The present invention relates to a geared motor for an electrically driven mobile object in which the entirety or part of propulsion power is drive power produced by a motor. In particular, the present invention relates to a geared motor for an electrically driven mobile object, such as an electric vehicle (BEV) in which only an electric motor is used as a drive source, a hybrid electric vehicle (HV) in which propulsion power produced by a motor is used in addition to propulsion power produced by an internal combustion engine, and a fuel cell electric vehicle (FCV) in which propulsion power is produced by a motor which rotates by using electric power derived from fuel.
Heretofore, a synchronous reluctance motor that uses ferrite magnets has been known as shown in Patent Document 1. Also, an interior permanent magnet synchronous motor (IPMSM) in which rare earth sintered magnets are embedded in a rotor has been known. However, rare earth elements are scarce resources. In particular, dysprosium (Dy) necessary for improvement of performances such as impartment of heat resistance is precious and is locally present in different regions of the earth, which has raised a problem associated with stable supply. In order to overcome this problem, there has been proposed a permanent magnet assisted synchronous reluctance motor (PMA-SynRM) in which, instead of rare earth sintered magnets requiring a large amount of a raw material, rare earth bonded magnets are embedded in a rotor as shown in Non-patent documents 1 and 2.
Also, there has been demand for realization of a high-efficient, high-output (power) motor which uses less neodymium (Nd) and which uses less or does not use heavy rare earth elements, including dysprosium (Dy), and the scientific and technological policy of Japan has set a development goal for such a high-efficient, high-output motor.
In Patent Document 1, instead of precious rare earth elements, ferrite is used. However, ferrite is easily demagnetized by a magnetic field produced by armature current. Therefore, Patent Document 1 proposes a structure for effectively preventing demagnetization. Also, when rare earth sintered magnets are used, in order to reduce magnetic fluxes produced by the magnets for the purpose of realizing high rotational speed, it is necessary to make the rare earth sintered magnets very thin. However, there is a limit to how thin the magnets can be made. Therefore, in the case where rare earth sintered magnets are used in a motor, it is almost impossible to increase its rotational speed to 20000 rpm or higher.
Meanwhile, in Non-patent Documents 1 and 2, rare earth bonded magnets are used as embedded magnets of a rotor of a motor. A maximum output of 75.6 kW to 92.7 kW is attained at 5000 rpm. Also, its maximum rotational speed is 17000 rpm, and its output decreases from 75.6 kW (the maximum output) to 44.5 kW at the maximum rotational speed.
As described above, in Non-patent Documents 1 and 2, rare earth bonded magnets are used in a magnet-embedded-type reluctance motor. However, the magnet-embedded-type reluctance motor fails to achieve a rotational speed of 17000 rpm or higher. Also, no studies at all have been conducted on the relation between the maximum rotational speed or the maximum output and the area of an outer circumferential surface which is optimal for the maximum rotational speed and the maximum output; i.e., the optimal amount of rare earth bonded magnets used in the rotor. Accordingly, Non-patent Documents 1 and 2 do not include a viewpoint of minimizing the amount of rare earth elements used in a motor, while maintaining the high performance of the motor.
In view of the above, the present inventors carried out studies while setting a goal of minimizing the amount of rare earth bonded magnets used in a motor without impairing the performance of the motor. As a result, the present inventors concluded that it is necessary to reduce the side area of the rotor as much as possible while maintaining a desired output. The present inventors has finally conceived a new idea of reducing the area of the outer circumferential surface of the rotor of a motor in order to achieve a maximum rotational speed of 20000 rpm to 45000 rpm, which has not conventionally been achieved by a high output motor whose output is 25 kW or greater, thereby reducing the amount of rare earth bonded magnets as much as possible, and increasing torque which decreases at high rotational speed by using a speed reduction gear, whereby torque in a usage range is obtained, and thus the motor can be used as a drive source of an electric automobile.
An object of the present invention is to realize a drive motor of an electric automobile which uses a reduced amount of rare earth bonded magnets while maintaining the performance of the motor as much as possible.
