A bearingless motor

Abstract

A bearingless motor 10 includes a rotor 12 having p pole pairs 18, and a stator assembly 14 including stator core 20 and stator winding 22. The stator winding is energised to generate a first magnetic

Description

B ACKGROUND A bearingless motor is an electrical motor that has a bearing system integrated within its magnetic circuit, and enables rotation of a rotor absent the need for conventional bearings such as ball bearings and the like. Bearingless motors usually typically have two sets of windings for torque and force production, supplied by independently connected and controlled inverters. SUMMARY A first aspect of the present invention provides a method of controlling a bearingless motor comprising a rotor having p pole pairs, and a stator assembly comprising a stator winding, wherein the method comprises: energising the stator winding to generate a first magnetic field having p pole pairs for generating torque to act on the rotor; and energising the stator winding to generate a second magnetic field comprising a component having q pole pairs for generating a translational force to act on the rotor, wherein q=p±l, wherein energising the stator winding to generate the second magnetic field comprises applying a modulation factor to the stator winding, the modulation factor comprising a factor of 2p±l. Application of the modulation factor comprising the factor of 2p±l to the stator winding, to generate the second magnetic field, may result in a shear stress at an outer surface of the rotor that leads to translational forces along two orthogonal axes. For example, where a factor of p±l is utilised, a translational force along one axis, say an x-axis of the rotor when viewed in plan view, may be zero, resulting in relatively poor control of a position of the rotor along the x-axis. In contrast, where a factor of 2p±l, translational forces may be applied to the rotor in both x- and y- axes of the rotor when viewed in plan view. This may provide for improved stability of the rotor position in use. Applying the modulation factor to the stator winding may comprise applying the modulation factor to currents induced in the stator winding. The factor may comprise a waveform defined by a cosine with an angular variation defined by 2p±l. The factor may comprise a multipole field with a variation of cos(2p+l) around an airgap between the rotor and the stator assembly. Modulation of current in the stator winding which is located in a positive half cycle of the cosine may be by a factor of 2p+l. Modulation of current in the stator winding which is located in a positive half cycle of the cosine may be by a factor of 2p-l. The translation force may comprise a force to act on the rotor along at least two axes within a plane orthogonal to a longitudinal axis of the rotor when viewed in a direction along the longitudinal axis of the rotor. The stator winding may comprise a first set of coils and a second set of coils. The method may comprise energising the first and second sets of coils in phase with one another to generate the first magnetic field. The method may comprise energising the first and second sets of coils with current of a same magnitude to generate the first magnetic field. The method may comprise energising the first and second sets of coils out of phase with one another to generate the second magnetic field. The method may comprise energising the first and second sets of coils with currents of different peak magnitudes to generate the second magnetic field. The first set of coils and the second set of coils may each form a three-phase winding. The stator winding may comprise a three-phase winding. For each phase of the first set of coils, coils of the phase may be connected in series with one another. For each phase of the second set of coils, coils of the phase may be connected in series with one another. Coils of corresponding phases between the first and second sets of coils may be connected in parallel with one another. Each of the first set of coils and the second set of coils may have a cardinality of 2p±l. The stator winding may comprise 2(2p±l) coils in total. The cardinality of each of the first and second sets of coils may be an integer multiple of 3. A total number of coils of the stator winding may be an integer multiple of 3. The first and second sets of coils may be interleaved with one another, such that every other coil of the stator winding belongs to a same one of the first and second sets of coils. The factor may be an integer multiple of 3. The second magnetic field may comprise a first component having q pole pairs, and a second component having r pole pairs, wherein r=3p±l. The method may comprise energising the stator winding to generate the second magnetic field comprising the first component having q pole pairs, and the second component having r pole pairs. Alternative modulation factors may comprise a lower r value, for example at r=2p±l, which may be more likely to cause loss in the rotor due to the harmonic component penetrating deeper into the rotor and causing excess loss. The bearingless motor may comprise an outer rotor motor, and the method may comprise energising the stator winding to generate the second magnetic field comprising a component having q=p-l pole pairs. The bearingless motor may comprise