Technical field

The invention relates to an induction motor with short-cir¬ cuited armature and a squirrel-cage formed of tubes - here¬ inafter pipe cage -, at which the bars of the cage are pro- jecting beyond the frontal side of the rotor at one or both ends thereof and on the projecting ends there are short- circuiting means arranged, furthermore in the bar section between the short-circuiting means and the frontal side of the rotor, at least on one side of the rotor each single rod is provided with a tube made of a magnetizable material, being mutually electrically short-circuited.

Background art

About 90 of the electromotors operated all over the world are induction motors and about 4/5 of them are squirrel-cage motors. This can be explained by the fact that the slip-ring ^• armature motors - although their operational properties are far more advantageous - are often causing break-down due to the slip-ring and the brushes, which easily get damaged; be¬ sides said motors are rather expensive, involving more costs than the induction motors with short-circuited armature.

The drawback of the. induction motors with short-circuited armatur>ss lies in that they are starting with a considerably higher current absorption, simultaneously the starting tor¬ que is less, they endure less startings, regulating of the number of revolutions involves high costs.

In order to reduce the high starting current absorption and to increase the starting torque of motors with short-circuit¬ ed armatures, at different points of the rotor magnetizable solid iron coatings or inserts have been generally used, whereby said magnetizable iron coatings have been arranged, on the party lying outside the rotor iron body - e.g. on the

coil heads -. Said iron inserts are improving operational conditions - however, to a slight extent - in so far as starting current absorption becomes less, but simultane¬ ously the starting torque is also reduced resulting in a worse efficiency of the motor. The HU-PS 1540S9 and the corresponding _. GB-PS 1,121,239, respectively,, describe a solution, at which in case of art induction twitn short-circuited armature, where the bars of the cage --the rotor bars ~ are protruding beyond the frontal side of the rotor at one or both ends thereof and on the ends of the single protruding bars there are short- circuiting means arranged, in the bar section between the short-circuiting means and the frontal side of the rotor, at least on one end of the rotor there is a tubular, ag- netizable, mutually electrically short-circuited material arranged. By using this solution, it has succeeded to re¬ duce starting current absorption and to increase starting torques, however, to obtain a far lower starting current and a higher starting torque,- the rotor bar ought to be considerably lengthened, involving a significant -deterior¬ ation of the efficiency of the motor and an increase in the dimensions of the motor case resulting in disturbing • effects.

Disclosure of Invention

The aim of the invention has been to eliminate the diff¬ iculties mentioned above and to improve the induction mo¬ tor with a short-circuited armature and a pipe cage, as previously described, at which partly the detrimental lengthening of the motors, partly the reduced efficiency caused by the overlong bars can be avoided.

The invention aims to provide a solution, by means of which at induction motors with a short-circui-'ed armature and a pipe cage, at which the cage-tars are protruding beyond the frontal side of the rotor at one or both ends o:

thereof and on said protruding ends of the bars short-cir¬ cuiting means are arranged, furthermore in o bar-section between the short-circuiting means and the frontal side of the rotor, at least on one side of the rotor each rotor bar is provided with magnetizable tubular materials in a mutual electrical short-circuit connection, the difficulties and drawbacks previously described can be eliminated.

In accordance with the invention the task set is solved in such a manner that by the proper selection of the wall- thickness, under the influence of the starting current of the rotor partly the tubes are reaching or at least well approximating the state of magnetic saturation, partly the Skin-effect is arising in them. It was found that after hav¬ ing exceeded the limit value of the magnetic saturation,the power factor becomes worse, an the other hand, when satura¬ tion doe.s not come up to said limit value, actually super¬ fluous masses will be present and unmotivatedly large space requirements have to be reckoned with.

