US1035373A - Alternating-current induction-motor. - Google Patents

Description

B. MOGOLLUM- ALTBRNATING CURRENT INDUCTION MOTOR. APPLIOATION FILED IARJI, 1 912.

1,035,373. Patented Aug. 13, 1912.

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COLUMIIA PLANOGRAFH 0a.,WASHINGTON. D. c.

UNITED STATES PATENT OFFICE.

BURTON MGCOLLUM, OF WASHINGTON, DISTRICT OF COLUMBIA.

ALTERNATING-CURRENT INDUCTION -MOTOR.

To all whom it may concern:

Be it known that I, BURTON MoCoLLUM, a citizen of the United States, and a resident of \Vashington, District of Columbia, have invented a new and useful Improvement in Alternating-Current Induction-Motors, of which the following is a specification.

In the construction of induction motors it is necessary to provide slots in the magnetic members in which the conductors are placed, and for this purpose three general classes of slots are in use, viz., (1) wide open slots, (2) partially closed slots, and (3) completely closed slots. Each of these forms possesses certain advantages and at the same time certain disadvantages The wide open slots give, as a rule, a lower magnetic leakage, but they impart a high reluctance to the magnetic circuit making a large exciting current necessary, and in general causing the reactance to pulsate with the movement of the rotor. Completely closed slots give, on the other hand, a low magnetic reluctance but tend to give rise to great magnetic leakage which is, as a rule, very objectionable. Partly closed slots on either or both memhers aiford a compromise between these two extremes, and this form has been most frequently used in practice. I have now devised a means whereby, without increasing the cost of the motor, substantially the same low magnetic reluctance and exciting current are obtained as can be obtained by the use of completely closed slots on the secondary member, and at the same time the leakage is reduced to even a lower value than is obtained by the use of wide open slots of the usual types. The means whereby I am enabled to accomplish this result is set forth in detail in the following specification, reference being made to the accompanying draw- 1ngs.

Of the drawings: Figure 1 illustrates the effect of wide open slots of the usual form in increasing the magnetic reluctance of the main magnetic circuit. Fig. 2 shows how this increase in magnetic reluctance may be avoided by a special design of the teeth and slots of the secondary member. Fig. 3 is an end elevation of a portion of the secondary member, showing a special case of a practical embodiment of my invention. Fig. 4 is a sectional view of a port-ion of the secondary member, showing a method of short circuiting the conductors of the secondary circuits. Fig. 5 is an end elevation of a por- Specification of Letters Patent.

Application filed March 11, 1912.

Patented Aug. 13, 1912.

Serial No. 682,838.

tion of a rotor, showing a general embodiment of my invention. Fig. 6 is a side elevation of the structure shown in part in Fig. 5.

In the following description of my invention, for the sake of simplicity and clearness, a special case is first described, after which the more general embodiments of my invention are disclosed.

Referring to Fig. 1, 1 and 2 represent respectively a tooth and slot of the primary member, and 3 and 4 a tooth and slot of the secondary member. The lines 5 show how the magnetic flux is bunched in a comparatively small portion of the tooth surface. It will readily be seen that the fraction of the area under the teeth of the primary available for carrying magnetic flux is substantiall equal to the ratio of the tooth width to the combined width of slot and tooth of the secondary member, wide open slots on the secondary being assumed. As a rule, in practice but little more than half of the area under the teeth is effective on the average for carrying fiux. It is important to note that in this instance the width of the slot is large compared to the depth of the air gap. If however, we use a very great number of teeth and slots on either member, preferably the secondary, and thus make the width of the slots small compared to the depth of the air gap, a very different condition is obtained. This is shown clearly in Fig. 2. In this figure, 1 represents a part of the tooth of theprimary greatly enlarged, 3 and 4' are respectively the teeth and slots of the secondary, and 5 shows substantially the distribution of magnetic flux in the air gap under the tooth face. Here it is seen that the magnetic flux is distributed practically uniformly over the entire air gap except at points very near the secondary where it begins to concentrate over the ends of the teeth. It is very evident here that the entire cross section of the air gap under the teeth of the primary is available at all times for carrying magnetic flux except at points so near the secondary that that portion of the air gap in which the flux is concentrated constitutes but a small fraction of the total depth of the air gap, and consequently the reluctance of the air gap is substantially the same as would result from the use of completely closed slots on the secondary member. At the same time the magnetic reluctance in a direction at small compared to the depth of the air gap,

