US984582A - Alternating-current motor. - Google Patents

Description

B. MoCOLLUM. ALTBRNATING QUBBENI' MOTOR. APPLIOATION FILED D180; 22, 1909. 984,582. Patenned Feb. 21, 1911.

2 SHEETS-SHEET 1.

fame/Mo's I fluri'an/ M follumw B. -MGGOLLUM- ALTERNA'IING CURRENT MOTOR.

I APPLICATION FILE D 13210.22, 1909. 984,582.

Patented Feb. 21, 1911.

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attomwf To all whom it may concern:

,ful Improvements in Alternating-Current at which a possible spark may occur, makes :tATlrlNT J Applicatien file-cl .Bccember 2&3, ZtZe.

Be it known that I, BURTON MoC-oLLUM, a citizen of the UnitedStates, residing at Lawrence, in the county of Douglas and State of Kansas, have invented new and use- Motors, of which the following is a specification. a

My invention relates particularly to induction motors of the squirrel-cage type, and has to do with the starting of these motors. However, as will appear hereinafter, my invention is not limited to its application to squirrel-cage motors, it being practicable to apply it with facility, and a. great resulting advantage, to coil wound rotors of induction motors, and in other ways.

The starting torque of squirrel-cage in duction motors is, as is well known, very poor when such motors have good running constants. Motors of the squirrel-cage type possess, however, many very excellent features, especially adapting them-for use in many situations. Chief among these vteatures is their good mechanical construction, and the absence of moving contacts of any sort. The absence of all moving contacts these motors particularly desirable for use in mines and in factories where 'there is danger of fire, while the absence of these contacts, and sturdy mechanical construction of the squirrel-cage motor, makes them least liable to breakdown, and adapts them to stand usage under the hardest conditions. But owing to their poor starting torque, their use has. beenmuch limited.

It is well known to those skilled in the art that an induction motor requires a high resistance in the secondary circuit, if it is to have good. starting torque, but this resistance must be low it the motor is-to have good running constants, suchas high eiiiciency, good speed regulation, and low temperature in operation. In order to accoin plish' this numerous devices have been pro posed, most of which contemplate the use of a rheostat resistance in series with the secondary winding, which is cut in instarti ng and cut in when the motor is running. These are'open to the objections that they usually are provided with movable contacts, are e10 pensive, mechanically complicated, and re quire a coil wound rotor, all of which conditions it is desirable to avoid. Most of the device's proposed are also subject to the oh i Specification of Letters- Eatent.

ing the motor. Attempts 1h jection that they are norsaii to produce a. motor harem these ttjet tionable features, and which would operate automatically in starting; out f these features is the making of the short circuiting ring of the squirrel-cage outer-"iron inert-l to take advantage of the'iskin effect 1 "found that this ineans givesgood g torque, but operates satisactorilyo1rly in' jno tors of small size, so small as two or horsepower. The meters of large-"size" equipped with this means of starting auto-F matically may have a good startingitorqupg but poor speed regulation. and pocirgefiici'enc The reasons for this will beapparentpupm a little study of the curves of Eigurea. the accompanying drawing. flhei-curve-nume's bered I on this sheet, is the saturation curve: of the average grade of iron emplojted i ftl llQ manufacture of machines in Ithe' electrical art. This curve drawn het-weenulgiloeliness of flux per square centimeter nd tlre'nazzigr-i netizing force in c. gmsumit. observed that the iron ,ifapiclly hecomessatii rated, the greatest cl'iange'in flu-ti Ede occurring before the ina'gnetizinghtoi reached 20 c. g. s. units Thereafter-their is;

only the very slightest-Change; in fl fi l? density, the saturationeu ciao approximately straight parallel to the horizontal. axis a magnetizing force of ten. been applied, there is relative nit-s has in the flux density. Now. var la ions ei'tect depend directlynpon variat' density, as well asupon a frequency of the circuit conductor. Speaking b proximate formula. .lev r trosmo'tive force impre sepia of a conductor. This willh t given instant for every eleime t ti-on of the conductor; Let e iii ii, F. in any eleme-nt of tl used at alternations of round that element. 6, acting on that eleme counter E. M. Ffs, or,

