GB2062976A

GB2062976A - DC motors

Info

Publication number: GB2062976A
Application number: GB8032500A
Authority: GB
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1979-10-09
Filing date: 1980-10-08
Publication date: 1981-05-28
Also published as: DE3037724A1; DE3037724C2; FR2467504B1; GB2062976B; JPS5656162A; FR2467504A1

Abstract

A DC, e.g. brushless, motor comprises a stator (1) which generates a magnetic flux having a density distribution Figure 2a including a wave-shaped fundamental component Figure 2b and a wave-shaped auxiliary component Figure 2c which is an even harmonic of the fundamental component. The motor has a rotor (3) including two magnets (5 and 6), a first one (5) of the magnets having poles disposed with even angular pitch around a shaft (4) and the second magnet (6) having a number of poles which is an even multiple of the number of poles of the first magnet (5), a peak of the flux density distribution Figure 2e of the second magnet (6) being at a different position from a peak of the auxiliary component when one pole of the first magnet faces the position of the peak of the fundamental component of the stator, whereby the motor can start even from what would normally be dead points of the rotor position by virtue of a torque generated by magnetic force between the second magnet (6) of the rotor (3) and the auxiliary component of the stator (1). The rotor magnets may be arranged sequentially in the axial direction as in Figure 4 (not shown), and the stator of the motor may include non-wound poles as in Figures 3, 5 (not shown). <IMAGE>

Description

SPECIFICATION DC Motors This invention relates to direct current (DC) motors.

It is known for DC motors to be provided with switching devices instead of conventional commutators. Most conventional D.C. motors having switching devices have a plural number ofstatorwindings and a rotor having the same plural number of permanent magnet poles which are disposed at the same angular pitch as those of the statorwindings.

The stator windings and the permanent magnets poles of the rotor are disposed with uniform angular pitch around the axis of the motor. In such a conventional D.C. motor, if the rotor is at a specified position in which the permanent magnet poles of the rotor face the poles of the stator and the polarities of the facing poles are the same, the rotor magnet is likely to be repelled in either direction of rotation.

However, if the facing poles face one another precisely, the balance of the magnetic repulsion force makes it impossible for the rotor to start rotating. If the facing poles are of opposite polarity, each of the rotor magnets is held in position by the stator. Therefore, there are two kinds of dead points in the conventional D.C. motor; the former is an unstable dead point and the latter is stable dead point. If the rotor is in such a dead point, the motor cannot start even if electric power is fed to the motor.

In order to remove such dead points, US Patent No. 3 299335 (Wessels et al.) proposes a motor construction which comprises a rotor which consists of magnetic parts and non-magnetic parts disposed about the rotor circumference, the latter being interposed between successive pairs of magnet poles of the magnetic parts. By virtue of such structure, the motor can start regardless of the angular position of the rotor. However, in this known DC motor, the rotor consists of several parts and two different kind of materials, namely the magnetic parts and non-magnetic parts, are bonded to each other.Therefore, this motor suffers from the disadvantage that manufacture of the rotor is not easy because different kinds of arc-shaped components have to be bonded together to form a ring which is accurately balanced for rotation, and the rotor is liable to break off by virtue of centrifugal force.

According to the invention there is provided a DC motor comprising a permanent magnet rotor and a stator having at least two exciting coils which, in use, are alternately switched so as to be fed with DC current in response to the angular position of the rotor, wherein: the stator has a predetermined plurality of main poles arranged to be excited by the exciting coils to provide a magnetic flux having a flux distribution, with respect to angular position around the axis of the stator, comprising a wave-shaped fundamental component and a wave-shaped auxiliary component which is an even-number harmonic wave of the fundamental component, the harmonic wave having a peak at the same angular position as a peak of the fundamental component; and the rotor has a first magnet and a second magnet, the first magnet comprising the same number of rotor poles as the number of main poles of the stator, the first magnet having a first flux distribution with respect to angular position around the axis of the rotor, the first flux distribution being of substantially the same shape as said fundamental component, and the second magnet comprising said even number of auxiliary rotor poles and having a second flux density distribution with respect to angular position around the axis of the rotor, the second flux distribution being of substantially the same shape as said auxiliary component, but a peak of the second flux distribution being shifted by a predetermined angle with respect to a peak of said auxiliary component, the first and second magnets being coaxially disposed about the axis of the rotor.

