MOTOR

Abstract: A motor comprises magnetic poles constituted by a permanent magnet magnetized in its radial direction and a core having salient poles placed inside or outside the magnetic poles wherein the magnetic flux distribution of the permanent magnet is controlled so that the cogging torque characteristic of the motor is rendered to be a specified condition.

Description: The present invention relates to a motor of the kind defined by the precharacterizing features of claim 1. Various types of machines, electric or electronic devices and apparatuses have been developed and improved, and demand of a small-sized motor of high quality, cogging-free and of a large torque has increased. Although a coreless motor has been used to reduce cogging produced in a motor, the coreless motor has not been able to provide a sufficiently large torque. Accordingly, it was necessary to manufacture a large-sized coreless motor to obtain a large torque. On the other hand, a motor of the kind defined at the preamble having salient poles as shown in Figure 1 is used to obtain a large torque. This conventional motor will be described with reference to Figure 1, although the motor shown in Figure 1 is applicable to the present invention. In Figure 1, a reference numeral 1 designates a core having a plurality of salient poles 2. Windings are omitted to simplify an illustration. A permanent magnet 3 in a ring form is magnetized in its radial direction to form an alternate arrangement of the N Poles and the S poles along the circumferential direction of the ringed permanent magnet. A reference numeral 4 designates a magnetic yoke. Either one of the core 1 and the magnetic yoke 4 is used as a stator and the other is used as a rotor. The motor as shown in Figure 1 is an outer rotor type motor in which the core 1 is used as the stator and the magnetic yoke 4 is used as the rotor. Although the above described motor having the salient poles 2 provides a large torque, it has the drawback that the cogging is produced. As a technique of reducing the cogging in the motor having the salient poles 2, a method of machining an armature iron core in a specified shape is proposed as in Japanese Examined Patent Publication 52426/1983. However, the method of machining the armature iron core is complicated. Further, reduction in the cogging to be attained by the above-mentioned method is insufficient. Recently, use of rare-earth magnet having a strong magnetic force as the permanent magnet accelerates miniaturisation of the motor. However, with the small-sized motor, a problem of the cogging is further conspicuous and it is disadvantage in practical use. A further motor of the kind defined at the preamble is known from the US-A-1 354 144. In order to reduce cogging, in this known machine the number of the stator salient poles is less than that of the rotor permanent magnet poles. A further motor of the kind defined at the preamble is known from the DE-A-28 23 208. In order to reduce cogging it is known from this document to provide recesses at the main surface of at least one of the magnetic poles of the permanent magnet. It is the object of the present invention to provide a motor of a kind defined at the preamble having a still further minimized cogging if compared with the prior art. This object is obtained by the characterizing features of claim 1. Advantages developments of the invention are given by the subclaims. Preferred embodiments of the present invention will be descrobed with reference to the accompanying drawing wherein: Figure 1 is a diagram showing a motor comprising four magnetic poles and six salient poles according to an embodiment of the present invention; Figure 2 is a diagram showing an embodiment of a magnetizing device used for magnetization of a permanent magnet shown in Figure 1; Figures 3 and 4 are respectively characteristic diagrams of the cogging torque of the motor shown in Figure 1 in which an angle of magnetization is used as a parameter; Figures 5 and 6 is characteristic diagrams showing another example of the cogging torque of the present invention; Figure 7 is a diagram showing another magnetizing device; Figures 8 and 9 are respectively characteristic diagrams showing a distribution of the magnetization of a magnetic pole in each of embodiments of the present invention; Figure 10 is a diagram showing a pattern of the cogging torque of a motor comprising 10 magnetic poles and 15 salient poles; Figures 11 and 12 are respectively diagrams showing power spectra obtained by frequency analysis of the cogging torque in which angles of magnetization are respectively  = 28° and  = 26°. In the following, the present invention will be described in detail. In the present invention, the magnetic flux distribution of a permanent magnet is controlled so that the cogging torque characteristic of a motor indicates a specified pattern. Usually, such magnetic flux distribution is obtained by controlling an angle of magnetization or a pattern of magnetization