In accordance with the invention, instead of lengthening the tubes, the increased resistance caused by the Skin-effect — which again results from the increased wall-thickness — has been utilized. Due to the Skin-effect, in dependence of the root ^' of frequency instead of the total cross-section of the tube, the current engages but a part of the wall-thick- ness - e.g. at a period of 50 -, amounting to 1 - 1,5 mm, ^• accordingly, in case of a wall-thickness of 6 mm a fourfold increase in resistance may be obtained, i.e. a quarter of the tube length proved to be sufficient.

As a consequence, by selecting the wall-thickness in accord- ance with the invention, the rotor, bars are to be length¬ ened only by the length-plus resulting from the coil heads arranged on both sides of the rotor of the slip-ring arma¬ ture motor, i.e. the length of the case of the slip-ring armature motor has not to be increased, nevertheless, an

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S * \ . 2 ^~ 2b _.

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equal, but even a batter efficiency can be achieved than at the slip-ring armature motors, since the tubes flattened to the form of a notch, lying on the rotor, but outside and having been separated from the same, are exhausting the majority of the heat of the rotor, thus reducing inner losses.

From the fact described here it becomes obvious that in or¬ der to produce the advantages involved in Skin-effect, num¬ ber of notches should be such that tubes with larger wall- thicknesses should be required.

A further advantage may be obtained by providing the tubes with one or more slits forming air gaps in longitudinal direction, preferably along the external and/or internal generatrix, which is lengthening the beginning horizontal starting section of the magnetizing curves.

Preferably the tubes are made of cast magnetizable material.

In σ preferred embodiment of the invention the tubes are formed with a cross-section having been flattened e.g. to the shape of a notch. In an other embodiment of the invention the tubes are pro¬ vided with one or more slits forming air-gaps in longitu¬ dinal direction, preferably along the external and/or in¬ ternal generatrix.-

The tubes and air-gaps made of magnetizable material can be formed in two or more sections of appropriate arc shape.

A further embodiment of the invention may be developed such that the tubes are attached to two tube supporting discs, one of which being arranged in such a manner that it could slide longitudinally along the shaft of the motor. The securing rings if required are attached to the outer part of the tubes.

Advantageously, fan-blades are arranged on the tubes or on a part thereof. In the environment of the fan-blades,

jαlousie-like blow-off openings can be formed in the wall of the motor casing.

By performing adjustment in the manner described, excita¬ tion occuring at the nominal number of revolutions can be avoided, yielding the possibility of eliminating the plus- slippage eventually occuring at the operational number of revolutions.

Utilizing the Skin-effect, by a moderate lengthening of the bars satisfactory increase in rotor resistance may be obtained, thus succeeding in eliminating the disturbing stalling point of the starting torque curve of induction motors with a short-circuited armature, simultaneously by the proper choice of the air-gap of the tubes - amounting to approx. 1 mm — the angle of inclination of the torque curve can be also changed, in so far as it can be dis¬ placed from the series towards the shunt (see the curve "c" in Fig.4).

As mentioned above, in the induction pipe-cage motor accord¬ ing to the invention, the rotor bars, are lengthened for the purpose of pulling the pipe-cage onto the bars. To obtain an efficiency not lower than that of the slip-ring armature motors, said lengthening of the rotor bars cannot be great¬ er than the cut stretch of the coil heads of the rotor of a slip-ring armature motor. The pipe-cage i.e. the squirrel- cage formed of tubes, however, causes unnecessary plus- resistance. For eliminating this, the insulation of the rotor bars is removed from their outermost surface in the area of the tubes near to the coil head.

By this way, the pipe cage will be gradually separated, i.e. disconnected during the acceleration of the motor onto its operational speed of rotation in consequence of the consider¬ able centrifugal force occured by the increased speed of ro _¬ tation pressing the rotor bars to the pipe cage and causing a short-circuiting at the uninsulated surfaces of the rotor

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bars mentioned above. Hereby the current in the other tube- parts gradually ceases to exist and the motor turns into a usual short-circuited armature motor.

