Method and device for compensating the armature reaction of a rotating exciter

#The present invention relates to a process for cancelling the reaction field, also called armature reaction, induced by a rotor which is rotating in a static magnetic field in a static outer core, wherein the rotor comprises at least two conductors aligned substantially parallel to the axis of the rotor. The present invention also relates to a static outer core for carrying out such a process and to a brushless exciter comprising such a static outer core.

#A synchronous machine is a dynamoelectric machine which may be utilised as a motor for driving a shaft or any load at a constant speed or as a generator for producing a voltage at a predetermined frequency depending on the speed of the driving shaft. When the device is used as a synchronous generator, it is customary to e.g. provide field excitation for the rotor through a synchronous brushless exciter generator. The exciter generator converts the direct current (DC) stator field into a polyphase alternating current (AC) armature voltage which is rectified by a set of rotating rectifiers mounted on or within the driving shaft to provide the DC excitation for the field windings of the synchronous generator, i.e. for the rotor of the generator.

#Thus, a rotating exciter is a reversed generator with the field winding, fed with DC current, implemented on the static parts. The armature is located on the rotating part and produces AC voltage. A set of diodes is used to rectify it to produce a DC current, required to energise the field winding of the synchronous machine, i.e. the rotor of the generator.

#A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

#FIG. 1 shows a schematic representation of the static magnetic field (B) as a function of the circumferential angle Theta;

#FIG. 2a) shows a schematic cut perpendicular to the axis of the exciter through the rotor according to the invention; b) schematically shows the electrical connectivity of the rotor;

#FIG. 2a) shows an axial cut through an exciter serving as a preferred embodiment. The rotor 4 is coaxially mounted in an static outer magnetic core 3, wherein in between there is an air gap 9. The static outer core 3 comprises a field winding 5 which induces a magnetic field as indicated with the lines 7. In this embodiment the induced magnetic field 7 is bipolar, i.e. the field winding 5 is fed with a current positive on the right side and negative on the left side or vice versa in FIG. 2a). It is however also possible to have a permanent magnet 18 giving rise to such a bipolar field. The use of a permanent magnet 18 does not allow to directly control the current induced in the rotor, which is necessary to control the output power of the generator, and therefore usually if permanent magnets 18 are used, there is also a field winding 5, which can be used either to increase or to decrease the static background magnetic field by feeding it with an appropriate current. The magnetic field 1 induced by the field winding 5 is schematically drawn in FIG. 1 as a function of the angle theta around the rotational axis 8 common to the rotor 4 and the static outer core 3. The ideal magnetic field 1 shows a rectangular characteristic, the sign change being located at the position where theta aims at the field winding 5, i.e. horizontally in FIG. 2a).

#The static outer core 3 comprises a field winding 5 which induces a magnetic field as indicated with the lines 7. In this embodiment the induced magnetic field 7 is bipolar, i.e. the field winding 5 is fed with a current positive on the right side and negative on the left side or vice versa in FIG. 2a). It is however also possible to have a permanent magnet giving rise to such a bipolar field. The use of a permanent magnet does not allow to directly control the current induced in the rotor, which is necessary to control the output power of the generator, and therefore usually if permanent magnets are used, there is also a field winding 5, which can be used either to increase or to decrease the static background magnetic field by feeding it with an appropriate current. The magnetic field 1 induced by the field winding 5 is schematically drawn in FIG. 1 as a function of the angle theta around the rotational axis 8 common to the rotor 4 and the static outer core 3. The ideal magnetic field 1 shows a rectangular characteristic, the sign change being located at the position where theta aims at the field winding 5, i.e. horizontally in FIG. 2a).

#As can be seen from FIG. 2a), basically the rotating part, i.e. the rotor 4, looks very much like the one of an induction machine with a squirrel cage rotor. On a one side all the conductors which in this case are given by bars 6 are welded or electrically connected to a ring 13 (see FIG. 2b)) which collects the current and dispatches it to the return bars 6. On the other side, each bar 6 is electrically connected individually to two rings 14 and 15 by means of diodes 16 with reversed polarity. These two rings are the DC terminals. All the bars 6 are identical and are evenly distributed around the circumference of the rotor 4. The crossed diodes 16 lead to a rectification of the alternating current induced in the bars 6 when the rotor is rotating in the static magnetic field 7, thus giving rise to a direct current (DC) in the two rings 14 and 15, which current 17 can subsequently be used for energising the rotor of the generator.