Permanent-magnet-embedded electric motor and compressor

A rotor of a permanent-magnet-embedded electric motor includes an annular rotor core having a plurality of magnet insertion holes formed in a circumferential direction, and permanent magnets inserted into the magnet insertion holes, respectively. The rotor core is formed by alternately stacking a first core block and a second core block in an axial direction of the rotor core, the first core block not having slits between each of the magnet insertion holes and a circumferential surface of the rotor core, and the second core block having the slits between each of the magnet insertion holes and the circumferential surface of the rotor core. One of the slits and one end of the permanent magnet are arrayed in a radial direction, and the other slit and the other end of the permanent magnet are arrayed in the radial direction.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national stage application of International Patent Application No. PCT/JP2015/067205 filed on Jun. 15, 2015, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a permanent-magnet-embedded electric motor in which permanent magnets are embedded in a rotor core, and a compressor that includes the permanent-magnet-embedded electric motor.

BACKGROUND

A recent increase in the awareness of energy saving demands highly efficient electric motors, and has led to many proposals for permanent-magnet-embedded electric motors achieving high efficiency by using rare earth magnets with high residual magnetic flux density and coercivity in a rotor. The permanent magnets are embedded in the rotor so as to be able to use not only magnet torque but also reluctance torque; therefore, the electric motor achieving high efficiency can be constructed. Because the reluctance torque is proportional to the difference between d-axis inductance and q-axis inductance, it is generally desired to have a structure that allows a q-axis magnetic flux to pass easily but does not allow a d-axis magnetic flux to pass easily in order to increase the reluctance torque. Here, the d axis is a radial axis passing through the center of the magnet and the q axis is an axis obtained by rotating the d axis by an electrical angle of 90°.

However, compared with a permanent-magnet-embedded electric motor not using the reluctance torque, the permanent-magnet-embedded electric motor with high usage of the reluctance torque is likely to have pulsation in a torque waveform and has an increased torque ripple. The torque ripple during operation of the permanent-magnet-embedded electric motor causes vibration and noise, and thus needs to be suppressed within a standard value.

Patent Literature 1 describes that the torque ripple is reduced by disposing a plurality of slits on the surface of a rotor in a permanent-magnet-embedded electric motor.

PATENT LITERATURE

Although the permanent-magnet-embedded electric motor described in Patent Literature 1 can reduce the torque ripple by disposing the plurality of slits on the surface of the rotor, the slits block a magnetic path of a magnetic flux to cause a reduction in torque obtained at the same current. In this case, the current required to generate the same torque is increased to thus result in reduced efficiency of the electric motor due to an increased copper loss.

SUMMARY

The present invention has been achieved in view of the above and an object of the present invention is to provide a permanent-magnet-embedded electric motor that can maintain the efficiency of the electric motor by preventing a reduction in the torque obtained at the same current as well as reduce the torque ripple.

In order to solve the above problems and achieve the object, a permanent-magnet-embedded electric motor according to an aspect of the present invention includes: an annular rotor core in which a plurality of magnet insertion holes are formed along a circumferential direction; and a plurality of permanent magnets inserted into the magnet insertion holes, respectively. The rotor core is configured by alternately stacking, in an axial direction of the rotor core, a first core block and a second core block, the first core block having no slit on an outer side of each of the magnet insertion holes in a radial direction of the rotor core, the second core block having a pair of slits on the outer side of each of the magnet insertion holes in the radial direction of the rotor core, and one of the pair of slits and one end of each of the permanent magnets in the circumferential direction are arrayed in the radial direction of the rotor core, and another of the pair of slits and another end of each of the permanent magnets in the circumferential direction are arrayed in the radial direction of the rotor core.

Advantageous Effects of Invention

The present invention produces an effect where it is possible to maintain the efficiency of the electric motor by preventing a reduction in the torque obtained at the same current as well as reduce the torque ripple.

DETAILED DESCRIPTION

A permanent-magnet-embedded electric motor and a compressor according to embodiments of the present invention will now be described below in detail with reference to the drawings. Note that the present invention is not to be limited to the embodiments.

First Embodiment

FIG. 1is a sectional view illustrating the structure of a permanent-magnet-embedded electric motor according to the present embodiment, andFIG. 2is a sectional view of a rotor core according to the present embodiment. As will be described later,FIGS. 1 and 2illustrate a sectional structure of a first core block out of the first core block and a second core block constituting the rotor core.