The present invention is a geared motor for an electrically driven mobile object. The geared motor is adapted to provide propulsion power to an electric automobile. The geared motor comprises a synchronous reluctance motor whose maximum rotational speed is 20000 rpm to 45000 rpm and in which rare earth bonded magnets are embedded in a rotor, and a speed reducer which reduces the rotational speed of the synchronous reluctance motor to a rotational speed in a usage range while increasing torque to a predetermined range.
In the present invention, the electrically driven mobile object may also be referred to simply as an electric automobile. The “electric automobile” is a concept including a battery electric vehicle (BEV) in which only an electric motor using electric power stored in a battery is used as a drive source, a hybrid vehicle (HV) in which propulsion power produced by an electric motor is used in addition to propulsion power produced by an internal combustion engine, and a fuel cell vehicle (FCV) in which electric power is generated from fuel in a tank and is supplied to a motor serving as a drive source. In short, an automobile in which propulsion power is obtained by using an electric motor is defined as an electric automobile.
In the present invention, the maximum rotational speed may be in a range of 25000 rpm to 45000 rpm, in a range of 30000 rpm to 45000 rpm, or in a range of 32000 rpm to 45000 rpm. Further desirably, the maximum rotational speed may be 34000 rpm to 45000 rpm.
In the present invention, when a radius of the rotor is represented by R (cm) and a length of the rotor in an axial direction is represented by L (cm), the rotor desirably has a size which satisfies conditions of 2 cm≤R≤6 cm and 2 cm≥L≤25 cm. When the length L is greater than 25 cm, the axis of the rotor may wave. Therefore, setting the length L to be greater than 25 cm is not realistic. Furthermore, the radius R and the length L may be set to satisfy conditions of 4 cm≤R≤6 cm and 2 cm≤L≤18 cm. When the radius R is greater than 6 cm, the size of the motor may become excessively large, which is not desired from the viewpoint of an installation space of the motor.
Also, when a maximum rotational speed of the synchronous reluctance motor is represented by n (rpm), a maximum output of the synchronous reluctance motor is represented by P (W), a radius of the rotor is represented by R (cm), a length of the rotor in an axial direction is represented by L (cm), and a side area of the rotor is represented by S (cm2) and obtained by an equation of S=2πRL, the rotor desirably has a radius R and a length L, which provide a side area within a range of 0.7S to 1.1S, where S satisfies the following equation:
S=kP/n,
The value of k changes depending on the residual magnetic flux density of the rare earth bonded magnets, the shape of each magnet, the arrangement of the plurality of magnets, the shape of each of slits which separate adjacent magnets, the permeability and resistance of a magnetic circuit, the arrangement of armature coils, the positional relation between the magnets and the coils, the magnitude of armature current, etc. The value of k can be rendered small by optimizing these factors. Namely, even when the maximum output is large or the maximum rotational speed is low, the side area can be reduced by rendering the value of k smaller. The range of k is set from this point of view.
As to the side area which determines the radius R and the length L, the smaller the side area S, the greater the degree to which the amount of a rare earth element to be used can be reduced. Although the output (power) and the torque can be increased by increasing the side area S, the amount of the rare earth element to be used increases. Therefore, the upper limit is determined as described above. The range of the side area S is determined by adding predetermined margins to the range of the side area S obtained, in accordance with the above-described equation, from the maximum output P, the maximum rotational speed n, and an arbitrary value of k which satisfies conditions of 120 cm2/W min≤k≤160 cm2/W min. The reason why the margin on the side toward which the side area decreases is set to be larger than the margin on the side toward which the side area increases is that it is desired to make the side area S as small as possible in the range where a desired maximum output P and a desired maximum rotational speed n are realized. From this point of view, the side area which determines the radius R and the length L may be in a range of 0.7S to 1.1S. Also, the side area may be in a range of 0.8S to 1.1S or in a range of 0.9S to 1.1S.
