US1044217A - Squirrel-cage rotor. - Google Patents

Description

MGGOLLUM. SQUIRREL GAGE ROTOR.

APPLICATION FILED 1330.6, 1911.

1,044,217. Patented Nov. 12, 1912.

UNITED STATES PAEENT OFFICE. A

. nun'rou McCOLLUM, or WASHINGTON, msrmcr or coLuMBrA.

. SQUIRREL-CAGE ao'ron.

To all 'wlwmitmay concern.

Be it known that I, BURTON M COLLUM, a citizen of the .United States, and a resident of Washington, District of Columbia, have invented a new and useful Improvement in Squirrel-Cage Rotors, of which the following is a specification.

My invention relates to the type of alter nating current induction motor having a rotor known as the squirrel cage rotor, and has for its object the improvement of the operating characteristics of such motors. In practice such motors possess certain disadvantages .in that they have a nearly constant resistance in the secondary circuit, for which reason, as is well known, it'is impossible to secure in one'motor both high starting torque and highrunning efficiency, together with close speed regulation. Further, such motors as ordinarily constructed have an inherently low power factor, which also is objectionable in practice. With a view of overcoming the first of these diiiiculties, it has heretofore been proposed to use as short circuiting rings of the rotor thin sheets of metal having a positive temperature coeificient of resistance, and adapt ed to be heated to a high temperature by the starting currents'of the motor. While it is true that this device has a tendency to produce a higher resistance at starting than un- 'der running conditions, it has not been pos sible with the construction heretofore used to obtain anything like the variation of resistance desired. This is due chiefly to the fact that the short circuiting rings which form a part of the rotor circuit are of necessity of considerable length, and' unless they are, made of large cross section they will introduce a high resistance into the rotor circuit even under running conditions, and this causes the motor to operate at a low efiiciency and with poor, speed regulation. If, on the other hand, the short circuiting rings are made of sufiiciently large cross section to give the desired low resistance they will contain so much metal and their capacity for absorbing heat will be so great that they will heat up very slowly when the current is turned on, thus causing the motor to start very slowly under heavy loads or to fail to start'altogether. Further, for the reason mentioned above, it is impossible with this construction to use anything but a metal having a high conductivity such as copper,'for these end rings, so that the Specification of letters ratent. Application filed December 6, 1911. Serial No. 664,219.

amount of metal may be kept small, but since copper and all otherknown materials having'a high conductivity, have also a rel atively' low temperature coefiicient of resistance, it will readily be seen that the re sults obtained by this construction cannot be" adequate. In a previous application, Serial'No. 612,118, filed Mar. 3, 1911, I have described one means of avoiding this'difiiculty, which means. consists essentially in inserting permanently in the short circuiting devices of the'rotor circuits, resistance elements of the type described below. I have now discovered other and simpler means by which the above mentioneddifiiculties may be avoided, and I am thereby enabled not only to produce a motor -havin at once a high starting torque and high e ciency, but owing to the peculiar properties of the re- Patented Nov. 12, 1912'.

sistance devices used by. me the power factor v of the motor under normal running conditions is greatly improved, and the maximum torque which the motor is capable of exerting is, materially increased. Further, owing to the greater mechanical simplicity of the resulting structure, the cost of the rotor is materially reduced. A full and complete description of my invention is contained in the annexed specification, reference being made to the accompanying drawings.

My invention consists essentially in inserting in each of the conductor bars of the squirrel cage winding a resistance element having a relatively small cross section anda positive temperature coefficient of resistance, preferably one characterized by an abrupt rise of resistance at a critical value of current due to the heating effect of the current in said resistance element.

Of the drawings: Figure 1 isa side elevation partly in section of the rotor of the motor towhich my invention applies, showing an arrangement of the resistors in the squirrel. cage winding that I have found element. Fig. 5 shows another form of re sistance element and a method of connecting the same to the conductors of the squirrel cage.

