US3340411A

US3340411A - Rotary step-by-step actuator

Info

Publication number: US3340411A
Application number: US406304A
Authority: US
Inventors: Parisoe Wilbert
Original assignee: Oak Electro Netics Corp
Current assignee: Oak Electro Netics Corp
Priority date: 1964-10-26
Filing date: 1964-10-26
Publication date: 1967-09-05
Anticipated expiration: 1984-09-05
Also published as: NL6513823A

Description

Sept. 5, 1967 w. PARlsoE 3,340,411

ROTARY STEPBY-STEP ACTUATOR Filed- 0G12. 26. 1964 United States Patent O 3,340,411 ROTARY STEP-BY-STEP ACTUATOR .Wilbert Parisoe, Highland Park, Ill., assignor to Oak Electro/Netics Corp., a corporation of Delaware Filed Oct. 26, 1964, Ser. No. 406,304 Claims. (Cl. S10-49) This invention rel-ates in general to a rotary actuator, and more particularly to a rotary actuator which may be operated in a step-by-step manner to effect a desired switching operation.

In known types of rotary actuators, it has been conventional to employ a rotary solenoid mechanism wherein means are provided for of a `plunger into the desired rotary movement. This type of actuator has proven to be undesirable in certain res pects, in that it has necessitated the use of rather cornplex mechanism in order to produce the desired results. Accordingly, the general purpose of the present invention is t-o provide a rotary actuator wherein the actuator shaft rotates without axial movement so as to obviate the need for rather complicated mechanisms to convert the axial vmovement of a solenoid plunger into a desired rotary movement. To attain this, the applicant has associated a novel step motor in combination with novel rotary actuator means in a manner so as to improve the operating characteristics of the rotary actuator.

An object of the present invention is to provide a rotary actuator with a novel step -motor so that the step motor can be assembled and operated n a trouble-free manner, thus contributing to the improved output of the rotary actuator.

Another object of the invention is to provide a step motor for a rotary actuator as described in the preceding paragraph with an improved terminal block construction so that the terminal block can be assembled into the step motor in an efficient manner.

A further object of the invention is to provide a rotary actuator utilizing a rotary step motor for step-by-step operation with a novel interruptor switch for interrupting the excitation of the motor field.

Still another object is to provide a step motor operated rotary actuator with toggle interrupter means for opening and closing an interrupter switch at definite and predetermined angles.

.A still further object of the invention is to provide a rotary actuator as defined in the preceding paragraph with an interrupter switch wherein the angle at which the switch opens is adjustable.

These and other objects of the invention will hereinafter become more fully apparent from the following description, taken in connection with the annexed drawings, Wherein:

FIGURE 1 is a side elevational view of bodiment of the invention;

FIGURE 2 is a sectional view taken generally along line 2-2 of FIGURE l;

FIGURE 3 is a sectional view taken generally along the line 3-3 of FIGURE l;

FIGURE 4 is a fragmentary side view of the rotary actuator illustrated in FIGURE l, taken from a different side from that shown in FIGURE l;

FIGURE 5 is a fragmentary sectional view taken generally centrally through the rotary actuator, with certain parts broken away for clarity;

FIGURE 6 is a sectional View taken generally along line 6 6 of FIGURE 5;

FIGURE 7 is a sectional line 7--7 of FIGURES;

FIGURE 8 is a sectional view taken generally along line 8 8 of FIGURE 5; and

a preferred emview taken generally along converting the axial movement l FIGURE 9 is a view similar to FIGURE 3 showing a modified form of nterrupter switch mechanism.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one specific embodiment and a modification thereof, with the understanding that the present disclosure is t-o be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. The scope of the invention will be pointed out in the appended claims.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, the rotary actuator embodiment illustrated in FIGURE l includes a step motor housed in a suitable casing 20. As may be best seen in FIGURES 5, 7 and 8 the step motor includes a stator indicated generally at 21, and a rotor indicated generally at 22. The stator 21 is formed of three