DC motors embodying the invention do not have dead points and can start regardless of the position of the rotor. The rotors of DC motors embodying the present invention can be formed with uniform material circumferentially about their axes, whereby the rotors can be manufactured very easily and are durable. If a magnetic sensor is used to detect rotation of the rotor, one of the rotor magnetsperse can be utilised to apply alternating magnetic flux thereto.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of the fundamental structure of a first DC motor embodying the present invention; Figures 2(a) to 2(f) are graphs showing flux density distributions of the motor of Figure 1 with respect to angular position around the axis of the rotor; Figure 3 is a plan view of a second DC motor embodying the present invention; Figure 4 is a perspective view of magnet rings of a rotor of a third motor embodying the present invention; Figure 5 is a plan view of a stator of the third motor embodying the present invention; Figure 6 is a sectional elevational view of the third motor embodying the invention; and Figures 7/at, 7(b) and 7(c) are graphs showing flux density distributions of the third motor embodying the invention with respect to angular position around the axis of the rotor.

The principles of construction and operation of embodiments of the invention will now be described with reference to Figures 1 and 2. Figure 1 shows a first DC motor embodying the present invention. The motor comprises a stator 1 of cylindrical shape which has two main poles 2a and 2b in diametricallyopposed positions, i.e. spaced 180" apart from each other with respect to the axis of the rotor. Field coils 2a' and 2b' are wound around the main poles 2a and 2b, respectively. A rotor 3 is directly or indirectly secured to a shaft 4 within the stator 1. The rotor 3 comprises two permanent magnets 5 and 6 which are coaxially connected or bonded along the shaft 4.

The permanent magnet 5 has two main poles (N- and S-poles) in diametrically-opposed positions, i.e.

spaced 1800 apart from each other with respect to the axis, as shown in Figure 1. The permanent magnet 6 has four auxiliary poles arranged as a cross in such a manner that poles of opposite polarity are disposed 90" apart from each other with respect to the axis. Each pole of the permanent magnet 5 is disposed between the poles of the permanent magnet 6 with respect to their angular positions around the axis of the rotor 3. The area of the permanent magnet 5 shown in Figure 1 is larger than that of the permanent magnet 6 and, conse quently, the magnetic force of the former is larger than that of the latter. The field coils 2a' and 2b' are alternately switched on and off, in dependence upon the angular position of the rotor 3, by a rotor position detector comprising, for example, a Hall-effect device.The switching intervals of the field coils 2a' and 2b' are synchronised with polarity changes of the magnetic flux at the main poles 2a and 2b by the magnet 5. In order to carry out such synchronised switching, the Hall-effect device or other rotor position detector is installed at a position precisely facing the magnet 5.

When the field coil 2a' is energised, a magnet flux O is induced in the stator 1, the flux density distribution with respect to the angular position û around the shaft 4 of the stator 1 being as shown in Figure 2(a). The angular position 8 marked on the abscissa of Figures 2(a) to 2(f) corresponds to the angular position shown in Figure 1. The flux density distrubtion of the stator 1 as shown in Figure 2(a) is of such a shape that it can be resolved into two components, one of which is a wave-shaped fundamental component shown in Figure 2(b) and the other of which is a wave-shaped auxiliary component shown in Figure 2(c).At the angular position where 6 = 0 , where the centre of the main pole 2a is disposed, both the fundamental component and the auxiliary component are at peak values of flux density distribution, and the auxiliary component has the shape of a second harmonic wave of the waveform of the fundamental component. At the time (angular position) shown in Figures 1 and 2, the main pole 2a (phase angle (angular position) 0 = 0 ) is excited to be an N-pole and the other main pole 2b is excited to be an S-pole.The permanent magnets 5 and 6 of the rotor 3 of Figure 1 have flux density distributions shown in Figures 2(d) and 2(e), respectively, whereby the distribution of flux of the permanet magnet 5 is substantially the same as that of the fundamental component of the stator 1 and that of the permanent magnet 6 is substantially the same as that of the auxiliary component, but the phase of the flux density distribution of the permanet magnet 6 is shifted by 45" from that of the auxiliary component, as can be seen from a comparison of Figures 2(c) and 2(e).

