard should be had to the avoidance of such dangerous places. There was still much to be done in the study of electrical phenomena in the air in order to give greater security to electrical installations. As a result of the paper, the Toulouse section of the French Association of Electricians has adopted a resolution recommending that all information regarding accidents arising from lightning, hail, and thunderstorms be concentrated at the Pic du Midi Observatory at Bagnères-de-Bigorre, in the Upper Pyrenees, and that an active campaign should be organised to induce public authorities, electric traction and electricity supply undertakings, and all technical and agricultural societies to assist in making the observatory a centre of information about the effects of lightning and storms. ⁸ See the author's article on "The Single-Phase Repulsion Motor," Electrician, May 1st, 1925. ⁹ U.S. Patents No. 758,667 and No. 758,663, May 3rd, 1904. ¹⁰ The mathematical theory of both the plain and compensated types of repulsion motor is given in the author's paper on "Operating Characteristics of Single-Phase Repulsion Motors," World Power, Vol. 4, December, 1925, pp. 306-313. | ||