Also, in the present invention, the maximum output P of the synchronous reluctance motor is desirably in a range of 25 kW to 180 kW, more desirably in a range of 40 kW to 180 kW. Also, the desirable maximum output P may be in a range of 40 kW to 150 kW or in a range of 40 kW to 100 kW. A motor having an output of 25 kW or greater and having a rotational speed of 20000 rpm or greater has not yet been realized. Although the torque of the synchronous reluctance motor at the maximum rotational speed depends on the maximum output, the torque is desirably in a range of 5 Nm to 86 Nm, more desirably in a range of 5 Nm to 70 Nm, further desirably in a range of 6 Nm to 60 Nm. Although the reduction ratio of the speed reducer is determined by the used rotational speed range of the output axis of the speed reducer, in consideration of use as a drive source for an electrically driven mobile object, the reduction ratio is desirably in a range of 3 to 30. The reduction ratio may be in a range of 12 to 30 or in a range of 19 to 27.
In the present invention, each of the rare earth bonded magnets desirably has a residual magnetic flux density of 0.6 T to 1.0 T. Also, the rare earth bonded magnets may be formed such that each magnet contains neodymium (Nd) and does not contain any element in a heavy rare earth element group containing dysprosium (Dy) and terbium (Tb). In particular, this is effective because a high performance motor can be realized without using dysprosium (Dy) or terbium (Tb), whose production amounts are limited.
According to the present invention, the rotational speed of the rotor is set to 20000 to 45000 rpm, and thus, it is possible to reduce the side area of the rotor, thereby reducing the amount of a rare earth element to be used, while realizing a maximum output of 25 kW to 180 kW.
The present invention will now be described on the basis of a specific embodiment. The present invention is not limited to the following embodiment.
Rare earth bonded magnets 14 and 15 are charged in these cavities 12 and 13 along the axial direction. Each rare earth bonded magnet is composed of particles of a rare earth element and a binder resin for binding the particles together. Each rare earth bonded magnet has an electric resistivity of 50 μΩm or greater, desirably 100 μΩm or greater, more desirably 500 μΩm or greater, further desirably 1000 μΩm or greater. Thus, eddy current loss can be reduced further. The rare earth element particles are not limited to a single type of particles and may be prepared by mixing a plurality of types of particles which have different compositions and/or particle size distributions. The magnet particles may be isotropic magnet particles or anisotropic magnet particles. In the case where anisotropic magnet particles are used, the bonded magnets may be molded in an orienting magnetic field. Needless to say, orientation and magnetization may be performed simultaneously. The binder resin may be a thermoplastic resin or a thermosetting resin. In the case where a thermosetting resin is used, a hardening heat treatment (curing treatment) is preferably performed after molding.
Although an arbitrary charging method can be used to charge the rare earth bonded magnets 14 and 15, there can be used a molding method of injecting a mixture of rare earth element particles and a resin into the rotor 10 and cooling the injected mixture for solidification. Alternatively, compression molding may be employed. After inserting semi-cured rare earth bonded magnets into the cavities 12 and 13, the rare earth bonded magnets may be heated for complete hardening. In the case of compression molding, it is possible to reduce the amount of the binder resin and increase the amount of the rare earth element particles. Therefore, magnets formed by compression molding can have higher magnetic flux densities as compared with magnets formed by injection molding.
Nd—Fe—B based magnet powder, Sm—Fe—N based magnet powder, Sm—Co based magnet powder, or the like can be used as anisotropic rare earth magnet powder. Magnetization may be performed after a kneaded mixture of magnet powder and a binder resin is charged into the cavities 12 and 13 of the rotor 10 and the rotor is placed in an orientating magnetic field so as to orient the magnetic powder. In the case of compression molding, the kneaded mixture is compressed in an orientating magnetic field, whereby rare earth bonded magnets are formed.
NdFeB based anisotropic rare earth magnet powder, which was Nd based magnet powder (coarse powder), and SmFeN based anisotropic magnet powder, which was Sm based magnet powder (fine powder), were used as anisotropic rare earth magnet powders. PPS (poly phenylene sulfide), which is a thermoplastic resin, was used as a binder resin. A compound (S5P-13MF listed in a catalog of Aichi Steel Corporation) containing these materials was used. This compound was charged into the rotor 10 by injection molding, whereby rare earth bonded magnets were embedded in the rotor and were united with the rotor.
