The present invention relates to a rotor to be used for a brushless motor and, more particularly, it relates to a rotor to be used for a brushless motor that is designed to reduce inertia without degrading the motor characteristics.
Brushless motors have been and being popularly used as drive sources for the electric equipment of automobiles such as electric power steering systems (to be referred to as EPS hereinafter).
The rotor 3 is formed by using a rotor core 6 fitted to a metal-made shaft 5 and a plurality of magnets 2 rigidly secured to the outer periphery of the rotor core 6. The stator 4, on the other hand, is formed by using a housing 7, a stator core 8 rigidly secured to the inner periphery of the housing 7 and a coil winding 9 formed by winding a wire around the stator core 8. A plurality of teeth are projecting from the inner periphery of the stator core 8 and the coil winding 9 is formed by winding a wire through the slots formed by the teeth.
A die cast bracket 21 of aluminum is fitted to an open end of the housing 7. A resolver mount unit 22 that is made of synthetic resin is interposed between the bracket 21 and the housing 7. A ring-shaped resolver stator 10a is fitted to the resolver mount unit 22. A coil 10b is formed around the resolver stator 10a by a wire wound around the latter, thus an exciting coil and a detector coil are provided.
A resolver rotor 10c is arranged in the inside of the resolver stator 10a and rigidly secured to the shaft 5. Thus, a resolver 10 is formed by the resolver rotor 10c and the resolver stator 10a. The resolver rotor 10c is formed by laying metal plates to produce projections projecting in three directions. As the shaft 5 rotates, the resolver rotor 10c also rotates in the resolver stator 10a. A high frequency signal is applied to the exciting coil of the resolver stator 10a and the phase of the signal output from the detector coil changes as the projections move toward and away from the detector coil. The rotary position of the rotor 3 is detected by comparing the detection signal and a reference signal. Then, the electric current being supplied to the winding 9 is appropriately switched according to the rotary position of the rotor 3 to form a rotating magnetic field, which by turn drive the rotor 3 to rotate.
When such a motor 1 is employed as drive source in an EPS, as the driver of the automobile operates the steering wheel, the motor 1 is driven under control according to the steering angle, the running speed and the like of the automobile. Then, the revolutions of the shaft 5 are transmitted to the steering column of the automobile by way of a reduction gear (not shown). As a result, steering assisting force is supplied to the steering column shaft so that the driver can operate the steering wheel with relatively small power.
However, as the inertia of the rotor 3 increases in such a motor in an EPS, there arises a problem that the responsiveness of the motor falls in the steering operation due to the inertia. In an EPS, as the driver operates the steering wheel and the steering column rotates, the rotor 3 also rotates. Therefore, if the inertia of the rotor 3 is large, the movement of the steering column is retarded under the influence of the inertia so that it may no longer be possible for the driver to get a quick and light steering feeling. Particularly, motors to be used for EPSs have been required to show a high output power in recent years to make the motors relatively bulky. Then, the rotor is made larger accordingly to consequently give rise to a large inertia in the rotor. Thus, the influence of the inertia is no longer negligible.
In an attempt of reducing the inertia of the rotor 3, Jpn. Pat. Appln. Laid-Open Publication No. 9-275652 proposes a rotor of a brushless motor whose weight is reduced by providing in the yoke thereof (which corresponds to the rotor core 6 in
Problems to be Solved by the Invention
However, when the rotor core is provided with such a space, the magnetic field magnetization can easily become uneven when magnetizing the magnets. Although the space is not used as magnetic path when driving the motor, it is not necessarily that the space is unnecessary at the time of magnetization. In other words, when a magnetic field is formed for the purpose of magnetizing the magnets, the magnetic field can become uneven due to the space. As the magnetic field of magnetization becomes uneven, the magnetic characteristics of the magnets can also become uneven to adversely affect the characteristics of the motor. Additionally, the time spent for magnetizing the magnets increases due to the uneven magnetic field to reduce the productivity and increase the energy consumption and hence the running cost of the magnetizing apparatus.
An object of the present invention is to reduce the inertia of a brushless motor without degrading the characteristics of the motor by magnetizing the magnets in a stable magnetic field of magnetization.
Means for Solving the Problems
A rotor for a brushless motor according to the present invention characterized by comprising a hollow-cylindrical rotor core to be fitted to a rotary shaft and magnets to be fitted to the outer peripheral surface of the rotor core, characterized in that the rotor core has: an outer ring section formed to extend from the outer peripheral surface of the rotor core toward the rotary shaft: with a predetermined thickness; a plurality of ribs formed inside the outer ring section and extending from the inner peripheral surface of the outer ring section toward the rotary shaft; and hollow sections formed between the ribs; and the outer diameter gn of the hollow sections is defined by φc−2×3 Wt≦φn≦φc−2×1.3 Wt, where φc is the outer diameter of the rotor core and Wt is the thickness of the magnets. In this case, preferably, the outer diameter of the hollow sections is defined by φc−2×2 Wt≦φn≦φc−2×1.3 Wt
According to the present invention, it is possible to secure a uniform magnetic field of magnetization around the magnets with a magnetic flux density sufficient for magnetization at the time of magnetizing the magnets by limiting the outer diameter of the rotor core to the above-defined range. Additionally, it is also possible to suppress any increase in the cogging torque and any reduction in the motor torque by limiting the outer diameter pn to the above-defined range.
