CZ308292B6 - Rotor cage of small asynchronous motors - Google Patents

Abstract

The invention relates to a short circuit rotor cage in which the cage material itself comprises 5 to 90 % by weight of an additive material with a specific electrical conductivity generally higher than that of aluminium. The filler material consists of elements of the shape selected from the group of long thin body, ball, ellipsoid, polyhedron, hyperboloid and the material is selected from copper, iron, bronze, brass.

Description

Rotor cage of small asynchronous motors

Field of technology

The invention relates to the conversion of electrical energy into mechanical energy using an asynchronous machine, in particular the rotor winding of small asynchronous motors.

Prior art

The rotor winding is an essential part of all electrical machines, which convert electrical energy into mechanical energy. It is made as a classic winding from individual conductors, then it is a so-called wound rotor, or it is made by spraying from aluminum, in this case it is a so-called short cage. In some cases, aluminum is replaced by copper and the individual bars are short-circuited to so-called short circuits during cooking.

The cage, which is briefly sprayed from aluminum, is simple in construction and easy to manufacture. From the mechanical point of view, in some cases it is damaged by the existence of various cracks, scratches, etc. The construction of a short-circuited cage from copper conductors is technologically and costly.

For higher and large outputs - the limit for cast aluminum and for welded C _{at the} cage, the determining factor is not only the reliability but also the price of the technology of production of these cages. The failures that occur during the operation of AM are caused by the fact that the stress caused by electrodynamic forces reaches such a magnitude and character that it is already beyond the limits of strength or fatigue of the material used.

During operation, the current-carrying parts, i.e. the cage, are stressed by mechanical forces which are caused by the currents flowing through them. Mechanical forces between coupled conductors always arise, even when this is undesirable. They can be harmful, especially when caused by large currents. These forces are caused by a magnetic field that arises around each conductor through which current flows. The current in the cage is created by magnetic induction, when the magnetic flux excited by the stator winding - flowing through a three-phase current, passes through a relatively small air gap into the rotor. There the rotating field of the stator wave, resp. its lines of force intersect the cage rods placed between the teeth of the magnetic circuit. The magnitude of these currents at a constant speed of the rotating field is given by the instantaneous magnitude of the magnetic field of the stator at the location of the rod. A system of currents is created in the cage, the magnitude of which is determined by the course of the field intensity. Because the rotor is at rest during start-up, the space wave generated by these currents will have the same position as the stator magnetic field wave and will rotate at the same speed as the field, and thus the stator current layer. The voltages induced in the rotor rods are proportional not only to the magnitude of the stator field wave, but also to the frequency, i.e. the speed of movement of the rotor relative to the rotating stator field. Inductive reactance causes a phase delay of the current behind the voltage, which leads to a reduction in the torque increase. Therefore, AM has a relatively small engagement torque at relatively large inrush currents.

The calculation of the magnitude and character of electrodynamic forces is performed from the law of conservation of energy or from the Biot - Savart law. In general, a force acts on the element of the current conductor dl flowing through the current i, which is located in the magnetic field with induction B:

dF = idl xBi (1)

With a wire length of 1, this force will reach:

- 1 CZ 308292 B6

F = i I dl xB (2)

The formation of electrodynamic forces in the cage of an asynchronous motor occurs:

- for multi-stream, especially parallel tracks (cage rods),

- when changing (curving, breaking) the current path (rod, circle),

- if the conductor is in close proximity to the ferromagnetic material,

- when changing the cross-section of the current path (rod - circle), practically identical to the variant ad. b.

When calculating the magnitude and course of electrodynamic forces, the cage rods are replaced by dimensionless current fibers and their parallelism is assumed. Z rov. (1) applies:

dF ₂ = i ₂ dl ₂ xB (3)

Where:

- F2 is the magnitude of the force acting on the element db with current 12,

- Bi is the magnitude of the magnetic induction induced by current ii in a conductor of length h.

For infinitely long conductors from Biot - Savart's law, it follows for Bi _:

Where: μ = μ ₀ . μ _Γ

Bi μ i _± r ^{+ co} dl _± xr

4π r ³ (4)

The cages are practically entirely "immersed" in leafed ferromagnetic material. The rings fit practically "close" to the iron. For them, we consider the same environment - usually Ei sheets. However, the relative permeability is not constant for this material, but depends on the instantaneous value of the magnetic induction in the rotor tooth. μ _Γ = f (B _zr ).

