Alternating-current motor.

Abstract

Claims

E. F. W. ALEXANDERSON. ALTBRNATING- GURREN T MOTOR. APPLICATION FILED MAY 1,1907. W itnessss q a M i Irwsritdr Erna FWAIe underson 1mm. y I UNITED STATES P TENT OFFICE. ERNST F. ,W. ALEXANDERSON, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC- TRIO COMPANY, A CORPORATION OF NEW YORK. ALTERNATING-CURBENT MOTOR. Specification of Letters Patent. Patented June 1, 1909. To all whom it mag concern: I Be it known that I, ERNST F. W. ALEXAN- DERsON, a citizen of the United States, residing at Schenectady, county of Schenectady, State of New York, have invented certain new and useful Improvements Alternating-Current Motors, of which the following is a specification. My invention relates to alternating-current motors of the repulsion type, in which the armature is short-circuited on a line displaced from the line of primary magnetization, and its object is to improve the operation of such motors, both in power-factor and commutation. ' The repulsion motor, as ordinarily arranged, is subject to certain disadvantages as compared with the compensated series motor. In the repulsion motor the secondary current is produced by induction from the )rimary, as in a transformer, but owing to t e displacement of the lines of magneti-- zation of the primary and secondary windmgs, conditions exist in the ordinary repulsion motor like those in a transformer in which the rimary and secondary coils are displaced rom each other on the core. Such an arrangement, as is well known in the transformer art, roduc'es ieakage fluxes and these leakage uxesin the repulsion motor lower the power-factor. Because of the position of the brushes relative'tio -the primary magnetization, in the ordinary repulsion motor the two conductors or sets of conductors which form the coil undergoing commutation are cutting fields, produced by the stator winding, which are of op osite polarity, so that the clectromotive orces lnduced in the coil, due to this cutting, are in series. This produces short-circuited currents, which have a disturbing effect on commutation. Furthermore, a field is reduced due to the rotor currents induce by rotation, which field is peaked in form and the coil undergoing commutation is passing through the peak of this field. Whilebelow synchronisin the electromotive force due to cutting this field is useful as op osing the electromotive force due to trans ormer action, the effect becomes excessive above syhchronism, and for this reason it is exceedingly difficult to secure satisfacto commutation in a-repulsion motor at spec s much above synchronism; . My invention (-fiIlSlStSill a. novel arrange- 'ment of the motor, such that the above-mentioned defects are eliminated.- I accomplish this by providing the motor with a fractional pitch rotor winding and so arranging the rotor brushes as to short-circuit the rotor winding on a line displaced from the line of primary magnetization by an angle, the complement of which bears to ninety electrical degrees a ratio approximately equal to'the pitch of the rotor winding. This statement may be explained as follows. It is well understood in the art that not all the conductors of a fractional pitch winding are magnetically effective, since in some of the slots the currents in the conductors are in opposite directions and neutralize each other. It is further well understood that the proportion of conductors that are effective is equal to the pitch; i. a, if the pitch is 75%, then 75% of the conductors are magnetically effective. N 0w, as will be hereinafter pointed out, it is desired that the magneto-motive forces of armature and inducing windings be similarly dist ibuted. To get this result the proportionof effective armature conductors, or, in other words, the fractional pitch, should be. equal to the proportion of the stator over which the inducing winding is distributed. Thiscan further be expressed in terms of the angle between the lineof the rotor short-circuit and the line of stator magnetization. The line of the rotor short-circuit should coincide with the line of magnetization of the inducing winding. The displacement therefrom of the line of stator magnetization is produced by the exciting winding and .is proportional to the amount of the stator over which the exciting winding is distributed. In other words, and sinc the portion of the stator covered by the exciting winding is thatnot covered by the inducing winding, the angle of said displacement bears to 90 electrical degrees a ratio equal to the proportion of the stator not covered by the inducing winding, and the complement of said angle bears to 90 electrical degrees aratio e ual to the proportion of the stator covered y the inducing winding. Butsince that proportion is equal to the fractional pitch, we reach the statement with which we started, that the angle between the lines of rotor shortcircuit and stator magnetization is'such that its complement bears to 90 electrical degrees a ratioek'ifial'to the pitch of the rotor w1nd- That is, if half-pitch is selected for the rotor winding, the displacement between the rotor short-circuit and the line of primary magnetization should be approximately forty-five degrees, since the complement of this angle, which is forty-five degrees, bears 'to ninety degrees the ratio of one-half. Siminetomotive forces involved in the induction of the rotor currents, thereby securing the same conditions as in a transformer in which the primary and secondary windings are exactly superposed; second, the opposite sides of a rotor coil undergoing commutation lie op osite the same pole of the stator so that the electromotive forces due to rotation in the stator'field tend to neutralize; and third; the field due to the rotor magnetomotive force is flattopped, instead of peaked, so that the speed can be carried much higher before the commutation limit is reached. My invention will best be understood by reference to the accompanying drawings, in which- Figure 1 shows diagrammatically a repulsion motor with its windings arranged in accordance with my invention; Fig. 2 shows diagrammatically the connection'of the motor winding; Fig. 3 shows the motor connected for operation as a series motor; and Figs. 4, 5 and .6 are explanatorydiagrams. In Fig. 1,'A represents the stator and B the rotor, which is provided with a commutator C and short-c rcuiting brushes 1) b. I have selected a bipolar motor for purposes of illustration and have shown each rotor coil with span of approximately 90 degrees, which in a bipolar motor is halfpitch.