GB2213326A - Adjustable brush pressure in an electric motor - Google Patents

Abstract

In a fractional horsepower PMDC motor a brush 40, (137) is supported in contact with a commutator on a brushleaf 39, (138) mounted on a frame 33, (113). A spring 44, (138) bears against the brushleaf 39, (138) to urge the brush 40, (137) against the commutator 27, (111). The spring pressure is adjustable to allow a user to maintain a uniform and optimum brush pressure as the brush wears. Adjustment is by means of the pegs 46 or by an adjusting screw (149, Fig 6). The frame 33, (113) is movable circumferentially of the commutator to adjust motor timing. <IMAGE>

Description

ADJUSTING BRUSH PRESSURE IN AN ELECTRIC MOTOR The present invention relates to the adjustment of brush pressure in an electric motor.

In small, fractional horsepower permanent magnet direct current motors or micro motors it is common for the brushgear design to allow the brush pressure to change substantially as the brush wears. It is common to over pressure the new brush so that the optimum pressure is reached at some point mid way through the life 0: the brushes.

For most applications this difficulty does not present too many problems. However when using motors with rechargeable cells it is importent to 2void wasting stored energy and the performance of the motor is mostly determined by the effectiveness o' the brush to commu ator interface. A careful balance has to be achieved between strong pressure to keep the interface resistance low and not too strong a pressure which will otherwise cause high frictional losses and mechanical wear on the brushes.

It has been common practice in the design of brushl eaves to use a material such as Beryllium Copper alloy which acts both as the spring and as the means of carrying current from the brush to the motor terminal.

It is now apparent that for some applications it is advantageous to separate the functions of providing pressure and carrying current. In this way much higher currents can be carried by the current carrying component since there will be little effect on the pressure characteristics if this component reaches high temperatures due to ambient and self heating from 12R (current) heating. The pressure delivering component no longer has to carry current and may therefore have a higher resistivity to electricity, hence, spring steel can be selected which is less temperature sensitive and thus unaffected by such high power inputs.

The present invention provides a fractional horsepower permanent magnet direct current motor having a housing, a rotor assembly rotatably mounted in the housing and comprising a shaft and a commutator and wound armature mounted fast on the shaft, and brushgear mounted on tne nousing for supplying current to the commutator the W-ushgecr comprising a brush mounted on an electric2l'v conducting brushleaf and a spring urging the brush against the commutator, wherein means is provided to adjust the pressure of the brush against the commutator by adjusting the pressure exerted by the spring.

By adjusting the spring pressure or tension a motor user can adjust the brush pressure to maintain it at an optimum throughout the life of the brush. Thus the motor is capable of operating with higher power inputs and efficiency during the lire cf the brush.

Preferably the spring pressure is adjustable without any disassembly of the motor.

The invention will be further described by way of example wi,h reserence to the 2ccomp2nying drawings, in which: Figure 1 is a side view of a first embodiment of a fractional horsepower PnDC 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; Figure 6 is a partly exploded perspective view of a second embodiment of a fractional horsepower PtlDC motor embodying the invention; Figure 7 is a plan view of the motor sr Figure 6 assembled; and Figure 8 is a side view of the motor of Figure 6 assembled.

Figures 1 to 5 of 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 having an end plate 4 at one end and an end cap cap 4 at its other end.

The central casing part 3 is made up of a stack of steel laminations ; each having a castellated outer periphery 6. -he stack of 12min2tiors 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 (two are shown in Figure 1) which.

form an opening into the housing 2 for air to pass.

The outer circufrential 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 14 in the form of a bridge piece of metal or alloy is mounted on the other end of the sack 3 and carries a ball bearing 15. The end cap 15 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 (two are shown in Figure 1). The outer circumferential surface of the shirt is castellated. Integrally formed with the rim 16 is a bridge 19 which carries the ball bearing 15. 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 end cap 14 thus provides two openings 59 between the rim 16 and bridge 19, through which access to a commutator 27 is gained.

The end plate 4, stack 3 and end cap 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 screv.ed into threaded holes 60 in the end cap 14 to hold the end plate 4, stack 3 and end cap 14 securely together.

