Electrical Machine

The invention relates to electrical machines of the kind having a stator and a rotatable armature, the winding of which is energized via a commutator or via slip rings. Such electrical machines are widely used as motors or as generators.

It has been found that in such machines the commutation performance is impaired after a long period of operation. Especially in small motors it may happen that the brushes cooperating with the commutator become damaged or broken.

It is therefore an object of the invention to improve an electrical machine of the kind referred to such that the commutation performance is not impaired even after a long period of operation.

It is another object of the invention to provide an electrical machine in which the brushes do not become damaged due to a relative axial displacement between the commutator on the one hand and the brushes on the other hand.

Those and other objects are achieved by an electrical machine in which the brushes are held by a holding plate, which in turn is supported so as to be axially freely shiftable but circumferentially stationary with respect to the axis of the machine. The holding plate butts against an element of the machine that is axially stationary with respect to the shaft of the armature or rotor. By means of this mounting arrangement, the brushes will follow any axial movement of the armature and of the commutator and thus will be in an axially stationary position unshiftable with respect to the commutator.

Preferably, the holding plate butts against an element of or on the rotor shaft that is in or near the plane in which the holding plate is positioned. By this, a different thermal expansion of the rotor shaft on the one hand and of the stator or housing of the machine on the other hand will not adversely affect the axial position of the brushes with respect to the commutator.

It has been found that after a long period of operation of an electrical machine having a commutator or a slip ring, the brushes form grooves in the same by abrasion at the contacting surface. In the electrical machine according to the invention it is assured that the brushes remain in thier initial axial position with respect to the commutator. Thus, when grooves are formed in the same after a long period of operation, the brushes are prevented from contacting only the rim of the grooves, which would cause increased sparking and a higher contact resistance. According to the invention, the brushes maintain an even contact with the surface of the commutator or of the slip rings, as the case may be, even if grooves have been formed on the same after a long period of operation. The most important advantages will be achieved in commutator machines, whereas in electrical machines having slip rings the favourable effect is not so significant; accordingly, the following detailed description has reference to commutator machines.

In a preferred embodiment, the holding plate butts against the rotor bearing which is provided nearest the holding plate and which should be capable of receiving axial forces. By this, the axial shifting of the brushes with respect to the commutator is considerably reduced or even totally eliminated.

Preferably, the holding plate may be resiliently biased towards the element against which the holding plate butts. By this, it is guaranteed that the axial play of the bearing of the rotor which supports the holding plate does not influence the relative axial position of the brushes with respect to the commutator.

It is advisable to provide at least two pressure springs between the holding plate and the adjacent end plate of the electrical machine in a circumferential equidistant relationship. The holding plate is thus biased without the risk of being tilted.

The maximum advantage will be achieved if the element or region of the rotor shaft, against which the holding plate butts, lies in or near the plane of the holding plate, since by this the effect of thermal expansion on the commutation performance is minimized.

In a specific embodiment, the holding plate comprises a central hollow hub, the inner wall of which receives one of the bearings of rotor and the outer wall of which slidingly fits within a correspondingly shaped recess of the corresponding end plate. The shiftability between the outer wall of the hollow hub on the one hand and the recess of the corresponding end plate on the other hand assures that the relative axial position of the brushes with respect to the commutator is maintained. The hollow hub may also comprise a radially inwardly projecting shoulder which serves as an abutment for the rotor bearing. A keyway coupling may be provided to prevent a relative rotational movement of the hollow hub with respect to the second end plate.

The single FIGURE shows a longitudinal sectional view of an electrical machine according to the invention taken through the axis of rotation of the machine.

The following description taken in connection with the accompanying drawing is directed to a preferred embodiment.

In the drawing there is shown an electrical machine 1 comprising a housing 2, at the inner wall of which an even number of permanent magnets are secured. In the specific embodiment illustrated, two diametrically opposing permanent magnets 3 and 3a are provided. A first, or front, end plate 4 is attached to one side of the tubular housing, and a second, or rear, end plate 5 is attached to the other side of the housing. Each of these end plates have a radially outwardly projecting shoulder 6 abutting against the corresponding end face of the housing 2, the end plates and the housing forming a stator frame for the machine 1.

The front end plate 4 is provided with a bore 7 which over a part of its length has an increased diameter and forms a seat for a ball bearing 8. The second or rear end plate 5 is provided with a central bore 9 which has a greater diameter than the outer diameter of the adjacent ball bearing 12 supporting the shaft 21 of the armature 20.

A circular holding plate 10 is provided which has a protruding hollow hub 11, the outer surface of which slidably fits in the central bore 9 and the inner surface of which snugly receives the ball bearing 12.

At its outwardly facing end, the hollow hub 11 is provided with a radially inwardly projecting shoulder 13 which serves as an abutment for the outwardly facing side face of the outer cage ring 14 of the ball bearing 12.

The rear end plate 5 is further provided with two diametrically opposed blind bores 15, which face into the interior of the machine. Each of these bores 15 is adapted to receive a pressure spring 16, the spring exerting a biasing pressure against the holding plate 10.

In the embodiment shown, the holding plate 10 is made of metal. Its side facing the armature carries support members, or spacers, 17 made from an insulating material, and each of these members holds a brush receptacle 18, into which a brush 19 and a spring fixed thereto is inserted. Each of the receptacles 18 is provided with a connecting tag, or electrical lead (not shown).

The shaft 21 supporting the rotor or armature 20 is rotatably held in the ball bearings 8 and 12 and is of a reduced diameter at its end portions, so that shoulders 22 are formed which serve as abutments for the inner cage rings of the ball bearings 8 and 12.

The armature 20 includes windings 20' which are connected to commutator 23 in conventional manner. The commutator is mounted on shaft 21 adjacent the armature in conventional manner and cooperates with the brushes 19.

The the exerted by pressure springs 16 tends to displace the holding plate 10 to the left as seen in the FIGURE. This axial displacement is restricted, however, by the ball bearing 12, which concurrently acts as a radial as well as an axial bearing. By the action of the pressure springs 16 the effect of any axial play of the ball bearing 12 on the relative axial position of the brushes 19 with respect to the commutator 23 is eliminated. If the shaft and the commutator fixed thereto moves axially, the ball bearing 12 and the holding plate 10 supporting the brushes will be displaced in the same amount.

Instead of ball bearings, any other bearing may be used that is adapted to transmit the small axial load of the springs 16.

It is advantageous to arrange the pressure springs in an equidistant angular relationship to one another so as to avoid the exertion of tiliting forces on the holding plate 10.

If bearings are used having a minute or neglectable axial play, the pressure springs may alternatively be omitted.

The sliding fit of the hollow hub 11 in the second end plate 5 should be as easy-going as possible in order not to jeopardize the free axial movement of the holding plate, which of course must be held circumferentially immovable, for instance by providing a pin 24 fitted in the hollow hub 11 and extending into a longitudinal groove 25 formed in the second end plate.

The invention is not limited to the aforementioned embodiment, but can be realized in any kind of electrical machines having commutators or slip rings and brushes of any kind cooperating therewith. The invention is also applicable to electrical machines, in which the rotor is adapted to perform a desired axial movement.