Background to the Invention A number of DC motor systems exist for use in electric vehicles of various kinds. In each case the motor includes a stator with an arrangement of electromagnets which are sequentially energised and de-energised in relation to a rotor with an arrangement of permanent magnets, in order to turn the wheels of the vehicle. Some of these systems attempt to retrieve the energy stored in the electromagnets, but in general the energy is wasted.

There has long been a need for an electric drive wheel that does not necessarily require mechanical gears, chains or belts and does not necessarily have a central hub and axle.

Such a wheel can be fitted with a tyre and used in an electric vehicle such as a scooter.

Ideally the wheel should be lightweight and energy efficient, and capable of delivering useful torque under a range of operating conditions.

Summary of the Invention It is an object of the invention to provide an improved DC motor for use in an electric wheel, or at least to provide a useful alternative to existing motors.

In one aspect the invention may broadly be said to consist in a motor including: a stator having a ring of electromagnets with radially aligned poles, a rotor placed concentric with and adjacent to the stator, having a ring of permanent magnets with radially aligned poles, and a switching circuit that connects a power supply to the stator according to relative orientation of the rotor and stator, creating electromotive torque by magnetic interaction between the poles, wherein the switching circuit connects the electromagnets to the power supply in groups that receive energy from and then return energy to the power supply as the motor runs.

Preferably each group of electromagnets is energised by connection to the power supply and then de-energised through a reverse connection to the power supply. Further, each group of electromagnets is energised and then de-energised by connection to the power supply while other groups of electromagnets are disconnected from the power supply.

Further, two groups of electromagnets are placed alternately around the circumference of the stator, the groups being successively energised and de-energised by alternate connections to the power supply.

In one embodiment the switching circuit operates an eight part cycle including: energising and de-energising one group of electromagnets with an initial polarity, energising and de- energising the other group of electromagnets with the initial polarity, energising and de- energising the first group of electromagnets with reversed polarity, and energising and de- energising the second group of electromagnets with reversed polarity.

Preferably the motor includes sensors that provide the switching circuit with information on orientation of the rotor. Preferably each electromagnet is a solenoid formed on a hollow cylindrical core. The power supply is not necessarily included.

In another aspect the invention may be said to consist in a motor including: a stator having a ring of electromagnets with radially aligned poles, a rotor placed concentric with and adjacent to the stator, having a ring of permanent magnets with radially aligned poles, and a switching circuit that connects a power supply to the stator according to relative orientation of the rotor and stator, creating electromotive torque by magnetic interaction between the poles, wherein substantially all of the electromagnets have at least partially hollow cores.

Preferably the electromagnets are formed by solenoids having substantially cylindrical hollow cores. Preferably each core has a hollow centre and the diameter of the hollow centre is from 10% to 90% of the diameter of the core. In one embodiment each core has a hollow centre and the diameter of the hollow centre is approximately one third or one half the diameter of the core. The switching circuit connects the electromagnets to the power supply in groups that receive energy from and then return energy to the power supply.

The invention may also be said to consist in an electric wheel incorporating a motor as defined above.

List of Figures Preferred embodiments of the invention will be described with reference to the accompanying drawings, of which: Figure 1 shows the housing of a DC motor, Figure 2a and 2b are cross-sectional views of the motor showing electromagnets in the stator and permanent magnets in the rotor, Figures 3a, 3b, 3c, 3d are further views of the stator and rotor, Figures 4a, 4b, 4c are views of a solenoid that may be used to form the electromagnets, Figure 5 schematically shows a switching circuit that may be used to control energising and de-energising of the electromagnets in relation to a power supply, Figure 6 is table summarising operation of the switching circuit, Figures 7a, 7b, 7c, 7d show four stages in operation of the switching circuit to control energising of the electromagnets, and Figures 8a-h show detail of current flow in an 8 part cycle of energising and de- energising electromagnets in the stator.

Detailed Description of Preferred Embodiments Referring to the drawings it will be appreciated that a DC motor according to the invention can be implemented in many forms for a range of different purposes. The overall construction and operation of a DC motor will be known to a skilled person and details have not been included in the examples that follow. The example given here involves construction of the motor in the form of a wheel.

Figure 1 is an external view of a DC motor configured for use in an electric wheel and centered around an axis 10. In this example, the mechanical parts of the motor are contained by a housing having an outer rim 11, an inner rim 12 and two face plates 13, only one of which is visible, held together by a series of bolts 14. A range of different or more complex housings are possible. A tyre may be fixed to the outer rim when the motor is used in an electric wheel. The motor may also be fixed to a larger structure in various ways, via the inner rim or the face plates, for example, or may be combined to operate

side-by-side or inline with other motors of the same kind. The space within the inner rim may remain empty or may be used to hold other components, such as switching circuitry and/or a battery.