Under these cicumstances, the permanent magnet 5 of the rotor 3 is in a position which, in a conventional motor, would be an unstable dead point, since the balance of the magnetic repulsion force between the permanent magnet 5 and the stator 1 is held as shown in Figures 2(b), 2(c) and 2(d). However, a magnetic force generated between the permanent magnet 6 and the stator 1, that is the magnetic force between the permanent magnet 6 and the auxiliary component of the flux density distribution, causes the rotor 3 to rotate in the clockwise direction as shown by an arrow A in Figure 1. As the rotor 3 rotates, a clockwise torque is generated between the permanent magnet 5 and the fundamental component of the flux density distribu tion, whereby the rotor 3 continues to rotate.When the rotor 3 rotates through 180 and reaches a position which, in a conventional motor, would be a stable dead point, in which the permanent magnet 5 is attracted by the stator 1 to be held still there, the excitation of the field coil 2a' is stopped and the field coil 2b' becomes excited. Then, the flux density distribution of the stator 1, energised by the field coil 2b', becomes as shown in Figure 2(f). At this time, by the switching of the excitation, a position which in a conventional motor would be an unstable dead point again occurs between the then N-excited main pole 2b and the pole N of the permanent magnet 5.

However, the rotor 3 continues to rotate in the clockwise direction by virtue of the magnetic force between the permanent magnet 6 and the stator 1 and the force of inertia. Thus, after passing through the position which, in a conventional motor, would be an unstable dead point, a clockwise torque is generated between the permanent magnet 5 and the fundamental component of the flux density distribution in similar manner to that described above.

Therefore, the motor continues to rotate.

Figure 3 shows a second DC motor embodying the invention. The motor, which is a two-pole inner rotor type motor, comprises a stator 11 having two diametrically-opposed main poles 21a and 21b and two supplementary poles 21cand 21ddisposed between the main poles 21a and 21b. The supplementary poles 21 c and 21 dare used to generate the auxiliary component of the flux density distribution of the stator 11. Field coils 21a' and 21b' are wound around neck parts of the main poles 21a and 21 b, respectively. The field coils 21a' and 21b' are connected at one end to switching transistors Trl and Tr2, respectively. The other ends of the field coils 21a' and 21b' are connected in common to a DC voltage source E.The rotor 31 of this embodiment is the same as the rotor 3 of the motor of Figure 1. A detecting means, for example a Hall-effect device HD, is disposed in front of the supplementary pole 21cfordetecting rotation of the rotor 31. Rotation signals developed by the Hall-effect device HD and synchronised with the change of magnetic flux of the rotor magnet 5 are fed to a switching circuit 71 which includes, for example, a flip-flop circuit, and which controls the switching transistors Tr1 and Tr2 so that they are alternately switched on. By virtue of such structure, the current two the field coils 21a' and 21b' is switched by the switching transistors Trl and Tr2 in synchronism with the rotation of the rotor 31.

Figures 4, 5 and 6 show a third motor embodying this invention, this motor being an outer rotor type DC motor. In this embodiment, an outer rotor 32 is in the form of a cylinder having a hole through it, that is a short tube or ring as shown in Figure 4. The rotor 32 comprises a first ring-shaped permanent magnet 52 having four poles and a second ring-shaped permanent magnet 62 having eight auxiliary poles.

The N and S poles of each of the permanent magnets 52 and 62 are disposed alternately around the periphery thereof, and every other pair of adjacent faces of the alternate poles of the permanent magnet 62 is aligned with those of the alternate poles of the permanent magnet 52, as shown in Figure 4. The permanent magnets 52 and 62 have the same outer and inner diameters and the same thickness. However, the height 'a' of the permanent magnet 52 is greater than the height 'b' of the permanent magnet 62, whereby the magnetic force of the permanent magnet 52 is greater than that of the permanent magnet 62. The ring-shaped permanent magnets 52 and 62 are bonded coaxially about the axis of the rotor 32.

