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 the theory and operation of various types of electric motors.
| Identifier | ExFiles\Box 61\2\ scan0047 | |
| Date | 27th September 1929 | |
| SEPTEMBER 27, 1929. THE ELECTRICAL REVIEW. 517 cage motors the best starting effects are obtained where high starting currents are allowed, but if the starting current is restricted to low values the monocyclic method does not quite give such good starting effects as the ordinary split-phase method. In the case of slip-ring motors with suitable rotor resistances the effects are con-verse, the best starting conditions, compared with the split-phase arrangement, being obtained with the lower starting currents. There are numerous methods of starting based on the foregoing principle.⁵ In one of these a condenser takes the place of one of the impedances, the resulting starting torque efficiency with this arrangement being superior to that of the polyphase induction motor. Such an arrangement is, however, rarely employed in practice, on account of the much greater bulk and expense as com-pared with the two impedances. It is usual entirely to disconnect the two impedances from the supply when the motor has accelerated to full speed; by this means the losses in these parts of the circuit are dispensed with, and it is possible to employ impedances of relatively small size, on account of the short period of duty. A device which has found a large sphere of use in America, but has not, as far as we are aware, appeared in this country, makes use of a condenser connected across two terminals of a three-phase machine. One of these two terminals and the free terminal are then connected to the single-phase supply. This circuit arrangement renders the motor self-starting, with an appreciable starting torque and high starting torque efficiency when a suit-able condenser is used. It is usual with this arrange-ment to leave the condenser connected during the run-ning period, the condenser being designed so as to com-pensate for the lagging magnetising-current of the motor, and hence produce approximately unity power-factor. One of the difficulties with such starting devices as we have described is occasioned by the fact that the motor circuit constants alter, for various reasons, during the accelerating period. It is thus impossible to obtain the desired conditions throughout the entire speed range, and some sort of compromise is usually effected when design-ing the starting gear. In the case of motors with a low-resistance short-circuited rotor, the total motor impe-dance and the power-factor at standstill are low, and increase very rapidly with increasing speed. With motors having a high-resistance rotor the conditions are converse, the power-factor in particular changing less from standstill to full speed; thus the effect of changing power-factor of the motor circuits upon the circuit con-stants of the starting device is not so great in the latter case. Unfortunately, however, high-resistance rotors are less efficient at normal speed, as they produce greater changes in speed for variations of torque than those of low resistance. It will be appreciated that this is an important point in view of what has been said with regard to the speed-torque characteristics of the single-phase machine. In practice, conditions are quite easily obtained wherein a considerable starting torque is developed, but falls off to zero and then reverses at some intermediate speed, so that the motor is unable to attain full speed with the starting device in circuit. From the foregoing remarks it is evident that many difficulties beset the path of the designer of single-phase motors. It becomes easy to agree with the dictum of a well-known authority on this subject :— “The single-phase motor has been the subject of perhaps more theoretical speculation than any other dynamo-electric machine, and the reason for it is, undoubtedly, that it is in its functioning the most complicated of all dynamo-electric machines.”⁶ The subject is undoubtedly a very extensive and diffi-cult one; it has been approached from every conceivable angle by the numerous investigators in America, France, and Germany, as well as this country, although the amount published on the subject in England has been very small. In connection with the history of the theory, the names of Dr. Behn-Eschneburg, Georges, Potier, Prof. Steinmetz, and Dr. A.{Mr Adams} Thomaelen are outstand-ing. To Dr. Thomaelen we owe the first highly accurate graphical analysis based on the two-field theory origin-ally suggested by Ferraris.⁷ A further method of operation, which makes use of a well-known principle connected with polyphase motor operation, may be included in the category of monocyclic devices :— If one terminal of a three-phase induction motor running under normal conditions is disconnected from the supply, the machine will continue to operate with a reduced overload capacity. These conditions represent what is known in practice as “single-phasing,” which results in reduced speeds under normal load in three-phase working, and serious overheating of the motor windings if the normal full load of the motor is sustained for any appreciable period. When running under these conditions, a tertiary e.m.f. is induced in the idle phase of the stator winding by the combined action of the stator and rotor currents. This tertiary e.m.f. is essentially wattless, as already explained above, but combines with the supply e.m.f. to produce conditions approximating to a three-phase dis-tribution of m.m.f. in the motor windings. It follows that a second three-phase machine, when connected to the terminals of a motor operating in this manner, will be self-starting and able to run up to speed without any special starting device. The starting torque developed by the second motor is, of course, not so great as would be obtained from the machine on a three-phase system, but is sufficiently in excess of the starting torque of a split-phase motor to recommend this system where a high starting torque is required on a single-phase supply. In practice this method has been used on large single-phase railway systems, where it has given satisfactory service, chiefly on account of the simplicity of the control arrangements. It has the disadvantage for traction work that the pilot motor or “phase convertor,” as it is then called, represents an idle dead weight, as com-pared with other systems, but the economies effected by the simplicity of motors and control gear usually compen-sates for this defect. An instance of the useful applica-tion of the method occurs in connection with the opera-tion of a number of single-phase motors in fairly close proximity to each other. In such a case, one of the machines, preferably of large size, may be started up first, by any of the monocyclic devices described above, it being then used to supply a three-phase system of e.m.f.’s to the remaining motors; these will require merely some form of direct switching device for connect-ing them in turn to the pilot motor leads. All the machines can be of the normal three-phase type, so that considerable economy is effected in both motors and control gear by the use of this method, apart from the superior operating characteristics obtained by its means. 3. Single-phase Commutator Motors.—Anything ap-proaching a complete discussion of the theory and appli-cation of the numerous types of single-phase commutator motor would require the scope of a large book. As in the case of other single-phase machines, this type has been the subject of considerable practical and theoretical investigation, with the result that the number of types available is very large, covering almost every conceiv-able method of operation. In modern practice, the most usual types are repre-sented by two categories :— (1) The plain series motor. (2) The repulsion motor. Of these two types, the latter is probably most fre-quently encountered. The chief difficulty with all a.c. commutator machines rests on the question of commutation. This is due to the ⁵ They are discussed very fully in a mathematical way by Prof. C. P. Steinmetz in his paper on “The Single-Phase In-duction Motor,” Transactions, A.I.E.E., Vol. 15, 1898. ⁶ From a paper by E.{Mr Elliott - Chief Engineer} F.{Mr Friese} W. Alexanderson, in the Trans-actions, A.I.E.E., Part I, 1918, p. 691. ⁷ See the author’s tract, loc. cit., pp. 12-13; also the author’s paper on the “General Theory of the Single-Phase Inductor Motor,” World Power, Vol. 6, October, 1926, pp. 193-196, where the two-field theory is examined mathemati-cally. | ||