an inner rotor motor, and the method may comprise energising the stator winding to generate the second magnetic field comprising a component having q=p+l pole pairs. The value of p may comprise an integer value. The value of p may be greater than or equal to 6. The value of p may be greater than or equal to 8, greater than or equal to 10, or greater than or equal to 12. The method may comprise controlling the bearingless motor such that the rotor rotates relative to the stator assembly at less than 5,000 rpm, for example less than 4,000 rpm, or less than 3,000 rpm. The method may comprise introducing a negatively rotating field relative to the rotor direction to generate the second magnetic field. A second aspect of the present invention provides a bearingless motor comprising: a rotor assembly comprising p pole pairs; and a stator assembly comprising a stator winding, wherein the stator winding is: configured to be energised to generate a first magnetic field having p pole pairs for generating torque to act on the rotor; and configured to be energised to generate a second magnetic field comprising a component having q pole pairs for generating a translational force to act on the rotor, wherein q=p±l; and configured to be such that a modulation factor is applicable to the stator winding to energise the stator winding to generate the second magnetic field, the modulation factor comprising a factor of 2p±l. The factor may comprise a waveform defined by a cosine with an angular variation defined by 2p±l. Modulation of current in the stator winding which is located in a positive half cycle of the cosine may be by a factor of 2p— 1. The factor may comprise a multipole field with a variation of cos(2p+l) around an airgap between the rotor and the stator assembly. Modulation of current in the stator winding which is located in a positive half cycle of the cosine may be by a factor of 2p-l. The translation force may comprise a force to act on the rotor along at least two axes within a plane orthogonal to a longitudinal axis of the rotor when viewed in a direction along the longitudinal axis of the rotor. The stator winding may comprise a first set of coils and a second set of coils, and the first and second sets of coils may be configured to be energised in phase with one another to generate the first magnetic field. The first and second sets of coils may be configured to be energised with current of a same magnitude to generate the first magnetic field. The first and second sets of coils may be configured to be energised out of phase with one another to generate the second magnetic field. The first and second sets of coils may be configured to be energised with currents of different peak magnitudes to generate the second magnetic field. The first set of coils and the second set of coils may each form a three-phase winding. For each phase of the first set of coils, coils of the phase may be connected in series with one another. For each phase of the second set of coils, coils of the phase may be connected in series with one another. Coils of corresponding phases between the first and second sets of coils may be connected in parallel with one another. Each of the first set of coils and the second set of coils may have a cardinality of 2p±l. The stator winding may comprise 2(2p±l) coils in total. The cardinality of each of the first and second sets of coils may be an integer multiple of 3. A total number of coils of the stator winding may be an integer multiple of 3. The first and second sets of coils may be interleaved with one another, such that every other coil of the stator winding belongs to a same one of the first and second sets of coils. The factor may be an integer multiple of 3. The second magnetic field may comprise a first component having q pole pairs, and a second component having r pole pairs, wherein r=3p±l. The stator winding may be configured to be energised to generate the second magnetic field comprising the first component having q pole pairs, and the second component having r pole pairs. Alternative modulation factors may comprise a lower r value, for example at r=2p±l, which may be more likely to cause loss in the rotor due to the harmonic component penetrating deeper into the rotor and causing excess loss. The bearingless motor may comprise an outer rotor motor, and the stator winding may be configured to be energised to generate the second magnetic field comprising a component having q=p-l pole pairs. The bearingless motor may comprise an inner rotor motor, and the stator winding may be configured to be energised to generate the second magnetic field comprising a component having q=p+1 pole pairs. The value of p may comprise an integer value. The value of p may be greater than or equal to 6. The value of p may be greater than or equal to 8, greater than or equal to 10, or greater than or equal to 12. The stator winding may be configured to be energised such that the rotor rotates relative to the stator assembly at less than 5,000 rpm, for example less than 4,000 rpm, or less than 3,000 rpm. The stator winding may be configured to introduce a negatively rotating field relative to the rotor direction to generate the second magnetic field. The motor may be a slotless motor, for example with the stator assembly comprising a stator core that does not define