Patent Publication Number: US-2007108169-A1

Title: Rotor using electrical steel sheet with low iron loss, rotor manufacturing method, laser peening method, and laser peening apparatus

Description:
TECHNICAL FIELD  
      The invention relates to a rotor using an electrical steel sheet with low iron loss.  
     BACKGROUND ART  
      A rotor disclosed by the publication of unexamined Japanese patent application, JP-A-2001-16809 shows a method of achieving high speed rotation by reducing stress concentration occurring in the corner of slot and simultaneously causing the reduced stress concentration to occur at a location offset from the shortest portion between the external periphery and the slots in order to reduce the size and the weight of a motor.  
      A rotor disclosed by the publication of unexamined Japanese patent application, JP-A-2002-112481 shows method of achieving high speed rotation by increasing the resistance of the rotor against centrifugal force by dividing each magnet pole into two sections and providing a bridge in the middle in order to reduce the size and the weight of a motor.  
     DISCLOSURE OF INVENTION  
      However, the maximum rotating speed of a motor depends on the strength of the electrical steel sheet used in the rotor so that it is mandatory to use electrical steel sheets with high mechanical strength. Electrical steel sheets with high mechanical strength have high iron loss, which means that cooling of the motor can be a problem. In other words, the rotor and the rotor shaft of such a motor need to be cooled.  
      It is an object of the present invention to provide a rotor using an electrical steel sheet with low iron loss for enabling high speed rotation of a motor, a manufacturing method of the rotor, a laser peening method, and a laser peening apparatus.  
      It is still more specific object of the invention to provide a rotor using an electrical steel sheet with low iron loss, including a bridge side on an inner circumference of a magnet insertion window of the rotor, in which a strength of the bridge side is improved by means of applying a laser peening of irradiating the bridge side with a laser through a liquid.  
      Another object of the invention is to provide a method of manufacturing a rotor using an electrical steel sheet with low iron loss, which includes applying a laser peening of irradiating with a laser through a liquid a bridge side on an inner circumference of a magnet insertion window of the rotor to improve a strength of the bridge side.  
      Another object of the invention is to provide a laser peening method of irradiating a rotor made of a low iron loss electrical steel sheet with a laser through a liquid, which includes irradiating with the laser a bridge side on an inner circumference of a magnet insertion window of the rotor while moving the rotor relative to an irradiation spot of the laser, to improve a strength of the bridge side.  
      Another object of the invention is to provide a laser peening apparatus with a laser irradiating device for irradiating a rotor made of low iron loss magnetic steel with a laser through a liquid, and a drive device for moving the rotor relative to an irradiation spot of the laser in such a way that the laser irradiates the rotor along a bridge side of an inner circumference of a magnet insertion window of the rotor.  
      The objects, features, and characteristics of this invention other than those set forth above will become apparent from the description given herein below with reference to preferred embodiments illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a plan view of assistance in explaining the shape of a rotor relative to a FEM elasticity analysis.  
       FIG. 2  is a plan view of assistance in explaining a method of measuring static strength of the rotor.  
       FIG. 3  is a graph of assistance in explaining a measurement result of the static strength (tensile property) of a bridge and showing a relation between displacement and load at point-A.  
       FIG. 4  is a side view of assistance in explaining a laser peening apparatus according to Embodiment 1.  
       FIG. 5  is a side view of assistance in explaining a traveling path of a laser irradiation spot.  
       FIG. 6  is a cross-section of assistance in explaining the relation between an incident angle of a laser and an inner circumference of a magnet insertion window.  
       FIG. 7  is a plan view of assistance in explaining a rotor according to Embodiment 1.  
       FIG. 8  is a graph of assistance in explaining the tensile property (yield strength) of a bridge of the rotor shown in  FIG. 7 .  
       FIG. 9  is a graph illustrating Vickers Hardness distribution on cross-section IX-IX of a center bridge shown in  FIG. 7 .  
       FIG. 10  is a graph illustrating Vickers Hardness distribution on cross-section X-X of an outer bridge shown in  FIG. 7 .  
       FIG. 11  is a plan view of assistance in explaining a rotor according to Embodiment 2.  
       FIG. 12  is a cross-section taken on line XII-XII of a center bridge shown in  FIG. 11 .  
       FIG. 13  is a cross-section taken on line XIII-XIII of an outer bridge shown in  FIG. 11 .  
       FIG. 14  is a cross-section of the center bridges as the rotors are stacked as shown in  FIG. 11 .  
       FIG. 15  is a cross-section of the outer bridges as the rotors are stacked as shown in  FIG. 11 .  
       FIG. 16  is a graph of assistance in explaining the relation between the tensile property (yield strength) and a step of the bridge.  
       FIG. 17  is a perspective view of assistance in explaining the method of manufacturing a rotor according to Embodiment 3 and illustrating how rotors are mounted on a holder.  
       FIG. 18  is a perspective view of assistance in explaining the method of manufacturing a rotor according to Embodiment 3 continuing from  FIG. 17  and illustrating how rotors are laser-peened.  
       FIG. 19  is a perspective view of assistance in explaining the method of manufacturing a rotor according to Embodiment 3 continuing from  FIG. 18  and illustrating how rotors are removed from the holder.  
       FIG. 20  is a cross-section of assistance in explaining a main part of the laser peening apparatus shown in  FIG. 18 .  
       FIG. 21  is a side view of assistance in explaining a laser peening apparatus according to Embodiment 4.  
       FIG. 22  is a side view of assistance in explaining a traveling path of a laser irradiation spot concerning the laser peening apparatus shown in  FIG. 21 .  
       FIG. 23  is a graph illustrating Vickers Hardness distribution of a rotor concerning the laser peening apparatus shown in  FIG. 21 .  