In a rotor for brushless motor according to the invention, the ribs may be formed at positions located inside the outer ring section relative to the inter-magnet spaces adjacently arranged magnets or the inter-magnetic-polar spaces so as to extend radially from the rotary shaft. In a rotor for a brushless motor according to the invention, the ribs may be arranged so as to be centered respectively at the line segments Lr passing through the middle points of the inter-polar spans Pw of adjacently located magnets and the center O of the rotary shaft. The angular pitch θr of arrangement of the ribs may be made larger than the central angle θm of the magnets (θr>θm).
Additionally, the hollow sections may be formed to show a substantially sector-shaped cross section and arranged on the prolonged lines passing through the polar centers of the magnets. The width Wv of the ribs may be smaller than the thickness Wt of the magnets (Wv≦Wt).
According to the present Invention, the rotor core of a rotor to be used for a brushless motor is made to have an outer ring section formed to extend from the outer peripheral surface of the rotor core toward the rotary shaft with a predetermined thickness, a plurality of ribs formed inside the outer ring section and extending from the inner peripheral surface of the outer ring section toward the rotary shaft and hollow sections formed between the ribs and the outer diameter φn of the hollow sections is defined by φc−2×3 Wt≦φn≦φc−2×1.3 Wt (where φc is the outer diameter of the rotor core and Wt is the thickness of the magnets). Thus, when magnetizing the magnets, it is possible to form a magnetic field of magnetizing that is not uneven and shows a sufficiently high magnetic flux density around the magnets. Additionally, it is possible to suppress any aggravation of the cogging torque and the motor torque when the outer diameter gn is defined to be within the above-cited range.
Thus, it is possible to efficiently magnetize the magnets by means of a stable magnetic field of magnetization, while reducing the inertia by means of the hollow section. Also, it is possible to suppress any aggravation of the cogging torque and the motor torque. Therefore, it is possible to reduce the inertia without reducing the power output while the productivity is improved for magnetization and the energy consumption rate of the magnetizing power source can be reduced.
[
[
[
[
[
[
Now, the present invention will be described in greater detail by referring to the accompanying drawings that illustrate preferred embodiments of the invention.
A plurality of (e.g., six) ribs (rib sections) 13 are arranged in the inside of the outer ring section 12. The ribs 13 extend from the inner peripheral surface 12a of the outer ring section 12 toward the shaft 5 and their inner ends are rigidly secured to the shaft 5. The ribs 13 are arranged at regular angular intervals with an angular pitch of θr (which is equal to 60° when the number of ribs is six). Thus, the ribs 13 extend in radial directions from the shaft 5. Each of the ribs 13 is arranged in such a way that a line segment Lr extending from the center of the shaft 5 to the middle point of two adjacently located magnets 2, or the middle point Cp of the inter-polar span Pw, agrees with the center line of the rib 13, and θr is larger than θm (θr>θm). The ribs 13 have a width Wv smaller than the largest thickness Wt of the magnets 2 (Wv≦Wt).
The ribs 13 axially extend along the entire length of the rotor core 6 and an axially extending hollow section 14 is formed between any two adjacently located ribs 13. The hollow sections 14 are located right below (above) the respective magnets 2, each of the hollow sections 14 has a sector-shaped cross section with its centerline agreeing with the prolonged line M passing through the polar center of the corresponding magnet 2 and the center of the shaft 5. The outer peripheries of the hollow sections 14 are inner peripheral surfaces 12a of the outer ring section 12 and the outer diameter of the hollow sections (the inner diameter of the outer ring section 12) is expressed by φn. If the outer diameter of the rotor core 6 is φc, φn is defined as φc−2×3Wt≦φn≦φc−2×1.3 Wt, preferably by φc−2×2 Wt≦φn≦φc−2×1.3 Wt. Note that φc is larger than the outer diameter (ps of the shaft 5 (φc>φs).
As seen from
On the other hand, the outer diameter pn of the hollow sections influences not only during the magnetizing process but also the cogging torque and the motor torque during the motor driving operation.
As seen from
Thus, the unevenness of the magnetic field of magnetization can be reduced by arranging sector-shaped hollow sections 14 immediately below the respective magnets 2 and so defining the outer diameter as to be within the above-cited range. Then, consequently, it is possible to efficiently magnetize the magnets 2 by means of a stable magnetic field of magnetization. Additionally, neither the cogging torque nor the motor torque is aggravated in the above-cited range. Thus, it is possible to achieve an inertia reducing effect by means of the hollow sections 14 without reducing the power output. Then, the productivity is improved and the energy consumption rate of the magnetizing apparatus can be reduced because it is possible to efficiently magnetize the magnets 2. Therefore, the magnetizing operation will be energy saving and it is possible to reduce the produce cost.
The present invention is applicable not only to rotors formed by using segment type magnets as in the case of Embodiment 1 but also to rotors formed by using a ring type magnet.
A ring magnet 17 is fitted to the outer periphery of the rotor core 6 in the rotor 16 of
As shown in
The present invention is not limited to the above embodiments but, needless to say, can be variously modified without deviating from the subject matter of the invention.
For example, the numerical values of φc and Wt of the above-described embodiments are only cited as examples and the present invention is effectively applicable to rotors having dimensions other than those cited above. While the present invention is applied to a brushless motor to be used for an EPS in each of the above-described embodiments, the present invention is also applicable to brushless motors to be used not only for EPSs but also for industrial machines such as industrial robots, personal computers, peripheral equipment of personal computers and other IT apparatus. While a motor adapted to detect the rotary position of the rotor by means of a resolver is described above as typical brushless motor by referring to
Number | Date | Country | Kind |
---|---|---|---|
2004-050370 | Feb 2004 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP05/02563 | 2/18/2005 | WO | 8/24/2006 |