WO2001028076A1

WO2001028076A1 - An external rotor brushless dc motor

Info

Publication number: WO2001028076A1
Application number: PCT/GB2000/003812
Authority: WO
Inventors: Henry Riddoch
Original assignee: Nmb (Uk) Limited
Priority date: 1999-10-08
Filing date: 2000-10-04
Publication date: 2001-04-19
Also published as: AU7543600A

Abstract

An external rotor brushless DC motor is disclosed having a Hall effect sensor (6A) for providing a rotor position signal indicative of the angular position of the motor's rotor assembly relative to the motor's winding assembly. A base plate (21) is disposed between the Hall effect sensor and the rotor assembly and the Hall effect sensor is provided with a probe (35) passing through the base plate (21) such that the probe may magnetically couple the Hall effect sensor to the magnetic field generated by the rotor assembly to sense the angular position of the rotor assembly relative to the winding assembly.

Description

AN EXTERNAL ROTOR BRUSHLESS DC MOTOR

This invention relates to an external rotor brushless DC motor.

For many years, external rotor brushless DC motors have been manufactured in accordance with a conventional topography. An example of such a conventional topography of a brushless DC motor is shown in Figure 1 of the accompanying drawings. The basic premise of this topography is that the stator assembly has a stator assembly base with a recess therein. The driving and control circuitry of the motor is attached to the stator assembly base in the recess, the stator windings being located above the circuitry such that the circuitry is sandwiched in the recess between the stator windings and the stator assembly base. Typically, the stator windings and the circuitry are permanently affixed to the stator assembly base.

As is well known, brushless DC motors must be commutated electronically by selectively energising the stator windings according to the angular position of the rotor, the angular position of the rotor conventionally being detected by position sensors usually in the form of Hall effect sensors. Conventionally, a Hall effect sensor or multiple Hall effect sensors are mounted adjacent the stator winding assembly in the magnetic field generated by the rotor assembly and are therefore usually remove from the control circuitry of the motor. As set out in co-pending U K Patent Application No 9923900 6 (filed 8th October, 1999), a new external lotor brushless DC motoi topography has been devised in which the statoi winding assembly and the electronic ciicuitry of the motor are sepaiated fiom one another by a base plate of the stator assembly This arrangement seeks to overcome several drawbacks of the conventional motor topography, such as making it easier to modify or repair the circuitry or components on the printed circuit board without at least partially damaging or destroying the stator windings, or the assembly base Another drawback with the conventional topogiaphy is that it is difficult to dissipate heat, generated by both the motoi and the ciicuitry on the punted circuit board, away from the rotor Anothei drawback of the conventional topography is that it is difficult to seal the printed circuit board and the components thereon from the ingress of dust and moistuie

However, when using the motoi topography descπbed in UK Patent Application No. 9923900 6 the oi each Hall effect sensor of the motor circuitry is separated from the rotor (which caπies permanent magnets) by the above- mentioned base plate Accoidingly, the effectiveness of the Hall effect sensors is reduced by virtue of their sepaiation from the magnetic field generated by the rotor Whilst it would of couise be possible still to locate the oi each Hall effect sensoi above the statoi base plate directly within the magnetic field generated by the rotoi, this itself has seveial disadvantages Fπstly, Hall effect sensoi s aie expensive and it is undesnable to subject them to the generally harsh environment which exists in the immediate vicinity of the spinning rotor It is theiefoie desnable to locate the Hall effect sensoi with the iest of the motor ciicuitry such that it is piotected fiom this haish envnonment by the stator base plate Secondly, signals fiom the Hall effect sensoi aie sensitive to electrical livterfeience if the Hall effect sensoi is i emote fiom the contiol circuitry Furthei, it can be inconvenient and sometimes difficult to seal the area wheie the Hall sensoi would have to pass thiough the base plate to be in proximity with the magnetic field generated by the iotoi

It is an object of the present invention to seek to provide an external rotor brushless DC motor which does not suffei fiom the above-mentioned drawbacks

Accoidingly, one aspect of the piesent invention piovides an external rotor brushless DC motoi comprising a winding assembly, a rotor assembly generating a magnetic field, the rotor assembly being mounted foi iotation externally of the winding assembly, and a Hall effect sensor foi providing a iotoi position signal indicative of the angular position of the rotor assembly relative to the winding assembly, wherein a base plate is disposed between the Hall effect sensor and the rotor assembly, the Hall effect sensor having a piobe passing through the base plate, the probe magnetically coupling the Hall effect sensor to the magnetic field generated by the iotoi assembly to sense the angulai position of the rotor assembly ielative to the winding assembly