In thus inserting the resistance elements in' series with the conductor bars of the- 100 satisfactory. Fig. 2 is an end elevation' squirrel cage, two important advantages are obtained. It is notthen necessary to use a fixed length of resistance element as is the case when the resistance element constitutes the short circuiting ring of the rotor winding, but it is permissible to use a resistance element as short as desired. For this reason I am able to make the resistance elements of very small cross section and, at the same time, to make them ofsuch short length that the resistance under norm'al running conditions will not be high@ enough to affect seriously the efficiency and speed regulation of the motor. In this way j Iain enabled to use metals having a much 1' higher specific resistance than has hereto- E fore been practicable, and can therefore use 5 those metals which give the greatest increase of resistance with rise of tempera- 1 ture, and at the same time the quantity of 1 metal in the resistors can be kept so small 1 that it heats very rapidly when the current l is turned 011, thus producing a very rapid E rise in resistance and a correspondingly rapid increase in the torque. Further, by I thus inserting the resistance elements in the conductor bars of the rotor, the current carried by each element will be relatively small compared to the current in the short circuiting rings, and the resistance elements can therefore more readily be made of small cross section and a more rapid rise in resistance is secured. Also, inthis case a single short circuiting ring only is required on each end of the rotor and a better mechanical construction is obtained.

I have experimented with many sub-E stances as resistance elements and have 1 found that the magnetic metals, iron, nickel and cobalt are much better than any others, in that they show a very great and abrupt, rise of resistance at a certain well defined critical value of current. It is characteristic of these metals that when used in comparatively pure state they show a relatively low and only slightly changing resistance as the current is increased up to a certain critical value, at which point any further increase in current causes the resistance to rise very quickly to a manifold value. My invention consists further in so proportioning the resistance element that the normal load currents in the rotor will be somewhat below the critical value necimportant improvements in the operating characteristics are obtained. Under normal operating conditions the frequency of the alternating currents in the secondary circuit of an induction motor is very low. Since the temperature, and therefore the resistance of the resistance elements, tends to vary with the current strength, it is evi-- they can be made of very small dimensions as described, their capacity for absorbing heat is so small that relatively great pulsations in resistance will occur with the low frequencies that usually prevailin the secondaries of induction motors in practice. Herefrom follows the important advantages referred to above.

Since the heat. in the resistance element is increasing and the resistance therefore rising as long as the square of the current in the element is above its meah value, ap-

proximately speaking, it follows that the resistance of the element will continue 'to increase for some time after the current has passed through its maximum value, and. as a result of this, the maximum of current is made to occur sooner than it would if the resistance were unvarying. The effective current in the rotor is therefore advanced in phase, a condition that improves the power factor of the motor and enables it to exert a higher maximum torque as mentioned above. Thls pulsation in resistance is greatly augmented by using materials having a relatively great tendency to increase their resistance with increasing current, such as the magnetic materials, and these metals are therefore best suited to securing all of the advantages mentioned above, viz: high starting torque combined with high efiiciency, good speed regulation, improved power factor and high maximum torque.

If iron or cobalt be not worked at too high a temperature they are quite stable in air, and'will, under such circumstances,.have an indefinite life. Then, however, it is desired to operate them at very high temperatures during the starting period in order to obtain the maximum increase in resistance, they may gradually oxidize and deteriorate. Nickel is much mor resistant to oxidation than iron or cobalt, and at moderately high temperatures either nickel alone, or iron or cobalt coated with'nickel makes a more satisfactory resistance element. When it is desired to push the temperatures at starting to still higher'values,

whichdoes not oxidize readily at the, temperature desired. This can readily be accomplished by numerous well known methods, Coatings of-gold or platinum are pure ticable examples ofthis embodiment of my invention.

Referring particularly to Figs. 1 and 2, 1 designates the laminated core of the rotor, 2 the conductor bars of the squirrel cage winding embedded in the slots of the rotor core, and 3the short circuiting end rings of the squirrel cage. It will be observed that in the case illustrated there are inserted on each end of the rotor in alternate conductor bars, resistance elements 4, of very-short lengthy and greatly reduced cross section, these being proportioned in accordance with the principles set forth .above. It should be understood that this alternate placing of the resistance elements on either end of the rotor is not essential to the satisfactory operation of the motor from an electrical standpoint. In so "far as the electrical operation is concerned, the resistance elements might all be, placed on one end of the-rotor, or they might be grouped in any 'ofa variety of ways that readilysuggest themselves. ,It is desirable, however,-.that

the heavy short circuiting rings 3, be each connected directly to some of the conductor bars in order to secure ample'mechanical strength and rigidity. If the rings, 3, be

otherwise suitably braced and held in position, any othergrouping of the resistance elements in the conductor bars will be satisfactory.

