BACKGROUND OF THE INVENTION The electric motor is one of the many advantageous uses of electricity. One of the most common electric motors used in electrical appliances is the direct current (DC) motor. A common electric motor is basically comprised of a rotor which is free to rotate within a stator, the rotor and the stator being provided with a plurality of magnetic poles. Some or all of the magnetic poles may be provided with electric windings through which electric current flows, thus inducing magnetic fields. As the rotor rotates with respect to the stator, it is necessary to switch the direction (polarity) of the magnetic fields by switching the direction of current flow within the windings.

In DC motors it is necessary to physically change the direction of the current within the windings in order to change the polarity of the poles. It is most common for the poles with windings to be located on the rotor. It is therefore necessary to provide the DC motor with a switching arrangement, such as a commutator and brushes, in order to switch the direction of the current flow in the windings of the rotor whilst the rotor rotates about its own axis. Such a motor is typically complicated in construction owing to the requirement for the additional mechanical components to ensure the switching of the electric current flow in the windings of the rotor.

Furthermore, the brushes are prone to wear and as a consequence the connection between the commutator and brushes becomes unreliable over time. Such wear reduces the efficiency of the motor by reducing the contact between the commutator

and the brushes. It is therefore often necessary for the brushes to require replacement on a regular basis. A failure to replace the worn brushes will eventually result in contact between the commutator and the springs biasing the brushes against the commutator, leading to destruction of the commutator.

It is also not uncommon for sparking to occur between the brushes and commutator, which may prove damaging to the motor and may cause significant interference with electronic equipment such as computers and with radio reception in the vicinity of the motor.

Due to the open connection between the brushes and the commutator, it is also impossible to use such a motor in a high moisture environment or in an application where the motor is submersed in a liquid without the need for sealing the motor from the environment within which it is to operate. In a standard direct current electric motor, in order to eliminate"shorting"the brushes must not have contact with liquid, and may be protected by rubber or plastic seals. However, as the shaft turns in the seal, the propensity for leaks is common due to the wear of the seal and/or the rusting of the seal retainer.

Furthermore, in a standard direct current electric motor the motor bearings usually require continual lubrication, thereby adding to the maintenance requirements if the motor is to continue operating as desired over a period of time.

It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY OF THE INVENTION According to the present invention there is provided an electric motor including:

a rotor capable of rotation about an axis, said rotor including a plurality of permanent magnets positioned around said axis of rotation; a stator having a plurality of coils capable of producing magnetic fields; a control means for switching the magnetic fields of said stator coils; wherein the rotor is mounted rotatably with respect to the stator and the control means switches the magnetic fields of the stator coils such that, in use, the rotor is caused to rotate about said axis of rotation.

In one preferred form, the present invention provides an electric motor including: a rotor capable of rotation about its main axis; a plurality of permanent magnets positioned around said main axis of the rotor on a face of the rotor such that magnetic poles of the magnets are parallel to the main axis of the rotor; a stator having a plurality of stator coils capable of producing magnetic fields with north-south orientation substantially parallel to the main axis of the rotor; wherein the rotor is mounted rotatably with respect to the stator and a control means switches the magnetic field of the stator coils such that, in use, the rotor is caused to rotate about its main axis.

Preferably the rotor comprises a disc, said disc capable of rotation about its main axis and including a plurality of permanent magnets positioned around said main axis on a face of the disc such that magnetic poles of the magnets are parallel to the main axis of the disc.

It is also preferred that the electric motor is provided with an even number of permanent magnets positioned on the rotor, and wherein the magnets are positioned around the axis of the disc such that the polarity of the magnets alternates around the axis. That is, the polarity of a permanent magnet is opposite to the polarity of the adjacent permanent magnets to each side.

It is preferred that the control means is capable of controlling the direction (polarity) of the magnetic fields of the stator coils by switching the direction of the current within the windings of the stator coils.

The electric motor may also be provided with sensors to detect the rotational position of the disc, such that the control means may switch the direction of the magnetic fields of the stator coils according to the position of the disc.

Advantageously, a direct current electric motor designed according to the present invention does not require brushes or any other form of mechanical commutation typical of conventional direct current electric motors.

