GB2169755A - D.C. electric motors - Google Patents

Abstract

A brushless d.c. motor has a rotor (20) of unmagnetised magnetic material and a parking magnet assembly (31) located axially close to one end of the rotor. The parking magnet assembly has two permanent magnet regions extending along a tubular body (32) which provides two opposite poles spaced diametrically opposite one another about the axis of rotation of the rotor. The cross-sectional shape of the tubular body is substantially the same as that of the rotor. The parking magnet assembly is oriented such that the rotor adopts a rest position that is at about 45 degrees with respect to the position the rotor adopts when the motor stator (10) is energised. <IMAGE>

Description

SPECIFICATION Electric motors This invention relates to electric motors.

The invention is more particularly concerned with brushless d.c. motors.

Brushless d.c. motors have a rotor that is mounted for rotation within a stator having an electrical coil assembly. By appropriately energising coils in the assembly, the rotor can be caused to rotate. One problem with such motors, however, is that they can be difficult to start since, if the rotor is in alignment with the stator coils when at rest, the first pulse applied to the stator will not cause movement of the rotor. Various arrangements to facilitate starting of such motors have been proposed but these are not applicable to motors with an unmagnetised rotor.

It is an object of the present invention to provide an electric motor in which starting is facilitated in a simple way by a relatively compact construction.

According to one aspect of the present invention there is provided an electric motor of the kind having a stator comprising an electrical coil assembly disposed about a rotor such as to cause rotation of the rotor about its axis upon appropriate energisation of the assembly, wherein the rotor is of an unmagnetised magnetic material and is of a construction such that it tends to adopt a predetermined orientation relative to said stator upon energisation of said assembly, wherein said motor includes permanent magnet means located axially of said rotor, said permanet magnet means having first and second poles spaced from one another about the axis of rotation of the rotor close to one end of said rotor, such that the rotor adopts a rest position in alignment with said poles when not acted on by said assembly, and wherein said poles are located such that said rest position is out of alignment with said predetermined orientation.

The stator may have two pole pieces that are located diametrically opposite one another, the first and second poles of said permanent magnet means being located diametrically opposite one another. The permanent magnet means may include two permanent magnet re- gions and a member of magnetic material spaced along the axis of said motor remote from said rotor, each permanent magneti region having a first pole located close to said rotor and a second pole located close to said member of magnetic material, the first pole of the two regions being of opposite polarity from one another.

Preferably the cross-sectional shape of the said one end of the rotor is substantially the same as the cross-sectional shape of the adjacent end of the permanent magnet means.

The permanent magnet means may be substantially tubular in shape or may have two bar-shape concave permanent magnets. The rest position may be at an angle at substantially 45 degrees with respect to the said predetermined orientation.

A d.c. brushless motor in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through the motor along its axis; Figure 2 is a cross-sectional view of the motor along the line ll-ll of Fig. 1; Figure 3 shows the rotor of the motor in perspective; Figure 4 is a cross-sectional view of the motor along the line IV-IV of Fig. 1; Figure 5 is a perspective view of a part of the motor; and Figure 6 is a perspective view of an alternative part of the motor.

With reference to Figs. 1 to 3, the motor has a cylindrical casing 1 that is closed at its forward and back ends by respective end plates 2 and 3. Within the casing 1 there is supported a stator assembly 10 which comprises a coil assembly having two electrical coils 11 and 12 extending about respective pole pieces 13 and 14. The pole pieces 13 and 14 are formed from flat iron plates 15 laminated together. The plates 15, as best seen in Fig. 2, comprise an outer ring 16 from which project inwardly two diametrically opposite arms 17 and 18. The end of each arm 17 and 18 is curved to follow the circumference of a common circle. The arms 17 and 18 of these plates together define the pole pieces 13 and 14 respectively of the stator.

Extending along the axis of the motor, intermediate the two pole pieces 13 and 14, there is a rotor 20, which is shown in Fig. 3. The rotor 20 is of generally cylindrical shape with flattened opposite faces 21 and 22 and is made up of laminated magnetic but unmagnetised metal plates 23 mounted on an axial shaft 24. The curved surfaces of the rotor 20 conform with the curvature at the ends of the pole pieces 13 and 14. The rotor shaft 24 is mounted for rotation about its axis by means of bearings 26 and 27 supported in the end plates 2 and 3 respectively of the motor.