I am able to secure the beneficial effects of completely closed slots on the secondary member, while at the same time, avoiding the disadvantage of large magnetic leakage arising from the use of closed slots. It will not generally be found practicable however, to use as large a number of teeth and slots as would be necessary to accomplish the results above described without departing from the usual method of construction in which the tooth is integral with the core of the secondary member, because the teeth would become so thin and frail that they could not be produced by the ordinary method of punching, and the cost of manufacture would in general be high. This objection is overcome however, in the modified construction shown in Fig. 3. Here the laminations which make up the core 6 of the secondary member are made up as plain circular disks without teeth. After these are assembled into a cylindrical core, there are placed on the surface of the core and approximately parallel to the axis of the cylindrical surface, thin strips of magnetic material 4. These strips are placed edgewise and project radially outward from the core, and take the place of the usual teeth as carriers of magnetism. These strips of magnetic material are spaced apart and are made to alternate with strips of non-magnetic material 3 of good conductivity, such as copper for example, which serve as the principal conductors of the induced currents. After these strips are in place, they are mechanically and electrically connected at both ends to rings 7 of conducting material, such as copper, brass, or other suitable material. One method of making this connection is shownmore clearly in Fig. 4. Here the longitudinal strips 4 and 3 are provided with a projection 8, which fits into a groove cut in the ring 7, and the longitudinal strips 3 and 4 are brought into mechanical and electrical connection with the ring 7 by means of soldering, welding, or by other suitable means. Numerous other methods of making these connections'will be apparent to any skilled mechanic. This affords a very inexpensive construction and has been found to give entirely satisfactory results. may be made from ordinary thin sheets or strips withparallel sides if desired, or they may be slightly thicker on the outside than on the inside, so as to fit closely andoccupy The longitudinal strips 3 and 4" all the space available for them. This lat ter not only gives a better economy of space, but also affords a firmer and more satisfactory mechanical structure. If desired, the strips 3 and 4 may be more or less insulated from each other and from the core 6, as by japanning or by other means, but as a rule this will not be necessary. In the form shown and described above, the strips 4 of magnetic material serve also'as conductors for part of the induced currents, but this is obviously not essential. They may, if desired, be insulated fro-m the strips 3 and from the ring 7. The former structure is, however, usually to be preferred.

In the case of rotor cores of ordinary length the strips 3 and 4 will need no other mechanical supports than the end rings 7.

When very long rotors are tobe used however, binding wires or other suitable reinforcements may be necessary at suitable intervals overthe outer surface. The strips 3 and 4 may be prevented from slipping on the core 6 by having an occasional strip somewhat wider than the restand projecting into a slot provided for it in the core 6 as shown at 9, Fig. 3. The same result canbe. accomplished by fixing the ring 7 to the core 6, one method of doing which is shown in Fig. 4, where pins or set screws 10 pass through the ring 7 and engage the core 6.

In the construction described above, it is necessary, in order to secure a low magnetic reluctance in the air gap, to make the width of the copper strips 4 quite narrow as compared'to the depth of theair gap, and since the air gap is itself usually quite small, a large number of strips becomes necessary. The thickness of the strips can be greatly increased and their number therefore reduced without increasing the magnetic reluctance of the air "a3 and without seri-- ously increasing the magnetic leakage, by the use of the'more general embodiment of my invention shown in Fig. 5. Here a thin sheet of magnetic material 11 is placed over the outer surface of the secondary member. In this case, the flux- 5 will be practically uniformly distributed over the air gap under the face ofthe primary tooth 1, and on reaching the magnetic sheet 11 that partof the flux which lies directly over the nonmagnetic strips 4 is conducted along the band 11 in either direction to the magnetic strips 3. Since the flux dividesat about the middle of the non magneticstrips 4, the thickness of the magnetic sheet 11 needs to be great enough to carry without over saturation, only one half of the flux lying directly over one of the non-magnetic strips 4. The thinner the strips 4 are made therefore, the thinner the sheet 11 can be made, and if the strips 4 are made sufficiently thin, the sheet 11 canbe dispensed with altogether, in which case the construction reduces to the special case described above. In using this construction, it is important not to make the sheet 11 unnecessarily thick, since this would increase the magnetic leakage unduly. By proper de sign however, the sheet 11 can be kept sufficiently thin to prevent any serious magnetic leakage and at the same time, the thickness of the magnetic strips t can be increased so that their number will not be great enough to make the construction tedious or expensive. It is apparent that the magnetic band 11 may also be used in connection with the structure shown in Fig. 3 in which the teeth 3 are integral with the core 6. By so doing, the teeth and slots may be made sufficiently wide to make it possible to make the laminations of the core by the usual method of stamping, without at the same time making, the band 11 so thick as to cause excessive magnetic leakage. This latter construction can obviously be used on either primary or secondary members, since it is quite as applicable to wound rotors as to rctors of special design described above.

