The present invention refers to a reluctance motor for one or more phases of switched direct current of the type described in the introduction to the attached main claim.

A reluctance motor consists of a stator for one or more phases,, in which each phase may have one or more poles, each incorporat¬ ing one or more windings. In each phase, the windings are either connected in series, in parallel or in series and parallel. The stator surrounds the rotor, which generally incorporates salient poles, which are symmetrical or unsymmetri ca I in des¬ ign in order to provide starting torque.

By feeding direct current pulses - with the aid of a "commut¬ ation device" - to each phase at certain predetermined angular intervals in the rotor, the required torque in the motor is obtained. These angular intervals in the rotor are scanned by means of a rotor-posit on sensor which transmits signals to a control device, which in turn controls a power-step for each phase.

In certain cases, it is desirable for a motor to be capable of being operated at low speed with a correspondingly higher torque. In known designs for motors, this is achieved by in¬ creasing the number of phases, and thus the number of poles on the stator. However, this results in more windings, necess¬ itating both additional labour and more copper, in turn entail¬ ing an often not-inconsiderable increase in price for the mot¬ or.

Thus the purpose of the present invention is to present a reluct¬ ance motor with few phases in relation to its speed, that is to say, with as few windings as possible.

The invention solves this problem in the manner detailed in the descriptive part of the attached main claim. In this, it is assumed that the reluctance motor according to the present invention incorporates a stator with windings for one or more phases arranged around two or more poles for each phase, and in such a way that the poles surround a rotor with two or more

rotor poles. The invention is characterized in that the motor is of the commutator type incorporating a rotor position sensor, together with the fact that the stator poles branch to form teeth, two or at most three per pole. By this means, the motor is capable of low speed with correspondingly high torque, but is still a simple and inexpensive design, since few winding coils are required and thus also relatively little copper.

In a particularly advantageous embodiment of the invention, the motor is two-phase, with two poles per phase and two teeth per pole; in addition, the distance between the two teeth on each pole is great¬ er than the distance between two adjacent teeth located on diff¬ erent but adjacent poles. In this embodiment, it is beneficial for the rotor poles to be supplemented by auxiliary poles in order to bring about a precisely-defined starting torque.

The invention is described in greater detail below, with refer¬ ence to the attached Fi ures 1 - 3, in which Figure 1 shows a reluctance motor according to known design, which is three- phase with six stator poles and four rotor poles, in which Figure 2 shows a different reluctance motor according to known design, which is two-phase with four stator poles and two rotor poles, and, finally, in which Figure 3 shows a reluctance motor according to the present invention, for two phases,with four stator poles and two rotor poles, and in which the stator poles branch into two teeth per pole. All three motors are assumed to be of the switched type, incorporating commutation and a rotor position sensor according to known design.

It is assumed that all three motors represented in the Figures operate at a speed of 3,000 r.p.m. The motor illustrated in Figure 1 incorporates six poles, and also six winding coils, 21. The motor is three-phase and the poles operate in pairs. The rotor, 22, has four rotor poles, 23-26. Auxiliary poles for starting torque are not necessary in the three-phase reluct¬ ance motor. In this motor type, the commutation frequency at 3,000 r.p.m. is 600 Hz.

Figure 2 shows a reluctance motor, also of known design. It is a two-phase motor with four poles and four winding coils in the stator. The two rotor poles, 31 and 32, and provided with auxiliary poles, 33 and 34, in order to achieve a defined starting torque. When this motor - like the variant shown in Figure 1 - rotates at a speed of 3,000 r.p.m., it provides a considerably lower torque than the variant shown in Figure 1. However, the commutation frequency is lower - 200 Hz, to be precise - which is an advantage in that eddy current losses are smaller, in turn improving the motor's efficiency.

Figure 3 illustrates an embodimentof the reluctance motor acc¬ ording to the present invention. The chosen example shows a two-phase motor with four poles and two teeth per pole. For certain purposes, it may be advantageous to select a different number of phases, a different number of poles per phase or to choose between two or three teeth per pole. The advantage of toothed poles is generally greatest where there are two or three teeth per pole. Where there are more teeth per pole, the commutation frequency is so high that the gain in terms of simplicity, slow speed and torque may be swallowed up by increasing eddy current losses.

In the embodiment of the invention illustrated in Figure 3, the stator, 1, is designed for two phases, in which each phase has two winding coils, 7, 9, and 8, 10, around each of its poles, 3, 5, and 4, 6. The poles are branched into two teeth, 12, 13, etc. The teeth are so arranged that, in the example shown, the teeth on the same pole are twice the distance from each other as from two adjacent teeth on different poles. Stator 1 also surrounds rotor 2, which, in the chosen configuration of the stator, is provided with six identical poles, 11, and with six auxiliary poles, 14, immediately adjacent to the latter. In the example shown, the motor rotates anti-clockwise.

The variant of the motor according to the present invention, as illustrated in Figure 3, corresponds as regards complexity and manufacturing cost to the motor according to known design which is illustrated in Figure 2, but, as the same speed, has approximately double the potential torque. The motor illustrated

in Figure 1 provides approximately the same performance as the one according to the invention and the version as illustrated in Figure 3. At the same speed, the motors have the same torque and commutation frequency; however, the motor accord¬ ing to the present invention is cheaper to produce.

The use of toothed poles is known in other types of motors, primarily stepper motors. Such motors, however, are not comm¬ utation motors, but are generally of the stepper motor type. An alternating current motor of this type is described in U.S. patent 4,035,680.

The commutation principle itself, by which a rotor position sensor, for example of the optical type, operates a control device to which output stages are connected, is known, and is described, for example in the British Patent Specification No. 1,597,790. It may also be mentioned at this point that a high commutation frequency places considerable demands as to the angular accuracy of the rotor position sensor. Switching the motor on or off too early or too late will result in reduced efficiency. •

It should also be taken as understood that it is not necessary to have the rotor surrounded by the stator. In principle, the rotor may equally well be on the outside.