Figures 2a and 2b are sectional views through the motor in Figure 1, perpendicular to the axis 10. Figure 2a shows a stator 20 having a series of circumferentially arranged electromagnets 21 mounted on the inner rim 12. A rotor 22 has a series of circumferentially arranged permanent magnets 23 mounted on outer rim 11. The electromagnets 21 are energised from a power supply through a switching circuit, neither of which are shown in this diagram. The permanent magnets 23 have alternate polarity around the rotor. Locating holes 24 are provided for bolts 14 and for a series axles that carry rollers which allow relative motion between the stator and rotor about axis 10. In use, a pneumatic tyre 25 may be mounted on outer rim 11 and form the rotatable part of a wheel.

In this example the permanent magnets 23 are cylindrical disk magnets set evenly in a non- magnetic matrix 18 around the internal circumference of the rim 11 of the rotor 22. The electromagnets 21 are evenly spaced around the outer circumference of the rim 12 of the stator, and set in a matrix of non-magnetic material 19. The permanent magnets and electromagnets are radially oriented around axis 10 and in close proximity but separated by an airgap 15. The number and arrangement of magnets and electromagnets and other components may be varied as required for the vehicle or other device in which the motor is used.

Figure 2b is alternative section through the motor in Figure 1. A radial cavity 26 in the stator 20 contains one or more sensors 27 that may be used to determine the relative orientation of the stator and rotor 22, such as a pair of Hall effect sensors that generate signals in response to movement of the magnets 23 in the rotor around the stator. The sensors are calibrated to generate an output signal that depends on the polarity of the closest magnet on the rotor. A circumferential cavity 28 provides a passage for wiring of the electromagnets in the stator with apertures 29 leading from the cavity to each of the electromagnets. Rollers 30 are mounted on the stator and extend across the airgap 15.

Rotor 22 rotates around the stator and is maintained by the rollers at a fixed distance from the stator.

Figure 3a shows the outside of the stator viewed radially inwards towards the axis 10 with the rotor having been removed. Outer ends of the electromagnets 21 can be seen. Face plates 13 are held in place by bolts 14 through alternate locating holes 24, with one of the face plates having been removed to remove the rotor, as indicated in dashed outline. Pairs of rollers 30 are held in place in cavities 31 on the stator in by bolts 32.

Figures 3b, 3c, 3d are sectional views perpendicular to the axis 10, through both the stator and rotor. Fixed magnets 23 are mounted on the outer rim 11 and in this example, a tyre 25 has been added outside the rim. The rollers 30 are held in place on the stator by bolts 32 and run in circumferential cavities 33 on the rotor to maintain the airgap 15.

Electromagnets 21 are mounted on the rim 12 of the stator by non-magnetic bolts 44, surrounded by matrix 19. Electrical wiring reaches each electromagnet from cavity 28 through a respective aperture 29.

Figures 4a, 4b, 4c show a preferred design of the electromagnets 21 and how they may be mounted on the stator 20. Each electromagnet has a winding 40 on a magnetisable core 41, preferably with a hollow centre 42. In this example the electromagnet is shown in the form of a solenoid having a cylindrical core with a cylindrical hollow centre, although a range of different shapes and dimensions of the core may be used in practice. Bolt 44 passes through the core and a base plate 45, and through the rim 12 of the stator. Head 46 of the bolt engages one end of the winding while a shaped nut 46 engages the bolt on inside surface of the rim 12 and serves to hold the electromagnet in place in the stator. The diameter of the hollow centre of a cylindrical electromagnet may range from approximately 10% to approximately 90% of the diameter of the core but is preferably approximately one-third or one half of the diameter of the core.

The hollow centre 42 of each core 41 may assist the magnetic performance of the respective electromagnet 21. The shape of the local magnetic field produced by energisation of the winding 40 is determined in part by the shape of the core. The shape of the magnetic fields around the stator determines in part the nature of the interaction between the electromagnets 21 and the permanent magnets 23 as the motor produces torque. The permanent magnets move in and out of alignment with the electromagnets as the rotor is moved around the stator. As the winding 40 in each electromagnet is de-

energised and its magnetic field collapses, the hollow or at least non-magnetisable centre of the core reduces registration of the nearby permanent magnet with the electromagnet.

The permanent magnet cannot register with the nearby electromagnet and is not attracted to the centre of the electromagnet, thereby allowing a relatively easy pass of the permanent magnets around the stator.

Figure 5 schematically shows a switching circuit that may be used to energise the electromagnets on the stator. A processor 50 receives input from sensors hl, h2 that generate signals dependent on the relative orientation of the stator and rotor, and also from a speed setting device pl. The processor determines the orientation and speed of the rotor from the sensors and controls the generation of current pulses into the electromagnets of the stator by driver chips 51 and 52, in accord with the required speed setting. The electromagnets are provided in two groups A and B that are energised through terminals cl, c2 and c3, c4 respectively. Driver 51 determines the current supplied to group A electromagnets through four FETs 53 controlled by gate lines la, 2a, 3a, 4a. Driver 52 determines the current supplied to group B electromagnets through four FETs 54 controlled by gate lines lb, 2b, 3b, 4b. A DC power supply is connected across terminals +VDC and GRND and regulated by a voltage regulator 55 also under control of the processor. Each of FETs 53 and 54 are reverse shunted by respective diodes 56 and 57, to enable current flow back into the power supply when the electromagnets in groups A and B are de-energised respectively.