       FIG. 24  is a perspective view of assistance in explaining deformation of an outer bridge due to laser peening.  
       FIG. 25  is a graph illustrating a relation between the thickness of a work piece and surface compression residual stress after laser peening.  
       FIG. 26  is a graph illustrating a relation between the diameter of a laser irradiation spot and the depth of the compression residual stress layer. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Embodiments of the present invention will be described below with reference to the accompanying drawings.  
      In the rotor manufacturing method according to Embodiment 1 in which electrical steel sheets with low iron loss are used as the rotor material, laser peening is applied to the bridge side on the inner circumference of the magnet insertion window by irradiating it with laser beams through liquid to improve the strength of the bridge side.  
      The rotor has internally placed permanent magnets in this case and it is applied, for example, to an interior permanent magnet synchronous motor (IPM motor). The IPM motor is typically used as the drive motor for electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV).  
      Let us describe the bridge side strengthened by the laser peening referencing the result of a basic analysis concerning the rotor.  FIG. 1  is a plan view of assistance in explaining the rotor shape relative to FEM elasticity analysis.  
      A rotor  100  is an eight pole rotor equipped with internally built in permanent magnets. Each magnet pole of the rotor  100  is divided into two pieces and two magnet insertion windows  101  and  102  are provided. When the rotor  100  rotates, a centrifugal force is applied to each magnet. A region  105  holding the magnet on the outer circumference side of the rotor  100  shall be called the outer bridge, and a region between the insertion ports  101  and  102  shall be called the center bridge  106  here.  
      An FEM elasticity analysis is applied to the rotor  100  and the stress distribution in the rotating condition, particularly the stress distribution based on the centrifugal force acting on the magnet, is studied. The result proved that there are points of stress concentration at the magnet side of the outer bridge  105  and the root section of the center bridge  106 .  
      Next, a dummy rotor made of a single electrical steel sheet was prepared, to which a rotor single plate spin test was applied to study the start and progress of plastic deformation in the rotor. Dummy magnets that are matched with the dummy rotor of one sheet were inserted into the magnet insertion windows and the configuration of the rotor was formed by the (wire cut) electrical discharge machining.  
      In the rotor single plate spin test, the stress analysis method was used to predict the rotating speed at which plastic deformation starts and progresses, while the study was conducted at various levels of stopping speed of rotation. The degree of plastic deformation is evaluated by means of the etch pit method, judging the size of the area where etch pits are occurring. In order to eliminate the effect of the friction heat due to the interaction with the atmosphere, the air in the chamber was purged beforehand to conduct the test under a vacuum and room temperature condition.  
      The plastic deformation started around the points of stress concentration in the outer bridges and the center bridges according to the Von Mises stress distribution. The plastic deformation started when the rotating speed reached a value at which the maximum stress (Von Mises stress) becomes equal to the yield stress of the rotor (material) obtained by the tensile test.  
      After having reached the rotating speed that initiated the plastic deformation, the plastic deformation progressed as the rotating speed increased further. Clearly noticeable deformation of the external shape was observed when the plastic deformation progressed further to cause it to penetrate through the bridge. This could be confirmed by observing the etch pit generating area.  
       FIG. 2  is a plan view of assistance in explaining a method of measuring static strength of the rotor. Since the rotor does not make any plastic deformation other than in the bridges, the strength of the rotor is represented by the strength of the bridges.  
      The rotor  110  (single rotor) shown here is a portion corresponding to one magnetic pole, or a range of 60 degrees and is constrained in the radial direction. Tools  112 ,  116  in the shape of a magnet are rotatably disposed in magnet insertion windows  111 ,  115 . The tools  112 ,  116  are contacting only with linear parts  111 A,  115 A that constitute the outer peripheries of magnet insertion windows  111 ,  115  in the rotor radial direction. At the centers of gravity of the tool  112 ,  116 , pin holes  113 ,  117  are formed, through which pins  114 ,  118  are inserted respectively.  
      The static strength of the bridge can be measured by the relation between the load (tensile force) F that pulls the pins  114 ,  118  in the upward direction (the radial direction of the rotor) and the displacement at the point A.  
       FIG. 3  is a graph of assistance in explaining a measurement result of the static strength (tensile property) of the bridge and showing a relation between displacement at point-A and load. The rotor used (as a comparative example) in the measurement has a shape for having six magnet poles and is a rotor with an outer diameter of 100 mm made from electrical steel sheets (35A300) by the die-punching. The thickness of the electrical steel sheet is 0.35 mm and the iron loss (W/kg) for a frequency of 50 Hz and a maximum magnetic flux density of 1.5 T is 3.00 (W 15/50 ) or less.  
      The displacement-load curve is similar to the stress-strain curve, rising with a sharp linear relation in the initial period when the deformation is small, but the gradient dulls down leaving the linear relation thereafter. This is due to the fact that yielding occurs in the stress concentration areas (plastic deformation occurs). As the displacement increases further, the material starts to produce plastic deformation causing work hardening at the same time.  
      The yielding force (or strength) is defined as the load when a 10 μm deviation occurs between the displacement-load curve and the straight line. For example, it is 210 N in the displacement-load curve of the comparative example shown in  FIG. 3 .  
      The static strength of the bridge is related to the strength in the rotor single plate spin test. For example, in the rotor single plate spin test, the diameter increases exponentially because of permanent deformation as the rotating speed increases. Therefore, if the usage limit rotating speed is defined as the speed that increases the diameter by a specified amount, for example, 20 μm, the usage limit rotating speed of the rotor (as the comparative example) applied to the static strength of the bridges is approximately 2.08×10 4  rpm.  
