TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to solid squirrel-cage rotors for induction machines.

Particularly, the invention relates to an improved design for such solid squirrel-cage rotor to thereby reduce harmonic losses from frequency converter drives. The invention relates also to a manufacturing method for such rotor.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

Solid rotor induction machines are usually used for high speed rotating applications because of the mechanical robustness. A rotor of such machine comprises typically a cylindrical solid rotor body made of steel. The rotor body comprises a number of axially extending slots e.g. equiangularly arranged around the circumference of the rotor body. Copper rods are arranged in the slots in a close fit and are short-circuited at the axial ends of the rotor by copper rings to thereby define the squirrel-cage structure. The squirrel-cage structure provides a good balance in electrical aspect and magnetic aspect. Therefore it has clear advantages on power factor and efficiency over other solid rotor structures without the squirrel-cage. Typically, the copper rods are bonded to the rotor body in a HIP (hot isostatic pressing) procedure in which the rotor is arranged within a steel capsule and is exposed to high pressure and high temperature, thereby forming a strong diffusion bonding layer between the steel and the copper materials that are in contact with one another.

A disadvantage of such rotor structure, however, is the high surface losses due to harmonic currents from a frequency converter which is used for driving the high speed induction machine. The voltage waveforms of the high speed converters have high fundamental frequency and are therefore usually far from sinusoidal. This is especially remarkable for high power converters, where the maximum device switching frequency is limited. If this voltage with high content of harmonics is directly applied to the terminals of a solid rotor machine, the losses due to eddy currents on the rotor surface will be considerably high. The losses increase quadratically with the harmonic currents measured as THDi (Total Harmonic Distortion of current). US5001380 discloses a motor wherein a solid ferromagnetic rotor core without a squirrel-cage structure is provided with a screen in order to increase the starting torque of the motor (see US5001380, col. 1, lines 52-55). The screen in US5001380 improves the conductivity of the rotor, thereby resulting in an increased current and consequently a higher torque. The screen layer needs to be relatively thick in order to improve the conductivity. In a squirrel cage rotor, on the other hand, a high conductivity of the rotor is ensured through axial (copper) bars of the squirrel cage. Therefore, from the starting torque point of view the screen of US5001380 has the same function as the axial bars of a squirrel cage rotor, namely to improve the conductivity in the axial direction of the rotor. Applying the screen according to US5001380 on a squirrel cage motor would not result to an increased starting torque, or at least such an increase would not be expected. On the contrary, since the axial bars of a squirrel cage rotor are often made on pure copper, applying the screen according to US5001380 on a squirrel cage motor would merely be expected to worsen the conductivity and consequently lead to a decreased starting torque.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a solid squirrel-cage rotor for an induction machine, which lacks the above disclosed disadvantage when being operated at high speed and high power.

It is a further object of the invention to provide such rotor, which is simple, robust, reliable, and of low cost.

It is still a further object of the invention to provide a manufacturing method of a solid squirrel-cage rotor for an induction machine, which fulfills the above objects.

It is yet a further object of the invention to provide such manufacturing method, which is simple, reliable, of low cost, easy to implement, and which adds a minimum of further process steps to an existing manufacturing method for manufacturing a prior art solid squirrel-cage rotor.

These objects, among others, are according to the present invention, attained by rotors and manufacturing methods as claimed in the appended claims.

According to a first aspect of the invention there is provided a solid squirrel-cage rotor for an induction machine comprising a cylindrical solid rotor body including a number of axially extending slots arranged in the rotor body, electrically conductive bars arranged in the slots in a close fit; and electrically conductive members at the axial ends of the rotor body, which short-circuit the electrically conductive bars, thus forming the squirrel-cage structure of the rotor. An electrically conductive shielding layer is arranged on the cylindrical surface of the rotor body and on any exposed portions of the electrically conductive bars to shield the rotor body and the electrically conductive bars from any harmonic electric fields present outside the rotor. The resistivity of the electrically conductive shielding layer material is less than 50 x 10 ^"8 Qm, preferably less than 10 x 10 ^"8 Qm, and most preferably less than 2 x 10 ^"8 Qm, at 20 °C in order to obtain the desired functionality.