A permanent-magnet-embedded electric motor1includes an annular stator2and a rotor3disposed radially inward of the stator2with a gap4therebetween.

The stator2includes an annular stator core5and a winding7wound around a plurality of teeth6formed on the inner peripheral surface of the stator core5. The teeth6are disposed at equal intervals in the circumferential direction of the stator2and extend in the radial direction of the stator core5.

The winding7is wound by adopting distributed winding. In the distributed winding, the winding7is wound across the plurality of teeth6.

The rotor3includes an annular rotor core10, a plurality of permanent magnets11embedded in the rotor core10, and a shaft12fitted in the center of the rotor core10. The permanent magnets11form magnetic poles of the rotor3, and the number of magnetic poles is equal to the number of the permanent magnets11. The number of the permanent magnets11is set to six, for example, but is not limited to six as long as a plurality of magnets are provided.

A plurality of magnet insertion holes13are formed in the rotor core10, where the number of the magnetic insertion holes is equal to the number of magnetic poles. The magnet insertion holes13are formed along the circumferential direction of the rotor3on the outer peripheral side of the rotor core10. The magnet insertion holes13pass through the rotor core10in the axial direction of the rotor core10. The permanent magnets11are inserted into the magnet insertion holes13. The permanent magnet11has a flat plate shape, for example, and a main portion13cof the magnet insertion hole13into which the permanent magnet11is inserted extends in a direction orthogonal to the radial direction of the rotor core10. Note that in the following description, a “circumferential direction” refers to a circumferential direction of the rotor core10, and a “radial direction” refers to a radial direction of the rotor core10. The circumferential direction is also the direction of rotation of the rotor3.

Both ends of the magnet insertion hole13in the circumferential direction are void portions13aand13b, which are disposed on both sides of the permanent magnet11in the circumferential direction. A direction of magnetization of the permanent magnet11corresponds with the radial direction. The direction of magnetization is alternately reversed in the circumferential direction. The void portions13aand13bare bent toward the outer periphery with respect to the main portion13cof the magnet insertion hole13into which the permanent magnet11is inserted, and extend in the radial direction toward the outer periphery of the rotor core10. A shaft hole14passing through the rotor core10in the axial direction is provided at the center of the rotor core10, where the shaft12is press-fitted into the shaft hole14.

The permanent magnet11inserted into the magnet insertion hole13can be a neodymium (Nd) or dysprosium (Dy)-based rare earth magnet, or a ferrite magnet composed mainly of iron oxide (Fe2O3). Because the rare earth magnet has high residual magnetic flux density and coercivity, the use of such rare earth magnet allows construction of a permanent-magnet-embedded electric motor having high efficiency and improved resistance to demagnetization. On the other hand, the ferrite magnet has residual magnetic flux density and coercivity that are one-third that of the rare earth magnet; therefore, in order to ensure the high efficiency and resistance to demagnetization similar to the case of using the rare earth magnet, the ferrite magnet being inserted has a larger volume than the rare earth magnet and is increased in size. However, the ferrite magnet is more inexpensive than the rare earth magnet and has high supply stability to thus enable construction of a permanent-magnet-embedded electric motor that is unaffected by a cost increase and a supply risk of the rare earth magnet.

FIG. 3is an enlarged sectional view illustrating a part of the first core block constituting the rotor core of the present embodiment, andFIG. 4is an enlarged sectional view in which the permanent magnet is inserted into the magnet insertion hole ofFIG. 3.FIG. 5is an enlarged sectional view illustrating a part of the second core block constituting the rotor core of the present embodiment, andFIG. 6is an enlarged sectional view in which the permanent magnet is inserted into the magnet insertion hole ofFIG. 5.FIG. 7is a perspective view illustrating a part of the rotor core of the present embodiment. Note that fromFIGS. 3 to 7, a component identical to that illustrated inFIGS. 1 and 2is denoted by the same reference numeral as that assigned thereto.

FIG. 3illustrates a sectional structure of a core block10a, which is the first core block, corresponding to one magnetic pole. Specifically, the figure illustrates the structure for the range of a central angle of 60° with respect to the center of rotation, which is the center of the shaft hole14in the sectional structure illustrated inFIG. 2.FIG. 4illustrates a state in which the permanent magnet11is inserted into the magnet insertion hole13inFIG. 3. Note thatFIG. 4illustrates a magnetic pole center30, which is the center of the permanent magnet11in the circumferential direction, and an inter-magnetic pole31, which is the middle of the adjacent magnetic pole centers30.FIG. 4illustrates an outer edge portion40, which is a portion of the core block10aon the outer side of the magnet insertion hole13in the radial direction.