of the permanent magnet. The shape of the permanent magnet is not limited as long as the permanent magnet is magnetized in its radial direction. However, the permanent magnet in a ring form as shown in Figure 1 is most preferably used. Further, it is recommendable to use a rare-earth magnet which utilizes an intermetallic compound of rare-earth elements and transition metals including cobalt as the main element from the viewpoint of requirement of a high magnetic properties. The ringed permanent magnet can be made by a sintering method or a resin-binding method. It is difficult to form the ringed magnet by the sintering method because of the shape of the magnet. Especially, an anisotropic treatment in the radial direction of the permanent magnet by using the sintering method adversely affects thermal expansion characteristic of the magnet, which result in breaking of the magnet. Accordingly, when the sintering method is used, the permanent magnet is subjected to large restriction in determination of the shape and the magnetic characteristic. On the other hand, the ringed plastic magnet formed by the resin-binding method is well fitted to the purpose of the present invention. In the following, a typical example of a magnet formed in a ringed shape of the present invention will be described. A ringed plastic magnet is generally manufactured as follows. In a mixture of at least two kinds of powder of ferrite and rare-earth magnetic materials, a resinous material is added and the mixture is kneaded. The kneaded mixture is formed into a ring-like molded product by an injection molding or a press molding. The molded product of a ring form is put in a magnetizing device to magnetize it so that plural pairs of the N poles and the S poles are magnetized in an alternate arrangement in the circumferential direction of the molded product. In the present invention, when the molded product is magnetized by using the magnetizing device, an angle of magnetization and a pattern of magnetization are precisely controlled. Namely, the molded product is magnetized so that regions of non-magnetization are provided, or magnetic properties are given to it with a specified distribution (the pattern of magnetization) in a range narrower than a value of (360°/the number of magnetic poles), as different from the conventional technique that magnetization is conducted on the entire surface of the ringed plastic magnet in the radial direction. Generally, a motor employing a ringed magnet having the entire surface magnetized gives extremely large cogging. Such a motor, produces, as cogging torque characteristics, a cogging torque having a period of a revolution angle (360°/i) in which two balanced stable points are given as reference points of the revolution angle, where i is the least common multiple between the number of the salient poles and the number of the magnetic poles. In other words, the cogging torque has two inflection points in such a period. Figure 3 shows typical cogging torque characteristics in the case that the angle of magnetization and the pattern of magnetization in a permanent magnet are changed. As shown in Figure 3, the values of torque are changed while the period of the revolution angle (360°/i) having balanced stable points as reference points is unchanged. In the following, the term revolution angle means that it has the balanced stable points as reference points. Further, the balanced stable points refer to points where the cogging torque becomes zero and there produces a force directing to the original position when the angle is advanced or delayed from the zero position. In Figure 3, the characteristic curves for the angles of 50° and 55° are indicated by shifting them by a half (1/2) period (15°). The present inventor has found that the cogging torque characteristics as shown in Figure 4 are obtainable within an extremely limited range by precisely controlling a magnetic flux distribution such as the magnetization angle and/or the magnetization pattern of the permanent magnet. Figure 4 shows an example of variation in the cogging torque characteristics. Namely, there is obtainable a pattern having four inflection points in the revolution angle (ϑi=360°/i). In this case, the cogging torque of the motor becomes extremely small, thus the purpose of the present invention can be attained. In the motor of the present invention, various patterns of the cogging torque characteristics are obtainable by controlling the magnetic flux distribution. Figures 5, 6 and 10 show examples of the patterns. As apparent from the Figures, there are always four inflection points in a period of the cogging torque of the revolution angle (360°/i). The inflection points refer to the points where the gradient is zero in a cogging torque curve. However, in order to minimize the cogging in the motor of the present invention, it is preferable that the four inflection points are not concentrated in a specified revolution angle, but they are dispersed equally in the revolution