Preferably, the electrically conductive bars, and optionally the electrically conductive shielding layer, are bonded to the rotor body in a HIP (high isostatic pressing) process. Hereby, very strong diffusion bonding layers in the interfaces between the electrically conductive bars and the rotor body, and optionally between the electrically conductive shielding layer and the rotor body, are obtained.

The slots may be arranged in the surface of the rotor body, e.g. equiangularly arranged around the circumference thereof, or as through holes within the rotor body.

The rotor is advantageously designed for high speed and optionally high power applications. Hence, during operation, the peripheral speed of the rotor is about 100 m/s or higher, the rotational speed of the rotor is about 6000 rpm or higher, and optionally the power of the induction machine, for which the rotor is provided, is about 1, 5, or 10 MW or higher.

It shall be appreciated that the electrically conductive bars of the squirrel-cage rotor are used to conduct the main rotor current to generate torque. The electrically conductive shielding layer will just protect the electrically conductive bars and the rotor body from harmonic fields present in the air gap between the rotor and the stator of the induction machine. To this end, the dimension of the electrically conductive bars in a radial direction is much bigger than the thickness of the electrically conductive shielding layer. Typically, the radial dimension of the electrically conductive bars is at least about 10 times bigger, and preferably at least about 20 times bigger, than the thickness of the electrically conductive shielding layer. According to a second aspect of the invention there is provided a manufacturing method of a solid squirrel-cage rotor for an induction machine, according to which a cylindrical solid rotor body is provided in which a number of axially extending slots are arranged in the rotor body. The electrically conductive bars are arranged in the slots in a close fit and electrically conductive members are arranged at the axial ends of the rotor body to short-circuit the electrically conductive bars. An electrically conductive shielding layer is arranged on the cylindrical surface of the rotor body and on exposed portions of the electrically conductive bars, if there are any, to shield the rotor body and the electrically conductive bars from any harmonic electric fields present outside the rotor.

Typically, the electrically conductive bars are bonded to the rotor body in a HIP (hot isostatic pressing) process, whereby diffusion bonding layers in the interfaces between the electrically conductive bars and the rotor body are formed. The electrically conductive shielding layer can be manufactured in a variety of processes.

In one embodiment, the electrically conductive shielding layer is bonded to the rotor body in the HIP process, whereby diffusion bonding layers in the interfaces between the electrically conductive bars and the rotor body are formed. The HIP process includes either to arrange the rotor body, the electrically conductive bars, and the electrically conductive shielding layer within a steel capsule and to expose them for high pressure and high temperature, after which the steel capsule is removed, or to expose the rotor body, the electrically conductive bars, and the electrically conductive shielding layer for high pressure and high temperature without using the temporary steel capsule.

In another embodiment the electrically conductive shielding layer is applied on the on the cylindrical surface of the rotor body and on any exposed portions of the electrically conductive bars by means of spraying.

Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description of embodiments of the present invention given herein below and the accompanying Figs. 1-3, which are given by way of illustration only, and thus are not limitative of the present invention.

Fig. 1 displays schematically in a radial cross-section a solid squirrel-cage rotor for an induction machine according to an embodiment of the invention.

Fig. 2 displays schematically in a radial cross-section a solid squirrel-cage rotor for an induction machine during manufacture according to an embodiment of the invention.

Fig. 3 displays schematically in a radial cross-section a solid squirrel-cage rotor for an induction machine during manufacture according to a further embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known techniques are omitted so as not to obscure the description of the present invention with unnecessary details.

The present invention relates to a high-speed solid squirrel-cage rotor for an electrical rotating induction machine. The electrical rotating induction machine comprises also a stator (not illustrated) arranged around the rotor with an air gap there in-between. The stator may comprise a plurality of stator plates stacked in the axial direction and stator windings. However, the stator may have various structures as readily known by a person skilled in the art. The stator will therefore not be further elaborated in this text.