      Moreover, the stress distribution obtained by applying the FEM elasticity analysis to the measurement of the static strength of the bridges was similar to the stress distribution in the rotating condition and, in particular, the locations of the stress concentration were identical.  
      As can be seen in the above, stress concentrations occur on the bridge side of the magnet insertion windows of the rotor (electrical steel sheet) on the inner circumference, so that those are the regions where higher strength is required for the purpose of high speed rotations of a motor.  
      Next, let us describe about laser peening.  
      In laser peening, green light pulse-like laser rays, for example, are irradiated on the work piece in a liquid such as water and oil. Laser produces high pressure plasma on the surface of the work piece. Since the plasma is in the liquid, any abrupt expansion is repressed and a reaction force is generated as a result. The reaction force is transmitted to the work piece as a shockwave to cause a compression residual stress, which resultantly increases the surface hardness of the work piece.  
      Therefore, when the rotor is treated by laser peening, its material strength increases due to the work hardening. While plasma is generated by ablation of the material surface caused by laser, plasma is quickly cooled in the liquid so that it produces fine metal particles, which remain in the vicinity of the material surface. Therefore, it is preferable to remove those fine particles in the vicinity of the material surface from a light path of the laser so as to minimize the energy loss due to scattering by feeding the liquid in the vicinities of the material surface, thus causing a flow of the liquid, using a hose, etc.  
      As can be seen from the above, the strength of the member can be improved by applying laser peening. Therefore, a necessary strength that is sufficient to withstand high speed rotations, will be secured in case of a rotor thus manufactured in accordance with the rotor manufacturing method of Embodiment 1, using even an electrical steel sheet with low iron loss with a low base material strength. In other words, it is possible to provide a method of manufacturing a rotor that allows high speed rotations of a motor, using an electrical steel sheet with low iron loss. Moreover, no effects of laser peening on the motor performances such as torque and efficiency can be found.  
       FIG. 4  is a side view of assistance in explaining a laser peening apparatus related to Embodiment 1.  
      A laser peening apparatus  600  has a main unit  610  (laser irradiating device) for irradiating the laser and a tank  660  in which a work piece  200  is located. The work piece  200  consists of several to several tens of rotors (made of electrical steel sheet with low iron loss) stacked, which are held together under mutual pushing condition by a pair of presser plates (pressurizing members)  220  to prevent any displacements. Each presser plate  220  has openings having similar shapes as those of magnet insertion windows  201  of the rotors  200  positioned to match the locations of the magnet insertion windows  201 .  
      The main unit  610  has a laser oscillator  620 , an output adjusting device  630 , a shutter  640 , and a lens  650 . The tank  660  containing water  680  as the liquid through which the laser is transmitted, is equipped with a window  670  located on the side and a two-axis moving table (not shown).  
      The laser oscillator  620  is equipped with a Q-switched modulator and generates infrared laser of a wavelength of 1064 nm (Q-switched YAG laser). The Q-switched YAG laser is most desirable as it produces extremely high-peak output pulse oscillations (with pulse widths of several to several 10 ns). It is preferable to convert the output in the near infrared laser region (wavelength: 1064 nm) into a secondary higher harmonic wave (wavelength: 532 nm) that is absorbed by water less likely, making it more energy efficient.  
      The laser used in the laser oscillator  620  is not limited to the Q-switched YAG laser but can be glass laser, copper vapor laser, excimer laser, etc. When it is necessary to make the laser pass through a long distance of water, it is preferable to use green pulse laser of the copper vapor laser or the YAG laser (second harmonic wave).  
      The output adjusting device  630 , which includes a mechanism with a polarizing element and a splitter combined together, adjusts the output of the laser  690  emitted by the laser oscillator  620  in order to control the energy of the laser  690  per pulse to a predetermined value. The shutter  640  should have a mechanical or electrical mechanism for shutting down the laser  690  from the output adjusting device  630  as needed.  
      The lens  650  is used to focus the laser  690  after passing the shutter  640  on the inner circumference of the magnet insertion window  201 . The window  670  is made of a material that is capable of transmitting the laser  690  and is used for introducing the laser  690  sideways into the inside of the tank  660 .  
      The two-axis moving table is used to two dimensionally move the rotor  200  as irradiated from its side by the laser  690  passing through the window  670 . For example, the two-axis moving table moves the rotor  200  held by the presser plates  220  in such a way as to allow the laser  690  to irradiate uniformly the specified locations of the inner circumference of the magnet insertion window  201 . It is preferable that the focus point of the laser  690  does not change while moving the rotor  200  tangentially as shown by an arrow in  FIG. 4 .  
      In other words, the two-axis moving table is a drive device for moving the rotor  200  relative to the irradiation spot of the laser  690  so that the laser  690  irradiates along the bridge side on the inner circumference of the magnet insertion window  201  of the rotor  200 .  
      As shown above, the laser peening apparatus in accordance with Embodiment 1 can easily irradiate with the laser from the main unit (laser irradiating device) the bridge side on the inner circumference of the magnet insertion window of the rotor, using the two-axis moving table (drive device). Therefore, it is possible to improve the strength of the bridge side by means of the laser irradiation and the rotor treated by laser peening apparatus has a sufficient strength for high speed rotations of the motor. In other words, a laser peening apparatus can be provided for manufacturing a rotor that allows high speed rotations of a motor, using an electrical steel sheet with low iron loss.  
      Next, let us describe how this laser peening apparatus operates.  
      Several to several tens of rotors  200 , which are stacked and held together under mutual pushing condition by the presser plates  220  to prevent their displacement, and disposed in the tank  660 , are mounted on the two-axis moving table.  