FIG. 5illustrates a sectional structure of a core block10b, which is the second core block, corresponding to one magnetic pole. Specifically, the figure illustrates the structure for the range of a central angle of 60° with respect to the center of rotation, which is the center of the shaft hole14in the sectional structure illustrated inFIG. 2.FIG. 6illustrates a state in which the permanent magnet11is inserted into the magnet insertion hole13inFIG. 5. Note thatFIG. 6illustrates the magnetic pole center30, which is the center of the permanent magnet11in the circumferential direction, and the inter-magnetic pole31, which is the middle of the adjacent magnetic pole centers30.

As illustrated inFIGS. 5 and 6, the core block10bincludes a pair of slits15aand15b, which is a pair of gaps formed between the magnet insertion hole13and the circumferential surface of the core block10b. Here, the circumferential surface of the core block10bcorresponds with the circumferential surface of the rotor core10. The slit15ais disposed radially outward of an end11a, which is one end of the permanent magnet11in the circumferential direction, and the slit15bis disposed radially outward of an end11b, which is the other end of the permanent magnet11in the circumferential direction. That is, the slit15aand the end11aare arrayed in the radial direction, and the slit15band the end11bare arrayed in the radial direction. Moreover, the slit15ais disposed adjacent to the void portion13aand is closer to the magnetic pole center30than the void portion13a, and the slit15bis disposed adjacent to the void portion13band is closer to the magnetic pole center30than the void portion13b. Note thatFIG. 6illustrates an outer edge portion41, which is a portion of the core block10bon the outer side of the magnet insertion hole13in the radial direction. The slits15aand15bare located at the circumferential ends of the outer edge portion41.

The slit15ahas a rectangular shape extending in the circumferential direction. That is, the circumferential length of the slit15ais longer than the radial length of the slit15a. The slit15bhas a rectangular shape extending in the circumferential direction. That is, the circumferential length of the slit15bis longer than the radial length of the slit15b. The slits15aand15bpass through the core block10bin the axial direction.

Other structures of the core block10bare the same as those of the core block10a. Moreover, the stator2ofFIG. 1is disposed around the core block10bin the permanent-magnet-embedded electric motor1.

As described above, the core block10adoes not have the slits15aand15bon the outer side of each of the magnet insertion holes13in the radial direction of the rotor core10, whereas the core block10bhas the slits15aand15bon the outer side of each of the magnet insertion holes13in the radial direction of the rotor core10.

FIG. 7illustrates the structure of the rotor core10for the angular range corresponding to one magnetic pole. As illustrated inFIG. 7, the rotor core10is formed by alternately stacking the core block10aand the core block10bin the axial direction of the rotor core10. Specifically, the rotor core10is formed of the core block10adisposed at the center in the axial direction and the pair of core blocks10bdisposed so as to sandwich the core block10afrom both sides thereof in the axial direction. One of the pair of core blocks10bconstitutes one end face of the rotor core10, and the other of the pair of core blocks10bconstitutes the other end face of the rotor core10.

The core block10ais formed by punching electrical steel sheets as illustrated inFIG. 3and stacking a plurality of the punched electrical steel sheets while crimping. Where a is the thickness of the electrical steel sheet, l1is the length of the core block10ain the axial direction, and n1is the number of electrical steel sheets that are stacked to constitute the core block10a, then l1=n1×a. Likewise, the core block10bis formed by punching electrical steel sheets as illustrated inFIG. 5and stacking a plurality of the punched electrical steel sheets while crimping. Where a is the thickness of the electrical steel sheet, l2is the length of the core block10bin the axial direction, and n2is the number of electrical steel sheets that are stacked to constitute the core block10b, then l2=n2×a. The total length of the core block10bin the axial direction is given by L2=(l2+l2)=2n2×a=N2×a. Here, N2=2n2is the total number of electrical steel sheets that are stacked to constitute the core block10b. The total length of the core block10ain the axial direction is given by L1=n1×a=N1×a. Here, N1=n1is the total number of electrical steel sheets that are stacked to constitute the core block10a.

Accordingly, the length of the rotor core10in the axial direction is (L1+L2), the total length of the core block10ain the axial direction is L1, and the total length of the core block10bin the axial direction is L2. The number of electrical steel sheets stacked in the rotor core10as a whole is (N1+N2), the number of electrical steel sheets stacked in the core block10ais N1, and the number of electrical steel sheets stacked in the pair of core blocks10bis N2.