angle (360°/i). In this respect, it is recommendable that the magnetic flux distribution is controlled to give a cogging torque characteristic in which there are two inflection points in the revolution angle of (1/2)(360°/i). In order to minimize the cogging and stabilize revolution of the motor, it is especially preferable that as shown in Figure 4, two inflection points are respectively formed in the positive polarity area and the negative polarity area with respect to the central point where the cogging torque becomes zero. Accordingly, it is desirable to control the magnetic flux distribution of the permanent magnet to provide five cogging-torque-zero points in either period of the cogging torque having the revolution angle of k·(360°/i). In this case, the k value is a coefficient in the range of 0.85 ≦ k ≦ 1.15 (the same definition is applied to the description stated below). This is because depending on condition of controlling of the magnetic flux distribution, there may occur a phenomenon as shown in Figure 5 or Figure 6 that the cogging torque curve does not indicate a zero point at the end of a period of the revolution angle (360°/i), but there is displacement of the curve. In other words, the k value means that the k value exists in the above-mentioned range, namely, when exists the fifth cogging-torque-zero point. Further, it is preferable in the motor having a small cogging torque and having the characteristic as above-mentioned that the five cogging-torque-zero points are not concentrated in a specified revolution angle, but are dispersed equally in the revolution angle k·(360°/i). Accordingly, it is recommendable to control the magnetic flux distribution in such a manner that three cogging-torque-zero points are provided in any period of the cogging torque of the revolution angle (k/2)(360°/i). The present invention is to provide a small-sized motor of high quality in which the cogging is minimized and a large torque is provided. From this viewpoint, the motor having a permanent magnet of 5 mm - 100 mm in outer diameter, 2 mm - 100 mm high and 0.5 mm - 10 mm in wall thickness can give remarkable effect. In the present invention, when a motor having a number of the magnetic poles and a number of salient poles is manufactured, the cogging can be relatively small without controlling the magnetization pattern and so on. Accordingly, the present invention is preferably applicable to a motor in which a relation of the number of the magnetic poles and the salient poles of the motor is given by the expressions n₁ (two magnetic poles and three salient poles), or n₂ (four magnetic poles and three salient poles), or n₃ (ten magnetic poles and six salient poles), where n₁, n₂, n₃ are respectively positive integars in the range of 1 ≦ n₁ ≦ 15, 1 ≦ n₂ ≦ 10, and 1 ≦ n₃ ≦ 5. As described above, the controlling of the magnetic flux distribution of the permanent magnet is extremely effective to reduce the cogging. Further, the controlling of the magnetic flux distribution is also useful to determine whether the cogging characteristic of the motor is excellent or not. Namely, if four inflection points are given in a period of the cogging torque of the revolution angle (360°/i) in measurements of the cogging torque characteristics of a given motor, then the motor can be provided for practical use without causing cogging. Thus, the present invention is directly applicable to the judgement of the cogging characteristics of motors. Preferred embodiments of the present invention will be described with reference to the drawings. Figure 1 is a diagram of an embodiment of the outer rotor type motor of the present invention. The motor comprises a core 1 having six salient poles, four magnetic poles 3 and a magnetic yoke 4 as the major parts. The magnetic poles 3 arranged in a ring form may be of any configuration. In this case, they are made of usual compositions and have a substantially uniform thickness in a cross-section taken along a line perpendicular to the center axis of the ring. Each of the magnetic poles has a pattern of magnetization such as a normal sine waveform or a trapezoidal form, provided that the magnetization pattern is controlled in conformity with the magnetic characteristic of the magnetic yoke 4 in such a manner that the period ϑi of a cogging torque is one half as much as the basic period ϑ'i of a normally produced cogging torque (where ϑ'i is an angle determined by (360°/i) when the least common multiple between the number of the magnetic poles 3 and the number of the salient poles is given as i). Namely, ϑi = 1/2 ϑ'i. The cogging is a ripple of rotation which is produced when the motor is driven. The ripple of rotation is resulted from a force (a cogging force) produced when the force acting on each part of the magnetic poles is not symmetric with respect to the rotation depending on a relative position of the rotation between the salient poles 2 and the magnetic poles 3. The cogging torque has a period of (360°/i) where