GB2221103A

GB2221103A - Electric motor with automatically actuable brake

Info

Publication number: GB2221103A
Application number: GB8916110A
Authority: GB
Inventors: Dieter Roshop; Edgar Zelle
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1988-07-23
Filing date: 1989-07-13
Publication date: 1990-01-24
Anticipated expiration: 2009-07-13
Also published as: IT1231162B; FR2634602A1; DE3825114C2; FR2634602B1; DE3825114A1; GB2221103B; IT8921250A0; GB8916110D0

Description

m 222 1,10 - 1 ELECTRIC MOTOR WITH AUTOMATICALLY ACTUABLE BRAKE The

present invention relates to an electric motor with an automatically actuable brake.

In DE-PS 34 07 731 there is disclosed an electric motor with an automatic brake, wherein part of the magnetic flux into the rotor of the motor is conducted to a ferromagnetic brake armature, which is thereby attracted to the rotor against the effect of a spring when the field is switched on. The armature is pressed against a locally fixed brake surface in opposite direction by the spring effect when the field is switched off, whereby the run-down movement of the rotor is braked. An increase in the braking moment is achieved in that the rotor and brake armature have wedge-shaped surfaces, which contact each other during the braking operation and by reason of their wedge angle press the brake armature in the direction of the brake surface. The motor can thus be brought to standstill within a very short time after the switching-off of the voltage supply.

The use of electric motors with brakes is prescribed for a number of appliances, particularly in the case of electrical tools and appliances driven by electric motors for household and D-I-Y uses. Although the arrangement disclosed in DE-PS 34 07 731 satisfactorily fulfils most requirements, it does have some defects which make an improvement desirable. Thus, for example, it is required to prevent premature response of the brake when there is a fall in the operating voltage of the motor. It is also necessary, without additional effort in terms of production, to provide the wedge surfaces with a smoother surface in order to prevent their seizure during the braking operation and to ensure the retraction of the brake armature.

There is therefore a need to ensure reliability of the braking 1 1 equipment even during voltage fluctuations and additionally to simplify production such that an improved braking effect is achieved with the elimination of previous difficulties caused by processing.

According to the present invention there is provided an electric motor provided with an automatically actuable brake comprising a fixed braking surface, a brake armature co-operable with the braking surface and mounted to rotate with the rotor during operation of the motor and to be movable towards and away from the rotor and also rotatable relative to the rotor through an angle limited by abutment means, the armature being drawn towards the rotor and away from the braking surface by magnetic flux of the motor when switched on, and resilient means effective when the motor is switched off to press the armature away from the rotor and against the braking surface, the abutment means being provided by a plurality of first wedge surfaces projectingfrom an end face of the rotor at an inclination to the axis of the rotor and a plurality of correspondingly inclined second wedge surfaces disposed at the armature and so co-operable with respective ones of the first wedge surfaces on relative rotation of the armature and rotor as to effect displacement of the armature towards the braking surface. 20 Preferably, the armature comprises a bent component in which the second wedge surfaces are defined by walls of stamped recesses directly contacted by a stamping die during formation of the part. For preference, the abutment means comprises a further such plurality of first wedge surfaces and a further such plurality of second wedge surfaces, the surfaces of the further pluralities having an inclination opposite to that of the surfaces of the first-mentioned pluralities and being co-operable on relative rotation of the armature r and rotor in a sense opposite to that for co-operation of the surfaces of the first-mentioned pluralities.

A motor embodying the invention may have the advantage that the wedge surfaces arranged at the rotor end face are constructed to be projecting out in axial direction of the motor from the plane of the rotor end face and of the short-circuit ring, respectively. Due to this construction, the short-circuit ring can retain the usual short axial height. The construction of the wedge surfaces at the rotor as projections causes the wedge surfaces at the armature to be constructed as concavities. The advantage results from this that the surface of the armature, which faces the rotor end face, can be disposed very close to the rotor lamination packet, whereby the magnetic flux flowing through the brake armature is greater than for the known constructions. A higher force of attraction of the armature results and is now sufficient. even in the case of a reduced voltage still able to operate the motor, to overcome the force of the resilient means, for example a compression spring, and prevent premature dropping-off of the brake armature. Moreover, retention of the usual small axial height of the short-circuit ring, on which the first wedge surfaces are present as projections pre- ferably in saddle-roof shape, entails a material saving for the winding compared with the known constructions. Due to the saddle-roof-shaped construction of the wedge surfaces, a universal use of the braking arrangement is achieved for motors of both directions of rotation and thus a simplification of production. Preferably, the rotor winding, short-circuit ring and braking surfaces are formed as an integral diecast component of aluminium alloy.

