GB2209881A

GB2209881A - Shifting brushgear of an electric motor

Info

Publication number: GB2209881A
Application number: GB8819352A
Authority: GB
Inventors: Roger Frederick Baines
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-09-11
Filing date: 1988-08-15
Publication date: 1989-05-24
Also published as: GB8819352D0; GB8721417D0; JPH0190255U; DE8811450U1

Abstract

In a small p.m.d.c. electric motor, brushgear is mounted on a frame 48 which is movable on an end cap 19 to adjust the commutation position without distributing a bearing 20 carried by the end cap 19. The frame 33 is adjusted by releasing a lockscrew 49 screwed into the end cap 19 and rotating a pinion 50 which engages a rack on the frame 48. <IMAGE>

Description

BRUSHGRAR FOR AN ELECTRIC MOTOR The present invention relates to a fractional horsepower permanent magnet direct current motor, and in particular to the mounting of brushgear in such a motor. Many fractional horsepower permanent magnet direct current motors are built to satisfy the market for model makers and enthusiasts.

Such model makers and enthusiasts are not content simply to mount a motor in their model and use it: they are interested in being able to service their motors and to 'tune' them for optimum performance.

One of the aspects of 'tuning' is the ability of the enthusiast to be able to adjust the commutation timing on his motor so that the motor can be optimised for naximum performance at a particular working point, which requires a particular orientation of commutation with respect to the stator magnetic field.

The motor brushgear is usually carried in an end cap on the motor housing. It has been common practice to provide arcuate slots in the end cap so that the enthusiast may loosen the end cap and rotate it with respect to the motor housing, thereby setting a new angle of commutation to the magnetic field.

There are drawbacks in this design. It is quite common for the motor to be located in a model in such a manner that there is limited adcess to it. Undoing two screws or nuts and rotating the end cap to an accurate position can be difficult. Furthermore it is usual for the motor shaft to be carried in ball bearings mounted in the housing ends; by disturbing the end cap, which carries a bearing as well as the brushgear, it is possible to upset the bearing alignment which is set by the motor manufacturer.

The present invention aims to provide a system which eliminates the need to disturb the setting of the bearings and which allows the enthusiast to make fine adjustments to the commutation timing even when access to the motor is restricted.

The present invention provides a fractional horsepower permanent magnet direct current motor comprising a housing having a permanent magnet stator mounted therein, a rotor assembly comprising a shaft and a commutator and a wound armature mounted fast on the shaft, the shaft rotating in bearings mounted on ends of the housing, and brushgear mounted on a frame and having brushes bearing on the commutator for feeding current from a power supply to the armature winding, wherein the housing has an opening adjacent the commutator, the opening being bound on one side by a rim which extends circumferentially of the motor shaft, the frame is mounted on the rim and is movable along the rim circumferentially of the shaft, means is provided for moving the frame along the rim, and means is provided for holding the frame in a fixed position on the rim.

Preferably the frame is moved by toothed means, such as a rack and pinion.

In this way the commutation may be adjusted by moving the frame relative to the housing, the bearings being undisturbed.

Other, preferred features and advantages of the invention will be apparent from the following description and the accompanying claims.

The invention will be further described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of a fractional horsepower PMDC motor embodying the invention; Figure 2 is a cross-section along the line II-II of Figure 1; Figure 3 is a view on arrow A of Figure 1; Figure 4 is a view on arrow B of Figure 1; and Figure 5 is a cross-section along the line V-V of Figure 1.

The drawings show a fractional horsepower permanent magnet direct current motor 1. The motor 1 comprises a housing 2 having a cylindrical central casing part 3 closed at one end by an end plate 4. The central casing part 3 is made up of a stack of steel laminations 5 each having a castellated outer periphery 6. The stack of laminations is held together during assembly by hollow rivets 7 passing through diametrically opposed holes 8 formed in inward projections 9 in the laminations 5. The projections 9 define between them recesses 10 which, in the formed stack 3, receive high-energy magnets 54 of the Neodymium-Iron-Boron type. Leaf springs 55 hold the magnets against one of the projections 9.

The end plate 4 is cast from metal or alloy and carries a ball bearing 11. The end plate has a skirt 12 which bears against the stack 3. The skirt 12 has four recesses 13 in its axial edge which form an opening into the housing 2 for air to pass. The outer circumferential surface of the end plate 4 is castellated in the same manner as the stack 3.

Tapped holes 56 are provided for mounting the motor.

An end cap in the form of a bridge piece 14 of metal or alloy is mounted on the other end of the stack 3.

The bridge piece 14 carries a ball bearing 15. The bridge piece 14 comprises an annular rim 16 with a skirt 17 which bears against the stack 3 and has four recesses 18 for the passage of cooling air into the motor. Integrally formed with the rim 16 is a bridge 19 which carries a ball bearing 20. Bridge 19 comprises two legs 21 which extend in the axial direction of the motor from the inner periphery of the rim 16 and a bearing housing 22 at the outer axial ends of the legs 21. The bridge piece 14 thus provides two openings 59 between the rim 16 and bridge 19, through which access to the commutator 27 is gained.

The end plate 4, stack 3 and bridge piece 14 are held together by two through bolts 23. As seen in Figure 2, heads 23' of the bolts 23 engage in recesses in the end plate 4 and extend through the end plate and hollow rivets 7. The bolts are screwed into threaded holes 60 in the bridge piece 14 to hold the end plate 4, stack 3 and bridge piece 14 securely together.