But 5 equals Qwfd; I quency of the nnpresse the maximum value 0 with the element under,

' Therefore simplicity, the center of the conductor. Hence, we have the ratio of the currents, will be '6 21rf From this equation we see that .if 0 is constant and 4 is constant, which would be the case with well saturated iron, then as the frequency f increases'the denominator decreases, thus increasing the ratio between the current near the circumference and that near the center, producing a greater inequality in the distribution of current and increasing the resistance. Obviously, from the equation, and the assumption made in regard to constant-impressed E. M. F., and constant flux b, the resistance would increase indefinitely with increasing frequency. Now let us apply these facts to an induction motor. If such a piece of iron is made a part of the secondary circuit of an induction motor as described-above, the impressed E. M. F. is not constant as assumed above, but varies with the frequency. In a constant potential motor the flux remains nearly constant, and such being the case the E. M. F. 6 is equal to E7, where E is the E. ltI. F. at unit frequency, and is constant.

Putting this in equation 3, we have V i Ef-27 rf E27r We see therefore, that the ratio of the current near the surface to that near the center is independent of the frequency and therefore the same distribution of current,

and hence the same resistance, will result at all frequencies and at all speeds. Hence, if

it is desired to secure an increase of resistance with decrease of speed we must make qt in the denominator increase as the speed falls off. That this is a physical impossibility where soft iron is used, is seen from the following consideration. The working current in the rotor of a squirrel-cage motor, must of necessity be quite large in order that the motor have the necessary power for which it is designed. Thus, in a ten-horsepower motor the normal load current in a rotor is such as to produce in a short-circuiting ring a magnetizing force of about tweney-five or thirty c. g. s. units, and for larger motors would be still larger. As we have seen, iron under this force is already well saturated, and any further increase of motor.

current cannot increase appreciably the value of (,5. Hence, when iron rings are used for the purpose stated, the resistance will be practically independent of the frequency, and the skin etl'ect will not change-appreciably from no load or low speed to full load or high speed. The fact that there is the same skin effect as at full speed at starting, gives rise to a large slip, poor speed rcgula tion, and low etliciency, giving a motor of a horse power above two or three. or a motor of the very smallest size, very poor running constants. V Another device used in connection with induction motors to accomplish the same ends as are accomplished in the use of ohmic starting resistances. and make the motor automatic, is an inductive resistance used in connection with a coil wound rotor, the inductive resistance being closed cir cuited and free from moving contacts, and having a core of a hysteresis material. The inductive resistance then varies with the variation of current in the coil wound rotor of the motor, varying in degree as the frequency of the rotor E. M. F. varies, and the action of the inductive resistances being magnified by the presence of the core of hysteresis material. The core of hysteresis material in this instance forms no part of the electric circuit of the rotor, being simply a part of the magnetic circuit ofthc inductive resistance. Th1s device is open to the objection that unless it is placed on or within the rotor itself, movable contacts have to he provided in order to get the taps away from the Necessarily the induction resistances with heavy hysteresis material cores are very heavy, and are too bulky to be placed within the core of the rotor and out of the way, where the taps can be led to them without the use of nioving contacts.

It is the object of my invention to pro-- duce an induction motor which starts automatically, which has good starting characteristics, good running characteristics, and good starting torque. ood running torque, and a good efficiency, and which at the same time is free from moving contacts which are liable to produce sparks or undue heating of some parts. and which has the sturdy mechanical features of the squirrel cage motor under, the hardest and most severe conditions. After many experiments I have succeeded in producing a motor which possesses all these features. and which ope 'ates in an entirely satisfactory manner in both small and large sizes.

In the accomplishment of my invention I providethe rotor of the squirrel'cage motor with short-circuiting rings of a material having a relatively high hysteresis resistance. By using a short-circuiting ring of this material, I obtain three-fold advantageous re sults. In the first place, I get a high skin ear-s82 efiect, which varies greatly between low speed and high speed of the motor. I have found that a material having a relatively high hysteresis resistance, also has a marked skin effect, which varies greatly ance due to the hysteresis of the material itself, the passage of the current directly through the conductor giving rise to this resistance. "This hysteresis resistance also -varice with the variation in the magnetizing force of the current passing through the conductor. .And in the third place the two preceding advantages are obtained in the mostsiinple manner imaginable, the resistance be in inherent in the conductor of relatively 'hig hysteresis material which is used as-the short-circuiting ring of the squirrelcage 'rotor. .This structure is extremely simple and most efficient.

