US1771936A - Induction motor - Google Patents

Description

29, 1930- w. J. MORRILL I 1,771,936 I INDUCTION IOTOB Filed Oct. 11. 1927 Inventor: \A Qg'ne J. Morril I,

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w July 29, 1930 UNITED STATES PATENT OFFICE wam I. KORE-ILL, OF FORT wam, INDIANA, ASSTGNOB '10 cmmm mmIO OOHPANY, A CORPORATION OI NEW YORK INDUCTION MOTOR Application filed October 11, 1827. goth-1 I0. 925,558.

of induction motors is considerably higher than the normal load operating current and that various expedients have been proposed to reduce the starting current and still obtain a high starting torque and desirable operating efliciency. The double squirrel cage motor is one of such expedients. Ordinarily, a high resistance squirrel cage is located near the periphery of the rotor and a lower resistance squirrel cage is located more deeply in the rotor core. Due to the location of the low resistance squirrel cage its reactance at starting is high because the secondary frequency at starting is high. Consequently at starting the high resistance, low reactance squirrel cage at the periphery of the core contributes the main starting torque. As the motor speeds up and the secondar frequency decreases, more and more of the ux cuts the lower uirrel cage so that at the operating speed slower resistance secondary characteristic is obtained. To increase the selective distribution of the secondary current between the two secondary windings between starting and running conditions it has also been proposed to provide movable magnetic ridges arranged to vary the reluctance of the flux paths between the two secondary windings as the motor speeds up. Such an arrangement is described and broadly claimed in application Serial No. 13,571, filed March 6, 1925, Hans Lund, assigned to the same assignee as the present invention. My invention relates to an improvement of this type of machine.

In carrying my invention into efiect, I make use of the combined action of centrifugal and magnetic forces to control the'movement of the magnetic bridges. In the preferred embodiment of my invention the magnetic bridges or wed es are maintained in such a osition as to en stantially close the flux path tween the two secondarywindings by magnetic force during the initial starting period and are moved to increase the reluctance of this path by the action of centrifugal force as the motor speed increases.

The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of my invention reference is made to the following description to the accompanying drawing in which Fig. 1 represents a perspective view partly in section of a rotor embodying my invention; 2 shows the details of a preferred slot arrangement for carrying out the invention; F gs. 3 and 4 illustrate the difl'erent flux distIIbIlt-lOIlSRIId magnetic wedge positions for starting and running conditions respectively; Fig. 5 shows an alternative form of wedge and Fig. 6 represents a complete motor equipped with the rotor of my invention.

One physical embodiment o my invention which I have found to be satisfactory will be described in connection with Figs. 1 and 2. In Fig. 1, 10 represents the rotor shaft, 11 the iron laminations of the rotor core, and 12 one of the end rings. In this instance the end rings comprise copper plates which, together with the two squirrel cage windings represented at 13 and 14, serve to clamp the rotor laminations together. The end ring plates v12 are provided with openingsconforming to the size and spacing of the squirrel cage bars and the bars extend through these openings as represented at 15 and 16 and after assembly the bar ends are brazed over and welded to the plate as represented at 17. The type of end ring construction is not important. The construction illustrated is advantageous because it serves to prevent the loose magnetic bars represented at 18 fromworking out endwise.

' The details of the slot construction and the arrangement of the parts therein is shown in Fig. 2. 19 shows the usual narrow slot opening at the periphe which may be filled up by a suitable wedge if desired. The high resistance squirrel cage bars constituting the main starting winding are contained in the top of the slot and are held in place by the end rings and the slot surfaces. The low resistance, high reactance bars 14; are placed in the bottoms of the slot. The slot space be-. tween the bars 13 and 14 is wider at the top than at the bottom and, except for the iron freely movable bar 18, forms a high reluctance air gap between the two windings. The lower converging portion of this slot space 1s dimensioned and shaped to conform to the general size and shape of the loose magnetic bar or wedge 18 in order that the we ge 18 may serve to substantially close this air gap when the wedge 18 is in the bottom portlon thereof. In this case the wedge 1S'I011I1d and the contacting surfaces of the slot at 20 are convex, the radius of the surface 20 bein slightly greater than that of the wedge. t will be obvious that other equivalent shapes for accomplishing the object might be used here. The round wedge and circular concave surface at 20 has some advantage because 1t is immaterial if the bar 18 should happen to turn over.