- The magnetic material of the stator is uniformly distributed around the rotor, and carries a winding which comprises two groups of coils, one of which, a a, is distributed over portions of'the stator winding corresponding to the pitch of the rotor winding,that is, over arcs of ninety degrees. The other group of coils a a is distributed over the remaining portions which, since the itch selected for the rotor winding is one-ha f, are also arcs of ninety degrees. The coils a a, which serve to induce the current in the rotor, as will hereafter be explained, roduce a fiel? which is vertical as viewed in ig. 1, while t 1e coils a a, produce the closs-magnetization or torque-producing field, which is horizontal as viewed in Fig. 1. Instantaneous directions of current in the several coils of the stator are indicated by arrow-heads. be seen that the direction of theline of 6 magnetization produced by the stator currents is as indicated by the dotted line a x. The purpose of dividing the stator winding into twogroups is, first, to facilitate reversal of the motorby reversing the relative connections of the two groups, and second, to permit the o eration of the motor as a series motor on eit er direct or alternating-current. With current flowing at any instant in the stator'coils a in the directions indicated by the arrow-heads, the directions of currentsin the rotor conductors will be as indicated by the arrow-heads, as may be seen by tracing a circuit through the rotor winding from one brush b to the other. It will be seen from the arrow-heads that, while the currents in the two conductors in each slot adjacent that portion of the stator covered by the coils a magneto-motive force as though a portion of. the rotor winding were cut out of circuit by the use of a plurality of sets of short-circuiting brushes. This. current distribution is shown diagrammatically in Fig. 2, in which the rotor conductors, which are effective in producing magnetization, are shown as black circles, while the conductors, each of which has its magnetic effect mutralized by the other conductor or conductors in the same slot, are shown as white circles. Fig, 2 shows that the direction of the rotor magnetization is vertical and the rotor brushes b. bgare laced in this diagram, on a vertical line to indicate this. This also. represents the posi tion they would in reality occu y with a Gramme ring armature. The di erence in the brush positions in Figs. 1 and 2 is due to the fact that in Fig. 1 the brushes are shown in the position they would in reality occupy with a drum-wound armature, due to the fact that the connections from the commutator segments to the coils are made at the center of the end connections of the coils, so that while in Fig. 1, as in Fig. 2, the brushes 1) b are, as far as the electrical operation is concerned, short-circuiting the motor on a vertical line, they actually lie on a horizontal line. It will be understood that hereinafter, when Ispeak ol the position of the brushes, I mean the eiiective electrical position, as indicated diagrammaticall not the physical positiomwh the arrangement of the ce immaterial in operation. It will be seen, from an i spection of Figs. 1 and 2, that not only is the line of magnetization of the rotor vertical, like that produced'by the stator coils a a, but that the jdepeiids, 011' ml which is Fig. 2, and portions of the rotor periphery, which are covered by conductors which are-effective in producing a magnetization, correspond exactly with theportions of the stator whichare covered by the coils a a. Consequently, the conditions in a motor thus arranged are the same as in a transformer, where the primary and secondar windings are exactly superposed, so that eakage fluxes are-avoided, and a high power-factor secured. In other words the armature reaction is exactly compensated. -'It will further be noted havinga distributed winding on the stator extending-over the-ent1reperipher ,and the usual full-pitch rotor winding. T e'relativedirections of current in'the stator A and in the rotor B are'shown 'by arrow heads. It will be seen that the ortions of both stator and rotor, which lie tween the dotted linesyassist inproducing'the cross-magnetization, and that the field due to the magneto-motive-force of'the rotor isapeaked and isnot completely neutralized by the stator. .' "Fig. 5 shows a diagram of a modified form of'repulsion motor, in which the stator-winding is distributed over only a portion of the stator A, as indicated by thesha'ding. The heavy lines represent the portions of the stator which are wound; the light ortions being supposed to be unwound. T e rotor purpose. B is supposed to be the usual-'ty-pewith fullpitclrwinding. inthis "motor the cross-magnetization is produced by the rotor alone; that portion of the rotor winding between the two dotted lines being effective for this In this motoralso the field due to the