GB2212002A

GB2212002A - Rotor starting polishing means for a brushless d.c. electric motor

Info

Publication number: GB2212002A
Application number: GB8725633A
Authority: GB
Inventors: Roger Frederick Baines
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-11-02
Filing date: 1987-11-02
Publication date: 1989-07-12
Also published as: GB8725633D0

Abstract

An axial air gap brushless d.c. electric motor for driving a viscous load, comprises a stator (10), a permanent magnet rotor (11), motor driving windings (13) on the stator, a positional detector and switching circuit (16), and at least one low reluctance member (24) on the stator. The low reluctance member extends to a greater extent in a peripheral direction than in a radial direction and is positioned such that the magnetic coupling between the permanent magnet rotor and the lower reluctance member will always cause the rotor to come to rest, when power to the motor is switched off, in a position in which torque will be developed when the power is next switched on. <IMAGE>

Description

Brushless d.c. electric motor This invention relates to an axial air gap brushless d.c. electric motor.

Single phase bi-polar or two phase unipolar windings are sometimes preferred to three phase windings in brushless motors because less positional detectors are required thus reducing the cost of the electronic components.

However, it is known that when single phase bi-polar or two phase unipolar windings are used in a brushless d.c. motor there are points of zero torque at the switching positions. Should a motor come to rest at this position it is likely not to start when power is switched on.

Various means of introducing harmonic torques are known some of which involve a secondary magnet system. In such systems there is a guarantee that torque is developed irrespective of the static position. This is useful when the motor is driving a coulomb load (velocity independent load) which is apparent at all times.

However when driving a viscous load (e.g. a fan) the motor has only to overcome its internal friction to start rotating. When motors are used to drive such loads the expense of the secondary magnet system is not justified.

It is known in radial air gap brushless motors to design assymetry into the salient poles of the stator so that the magnetic field developed in the air gap pulls the rotor to a position which is not a position at which zero torque is developed.

However, in an axial air gap motor it is not usual to have any iron acting as a salient pole.

According to a first aspect of the invention, there is provided an axial air gap brushless d.c. electric motor for dirving a viscous load, comprising a stator, a permanent magnet rotor, motor driving windings on the stator, means for selectively energising the windings in response to means detecting the position of the rotor relative to the stator, and at least one low reluctance member which extends to a greater extent in a peripheral direction than in a radial direction and which is positioned such that the magnetic coupling between the permanent magnet rotor and the low reluctance member will always cause the rotor to come to rest, when power to the motor is switched off, in a position in which torque will be developed when the power is next switched on.

Preferred and/or optional features of the first aspect of the invention are set forth in claims 2 to 6, inclusive.

According to a second aspect of the invention, there is provided an axial air gap brushless d.c. electric motor, comprising a stator, a permanent magnet rotor, motor driving windings on the stator, means for selectively energising the windings in response to means detecting the position of the rotor relative to the stator, and at least one low reluctance member on the stator at a position such that the magnetic coupling between the permanent magnet rotor and the low reluctance rotor will always cause the rotor to come to rest, when power to the motor is switched off, with the windings out of registry with the magnetic poles of the permanent magnet rotor.

The invention will now be more particularly described by way of example with reference to the accompanying drawing which is an exploded perspective view of one embodiment of an electric motor according to the invention.

Referring to the drawing, the motor shown therein comprises a stator 10 and a rotor 11. The stator 10 comprises a disc-like winding support 12, typically of plastics mateial, and four discrete winding coils 13 fixed to the support, such as by glue, in equi-angularly spaced relationship. The support 12 has a central hole 14 in which a journal bearing 15 is mounted. A positional detector and switching circuit 16 is mounted on the support 12 between two of the winding coils 13.

The rotor 11 comprises a shaft 17 mounted for rotation in the bearing 15, and a disc-like metal plate 18 supporting four segmental permanent magnets 19. The magnets 19 are glued to the plate 18 and adjacent magnets are magnetised in opposite axial directions.

The stator 10 and rotor 11 are mounted in a drawn shallow metal can 20 closed at one end by an integral end plate 21 which provides a thrust face for a ball end 22 of the rotor shaft 17. The can is closed at its other end by a metal end cap 23.

Diametrically opposite winding coils 13 are connected electrically in series and the positional detector and switching circuit 16 includes a Hall effect device which detects the position of the rotor relative to the stator and two electronic switches which operate in response to the Hall effect device to alternately energise the two pairs of series connected winding coils 13, as the magnets 19 come into register with the coils 13.

Two low reluctance members 24, conveniently of ferromagnetic material, are mounted on the support 12. The members 24 are arranged diametrically opposite one another and extend to a greater extent in a peripheral direction than a radial direction.

Preferably and as shown, the members 24 are arcuate with their radii of curvature centred on the axis of the motor.

The members 24 are positioned such that the magnetic coupling between the magnets 19 and the members 24 will always cause the rotor 11 to cog to a start position, when power is switched off, i.e. to a position in which torque will be developed when the power is next switched on. Hence, the motor should come to rest with the winding coils 13 out of registry with the magnets 19. It is advantageous to arrange for the motor to come to rest at a position which will ensure maximum starting torque and this can be achieved by positioning the members 24, as shown, at a position which is displaced angularly with respect to the coils 13 by 45 electrical degrees.

The members 24 introduce a pole couple which, during rotation of the rotor, is an alternating couple and which adds up to zero in a complete cycle. It therefore neither adds nor subtracts from the torque developed by the phases. However, its presence does modify the waveform of the phase torque reducing the peak torque and enhancing the zero torque.

The value of the pole sensitive torque can be made as low as will just ensure that a de-energised rotor will be rotated into any one of a number of appropriate start positions.

The above embodiment is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention defined by the appended claims. For example, a single low reluctance member 24, could be used, although the advantage of using two such members is that the moments applied to the shaft by each member will balance. Moreover, the motor could have, for example, just two magnets and two winding coils.

Claims

1. An axial air gap brushless d.c. electric motor for driving a viscous load, comprising a stator, a permanent magnet rotor, motor driving windings on the stator, means for selectively energising the windings in response to means detecting the position of the rotor relative to the stator, and at least one low reluctance member on the stator, the low reluctance member extending to a greater extent in a peripheral direction than in a radial direction and being positioned such that the magnetic coupling between the permanent magnet rotor and the low reluctance member will always cause the rotor to come to rest, when power to the motor is switched off, in a position in which torque will be developed when the power is next switched on.

2. A motor as claimed in claim 1, wherein the low reluctance member is arcuate.

3. A motor as claimed in claim 2, wherein the radius of curvature of the arcuate member is centered on the axis of the motor.

4. A motor as claimed in any one of the preceding claims, wherein the low reluctance member is formed of ferro-magnetic material.

5. A motor as claimed in any one of the preceding claims, wherein the low reluctance member is displaced angularly with respect to the windings by 45 or substantially 45 electrical degrees.

6. A motor as claimed in any one of the preceding claims, comprising at least one pair of said low reluctance members arranged diametrically opposite one another with respect to the axis of the motor.

7. An axial air gap brushless d.c. electric motor, comprising a stator, a permanent magnet rotor, motor driving windings on the stator, means for selectively energising the windings in response to means detecting the position of the rotor relative to the stator, and at least one low reluctance member on the stator at a position such that the magnetic coupling between the permanent magnet rotor and the low reluctance member will always cause the rotor to come to rest, when power to the motor is switched off, with the windings out of registry with the magnetic poles of the permanent magnet rotor.