The part indicated at 21 is a thin non-magnetic guide or spacer for the loose wedge 18 25 when it is in the upper portion of the air gap.

This prevents the wedge 18 from clinging to the sides of the slot by magnetic attraction.

There may be two or three of such guides,

one near each end of the rotor and one at the middle'of the rotor. They are stacked with the laminations of the rotor as indicated at 21 in Fig. 1. It is unnecessary that these guides extend to the periphery of the rotor. As indicated in Figs. 1 and 2 these guide members extend only to the bottom of the bars 13 and have U-shaped slots cut in their peripheries opposite to the rotor slots and have openings for the lower bars 14. I do not consider that such guides are essential but prefer to use them or some equivalent arrangement as an assurance that the magnetic bars 18 will not stick to the iron laminations while in the upper portions of the air gap. When used. these spacers may be made of sheet copper or other non-magnetic material.

When the motor is at rest with the stator winding deenergized, the loose iron bars 18 will take a position dependent solely upon gravity. If the rotor shaft is horizontal the bars on the upper part of the rotor will be in the lower portions of the air gaps between the two squirrel cage windings as indicated in full lines in Fig. 2, and the bars on the lower portion of the rotor will rest against the inner side of the squirrel cage bars 13 in some such position as is indicated in dotted lines at 18 in Fig. 2. When the motor is energized with the rotor stationary a heavy current will be induced in the secondary windings. All the loose bars that are not already in the bottoms of the slots, namely in the position represented in full lines in Fig. 2, will be substantially instantaneously G5 drawn there by a strong magnetic pull produced by the flux crossing the air ga be tween the two windings and caused c iefly by the flux surrounding the lower squirrel cage bars and produced by the current flowing in this winding. This magnetic force is proportional to the rotor flux. The motor will then start, the greater portion of the torque being produced by the upper high resistance squirrel cage. This is because of the fact that the flux path between the two windings is now of fairly low reluctance, thereby shutting the flux from the lower winding to a large extent and also because the secondary fre uency is high. Because the motor starting ux is high, some flux will thread the lower squirrel cage and current therein will produce an additional flux through the loose magnetic wedges tending to hold the wedges in the bottoms of the slot spaces. The flux conditions during the accelerating period may be represented as shown in Fig. 3. As the motor accelerates the centrifugal force,tending to throw the loose magnetic wedges outward, increases and the magnetic force which holds the wedges in the narrow portion of the air gap decreases until finally these opposing forces become such that the wedges fly out and the operating condition represented in Fig. 4. is reached. The lower squirrel cage then becomes fully active resulting in a very material decrease in the efiective secondary resistance. I have found that this arrangement which utilizes the action of magnetic forces to move the wedges to their starting positions and maintain them there against the action of centrifugal force during the accelerating period is effective in accomplishing these desirable results. No springs or other mechanical means are required to move or hold the wedges in the bottoms of the slots, this being accomplished solely by the magnetic forces described above. The starting characteristics may be changed by very slight changes in the diameter of the loose magnetic wedges or the shape of the contacting surfaces of the slot. The centrifugal force acting upon the wedges at any particular speed and rotor diameter may be altered by altering the weight of the magnetic wedges. For example, these bars may be replaced by iron tubes of the same external diameter as is shown in Fig. 5 or alternate slots may con tain solid wedges and the intermediate slots may contain magnetic tubes to arrive at some intermediate result due to the fact that different weight wedges will fly out at different speeds.