The residual magnetic flux density Br of the rare earth bonded magnets was set to 0.67 T. It is preferred to set the residual magnetic flux density Br to fall within a range of 0.6 T to 1.0 T.
1. Torque-Rotational Speed Characteristic
2. Field-Weakening Control
In
3. Relational Expression of Motor
Equation (1) holds for the relation among the torque T (Nm), rotational speed n (rpm), and output P (W) of the motor. The torque T of the motor is expressed by Equation (2), where Pn is the number of pole pairs, and, as shown by Equation (3), Φa is a value (d-axis component) obtained by converting the interlinkage flux (flux linkage) Φm of the armature coil by the magnetic fluxes of the bonded magnets to d-axis component in a dq coordinate system. Here, the interlinkage flux Φm is a maximum (DC) interlinkage flux when the rotor is stopped and a peak value when the rotor rotates. Id and Iq are the d axis (direct axis) component and the q axis (quadrature axis) component of the armature current, respectively. In the present embodiment, the number Pn of pole pairs is 4. However, the number Pn of pole pairs may be an arbitrary number other than 4; for example, 1, 2, 3, or 6. Ld and Lq are the d-axis inductance and the q-axis inductance of the armature coil, respectively. As shown in
Equation (2) can be transformed to Equation (4) by using the area S of the cylindrical side surface of the rotor (hereinafter referred to as the “side area S”). In Equation (4), Φa0 is the interlinkage flux per unit side area (interlinkage flux density) (the d-axis component). Ld0 and Lq0 are the d axis inductance and the q axis inductance, respectively, of the armature coil per unit side area.
[Equation 4]
T=SPn[Φa0Iq−(Lq0−Ld0)IqId] (4)
Here, k is defined by Equation (5). T in Equation (4) is expressed by using k and is substituted in Equation (1), whereby the side area S can be expressed by Equation (6).
In Equation (4), each of Ld0, Lq0, Id, and Iq at the maximum rotational speed can be regarded to be constant irrespective of the maximum output and the side area because of the limit of the field-weakening control at the maximum rotational speed. In a motor of the embodiment whose maximum output was 50 kW, the side area S was 218.7 cm2, and a maximum rotational speed of 34000 rpm was obtained. The value of k at this maximum rotational speed obtained from Equation (6) is 148.7 cm2/W min.
4. Relation Between the Maximum Rotational Speed and the Side Area Obtained by Using the Maximum Output as a Parameter
Since Equation (5) shows that, at the maximum rotational speed, the value of k does not depend on the side area S, the value of k can be regarded to be constant irrespective of the maximum output. When the maximum rotational speed is substituted as the value of n in Equation (6), the relation between the maximum rotational speed n and the side area S can be obtained by using the maximum output P as a parameter.
5. Relation Between the Maximum Output and the Side Area Obtained by Using the Maximum Rotational Speed as a Parameter
5. The Relation Between the Maximum Rotational Speed and the Maximum Output Obtained by Using the Side Area as a Parameter
6. The Relation Between the Maximum Rotational Speed and the Maximum Output Obtained by Fixing the Side Area and Using k as a Parameter
As is apparent from Equation (5), the value of k is proportional to the reciprocal of Φa0Iq−(Lq0−Ld0)Iq−Id, where Lq0−Ld0>0, Iq>0, and Id<0. The first term represents a magnet torque per unit pole pair number and unit side area, and the second term represents a reluctance torque per unit pole pair number and unit side area. In general, the reluctance torque is greater than the magnet torque. However, since phase current increases as the rotational speed decreases, the degree of increase of the reluctance torque becomes greater than that of the magnet torque. Since phase current decreases as the rotational speed increases, the rate of reduction of the reluctance torque to the rotational speed is larger than that of the magnet torque. Therefore, as the rotational speed increases, the ratio of the magnet torque to the total torque increases gradually, the influence of the interlinkage flux Φa0 per unit area on the value of k increases. The interlinkage flux Φa0 depends on the residual magnetic flux density Br of the bonded magnets, the shape of each bonded magnet, the arrangement of a plurality of bonded magnets, the number of layers of bonded magnets, the interval between bonded magnets of adjacent magnetic poles, the slit width of the bonded magnets, the permeability of a magnetic circuit, the positional relation between the bonded magnets and the armature coils, etc. In the present invention, the value of k is set within the range of 120 cm2/W min to 160 cm2/W min, in consideration of the fact that the residual magnetic flux density Br at the time when the value of k is set to 148.7 (cm2/W min) is 0.67 T, and that the practical range of the residual magnetic flux density Br is 0.6 to 1.0 T. The smaller the value of k, the greater the degree to which the side area S can be reduced for the same maximum rotational speed and the same maximum output, and the greater the degree to which the amount of bonded magnets used can be reduced.