any slots for location of the stator winding. The motor may be a slitted motor, for example with the stator assembly comprising a stator core that defines slots for location of the stator winding. The stator winding may comprise a double layer stator winding. The stator winding may comprise a single layer stator winding. The stator winding may comprise a distributed stator winding, for example with each coil of the stator winding comprising a first arm located at a first winding position about a periphery of a stator core of the stator assembly, and a second arm located at a second, different, winding position about the periphery of the stator core assembly. The first winding position may be spaced apart from the second winding position along the periphery of the stator core, with at least one intermediate winding position between the first winding position and the second winding position. The first winding position may be spaced apart from the second winding position along the periphery of the stator core, with exactly one intermediate winding position between the first winding position and the second winding position. The stator winding may comprise a toroidally wound winding, for example with first and second arms of each coil of the stator winding located at a same winding position about a periphery of the stator core. Toroidal windings may lead to shorter end windings and lower coil loss. The bearingless motor may comprise control electronics configured to energise the stator winding. The control electronics may comprise at least one of a dual purpose no voltage inverter, two isolated three-phase inverters, a six phase inverter, and six independently controllable H-bridges. A third aspect of the present invention provides an air moving device comprising a bearingless motor according to the second aspect of the present invention. The air moving device may comprise any of a fan, an airflow heater, an air purifier, and an air humidifier. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an example of a bearingless motor; Figure 2 is an enlarged view of a portion of the bearingless motor of Figure 1; Figure 3 shows an example stator winding configuration; Figure 4 shows a table corresponding to the example stator winding configuration of Figure 3; Figure 5 shows a schematic view illustrating the example stator winding configuration of Figure 2 connected to control electronics; Figure 6 shows example stator field harmonics for torque production; Figure 7 shows example stator field harmonics for force production; Figure 8 shows a table illustrating combinations of rotor pole pair number and corresponding modulation factors; Figure 9 shows an example of an air moving device comprising the bearingless motor of Figure 1; and Figure 10 shows an example of an alternative stator winding configuration. DETAILED DESCRIPTION A bearingless motor 10 is illustrated schematically in Figure 1. The bearingless motor 10 comprises a rotor 12, a stator assembly 14, and control electronics 16. The rotor 12 comprises a magnet, or a plurality of magnets, having sixteen magnetic poles 18. Thus the rotor 12 comprises a pole pair number, p, of eight. The rotor is annular in form. The stator assembly 14 comprises a stator core 20 and a stator winding 22. The stator core 20 is generally annular in form, and the rotor 12 sits outside the stator core 20. The stator core 20 is a slotless stator core, formed without any teeth. The stator winding 22 is illustrated schematically in Figure I through 5. The stator winding 22 is formed from thirty coils 24, with the number of coils 24 being equal to 2(2p-1), as will be discussed in more detail hereinafter. The coils 24 are split into a first set of coils 26 and a second set of coils 28. The first set of coils 26 has a total of fifteen coils 24, which is equal to (2p-1). The first set of coils 26 defines a three-phase winding, with a first subset 30 of five coils 24 in phase A, a second subset 32 of five coils 24 in phase B, and a third subset 34 of five coils 24 in phase C. Each of the coils 24 in the first subset 30 are connected in series with one another. Each of the coils 24 in the second subset 32 are connected in series with one another. Each of the coils 24 in the third subset 34 are connected in series with one another. The second set of coils 28 has a total of fifteen coils 24, which is equal to (2p-l). The second set of coils 28 defines a three-phase winding, with a first subset 36 of five coils 24 in phase A, a second subset 38 of five coils 24 in phase B, and a third subset 40 of five coils 24 in phase C Each of the coils 24 in the first subset 36 are connected in series with one another. Each of the coils 24 in the second subset 38 are connected in series with one another. Each of the coils 24 in the third subset 40 are connected in series with one another. The first subset 30 of the first set of coils 26 is connected in parallel with the first subset 36 of the second set of coils 28 to form a first parallel winding branch 42 for phase A. The second subset 32 of the first set of coils 26 is connected in parallel with the second subset 38 of the second set of coils 28 to form a second parallel winding branch 44 for phase B. The third subset 34 of the first set of coils 26 is connected in parallel with the third subset 40 of the second set