Where different weight wedges are employed in the same rotor care should be taken to distribute them uniformly so as not to unbalance the rotor or the secondary current distribution.

Such a rotor may obviously be used with single phase or polyphase motors and in Fig. 6 I have represented a single phase motor equipped with the rotor of my invention. The main stator winding is represented at 23. 24 represents a second stator winding arranged to produce a split phase starting characteristic. Winding 24 is connected in series with a transformer 25 the secondary of which is connected across a condenser 26. This is equivalent to placing a condenser directly in series with the winding 24 to advance the phase of the current in winding 24 with respect to that in winding 23. However, the connection of the condenser through the transformer has the advantage that the voltage across the condenser may be stepped up so that a smaller and less expensive condenser maybe used and also the capacity of this branch of the motor circuit may be easily varied between starting and running conditions by means of transformer taps at 27 and thus enable the same condenser to be utilized for obtaining a split phase starting effect and for power factor correction during normal operation. The kva of this condenser circuit should be higher for starting conditions than for running conditions and the changeover of the transformer connections should occur at the proper time in the acceleration of the motor. Consequently it is advantageous to employ an automatic speed responsive device such as the centrifugal governor switch represented at 28 to accomplish this change in con nections. At starting the automatic switch is closed to the left on tap 27 giving the maximum kva capacity of the circuit 24-25, and as the motor comes up to speed the kva capacity of this circuit is reduced bythe automatic closure of switch 28 to the right connecting this circuit through the entire r!- mary Windin of the transformer 25. us I obtain, by the same condenser, ideal conditions for split phase starting and high power factor running conditions. The changeover is accomplished automatically and the switch should be adjusted to respond to approximately the same speed as the changeover from starting to running conditions in the rotor by means of the movable magnetic Wedges. The combination is particularly advantageous for medium sized motors for operation on sin 1e phase circuits because the motor may in thrown directly upon the line without the use of a starting compensator. The starting current is reasonably low with a good starting torque and the ower factor of the combination is high. T ese results are accomplished entirely automatically by the two automatic features which are properly adjusted with respect to each other to obtain the best results under the particular load conditions to which the motor is subjected. It will be evident that quite a wide variety of starting oonditions may be obtained by this combination by merely changing the size and weight of the movable magnetic wedges and varying the settin of t e centrifugal switch and the trans ormer taps.

In accordancewith the provisions of the patent statutes I have described the principle of operation of my invention, together with the apparatus which I consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown and described is only illustrative and that the invention may be carried out by other means. What I claim as new and desire to secure by Letters Patent of the United States, 1s 1. A double squirrel cage rotor element for induction machines comprisin a magnetic core containing radial slots, a hlgh resistance squirrel cage winding located in the tops of said slots, a low resistance squirrel cage winding located in the bottoms of said slots, said windings being se arated by slot spaces,

circular magnetic we ges radially movable in said slot spaces,v said slot spaces being wider than said wedges at the top and shaped to substantially fit the wedges at the bottom.

2. A rotor element for dynamo electric machines comprising a magnetic core member containing radial slots windings in the tops and bottoms of said s ots separated by slot spaces, said slot spaces bein wider at the top t an at the bottom, radial y movable magnetic wedges in said slot spaces dimensioned to substantially fill the narrow portion of said slot spaces, and guide members for maintaining said wedges in the central portion of the slot spaces when in the Wider portions thereof.

3. A rotor member for dynamo electric machines comprising a laminated magnetic core member containing radial winding slots, radially movable magnetic bridges in said slots, and non-magnetic laminations in said core member having radial slots dimensioned to guide the ma etic bridges in their radial movements an thus prevent undesirable stickin of the bridges on the sides of the magnetlc slot surfaces.

In witness whereof, I have hereunto set my hand this 7 day of October, 1927.

' WAYNE J. MORRILL.

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