Journalled in the bearings 11, 20 is a rotor 24 of standard configuration for a PMDC motor, namely a shaft 25 carrying a laminated armature stack 26 wound with armature coils (not shown) connected to a commutator 27 mounted fast on the shaft. One end, 28, of the shaft 25 has a flat 29 for connection to a drive train.

The housing and armature dimensions are arranged so that the commutator 27 extends between the legs 21 of the bridge 19 and hence is accessible from either side of the bridge 9.

An annular guide wall 30 is formed on the rim 16.

The wall is spaced from the inner and outer edges of the rim 16 so as to form inner and outer circumferential ledges 31, 32.

A frame 33 carrying brushgear is mounted on the wall 30. As seen in Figures 2 and 3 the frame is semi-circular and sits astride the wall 30 such that it will slide along the wall 30, circumferentially about the motor axis. The frame 33 comprises a semi-circular base 34 having skirts 35, 36 which extend down from the inner and outer circumferential edges to fit snugly about the wall 30, the skirts 35, 36 sliding on the ledges 31, 32. The frame is of electrically insulating material, such as plastics or anodised aluminium. The latter is preferred for greater dissipation of heat from the brushgear. Two electrically conducting terminals 37 are mounted in the base 34, for example by force fitting into slots in the base. The terminals have shaped outer ends 38 for receiving female tags of power supply leads.

Brushleaves 39 each carrying a brush 40 are rivetted to respective terminals 37 by rivets 41. The brushleaves are of resilient metal strip stock as is usual in the art. The brushleaves and brushes are urged against the commutator 27 by coil springs 42 mounted on posts 43. One leg 44 of a spring 42 bears against a leaf 39, and the other leg 45 bears against a post 46. Three such posts 46 are provided for each spring 42. This enables the motor user to adjust the brush pressure on the commutator by hooking the leg 45 behind a different post 46.

The frame 33 is adjustably mounted on the wall 30 by means of a rack and pinion which allows for controlled adjustment of the position of the frame.

A kidney shape slot 47 is provided in the base 34. A rack 48 is formed on the radially outer wall of the slot 47. A locking screw 49 is screwed into one of the threaded holes 60 and holds a pinion 50 in the slot 47. The pinion 50 sits in the slot 47 with its lower end just clear of the wall 30. The pinion 50 engages with the rack 48 and has a collar 51 which extends past the head 52 of the screw 49. Slots 53 are formed in the collar 51.

To adjust the motor commutation, that is the position of the brushes 40 relative to the motor stator, i.e.

the magnets, the screw 49 is loosened. The pinion 50 is then turned to drive the frame along the wall 30, thus moving the brushes about the motor axis. When the desired commutation position has been reached, the lockscrew 49 is tightened, forcing the pinion against the frame and thus the frame against the wall, preventing further turning of the pinion 50 and holding the frame in position against the rim of the bridging piece.

Markings 57 on the circumferential surface of the frame and a centre line 58 on the stack 3 and bridge piece 14 enable the user to gauge accurately the position of the brushes.

Various modifications may be made to the described embodiment and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims

1. A fractional horsepower permanent magnet direct current motor comprising a housing having a permanent magnet . stator mounted therein, a rotor assembly comprising a shaft and a commutator and a wound armature mounted fast on the shaft, the shaft rotating in bearings mounted on ends of the housing, and brushgear mounted on a frame and having brushes bearing on the commutator for feeding current from a power supply to the armature winding, wherein the housing has an opening adjacent the commutator, the opening being bound on one side by a rim which extends circumferentially of the motor shaft, the frame is mounted on the rim and is movable along- the rim circumferentially of the shaft, means is provided for moving the frame along the rim, and means is provided for holding the frame in a fixed position on the rim.

2. A motor as claimed in claim 1, wherein said brushes are each supported at one end of a brushleaf, the brushleaves being mounted at their other end on the frame.

3. A motor as claimed in claim 1 or 2, wherein a spring is mounted on the frame and bears on the brushleaf to urge the brush against the commutator.

4. A motor as claimed in claim 1, 2 or 3, wherein the rack is provided on the frame and the pinion is mounted on the housing.

5. A motor as claimed in any one of claims 1 to 4, including means for locking the pinion in position to lock the frame in position on the rim.

6. A motor as claimed in any one of claims 1 to 5, wherein the frame is mounted on a removable end cap on the housing, the end cap carrying a said bearing.

7. A motor as claimed in any one of claims 1 to 6, wherein the frame is of plastics material.

8. A motor as claimed in any one of claims 1 to 7, wherein the housing has a cylindrical casing part carrying said magnets, said casing part being formed by a stack of laminations.

9. A motor as claimed in claim 8, wherein the outer surface of said stack is ribbed.

10. A motor as claimed in claim 9, wherein the outer surface of said stack is castellated.

11. A motor as claimed in claim 1, wherein an end of the housing adjacent the commutator has an end cap carrying said bearing for the motor shaft, said end cap comprising a circular rim arranged in a plane transverse to the shaft and a plurality of legs extending axially from the rim to a housing for the bearing, and said frame is mounted on the rim.

12. A motor as claimed in claim 11, wherein the frame straddles an arcuate wall on the rim, the wall extending along a circular arc about the motor axis.

13. A motor as claimed in claim 12, wherein the brushes are urged against the commutator by springs whose pressure is adjustable.

14. A motor as claimed in any one of claims 1 to 13, wherein the means for moving the frame is toothed.

15. A motor as claimed in claim 14, wherein the means for moving the frame comprises a rack and pinion.

16. A fractional horsepower permanent magnet direct current motor substantially as hereinbefore described with reference to the accompanying drawings.