Piefeiably, the piobe is manufactuied fiom a magnetic matenal and the base plate oi moie specifically the aiea of the base plate ai ound the piobe is manufactuied fiom a non magnetic matenal

Conveniently, the permeability of the piobe is less than the permeability of the aiea of the base plate aiea aiound the pi obe

Advantageously, the piobe compiises a feπ omagnetic matenal Conveniently, the probe compiises any material of low permeability such as low carbon steel

Advantageously, the base plate is manufactured from any non-magnetic non-ferrous material such as aluminium or zinc or plastic

Preferably, the motor has a Hall effect sensor associated with the probe

Alternatively, the motoi may have a pluiality of said Hall effect sensors each associated with a lespective piobe

Advantageously, the or each piobe is hermetically sealed to the base plate

Conveniently, the 01 each piobe is hermetically sealed to the base plate by adhesive, an mterfeience fit with the base plate or preferably a combination of both adhesive and mterfeience fit

Pieferably, the base plate has a side wall, the side wall defining a lecess within which the oi each Hall effect sensoi is located

Advantageously, the base plate is piovided with a covei, the oi each Hall effect sensor being located between the base plate and the covei

Conveniently, the oi each Hall effect sensoi is housed within an enclosuie

Piefeiably, the enclosuie is hermeticall) sealed Advantageously, the base plate is made from aluminium or other thermally conductive material.

In order that the present invention may be more readily understood, and so that further features thereof may be appreciated, embodiments will now be described, by way of example, with reference to the accompanying drawings, in which :-

Figure 1 is a cross-section thiough a conventional external rotor brushless DC motor; and

Figure 2 is a cross-section through an embodiment of an external rotor brushless DC motor in accordance with the present invention.

Referring initially to Figure 1 of the accompanying drawings, an external brushless DC motor 1 is illustrated having a generally conventional topography. The motor 1 comprises a stator assembly and a rotor assembly. The stator assembly has a stator assembly base which comprises a substantially circular base 2 having an outer upstanding cylindrical side wall 3. A central cylindrical column 4 is also upstanding from the centre of the circular base 2 and is concentric with the cylindrical side wall 3 so as to define an annular space between the cylindrical side wall 3 and the column 4. It is conventional for the driving and control circuitry of the motor to be housed within this annular space, typically on an annular printed circuit board 5 with the main surface of the printed circuit board 5 which carries all of the components 6 being oriented towards the base 2. The printed circuit board 5 is typically permanently connected to and attached to a plurality of multipole stator laminations with windings around each pole forming a stator winding assembly 7 which is attached to the stator assembly base and, more particularly, to the exterior of the column 4 diiectly above the base 2, the printed circuit boaid 5 thereby being sandwiched between the stator winding assembly 7 and the base 2 The stator winding assembly 7 is permanently attached to the column 2, prefeiably by thermoset lesin oi othei permanent adhesive It will be appreciated that the annulai space within which the printed circuit board 5 is to be affixed opens towards the statoi winding assembly 7

The rotor assembly of the conventional topography motor 1 comprises a rotor cup 8 having a senes of permanent magnet poles of opposite polarity 9 arranged aiound the internal penpheiy theieof The cup 8 is rotatably engaged in the column 4 by means of a shaft 10 held between two pans of bearings 1 1 withm the column 4 The iotoi assembly is thus journalled to the stator assembly which results in the iotoi assembly being mounted for rotation externally of the winding assembly

In older to piovide information legaiding the angulai position of the rotor assembly relative to the winding assembly, one oi more Hall effect sensors 6a aie provided on the punted cπcuit board 5 The oi each Hall effect sensor 6a is located in the close vicimts of the magnet poles 9 of the rotor assembly such that the oi each Hall effect sensoi is located within the magnetic field generated by the magnet poles 9 of the rotoi assembly The Hall effect sensoi s 6a, as is well known in the ait, pi ovide a iotoi position signal indicative of the angulai position of the i otoi assembU i elative to the winding assembl) This iotoi position signal is then used to commutate electronically the statoi windings

