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).
Article from 'The Electrical Review' about the characteristic features of available types of single-phase motors.
| Identifier | ExFiles\Box 61\2\ scan0045 | |
| Date | 27th September 1929 | |
| X75. 516 THE ELECTRICAL REVIEW. SEPTEMBER 27, 1929. C.A.P. 70/29 Hew. PJ Notes on Single-phase Motors. A General Discussion of the Characteristic Features of the Available Types of Machine. By G.{Mr Griffiths - Chief Accountant / Mr Gnapp} WINDRED. THE number of single-phase electricity supply systems available in this country is certainly large; but not, perhaps, so large as in the majority of other countries. The problem of utilising single-phase power efficiently is therefore a very real one, which has occupied the attention of engineers for many years, and is still without a complete solution. It is the object of the present article briefly to consider the various aspects of this problem, and to indicate the lines upon which development has proceeded. It will first be necessary to consider the different forms which have been assumed by the single-phase motor during its period of existence. In approximate order of development they are respectively :— (1) The split-phase induction type. (2) The monocyclic induction type. (3) The commutator type. 1. The Single-phase Split-phase Induction Motor.— In the ordinary split-phase motor, which is usually of the squirrel-cage type, the stator is provided with two windings, which are connected to the supply in such a manner as to give rise to a rotating stator field approaching to sine-wave form. In practice the field form differs very considerably from sine form on account of the interactions which are set up in the rotating rotor flux, which make it impossible to preserve the field form during the starting period. The effect of the rotating stator field is to render the motor self-starting, and when the rotor has attained sufficient speed the auxiliary or starting winding on the stator is disconnected from the line. The rotating stator field is now produced solely by the interaction of the main alternating stator flux and the quadrature flux induced in the rotor circuit by the action of the rotor conductors in cutting the lines of primary flux. It follows that this torque-producing auxiliary flux is proportional to the speed of the rotor, and that if this speed is reduced by the application of load the auxiliary field strength will decrease, with a resulting loss of torque. For this reason, the overload capacity of the split-phase machine is very low, as compared with the polyphase machine, in which the rotor flux is not relied upon for the production of a torque-producing flux. There are many methods of connecting the starting and running windings of a split-phase motor to the supply. In the majority of cases for machines of medium and large outputs a choke coil and resistance are employed for starting purposes, the former being connected in series with the starting winding, and the latter in series with the running winding. This method is by no means general, as occasionally the order of connection of the choke and resistance is reversed, depending upon which arrangement is best suited to the motor windings for the production of the maximum starting torque. Such matters are almost invariably determined by experiment, on account of the extreme difficulty of determining the starting performance to any degree of accuracy.¹ Some makers even arrange for the two windings to be connected in series for starting, but this method is confined to motors of small output. A further method makes use of the "shading coil" principle, in which approximately two-thirds of the stator pole pitch is surrounded by a short-circuited coil of large cross section having only a few turns. An e.m.f. of self-induction is set up in this coil by the action of the main flux, and the m.m.f. thus produced gives rise to an auxiliary flux which combines with the main flux to produce, in the manner already described, a rotating stator field which renders the motor self-starting. This method is very inefficient on account of the continuous losses in the short-circuited starting winding, a defect which has been overcome by bringing out the ends of the starting winding and short-circuiting them only during the starting period. So far as we are aware, this principle has not been employed in motors exceeding fractional horsepower outputs; its greatest application is in connection with fan motors, &c., where the starting torque required is small, and overloads are not normally encountered. The single-phase induction motor starting problem is a very complicated one. For machines of medium outputs the best method is probably the split-phase method already described. Unfortunately, however, the output frame size efficiency is very low. This is on account of the fact that the stator slots containing the starting winding cannot be usefully employed for the running winding; thus the output of a motor carcase wound for single-phase is only some 70 per cent. of that obtainable from a corresponding three-phase wound carcase. Adequately to describe the complicated points in motor theory which are necessary for a thorough understanding of the starting problem would require more space than is available at present.² We pass on to an examination of other methods of starting. 2. Single-phase Monocyclic Starting Devices.—The "monocyclic" method seems to have been introduced by Steinmetz; it is described in his patents Nos. 620,988 and 620,989 of 1895, and is undoubtedly the best all-round method from the viewpoint of operating characteristics. In this method two impedances of different power-factors are employed, connected across the single-phase mains in series. From the mid-point of this circuit is connected one-phase of an ordinary three-phase induction motor, whose remaining two phases are connected to the single-phase mains.³ This circuit arrangement results in the production of a polyphase system of currents in the three phases of the induction motor, and renders the motor capable of producing an appreciable starting torque, the actual value of which is dependant upon the circuit constants, and may be made a maximum by the suitable proportioning of the external circuit impedances. All the e.m.f.'s. produced in this system, with the exception of one in phase with the supply, are wattless, and there is thus only a single-phase supply of energy available. The interactions of these wattless e.m.f's. give rise, however, to the rotating magnetic fields necessary for starting, and the starting torque efficiency of the arrangement is actually very high.⁴ It is possible to employ either squirrel-cage or slip-ring machines in this way. In the case of squirrel- ¹ A theoretical examination of the split-phase method of starting is contained in the author's short treatise on "The Operation of Single-Phase Induction Motors," published by the A.E.S.D. ² The interested reader will find much of value in the paper on "The Starting of Single-Phase Induction Motors," by F.{Mr Friese} A.{Mr Adams} Lauper, Journal I.E.E., Vol. 65, p. 166. See also a valuable article on "Single-Phase Induction Motors and the Starting Problem," by F.{Mr Friese} W. Davies, ELECTRICAL REVIEW, January 4th, 1924. ³ There are numerous modifications of this principle. Several methods are described in the author's article on "The Operation of Polyphase Motors from a Single-Phase Supply," in the ELECTRICAL REVIEW, October 12th, 1923. ⁴ The theory of this circuit arrangement is examined mathematically in the author's tract on "The Operation of Single-Phase Induction Motors," pp. 20-24. | ||