The rotor as shown in Fig. 1 comprises a cylindrical solid rotor core or body 11 including a number of axially extending slots 11a around the circumference l ib of the rotor body 11 and electrically conductive bars 12 arranged in the slots 11a in a close fit. Preferably, but not necessarily, the slots 11a are arranged equiangularly around the circumference of the rotor body 11. Note that, Fig. 1 only shows two of the electrically conductive bars 12 of the rotor for reasons of simplicity. The rotor body 11 is preferably made of a magnetically soft material having high magnetic permeability and a relatively low electric conductivity, such as e.g. steel, while the conductive bars 12 are low in the magnetic permeability and relatively high in the electric conductivity, such as e.g. copper or a copper based alloy. The rotor body 11 and the conductive bars 12 are integrated with each other and the entire surface forms a smooth cylindrical surface. The peripheral speed of the rotor can be about 100 m/s or higher or the rotational speed of the rotor can be about 6000 rpm or higher.

In the axial ends, the rotor may be provided with electrically conductive members or rings (not illustrated), e.g. copper rings, thereby short-circuiting the electrically conductive bars 12. Hereby the squirrel-cage structure of the rotor is formed.

Further, an electrically conductive shielding layer 13 is arranged on the cylindrical surface of the rotor body 11 and on any exposed portions of the electrically conductive bars 12 to shield the rotor body 11 and the electrically conductive bars 12 from any harmonic electric fields present outside the rotor, e.g. in the air gap between the rotor and the stator, as caused by e.g. a frequency converter for driving the rotating induction machine. The resistivity of the material of the electrically conductive shielding layer 13 is preferably less than 50 x 10 ^"8 Qm, more preferably less than 10 x 10 ^"8 Qm, and most preferably less than 2 x 10 ^"8 Qm, at 20 °C, in order to obtain the best shielding functionality.

Further, the electrically conductive shielding layer 13 is advantageously continuous and covers an entire operative surface of the rotor (rotor body 11 and electrically conductive bars 12). Typically, the layer is about 0.5-2 mm thick and is made of copper or a copper alloy. Preferably, the electrically conductive bars 12 and the electrically conductive shielding layer 13 are of the same material.

The problem with high surface losses due to current harmonics is particularly severe for high power induction machines such as those with a power of about 1, 5, or 10 MW or higher. For such machines the present invention is particularly useful. It has been shown that the harmonic losses can be reduced to 20%.

The invention allows for the use of converters presently manufactured by Applicant for high speed and high power drives. The invention renders it unnecessary or at least less necessary to (i) use three-phase filters or reactors between the frequency converter and the induction machine, (ii) derate the converter and increase switching frequency to get a better shaped waveform, and (iii) to use multilevel converters to ensure achievement of a sinusoidal waveform. The electrically conductive bars 12 are arranged to conduct the main rotor current to generate torque whereas the electrically conductive shielding layer 13 will merely serve as a protecting shielding layer. Therefore, the dimension of the electrically conductive bars 12 in a radial direction is at least about 10 times bigger, and preferably at least about 20 times bigger, than the thickness of the electrically conductive shielding layer.

The electrically conductive bars 12 and optionally the electrically conductive shielding layer 13 are preferably attached to the rotor body 11 by means of metallurgical bonding. Advantageously, in each of the interfaces between the electrically conductive bars 12 and optionally the electrically conductive shielding layer 13 and the rotor body 11 there is formed a diffusion bonding layer comprising e.g. steel and copper and additionally an additive material. How such layers can be formed will be disclosed below.

The invention refers also to an electrical rotating induction machine comprising the rotor as disclosed above and a stator.

Simulations show that rotor time harmonic losses caused by converter harmonics (2200 Hz) can be reduced by up to 80% for THDi of 0.153.