With reference now also to Figs. 4 and 5, the back end plate 3, shown to the right of Fig. 1, supports a parking magnet assembly 31, which is shown in Fig. 5. The assembly 31 consists of a tubular permanent magnet 32 that is joined at its rear end to a disc-shape soft-iron end piece 33; the permanent magnet being formed with parallel flats 34 and 35 on its outer surface. The permanent magnet 32 is magnetised along its length to form two permanent magnet regions of opposite polarities, that is, one having a north pole close to the rotor 20 and other having a south pole close to the rotor. The magnetic circuit of the assembly 31 is completed at one end by the end piece 33, and at the other end by the rotor 20.The two opposite poles at the forward end of the assembly 31 are thereby spaced from one another on diametrically opposite sides of the motor axis, the end of the assembly having substantially the same size and shape in section as the rotor. The parking magnet assembly 31 is oriented about the motor axis so that it is out of alignment with the stator pole pieces 13 and 14 by an angle 6 of about 45 , as best seen in Fig. 4.

An alternative parking magnet assembly 41 is shown in Fig. 6. This is similar to the assembly 31 shown in Fig. 5 except that, instead of a tubular magnet 32 the permanent magnet regions are formed by two separate bar-shape concave permanent magnets 42 and 43. In effect, the operation of both assemblies 31 and 41 are identical.

The motor may also include a sensor (not shown) that provides an output in accordance with the position and, or alternatively, the speed of the rotor 20.

In operation, d.c. pulses are applied to the stator coils 11 and 12 thereby producing a magnetic field between the pole pieces 13 and 14. The pulses are applied when the rotor 20 is out of alignment with the pole pieces 13 and 14 so that the magnetic field tends to rotate the rotor to a position of alignment.

The pulses terminate before alignment is achieved so that the momentum of the rotor causes it to rotate past the alignment position to the next position at which a pulse is applied. Providing the pulses are applied at the correct position in the rotation of the rotor, the rotor will continue to rotate.

The problem with d.c. brushless motors, especially where rotor is unmagnetised, is that, when starting from rest, the rotor could be in alignment with the stator so that no momentum is imparted to the rotor by the energising pulses. The parking magnet assemnly 31 of the present invention ensures that the rotor 20 is out of alignment with the pole pieces 13 and 14 when at rest, so that the initial drive pulse applied to the stator 10 will give the required momentum to the rotor.

It can be seen that, because the two poles at the forward end of the magnet assembly 31 are located on opposite sides of the motor axis, the rotor 20 will adopt an orientation such as to complete the magnetic circuit of the assembly by coming into alignment with the ends of the magnets 32 and 33. Because the parking magnet assembly 31 is oriented at an angle 6 to the stator pole pieces 13 and 14, the rotor will also adopt this orientation.

The configuration of the parking magnet assembly 31 or 41 gives it a high efficiency but in a compact form. It can be seen that, because the assembly is positioned axially and interacts with the end of the rotor 20, it does not require any increase in the overall diameter of the motor.

Claims (10)

1. An electric motor of the kind having a stator comprising an electrical coil assembly disposed about a rotor such as to cause rotation of the rotor about its axis upon appropriate energisation of the assembly, wherein the rotor is of an unmagnetised magnetic material and is of a construction such that it tends to adopt a predetermined orientation relative to said stator upon energisation of said assembly, wherein said motor includes permanent magnet means located axially of said rotor, said permanent magnet means having first and second poles spaced from one another about the axis of rotation of the rotor close to one end of said rotor, such that the rotor adopts a rest position in alignment with said poles when not acted on by said assembly, and wherein said poles are located such that said rest position is out of alignment with said predetermined orientation.

2. An electric motor according to Claim 1, wherein the stator has two pole pieces that are located diametrically opposite one another, and wherein the first and second poles of said permanent magnet means are located diametrically opposite one another.

3. An electric motor according to Claim 1 or 2, wherein the permanent magnet means includes two permanent magnet regions and a member of magnetic material spaced along the axis of said motor remote from said rotor, wherein each permanent magnet region has a first pole located close to said rotor and a second pole located close to said member of magnetic material, and wherein the first pole of the two regions are of opposite polarity from one another.

4. An electric motor according to any one of the preceding claims, wherein the crosssectional shape of the said one end of said rotor is substantially the same as the crosssectional shape of the adjacent end of said permanent magnet means.

5. An electric motor according to Claim 4, wherein said permanent magnet means is substantially tubular in shape.

6. An electric motor according to Claim 4, wherein said permanent magnet means has two bar-shape concave permanent magnets.

7. An electric motor according to any one of the preceding claims, wherein the said rest position is at an angle of substantially 45 degrees with respect to the said predetermined orientation.

8. An electric motor substantially as hereinbefore described with reference to Figs. 1 to 5 of the accompanying drawings.

9. An electric motor substantially as hereinbefore described with reference to Figs. 1 to 5 as modified by Fig. 6 of the accompanying drawings.

10. Any novel feature or combination of features as hereinbefore described.