Journal led in the bearings 11, 15 is a rotor 24 of standard configuration for a PUDC motor namely m 0 t 0 r , n a m e 1 y a shaft 25 carrying a laminated armature stack 26 wound with armature coils 26' connected to the commutator 27 mounted fast on the shaft 25. One end, 28, of the shaft 25 has a flat 29 for connection to a drive train.

The commutator 27 lays between the legs 21 of the bridge 19 and hence is accessible from ei'l-ier side of the bridge 19.

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 3 cop' ses 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, 35 sliding on the ledges 31, 32. The frame ij of electrically insulating material, such as plastics or anodised aluminium. The latter is preferred for greater dissipation of heat from the brush gear. Two electrically conducting terminals 37 are mounted in the base 34, for example by force fitting into slots in the base. The terminals have shape: outer ends 38 for receiving female tags of power supply leads (not shown). 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 respective posts 43. One leg 44 of a spring 42 bears against a leaf 39, and the other leg 45 bears against a post 46. Leg 44 may bear on the end of a brush 40. Three such posts 46 are provided for each spring 42. This enables the motor user to adjust the preload on the spring, and hence the brush pressure on the commutator by hooking the leg 45 behind a different post 46, the greater pressure being applied when the leg is hooked behind the radially inner post 46a.

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 50 and holds a pinion 9 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 5 the position of the brushes 40 relative to the motor stator, i.e.

the magnets 54, the screw 49 is loosened. The pinion 50 is then turned with, for example, a screwdriver blade in the slots 53, 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 end cap.

Markings 57 on the circumferential surface o; the frame and a centre line 58 on the stack 3 and end cap 14 enable the user to gauge accura'ely 'he posi.icn of the brushes.

Figures 6 to 8 show a fractional horsepower permanent magnet direct current moXor comprising a deepdrawn can-like steel casing 102 closed at one end 103 and carrying a metal end cap 104 at its other end 15.

The end cap 104 is in the form of an annular rim 105 which sits on the casing end 1 CS and a bridge 10 integrally formed with the rim 106. The bridge 107 carries a bearing 108 which supports a shaft 109 of a rotor 110 of the motor. The shaft 109 is also supported in a bearing (not shown) carried on casing end 103.

The rotor 110 further comprises a commutator 111 and a wound armature 112 mounted on the shaft, the windings of the armature 112 being connected to the commutator in the usual way.

Two permanent magnets (not shown) are mounted on the inside of the casing, opposite the armature.

Current is supplied to the rotor 110 by a brush gear assembly which forms a particular feature of this invention. The brushgear is carried on a plastics holder 113 having a recess 114 in its underside, formed between inner and outer walls 115, 116, the recess 114 straddling a wall 117 formed on the collar rim 106. The holder 113 is held on the wall 117 by a lockscrew 118 which passes through a pinion 119 and a slot 120 formed in the outer wall 116 to engage in a threaded hole 121 in the wall lli. he head of the screw 118 engages a ledge 128 on the inside of the pinion 119. The pinion 119 has teeth 122 on its outer surface which engage wilt a rack 123 f 0 r m e d n the underside of a step 124 formed cn t n e outer wall 116. The holder 113 is held firmly in position by tightening the screw 118. The holder 113 can be moved circumferentially on the wall 117 by releasing screw 118 slightly and rotating the pinion 119, the pinion 119 having a slot 125 in its outer end to allow it to be turned with a screw driver. By moving the holder 113 the timing of the motor can be adjusted.

The brushgear carried by the holder 113 comprises a pair of metal frames 126 which are a force fit in respective blind slots 127 in upper surface of the holder 113.

Each frame 126 comprises a lower portion 133 which fits in the slot 127, and two upwardly extending planar legs 129, 132. A threaded hole 134 , 136 is formed in each leg 129, 132 by drawing and cutting, the frame 126 being formed by a stamping operation.

The first leg 129 has a flange 130 at an edge 131 to stiffen the leg 129 against out of plane bending, and the second leg 132 is spaced from and formed at an obtuse angle to the first leg 129.

