Brush-operating Gear For Electrical Machines

Abstract

The commutator brushes required for the starter operation of a combined starter-generator are lifted through cam action on completion of the starting operation by a spring-urged ring member which, at the beginning of the starter operation is turned against the spring bias by a rotary solenoid to disengage the brush holders thus allowing the brushes to be applied by application springs. The rotary solenoid is preferably of the type having inner and outer stator poles between which the pole pieces of an annular rotor penetrate, the resulting gaps, or at least one of them, being wedge shaped to maintain throughout the stroke a torque sufficiently high to overcome the progressively loaded return springs, sufficient initial torque being ensured by asymmetric location of the solenoid rotor pole pieces relative to the solenoid stator pole pairs in the deenergized position.

Description

This invention relates to brush-operating gear for electrical machines and has for an object to provide improved brush-operating gear which is particularly suitable for application to electric engine-starter motors which are intended to continue running after starting of an engine, for example because their armature is arranged thereafter to perform another operation such as the generation of electric current. In such cases it is desirable for the starter brushes to be lifted off the commutator when the starting operation has been completed and to be maintained in the lifted condition during the normal running of the engine which has been started, thus considerably reducing wear of the commutator and brushes.

The present invention provides for this purpose brush-operating gear including a brush-lifting element which is biased by spring means to a position of engagement with the brush mechanism in which the brushes are lifted off the commutator against the action of brush-application springs, and a solenoid operable to move the brush-lifting element against the action of the bias spring means, thus allowing the brushes to be placed into contact with the commutator by the application spring and withdrawing the brush-lifting element from operative engagement with the brush mechanism.

One object of the invention is to provide a brush-operating gear of the kind set forth which can be accommodated in a commutator-type electric machine with minimum increase of the dimensions of the machine. With this object in view, the solenoid is preferably of the rotary type and preferably has a stator provided with a set of outer pole pieces spaced circumferentially of the solenoid, a set of inner pole pieces similarly spaced and arranged to face the outer pole pieces across a radial gap, a magnetizing member, preferably a magnetizing winding, arranged to produce a magnetic field in which the inner and outer pole pieces are respectively magnetized with opposite polarity, and a rotor member having a set of magnetizable pole pieces which are angularly spaced similarly to the stator pole pieces and are so arranged so as to be able, by rotation of the rotor, to each enter the gap between an inner and outer stator pole piece, leaving residual radial gaps at their inner and outer surfaces, at least one of these residual gaps varying in radial width from a maximum operative when the rotor pole piece is substantially withdrawn rotationally from the gap between two coordinated stator pole pieces, to a minimum operative when the rotor pole piece has entered the gap between the two coordinated stator pole pieces to the maximum extent.

This permits the achievement of a torque characteristic of the rotary solenoid corresponding to the torque required at each position, thus minimizing the necessary dimensions of the solenoid system.

The accompanying drawings illustrate one embodiment of the invention applied to a combined alternator-generator and starter-exciter unit as described and claimed in our copending British Pat. application No. 18,692 of 19th Apr. 1963 (Inventors S.S. Hall, R. Miller, P. Riley) and corresponding applications in other countries.

FIG. 1 is an elevation in axial section of the complete unit,

FIG. 2 is a cross section on line 2-2 of FIG. 1, the sector A being shown in the brush-engaged position and the remainder of the figure being shown in the brush-lifted position, and

FIG. 3 is a cross section substantially on line 3-3 of FIG. 1.

Referring now to the drawing, the arrangement comprises a two-part housing 1, 2 in which two rotor systems 9 and 17, carried by a common shaft 7, are rotatable within a stator-iron structure 11 and a field structure 18 respectively. The winding 19 on the rotor 17 has connections to a commutator 20 which, when the rotor is running as a motor armature for starting an engine coupled to the shaft 7 by a stud shaft 23, cooperates with a set of brushes 21, only one of which is shown, and brush-operating gear 30 is provided for lifting the brushes 21 off the commutator except when the motor 17, 18 is energized for starting an associated engine and its rotor speed is below a predetermined value lying between the speeds respectively corresponding to the self-sustaining speed and the normal running speed of the engine to be started. The construction so far mentioned is described in more detail in our said copending application, but the brush-operating gear will now be described in more detail with particular reference to the accompanying drawings.