i is the least common multiple between the number of the salient poles and the number of the magnetic poles. The motor shown in Figure 1, has six salient poles and four magnetic poles. Accordingly, i = 12, hence, the period of the cogging torque is 30°. Figure 2 is a diagram showing a magnetizing device 10 to magnetize a ringed permanent magnet to form four magnetic poles. In Figure 2, a reference numeral 11 designates a yoke having four outer cores 12 in which exciting coils 13 are provided. A numeral 14 designates an inner core. A magnetic material 15 having a ring form is put between the outer cores 12 and the inner core 14, and the magnetic material 15 is magnetized to form magnetic poles 3. The width of magnetization of the magnetic material 15 is controlled by forming slant portions having an angle of  in respective projecting parts 12A of the outer cores 12 with respect to the center O. When each of the projecting parts 12A is magnetized by changing the angle , the cogging torque characteristic with respect to the revolution angle of the rotor (consisting of magnetic poles 3 and the magnetic yoke 4) of the motor shown in Figure 1 is shown in Figure 3. In Figure 3, the cogging torque is zero at positions of the revolution angles of 0°, 15° and 30°. However, scattering in the period may take place due to scattering in magnetization. As apparent from Figure 3, the magnitude of the cogging torque varies by changing the angle  of magnetization. The cogging torque becomes smaller in the range-between 55° and 60°. The phase of the cogging torque is inversed in the angle ranging 55° - 60°. Figure 4 shows in detail the cogging torque characteristics between 55° and 60°. Namely, in the region of phase transition, the cogging torque characteristic assumes a torque pattern in which torque patterns having different phases are composed, whereby a period ϑi of the cogging torque is one half of the original period ϑi. In other words, there are four inflection points in a period of the revolution angle of 30° and there are five cogging-torque-zero points. More specifically, two inflection points are provided in a period of the revolution angle of 15° and there are three cogging-torque-zero points. The motor having the above-mentioned characteristic minimizes occurrence of the cogging. In the cogging torque characteristic shown in Figure 4, the torque pattern sometimes does not show a correct form, but it may show a complicated pattern as shown in Figure 5 or Figure 6. However, the torque pattern has four inflection points in the period of the revolution angle of 30°. The motor having the above-mentioned characteristic remarkably reduces the cogging unlike the conventional motor having the cogging torque characteristic as shown in Figure 3. Accordingly, it is possible to control the magnetization pattern by changing the shapes of the outer cores 12 and the inner core 14 of the magnetizing device 10 and by changing a current for magnetization while observing variation in phase of the cogging torque so as to obtain the period of the cogging torque as above-mentioned. In the region having such torque pattern, the cogging torque can be sufficiently reduced and therefore, a micro-motor of a high performance can be designed. Figure 7 shows the magnetizing device 10 for explaining another method of magnetization. Grooves 14A are formed in the inner core 14 so that one angle ' is continuously changed at the inside of the magnetic material 15 of a ring form. Change in the angle ' is conducted in such a manner that a number of the inner cores 14 each having a different angle are previously prepared and the inner core 14 is replaced by the other in accordance with an angle required. The method of changing the angle may be any suitable one as long as the strength of magnetization for each of the magnetic poles can be continuously changed. In measurements of the strength of magnetization for each of the four magnetic poles 3 in a ring form, magnetization curve as indicated by solid lines or broken lines as in Figures 8 and 9 can be obtained. In the following, another embodiment of the present invention will be described. Figure 10 shows the cogging torque characteristic of a motor having ten magnetic poles and fifteen salient poles. In Figure 10,  refers to an angle formed at the end portion or each of the outer cores 12 in the magnetizing device 10 in the same manner as in Figure 2. The phase of the cogging torque is changed by changing  as in Figure 3. When the magnetization angle  is 26°, there are four inflection points in the basic period ϑ'i = 12° of the cogging torque. When the motor has a certain number of magnetic poles and salient poles, the absolute value of the cogging torque having the period as above-mentioned is sometimes smaller than the cogging torque having a period of 2ϑ'i which is produced due to scattering of the strength of the magnetization of the N poles and the S poles. Accordingly, sometimes, it is difficult to determine the period of the