of coils 28 to form a third parallel winding branch 46 for phase C. The arms of each of the first parallel winding branch 42 for phase A, the second parallel winding branch 44 for phase B, and the third parallel winding branch 46 for phase C, that correspond to the first set of coils 26, are connected to one another at a neutral point N in a star configuration. The arms of each of the first parallel winding branch 42 for phase A, the second parallel winding branch 44 for phase B, and the third parallel winding branch 46 for phase C, that correspond to the second set of coils 26, are each connected to one of three legs of a suspension inverter 62 of the control electronics 16. Points between the arms of each of the first parallel winding branch 42 for phase A, the second parallel winding branch 44 for phase B, and the third parallel winding branch 46 for phase C, are each connected to one of three legs of a drive inverter 60 of the control electronics 16. A distribution of the first set of coils 26 and the second set of coils 28 about a periphery of the stator core 20 is illustrated in Figures 1 and 2. The coils 24 are arranged in a double layer, such that each coil 24 has a first portion 48 located in a radially outer position 50 with respect to the stator core 20, and a second portion 52 located in a radially inner position 53 with respect to the stator core. Each coil 24 spans two winding positions, such that the first portion 48 is located in the radially outer position 50 of a first winding position 54, and the second portion 52 is located in the radially inner position 53 of a second winding position 56 that is spaced circumferentially from the first winding position 54 with an intermediate winding position 58 therebetween. Thus each coil 24 has a span of two winding positions. Each coil 24 of the first set of coils 26 is located at an odd numbered winding position, whilst each coil 24 of the second set of coils 28 is located at an even numbered winding position. Generalised stator winding configurations can be seen in Figures 3 and 4. In the example of Figure 3, each coil spans three winding positions. Input and output connections for phase A for each of the first set of coils 26 and the second set of coils 28 are shown. Similar input and output connections are envisaged for phases B and C, with an input connection for phase B for the first set of coils 26 starting at winding position seventeen, and an input connection for phase C for the first set of coils 26 starting at winding position seven. The term “switching function” in Figure 3 relates to application of a modulation factor to the stator winding 22, as will be discussed in more detail hereinafter, and may also be referred to as an inversion state. In the example of Figure 4, start coil positions, phases, and switching functions, of the coils 24 are shown. It will be appreciated that the configuration shown in Figure 3 can be applicable to distributed double layer windings with any number of span, or alternatively to a single layer toroidal winding. The control electronics 16 comprises a drive inverter 60, a suspension inverter 62, and a controller 64. Each of the drive inverter 60 and the suspension inverter 62 is a three-phase inverter. With the aforementioned connection of the drive inverter 60 and the suspension inverter 62 to the first set of coils 26 and the second set of coils 28, a dual purpose no voltage (DPNV) inverter is achieved. The controller 64 is connected to the drive inverter 60 and the suspension inverter 62, and is configured to control the drive inverter 60 and the suspension inverter 62 to energise the stator winding 22, as will be described in more detail hereinafter. In use, the bearingless motor 10 is controlled to produce a first magnetic field for generating torque to act on the rotor 12, and to produce a second magnetic field comprising a component for generating a translational force to act on the rotor 12. What follows is, in part, a general discussion applicable to bearingless motors, and so reference numerals may be omitted in places for sake of clarity. It has previously been proposed that to produce a torque, the number of pole pairs, p, of the rotor of a bearingless motor and the number of pole pairs of a magnetic field generated by energising the stator winding must be the same, with maximum torque per amp achieved when the rotor magnet field angle, a, and the armature field angle, P, are aligned. It has also previously been proposed that to produce a translational force to act on the rotor, the number of pole pairs, q, of a magnetic field generated by energising the stator winding, must be equal to p±l. It has been found, however, for relatively high numbers of pole pairs, p, for example pole pair numbers of six and above, that controllable translational forces can be difficult to produce. In particular, at any point on the surface of a rotor of a bearingless motor, there is shear stress in a circumferential direction. A magnitude of the shear stress is given by the product of the air-gap normal (radial) flux density produced by the rotor magnet and the tangential field strength produced by the axial current in the winding at that angle. This can be represented by: = Considering the shear stress on the rotor surface at a particular angle, 0, then the shear stress can be composed into two components: 5¾ = — <7 sin 9 = ¢7 cos 0 The total circumferential force on the rotor is then given by: Fc = Jo~" BnHt rd0 where la is the axial length of the rotor in a direction orthogonal to the plane of the rotor field, and r is the radius of the rotor. Thus: _ rr >57 Fx = lt, L” ayrd® = lc ,L —asm &rdd zv tv y tv v- y This component will sum to zero for all space harmonics of a except a component with a one pole pair variation, for example p±l. The shear stress can also be represented as: a = B„ cos(p8 - a) cos[qff — a = Bn cos(»S — a) .