      Next, the laser oscillator  620  is activated to generate the laser  690 . The output of the laser  690  from the laser oscillator  620  is adjusted by the output adjusting device  630 . The laser  690  whose energy per pulse is controlled to a predetermined value passes through the shutter  640 . The focus of the laser  690  is adjusted by the lens  650 , passes through the window  670 , and is laterally introduced into the tank  660 .  
      The laser  690  irradiates the bridge side on the inner circumference of the magnet insertion window  201  and generates high pressure plasma. Shock waves caused by the plasma generation are transmitted to the bridge side, generate compression residual stress, and increase the surface hardness of the bridge side as a result.  
      The rotor  200  held by the presser plate  220  is driven by the two-axis moving table. As a consequence, the irradiation spot of the laser  690  moves along the bridge side to process the locations of the rotor where high stress is generated due to the centrifugal force when the rotor rotates. Thus, the process efficiently increases the strength of the locations where strength is required.  
      Each electrical steel sheet of the rotor  200  is provided with insulation coating on the surface to form an insulation layer and improve its electrical characteristics. Therefore, the insulation layer can be damaged by ablation due to plasma if the surface of the rotor  200  is directly irradiated by the laser  690 . On the other hand, the destruction of the insulation layer is held within 3 μm or so from the edge as the laser is radiated at an angle of θ relative to the inner circumference of the magnet insertion window  201  in Embodiment 1. Therefore, the insulation deterioration of the rotor  200  can be practically prevented.  
       FIG. 5  is a side view of assistance in explaining a traveling path of the laser irradiation spot.  
      The irradiation spot S of the laser  690  starts from the side of the presser plate  220  and moves in the circumferential direction of an inner circumference of the magnet insertion window of the rotor  200  (the inner circumference direction). The moving pitch is, for example, 0.149 mm.  
      When the process for a specified distance is completed after repeating the movement of a specified pitch and reaches the turning point on one side, the irradiation spot S of the laser  690  is caused to move in the direction rectangular to the inner circumference direction, which is the stacking direction of the rotor  200 . The moving pitch is, for example, 0.149 mm. When the feed of the specified pitch is completed, the irradiation spot S of the laser  690  moves in the opposite direction along the inner circumference direction of the rotor  200  toward the turning point on the other side.  
      When the irradiation spot S of the laser  690  reaches a specified point on the presser plate  220  located on the other side of the presser plate  220  where the process was started as a result of repeating the above mentioned movements and feeds, the process is completed.  
      Namely, the irradiation spot S of the laser  690  is controlled in such a way that it repeats the cycle of being fed in the stacking direction each time when it reaches one of the turning points located along the inner circumference direction and also being caused to move a specified pitch along the inner circumference direction, and the laser  690  is, thus, to irradiates the entire regions where strength is required. The diameter, the feeding pitch, and the moving pitch of the irradiation spot of the laser  690  are chosen properly not to cause any regions not irradiated (gaps).  
      Irradiating the region including the sides of the presser plates  220  with the laser  690  is preferable to process the inner circumference of the magnet insertion windows  201  uniformly. Since a plurality of the rotors  200  are stacked in Embodiment 1, it is labor saving compared to a case of processing one by one. Also, the laser  690  can be more efficiently used as the number of irradiations (number of pulses) for the presser plate  220  can be reduced so that it contributes to a more efficient use of the laser  690 .  
       FIG. 6  is a cross section of assistance in explaining the relation between an incident angle of a laser and an inner circumference of a magnet insertion opening.  
      Although it depends on the size and shape of the magnet insertion window  201 , it is generally possible to irradiate the inner circumference of the magnet insertion window  201  with the laser while stacking together up to 30-40 pieces of rotors with a common thickness of 0.35 mm using presser plates  220  of a thickness of 3 mm and maintaining an incident angle of the laser  690  at 60 degree.  
      If any macro distortion occurs due to plastic deformation of the bridge side on the inner circumference of the magnet insert port  201  as a result of the irradiation of the laser  690 , uniform irradiation with the laser may become difficult. Therefore, it is preferable to have the presser plates  220  be kept under a pressure of approximately 5-10 kgf/cm to retain the stacked rotors  200 .  
      As shown above, laser peening method in accordance with Embodiment 1 makes it easy to irradiate with laser the bridge side on the inner circumference of the magnet insertion windows of the rotor by causing the rotor to move relative to the irradiation spot of the laser. As a result of laser irradiation, the strength of the bridge side is improved, hence providing the strength necessary for high speed rotations of the motor. In other words, it is possible to provide a laser peening method for manufacturing a rotor using an electrical steel sheet with low iron loss for enabling high speed rotation of a motor.  
      Although it depends on the overall positional relation, the same effect can be achieved even if the laser irradiation position shifts ±5 mm when a lens  650  with a focal distance of approximately 200 mm is used. Therefore, it is possible to choose a vertical direction as the moving direction of the rotor  200 . Although the rotor  200  is designed to move while keeping the irradiation spot of the laser  690  stationary in Embodiment 1, it is also possible to arrange in such a way as to make the irradiation spot of the laser  690  to move while maintaining the rotor  200  stationary.  
      It is also possible to change the traveling path of the irradiation spot of the laser  690 . For example, it is also possible to arrange the turning points in the stacking direction and to repeat the cycle of feeding the irradiation spot S of the laser  690  in the inner circumference direction and causing it to move a specified pitch in the stacking direction every time when it reaches one of the turning points.  
       FIG. 7  is a plan view of assistance in explaining a rotor according to Embodiment 1.  
      The rotor  200  has a shape for having six magnet poles and is made by the die-punching from electrical steel sheets (35A300). The rotor  200  is sized to have an outer diameter of 100 mm and a thickness of 0.35 mm. The rotors  200  are stacked (10 sheets) to be processed together during laser-peening.  