The following applies to the magnitude of force F2:

_μ _ο μ _Γ Η C ^{+ c} ° sinB - - I 5

4π J_ _rr , r ² f ₂

BoBriih f ² f ^{+ co} sin /? siná

II úi V d CivO

4π J _o J_ _m r ² μομμ ^ f ° sin psina r? = --------- II ----------- ap al?

4π J _o 4 al μ _Γ

F = ---- ú i ₂ * 10 ⁷ [N; m, m, A] (5)

If the conductors are not straight, then the relationship is used to determine the magnetic induction:

Mo / M

B = ---- (cos So - cosSi)

4πa vrr ⁷

-2 CZ 308292 B6

PoPrG

4πχ (C0S /? ₂ - COS / ^) cos /? ₂ = l _± - x yjd ² · + (l \ ~ ^% ) ² cos = —xs / a ² + x ²

The following applies to the strength Fi:

μ _ο μ _Γ C ¹ l ± - xx —---- lily I, ⁼ 4, =

4πα J _o [^ a ² + (1 ^ - x ') ² / a ² + x ²

21 ₁ μ _Γ

Ei = ------- yes

. 10 “ ⁷ (6)

The value in parentheses for the dimensions - groove spacing, bar length, common for those cage sizes where the effect of electrodynamic forces on mechanical strength is applied, is close to 1. Most groove shapes on larger machines with a vortex anchor are close to a rectangle with different ratios. hillside. For conductors with a rectangular cross-section, a correction factor kp was derived, by which the calculated force is multiplied. The following applies to the usual dimensions for bars:

0.12 a - b b

OTb + h (7)

The magnetic induction in the rotor tooth changes periodically with the slip frequency and the half-waves alternate with twice the frequency. This means that during each half-wave the magnetic induction in the rotor tooth definitely reaches twice the value when μ _Γ = max. than if they were in the air. Due to their construction and production technology, the mechanical stress in cast cages is not evenly or linearly distributed. The crystal structure, which is very often still disturbed by dendrites or foreign inclusions, is not always ideal and the crystals can be differently oriented. When such a material is stressed, it does not transfer all the crystals evenly. It is almost always an inhomogeneous material. The bars also have a substantially rougher surface, given by the interposition of the individual sheets in the groove, than the test bar, from which the material constants and the individual material limits are derived. This means that rotor bars should be considered in terms of fatigue limit, and not only in terms of strength limit. Cyclic - oscillating stress significantly reduces the strength limit or fatigue limit of the material. Small and short, but often recurring plastic or only elastic deformations can lead to the formation of cracks. In this case, only a few grains affected by microplastic deformation are sufficient. Crack propagation also promotes noise, which can reach values of up to 100 dB at the rotor location. It does not matter its origin - electromagnetic or aerodynamic. The onset of the fatigue crack is always on the surface of the bar. This is due to the fact that:

- in the case of cast aluminum, the surface layers are always more stressed during bending and torsion,

- have many surface defects.

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Cracks can also occur, for example, during cooling of the cast cage, or cooling of the cage heated during start-up, in transient states, etc. This is due to the different coefficient of longitudinal expansion AI and the rotor plate bundle. Usually chosen:

- for alternating thrust and pressure o _t c = 0,35 ση,

- for transient thrust and pressure

- for alternating bending

- for a transient bend

Othc = 0.61 opt, otc = 0.43 op _t , Oohc = 0.74 ort.

It has been experimentally shown that an increase in temperature leads to a further decrease in strength. E.g. at 500 ° C the strength decreases by 30%. The tensile strength of the alloy Al 99.5, which is most often used on cast cages, has a theoretically determined strength of 100 MPa on the test rod. Thus, under certain circumstances, the strength may drop to 20 to 30 MPa. Thus, cage failures occur because its electromagnetic stress is often much higher than the yield strength.

The causes of failures of cast aluminum cages of asynchronous machines can be briefly divided into:

1. Faults that occur at the manufacturer - preferably before mounting in the stator, are very difficult to detect and occur unambiguously:

- non-compliance with rotor bundle production technology or casting technology,

- unsuitable shape of the casting, eg sharp transitions of different cross-sections, etc.,

- unsuitable technological properties of the used metal, resp. alloys.