Figure 6 is a table summarising the operation of processor 50 when controlling the drivers 51 and 52 in response to signals from the sensors hl, h2. Figures 7a, 7b, 7c, 7d are schematic diagrams indicating the relationships between the rotor and stator for each part of the table in Figure 6. They show a number of electromagnets 21 on a small portion of the stator 20 with their connections to the switching circuit of Figure 5, and a number of permanent magnets 23 on a small portion of the rotor 22. A few electromagnets from each of groups A and B are shown. Motion of the rotor with respect to the stator is indicated by the large arrow in each diagram. Pulses of direct current are passed through the windings of each electromagnet from the power supply to determine the polarity of the electromagnets at an appropriate time in relation to the permanent magnets, to cause movement of the rotor through magnetic propulsion and attraction.

In the preferred embodiment, the polarity of the electromagnets is calculated to change when the mid point between adjacent permanent magnets as approximately midway between the spacing of adjacent electromagnets. The North pole of an electromagnet is created to attract the South pole of an approaching permanent magnet, while the South pole of an electromagnet is created to attract a North pole of an approaching permanent magnet.

In general, while the electromagnets of group A are energised, the electromagnets of group B are de-energised and return energy to the power supply, and vice versa.

In Figure 7a for example, when the sensors hl and h2 produce high and low signals respectively, the electromagnets in group B are energised by high signals at gate lines la, lb sending current from the power supply through terminals c3, c4. Meanwhile the electromagnets in group A are de-energised. Back emf and currents generated in the windings of electromagnets in group A may be returned to the power supply through cl, c2 and appropriate diodes 56,57.

Figures 8a-8h indicate in more detail how the electromagnets are energised through the switching circuit, and specifically how back emf caused by de-energisation of an electromagnet may be used to partly recharge the power supply. The paths of current flow along the main lines and the switching circuit are indicated by additional shading. These diagrams show an 8-part cycle in which the coil A of an electromagnet in group A is first energised and then de-energised, followed by coil B of an electromagnet and group B, after which the coils are again energised and de-energised in sequence but with reversed polarity. Other groupings of the electromagnets and other cycles are also conceivably useful.

Figure 7a-North and South Pole sensed Hall sensor #1 (Hl) senses a North Pole; Hall sensor #2 (H2) senses a South Pole. HI triggers 7C7 which control IC4 ald IC4, to switch MOSFETS which power Coil A creating a South magnetic field thus repelling the North Magnetic Pole of the permanent magnet; at the same time the following South Permanent Magnet is attracted. The South Pole of the permanent magnet switches H2 low. Coil B is de-energised and resting.

Figure 7b-Back EMF generation of Coil A An IC timing circuit (IC3) also controls IC4 and IC5 switching off the coil driving

MOSFETS, which allows the current generated by the Back EMF to flow into the power supply through another MOSFET. The Back EMF is caused by the collapsing magnetic field of Coil A and is further increased by the approaching South Pole of the permanent magnet.

Figure 7c-North and North Pole sensed Coil A is now resting. Hl and H2 are both high because they are both over North Poles.

The IC3 now allows the MOSFETS to receive current from the power source. Hl and H2 trigger IC4 and ICS switching on the MOSFETS thus supplying current to Coil B. This creates a North Magnetic Pole in Coil B repelling the North Pole of the permanent magnet and attracting the following permanent magnet South Pole.

Figure 7d-Back EMF generation of Coil B IC3 switches off the coil driving MOSFETS and allows current generated by the Back EMF to flow into the power supply through another MOSFET. The Back EMF is caused by the collapsing magnetic field of Coil B and is further increased by the approaching South Pole of the permanent magnet.

Figure 7e-South and North Pole sensed Hl senses a South Pole ; H2 senses a North Pole. The South Pole of the permanent magnet switches HI low. H2 triggers the IC's to switch on MOSFETS which power Coil 1, creating a North magnetic field thus repelling the South magnetic pole of the permanent magnet, at the same time the following North permanent magnet is attracted. Coil B is resting.

Figure 7f-Back EMF generation of Coil B IC3 switches off the coil driving MOSFETS and allows current generated by the Back EMF to flow into the power supply through another MOSFET. The Back EMF is caused by the collapsing magnetic field of Coil A and is further increased by the approaching North Pole of the permanent magnet.

Figure 7g-South and South Poles sensed Coil A is now resting. HI and H2 are both low because they are both over South poles.

The IC's switch on the MOSFETS thus supplying current to Coil B. This creates a South

magnetic pole in Coil B repelling the South Pole of the permanent magnet and attracting the following permanent magnet's North Pole.

Figure 7h-Back EMF generation of Coil B IC3 switches off the coil driving MOSFETS and allows current generated by the Back EMF to flow into the power supply through another MOSFET. The Back EMF is caused by the collapsing magnetic field of Coil B and is further increased by the approaching North Pole of the permanent magnet.