      The regions to be irradiated with the laser on the inner circumferences of the magnet insertion windows  201 ,  202  are magnet insertion window side  203  of the outer bridges  205  and magnet insertion window side  204  of the center bridges  206 . The energy of the laser is 60 mJ. The diameter of the laser irradiation spot is Φ0.4 mm. The pulse density of the laser is 135 pulses/mm 2 .  
       FIG. 8  is a graph of assistance in explaining tensile property (yield strength) of the bridge of the rotor shown in  FIG. 7 .  
      The yield stress of the bridges of the rotor manufactured with the above condition is 278 N and the yield stress of the comparative example manufactured without laser peening is 210 N. Namely, the strength of the rotor  200  was increased. The usage limit rotating speed of the rotor  200  is approximately 2.4×10 4  rpm, and the usage limit rotating speed of the comparative example is approximately 2.08×10 4  rpm.  
       FIG. 9  is a graph illustrating Vickers Hardness distribution on cross-section IX-IX of a center bridge shown in  FIG. 7 , and  FIG. 10  is a graph illustrating Vickers Hardness distribution on cross-section X-X of an outer bridge shown in  FIG. 7 .  
      The origin of the distance and the 900 μm point in  FIG. 9  correspond to the edges of the center bridge (the magnet insertion window side). The origin of the distance and the 1000 μm point in  FIG. 10  correspond to the edges of the outer bridge (the magnet insertion window side and the outer circumference). The weight used in measuring the Vickers hardness was 25 gf. The hardness measurement was performed at the center of the cross-section.  
      The center bridge  206  is sandwiched by the magnet insertion windows  201 ,  202  and has the magnet insertion window side  204 , which is treated with laser peening, on both sides. As a result, the center bridge  206  is hardened up to the points 0.3-0.4 mm apart from the edges (origin and 900 μm point) and shows the highest hardness at the edges. The Vickers Hardness of the electrical steel sheet (35A300) is approximately 200.  
      The magnet insertion windows  201 ,  202  exist only on one end of the outer bridges  205  and the magnet insertion window side  203  as laser-peened exists only on one side. As a result, the outer bridge  205  is hardened up to the point 0.3-0.4 mm apart from the edge (origin) and shows the highest hardness at the edge (origin). Hardened region extends also up to the point 0.2-0.3 mm apart from the end which is not laser-peened (1000 μm point). This hardening is due to punching strain.  
      When a motor using the rotor according to Embodiment 1 is compared in terms of the efficiency at a condition of 18000 rpm and 60 kW with a motor using a rotor that is not laser-peened, the motor using the rotor according to Embodiment 1 had a better efficiency than the other one. It is believed that the main cause of the result is an increase in the torque. Therefore, it is believed that the increase in the rotor core iron loss due to laser peening is small. Moreover, the temperatures applied to the rotor during its manufacturing process such as the shrink-fit temperature and the magnet adhesive curing temperature, and the motor operating temperature does not affect the work hardening due to laser peening.  
      As can be seen from the above, the rotor according to Embodiment 1 has an improved strength of the bridge at the inner circumference of the magnet insertion window where the strength is most needed as a result of the laser irradiation. Therefore, even if electrical steel sheets with low iron loss which have a low base strength are used as the raw material for the rotor, strength required for high speed rotations of the motor can be secured. In other words, it is possible to provide a rotor using an electrical steel sheet with low iron loss for enabling high speed rotation of a motor.  
      The sheet thickness of the rotor raw material (electrical steel sheet) is not limited to 0.35 mm, but other sheet thicknesses such as 0.20 mm can be used as well. Since the material is irradiated with the laser in an angle, the laser irradiation spot is elliptical and is larger than in the case of irradiating perpendicular to the surface of the material. Therefore, it is possible to achieve an identical effect as in the case of perpendicular irradiation by adjusting the distance between the lens  650  and the inner circumference of the magnet insertion window  201 .  
       FIG. 11  is a plan view of assistance in explaining a rotor according to Embodiment 2,  FIG. 12  is a cross-section taken on line XII-XII of a center bridge shown in  FIG. 11 ,  FIG. 13  is a cross-section taken on line XIII-XIII of an outer bridge shown in  FIG. 11 ,  FIG. 14  is a cross-section of the center bridges as the rotors are stacked as shown in  FIG. 11 , and  FIG. 15  is a cross-section of the outer bridges as the rotors are stacked as shown in  FIG. 11 .  
      The rotor  300  according to Embodiment 2 is substantially different from the rotor  200  according to Embodiment 1 in that it has steps on the bridges. Specifically, steps  303 A,  304 A are formed on one side of the surfaces of the magnet insertion side  303  of the outer bridge  305  and the magnet insertion side  304  of the center bridge  306  on the inner circumferences of the magnet insertion windows  301 ,  302  of the rotor  300 , respectively.  
      In Embodiment 2, the material thickness “t” of the rotor  300  before the steps are formed is 0.35 mm, and the ratio (step ratio) of the step thickness Δt divided by the material thickness “t” is 3%. The steps  303 A,  304 A can be formed by the pressing.  
       FIG. 16  is a graph of assistance in explaining the relation between the tensile property (yield strength) and the step of the bridge.  
      The rotor  300  is laser-peened on the magnet insertion side  303  of the outer bridge and the magnet insertion side  304  of the center bridge after the stacking. The energy of the laser is 60 mJ. The diameter of the laser irradiation spot is Φ0.4 mm. The pulse density of the laser is 45 pulses/mm 2 . The feeding pitch of the irradiation spot relative to the stacking direction of the rotor and the moving pitch of the irradiation spot relative to the inner circumference direction is 0.149 mm.  
