GB2251131A

GB2251131A - Balancing armature of an electric motor

Info

Publication number: GB2251131A
Application number: GB9027810A
Authority: GB
Inventors: Georg Strobl
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 1990-12-21
Filing date: 1990-12-21
Publication date: 1992-06-24
Also published as: GB9027810D0

Abstract

An armature is provided with balancing material, usually lead rods, inserted in apertures in the radial arms of the armature and arranged to extend beyond the rotor core surface. To compensate any out-of-balance following assembly of the armature, the ends of the rods 25 are machine trimmed to selectively adjust their protruding lengths. <IMAGE>

Description

Electric Motor The invention relates to electric motors.

The invention relates more particularly to fractional horsepower electric motors which often run at high rotational speeds and are manufactured in large quantities. The inertia of the motor armature is therefore important and it is desirable to keep the inertia as low as possible.

It is also desirable to ensure that the armature is well-balanced and this is not easy to achieve simply by setting close tolerances on the armature component dimensions which often or conventionally comprise laminations. Further, the amount of wire wound on each pole is not necessarily or always exactly the same.

According to one aspect of the invention there is provided an armature for an electric motor having arms extending radially from the axis of rotation of the armature for carrying armature windings including apertures therein remote from and extending parallel to the axis of rotation to reduce the effective rotational inertia of the armature.

In use, magnetic flux is concentrated generally adjacent the outer surfaces of the arms. The apertures are preferably positioned as far as possible from the armature axis, to have maximum effect in reducing the inertia, but distanced somewhat from the peripheral surfaces such the absence of material in the arms has the least detrimental affect on the flux path.

According to another aspect of the invention there is provided an armature for a fractional horsepower electric motor having arms extending radially from the axis of rotation of the armature for carrying armature windings, including apertures therein remote from and extending parallel to the axis of rotation of the armature into which separate armature balancing material is inserted.

The balancing material may comprise rods of readily machinable material which extend exterior of the end or ends of the armature so that their effective length and mass can be adjusted after assembly in the apertures. The rods may extend out of one side of the aperture and support a motor cooling fan assembly.

The balancing material may extend at least the full length of one or more of the apertures.

Balancing material may be placed only at one end of at least one of the apertures. The balancing material may comprise lead.

According to a further aspect of the invention there is provided a method of balancing a fractional horsepower electric motor armature comprising inserting balancing material in apertures in the arms of the armature which extend to some extent beyond ends of the armature, and selectively machining the material to balance the motor.

Embodiments of the invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of part of a fractional horsepower PMDC motor embodying the invention without showing an end cap; and Figure 2 is a view of an armature for a fractional horsepower motor.

Referring to the drawings in Figure 1, the fractional horsepower PMDC motor has a housing comprising a deep drawn can-like steel casing 10 carrying permanent magnets 11, and a bearing 12 in an end wall 13. The casing 10 is closed by an end cap (not shown) which carries a bearing. A rotor 19 of the motor comprises a shaft 20, a commutator 21 and wound armature 22 mounted on the shaft 20. The armature 22 comprises a stack 23 of steel laminations which are a tight fit on the shaft 20 and a wire winding 24 wound about the arms of the armature and connected to the commutator 21.

In the assembled motor the shaft 20 runs in the bearing 12 and a bearing supported in the end cap, the end cap (not shown) closing the casing 10.

Brushes (not shown) bear on the commutator 21.

The armature 22 in this example has five radially extending arms as shown which support respective of the windings 24. Apertures extend through each of the arms and lead rods are inserted and held in each aperture. The rods 25, shown only at one end of the armature extend to some extent beyond the end of the armature as shown.

In use, the armature is rotated after initial assembly and winding of the wires on the armature.

The out-of-balance of the armature due to manufacturing tolerances and assembly tolerances of the components are assessed, normally by balancing equipment. Then the ends of the rods 25 are selectively machined to reduce the length of their exposed ends, and so reduce their effective mass, to balance the armature both rotationally and axially; axially by comparative adjustment of the exposed length at each end of the armature.

The apertures themselves, in the armature, are provided in this embodiment to reduce the overall inertia of the armature and are used to hold the balancing rods, or other inserts. It will be appreciated that it is possible to use only some of and not all of the apertures in some embodiments and it is not necessary to insert balancing material in all the arms to achieve satisfactory balancing of the armature. Further in most cases only short rods or pieces of balancing material are used so that the apertures are not completely filled with balancing material along the full length of the apertures and so that the overall inertia of the armature is not greatly increased by adding the balancing material.

The rods 25 may be formed as inserts which have a different diameter along their lengths. The diameter of the rod parts which extend beyond the end of the armature may for example be somewhat greater than the diameter of the aperture. This means that removal of some of the extending part per unit length has somewhat more effect than otherwise when balancing the armature.

It is also possible in some embodiments to provide the balancing material in the apertures and to selectively drill out, or otherwise remove some of the balancing material, to adjust the effective mass of the balancing material. So in such embodiments, the material is partially removed as required from inside the apertures to balance the armature if and as required.

In Figure 2, a similar armature is shown except that in Figure 2 the armature stack has three, instead of five, radial arms to form three electrical poles of an electric motor. The radial arms 30 each have an aperture 31 extending parallel to the axis of rotation of the armature. Two of the apertures 31 have inserted in their ends rods 32 of lead alloy which extend to some extent out of the ends of the armature stack. These ends are trimmed by a suitable machining process to effect balance of the armature in the manner as described earlier.

Figure 2 also serves to illustrate an armature (or lamination) in which the rotational inertia is simply reduced by providing the apertures 31. The armature laminations are each provided with a suitably placed hole in the arms during manufacture of the laminations to form the apertures 31. The absence or removal of material to form the apertures 31 reduces the inertia. The apertures 31 are positioned well away from the rotational axis of the armature but somewhat distanced from the outer peripheral surfaces of the arms 30. This means that the flux generated in use and concentrated adjacent the outer surfaces of the arms 30 is least affected by positioning the apertures 31 symmetrically and where shown in Figure 2. Such an armature can be used, without the balancing material inserts, for example in the motor described with reference to Figure 1.

Claims

1. An armature for a fractional horsepower electric motor having arms extending radially from the axis of rotation of the armature for carrying armature windings, including apertures therein remote from and extending parallel to the axis of rotation of the armature into which separate armature balancing material is inserted.

2 An armature according to claim 1, in which the balancing material comprises rods of readily machinable material which extend exterior of the end or ends of the armature so that their effective length and mass can be adjusted after assembly in the apertures.

3. An armature according to claim 2 in which the rods extend out of one side of the aperture and support a motor cooling fan assembly.

4. An armature according to claim 3 in which the balancing material extends at least the full length of one or more of the apertures.

5. An armature according to claim 3, in which balancing material is placed only at end end of at least one of the apertures.

6. An armature according to any one of claims 1 to 5, in which the balancing material comprises lead.

7. An armature for an electric motor according to claim 1 substantially as herein described with reference to any one or more of Figures 1 and 2 of the accompanying drawings.

8. A fractional horsepower motor having an armature according to any one of claims 1 to 7.

9. A method of balancing a fractional horsepower electric motor armature comprising inserting balancing material in apertures in the arms of the armature which extend to some extent beyond ends of the armature, and selectively machining the material to balance the motor.

10. A method of balancing an armature of an electric motor substantially as herein described.

11. An electric motor having arms extending radially from the axis of rotation of the armature for carrying armature windings including apertures therein remote from and extending parallel to the axis of rotation to reduce the effective rotational inertia of the armature.

12. An armature according to claim 11 substantially as described with reference to Figures 1 and 2 of the accompanying drawings.