The construction of the brakinq armature with wedge surfaces, which 4 are preferably roof-shaped and concave in the axial direction of the motor, entails the advantage of simplified production. If the brake armature is made, as usual, of ferromagnetic sheet metal as a punchbent part in a deep-drawing process, then a material straightening or stretching takes place at the wedge surfaces whilst the material surface, by reason of the high pressing pressure, is solidified on the workpiece side acted on by the stamping die. In practice this means that a smooth and clean surface with small depth of roughness arises solely through the pressing operation and requires no further finishing treat- ment. In the known construction, in which the side stretched during the deep-drawing operation serves as a bearing surface, a finishing treatment of the surface is necessary because the depth of roughness is too great and the danger of wedging of the first and second wedge surfaces together, such as to be no longer releasable by the spring pressure, is very high.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, the single figure of which is a schematic sectional elevation of part of an electric motor embodying the invention, in the operating state of the motor.

Referring now to the drawing, there is shown part of an electric motor comprising a shaft 1 which carries a rotor lamination packet 2, at an end face 3 of which a compression spring 4 is centrally arranged. A brake armature 5 is arranged on the shaft 1 and comprises a pot- shaped portion 6 which carries brake linings 7. Connected with the portion 6 is a further, preferably pot-shaped portion 8 which is, however, turned through 1800 relative to the portion 6 and which has a collar 9 with several roof-shaped concavities 13.

1 A short-circuit ring 10, which interconnects winding rods of the rotor windings, is situated at the end face of the rotor lamination packet. The ring 10 corresponds in its dimensions to those of a motor of normal mode of construction and is constructed to be as flat as possible in order to minimise the overall length of the rotor. Arranged on the ring 10 are several wedge surfaces 11, which project in the axial direction of the motor and are constructed in roof 'shape, in the described embodiment, in order to be effective for motors of either direction of rotation. Preferably, the wedge surfaces 11 are formed by projections 12 which are produced integrally with the ring 10 and the winding rods in a die-casting process from aluminium alloy.

The wedge surfaces 11 correspond with the concavities 13 at the collar 9 of the pot-shaped portion 8, which form wedge-shaped surfaces 14. On braking of the motor, the wedge surfaces 11 and 14 respect ively associated with one direction of rotation are pressed one against the other with the effect that the brake armature 5 is pressed,with an additional force, in axial direction of the motor against a locally fixed braking surface (not shown).

Since the ring 10 is constructed to be very flat, the collar 9 of the armature 5 lies very close to the rotor lamination packet during operation of the motor. Thereby, the greater field density in the proximity of the packet has the effect that the force of attraction acting on the armature is appreciably greater than in known constructions which have a greater minimum spacing between rotor and armature.

Thus, in advantageous manner there is provided a moment of attraction which securely prevents dropping-off of the armature at the lowest permissible voltage for motor operation.

Since the wedge surfaces 11 at the rotor consist of a softer materi a] than the surfaces 14 of the armature 5, the depth of roughness of the harder surfaces should remain within predetermined limits in order to prevent wear and, above all, seizing of the interengaged wedge surfaces, as such seizing would prevent release of the surfaces when the motor is switched on again. For this purpose, the crystalline structure of the constituent material, preferably iron plate, of the collar 9 is changed during deep-drawing thereof to provide the shape of the wedge surfaces 11. The change in structure is utilised both for simplification of the production process and for surface improvement. Since the wedge surfaces 14 are pressed into the collar 9, preferably by means of a stamping die, in direction towards the pot-shaped portion 6, a compaction takes place of the surfaces 14 which face the stamping dieandthus receive a smooth, clean surface of small depth of roughness.

