The present invention relates to a brushless motor having a permanent-magnet rotor. Figure 1 illustrates a four-pole brushless motor 1 having a permanent-magnet rotor 2. The stator 3 comprises a circular back 4 and four pole arms 5 that project radially inward from the back 4. A coil 6 is wound about each pole arm 5, and the coils 6 are coupled together to form a single phase winding. A problem with this particular design of motor 1 is that the fill factor of the stator 3 is relatively poor. Additionally, it is generally difficult to wind the coils 6 onto the pole arms 5.

In a first aspect, the present invention provides a brushless motor comprising a four- pole permanent-magnet rotor and a four-pole stator, the stator comprising two stator elements arranged on opposite sides of the rotor, each stator element comprising a c- shaped core having a back and two arms that extend from opposite ends of the back, and a coil wound about the core, wherein the free end of each arm defines a pole tip, and the pole tips collectively have four-fold rotational symmetry about the rotational axis of the rotor. By employing two c-shaped cores, a stator having a relatively high fill factor may be achieved. Consequently, a more efficient and/or smaller motor may be realised. Additionally, winding the coil onto the cores is made easier.

The motor is bipolar and thus the direction of the magnetic flux through the stator is required to reverse with every 90 degrees of rotation made by the rotor. The stator, on the other hand has a rotational symmetry of 180 degrees. By employing pole tips that have four-fold rotational symmetry (i.e. a rotational symmetry of 90 degrees), the stator flux seen by the rotor is the same over each 90 degrees of rotation, in spite of the fact that the rotational symmetry of the stator is 180 degrees. Each arm may comprise a straight section that extends between the back and the pole tip. A slot opening defined between the pole tips may then be smaller than a slot width defined between the straight sections, i.e. the distance between the pole tips may be smaller than the distance between the straight sections. Additionally or alternatively, the width of the arm at the pole tip may be greater than that at the straight section. As a result, the pole tips are able to capture more of the rotor flux whilst the straight sections may be spaced apart so as to provide a relatively large slot. By providing a larger slot, the required number of turns may be achieved using a thicker coil, thereby reducing copper losses.

A coil may be wound about each arm of the core. This then has the benefit of reducing flux leakage between the arms, thereby reducing the inductance of the stator.

The leading edge and/or the trailing edge of each pole tip may be chamfered. This then reduces stator flux leaking between the pole tips and thus reduces the inductance of the stator.

In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 illustrates a conventional brushless motor;

Figure 2 is a sectional view of a brushless motor in accordance with the present invention;

Figure 3 illustrates the magnetic flux of a four-pole magnet in isolation; and

Figure 4 illustrates the magnetic flux of a hypothetical four-pole magnet in isolation but reflecting the shape of the stator of the present invention when in the aligned position. The brushless motor 10 of Figure 2 comprises a rotor 11 and a stator 12. The rotor 11 comprises a shaft 13 to which a four-pole permanent magnet 14 is mounted. The stator 12 comprises two stator elements 15,16 arranged on opposite sides of the rotor 11.

Each stator element 15,16 comprises a core 17, a pair of bobbins 18,19, and a pair of coils 20,21.

The core 17 is c-shaped and comprises a back 22 and two arms 23,24 that extend from opposite ends of the back 22. Each arm 23,24 extends toward the rotor 11 and has a free end that defines a pole tip 25. The leading edge 26 and the trailing edge 27 of each pole tip 25 are chamfered. This then reduces flux leakage between the pole tips 25, thereby reducing the inductance of the stator 12. Each coil 20,21 comprises a wire that is wound about a respective bobbin 18,19, each bobbin 18,19 surrounding a respective arm 23,24 of the core 17. A single wire may be used for both coils 20,21 of a stator element 15,16. Alternatively, separate wires may be used for each coil 20,21. The coils 20,21 of the two stator elements 15,16 are coupled together to form a single phase winding.

Each arm 23,24 of the core 17 comprises a straight section 28 that extends between the back 22 and the pole tip 25. The width of the arm 23,24 at the pole tip 25 is greater than that at the straight section 28. Additionally, the slot opening 29 defined between the pole tips 25 is smaller than the slot width 30 defined between the straight sections 28. As a result, the pole tips 25 are able to capture more of the rotor flux whilst the straight sections 28 may be spaced apart so as to provide a relatively large slot. By providing a larger slot, the required number of turns may be achieved with a thicker wire, thereby reducing copper losses. By employing a stator 12 having two c-shaped cores 17, a relatively high fill factor can be achieved. In particular, it is generally possible to achieve a higher fill factor than that illustrated in Figure 1. With the stator 3 of Figure 1, the slot between each pair of pole arms 5 is arc-shaped. As a result, it is generally difficult to fill the slot. In contrast, each slot of the stator 12 of Figure 2 is rectangular and thus more of the slot may be filled. In achieving a higher fill factor, a more efficient motor may be achieved for a given size. Alternatively, a smaller motor may be achieved for a given efficiency.

