This invention is concerned with the design of a D.C. Motor which is particularly, but not exclusively, suited for use in fans which are of relatively small size and are intended for use within housings of computers or other electronic equipment for cooling purposes.

From one aspect this invention provides a brushless D.C. Motor incorporating an electronic commutation system incorporating a sensor for sensing the angular position of the rotor magnet and circuitry for controlling switching of the current to the stator windings, the electronic components being mounted on a printed circuit which is integrally moulded with a body providing a bearing housing for the rotor shaft. The integral formation of the printed circuit with the bearing housing for the rotor shaft enables the rotor to be mounted with very great accuracy (both linear and angular) with respect to components carried by the printed circuit. In particular, it is desirable that the body shouldprovide locating portions for predetermined positioning of the stator and the sensor, since the relationship of the rotor to the stator, and particularly to the sensor, is critical for optimum operation of the motor. In a preferred arrangement the locating portions include upstanding

posts which limit axial movement of the 'stator and locate within interpole slots in the stator to determine rotational position thereof. Desirably the locating portions will incorporate a mount for receipt of the sensor in one of two alternative positions. This enables the sensor to be mounted in the correct position for either rotation of the rotor, as required.

Ideally the bearing housing will incorporate a bearing assembly for the rotor shaft.

The body can incorporate locating means for retaining an end cap which is positioned over the end of the body to cover the electronic components. This end cap can locate over and seal the end of the bearing housing and thus, for example, retain lubricant.

The printed circuit will ideally be embedded within a body formed from moulded plastics. The preferred form of sensor comprises a Hall effect device.

The invention also extends to a stator and rotor assembly for a D.C. motor wherein the rotor incorporates an annular magnet surrounding the stator and one or more thin steel arcuate strips are positioned around the magnet to act as flux return paths between poles of adjacent magnetic

portions of the magnet.

The provision of these flux return strips obviates the need for the conventional fairly thick steel surrounding cage which provides a flux return path, resulting in economy in both weight and size.

Advantageously each steel strip will be stressed to provide frictional location of the magnet within the rotor housing. The magnet will then ideally be shaped to provide for the accurate positioning and location of each steel strip.

From a further aspect the invention provides a stator and rotor assembly for a D.C. motor wherein the rotor incorporates a magnet which surrounds the stator and embodies castellations or other shape modifications to create a desirable offset of the magnetic field.

Offsetting of the magnetic field can resuJLt in the achievement of advantageous supporting effects for the rotor with respect to the stator. This is particularly so if the magnet is physically positioned relative to the stator in a generally symmetrical attitude in the axial plane so that the magnetic field is axially offset from the centre plane of the stator windings. In order to create these complex shape forms, the preferred embodiment incorporates a moulded magnet. Such a construction has a further

advantage since a magnetising process may be used that reduces the stray magnetic flux on the surfaces not adjacent to the stator. This in turn reduces the need for a heavy flux return circuit, in order to reduce the stray flux on the outer surfaces of the motor.

The elimination of the thick, steel surrounding cage permits the use of alternative materials, such as moulded plastic, for the motor housing. The use of the moulding process permits complex forms to be incorporated which may be both functional and efficient,

In any of the stator and rotor assemblies defined above it is desirable for the rotor housing to ^" incorporate a flexible retaining rim enabling the magnet to be installed within the rotor housing as a snap fit.

The brushless D.C. motor of this invention as hereinbefore defined will ideally incorporate the forms of stator and rotor assembly of this invention as defined above. Since all rotating components may be produced as 'balanced' injection mouldings, the resultant light weight rotor assembly requires far less correction to achieve a given balance standard.

The invention may be performed in various ways and preferred embodiments thereof will now be described with reference to the accompanying drawings,

in which :-

Figure 1 is an exploded perspective view from ^■ one side of parts of an assembly for a brushless D.C. motor of this invention; Figure 2 is an exploded view from the other side of the same assembly; and

Figure 3 is a vertical cross-section through the assembly illustrated in Figures 1 and 2.