The problem is that a defective casting does not require the cooperation of all the stated causes, but a single cause is sufficient for this. It is also not possible to prefer any of the casting methods - vibrating, pressure or centrifugal. Technological indiscipline will affect everyone.

2. Faults that only appear after a while - in operation. Their consequence is usually the shutdown of the machine. These faults are to a large extent "prepared" by reasons analogous to those in point 1. In addition, the effect of electrodynamic forces is applied during operation.

Technological properties of alloy A199.5

This A199.5 alloy is purchased in RF. The typical composition of the new unmelted material is 99.5% Al, 0.3% Fe, 0.1% Si, and 0.1% is a mixture of Cu, Mn, Ni, Ti, Zn. Melting slightly increases the proportion of Fe. Fici temperature is in the range of 760 to 765 ° C and solidification temperature is 658 ° C. It is not very suitable for larger and more strenuous machines. Cold mechanical strength - 20 ° C does not exceed 100 MPa. At temperatures above 100 ° C it drops by up to half. The modulus of elasticity has a value of 7.10 ⁴ MPa, the electrical conductivity is relatively good, about 34 Sm ¹ . However, it depends on the impurity content as well as the thermal conductivity. The coefficient of longitudinal thermal expansion is about twice as high as the coefficient of the sheet bundle. The shrinkage is about 1.0 to 1.1%. The presence of Fe in Al has an adverse effect on plasticity and increases the possibility of hot and cold cracking. On the contrary, a higher proportion of Si and also Cu has a very good effect on the mechanical strength, especially at higher temperatures.

The viscosity of the molten metal is very low, but the castability of Al is affected by thin oxide coatings which significantly degrade it. Silicon alloy is suitable for aluminum and magnesium is unsuitable.

In the case of the use of silumin, a sub-eutectic alloy with a Si content of 12.5% was originally considered an undesirable additive and alloys with a higher Si content were not used for considerable brittleness. The use of silumin was made possible only after the introduction of the so-called "inoculation", when the addition of sodium or a mixture of salts, etc. is added to the melt just before casting.

-4 CZ 308292 B6

The filling of the mold is only slightly dependent on temperature, but is inversely proportional to the surface tension of the molten metal. Grain refinement improves the mechanical properties of the alloy and increases the resistance of the material to hot cracking. The presence of H2 in the alloy is undesirable because it is soluble in heat and causes porosity when solidified.

Technological problems

The resulting rotor quality can be affected by:

- cage shape (existence of sharp rectangular transitions between bar and circles),

- quality of compression of the rotor bundle,

- the effect of rounding of delicate transitions and the possibility of undisturbed shrinkage of the casting during cooling.

This last condition is very difficult to meet, as it is prevented by a very uneven surface along the entire wall of the rods and a greater compression of the rotor bundle than would correspond to the shrinkage of the aluminum casting. A pressure in the range of 1.0 to 2.0 MPa is used to compress the beam. Calculating the pressing pressure is very difficult. As a rule, its size is chosen on the basis of experience. If the bundle is shrunk more than the AI shrinkage, then the cooling cage will shrink the sheets as long as the bundle allows. When the sheets cannot be pulled down, then the bars first reach a state of elastic stress, for the A199.5 alloy this area is small, and during further shrinkage the bars will be stressed at the strength limit, or only behind it. There will be a violation, or. rupture of one or even several bars. The defect occurs at the weakest point - narrowed cross-section, indistinct profile, lunkr, etc., or at the point of sudden change of cross-section (notched strength, disturbances of the crystallization process, etc. The length difference during cooling is about 0.75%, depending on the material The stresses that can occur in the bars during cooling from a temperature of about 660 ° C reaches up to 1000 MPa, with a rotor length of 500 mm the shrinkage is eg 3.6 mm.

Operational problems. During heavy start-up, large currents flow through the cage and the resulting electrodynamic forces cause large mechanical stresses, especially in the transition of the rod into a short circuit. This is compounded by the stresses caused by the temperature difference between the rod and the iron. Thanks to the skin effect, the entire current is concentrated in the upper part of the rod during start-up, where the temperature reaches up to 500 ° C. The temperature of the lower edge at a given moment does not exceed 80 ° C. This temperature difference in the rod tries to bend it. With regard to its attachment in the groove is. Mechanical deformation is made more difficult and elastic-plastic stress is created in the rod. Fatigue of the material, often with repetitive plastic deformations, can lead to cracks.