In addition to achieving a higher fill factor, it is generally easier to wind the coils 20,21 onto the cores 17. In order to wind the coils 6 onto the stator 3 of Figure 1, a winding machine having an additional axis of movement is required in order to follow the arc- shaped slots. Additionally, the winding machine is required to operate within the central bore of the stator 3. As a result, the length of each coil 6 along the pole arm 5 is limited by the diameter of the bore. With the stator 12 of Figure 2, on the other hand, the coils 20,21 may be wound onto the cores 17 using a winding machine having one less axis of movement, which is both cheaper and faster. Furthermore, the winding machine is not required to operate within a bore and thus no limit is imposed on the length of the coils 20,21 along the arms 23,24.

In spite of the aforementioned advantages, it is by no means obvious to employ two c-shaped stator elements in a permanent-magnet motor. Figure 3 illustrates the magnetic flux of a four-pole permanent-magnet rotor in isolation. It can be seen that the magnetic flux from each North pole divides and follows one of two return paths. The stator 3 of Figure 1 reflects the shape of the rotor flux. Consequently, when the rotor 2 is in the aligned position, the flux from each North pole travels along a pole arm 5 and divides in two at the back 4. One half of the flux then returns to the rotor 2 via a left- hand loop (i.e. anti-clockwise around the back 4 and then along the next pole arm) and the other half returns via a right-hand loop (i.e. clockwise around the back 4 and then along the next pole arm). In contrast, the stator 12 of Figure 2 is broken into two distinct stator elements 15,16. Consequently, when the rotor 11 is in the aligned position, the flux from each North pole follows one return path only, namely around one of the stator elements 15,16. One half of the flux from each North pole is therefore forced in a completely different direction to a completely different pole. As a result, the magnetic flux of the rotor 11 resembles that illustrated in Figure 4. The path taken by the rotor flux is therefore very different for the two stators 3,12. The rotor flux induces a back EMF in the phase winding, which then influences the power and efficiency of the motor. Since the path taken by the rotor flux is very different for the two motors 1,10, one would naturally assume that the back EMF for the two motors 1,10 is also very different. Indeed, it is not at all obvious that the back EMF for the motor 10 of Figure 2 would resemble a conventional waveform.

In addition to the rotor flux, there is also the problem of the stator flux. A permanent- magnet motor is bipolar and thus the direction of the stator flux must reverse as the rotor rotates. In the case of a four-pole permanent-magnet motor, the direction of the stator flux reverses with every 90 degrees of rotation made by the rotor. With the motor 1 of Figure 1, the stator 3 has a rotational symmetry of 90 degrees. Since the stator flux reverses every 90 degrees and the rotational symmetry of the stator 3 is 90 degrees, the stator flux seen by the rotor 2 is the same over each 90 degrees of rotation. With the motor 10 of Figure 2, on the other hand, the stator 12 has a rotational symmetry of 180 degrees. As a result, one would expect the stator flux seen by the rotor 11 to be different over each 90 degrees, i.e. the stator flux seen by the rotor 11 when rotating from 0 to 90 degrees will differ from that seen by the rotor 11 when rotating from 90 to 180 degrees. Such behaviour in the stator flux would adversely affect the performance of the motor 10. In particular, torque ripple will be greater. Additionally, any variance in the stator flux will have a knock-on effect on the back EMF induced in the phase winding. As a result, the power and/or the efficiency of the motor 10 will be adversely affected.

Although the stator 12 of Figure 2 has a rotational symmetry of 180 degrees, the pole tips 25 are specifically shaped and spaced about the rotor 11 so that collectively the pole tips 25 have a rotational symmetry of 90 degrees. That is to say that the pole tips 25 collectively have four- fold rotational symmetry about the rotational axis of the rotor 11. Accordingly, in spite of the fact that the stator 12 has a rotational symmetry of 180 degrees (i.e. two-fold rotational symmetry), the stator flux seen by the rotor 11 is nevertheless the same over each 90 degrees of rotation.

A stator having two c-shaped stator elements has previously been employed in a switched reluctance motor. However, unlike the motor 10 of the present invention, the rotor of a switched reluctance motor does not generate magnetic flux. Furthermore, a switched reluctance motor is unipolar and thus the direction of the stator flux does not change. Consequently, the aforementioned problems associated with the rotor flux and the stator flux of a permanent-magnet motor are not a concern for a switched reluctance motor. A person skilled in the art would not therefore conclude that the stator of the switched reluctance motor might equally be used with a permanent-magnet motor. Indeed, for the reasons set out above, the skilled person would simply fail to consider such a stator for a permanent-magnet motor. In the embodiment described above, the width of each arm 23,24 at the pole tip 25 is greater than that at the straight section 28. This then has the benefit that the pole tips 25 are able to capture more of the rotor flux. Conceivably however, the width of each arm 23,24 may be uniform. This then enables bobbins having pre-wound coils to be slid onto each arm 23,24, thus further simplifying the assembly of the motor 10.

The coils 20,21 of the above-described stator 12 are wound onto the arms 23,24 of each core 17. This then has the advantage that the coils 20,21 reduce flux leakage between the arms 23,24. Nevertheless, if required, rather than winding the coils 20,21 onto the arms 23,24, a single coil may be wound onto the back 22 of each core 17.