The motor assembly illustrated in the drawings incorporates a stator 1 and a rotor 2 together with a support body 3. The support body 3 incorporates a printed circuit on which is mounted various electronic components 4, including,a Hall effect device 5. The body 3 is integrally moulded about the printed circuit and part of the moulding defines a bearing housing 6 which receives a bearing system and a rotor shaft 7 projecting from the outer housing 8 of the rotor 2. Installed within the housing 8 is an annular magnet 9 provided with flux return segments in the form of arcuate steel strips 10. The insulated stator, carrying coils 11 is precisely located onto the support body 3 by upstanding lugs 12. These define the axial location of the stator 1 and a projection 13 fits between one of the interpole slots 14 in the stator to prevent rotational displacement of the stator. Mounting arms 15 provide for precise location (both linear

and angular) of the Hall effect device 5. An alternative mounting arm 15A allows for the Hall effect device to be located in a different position which will apply when the stator is mounted in an inverted position to cause rotation of the rotor in the opposite direction.

In use the Hall effect device 5 senses a change in magnetic state during rotation of the rotor magnet 9 and controls switching of the current to the coils 11 by means of the electronic circuitry mounted on the printed circuit. Because the stator 1 and the Hall effect device 5 are precisely located with respect to the bearing housing 6 by means of the integrally moulded body 3, the motor can be operated to optimum efficiency, since the rotor magnet 9 is also precisely located through its connection to the rotor ^" shaft 7 which is located within the bearing housing 6. This particular motor is intended for driving a fan and the rotor housing 8 will therefore carry impeller blades (not shown) .

The impeller will then rotate within an outer housing (not shown) to which the body 3 will be secured by screws 16 (or rivets or the like) passing through holes 17 to a central ring 18 of the outer housing. This arrangement will also provide a degree of

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vibration isolation between the body 3 and outer . housing.

The design of the magnet 9 is such that it will occupy minimal space and be of low weight. The conventional fairly thick steel surrounding cage has been replaced by the arcuate steel strips 10 which provide little bulk but achieve an adequate flux return path. As can be seen from Figure 2, the strips 10 are located within grooves defined by ribs 19 on the outer surface of the magnet 9. The magnet 9 also carries downwardly projecting castellations 20 which provide a dual effect. Firstly the castellations 20 are the parts of the magnet which are sensed by the Hall effect device 5. Also, because part of the magnet is effectively absent between the castellations, the magnetic field is axially offset above the central plane of the stator 1 , thus creating a degree ?f down¬ ward thrust of the rotor 2. The end of the rotor shaft 7 will then press against a thrust bearing 21. The symmetrical positioning of the rotor with respect to the stator by means of the precise location of the parts on the body 3, and the downward thrust produced, results in a "floating effect" for the rotor 2 so that there is very low force applied to the main bearing system 22 (Figure 3) by the rotor shaft 7, and thus low frictional losses.

The magnet 9 is held within the housing 8 as a snap fit within a flexible retaining rim 23 near the open end of the housing 8. Furthermore the steel strips 10 (which may be lightly sprung outwardly) act as a radial tolerance ring to hold the magnet concentric with the housing 8 so that there is no relative rotation between the magnet 9 and the housing 8.

The end of the assembly is closed by an end cap 24 having arms 25 which are secured as a snap fit over ledges26 after passing through openings27 in the moulded body 3. The end cap 24 will then also close off the bearing housing 6 and provide a support surface for the thrust, bearing 21. The closed region 28 may be filled with a lubricant if desired.

Ideally the sensor will comprise a bipolar Hall effect device, since this reduces or eiiminates the need to build up selective magnetic reluctance areas on the stator magnet for starting purposes. The bipolar device senses both poles with similar sensitivity and thus achieves a correct mark/space ratio for each full revolution. Also if, on starting the rotor is caused to rotate initially in the wrong direction, it will be reversed almost immediately to rotate con- tinuously thereafter in the correct direction once the sensor senses the next change. For certain purposes it

may be desirable to incorporate two sensors 5 at predetermined spaced positions and if this is so, further mounting arms 15 can be provided on the body 3 at the required position. It will be appreciated also that the printed circuit, which is formed integrally with the body 3, can be of various forms. Thus it could be a separate printed circuit board about which the body 3 is sub¬ sequently moulded or it could, for example, be formed by deposition onto a previously moulded body 3.

The D.C. motor of this invention can be used for a variety of purposes. Thus, for example, it may be used in connection with paper feeds on photocopiers as well as within computers for cooling purposes.