¾ cosfgff — F) Thus to produce translational force the magnetic field generated by energising the stator winding must have a harmonic component of one more or one less poles than the rotor pole number p. To modify the original p pole pair magnetic field produced by the stator winding to generate torque, a modulation factor, m, is introduced. Then the tangential component of H becomes: — $) cos(m5) = ((cos(p + m)5 — / ?) + cos((p — m) 9 — ^)) To introduce translational force, m = 1, orm = -l. For example, considering a rotor with four pole pairs, p = 4, then a magnetic field of 3 pole pairs of 5 pole pairs is needed to provide a translational force on the rotor. By introducing a modulation factor of m = 1: a. ... .,. = —-— (cos(5S — p) + cos{3 0 — / 3)) cos(40 — a) R H ■ = (cos(5S — / 3) + cos(3 5 — p)) cos(40 — a) The shear stress is then given by: er = (cos(50 — / 3) + cos(3 5 — / ?)) cos(49 — a) B„Htr , . . o' = ~ [costW — p — a) + cosfd + a — / 3) + cos(70 — a — / 3) + cos(—0 + a — £?)] 4' Only the two pole component gives rise to a net translational force. tr = —[cos(0 + a — p) + cos(—0 + a — ] 4 [cos5cos(a — ^) — s'indsin (a —• p} + cos(—S')cos (a — / ?) — sin(— S) sin (er — / 3)] [cos£cos(a — / 01 Then Fx becomes zero, as: | cosOsin&dd = 0 -■'o This means that, if the modulation factor is in the cos0 direction then force can only be produced in the y direction. The magnitude can be changed by changing the phase angle of the current or B, but force in the x direction cannot be controlled. The underlying reason for this is that the 2 pole stress field is produced by two contra-rotating 2 pole fields, which combine to produce a pulsating field. The present invention therefore introduces a modulation factor of 2p±l to generate a magnetic field comprising a component having q pole pairs for generating a translational force to act on the rotor, wherein q=p±l, as will now be described. For the rotor 12 of the bearingless motor 10 of Figure 1, which has 8 pole pairs, p, then: cos(p0 — / 3) cos(m&) = f(cos(p + m)ff — / 3) + cos((p — m) 9 — / 3)) With m = 2p-l = 15, then: Jftcos(80 — / 3) cos(l50) = (cos(70 + T cgs(23 9- / 3)) The following component is a harmonic corresponding to q=p-l: cos(70 + The shear stress is then given by: a =...............—.................- (cos(70 + + cos(23 0 — / 3)) cos(80 — a) c = [cos(15S — a + + cos(-S + + a) + cos(310 — a — / ?) + cos(150 - ? - ^)] As before, only the two pole component gives rise to a net translational force. Translational force in the y-direction is given by: Fv = jj ” = la jg” -^cqs (S + $ + a)cos8rd8 Fy = (cost? cos(a + $) cosd — sin£siii(a + / ?)cos&)rd& = la cos(a + Translational force in the x direction is given by: r L 1(^-^005(0 + 0 + a)«nt?rd0 * Y - u 4. ’ ' 2 JT S J-f * f = t, L“ (cos0 cos(« + / ?)sin &— sin0sin(a + 0)sin0)rd0 = — sin(a A -1¾ As can be seen, the magnitude of the force can be controlled by controlling the magnitude of Ht and the direction of the force can be controlled by controlling its angle relative to the rotor 12. To keep the force constant in space, a+p need to be kept constant. As above, there are the following components of the electric and magnetic fields to consider: SK cos(pff — a) and cos(q5 + $) The rotor at any one time is at an angle: where ©r is the rotational velocity of the rotor magnetic field and ipr is the rotational angle of the rotor magnetic field relative to a fixed point in space. The stator H field to produce translational force is at: = togt — where ©h is rotational velocity of the stator magnetic field and (pu is the rotational angle of the stator magnetic field relative to the same fixed point in space as the rotational angle of the rotor magnetic field. Then for a+p to be kept constant: a + / 3 = (t<jr + — tpn = constant^ Thus COr = - ©H. It is also desirable to keep the translational force rotating with the rotor 12. As above, there are the following components of the electric and magnetic fields to consider: cosfpS — a) and H* cos(g5 + ^) The rotor at any one time is at an angle: a = where ©r is the rotational velocity of the rotor magnetic field and q>r is the rotational angle of the rotor magnetic field relative to a fixed point in space. The stator H field to produce translational force is at: P — — where coh is rotational velocity of the stator magnetic field and cpn is the rotational angle of the stator magnetic field relative to the same fixed point in space as the rotational angle of the rotor magnetic field. Then for a+p to be kept constant: a + P = + = -r constant Thus COH = - 2(Dr. Taking the above into consideration, the control electronics 16 of the bearingless motor 10 can be used to energise the stator winding 22 of the stator assembly 14 to generate appropriate magnetic fields to both produce