rotor magneto-motive "force is peaked and is not com letely neutralized by the stator. Fig. 6 s ows a similar diagram of a repulsion'motor arranged in accordance with my invention. The portion of the rotor windingy'which is effective in producing magnetization, is shown in heavy lines, and'lines outside of the dotted lines. The stator winding is distributed over all the stator. In this motor, as in the compensated series motor, the cross-niagnetization is produced wholly by the stator winding, the field due to the rotor magneto-motive force is flat-topped and is wholly neutralized by the state a as is clearly shown by Fig. 2. Returning to Fig. 1, it will be seen that the conductors composing a coil undergoing commutation both lie opposite the same pole of the field produced by the stator coils Zy This field may be considered as-exte'ndi approximately forty-five degrees on each: side of a horizontal line passing through the axis of the machine. The two conductors forming -a coil undergoing commutation lie under the two ti s of the same pole of this field, so that the-electromotive forces induced in these conductors by cutting this field are opposed in the coil, and consequently the disturbing effect on commutation of cutting this field is eliminated. The back connections of the coils undergoing commutation are shown in dotted lines in-Fig. 1, the back connections of the other coils being omitted in order to avoid confusion in the diagram. It has been seen, by comparing Figs. 4 and :6, that in the ordinary repulsion motor represented'by F i'g. 4, in whiclrall the rotor conductors are effective in producing the rotor magnetization or armature reaction, the shape of the 'field produced by'the'retor cur- ,rents induced by rotation is peaked, and the oint of the peak is-at the'brushes, while in ig. 6, on the other hand, since only the por tion of the rotor indicated by'heavy lines is effective in producingmagnetization, the .armaturemagneto-motive force produces a field that is fiat-topped in shape. Consequently, the electromotive foree induced in a :coil short-circuited by'the brusgles by cutting the rotor field produced by t e rotor currents due to rotation is much less in F ig. 6 than in Fig. 4. In any repulsion motor, the electromotive forces induced in a coil shortcircuited by the brush aredue to two factors; first, the electro-motive force induced by transformer action, and second the electroinotiveforce d'ue'to cutting the field in which the coil conductors lie at that instant. [it synchronism these two'electro-metive forces .are equal and opposite, so that commutation is excellent. Above synchronism the electromotive force due to cutting the field increases, and since in the ordinary arrangement commutation takes place in a peaked field, as above explained, the electro-n1otive force due to cutting this field increases rapidly, so that the commutation becomes very poor as the speed of the motor is increased. zvith the flat-topped field produced by my invention, the speed may be-increased much further before the limit of commutation is reached. The flat-topped field produced by my invention has a still further advantage in respect to commutation. In any repul sion motor the inducing stator-ampere'turns are slightly in excess of the induced rotor ampere turns, since it is this excess which produces the inducing or transformer-fielddn the motor. The electro-1not1veforce 1nduced in'the coil undergoing commutation by cutting this field is approximately ninety I, field; and consequently has -not beenconsid- -ered in the' above discussion. The electromotive force of commutation reactance, - however, is also ninety degrees out of phase with the electro-motive force induced by transformer action in the short-circuited coil, and consequently, if the electromotive force produced by cutting the excess inducing field above mentioned is in the right direc tion, it assists in commutation. In the ordi nary repulsion motor, however, 'asshown in Fig. 4, although the inducing stator ampere turns are in excess ,of the induced rotor ampere turns, nevertheless, because of the difference in distribution of these two magneto-motive forces, and because therotor magneto-motive force is greatest at the brushes, the direction of the field at, the brushes is that of the rotor magneto motive force, and this is in. precisely the wrong direction to produce in the coil short-circuitedby I in shape as i the brush an electro-motive force opposing the commutation reactance. In a motor arranged-in accordance with my invention, on the other hand, the field produced by the stator inducing coils a a is precisely the same the field produced by the rotor induced ampere turns. Consequently the slight excess of the stator ampere turns produces a field cut by the conductors of the coil undergoing commutation which has the direction of the statormagneto-motive force. The electro-motive force induced in the short-circuited coil consequently in the proper direction for neutralizing commutation reactance. As has been set forth, the purpose of dividing the stator winding into two groups is both to facilitate reversal of the motor, and also to render it possible to operate the motor as a series motor on either direct or alternatingcurrent. Fig. 3 shows the motor connected as a series motor for direct-current operation. The rotor brushes, instead of being shortcircuited, are connected in series with the stator windings. Furthermore, the crossmagnetizing or torqueroducing coils a, which in Fig. 2 I have sliown connected in parallel, are shown in Fig. 3 connected in series, so as to produce a relatively stronger cross-field. The reason for this change of connections, is fully set forth in my former