, As "regards the first of the advantageous results just mentioned, referring to curve II of Fig. Jga simple examination of this curve in connection with what has been said above as to skin effect in conductors and particularly in iron conductors, will sufiice to show the existence of this advantage. Curve II is the saturation curve of a material' having a relatively high hysteresis resistance. Instead of rising abruptly, as is thecase with curve No. I thiscurve rises gradually and approximates a straight line from the zero pointclear upto and past the 150 c. g. s. units point to, approximate saturation. The flux density per square centimeter of a material of this nature, thus varies widely with varying magneto motive force, and does not remain substantially constant as is the case with material such as iron. With reference to the resulting equation No, 4 above given, it is quite clear that a skin eiiiect is had in a conductor having arelatively high hysteresis resistance, which shin eiiect 1S not approximately constant, but which is high at low speed of the motor when the magneto motive force is high, and

low at high speed of the motor'when the magneto motive force is low. That the sec ond advantage mentioned above follows primarilyyfrom the employnnent of a material having'a relatively high hysteresis resistance a short-circuiting ring, is striking andeasilyv discernible from inspection of the curves of Fig.4. The loss of energy due to hysteresis magnetic material is expressed by the formula:

Pinf where P is thepower lost due to the hysteresis eflect in the iron, 12. is a constant depending on the quahtycf the material and size and shape of the magnetic circuit, fis the frequency of the alternations of current, and 13 1s the magnetic flux density in the circuit. This energy loss maybe supposed to be expended in an additional. resistance inserted in the circuit, in which case the power lost would be given by the equation,

where I equivalent resistance necessary to produce the same energy loss as the hysteresis effect.

Equating and (6)"we have s Equation (8) gives the eiiective resistance of the circuit due to hysteresis. As has'been heretofore stated, the induction motor ncc turn, the current carried by the short-circultmg rings were of necessity very large, as they are in all 1ndu'ct1on motors. IVhen is the currentzflowi'ng and-1* is the For example, a 'ten-' s. units,rand since in the: squirrel cage rotor there is but-oneeftective' starting the magnetormotive force was the equivalent of c. g. s. units. Both starting and load currents are therefore large, and the starting current may be said to be four times the full load running current.

This motor also had a slip of about 5.- per cent. Applying formula No, 8 to this par- .ticular motor we may. write the formula with zero subscripts, numerals, etc., writing W as the effective starting resistance of the rotor as follows: I

mB -e 0 Subscript characters represent variable factors at the starting of the motor. We may likewlse write the value ofthe efie-ctive resistance a" at full load,as f 'ollowsz' 4 air- .7'1= ;[12 I I In this equation the subscript 1', is used to load. Now let us substitute in this equation the value ofthe variables as we know them.-

of iron, and referring to curve No.1 I of Fig.

indicate the value. of the variables at full we find that the value B does not change materially between the valuesiZo and 100 of the magnetomotivc force. We may thereance. therefore,

fore treat this quantity as a constant one for Therefore, for a motor equipped with an iron shortcircuiting ring 1, equals .S7' Hence the value of the running resistance is nearly as large as the starting resistance, and unless this value is made low the machine could not attain so small a slip as was assumed. But when we make the value of the resistance low, the starting torque of the machine will be only slightly better than the ordinary squirrel cage rotor with copper or brass rings. Since the resistance of the iron short 'circuiting ring is reduced only about 20 per cent. between low speed and high speed of the motor under load, and about half of the total resistance of the rotor is due to the copper. bars, the total resistance is changed only about 1/10 or 10 per cent. Consider now the case. of this same motor provided with shortcircuiting rings of a material having a relatively high hysteresis resistance, which material is that I'use in the accomplishment of my invention. Referring to curve No. II, which is the saturation curve of a material of this character used by me, it will be found that the value ofB,when the magneto motive force is 25 units is about 2,000 lines per square centimeter, while the value when magneto motive force is 100 c. s. units 14,500 lines. B equals T1 'stlfrom which B power equals flBQ-i'. Substituting these values in our preceding equation we have,

a 13 ,r 1 1.: 3 15. :2 ?l-" 3:% W}? K 352; 1,.

