GB2208190A - Rotor position sensing in brushless d.c. electric motor - Google Patents

Abstract

Detectors (16) for sensing the initial position of the rotor relative to the windings comprise primary (26) and secondary (26) windings on a common magnetic core (23). The core (23) is arranged to be just saturated by rotor field magnets opposite flux concentrates 24 magnet shield 27 preventing mutual inductance between the coils via the air. The primary winding is energised by a d.c. input voltage to the motor and the secondary winding generates an output voltage, the magnitude of which is dependent on whether the current seeks to strengthen or oppose the magnetic flux in the core which in turn depends on whether the detector is under the influence of a North or South rotor pole. Means (28) selectively energise the motor driving windings in response to detectors (16) which may also detect the position of the rotor relative to the stator during operation. <IMAGE>

Description

Brushless Electric Motor This invention relates to a brushless d.c. electric motor.

Brushless electric motors utilize electric circuit means for selectively energising the motor driving windings in response to means detecting the position of the rotor relative to the stator. It is known to use Hall effect and photoelectric devices as the position detecting means but these arrangements are complicated and expensive.

It is also known to include supplementary windings on the stator to detect back e.m.f. in order to detect position, but this only works when the rotor is rotating. Consequently, the motor will not start automatically. GB Patent Specification GB 2153599B discloses the use of position detecting brushes fitted to the rotor and co-operating position detecting conductors fitted to the stator to detect position at the start of rotation, the brushes moving out of contact with the conductors as the rotor gains speed to reduce the wear on the brushes. This system is, however, noisy.

According to the present invention there is provided a brushless d.c. electric motor comprising: (a) a stator, (b) a rotor mounted for rotation relative to the stator, (c) motor driving windings on the stator, (d) means for selectively energising the motor driving windings in response to means detecting the position of the rotor relative to the stator, the position detecting means comprising at least one positional detector having primary and secondary windings mounted on a common magnetic core which is magnetizable in opposite directions by North and South poles, respectively, on the rotor, the primary winding being energisable by a d.c. input voltage to the motor and the secondary winding generating an output voltage, the magnitude of which is dependent on whether the current in the primary winding seeks to strengthen or oppose the magnetic flux in the core, which is in or is close to saturation.

The positional detector can be used on start up only in which case supplementary windings can be provided on the stator for detecting back e.m.f. and hence the position of the rotor when the rotor is rotating.

However, more preferably, the position detecting means comprises a plurality of positional detectors equal in number to the number of stator phases and each detector operates switching means to selectively energise an associated stator phase at an appropriate time.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of one embodiment of an electric motor according to the present invention, Figure 2 is an enlarged view of one of the positional detectors of Figure 1, and Figure 3 is a simplified circuit diagram of the motor of Figure 1.

Referring to the drawings, 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 material, and a three phase winding in the form of three 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.

Three positional detectors 16 are fixed to the winding support 12 between 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 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 20 is closed at its other end by a metal end cap 30.

As best shown in Figure 2, each positional detector 16 comprises a soft ferrite core 23 having an enlarged head 24 which serves as a flux collector. A primary winding 25 and a secondary winding 26 are mounted on the core 23 in axially spaced apart relationship and an annular shield 27 of high permeance material, e.g. soft iron, is interposed between the windings 25 and 26 to minimise mutual inductance through the air.

The core 23 is designed so as to be in or close to magnetic saturation when under at least one of the poles of the rotor 11, say in this case under a South pole. If need be a small permanent magnet is provided at the end of the core 23 distal from its head 24 to ensure that this is so. The primary winding 25 is energised by a d.c. input voltage to the motor and the winding 25 is wound on the core 23 such that the current passing through the winding 25 will seek to strengthen the magnetic flux produced in the core by a rotor South pole. Thus, when current is supplied to a detector 16 under a South pole at motor start-up the voltage induced in the secondary winding 26 is extremely small since the change in flux in the core 23 is very small.However, when current is supplied to a detector 16 under a North pole at motor start-up, the flux generated by the current flowing through the primary winding 25 is in opposition to the flux in the core and a more substantial change of flux takes place with the result that the voltage induced in the secondary winding 26 is much greater.