As a rule, it is desirable that the sheet 11 shall not be electrically continuous in all directions over the surface of the rotor, since in that case objectionable eddy currents would be induced therein. In practice, the sheet 11 may be given the form shown in Fig. 6, in which a thin tape of magnetic material such as iron, nickel or cobalt having any suitable width is wound spirally over the surface of the rotor. This breaks up the electrical continuity in a direction parallel to the axis of the rotor, and thus effectually eliminates eddy currents. At the same time, the tape, being mechanically continuous in a direction tangential to the surface, serves also to oppose the centrifugal forces acting on the strips 3 and 4 at high speed. This is an important function, especially when the distance bet-ween the end rings is large. In order to further eliminate eddy currents, it is desirable that the magnetic covering 11 shall not make good electrical contact with the rotor conductors 3 and 4' at many points. To prevent this, a very thin coat of japan or other thin insulating material may be interposed between the outer covering 11 and the rotor conductors. Since the voltage tending to produce eddy currents is very low, a low degree of insulation will suffice, and in many cases the insulation may be dispensed with altogether.

It is obvious that the foregoing structure may be inverted, the secondary member being placed on the stationary part and the primary member on the rotating part without departing from the generic princlple of my invention.

I claim:

1. In an induction motor comprising rimary and secondary members, a seconc ary member comprising a magnetic core, thin strips of magnetic material alternating with strips of nonmagnetic material placed on said core and interposed edgewise between said core and the primary member, means for short circuiting, said strips of magnetic and non magnetic material at both ends and a thin band of magnetic material on the outer surface of said magnetic and non magnetic strips.

2. In an induction motor comprising primary and secondary members separated by an air gap, the secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of non-mag netic material and placed edgewise on said core, means for short circuiting said magnetic and non-magnetic strips at both ends, a band of magnetic material covering the outer surface of said strips, said strips being thinner than the depth of the air gap between primary and secondary members.

3. In an induction motor comprising primary and secondary members separated by an air gap, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of nonmagnetic material placed on said core, a thin band of magnetic material covering the outer surface of said strips, means for short circuiting said magnetic and non-magnetic strips at both ends, the thickness and composition of said magnetic band being such that the magnetic flux required to produce saturation in said band in a tangential direction will be approximately the flux carried by a port-ion of the air gap equal in width to one half the thickness of one of said nonmagnetic strips.

4. In an induction motor, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of non-magnetic material placed edgewise on said core between primary and secondary members, means for short circuiting said magnetic and non-magnetic strips at both ends, said strips being thin compared to the depth of the air gap.

5. In an induction motor, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of non-magnetic material placed edgewise on said core, means for short circuiting said magnetic and non-magnetic strips at both ends, a band of magnetic material covering the outer surface of said strips, a portion of said strips projecting into slots provided for them in said magnetic core.

6. In an induction motor comprising pri- 'mary and secondary members separated by an air gap, a secondary member comprising a magntlc core, thin strips of magnetic ma terial alternating with strips of non-magnetic material placed edgewise on said magnetic core, means for short circuitmg said magnetic and non-magnetic strips at both .ends, and' short circuited at both ends, said non-magnetic strips being thinner than the depth of the air gap.

7. In an induction motor comprising primary and secondary members separated by an air gap, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of non-mag.- netic material placed edgewise on said core, means for short circuiting said magnetic and non-magnetic strips at both ends, said strips being thin compared to the depth of the air gap, and certain of said strips projecting into slots provided for them in said magnetic core.

8. in an induction motor comprising primary and secondary members separated by an air gap, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of n0nmagnetic material placed edgewise on said magnetic core, means for short circuiting said magnetic and non-magnetic strips at both ends, said non-magnetic strips being thinner than the depth of the air gap.

9. In an induction motor, a secondary member comprising amagnetic core, thin strips of magnetic material alternating with strips of non-magnetic material placed edgewise on said core, means for short circuiting said strips at both ends, which means consists of end connections of conducting ma terial soldered to the ends of said strips.

10. In an induction motor, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of nonmagnetic material attached to said core in an edgewise position, means for short circuiting said strips at both ends, 40 which means consists of end connections of conducting material soldered to said strips, said end connections being made to engage said strips by means of a mortised construction.

11. In an induction motor, a secondary member comprising a magnetic core, thin strips of magnetic material alternating with strips of non-magnetic material placed on said core in an edgewise position, a band of magnetic material covering the outer surface of said strips, means for short circuiting said strips at both ends, which means consists of end connections of conducting material soldered to said strips, said end connections being made to engage said strips by means of a mortised construction.

BURTON MCCOLLUM;

Vitnesses J. G. MoGoLLUM, H. D. HUBBARD.

Copies ofthis patent may be obtained for five cents each, by addressing the Commissioner of Iatents, Washington, I). C.

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