torque and translational force to act on the rotor 12. To generate a first magnetic field for generating torque to act on the rotor 12, the drive inverter 60 supplies current in phase, and at the same magnitude, to the first set of coils 26 and the second set of coils 28. As can be seen, current flowing from one leg of the drive inverter flows in the same direction through the arm of the first parallel winding branch 42 for phase A corresponding to the first set of coils 26, and the arm of the first parallel winding branch 42 for phase A corresponding to the second set of coils 28, in the same direction. Current flowing from one leg of the drive inverter flows in the same direction through the arm of the second parallel winding branch 44 for phase B corresponding to the first set of coils 26, and the arm of the second parallel winding branch 44 for phase B corresponding to the second set of coils 28, in the same direction. Current flowing from one leg of the drive inverter flows in the same direction through the arm of the third parallel winding branch 46 for phase C corresponding to the first set of coils 26, and the arm of the third parallel winding branch 46 for phase C corresponding to the second set of coils 28, in the same direction. To generate a second magnetic field for generating a translational force to act on the rotor , the modulation factor of 2p-l is applied to the stator winding 22 by inverting the current flowing through the second set of coils 28. In such a manner current is inverted in 2p-l, i.e. fifteen, coils 24. This is achieved by using the suspension inverter 62 to energise the stator winding 22, whilst the drive inverter 60 is also being used to energise the stator winding 22. In some examples, the suspension inverter 62 can be sued to energise the stator winding 22 without using the drive inverter 60 to energise the stator winding 22, to control the translational position of the rotor 22 without producing torque. The modulation factor can either be thought of as a cosine variation around the coils 24, where the minima and maxima of the cosine align with the coil positions, or as a discrete function of length equal to the number of coils which are inverted 2p±l times around the airgap between the rotor 12 and the stator assembly 14. Current injected using the suspension inverter 62 flows in a first direction through the arms of the first parallel winding branch 42 for phase A, the second parallel winding branch 44 for phase B, and the third parallel winding branch 46 for phase C, corresponding to the second set of coils 28. The current injected using the suspension inverter 62 flows in a second, opposite direction through the arms of the first parallel winding branch 42 for phase A, the second parallel winding branch 44 for phase B, and the third parallel winding branch 46 for phase C, corresponding to the first set of coils 26. Application of the modulation factor m of 2p-l in such a fashion provides the second magnetic field having a component having q = p-1 pole pairs, which can interact with the rotor 12 to provide a translational force to the rotor 12. Such a translational force can stabilise the rotor 12 in a plane orthogonal to a longitudinal axis of the rotor 12, when viewed in a direction along the longitudinal axis of the rotor 12. Example plots of field strength for harmonics of the stator field for torque production and force production in the bearingless motor of Figure 1 can be seen in Figures 6 and 7 respectively. Here the 8p harmonic is prevalent for torque production in Figure 6, whilst the 7p harmonic is prevalent for force production in Figure 7. It will be appreciated that force demand can be determined by monitoring of a position of the rotor 12 using an appropriate position sensor, with such information fed back to the control electronics 16. It will be appreciated that the above can be generalised for a bearingless motor comprising any number of pole pairs, p. Further examples of bearingless motor combinations, alongside associated numbers of windings, are shown in Figure 8. The above-mentioned techniques may find particular utility in bearingless motors having at least six pole pairs. Bearingless motors having a relatively high number of pole pairs, such as at least six pole pairs, may also find particularly utility at relatively low speed applications in view of relatively complex switching requirements. The bearingless motor 10 of Figure 1 is intended to be driven at less than 5,000 rpm. Relatively low speed bearingless motors may find particular utility in air moving devices, such as fans, purifiers, humidifiers, or airflow heaters. An exemplary fan 100 is illustrated in Figure 9. Although described above in relation to a slotless motor, the techniques discussed herein can also be applied to slotted motors. Similarly, although described above in relation to a double layer distributed stator winding, the techniques discussed herein can also be applied to a single layer toroidal winding. An exemplary toroidal winding scheme 200 is illustrated schematically in Figure 10. Whilst particular examples and embodiments have thus far been described, it should be understood that these are illustrative only and that various modifications may be made without departing from the scope of the invention as defined by the claims.