If the current in the primary winding 25 is just sufficient to saturate the core 23 in the absence of a magnetic field, then as a given detector 16 moves from the influence of a South pole to the influence of a North pole the fields will approximately cancel out and the flux in the core 23 will change from saturation to about zero thereby inducing a voltage in the secondary winding 26. Subsequently, as the core 23 of the given detector 16 moves under the influence of a South pole the core 23 will be driven into saturation inducing a voltage of opposite polarity in the secondary winding 26.

Thus it will be appreciated that the voltages induced in the secondary windings 26 of the detectors 16 can be used to switch in an appropriate winding coil 13 at start-up and after start-up these induced voltages can be used for selective switching of the winding coils 13.

Figure 3 illustrates an example of a simplified circuit of the motor described above. As shown, one end of each winding coil 13 is connected to a star point and the other end of each winding coil 13 is connected to earth via an NPN Transistor 28. The primary winding 25 of each positional detector 16 derives a d.c. voltage from the d.c. voltage supply of the motor connected between the star point and earth. Each secondary winding is connected between earth and the base of its respective transistor 28.

A capacitor 29 is connected across the base-emitter junction of each transistor 28.

On start-up, voltage induced in the secondary winding 26 of a detector 16 under a North pole will switch on its respective transistor 28 so that current can flow through an associated winding coil 13. As the detector moves under a South pole the core 23 will be driven into saturation inducing a voltage of opposite polarity which will switch off its respective transistor 28. When the detector again moves under a North pole the flux will change from saturation to about zero and the voltage induced in the secondary winding will again switch on its respective transistor 28. The transistors 28 will, therefore, selectively energise the winding coils in response to the positional detectors 16.

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 as defined by the appendant claims.

For example, the invention can be applied not only to an axial air gap motor as described, but also to a radial air gap motor. Moreover, the positional detectors could be used only for detecting the relative positions of the rotor and stator at start-up, in which case supplementary windings could be provided on the stator for detecting back e.m.f.

and hence the position of the rotor when the rotor is rotating as is known in the art.

Claims (7)

1. Claims

   A brushless d.c. electric motor, comprising: (a) a stator, (b) a rotor mounted for rotation relative to the stator, (c) motor driving windings on the stator, (d) means for selectively energising the motor driving windings in response to means detecting the position of the rotor relative to the stator, the position detecting means comprising at least one positional detector having primary and secondary windings mounted on a common magnetic core which is magnetizable in opposite directions by North and South poles, respectively, on the rotor, the primary winding being energisable by a d.c. input voltage to the motor and the secondary winding generating an output voltage, the magnitude of which is dependent on whether the current in the primary winding seeks to strengthen or oppose the magnetic flux in the core, which is in or is close to saturation.
2. 2. A brushless d.c. electric motor as claimed in claim 1, wherein the position detector is used to detect the position of the rotor during start up and wherein supplementary windings are provided on the stator for detecting back e.m.f. and hence the position of the rotor when the rotor is rotating.
3. 3. A brushless d.c. electric motor as claimed in claim 1, wherein the position detecting means comprises a plurality of positional detectors equal in number to the number of stator phases and each detector operates switching means to selectively energise an associated stator phase at an appropriate time.
4. 4. A brushless d.c. electric motor as claimed in any one of the preceding claims, wherein the core of the or each positional detector has an enlarged head at an end adjacent to the rotor to serve as a flux collector.
5. 5. A brushless d.c. electric motor as claimed in any one of the preceding claims, wherein the core of the or each positional detector is magnetically saturable with no d.c. input voltage applied.
6. 6. A brushless d.c. electric motor as claimed in any one of the preceding claims, wherein the primary and secondary coils of the or each positional detector are axially spaced apart on said common core and a shield of high permeance material is provided between the coils to minimise mutual inductance through the air.
7. 7. A brushless d.c. electric motor, substantially as hereinbefore described with reference to the accompanying drawings.