      As shown in the graph, the rotor  300  according to Embodiment 2 has a higher yield strength compared to that of the rotor  200  according to Embodiment 1. The short dashes line shows the result of a comparative example manufactured without laser peening.  
      As can been seen from the above, the strengths of the bridges can be further improved in Embodiment 2 by having the steps in the bridges. The step does not have to be provided only on one side, but can be provided on both sides.  
       FIG. 17  through  FIG. 19  are perspective views of assistance in explaining the method of manufacturing a rotor according to Embodiment 3, and  FIG. 20  is a cross-section of assistance in explaining the main parts of the laser peening apparatus shown in  FIG. 18 . FIG.  17  shows the mounting of the rotor on the holder,  FIG. 18  shows laser peening of the rotor as a continuation from  FIG. 17 , and  FIG. 19  is a removal of the rotor from the holder as a continuation from  FIG. 18 .  
      The laser peening apparatus  700  has a laser generating unit  710 , a mirror  720 , an irradiation head  730 , and a drive device  735  for the irradiation head  730  as shown in  FIG. 18 . The laser generating unit  710  has a laser oscillator and an output adjusting device for generating pulse-like laser  790  having a specified energy. The mirror  720  is a laser transmitting part for changing the direction of the laser  790  emitted by the laser generating unit  710  and guiding it into the irradiation head  730 . The laser transmitting part is not limited to the mirror  720  but can be an optical fiber.  
      The irradiation head  730  of a substantially long cylindrical shape with a small diameter has a window  770 , a mirror  740 , an opening  755 , a water supply tube  750 , and an exhaust tube  760  as shown in  FIG. 20 . The window  770  is located at one end of the irradiation head  730  and used for introducing the laser  790 , whose direction is changed by the mirror  720 , into the inside of the irradiation head  730 .  
      The mirror  740  is aspherical and is located on the other end of the irradiation head  730 . The mirror  740  is made of a high thermally conductivity metal such as copper, aluminum, silver and gold covered with a dielectric multi-layer coating on the reflection surface. The opening  755  is located on the side of the irradiation head  730  close to the other end adjacent to the mirror  740 .  
      The mirror  740  changes the direction of and condenses the laser  790  transmitted through the window  770 . The reflected laser  790  is irradiated through the opening  755  on the bridge side  403  on the inner circumference of the magnet insertion window  401  of the rotor  400 .  
      The water supply tube  750  is used to introduce water  780  into the inside of the irradiation head  730  and discharge it through the opening  755  during the irradiation of the laser  790 . Therefore, the laser  790  is irradiated onto the bridge side  403  through the flowing water  780 .  
      In other words, the laser peening apparatus  700  is equipped with a liquid flow device for causing the liquid located in the irradiation plane of the laser  790  to flow. Therefore, metal particles, which are caused by the plasma created by the laser irradiation and floating close to the surface of the bridge side  403 , are efficiently removed from the light path of the laser  790 . Therefore, it is possible to prevent the energy loss due to scattering of the laser  790 .  
      The rotor  400  is first stacked on the holder  420  as shown in  FIGS. 17, 19  and then laser-peened. The holder  420  has a cylindrical part  430  which fits a center hole  410  of the rotor  400 , and presser plates  440  for keeping a plurality of stacked rotors  400  together under mutual pushing condition. Each presser plate  440  has an opening  445  for allowing the irradiation head  730  to pass through. The opening  445  has a shape substantially similar to the magnet insertion window  401  of the rotor  400  and its location is substantially similar to that thereof. The center hole  410  of the rotor  400  is provided with a notch (not shown) for positioning the rotor  400  relative to the rotational direction in advance.  
      The exhaust tube  760  is used for extracting air inside the irradiation head  730  before using the device and to discharge air that is separated as bubbles due to continuous prolonged usage of the device.  
      The drive device  735  has a function of moving the irradiation head  730  in the axial direction and a function of rotating the irradiation head  730  in the radial direction in order to move the rotor  400  relative to the irradiation spot of the laser  790  so that the laser  790  irradiates along the bridge side  403  on the inner circumference of the magnet insertion window  401  of the rotor  400 . A similar function can be achieved by moving the rotor  400  vertically instead of moving the irradiation head  730  in the axial direction.  
      Next, the method of manufacturing the rotor according to Embodiment 3 will be described.  
      The received rotors  400  are positioned by the notch relative to the rotational direction, and are inserted one by one onto the cylindrical part  430  of the holder  420 , and are held together under mutual pushing condition by the presser plates  440  when the prescribed number of rotors is stacked (see  FIG. 17 ).  
      Next, the irradiation head  730  is inserted in the axial direction into the magnet insertion windows  401  of the stacked rotors  400  via the opening  445  of the presser plate  440  (see  FIG. 18 ). Water  780  is then introduced into the inside of the irradiation head  730  from the water supply tube  750 . On the other hand, the laser  790  emitted by the laser generating unit  710  passes through the mirror  720 , the window  770 , and the inside of the irradiation head  730 , and is reflected and condensed by the mirror  740  (see  FIG. 20 ). The reflected and condensed laser  790  passes through the opening  755  and irradiates the bridge side  403  through the flowing water  780 . On the other hand, the irradiation head  730  is controlled by the drive device  735 , moves along the bridge side  403  repeating rotations and up/down movements in the axial direction in order to improve the strength of the required regions.  
      When the process on the magnet insertion window  401  as the current target is completed, the irradiation head  730  is taken out of the magnet insertion window  401 , turned in the radial direction, and is positioned to align with the next magnet insertion window  401 . The above process cycle by the irradiation head  730  is repeated.  
      When all the magnet insertion windows  401  formed on the rotors  400  are processed, the rotors are removed from the holder  420  for delivery ( FIG. 19 ).  