By removing the insulation of the rotor bars from the said surfaces, the short-circuiting action can be effected full sparkless and contact surfaces can be maintained constant by making the end parts of the tubes being next to the coil head anti-rust and anti-corrosive, e.g. by electroplating.

By the utilization of the Skin-effect arising in the tubes having thick walls, further essential advantates my be achieved:

- the tubes may be arranged outside the coil heads, accord¬ ingly, their heating-up at the start does not influence the inner coiled part of the motor, as a consequence closed arrangement becomes possible;

- at an operational speed of rotation the tubes placed onto the rotor bars do not exert a transforming influence as a consequence, the original low rotor resistance will be restored, the tubes are cold, simultaneously the lengthening of the rotor bars quasi "exhaust" the heat from the rotor. Accordingly, the stator will be also colder by about 20 , enduring better the overload;

In case of an overload of dangerous extent - iπspite of the slitting of the tubes - transformation is taking place and the motor decelerates; in this case overload will be distributed among the tubes;

- by the thick-walled tubes a high heat-capacity can be ensured for difficult and frequent 'startings;

Brief Description of Drawings

The invention will be explained in detail by way of example only, with reference to a preferred embodiment illustrated in the accompanying drawings, wherein:

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Fig. 1 is α schematically illustrated longitudinal section of the embodiment according to the invention;

Fig. 2 is showing the siitted tube in a part-sectional longitudinal view; Fig. 3 is a cross-section of the tube according to Fig.2;

Fig. 4 is representing the load-torςue curve of the motor according to the invention in dependence of the speed of rotation;

Fig. 5 is showing the power consumption of.the tubes of the motor in dependence of the current of the rotor bars.

Best 1'ode of Carrying out the Invention

Referring to Fig. 1, at the induction motor with the short- circuited armature illustrated therein, on one side, in this case on the left side, the rotor bars 1 are axially lengthened and thus protruding from the iron body of the rotor. On the rotor bars 1 the tubes 2 are arranged; the tubes are made of a magnetizable material, e.g. cast steel, Between the rotor bars 1 and the ubes 2 the insulation 3 can be found advantageously made of raicafoil. The insulation 3 of the rotor bars 1 is removed from their outermost sur¬ face in the area la of the tubes 2 near to the coil head. The end parts of the tubes 2 being next to the coil head are made anti-rust and anti-corrosive, e.g.. by electro- plating.

Each of the tubes 2 have been fixed in two tube supporting discs .4. Said supporting discs 4 simultaneously fulfil the task of short-circuiting the bars 1, accordingly,, the tubes 2 should be attached bv welding, ϊa ina into consideration that at the tubes 2 - due to changes in temperature - longitudinal deformation may also accur, in order to com¬ pensate the changes in dimension, p..\-. ^~ tub- supporting

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discs 4 to be seen on the left side are arranged in a slideable manner, displaceable longitudinally along the shaft 13 of the motor.

The securing rings 9, 10 serve for stabilizing the posi- tion of the tubes 2, as v/ell as for compensating the cent¬ rifugal force arising in course of rotation; said rings 9, 10 are expediently welded to the tubes 2. At ecch one of the tubes 2 there is a fan-blade 5 arranged, serving for the simultaneous cooling of the rotor bars 1 and the tubes 2, respectively. To the outer part of the securing rings 9, 10 the sealings 6, 11 have been connected with an air-gap 8 of 1 mm between them. By this solution the closedness of the motor according to any of the standards, e.g. IP 44 can be obtained. As it is to be seen, the inner motor - i.e. the rotor - has been arranged by means of the internal, e.g. by the right- sided supporting disc 4, the sealing 6 or by meany of a separate cover plate in a closed manner, ensuring the closedness in accordance with the prescriptions of the standard IP 44. In such a manner a closed motor may be obtained, from which the heat is led away by means .of the rotor bars 1, as a consequence, a considerable improvement can be achieved in relation to the efficiency of the motor, also resulting in a decrease in temperature by about 20 C σt the stator, i.e. using the same copper quantity a higher efficiency may be achieved. In Fig. 1 the directions of the air current are indicated by arrows.