7. The Relation Between Radius and Axial Length for the Case where Predetermined Margins are Imparted to the Determined Side Area
The optimum side area S at the time when a maximum output of 50 kW and a maximum rotational speed of 34000 rpm were realized was 218.7 cm2.
The optimum side area S for the case where the maximum output is 100 kW and the maximum rotational speed is 34000 rpm is 437.4 cm2.
The optimum side area S for the case where the maximum output is 150 kW and the maximum rotational speed is 34000 rpm is 656.0 cm2.
8. The Relation Between the Radius and the Axial Length Determined by Using the Side Area as a Parameter
9. The Relations Between the Radius and the Axial Length Determined for Different Maximum Outputs by Using the Maximum Rotational Speed as a Parameter
In the case where the maximum output is 25 kW, conditions of 2 cm≤R≤6 cm and 2 cm≤L≤25 cm are satisfied in the entire maximum rotational speed range of 20000 rpm to 45000 rpm, and the degree of freedom is high. It is understood that, although the ranges of the radius R and the axial length L which satisfy the conditions of 2 cm≤R≤6 cm and 2 cm≤L≤25 narrow as the maximum output increases, the optimal ranges of the radius R and the axial length L expand when the maximum rotational speed is increased (
10. Optimization for Realizing the Maximum Rotational Speed
When a maximum output P and a maximum rotational speed n are given, a torque T can be obtained from Equation (1). This torque means the maximum torque for the maximum output P and the maximum rotational speed n. Namely, this maximum torque means the limit torque at that output and that rotational speed, and toque greater than the limit torque cannot be output. Meanwhile, the torque T given by Equation (4) are determined by the values of Ld0, Lq0, Id, and Iq at the maximum rotational speed of the motor. Accordingly, when the values of Φa0, Ld0, Lq0, Id, and Iq at the maximum rotational speed are determined such that the torque T given by Equation (4) becomes the limit torque, the maximum rotational speed n which satisfies Equation (1) can be realized.
11. Gear Speed Reduction
Next, the characteristic of a speed reducer which reduces the rotational speed of the motor to the rotational speed of a drive shaft for propelling an automobile (the output axis of the speed reducer) will be described for the case where the maximum output is 50 kW.
12. Prototype Machine
The radius R of the rotor is 4 cm, the axial length L of the rotor is 8.7 cm, and the side area S is 218.7 cm2. As shown in
The relation among rotational speed (rpm), torque T (Nm), and output (kW) of the motor of the above-described prototype machine alone was measured. Table 1 shows the measured values and values converted from the phase current effective value Ie.
It is found that the measured rotational speed n-torque T characteristic of
Also, in the above-described prototype machine, the speed reducer shown in
The above-described prototype machine reveals that the side area S of the rotor of the motor and the value of k are important factors for realizing the maximum output of 25 kW to 180 kW and the maximum rotational speed of 20000 rpm to 45000 rpm, and thus, the present inventors have completed the invention of minimizing the amount of the rare earth element by determining the side area, the radius, and the axial length of the rotor in the above-described manner.
The present invention can be applied to a drive motor for an electrically driven mobile object in which the entirety or part of propulsion power is drive power produced by the motor.
Number | Date | Country | Kind |
---|---|---|---|
2020-202611 | Dec 2020 | JP | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/JP2021/044779 | 12/6/2021 | WO |