FIG. 8is a graph illustrating the results of torque and torque ripple calculated for the same current with respect to the ratio in the axial direction of the first core block in the rotor core according to the present embodiment. That is, the horizontal axis represents L1/(L1+L2), which is the ratio of the total length of the core block10ain the axial direction to the length of the rotor core10in the axial direction. Since L1/(L1+L2)=N1/(N1+N2), the horizontal axis also represents the ratio of the total number of electrical steel sheets stacked in the core block10ato the number of electrical steel sheets stacked in the rotor core10as a whole. The vertical axis represents the torque or torque ripple.

As illustrated inFIG. 8, the torque ripple has the smallest value when the ratio of the total length of the core block10ain the axial direction to the length of the rotor core10in the axial direction falls within the range of 35 to 45%, i.e., 0.35≤L1/(L1+L2)≤0.45.

On the other hand, the torque decreases monotonically as the ratio of the core block10aincreases from 0 to 100%, where the torque is 1.0% lower when the ratio of the core block10ais 100% than when the ratio of the core block10ais 0%. The reduction in torque can be kept within the range of 0.35 to 0.45% when the ratio of the core block10ais in the range of 35 to 45%.

Note that the torque ripple is slightly larger when the ratio of the core block10ais 35% than when the ratio of the core block10ais 45%, and the torque is slightly larger when the ratio of the core block10ais 35% than when the ratio of the core block10ais 45%. That is, although the torque ripple is slightly larger when the ratio of the core block10ais 35% than when the ratio of the core block10ais 45%, the torque can be increased correspondingly to improve the efficiency of the electric motor.

In the present embodiment, the rotor core10is formed by stacking the core block10anot provided with the slits15aand15band the core block10bprovided with the slits15aand15bin the axial direction. The torque acting on the rotor core10in such a structure is a synthesis of the torque acting on the core block10aand the torque acting on the core block10b. In this case, the torque waveform of the torque acting on the core block10aand the torque waveform of the torque acting on the core block10bbecome out of phase due to the effect of the slits15aand15b; therefore, the peak values of the torque waveforms are cancelled out in the rotor3as a whole to be able to reduce the torque ripple in the electric motor during operation.

Moreover, as illustrated inFIG. 8, the effect of reducing the torque ripple can be maximized by forming the rotor core10so as to satisfy 0.35≤L1/(L1+L2)≤0.45 or 0.35≤N1/(N1+N2)≤0.45.

Furthermore, as illustrated inFIG. 8, the reduction in torque can be further suppressed by forming the rotor core10by stacking the core block10aand the core block10bin the axial direction.

In the present embodiment, the slit15ais disposed radially outward of the end11a, which is one end of the permanent magnet11in the circumferential direction, and the slit15bis disposed radially outward of the end11b, which is the other end of the permanent magnet11in the circumferential direction. That is, the slit15aand the end11aare arrayed in the radial direction, and the slit15band the end11bare arrayed in the radial direction. Such arrangement of the pair of slits15aand15bincreases magnetic resistance at the inter-magnetic pole31and decreases a leakage flux passing through the inter-magnetic pole31; therefore, the magnetic flux of the permanent magnet11can be used effectively.

The present embodiment provides the two slits15aand15bper magnetic pole. This allows the torque to be larger than when three or more slits are provided.

Note that the core block10bcan also be provided with one or three or more slits per magnetic pole. Moreover, the position, shape, and size of the slit provided in the core block10bare not limited to the illustrated example. That is, provision of the slit in the core block10bcan obtain a torque ripple reduction effect similar to the above to one degree or another.

The present embodiment as described above can maintain the efficiency of the electric motor by preventing a reduction in the torque obtained at the same current as well as reduce the torque ripple.

The winding7is wound by adopting the distributed winding in the present embodiment. Compared to concentrated winding, the distributed winding is advantageous in increasing the torque at the same current because the reluctance torque can be generated effectively in addition to the magnet torque, but increases the torque ripple due to the reluctance torque. By applying the rotor3of the present embodiment to the stator2adopting the distributed winding, it is possible to construct the permanent-magnet-embedded electric motor1achieving high efficiency by effectively using the reluctance torque while reducing the torque ripple. Note that the winding7can instead adopt the concentrated winding.