      As can be seen from the above, Embodiment 3 is also cable of improving the strength of the bridge on the inner circumference of the magnet insertion window where strength enhancement is required.  
       FIG. 21  is a side view of assistance in explaining a laser peening apparatus according to Embodiment 4. Those members that have the identical functions as those in Embodiment 1 will be denoted with the identical reference numerals in order to avoid duplicating their descriptions.  
      The laser peening apparatus  800  is substantially different from the laser peening apparatus  600  of Embodiment 1 in that the former has a position detecting device  810  for detecting the position of the rotor  850  relative to the irradiation spot of the laser  690 , and a condition detector  820  for detecting laser peening condition.  
      The position detecting device  810  has an imaging device  811  for optically monitoring the vicinities of the irradiation spot of the laser  690 , and a window  812  formed on the side of the tank  660 . The imaging device is, for example, a video camera and is provided close to the window  812 .  
      The position detecting device  810  is capable of detecting the position of the rotor  850  relative to the irradiation spot of the laser  690  by means of utilizing the image taken by the imaging device  811  through the window  812 , for example, analyzing the image frame. The installation location of the imaging device  811  can be, for example, the inside or above of the tank  660 .  
      The condition detector  820  has a sound measurement device  821  for measuring the sound induced by the plasma generated by the irradiation of the laser  690 . The sound measurement device  821  is a microphone using a piezoelectric element, for example, and is placed near the rotor  850 .  
      The difference in laser peening condition makes difference in the sound. Therefore, the condition detector  820  can detect laser peening condition from the sound measured by the sound measurement device  821  by analyzing the waveform of the signal obtained by AD conversion of the output or by analyzing visualized images of the sound obtained by shadow graphs.  
      On the other hand, laser peening condition is affected by the constitution of the irradiation plane of the laser  690 . For example, laser peening condition is different between when the irradiation spot of the laser  690  is passing through the middle of the thickness of the rotor  850  and when it is passing through a region that contains an overlapping surface of the rotor  850 .  
      Therefore, the condition detector  820  is capable of identifying the position of the rotor  850  relative to the irradiation spot of the laser  690  in real time with a high precision based on the detection of laser peening condition. The installation location of the sound measurement device  821  can be, for example, the outside of the tank  660 .  
      The laser peening apparatus  800  controls a two-axis moving table (not shown), which is a drive device for causing the rotor  850  to move relative to the irradiation spot of the laser  690 , using the detection results of the position detecting device  810  and the condition detector  820 . Thus, it is possible to control the traveling path of the irradiation spot of the laser  690  with a high precision. It is also possible to omit one of the position detecting device  810  and the condition detector  820  as needed, or to provide a plurality of the position detecting device  810  and/or the condition detector  820 .  
      Differences of laser peening condition can also be created by the irradiation spot of the laser  690  passing through a region that contains a overlapping plane between the rotor  850  and the presser plate  220 , or the middle of the thickness of the presser plate  220 , or the outer edge of the presser plate  220 . Therefore, it is possible to use the condition detector  820  as a detector for control abnormality by properly setting up the threshold value for identifying laser peening condition.  
      The detection of laser peening condition should not necessarily depend on sound. For example, the sound measurement device  821  can be replaced with a light measurement device that measures the plasma lighting caused by the laser irradiation. The light measurement device which can typically consist of devices such as a color filter, an image intensifier, and a CCD is so disposed as to measure the plasma lighting distribution through a window formed on the side of the tank  660 .  
       FIG. 22  is a side view of assistance in explaining a traveling path of a laser irradiation spot concerning the laser peening apparatus shown in  FIG. 21 .  
      The rotor  850 , which is the work piece of laser peening, is formed by punching an electrical steel sheet (35A230) with a thickness of 0.35 mm. The energy of the laser is 70 mJ. The diameter of the laser irradiation spot S is Φ0.4 mm. The pulse density of the laser is 50 pulses/mm 2 .  
      The irradiation spot S is controlled to repeat the cycle of being fed with a specified pitch in the stacking direction each time when it reaches one of the turning points arranged in the inner circumference direction and moving with a specified pitch along the inner circumference direction. Two different conditions are set up for the feeding pitch of the irradiation spot S relative to the stacking direction of the rotor  850  and the moving pitch of the irradiation spot S relative to the inner circumference direction.  
      In the condition  1 , the feeding pitch is 0.35 mm, which substantially matches with the thickness of the rotor  850 , and the moving pitch is 0.141 mm. In the condition  2 , the feeding pitch and the moving pitch are both 0.141 mm.  
      In the condition  1 , the two-axis moving table is controlled in such a way that the irradiation spot S to pass through the thickness center of the rotor  850  extending along the overlapping surface of the rotor  850 . Therefore, the irradiation spot S passes each rotor  850  only once relative to the circumferential direction.  
       FIG. 23  is a graph illustrating Vickers Hardness distribution of a rotor concerning the laser peening apparatus shown in  FIG. 21 . The weight used in measuring the Vickers Hardness was 50 gf.  
      The Vickers Hardness distribution in the condition  1  substantially matches with that of the condition  2 . The feeding pitch of the condition  1  is 0.35 mm and that of the condition  2  is 0.141 mm, so that the value of the feeding pitch of the condition  2  divided by the feeding pitch of the condition  1  is 0.402. Therefore, the condition  1  can reduce the number of irradiations for laser peening approximately 40%, and also reduce the manufacturing cost by shortening the process time compared to the condition  2 .  