, In the construction illustrated in Figs. 1 and 2, it will be'seen that the resistance ele ments are not integral with the conductor bars, but consist of separate pieces of metal fastened on one. end to the conductor bars, and on the other, to the end rings by soldering or other suitable means. In this case the resistance element may be of the same or different material from that of the conductor bar.

In Fig. 4 is shown a modified construction of the resistance element in which a-shortportion, 4, near the end of the conductor bar 2 is greatly reduced in cross section,

iii

the bar being connected to the end ring3,

by means of the screw In Fig. 5 is shown a still further modification of the resistance element. Here, the resistance element t is not integral with the conductor bar 2", and may be of the same or different material, and is connected therewith at one end by means of the screw 6. The other end of the resistance element 4: is connected to the lug 7 by means of the screw 8, and this lug is in turn connected rel cage winding},

to the-,sliort circuitingj ring 3 by means of the screws 9. It will be understood that any combination of the essential elements of the construction shown may be used with equal .satisfaction, the resistance elements being connected at either or both ends by means of solder, screws, rivets, bolts or clamps, or by any other suitable means designed to give a good and permanent electrical and mechanical connection.

In the annexed claims, I cover broadly the use of resistance elements of reduced cross section and positive temperature coefficient of resistance inserted in the conductor bars ofthe squirrel cage, and claim specifically the use of iron as the essential ele ment in such'resistors. That portion of the above disclosure which relates to the use of cobalt and nickel as the essential elements in theresistors is made the basis of two sepa rate applications, .Serial Nos. 684,419 and 684,420, filed Mar. 18, I912, both entitled Imp'rovementin induction motors.

Iclaim:

1 .In an induction motor having a squira series resistance in each of the bars of t e winding, said resistances comprising portions of reduced cross secnon having a positive temperature coefiicient of res stance, and characterized by an abrupt rise of resistance at a critical value of current. v 2. In an induction motor having a .squirrel cage winding, a series resistance in each of the bars of the winding, .said resistances comprising port ons of reduced cross secti'on having a positive temperature coeflicient of resistance, and characterized by "an abruptrise of resistance at a critical current value, and so proportioned that the normal starting currents in the rotor bars will be above the critical current value, and the normal load currents rent value. I

In' an induction motor having a squirrel cage winding, a series resistance in each of the bars of the winding, said resistances comprising portions of reduced cross section composed. of a magnetic .material, and

adapted to be. heated to a relatively high temperature by the starting currents in: the

rotor bars.

4. In an induction motor having asquirrel cage winding, a series resistance in each of the barsof the winding, said resistances comprising portions of reduced cross. section comp'osed of iron, and adapted to be heated to a relatively high temperature by the starting currents in the'rotor bars.

5. In an. induction motor having a squirrel cagewinding, a series resistance in each of the bars of the winding, said resistances comprising portions of reduced cross section.

having a positive temperature coeflicient of resistance and coated with a material subbelow the critical curof the bars of the Winding, said resistances comprising portions of reduced cross section, composed oi: magnetlc material coated with a material substantially non-oxidizable at the temperatures normally reached b y the resistances.

7. In an mductlon motor, and 1n combination, a squirrel cage Winding, a series resistance in each of the bars of said Winding, said resistances comprising portions of reduced cross section, composed of a magnetic material coated with nickel,

8. In an induction motor, and in combination, a squirrel cage Winding, a series resistance in each of the bars of said winding, said resistances comprising portions of reduced cross section, composed of'a conductor of iron coated with nickel.

' BURTON MGCOLLUM.

Witnesses v JESSIE C. MCCOLLUM, R. H. KEssNER. a

Images