The brush-operating gear comprises a rotary solenoid having a stator-core structure 33 formed as a ring whose profile is substantially a U-section the open side of which faces the armature 17, and which accommodates, at the bottom of the U, a magnetizing winding 34 coaxial with the shaft 7. At the open end of the U its outer wall is subdivided to form a number of uniformly spaced outer pole pieces 35 each of which is faced in radially spaced relation by an inner pole piece 36 connected to the radially inner wall of the U-section annular body 33. This stator-core structure is supported at 37 in a stationary housing body 38 attached to the outer end of the starter housing 2, and has a central bore 39 whose wall is radially spaced from the shaft 7. Supported in this bore 39 by needle rollers 40 is a sleeve 41 from which extends an annular flange 42 which is thus mounted for rotation about the axis of the core structure 33, and which carries magnetizable armature pole-piece plates 43 which project from the flange into the annular space defined by the radially inner surface of the outer pole pieces 35 and the radially outer surface of the inner pole pieces 36, the radial thickness of the armature pole pieces 43 being somewhat less than the minimum radial distance between the said surfaces so as to leave an airgap both at the radially inner and at the radially outer side of each armature pole piece when pole piece has moved to its maximum depth into the space between said inner and outer stationary pole pieces 35, 36.

As shown more clearly in FIG. 2, a number of spiral-strip springs 44 each have their center fixed in the stationary housing body 38 and their outer end attached to a block 45 which is secured to the flange 42 so that when, due to energization of the winding 34, the armature pole pieces have moved to the position 43a shown in sector A of FIG. 2, the spiral springs 44 are tensioned due to the extension of their outer end portions 46 so that upon cessation of the energization of winding 34, the springs 44 will return the flange plate 42 to the angular position in which its armature pole pieces are in the position shown at 43 in the remainder of the same FIG. 2 outside the sector A. The commutator brushes 21 are mounted in brush-holder arms 47 which, as shown in FIG. 3, are pivotally mounted at 48 in the stationary structure, and the arms 47 are urged by springs 49 to apply the brushes 21 to the commutator 20. Each brush-holder arm 47 is equipped with a cam block 50 having suitably inclined lifting-cam surface 51, and four lifting rollers 52 are carried on the flange plate 42 in such relation that when, at the end of a starting operation, the plate 42 moves to displace its pole pieces from the position 43a, shown in sector A of FIG. 2, to the position 43 shown outside sector A in FIG. 2, each brush-holder arm 47 is engaged by one of the rollers 52. After engaging the surface 51, this roller cooperates with the surface 51 to lift, by means of the cam block 50, the brush-holder arm 47 and thus to lift the brush 21 from the armature 20. The winding 34 may be connected in parallel with the circuit including the brushes, and armature winding 19, and series field winding 22 so that, when the engine is to be started, and before a voltage is applied to the starter motor via brushes 21, the winding 34 is energized, and the resulting magnetic flux will cause the flange 42 to be rotated to move the solenoid armature pole pieces 43 from their normal position illustrated in FIG. 2 outside Sector A to the position 43a illustrated in sector A, causing the flange 42 to rotate in the direction of the arrow X, thereby moving the rollers 52 along the cam surfaces 51 against the action of the spiral-strip springs 44 so as to allow the brush-pressure springs 49 to apply the brushes 21 to the commutator 20. When as a result of this starter operation, the engine has begun to fire and the engine speed has reached a value intermediate between the self-sustaining speed and the minimum normal running speed of the engine, a speed-responsive switch 53, controlled for example by the appearance of an output voltage in the terminals 14 or 16 of the alternator-generator part of the unit, cuts off the starter current and at the same time the energization of the winding 34, thus allowing the springs 44 to return the flange 42 to its position shown outside sector A, and during this movement the rollers 52 cooperate with the cam surfaces 51 of the blocks 50 to raise the brush-holder arms 47 against the action of the brush-pressure springs 49 and thus lift the brushes 21 clear of the commutator 20.

In one example, in which the unit is intended for application to a power takeoff pad of an aircraft gas-turbine engine whose normal running speed is 12,000 r.p.m., the brush-lifting mechanism was arranged to lift the starter brushes at a speed corresponding to 7,000 r.p.m. of the engine, which, although only seven-twelfths of the full running speed of the engine, was about twice the self-sustaining speed, thus on the one hand minimizing the risk of starting failure and on the other ensuring that the brushes will never be in contact with the commutator at anything like the full engine speed, a feature which greatly assists in obtaining a long service life of the commutator and brushes between overhauls.