The stator 12 of this embodiment (see Figure 5) has four main poles 22a,22b, 22c and 22d and four supplementary poles 22e, 22f, 22g and 22h which are alternately disposed. The supplementary poles 22e, 22f, 22g and 22h serve to generating the auxiliary component of the flux density distribution. Field coils 22a' and 22c', which are wound around the main poles 22a and 22c, respectively, are connected in series between a DC voltage source E and a switching transistor Tr3. Field coils 22b' and 22d', which are wound around the main poles 22b and 22d, respectively, are connected in series between the DC voltage source E and a switching transistor Tr4. A Hall-effect device HD for detecting rotation of the rotor 32 is mounted adjacent to the supplementary pole 22f and send signals to a switching circuit 72.The switching circuit 72 includes, for example, a flipflop and makes the switching transistors Tr3 and Tr4 turn on alternately during rotation of the rotor 32.

As shown in Figure 6, the stator 12 is fixed on a motor housing base 13. The rotor 32 is disposed around the stator 12 and fixed in a flange part 33 of a cover 34. The cover 34 is rotatably mounted on the motor housing base 13 by a shaft 2 journalled in a bearing 14.

In this embodiment, the flux density distribution of the stator 12 at the time when the field coils 22a' and 22c' are energised is shown in Figure 7(a). As shown in Figure 7(a), the shape of this flux density distribution is similar to that of Figure 2(a), but has a frequency which is twice as high. In similar manner to the first two embodiments, the flux density distribution of the stator 12 can be resolved into two components, namely the fundamental component and the auxiliary component. The flux density distribution of the permanent magnets 52 and 62 of the rotor 32, shown in Figures 7(b) and 7(c), are also similar to those of Figures 2(d) and 2(e), but both have a double frequency. Therefore, the motor of this embodiment can start, regardless of the angular position of the rotor, in similar manner to that described above.

In the above embodiments, the auxiliary comporient of the flux density distribution of the stator is a second harmonic of the fundamental component thereof. However, the auxiliary component may be an even-number harmonic of the fundamental component.

The DC motors embodying the present invention and described above do not have dead points, as do conventional motors, whereby they can start at any angular position of the rotor. Furthermore, the rotor of motors embodying the present invention can comprise only two ring-shaped elements of magnetic material, whereby the rotor is highly resistant to strain caused by centrifugal force, whereby the motor is easily manufactured and is very tough.

Furthermore, if a Hall-effect device is used for the rotation detecting means, it is not necessary to use another magnet or magnets to apply an alternating magnetic flux to the Hall-effect device to effect synchronisation with the rotation of the rotor.

Claims

1. A DC motor comprising a permanent magnet rotor and a stator having at least two exciting coils which, in use, are alternately switched so as to be fed with Do current in response to the angular position of the rotor, wherein: the stator has a predetermined plurality of main poles arranged to be excited by the exciting coils to provide a magnetic flux having a flux distribution, with respect to angular position around the axis of the stator, comprising a wave-shaped fundamental component and a wave-shaped auxiliary component which is an even-number harmonic wave of the fundamental component, the harmonic wave having a peak at the same angular position as a peak of the fundamental component; and the rotor has a first magnet and a second magnet, the first magnet comprising the same number of rotor poles as the number of main poles of the stator, the first magnet having a first flux distribution with respect to angular position around the axis of the rotor, the first flux distribution being of substantially the same shape as said fundamental component, and the second magnet comprising said even number of auxiliary rotor poles and having a second flux density distribution with respect to angular position around the axis of the rotor, the second flux distribution being of substantially the same shape as said auxiliary component, but a peak of the second flux distribution being shifted by a predetermined angle with respect to a peak of said auxiliary component, the first and second magnets being coaxially disposed about the axis of the rotor.

2. A DC motor according to claim 1, wherein the auxiliary component of the flux density distribution of the stator is a second harmonic wave of the fundamental component.

3. A DC motor according to claim 1 or claim 2, wherein the first magnet has a greater magnetic force than the second magnet.

4. A DC motor according to claim 1, claim 2 or claim 3, wherein the first and second magnets of the rotar are permanent magnets.

5. A DC motor according to any one of the preceding claims, wherein the stator has supplementary poles between the main poles.

6. A DC motor according to any one of the preceding claims, wherein the first and second magnets are bonded to each other.

7. A DC motor according to any one of the preceding claims, wherein a switching circuit is connected in series with the exciting coils, the switching circuit comprising means to detect changing of the magnetic flux of the first magnet of the rotor, thereby to control switching thereof.

8. A DC motor substantially as herein described with reference to Figures 1 and 2, Figure 3 or Figure 4 to 7 of the accompanying drawings.