AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS. FOURTEENTH GENERAL MEETING. GREENACRE-ON-THE-PISCATAqua, ELIOT, ME., July 26, 27 and 28, 1897. The opening session of the Fourteenth General Meeting of the INSTITUTE was called to order by President Crocker at the "Eirenion," Greenacre, on Monday, July 26th at 2 P. M. The Secretary announced that the papers to be presented had already been printed in the May, June and July issues of the TRANSACTIONS with the exception of the last three appearing on the programme. The following Associate Members were elected and transferred at the meeting of Council, June 23, 1897. Name. ATKINS, HAROLD B. Address. Engineer, A. K. Warren & Co., COPELAND, CLEMENT A. Manager of Electric Lighting Sta DAMON, GEO. B. EDMANDS, I. R. ELLARD, JOHN W. SCOTT, Wм. M. tion and Electrician, Copper Consulting Engineer, F. P. Sheldon Electrical Engineer. The Cutter Endorsed by. Townsend Wolcott C. O. Mailloux. A. K. Warren. Edw. L. Nichols. C. F. Bancroft. Chas. P. Steinmetz J. F. Morrison. Mr. Charles P. Steinmetz then read the following paper on "The Alternating Current Induction Motor." the American Institute of Electrical Engineers, Eliot, Me., July 26th, 1897. President Crocker in the Chair. THE ALTERNATING CURRENT INDUCTION MOTOR. BY CHARLES PROTEUS STEINMETZ. POLYPHASE INDUCTION MOTOR. § 1. Load Curves.-In its general behaviour the alternating current induction motor is analogous to the continuous current shunt motor. Like the shunt motor it operates at approximately constant magnetic density. It will run at fairly constant speed, slowing down gradually with increasing load. The main difference, however, is that in the induction motor the current is not passed into the armature by a system of brushes, as in the continuous current motor, but induced in the armature by the alternating field, and in consequence thereof, the primary circuit of the induction motor fulfils the double function of an exciting circuit corresponding to the field circuit of the continuous current shunt motor, and an inducing circuit producing the current in the armature by electro-magnetic induction. In its electro-magnetic features, however, the induction motor is essentially a transformer. That is, it consists of a magnetic circuit interlinked with two electric circuits, the primary or inducing, and the secondary or induced circuit. The difference between transformer and induction motor is that in the former the secondary is fixed regarding the primary, and the electrical energy induced in the secondary is made use of, while in the latter the secondary is movable regarding the primary and the mechanical force acting between primary and secondary is used. The secondary or armature of the motor consists of two or more circuits displaced in phase from each other so as to offer a closed secondary to the primary circuits, irrespective of the relative motion. The primary consists of one or several circuits. In consequence of the relative motion of the primary and secondary, the magnetic circuit of the induction motor must be arranged so that the secondary while revolving does not leave the magnetic field of force. That means the magnetic field of force must be of constant intensity in all directions, or in other words the component of magnetic flux in any direction in space be of the same or approximately the same intensity but differing in phase. Such a magnetic field can either be considered as superposition of two magnetic fields of equal intensity in quadrature in time and space, or it can be represented theoretically by a revolving magnetic flux of constant intensity, or simply treated as alternating magnetic flux of the same intensity in every direction. In the polyphase motor this magnetic field is produced by a number of electric circuits displaced from each other in position in space, and excited by currents having the same displacement in phase as the exciting coils have in space. In the monocyclic motor the one of the two superimposed quadrature fields is excited by the primary energy circuit, the other by the magnetizing or teaser circuit. In the single phase motor the one of the two superimposed magnetic quadrature fields is excited by the primary electric circuit, the other by the induced secondary or armature currents carried into quadrature position by the rotation of the secondary. In either case, at or near synchronism the magnetic fields are identical. The transformer feature being predominant, in theoretical in- · vestigations of induction motors it is generally preferable to start therefrom. The characteristics of the transformer are independent of the ratio of transformation, other things being equal. That is, doubling the number of turns for instance, and at the same time reducing their cross-section to one-half, leaves the efficiency, regulation, etc., of the transformer unchanged. In the same way in the induction motor it is unessential what the ratio of primary and secondary is, or in other words the secondary circuit can be wound for any suitable number of turns, provided the same total copper cross-section is used. In consequence hereof the secondary circuit is mostly wound with one or two bars per slot, to get maximum amount of copper, that is minimum resistance of secondary. |