The running the material haw-in; the 1 esis resist the starting resistance. as before, the copper resistance constitutes about one-halt of the total, giving the total starting resistance about fifteen times the running resistance, as

change of only about ten iron ring ls'used. It: is ans result which may be compared with the per cent. when an this most advant said to be prom connection with the result from the greatly c. which has secured to isiin'ient of my invention, to a tain its objects. That I acts of my invention Q from inspection of of Fig. 5. Curve auu inentwl me the ac and enabl the e .where its slip is quite one per cent. smaller ced in degree, taken in life-1,682

torque curve of a motor equipped with the shortcircuiting rings of my invention. More than double full load torque is available at starting in. my motor. The tor ue increases slightly at the point of twentyve per cent. slip, and then decreases to the tall load point than the slip of the motor with the copper shortcircuiting rings.

In the accomplishment of my invention '1 have experimented with many materials. I have found that not all substances or materials which have a relatively high hysteresis resistance are adapted for use in the electric circuits of motors. Besides having a relatively high hysteresis resistance. it is necessary that the material used should have a low ohmic resistance at full load. It is also desirable that the resistanceof the material used as a conductor should be very low. Ordinary carbon steel when hardened to present a relatively high hysteresis resistance, I found has too high an ohmic resistance. Likewise manganese steel and numerous others have too high a. resistance or reactanceo Among the various materials with which I have experimented, I have found several which are most admirable for the purpose, chief among which are steels alloyed respectively with chromium, molybdenum, and tungsten, '01 combinations of these, and of these I have found tungsten steel to -be the best. It is characteristic of these materials that they have a relatively high skin effect and hysteresis resistance at low speeds of the motor, and a skin effect and hysteresis resistance relatively lower than castiron at higher speeds of the motor, and have a relativel lower ohmic resistance. All of these steels temper very hard and then draw the temper until the bestresultsare secured. Curve No. II of Fig.4 is the and a half per cent, which proportion I have found to be most desirable for motors of ordinary size. Curve No; IV-is the torque curve ofa motor equipped with tungsten steel rings This material has an extremely low ohmic resistance, and as has been seen,

fulfils admirably the conditions'of service. That the third of the three-fold advantageous results 'n'ientioned, namely, simplicity and hardiness, or mechanical. sturdiness and safety, results from the use of this particular material, will be apparent from a consideration of the accompanying drawings, and particulariv of Figs. 1 to Of the drawings, Fi 1 a an end view of an induction mote squirreicage type, equipped with a :ircuiting ring of the material of my invention.

Fig. 2 is I tioned. Fig. is a figure in which is delineated torque curves of a motor equipped with ordinary copper rings and with the rings of my invention above mentioned.

.motdrfwhile 11 indicates the squirrel-cage at copper bar conductors 12.

rings 13 of a material having'a relatively 10 indicates the stator winding of this This rotorfisprovided with the usual Instead of the usual copper shortcircuiting rings, how-ever. itis provided at its ends with shortcircuitinq high hysteresis resistance, such as tungsten steel of the grade mentioned above All that is necessary in starting the motor equipped with theserings is to connect the leads 15 to a source of power by a suitable switch. The motor will then start under load. and have throughout its operation the good characteristics of a motor of this type. There are no moving cont-actsfall the connections of the motor being mechanically and electrically secure and permanent. As the stator bf the induction motor has no movable contacts, the induction motor of this invention is entirely-free from them. That the use of this material as a shortci'rcuiting ring is at once the simplest, neatest, and most efiicient arrangement, is clearly apparent. The combined skin effect and hysteresis resistance is had at one and the same time in this shortcircuiting ring, which forms a part of the electric circuit of the rotor.