application, Serial No. 354,259, filed January 26, 1907, and, briefly stated, is that a stronger cross-magnetization is required for satisfactory operation in a direct-current series motor than is desirable in the same motor operated on alternating current, whether as seriesor repulsion motor. It will be seen from Fig. 3 that owing to the identity in the distributions of the magneto-motive forces of the rotor and of the winding (1 a, which, with the series connection, serves as a compensating winding, perfect compensation for armature reaction is secured. It will be understood that I have illustrated my invention diagrammatically, and by cutting this field is that in practice any well known forms of motor construction .and windings may be emlployed. do not desire to limit myself to the particular construction and arrangement of parts here shown, but aim in the appended claims to cover all modifications which are within the scope of my invention. What I claim as new and desire to secure by Letters Patent of the United States, is, 1. In adynamo-electric machine, a stator winding, a fractional pitch rotor winding provided with a commutator, and brushes and connections arranged to short-circuit the fotor winding on a line displaced from the line of primary magnetization by an angle the com lement of which bears to ninety electrica degrees a ratio approximately equal to the pitch of the rotor wind g; 2. In a dynamwelectric,machine, adis tributed stator winding, a fractional pitch rotor winding provided with a commutator, and brushes and connections arranged to short-circuit the rotor winding on a hne displaced from the line of primary magnetization by anangle the complement of-which bears to ninety electrical degrees a ratio approximately equal to the pitch of the rotor winding. 3'. In a dynamo-electric machine, afrac tional pitch rotor. winding provided with a commutator, a stator winding comprising two groups of coils, one distributed over portions of the stator corresponding approximately to the itch of the rotor winding and the other distributed over the remaining portions, and commutator brushes and conneci tions arranged to short-circuit the rotor on a line parallelto the line of magnetization produced by the first mentioned group of stator coils. 4. In a dynamo-electric machine, a fractional pitch rotor winding provided with a commutator, a stator winding comprising two groups of coils, one distributed over portions of the stator corresponding approximately to the pitch of the "rotor winding and the other distributed over the remaining portions, and commutator brushes and shortcircuiting connections arranged symmetrically with respect to both groups of stator coils. 5. In a dynamo-electric machine, a fractional pitch-rotor winding, a stator haylng its' magnetic material distributed uniformly around the rotor, a stator wlnding comprising two groups of coils, one distributed over portions of the stator corresponding approximatelyto the pitch of the rotor wind ng and the other distrlbuted over the remaining portions and commutator brushes arranged symmetrically with respect to both groups .of statoncoils. 6. An alternating-current repulsion mo'- tor, comprising an armature provided with a commutator and a fractional pitch winding, brushes and connections short-circuiting the armature on a definite line, a stator, stator coils in inductive relation to the armature short-circuit distributed over portions only of the stator corresponding to the pitch of the armature coils, and cross-magnetizing coils occupying the portions of the stator not occupied by H otherstator coils. 7. In a repulsion motor, a rotor Winding provided with a comn'iutator, a single set of commutator brushes and connections shortcircuiting the Winding on a definite line, 'the winding having a fractional pitch whereby a portion of its conductors are at any instant magnetically inefl ective, and a stator Winding having two groups of coils, one distributed over portions of the stator corresponding a pproximately to the portions of the rotor the conductors of which are magnetically effective, land the other occupying the remaining portions of the stator. 8. In a repulsion motor, a rotor winding provided With a commutator, a single set of commutator brushes and connections shortcircuiting the winding on a definite line, the. Winding having a fractional pitch whereby a portion of its conductors are at any instantmagnetically ineffective, and a stator Winding having its line of magnetization displaced from the line of the rotor short-circuit by an angle corresponding approximately to oneally inel 'lective conductors on the rotor. E). ln a repulsion motor, a rotor Winding provided with a commutator, a single set of commutator brushes and connections shortcircuiting the winding on a definite line, the winding having a fractional pitch whereby a portion of its conductors are at any instant magnetically inefi'ective, and a stator Wind ing comprising coils arranged to produce in conjunction with the rotor Winding a fiattopped field in the direction of the line of the rotor short-circuit and other coils arranged to produce a erossdield. 10. In a repulsion motor, a fractional pitch rotor winding provided with commutator, brushes, and connections slwrt-circuiting the Winding on a definite line, and a stator Winding comprising coilsarranged to produce in conjunction with the rotor winding a flat-topped field in the direction of the line of the rotor short-circuit and other coils arranged to produce a cross-field. In Witness whereof, I have hereunto set my hand this 30th day of April, 1907. ERNST F. W. ALEXANDERS ON. Witnesses 1 V BENJAMrv B. HULL, HELEN Onronn.

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