A manufacturing method of the solid squirrel-cage rotor according to a first embodiment will be described with reference to Fig. 1. The cylindrical solid rotor body with a number of axially extending slots 11a in the rotor body is formed and the electrically conductive bars 12 are placed in the slots 11a in a close fit. At the axial ends of the rotor body 11 the electrically conductive bars 12 are connected to one another by electrically conductive members. The electrically conductive shielding layer 13 is arranged on the cylindrical surface of the rotor body 11 and on the exposed portions of the electrically conductive bars 12. Typically, the electrically conductive shielding layer 13 is provided as a rectangular sheet which is wrapped around the cylindrical surface such that its end sides meet one another. These are welded together by an axially extending welding joint 14. In this manner the electrically conductive shielding layer 13 resembles a capsule used in a HIP (hot isostatic pressing) process, wherein diffusion bonding layers in the interfaces between the electrically conductive bars 12 and the rotor body and in the interfaces between the electrically conductive shielding layer 13 and the rotor body 11 are formed, and thus no customary steel capsule is used. The above structure is exposed to high pressure and high temperature to form the diffusion bonding layers. In this embodiment the electrically conductive bars 12 and the electrically conductive shielding layer 13 are attached to the rotor body in the same process step. The provision of a separate temporary steel capsule during the HIP process is avoided. In principle, this manufacturing method is not more complex than a prior art manufacturing method without the electrically conductive shielding layer 13.

A manufacturing method of the solid squirrel-cage rotor according to a second embodiment will be described with reference to Fig. 2. This method differs from the one disclosed with reference to fig. 1 in that the electrically conductive shielding layer 13 is not closed by a welding joint. Instead a separate capsule 21, preferably made of stainless steel is arranged around the rotor and is closed by a welding joint 22 during the HIP process, after which the capsule 21 is removed. Also in this embodiment the electrically conductive bars 12 and the electrically conductive shielding layer 13 are attached to the rotor body in the same process step. The provision of a separate temporary steel capsule during the HIP process is made as in the prior art method, but the difference is here that also the electrically conductive shielding layer 13 is attached/bonded to the rotor body in the same HIP process. In principle, this manufacturing method requires only one additional process step as compared to the prior art manufacturing method; i.e. the arranging of the electrically conductive shielding layer 13 at the rotor.

Finally, a manufacturing method of the solid squirrel-cage rotor according to a third embodiment will be described with reference to Fig. 3. In this embodiment the electrically conductive bars 12 are bonded to the rotor body 11 in a prior art HIP process by aid of a separate temporary capsule being arranged around the rotor. After having removed the capsule, the electrically conductive shielding layer is applied onto the cylindrical surface of the rotor body 11 and on any exposed portions of the electrically conductive bars 12 by means of applying a spray 31 from a spray nozzle 32 while the rotor is rotated in a separate process step. This embodiment differs from the previous ones in that the electrically conductive bars 12 are bonded to the rotor body 11 before the electrically conductive shielding layer 13 is applied. The attachment of the electrically conductive shielding layer 13 is advantageously obtained during spraying, but optionally the application of the electrically conductive shielding layer 13 will be followed by a further attachment step. This embodiment may be slightly more complex than the previous ones in that it requires more process steps.

It shall be appreciated that the rotor may be manufactured by yet different methods such as e.g. explosion welding or electroplating.

Note that while also each of Figs. 1 and 2 only shows two of the electrically conductive bars 12 of the rotor for reasons of simplicity the electrically conductive bars 12 can be provided in a greater number e.g. arranged equiangularly around the circumference of the rotor body 11.

It shall also be appreciated that the slots can be provided as a number of through holes drilled in the rotor body in an axial direction around a center axis of the rotor body 11. In such embodiment the electrically conductive bars 12 are embedded within the rotor body 11 and have no exposed surfaces to be covered by the conductive shielding layer 13. The conductive shielding layer 13 is, as above, covering the operational surface of the rotor body 11.

It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.