hen it is desired to build motors of large size, the current in the rotor circuit may become'so great that, as above described, the normal fullloadourrent will produce practically a condition of saturation of. rings of normal proportions, so that with increasing current due to decreasing speed the valve of qS ,in equationta) will be nearly constant, as is the case with iron rings in very small motors, and hence there will not be nearly as great a tendency for the resistance to increase with decreasing speeds. In order to overcome this defect, and avoid using abnormally heavy rings, when very large motors are to be built, I make use of the modified squirrel-cage construction, as shown in Fig. 3. It is made upjof two intermeshed cages, preferably electrically independent, one of them being composed of the bars 16, the copper short. oircuiting ring 17, and the ring of relatively high hysteresis resistancematerial 18; and the other of the bars 16 and the'rings 17 and 18. By using this construction half the Working current is made to flow in one ring 18 and half in the other, and hence much larger working currents maybe employed before the normal load current produces a' condition of ap at higher speeds of the motor.

proximate saturation. In this type of con struction, therefore, it is possible to m"; a use of the principles hereinabove set forth, in motors of much larger size than is possi-? ble with a single squirrel-cage, without unduly increasing the size and weightof the shortcircuiting rings for themostadvantageous normal operation. It'is, of course, evident that I may form both rings 17 an dsrl' 18 as rings of relatively high 'hysteresi 7 'sistance material. The prime feature o'f t is modification is the division of the-work current by the use of. the double ui'rrelcage construction. Obviously the ca g J c ou'fldti'. be made in a multiple number of 'interm'edi- 0 ate sections greater than. two.

While I'have described the bestfornnand embodiment of my invention now vknownlto 'me, I desire to have it tlntlelFtOtXltliat iaiil'ii the annexed claims it is intended that all modifications not departing from the generic spirit of my invention be covered.

What I claim. is:

1. In an'induction motor of the squirrel cage type, a short circuiting ring of a material having a high hysteresis resistance and a high skin effect at low speeds of the motor and having a hysteresis resistance and skin effect relatively lower than cast iron 2. An induction motor comprising primary and secondary windings. said secondary winding having parts of its electrical circuit formed of a conductor of magnetic material which has a lower ohmicresistance than cast iron, and a relatively high hysteresis resistance and skin effect at low speeds of the motor and hysteresis resistance and skin effect relativelylower-than cast iron at higher speeds of the motor.

3. In an induction motor having primary and secondary members. a conductor of fixed dimensions forming part of the secondary of said-motor and which has high hysteresis resistance and high skin effect at low speeds of the motor and has a hysteresis resistance and skin effect relatively lower than cast iron at higher speeds of the motor.

4. In an induction motor. primary and secondary members, one of said members being provided with a conductor of tungsten alloy steel forming-a part of the electriccircuit of said member.

5. In an induction motor, primary and secondary members, one of said members being provided with a conductor of tempered alloy steel of low ohmic resistance forming apart of the electric circuit of said member.

6. In an induction motor, primary and secondary members, one of said members being provided with a conductor of tungsten alloy steel forming part of the electric circult of said member, said conductor contaming two and one half per cent. or over of tungsten.

7. In an induction motor, primary and secondary members one of said members having permanently in its electrical circuit a conductor of a magnet material having a saturation curve which approximates closely .a straight line between zero point on the curve and approximate saturation.

8. In an induction motor, primary and secondary members, one of said members havin two windings of ion ohmic resistance isposed around the member and each winding being permanently short- .-ircuited by a conductor having a relatively high hysteresis resistance at low speeds of the motor, and a hysteresis resistance relatively lower than cast iron at higher speeds of the motor.

9. In an induction motor, a squirrel cage rotor formed of two electrically independent cage sections, each of which is provided with two short circuiting rings, one at each end, one of the short circuiting rings of ca ch cage section being composed of a material having a relatively high hysteresis resistance and skin effect at low speeds of the motor and laving a hysteresis resistance and skin efioct relatively lower than cast iron at higher speeds of the motor.

10. In an induction motor, a squirrel cage rotor comprising a magnetic core provided with slots, and two intermeshed cage sections composed of bars in said slots, and two sets of short-circuiting end rings, the bars in some of said slots being connected to one set of end rings, and the remaining bars to the other set of end rings, the bars of one set overlapping an end ring of the other.

In testimony whereof I have hereunto setmy hand in presence of two subscribing witnesses.

BURTON IBICOOLLULI.

\Vitnesses J. R. TARBOX, .iiin'rnnn L. Bar-ANT.

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