cogging torque of the present invention in view of one cogging torque characteristics. In this case, when the basic period ϑ'i of the cogging torque is 12°, the cogging torque pattern at or near the revolution angle  = 26° which causes inversion of the phase of the torque in the instant period shows a complicated shape. At a glance, the torque pattern seems to have the period of 2 ϑ'i. This is because the absolute value of the cogging torque having the above-mentioned period is extremely small due to increase in number of the magnetic poles and the salient poles of the motor, and it is smaller than the cogging torque produced due to scattering of the strength of the magnetization of the N poles and the S poles. However, when viewed in detail, the cogging torque of ϑ'i is found on the cogging torque of the period of 2ϑ'i. Figures 11 and 12 are respectively power spectrum diagrams obtained by frequency analysis of the cogging torque at the revolution angles  of 28° and 26° in Figure 10. When  = 28°, the strength of the spectrum b corresponding to the basic period ϑ'i is extremely large. Namely, it is about 70 times as much as the strength of the spectrum d corresponding to the period ϑi and is about 6 times as much as the strength of the spectrum a corresponding to the period of 2ϑ'i. When  = 26°, the strength of the spectrum b is about 12 times as much as the strength of the spectrum d. Accordingly, the spectrum a shows the greatest value. The fact that the strength of the spectrum b is greater than the spectrum d mainly owes to high frequency components produced when the cogging torque pattern having the period of 2ϑ'i is deviated from a sine wave. The cogging torque can be judged by a power spectrum. When the strength of the spectrum b corresponding to the basic period ϑ'i of the cogging torque is about 30 times or smaller than the strength of the spectrum d corresponding to the period ϑi of the cogging torque of the present invention, the magnetic poles having the value of magnetization can be used for a motor which minimizes the cogging torque. Thus, the magnetization angle and the magnetization pattern can be precisely controlled by changing the power spectrum. In summary, the phase of the cogging torque having the period of ϑ'i is detected and at the same time, the cogging torque having the period of ϑ'i is reduced, with the result of obtaining the cogging torque having the period of ϑ'i as a torque pattern.

Claims: A motor comprising magnetic poles constituted by a permanent magnet magnetized in its radial direction and a core (1) with salient poles (2) placed inside or outside the magnetic poles (3) of said permanent magnet, characterized in that said permanent magnet has a magnetic flux distribution such that in the cogging torque characteristics of said motor, there are four inflection points in a period of the cogging torque of a revolution angle (360°/i) having balanced stable points as reference points, where i is the least common multiple between the number of said salient poles (2) and the number of said magnetic poles (3). The motor according to Claim 1, wherein there are two inflection points in the period of the cogging torque of a revolution angle (1/2) (360°/i) having balanced stable points as reference points. The motor according to Claim 1, wherein there are five cogging-torque-zero points in a period of the cogging torque of a revolution angle k ·(360°/i) having balanced stable points as reference points, where k is the number in the range of 0.85 - 1.15. The motor according to Claim 3, wherein there are three cogging-torque-zero points in a period of the cogging torque of a revolution angle (k/2) (360°/i) having balanced stable points as reference points. The motor according to Claim 1, wherein said magnetic flux distribution is controlled by adjusting an angle of magnetization. The motor according to Claim 1, wherein said magnetic flux distribution is controlled by adjusting a pattern of magnetization. The motor according to Claim 1, wherein said permanent magnet constituted by the magnetic poles (3) is in a ring form. The motor according to Calim 1, wherein said permanent magnet constituted by the magnetic poles (3) is a plastic magnet. The motor according to Claim 1, wherein said magnetic poles (3) and said salient poles (2) of the motor have a relation of n₁ (two magnetic poles (3) and three salient poles (2)), or n₂ (four magnetic poles and three salient poles, or n₃ (ten mangetic poles (3) and six salient poles (3)), where n₁, n₂, n₃ are respectively positive integers in the ranges of 1 ≦ n₁ ≦ 15, 1 ≦ n₂ ≦10 and 1 ≦ n₃ ≦5. The motor according to Claim 7, wherein said permanent magnet is in a ring form in which the outer diameter is in the range of 5 mm - 100 mm, the heigth is in the range of 2 mm - 100 mm and the wall thickness is in the range of 0.5 mm - 10 mm.

Inventors: ARITA, YOJI
Applicants: MITSUBISHI KASEI CORPORATION
Application year: 1986
Main classification: H02K37
IPC codes: H02K21, H02K29, H02K37