Each leg 132 carries a brushleaf 136, spring 138 and wire grip 141. The brushleaf 136 is of generally L-shaped configuration and has a brush 137 mounted at one end 136a in a manner known in the art. The bar 139 of the leaf 136 is held flat against the outer surface of the leg 129 (vide hereinafter), and the leaf 136 projects through the gap between the legs 129, 132. The steel spring 138 is secured at one end 38a to the leg 129 and bears at its other end 40 on the brushleaf 136 to urge the brush 137 against the commutator 111. The wire grip 141 has an arcuate end 142 and a wire lead 143 from a power supply is trapped between the end 142 and the leg 129. The brushleaf 136, spring 138 and wire grip 141 are held on the leg 129 by a screw 144 which is screwed into the threaded hole 134, the shaft 145 of the screw 144 passing through holes 146, 147 and 148 in the brus hl ea f , spring and wire gr'.

The spring 138 is arranged to just bear on the brushleaf 136 when it is secured in place by the screw 144.

To adjust the pressure exerted on the brushleaf 136 by the spring 138, a second screw 149 is provided.

The shaft 150 of the screw 146 passes through a hole 151 in the spring 138 and engages n the hole 136.

To increase the pressure exerted on the brushleaf 136 by the spring 138, and hence the pressure of the brush 137 against the commutator 111, the screw 149 is tightened, urging the adjacent spring portion towards the leg 132.

To assemble the motor, the casing 12, rotor assembly 110 and end cap 14 are assembled in the usual way.

The frames 126 are inserted in the slots 127 in the holder 113 and the holder 113 attached to the rib 117 on the end cap 14 by screw 118 and pinion 119. A brush and brushleaf 137, 136, spring 38 and wire grip 41 are then attached to the frame 126 by a screw 144, the brush 37 bearing lightly on the commutator 111.

Screw 49 is inserted through hole 151 in the spring 138 and screwed into threaded aperture 136 to achieve the desired spring pressure.

When the motor is Installed the screw 144 is loosened slightly to allow a supply lead 143 to be inserted between the grip 141 and leg 129.

In use, the user can "tune" the motor by adjusting the timing of the motor by moving the holder 113 circumferentially of the commutator 111, and the brush pressure can be adjusted by screw 149 to retain optimum pressure as the brush 137 wears.

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

Claims (9)

1 . A fractional horsepower permanent ma magnet direct current motor, comprising a rotor assembly rotatably mounted in a housing and comprising a shaft and a commutator and wound armature mounted fast on the shaft, and brushgear mounted on the housing for supplying current to the commutator, the brushgear comprising a brush mounted on an electrically conducting brushleaf, and a spring for urging the brush against the commutator, wherein adjustment means is provided to adjust the pressure of the brush against the commutator by adjusting the pressure exerted by the spring.

2. A motor as claimed in claim 1, wherein the brum gear is mounted on a holder which is movable circumferentially of the commutator.

3. A motor as claimed in claim 1 or 2, wherein the spring is an arcuate leaf spring which is mounted at one end on the housing and bears at its other end on the brushleaf or brush.

4. A motor as claimed in claim 3, wherein the adjustment means is arranged to urge the spring against the brushl eaf or brush to increase the brush pressure.

5. A motor as claimed in claim 4, wherein the spring pressure is adjusted by means of a screw.

6. A motor as claimed in claim 1 or 2, wherein the spring is a coil spring having a first leg which bears on the brush or brushleaf and a second leg arranged to be held in different position, to impart respective different preloads to the spring.

7. A motor as claimed in claim 6, wherein sil second leg is engageable against one of a plurality of stops, said stcps being arrange to impart a respective different reload to the spring.

8. A motor as claimed in any one o f claims 1 to 7, wherein the adjustment means is accessible from outside of the motor housing.

9. A fractional horsepower permanent magnet direct current motor substantially as hereinbefore descrid it wit, reference to Figures 1 to 5 or Figures 6 to 8 of the accompanying dra.sings.