      The Vickers Hardness distribution shown in  FIG. 23  is slightly different from the Vickers Hardness distributions shown in  FIG. 9  and  FIG. 10 . However, the Vickers Hardness tends to increase 10 or so in measured values when the measuring load is 25 gf instead of 50 gf. The Vickers Hardness of the electrical steel sheet (35A300) is approximately 200 and the hardness of the electrical steel sheet (35A230) is approximately 210. The difference shown above seems to come from the difference in the measuring weight of the Vickers Hardness and the difference in the base materials of the rotor  850 .  
      As can be seen from the above, Embodiment 4 has a position detecting device and a condition detector so that the drive device for moving the rotor relative to the laser irradiation spot can be controlled with a high precision. Consequently, it is possible to reduce the number of irradiations for laser peening, the process time, and the manufacturing cost easily by controlling the traveling path, the feeding pitch, and the moving pitch of the laser irradiation spot with a high precision.  
      The diameter of the irradiation spot must be larger than the thickness of the rotor when the feeding pitch of the laser irradiation spot matches with the rotor thickness approximately. Furthermore, the plasma pressure tends to reduce at the edge of the irradiation spot. Therefore, the irradiation spot diameter is preferably 1.1 times of the rotor thickness, or more preferably 1.2 times or larger.  
      On the other hand, an increase of the irradiation spot diameter requires a higher capacity of the laser oscillator, thus an increase of the equipment cost. Considering the capacities and the prices of available laser oscillators, the irradiation spot diameter is preferably less than 3 times, or more preferably less than 2.5 times of the rotor thickness.  
      Therefore, the value obtained by dividing the irradiation spot diameter with the rotor thickness should better be within 1.1-3, or more preferably 1.2-2.5.  
      Furthermore, it is preferable that the center of the laser irradiation spot be located within a range corresponding to one half of a thickness of the rotor in a middle portion in the thickness of the rotor. This makes it possible to achieve the necessary strength securely.  
      It is also possible to change the traveling path of the irradiation spot of the laser  690  as in Embodiment 1. It is also possible to arrange the turning points in the stacking direction and to repeat the cycle of feeding the irradiation spot S of the laser  690  in the inner circumference direction and causing it to move a specified pitch in the stacking direction every time when it reaches one of the turning points.  
      In order to improve the electromagnetic performance of the motor, it is preferable to position the magnet of the rotor as close as possible to the stator and to make the width of the outer bridge as narrow as possible. On the other hand, the width of the outer bridge is preferably as wide as possible in order to sustain the strength during high speed rotations of the motor. Thus, in consideration of both effects, the width of the outer bridge is chosen to be approximately 1 mm.  
       FIG. 24  is a perspective view of assistance in explaining deformation of an outer bridge due to laser peening.  
      In some cases, laser peening forms a compression residual stress layer from the surface to the depth of approximately 1 mm. Therefore, laser peening the outer bridge side  503  on the inner circumference of the magnet insertion window  501  of the rotor  500  having an outer bridge  505  with a width of approximately 1 mm causes the entire outer bridge  505  a plastic deformation extending in the circumferential direction, resulting in a three dimensional deformation.  
      When the outer bridge  505  deforms, the stacking ratio of the rotor deteriorates, causing deterioration of the overall strength as well, and may cause a large deformation due to the centrifugal force. Since such a deformation does not necessarily occur in a uniform pattern, so that it may affect the balance during high speed rotations to generate vibrations and increase the breakdown risk. Moreover, it releases the compression residual stress formed by laser peening and suppresses laser peening effect.  
       FIG. 25  is a graph illustrating a relation between the thickness of a work piece and surface compression residual stress after laser peening. The thickness of the work piece is 3-14 mm, and the laser peeing is applied under the same condition. The material of the work piece is Fe-3% Si alloy.  
      As shown in the figure, the deformation releases the surface compression residual stress if the work piece is thin. If the data of  FIG. 25  is extrapolated and is applied to a case in which laser peening is applied to the outer bridge side of the outer bridge with a width of 1 mm, the resultant surface compression residual stress is expected to be less than 100 MPa. According to various experiments and analyses conducted using work pieces made of Fe-3% Si alloy, conspicuous deformations occur and the surface compression residual stress is released when the thickness of the compression residual stress layer due to laser peening exceeds 10-20% of the thickness of the work piece.  
      Therefore, it is preferable to control the thickness of the compression residual stress layer to be less than 0.2 mm in order to suppress the deformations in case of laser peening the outer bridge side of an outer bridge with a width of 1 mm. When the rotor is held under mutual pushing condition, the outer bridge is constrained so that it is possible to increase the thickness of the compression residual stress layer due to laser peening. The pressing force to maintain the rotor under mutual pushing condition is preferably, for example, 5 kgf/cm 2  or higher, and the effect is inconspicuous if it is below 5 kgf/cm 2 .  
       FIG. 26  is a graph illustrating a relation between the diameter of a laser irradiation spot and the depth of the compression residual stress layer. The work piece in this example is Fe-3% Si alloy.  
      As can be seen from the graph, the thickness of the compression residual stress layer is approximately equal to the diameter of the irradiation spot. Thus it is possible to maintain the three dimensional deformation of the outer bridge (not held under mutual pushing condition) sufficiently small, when the pulse energy is assumed to be 20 mJ and the irradiation spot diameter to be approximately 0.2 mm.  
      It is obvious that this invention is not limited to the particular embodiments shown and described above but may be variously changed and modified without departing from the technical concept of this invention.  
      For example, it is possible to apply the step related to Embodiment 2 to the bridge related to Embodiment 1 or Embodiment 3. Also, it is possible to provide a liquid flow device for the laser peening apparatus related to Embodiment 1 or Embodiment 4.  
      This application is based on Japanese Patent Application No. 2003-330686 filed on Sep. 22, 2003 and No. 2004-273677 filed on Sep. 21, 2004, the contents of which are hereby incorporated by reference.