Brushless Electric Motors

This invention relates to brushless electric motors, particularly brushless motors having a field coil system and a permanent magnet rotor.

Such motors have on the rotor, in addition to the permanent drive magnets that effect the rotation by reaction with the field coils in the stator, a set of sensor magnets that effect, through sensors, the activation of the field coils in the correct sequence to drive the rotor. It is now found that these sensor magnets can be dispensed with, simplifying the construction of the motor and making it more compact and less costly. This is of particular importance in one area of use of this type of motor, namely as a drive motor for wheelchairs and invalid carriages.

The invention comprises a brushless electric motor having a permanent magnet rotor, having magnets contained within an annular wall, and a stator, housing field coils, in which the field coils are energised in accordance with control signals sensed by a field sensor arrangement, characterised in that signals are derived from a field leaked from the drive magnets through the rotor wall. The leakage field may be afforded by means of a reduction in rotor thickness at a sensing position, which may conveniently be located at an end of the rotor.

The reduction in rotor thickness will be such as will permit a sufficient leakage field to actuate the field sensor arrangement, which may comprise a Hall Effect Device arrangement.

The rotor may comprise a cylindrical cup having internal slots for the drive magnets. The cylindrical cup may have a base by which it is mounted on a motor shaft and an open end which has a reduced external diameter affording a leakage path for magnetic field from the drive magnets.

The invention also comprises a brushless electric motor as set forth above, having control circuitry at least in part contained within the motor casing. A control circuit board may be mounted in a housing of the motor casing so as to extend parallel to the motor shaft.

The invention also comprises a motor and gearbox combination comprising a motor having the features referred to above, and further comprises a vehicle powered by one or more such motors. The motors and motor and gearbox combinations are especially suitable for powering wheelchairs, and the invention comprises a wheelchair so powered.

A motor, a motor and gearbox combination, and a wheelchair according to the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is an axial section through one embodiment of motor;

Figure 2 is a view on Arrow 2 of Figure 1 ;

Figure 3 is a perspective view of one embodiment of motor/gearbox combination;

Figure 4 is a view like Figure 3 of a second embodiment of motor/gearbox combination;

Figure 5 is a view of the combination of Figure 5 mounted to drive a wheelchair wheel; and Figure 6 is a rear view of a wheelchair with a motor driving each of its two main wheels.

The drawings illustrate a brushless electric motor 11 having a permanent magnet rotor 12 and a stator 13 housing field coils 14, in which the field coils 14 are energised in accordance with control signals sensed by a field sensor arrangement 15 which signals are derived from leakage field from the rotor drive magnets 16.

The leakage field is afforded by means of a reduction in rotor thickness at a sensing position 17 which is located at an end 12a of the rotor 12.

The reduction in rotor thickness is such as will permit a sufficient leakage field to actuate the field sensor arrangement 15, which comprises a Hall Effect Device arrangement.

The rotor 12 comprises a cylindrical cup 18 having internal slots 19 for the drive magnets 16. The cylindrical cup 18 has a base 18a by which it is mounted on a motor shaft 21 and its open end 12a has a reduced external diameter affording a leakage path for magnetic field from the drive magnets 16.

The motor can thus be shorter than conventional such motors which have separate sensor magnets on the shaft 21. Fractional horsepower motors of this kind are used inter alia to drive wheelchairs, and are usually provided with a gearbox. The reduced motor dimensions, resulting from dispensing with the control magnets, mean a smaller motor/gearbox assembly, which facilitates incorporation into a wheelchair in such fashion that permits enhanced foldability thereof.

Moreover, a control circuit board 22 is mounted in a housing 23 of the motor casing 24 so as to extend parallel to the motor shaft 21. The field sensor arrangement 15 is attached directly to the circuit board 22. This reduces the need for external control circuitry, which can result in a more compact joystick arrangement for controlling a wheelchair

Figures 3 and 4 illustrate motors 11 according to the invention with attached gearboxes 41, showing how the gearbox 41 can be arranged in different positions with respect to the motor 11. The gearbox output shaft 42 can also project either side of the gearbox 41. Figure 5 illustrates how a motor/gearbox combination 61 can be fitted to a wheel 62 and framework 63 of a wheelchair, and Figure 6 shows a rear view of a wheelchair 71 with

motor/gearbox combinations 61, with oppositely projecting output shafts, fitted for powering both main wheels 62. It is clear that the small size of the motor/gearbox combination 61 greatly facilitates folding of the wheelchair, the two combinations occupying less than 15% of the width of the wheelchair.