Patent GB-190422631-A (GB)
Title: Improvements in Methods of Exciting Electro-magnets by Alternating Electric Currents
Inventor: LATOUR MARIUS
Filed: 1904-10-20
Granted: 1905-10-19
Abstract: 22,631. Latour, M. Oct. 20, A.D. 1903, [date applied for under Patents Act, A.D. 1901]. Electromagnets ; field magnets. - A constant unidirectional magnetic flux is produced in a magnetic circuit from monophase or polyphase alternating c u r - rents by superimposing two sets of magnetomotive forces, each varying in accordance with the squares of the sines of the phase angles of one of the corresponding phases. The polyphase &c. voltages are each led into an exciting winding or windings, the effective number of turns of which is synchronously varied to correspond with the variation in the sines of the phase angles of each voltage. The invention is particularly designed for the purpose of rendering alternating-current generators self-exciting, and is shown in the Figure as applied to a two-phase source of current. The exciting-winding A is divided into a number of sections a<1>-a<6>, having approximately the same ohmic resistance and connected at their extremities to the segments of a six-part commutator furnished with two pairs of short-circuiting brushes B, B and B', B' situated at right-angles to one another and fed with current from a twophase source D. The number of turns in each section successively varies in a sinusoidal manner, and may be calculated from the expression k sin { a + (2#/n) (r-1) } where k is a constant, n the total number of sections, a the angle of phase at any instant, and r the number of each section in the series. In the diagram, a is taken as being equal to #/n, n as 6, and k as 2, giving the number of turns in each section, taken in order, as:- 1, 2, 1,-1,-2,-1, those sections with a negative number of turns being wound in the opposite direction to the others. With the brushes in the position shown, the ampere-turns at any one instant are equal to 4 I sin a, where I represents the maximum value of the current. If, however, the brushes on the commutator are rotated synchronously with the alternation of the currents entering the brushes, the effective number of turns traversed by the current is proportional to sin a, thus making the resulting ampÞre-turns at any instant proportional to 4 I sin <2>a. Since the second pair of brushes are displaced by a rightangle from the first set, a second flux corresponding to an instantaneous value of the ampere-turns of 4 I. cos <2>a will be impressed upon the circuit, producing a resultant flux of 4 I, i.e., the flux is constant and in one direction. If a simple alternating current is supplied to one pair of brushes B, B and the other pair is short-circuited, a similar result will be obtained. Two or more windings A may be employed. When the exciting-winding is located in slots, as in the field magnets of alternators coupled to steam turbines, the number of turns in each section of the winding may be constant if the successive sections are placed in pairs of slots at the extremities of chords which vary in magnitude sinusoidally. In multipolar machines, the corresponding sections may be connected in parallel or in series, or according to any combination of these two arrangements. The winding may consist of several successive coils having an equal number of turns and grouped into sections which follow a sinusoidal law. In the case of slow-speed alternators, in place of exciting the field of the alternator by the above method, it is preferable to excite directly the field of an exciter, and by causing the exciter to work on the rectilinear part of its characteristic curve, the excitation of the alternator can be regulated under the same conditions i shunt, in series, or in compound with the network.