Since the torque opposed by the spiral-strip springs 44 to the angular displacement of the flange plate 42 increases with progressive movement of the plate from the brushes-lifted position to the brushes-applied position, it is desireable to ensure that the torque of the rotary solenoid follows a somewhat similar law and at least does not unduly decrease during the progress of the brush-application movement. In order to approximate such condition, the mutually facing surfaces of the solenoid-armature pole pieces 43 and of the pole pieces of the annular body 33 are, in the illustrated preferred form of the invention, angularly displaced in the circumferential direction in such a manner that the gap at least at one of the radially inner and outer surfaces of the pole pieces 43 becomes narrower as these pole pieces enter, due to the rotation of flange plate 42, into the space between the inner and outer pole pieces 35 and 36 of the solenoid stator body 33. This feature can be clearly seen in FIG. 2 at the radially outer surface 36a of the pole pieces 36.

Suitable stop means, not shown, are preferably provided for limiting the movement of the flange plate 42 in the direction of the arrow X under the action of the solenoid winding 34 and also to limit its movement in the opposite direction under the action of the spiral-strip springs 44; the latter limit is so chosen that that end of each armature pole piece 43 which is leading during movement in the direction X, is much closer to the nearest pair of pole pieces 35 and 36 than the opposite end of the same pole pieces is to its next adjacent pair of pole pieces 35 and 36. This will ensure that a large torque in the desired direction is produced when the winding 34 is first energized.

Claims

What we claim is:

1. An electromagnetic device for lifting from the commutator of an electric machine brushes which are normally applied to the commutator by brush-loading spring means, the device comprising lifting means including a brush-lifter ring operable by rotation about the axis of the commutator to withdraw the brushes from contact with the commutator against the action of said brush-loading spring means, lifting spring means associated with said brush-lifter ring and urging said ring in the direction of such brush-lifting operation with a torque sufficient to move the ring from an inoperative position allowing the brushes to engage the commutator to an operative position in which the brushes are lifted off the commutator, and a rotary solenoid operative when energized to return said brush-lifter ring from its operative position to its inoperative position against the action of said lifting-spring means, said rotary solenoid including an annular yoke and a magnetizing coil, both coaxial with the commutator, and an armature member coaxially rotatable relative to the yoke and mechanically connected to said brush-lifter ring, said yoke having circumferentially spaced pole pieces projecting at one side of the coil to form, when the coil is energized, magnetic poles of one polarity, and said armature member including armature elements spaced circumferentially for respective cooperation with said pole pieces and each having two surface portions extending approximately in the direction of movement of the element about the common axis of the commutator and armature member and facing respectively one of said pole pieces and a surface portion of said yoke at the opposite side of the coil, the mutually facing surface portions of said armature element and pole piece being inclined relative to the direction of their relative movements over at least part of their length in such manner as to cause the effective airgap between the armature element and yoke to decrease as the armature member moves, when the coil is energized, the brush-lifting ring from its inoperative position towards its operative position.

2. Device as claimed in claim 1, wherein the yoke has a set of outer pole pieces spaced circumferentially, a set of inner pole pieces similarly spaced and arranged to face the outer pole pieces across a radial gap, with the coil arranged to produce a magnetic field in which the inner and outer pole pieces are respectively magnetized with opposite polarity, the armature member having a set of magnetizable armature pole pieces which are angularly spaced similarly to the pole pieces of the yoke and are so arranged so as to be able, by rotation of the armature member, to enter each gap between an inner and outer pole piece of the yoke, leaving residual radial gaps at their inner and outer surfaces, at least one of these residual gaps varying in radial width from a maximum operative when the armature pole piece is substantially withdrawn rotationally from the gap between two coordinated pole pieces of the yoke, to a minimum operative when the armature pole piece has entered the gap between the two coordinated pole pieces of the yoke to the maximum extent.

3. A device as claimed in claim 1 for use with an engine starter, wherein the solenoid is arranged to be energized automatically prior to the energization of the circuit including the commutator brushes and the starter-field winding in series with the starter armature the solenoid-energizing circuit including a switch opening the circuit when and as long as the speed of the machine exceeds a predetermined value.