As explained in co-pending U k Patent Application No 9923900 6, theie aie seveial di awbacks associated w ith the above-descnbed conventional motoi topogiaphy, all of which stem iiom the conventional location of the printed circuit board 5 at a position between the housing base 2 and the stator assembly 7 which are typically both permanently affixed to the other. It is, therefore, extremely difficult to modify or repair the circuitry or components on the printed circuit board without at least partially damaging or destroying the stator winding assembly or the bases. Other drawbacks include the difficulty of sealing the printed circuit board and the components thereon from the ingress of dust and moisture and the difficulty of dissipating heat, generated by both the motor and the circuitry on the printed circuit board, away from the motor and isolating the elechonic components from the heat generated within the stator winding assembly 7. Therefore, our above-mentioned co-pending U.K. Patent Application provides a departure from the conventional topography used in external rotor brushless DC motors.

Figure 2 of the accompanying drawings illustrates the general topography to which the above-mentioned U.K. Patent Application is directed, but also illustrates the present invention applied thereto.

It will be apparent from a study of Figure 2 that the rotor assembly and stator winding assembly 7 are affixed to the column 4 in a conventional manner (as compared to the known arrangement of Figure 1). However, the difference between the motor arrangement to which the above-mentioned U.K. Patent Application is directed and the conventional motor assembly shown in Figure 1 lies in the base portion of the stator assembly.

The stator assembly of base 20 of the motor 1 illustrated in Figure 2 comprises an effective inversion of the arrangement shown in Figure 1. The stator assembly base compiises a conventional column 4 upstanding from a circular base plate 21. A cylindrical side wall 22 projects from the base plate 21 in the opposite direction to the column 4, the side wall 22 thereby projecting away fiom the statoi winding assembly 7, in the opposite direction to the column 4 Thus, theie is no recess between the stator winding assembly 7 and the base plate 21 within which the printed cncuit board can be mounted

In contrast to the conventional topogi aphy illustrated in Figure 1, the side wall 22 which depends away from the statoi winding assembly 7 defines a space on the opposite side of the base plate 21 to the stator winding assembly 7 within which the circuitry associated with the motoi can be mounted Thus, the recess defined by the side wall 22 and the base plate 21 opens away from the statoi winding assembly 7

A cover plate 23 is seated on an internal step formed around the free edge of the side wall 22 An enclosure 24 is theieby defined between the base plate 21 and the covei plate 23 which is bounded by the side wall 22 Preferably, further bosses 25 or partition walls (not shown) are formed within the side wall 22 so as to piovide, respectively, means by which the cover plate

23 can be seemed to the base plate 21 and a means to divide the main enclosure

24 into vaπous sub-enclosuies within the boundary of the side wall 22

In the embodiment of the motoi lllustr ated in Figure 2, the pπnted circuit board 5 is attached to the base plate 21 The components 6 on the printed circuit boaid 5 aie positioned on the opposite surface of the pπnted circuit boai d 5 to that adjacent to the base plate 21 Thei efoie, the components 6 aie readily accessible upon iemoval of the covei plate 23 Thus, the components 6 and ciicuitry of the punted cncuit boaid 5 can be easily lepaned or maintained without any need to cause damage to the winding assembly 7 or any othei part of the statoi assembly Suitable wning 7a connects the winding assembly 7 to a punted cncuit boaid 5 thiough the base plate 21 The opening thiough which the wiring 7a passes through the base plate 21 is sealed by a rubber seal or the like.

In the embodiment illustrated in Figure 2, the enclosure 24 is hermetically sealable by a rubber O-ring 33 or other similar sealing arrangement so as to protect the circuitry and components within the enclosure 24 from the ingress of dust or moisture. Additionally, the thermal transfer properties of the motor are enhanced in that heat from the motor can be more readily dissipated through the stator assembly, along the base plate 21, down the side wall 22 and from the cover plate 23. External heat sinks or the like may be connected to the cover plate 23 to facilitate further heat sinking.