Furthermore, a direct current electric motor designed in accordance with the present invention can offer improved energy efficiency and reliability when compared to conventional direct current electric motors which rely upon a mechanical commutator and brushes.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 illustrates a cross-sectional side view of a preferred embodiment of a direct current electric motor in accordance with the present invention;

Fig. 2 illustrates a cross-sectional view of the motor depicted in Fig. 1 along SectionA-A; Fig. 3 illustrates a cross-sectional view of the motor depicted in Fig. 1 along Section B-B; Fig. 4a illustrates a plan view of a sensor disc used in the present invention to indicate the position of the rotor of the electric motor; Fig. 4b illustrates a cross-sectional side view of the sensor disc depicted in Fig. 4a.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to Fig. 1, a cross-sectional side view of an electric motor 1 which employs the principles of the present invention is shown. The electric motor 1 is provided with a motor casing 3 in which a rotor having a disc 10 is mounted for rotation about a longitudinal axis of shaft 11. The shaft 11 is mounted along a longitudinal axis 12 of the motor and is mounted for rotation within bearings 4 and 5.

A load may be connected to the outer end of the shaft 11. In the embodiment depicted, the electric motor is shown in combination with an impeller 2 to provide a liquid agitator. The agitator is attached to the shaft 11 by a suitable fixing means, such as mounting screw 13.

Referring to Fig. 2, the rotor disc 10 is provided with a plurality of permanent magnets 7 spaced around the face of the disc. In the preferred embodiment shown the magnets 7 are mounted in recesses in the face of the rotor disc 10. The permanent magnets 7 may be rare earth magnets. The permanent magnets are preferably equidistant from the centre of the disc and may be spaced with equal angularity around

the axis of the disc. However in one form of the invention it may be desirable for at least one magnet to be offset in position in order to aid in the starting of the motor.

That is, whilst the majority of the permanent magnets may be equi-spaced around the axis of the disc, one magnet may be positioned such that it is closer to one adjacent magnet than it is to the other adjacent magnet on its opposite side. For example, magnet 7e may be positioned closer to magnet 7f than to magnet 7d. The permanent magnets 7 are mounted within the disc 10 such that the north-south axis of each of the magnets is parallel to the longitudinal axis of shaft 11. In the preferred embodiment illustrated there are provided eight magnets around the disc. However it should be noted that the number and/or size of the magnets may be varied according to the output requirements of the motor. The magnets are mounted such that the direction of their magnetic poles alternate around the circumference of the disc. That is, permanent magnets 7a, 7c, 7e and 7g have their north poles pointing in one direction perpendicular to the plane of the disc, whilst poles 7b, 7d, 7f and 7h have their north poles pointing in the opposite direction.

Referring to Fig. 1, there is further provided within the casing 3 a plurality of stator coils 8 which are affixed to the casing 3 by mounting pins or bolts 14.

Preferably the number of stator coils is equal to the number of permanent magnets.

The stator coils 8 are spaced near the circumference of the casing at the same radius from the shaft 11 as the permanent magnets 7, such that the permanent magnets 7 are capable of being positioned directly adjacent the stator coils 8.

Referring to Fig. 3, the coils 8 may be spaced with equal angularity around the central axis of the stator. However in one form of the invention it may be desirable for

at least one coil to be offset in position in order to aid in the starting of the motor.

That is, whilst the majority of the coils may be equi-spaced around the axis of the stator, one coil may be positioned such that it is closer to one adjacent coil than it is to the adjacent coil on its opposite side. For example, coil 8b may be positioned closer to coil 8c than to coil 8a.

Each of the coils 8 includes conductive wire wound around a core such that the magnetic field which is generated by an electric current flowing through the wires of the coil will induce a magnetic field which has a north-south pole which is parallel to, and preferably in alignment with, the magnetic axis of the permanent magnets 7 and the shaft 11. Preferably the coils are electrically connected to each other in a series/parallel configuration.

Each of the coils is connected to a control circuit (not shown). The control circuit is capable of switching the direction of current flow within the coils such that the polarity of the magnetic field produced by the coils can be alternated so that the permanent magnets 7 of the rotor may be attracted or repelled by the stator coils 8 at various stages during a complete cycle of the motor.

Preferably the control system is designed to ensure that there is an equal flow of current to each coil whilst also ensuring there is a minimum dispersion of heat.

Preferably the control system ensures the accurate speed of operation of the motor. In many applications it is desirable for the motor to operate at a speed of 1,000 revolutions per minute or greater.

There is also connected to the shaft 11, at the opposite end to the agitator, a sensor disc 15. The sensor disc is affixed to the shaft 11 with a suitable fixing means,

such as mounting screw 16. Referring to Figs. 4a and 4b, a plurality of small permanent magnets 17 are positioned in a face of the sensor disc. Preferably the number of permanent magnets 17 in the sensor disc 15 equals the number of permanent magnets 7 in the rotor 10. The permanent magnets 17 are configured around the sensor disc with alternating polarity in the same manner as the permanent magnets 7 are arranged in the rotor 10.