In Fig. 2 and 3 the longitudinal and cross_sections of the tube 2 have been illustrated. The cross-section of the tube 2 has a shape identical with the notch; for the sake of a better cooling and in order to be able to place more tubes, the cross-section of the tube 2 is shaped in c flat and oval form. The* size of the slitted air-oap 12 used to amount to 0,5 - 1,5 ι.ι. The task of said air-gap 12 is to inhibit the occurance of the trancvorπiotion affect, in course of normal oporction of the r-.otor, accorcϋnriy

transformation cannot take place. As a consequence, at a certain overload of the motor reduction of the speed of rotation does not take immediately place, i.e. the motor becomes less sensitive to overload. By increasing the size of the air-gap, insensitiveness will also increase, but after having exceeded a certain limit value, cos <• becomes worse. According to our observations this limit value lies well over 1 mm.

Advantageously the tubes 2 are made of cast steel expedi- ently' with a carbon content of 0,17 '.-- It was p rroved r, however, that shorter tubes with a higher efficiency can be produced, when the tubes are made of cast steel with a carbon content of 0,'02. '2,

Referring to the Fig.4, from the torque curves illustrat- ed here, the curve "a" belongs to the slip-ring armature motor, without rheostat, the curve "b". to the motor with double squirrel-cage, while the curve "c" belongs to the motor with pipe cage formed in accordance with the invention.

It may be well seen that in contrast to the curves "a" and ⁿ b" the curve "c" does not show a stalling point, i.e. by increasing the overload, the speed of rotation steadily • decreases.

At last, in Fig. 5 the development of the power consumption of tb© pipe cage has been illustrated in dependence of the • current of the rotor bars, particularly, when using tubes withja wall-thickness of 5, 6 and 7 ram respectively. The curve drawn in a dashed line is showing the modification of the curves caused by slitting the tubes 2.

The advantages of the induction motor with a short-circuited armature and a pipe cage formed according to the invention may be summarized, as fellows;

- when applying the voltage, the sudden increase of the current results in an increased Skin-effect, the tubes

« are performing only choking, accordingly, a significant transient increase in current does not take place. The

same stands for the case of reversing, power consumption slightly increases;

- by using a regulation v/ith semi-conductors, the tubes are smoothing, as a consequence, compared to sinusoidal

5 control, reduction at the torque cannot be observed;

- in course of rotation the loss in iron decreases, upper harmonics of the teeth and consequently accessory dis¬ turbing sound effects are filtered;

- at a lower speed of rotation and when regulating the 10 speed of rotation, the Skin-effect improves the power factor. In case of starting with a lower current, cos may approximate 1.

- in the same manner, due to the Skin-effect, in course of reversing - braking the force does not decrease;

15. - the size of the motor does not surpasse the dimensions of the slip-ring-σrmature motor, on the other hand it is considerably cheaper, costs of production hardly surpass those of an induction motor with short-circuited armature; - perhaps the operational safety, being better by orders of magnitud.e when compared to known motors, can be con¬ sidered as the most significant advantage. From this fact it results that:

- starting current becomes lower and control of the same becomes possible, simultaneously starting torque in¬ creases;

- the heat involved in starting and regulation - similcrly to slip-ring-armature motors - remains outside the inner parts of the motor, but of course, the use of a slip ring is not necessary.

- Taking into consideration that at an operational speed of rotation the pipe cage is performing cooling, possi¬ bility of loading becomes greater; below operational

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speed of rotation loading heat is distributed by means of transformation;

- when overloaded, the induction motor with the short-cir¬ cuit armature and pipe-cage formed according to the in¬ vention does not "stall", it only decelerates;

- due to the flattened shape and the air-gap between them, the tubes perform self-ventilation in a considerable extent, they are insensitive and endure a degree of temperature of several hundred centigrades.