Having separated the electronic circuitry of the motor arrangement of Figure 2 from the winding assembly 7 (and hence also the motor assembly with its associated permanent magnets 9), it is advantageous also to locate the Hall effect sensor or sensors 6a on the opposite side of the base plate 21 so as also to protect the Hall effect sensor or sensors 6a which, as noted above, are sensitive and easy to damage. However, locating the Hall effect sensors 6a on the opposite side of the base plate 21 to the permanent magnets 9 of the rotor assembly reduces the effectiveness of the Hall effect sensor because at the very least it is no longer located in the strongest part of the magnetic field generated by the magnets 9, and at worse, is not affected at all by the magnetic field generated by the permanent magnets 9.

In order to solve the above-mentioned problem of the reduced effectiveness of the Hall effect sensors when located on the opposite side of the base plate 21 to the permanent magnets 9, it is proposed to provide each Hall effect sensor 6a with a probe 35 passing through the base plate 21 and terminating at a position indicated generally at 36 which is located well within the magnetic field generated by the permanent magnets 9 of the rotor. The probes 35 extend through the base plate 21 to a point generally corresponding to the point at which the Hall effect sensors of the conventional motor topography illustrated in Figure 1 are located.

The or each probe 35 is adapted to couple magnetically its respective Hall effect sensor to the magnetic field generated by the rotor assembly, such that the Hall effect sensor is still effectively influenced by the magnetic field in the usual way so as to provide a rotor position signal indicative of the angular position of the rotor assembly relative to the winding assembly, which signal can then be used for electronically commutating the motor. The probe 35 shown on the right hand side of Figure 2 is proud at the base plate 21. Preferably, the probe 35 as shown on the left hand side of Figure 2 is flush with the base plate 21. Also, it will be noted that the Hall effect sensor 6A on the left hand side is located within a through hole 37 in the printed circuit board 5 thereby shortening the length of the probe 35.

A preferred embodiment of the present invention uses probes 35 made of a magnetic material mounted in a base plate of a non magnetic material. More preferably, the probes 35 comprise a low remnance magnetic material such as, for example, a low carbon steel or any other magnetically conductive material. The material of the plate 21 is a non-magnetic thermally conductive material such as aluminium or zinc, but could also be made of plastic for lower power motors.

As explained previously, the cover plate 23 is preferably hermetically sealed to the downwardly depending side wall 22 of the base plate 21 so as to ensure good protection of the electronic circuitry therebetween. Therefore to ensure that the enclosure as a whole is hermetically sealed, the or each probe 35 is preferably hermetically sealed to the base plate 21 by way of, for example, adhesive, an interference fit in the base plate 21 or a combination of adhesive and interference fit.

In the present specification "comprise" means "includes or consists of and "comprising" means "including or consisting of .

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1 An external rotor brushless DC motoi comprising a winding assembly, a rotor assembly generating a magnetic field, the rotor assembly bemg mounted foi rotation externally of the winding assembly, and a Hall effect sensor for pioviding a iotor position signal indicative of the angular position of the rotor assembly i elati ve to the winding assembly, wherein a base plate is disposed between the Hall effect sensor and the rotor assembly, the Hall effect sensor having a probe passing through the base plate, the probe magnetically coupling the Hall effect sensor to the magnetic field generated by the iotor assembly to sense the angular position of the rotor assembly relative to the winding assembly

2 A motor according to claim 1, wherein the probe comprises a ferromagnetic material

3 A motor according to claim 2, wheiein the probe compπses any magnetically conductive matenal of low permeability

4 A motoi accoiding to any pieceding claim having a pluiahty of said Hall effect sensors each associated with a iespectn e piobe

5 A motoi accoiding to any pieceding claim, wheiein the oi each probe is hermetically sealed to the base plate

6 A motoi accoiding to claim 5, wheiein the piobe is hermetically sealed to the base plate by an adhesive

7. A motor according to Claim 5 or 6, wherein the probe is hermetically sealed to the base plate by an interference fit in the base plate.

8. A motor according to any preceding claim wherein the base plate has a side wall, the side wall defining a recess within which the or each Hall effect sensor is located.

9. A motor according to any preceding claim, wherein the base plate is provided with a cover, the or each Hall effect sensor being located between the base plate and the cover.

10. A motor according to any preceding claim, wherein the or each Hall effect sensor is housed within an enclosure.

11. A motor according to claim 9, wherein the enclosure is hermetically sealed.

12. A motor according to any preceding claim, wherein the base plate is manufactured from any non-magnetic, non-feπous material.

13. A motor according to any preceding claim, wherein the base plate is made from aluminium, zinc or any other thermally conductive material.