Moreover, the present embodiment is provided with the void portions13aand13b. This prevents a short circuit of the magnetic flux at both ends of the permanent magnets11and allows the magnetic flux to reach the stator2more easily, whereby the torque can be increased. It is also possible to not include the void portions13aand13b.

The electrical steel sheets constituting the rotor core10has the same thickness in the present embodiment but need not have the same thickness. Moreover, the core blocks10aand10bare each formed by stacking the electrical steel sheets but may each be formed integrally.

FIG. 9is a perspective view illustrating a part of a rotor core according to a first variation of the present embodiment. As illustrated inFIG. 9, the rotor core10of the present variation is formed of the core block10bdisposed at the center in the axial direction and a pair of core blocks10adisposed so as to sandwich the core block10bfrom both sides thereof in the axial direction. One of the pair of core blocks10aconstitutes one end face of the rotor core10, and the other of the pair of core blocks10aconstitutes the other end face of the rotor core10. In this case as well, similar to the present embodiment, the torque ripple can be reduced while preventing a reduction in the torque obtained at the same current. Specifically, the effect of reducing the torque ripple can be maximized by forming the rotor core10so as to satisfy 0.35≤L1/(L1+L2)≤0.45 or 0.35≤N1/(N1+N2)≤0.45.

FIG. 10is a perspective view illustrating a part of a rotor core according to a second variation of the present embodiment. As illustrated inFIG. 10, the rotor core10of the present variation is formed by alternately stacking the core block10aand the core block10bto form five layers in total. The number of the core blocks10bis three and the number of the core blocks10ais two in the illustrated example. One of the three core blocks10bconstitutes one end face of the rotor core10, another one of the three core blocks10bconstitutes another end face of the rotor core10, and the remaining one of the three core blocks10bis disposed at the center in the axial direction of the rotor core10. In this case as well, similar to the present embodiment, the torque ripple can be reduced while preventing a reduction in the torque obtained at the same current. Specifically, the effect of reducing the torque ripple can be maximized by forming the rotor core10so as to satisfy 0.35≤L1/(L1+L2)≤0.45 or 0.35≤N1/(N1+N2)≤0.45.

Note that the configuration of the rotor core10is not limited to the example inFIG. 7, 9, or10, and the rotor core can be formed by alternately stacking the core block10aand the core block10b. The number of electrical steel sheets stacked need only be plural regardless of whether the number is even or odd. The total number of the core blocks10aand the core blocks10bconstituting the rotor core10may be larger than five layers. In this case as well, similar to the present embodiment, the torque ripple can be reduced while preventing a reduction in the torque obtained at the same current. Specifically, the effect of reducing the torque ripple can be maximized by forming the rotor core10so as to satisfy 0.35≤L1/(L1+L2)≤0.45 or 0.35≤N1/(N1+N2)≤0.45.

Second Embodiment

FIG. 11is a view illustrating the structure of an air conditioner according to the present embodiment. An air conditioner100according to the present embodiment includes an indoor unit101and an outdoor unit102connected to the indoor unit101. The outdoor unit102includes a compressor103according to the present embodiment. The compressor103uses the permanent-magnet-embedded electric motor1of the first embodiment. Note that the permanent-magnet-embedded electric motor1includes the first and second variations described in the first embodiment.

The air conditioner100is required to have energy saving performance and thus needs to be highly efficient. Moreover, in order to keep vibration and noise generated from the air conditioner100to be equal to or lower than the standard values, vibration and noise generated from the compressor103need to be reduced. The permanent-magnet-embedded electric motor1according to the first embodiment can reduce torque ripple while preventing a reduction in torque obtained at the same current. Therefore, the permanent-magnet-embedded electric motor1is applied to the compressor103so as to be able to construct the compressor103and the air conditioner100that can reduce the vibration and the noise caused by the torque ripple while preventing a reduction in the efficiency of the electric motor.

Note that the permanent-magnet-embedded electric motor1of the first embodiment can be applied not only to the compressor103but to each of a fan104of the indoor unit101and a fan105of the outdoor unit102. In this case as well, the effect similar to that described above can be obtained.

The permanent-magnet-embedded electric motor1according to the first embodiment can also be applied to electrical devices other than the air conditioner, in particular to a compressor of an electrical device having a refrigeration cycle other than the air conditioner100. In these cases as well, the effect similar to that described above can be obtained.

The configurations illustrated in the aforementioned embodiments merely illustrate examples of the content of the preset invention, and can thus be combined with another known technique or partially omitted and/or modified without departing from the gist of the present invention.