The disadvantage of a short-circuited cage made of aluminum is that higher efficiency and power factor cannot be achieved.

The essence of the invention

The above drawbacks are largely eliminated by the rotor cage of the short asynchronous motors according to the invention. Its essence is that the actual material of the cage briefly contains 5 to 90% by weight of additional material with a specific electrical conductivity higher than that of aluminum.

The additive material is preferably formed by elements in the shape selected from the group of long thin body, ball, ellipsoid, polyhedron, hyperboloid and in a preferred embodiment is selected from the group of materials copper, iron, bronze, brass.

The elements of additional material are preferably provided on their outer surface with grooves, which can have a different geometric shape - longitudinal, at an angle of 1 to 90, or they can have a helical shape - even multiple. Balls, long thin bodies, ellipsoids and hyperboloids can be differently situated in the axial direction - turned, they can be contained in a rod or cage in various numbers, etc.

-5 CZ 308292 B6

The essence of the technical solution, leading to the improvement of technical and economic parameters of asynchronous machines with a short-circuit armature, is the enrichment of the cage's own material for a short time with another material, usually with a higher electrical conductivity. This material can advantageously be implanted into the base material - Al in the form of long thin bodies, spheres, ellipsoids, etc. suitably oriented in the axial direction of the rotor groove.

This modification can achieve the necessary change in the electrical conductivity of the rotor cage, thus improving the engagement torque and start-up time.

Explanation of drawings

The short-circuited rotor cage of small asynchronous motors according to the invention will be described in more detail in a specific exemplary embodiment with the aid of the accompanying drawings, in which Fig. 1 shows an embodiment of a short-circuited electric motor rod with implemented rods. Fig. 2 is an embodiment of a short circuit of an electric motor with implemented balls. Fig. 3 is an embodiment of a short circuit of an electric motor with implemented ellipsoids. Fig. 4 is an embodiment of a short circuit of an electric motor with implemented hyperboloids. And Fig. 5 is an embodiment of a shorted electric motor rod with implemented combinations of ellipsoids interconnected by rods. Figures 6 to 10 show the above embodiments in side view.

Examples of embodiments of the invention

In the case of short-circuited rotor cages for small asynchronous motors, the actual short-circuited cage material contains 5 to 90% by weight, an additional material with a specific electrical conductivity generally higher than that of aluminum.

The additive material consists of elements in the shape selected from the group of long thin body, ball, ellipsoid, polyhedron, hyperboloid and selected from the group of materials copper, iron, bronze, brass. Elements made of additional material are provided with grooves on their outer surface. The grooves have different geometries - longitudinal, at an angle of 1 to 90, or they can have the shape of a helix - even multi-pass. Balls, long thin bodies, ellipsoids and hyperboloids can be differently situated in the axial direction - rotated, they can be contained in a rod or cage in various numbers, etc.

Individual designs of short-circuited rotor rods with implemented elements of another material can be seen from the enclosed figs.

Industrial applicability

Rotor rods according to this technical solution will find application in the construction of electric motors, especially low power.

Claims (5)

A rotor cage for small asynchronous motors, characterized in that the actual cage material briefly contains 5 to 90% by weight of an additional material with a specific electrical conductivity higher than that of aluminum, which is selected from the group of materials copper, iron, bronze, brass and the additive material consists of elements in the shape selected from the group of long thin body, ball, ellipsoid, polyhedron, hyperboloid. -6 CZ 308292 B6 Rotor cage according to Claim 1, characterized in that the elements of additional material are provided with grooves on their outer surface. Rotor cage according to claim 2, characterized in that the grooves have different geometric shapes. Rotor cage according to any one of the preceding claims 1, 2 and 3, characterized in that the elements of additional material with their own axis of symmetry have their axis rotated relative to the axial axis of the rotor cage by 1 to 90 °. me Rotor cage according to any one of the preceding claims 1, 2, 3 and 4, characterized in that the additional material elements are rotated relative to the axial axis of the rotor cage so that the rotated elements are helical in shape with a pitch angle of 0.1 to 90 °.