Referring to Fig. 1, within chamber 18 and located in the proximity of the sensor disc 15 there is provided a magnetic field sensor (not shown). The sensor is preferably a Hall-effect sensor which is capable of detecting the proximity of one of the small magnets 17. The Hall-effect sensor creates an electronic impulse when a magnet 17 passes within its proximity. The magnet sensor is connected to the control circuit so that the control circuit is capable of determining the rotational position of the rotor 10.

In use the electric motor is operated by supplying direct current to the stator coils 8 which in turn produce magnetic fields in the separate coils with alternating polarity. For example, coils 8a, 8c, 8e and 8g are provided with current from the control circuit such that the north poles of the coils are in a common direction parallel to the shaft 11, whilst the coils 8b, 8d, 8f and 8h are provided with current such that their north poles point in a common direction, opposite to those of coils 8a, 8c, 8e and 8g. This will result in the magnetic fields of the coils 8 and the magnetic fields of the permanent magnets 7 attracting or repelling each other and thereby resulting in the agitator 2, the shaft 11, the rotor disc 10 and the sensor disc 15 rotating until a permanent magnet 7 is at its closest proximity to an attracting coil 8. When the position detector detects the magnets 7 being at a certain position with relation to the

coils 8, the currents within the coils 8 are switched to switch the direction of the magnetic fields of the coils 8 by 180 degrees, such that permanent magnets 7 which were previously attracted to a coil 8, will now be repelled. This will rotate the disc 10 further, to a position where each permanent magnet 7 will approach another coil 8 to which it is now attracted. Thus if the magnetic fields of the coils 8 are switched at the appropriate moments in time by the control circuit, the magnets 7 within the rotating disc 10 will be repelled and attracted by different coils, thereby inducing further rotation of the disc.

The design of the present invention is advantageous in that it is a brushless motor since it is not necessary to transfer current to the rotor. It is therefore not necessary to use a mechanical commutator to switch the currents which induce the magnetic fields. The control circuit is able to switch the currents at the appropriate time with greater accuracy and reliability than a mechanical commutating system.

The result of this is that the electric motor produces very little static electricity which might interfere with sensitive electronic equipment or radio equipment in close proximity to the motor.

The preferred embodiment of the electrical motor is further advantageous in that it is possible to use the motor in moist environments. The only components requiring electricity within the casing of the motor are the coils, but the coils may be insulated so that liquid may freely flow inside the motor cavity. This removes the need for seals to protect the inside of the motor from moisture, and thus removes the problem of faults and deterioration of mechanical seals within a motor.

The preferred embodiment of the electric motor illustrated in Fig. 1 can be submersed and operated in a liquid without any detrimental effect. Whilst submersed, the liquid can flow continuously through the shaft bearings 4,5 and shaft cavity 20 of the motor so as to provide continuous self-lubrication of the shaft and bearings and a continuous cooling of the rotor, bearings, shaft and windings. In this application, mechanical seals need not be used. In contrast, in a standard direct current electric motor the motor bearings require lubrication and mechanical seals are typically employed so as to retain the lubrication, to prevent the ingress of contaminants to the bearings, and to prevent the ingress of moisture into the motor. The chamber 18 may be sealed by one or more seals, such as an O-ring seal 21 shown in Fig. 1, so as to prevent the ingress of moisture into the sensors and control circuitry of the motor.

It is possible for the motor of the preferred embodiment to be operated using very low DC voltages such as, for example, 35 volts or less. Therefore, in the event of faulty insulation of the coils, the operating voltages of the motor are low enough to prevent dangerous electrical shocks or arcing within the motor.

The design of the motor is further advantageous in that the motor is not susceptible to burn out if the motor is excessively loaded or jammed accidentally.

Advantageously, the motor may be configured to provide variable speed and/or bi-directional operation. It is possible to vary the speed of the motor by varying the magnitude of the voltage provided to the coils. It is also possible to change the direction of rotation of the motor by means of altering the polarity of the voltage provided to the coils.

Advantageously, the present invention enables a compact construction of electric motor. Due to the alignment of the magnetic axes of the coils and permanent magnets it is possible to produce a small, compact electric motor which is ideal for applications where there is limited space, such as in a"down the hole"bore pump used to pump water from a bore hole.

The above preferred embodiments of the present invention have useful applications in moist environments such as in refrigeration units, in washing machines and clothes driers.

The preferred embodiment of the present invention is particularly suitable for applications where it is desired to operate the motor within a liquid environment. An example of such an application is that disclosed in the inventor's United States Patent No. 5,191, 773 where the motor is submersed in a bath of cooling fluid. Further potential applications are in marine environments and in environments where there are combustile fumes and a consequent risk of ignition.

Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.