Claims

1. A method of controlling a bearingless motor comprising a rotor having p pole pairs, and a stator assembly comprising a stator winding, wherein the method comprises:energising the stator winding to generate a first magnetic field having p pole pairs for generating torque to act on the rotor; andenergising the stator winding to generate a second magnetic field comprising a component having q pole pairs for generating a translational force to act on the rotor, wherein q=p±l,wherein energising the stator winding to generate the second magnetic field comprises applying a modulation factor to the stator winding, the modulation factor comprising a factor of 2p±l.

2. A method according to Claim 1, wherein the stator winding comprises a first set of coils and a second set of coils, and the method comprises energising the first and second sets of coils in phase with one another to generate the first magnetic field.

3. A method according to Claim 2, wherein the method comprises energising the first and second sets of coils out of phase with one another to generate the second magnetic field.

4. A method according to any one of Claims 2 to 3, wherein the first set of coils and the second set of coils each form a three-phase winding.

5. A method according to Claim 4, wherein for each phase of the first set of coils, coils of the phase are connected in series with one another, for each phase of the second set of coils, coils of the phase are connected in series with one another, and coils of corresponding phases between the first and second sets of coils are connected in parallel with one another.

6. A method according to any one of Claims 2 to 5, wherein each of the first set of coils and the second set of coils has a cardinality of 2p±l.

7. A method according to any one of Claims 2 to 6, wherein the first and second sets of coils are interleaved with one another, such that every other coil of the stator winding belongs to a same one of the first and second sets of coils.

8. A method according to any one of Claims 1 to 7, wherein the second magnetic field comprises a first component having q pole pairs, and a second component having r pole pairs, wherein i=3p±l.

9. A method according to any one of Claims 1 to 8, wherein p is greater than or equal to 6.

10. A bearingless motor comprising:a rotor assembly comprising p pole pairs; and a stator assembly comprising a stator winding, wherein the stator winding is:configured to be energised to generate a first magnetic field having p pole pairs for generating torque to act on the rotor;configured to be energised to generate a second magnetic field comprising a component having q pole pairs for generating a translational force to act on the rotor, wherein q=p±l; andconfigured to be such that a modulation factor is applicable to the stator winding to energise the stator winding to generate the second magnetic field, the modulation factor comprising a factor of 2p±l.

11. A bearingless motor according to Claim 10, wherein the stator winding comprises a first set of coils and a second set of coils, and the first and second sets of coils are configured to be energised in phase with one another to generate the first magnetic field.

12. A bearingless motor according to any one of Claims 10 to 11, wherein the first set of coils and the second set of coils each form a three-phase winding.

13. A bearingless motor according to Claim 12, wherein for each phase of the first set of coils, coils of the phase are connected in series with one another, for each phase of the second set of coils, coils of the phase are connected in series with one another, and coils of corresponding phases between the first and second sets of coils are connected in parallel with one another.

14. A bearingless motor according to any one of Claims 10 to 13, wherein each of the first set of coils and the second set of coils has a cardinality of 2p±l.

15. A bearingless motor according to any one of Claims 10 to 14, wherein the first and second sets of coils are interleaved with one another, such that every other coil of the stator winding belongs to a same one of the first and second sets of coils.

16. A bearingless motor according to any one of Claims 10 to 15, wherein the second magnetic field comprises a first component having q pole pairs, and a second component having r pole pairs, wherein r=3p±l.

17. A bearingless motor according to any one of Claims 10 to 16, wherein p is greater than or equal to 6.

18. A bearingless motor according to any one of Claims 10 to 17, wherein the motor is a slotless motor.

19. A bearingless motor according to any one of Claims 10 to 18, wherein the stator winding comprises a distributed stator winding.

20. An air moving device comprising a bearingless motor according to any one of Claims 10 to 19.

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