The opposite side, i.e. the MrawC side, has an appreciably greater depth of roughness, and could serve for the wedge surfaces only if smoothed with appreciable effort as is necessary in the prior art construction.

1

Claims

CLAIMS 1. An electric motor provided with an automatically actuable brake

comprising a fixed braking surface, a brake armature co-operable with the braking surface and mounted to rotate with the rotor during operation of the motor and to be movable towards and away from the rotor and also rotatable relative to the rotor through an angle limited by abutment means, the armature being drawn towards the rotor and away from the braking surface by magnetic flux of the motor when switched on, and residual means effective when the motor is switched off to press the armature away from the rotor and against the braking surface, the abutment means being provided by a plurality of first wedge surfaces projecting from an end face of the rotor at an inclination to the axis of the rotor and a plurality of correspondingly inclined second wedge surfaces disposed at the armature and so co-operable with respective ones of the first wedge surfaces on relative rotation of the armature and rotor as to effect displacement of the armature towards the braking surface.
2. An electric motor as claimed in claim 1, wherein the armature comprises a bent component in which the second wedge surfaces are defined by walls of stamped recesses directTy contacted by a stamping die during formation of the part.
3. An electric motor as claimed in either claim 1 or claim 2, the abutment means comprising a further such plurality of first wedge surfaces and a further such plurality of second wedge surfaces, the surfaces of the further pluralities having an inclination opposite to that of the surfaces of the first-mentioned pluralities and being co-operable on relative rotation of the armature and rotor in a sense opposite to that for co-operation of the surfaces of the firstmentioned pluralities.
4. An electric motor as claimed in any one of the preceding claims, wherein the first wedge surfaces are provided on a short-circuit ring of a winding of the rotor.
5. An electric motor as claimed in claim 4, wherein the first wedge surfaces are of aluminium alloy.
6. An electric motor as claimed in any one of the preceding claims, wherein the second wedge surfaces are of deep-drawable ferromagnetic mater i a].
7. An electric motor substantially as hereinbefore described with reference to the accompanying drawing.

Published 1990 at The Patent Office, State House, 6671 High Holborn, London WC1R 4TP. Further coplesmaybe obtainedfrom The Patent Office. Wes Branch, St Mary Cray, Orpington, Kent BR5 ZED. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187

7. An electric motor substantially as hereinbefore described with reference to the accompanying drawing.

- q - Amendments to the claims have been filed as follows 1. An electric motor provided with an automatically actuable brake comprising a fixed braking surface, a brake armature co-operable with the braking surface and mounted to rotate with the rotor during operation of the motor and to be movable towards and away from the rotor and also rotatable relative to the rotor through an angle limited by abutment means, the armature being drawn towards the rotor and away from the braking surface by magnetic flux of the motor when switched on, and resilient means effective when the motor is switched off to press the armature away from the rotor and against the braking surface, the abutment means being provided by a plurality of first wedge surfaces projecting in the direction of the rotor axis from the plane of a shortcircuit ring of the rotor winding and at an inclination to the plane of rotation and a plurality of corresponding second wedge surfaces disposed at the armature and so co-operable with respective ones of the first wedge surfaces on relative rotation of the 'armature and rotor as to effect displacement of the armature towards the braking surface.

2. An electric motor as claimed in claim 1, wherein the armature comprises a bent component in which the second wedge surfaces are defined by walls of stamped recesses directly contacted by a stamping die during formation of the part.

3. An electric motor as claimed in either claim 1 or claim 2, the, abutment means comprising a further such plurality of first wedge surfaces and a further such plurality of second wedge surfaces, the surfaces of the further pluralities having an inclination opposite to that of the surfaces of the first-mentioned pluralities and being -10 co-operable on relative rotation of the armature and rotor in a sense opposite to that for co-operation of the surfaces of the first mentioned pluralities.

4. An electric motor as claimed in any one of the preceding claims, wherein the first wedge surfaces are formed integrally with the shortcircui.t ring.

5. An electric motor as claimed in claim 4, wherein the first wedge surfaces are of aluminium alloy.

6. An electric motor as claimed in any one of the preceding claims, wherein the second wedge surfaces are of deep-drawable ferromagneti material.