sections

23, 24 and 25, with

sections

23 and 25 being generally identically shaped and best illustrated in FIGURE 7. The shape of section 24 is best illustrated in FIGURE 8. Each of sections 23-25 is generally rectangular in cross-section, and each section has approximately the same outer dimensions, so that they will snugly fit within casing 20. Each of sections 23-25 is provided with a hole 26 adjacent each corner thereof, and a mounting stud 27 extends through each of the aligned holes to retain the sections 23-25 in their desired assembled relationship. The forward ends of studs 27 extend outwardly from the forward end of the casing 20, so that the rotary actuator may be mounted to a support, not shown. Each of the sections 23-25 includes respective parallel faces 23a-23h, 24a-24h and 25a-25h. As can be best seen in FIGURE 5, faces 23b and 24a are positioned in abutting relationship, while faces 24b and 25a are also positioned in abutting relationship. A front wall 28 abuts against face 25b, while a rear Wall 29 abuts against face 23a.

Walls

28 and 29 define the front and rear walls of the casing 20, and will hereafter be more fully described.

Since

sections

23 and 25 are identical, a detailed description will be given of section 25 only, with it being understood that corresponding parts are also provided on section 23. A central bore 31 is provided in section 25 for reception of rotor 22, and the axis of bore 31 is positioned at right angles to the plane of faces 25a and 2511. A plurality of radial openings or notches 32 are provided around bore 31, to -define therebetween a plurality of poles 33. Poles 33 are equiangularly spaced around bore 31, and any number may be provided, with five being shown in FIGURE 7 for purposes of illustration only. It will be understood, of course, that

stator sections

23 and 25 are formed of a material having effective magnetic properties, such as, for example, pressed and sintered pure iron powder.

As can be best seen in FIGURE 8, stator section 24 is provided with a central bore 34 of relatively large diameter, so that section 24 is shaped generally in the form of an annulus. Annular section 24 is discontinuous in that an opening 35 is provided in the upper portion thereof. Opening 35 is defined by facing surfaces 36 and 37, each of which has an aligned respective dovetail slot 38 and 39, for reception of a terminal block to be hereafter described. Like

sections

23 and 25, section 24 is also formed of a material having effective magnetic properties, such as pressed and sintered 100% pure iron powder or the equivalent. A coil 41 is carried by a suitable coil bobbin which is fixed within `bore 34. The inner diameter 42 of the coil bobbin is substantially the same as the inner diameter of bore 31, and the rotor 22 is spaced from the bobbin and from the bore 31, so that it is free to rotate relative to Ithe stator. It will be understood, of course, that the coil 41 is formed of a length of wire having opposite ends, each of which is adapted to be connected to a source of direct current power, not shown. To this end, a terminal block 44 is provided, having a pair of

terminals

45 and 46 to which the respective ends of the wire are adapted to be secured. As can be best seen in FIGURE 8, suitable dovetail extensions are provided on

opposed faces

47 and 48 of the terminal block, and are slidably received within the dovetail recesses 38 and 39. The complementarily shaped dovetailed portions on the terminal block 44 and the stator section 24 are sized so as to provide a sliding iit between the terminal block and the stator section. Thus, the terminal block can be slid in place in stator section 24 without difficulty. As can be best seen in FIGURE 5, terminal block 44 has a width which is substantially the `same as the thickness of stator section 24, so that the terminal block is firmly held in place between the faces 23b and 25a of

stator sections

23 and 25 respectively, when the stator unit is assembled. As is well known in the art, terminal block 44 is formed of a suitable insulating material.

Rotor 22 is a generally cylindrical member, and includes a plurality of generally radially extending, axially elongated poles 51. Rotor 22 is movable between two positions, with the first or normal position being shown in FIGURES 7 and 8. In the normal position, each pole 51 is ypositioned in `general alignment with an opening 32 between adjacent stator poles 33, with each pole 51 being offset a slight angular amount, as for example toward the adjacent stator pole 33 in a clockwise direction. Thus, when the iield coil 41 is energized, the rotor poles 51 will be attracted to their closest stator poles 33, and the -rotor 51 will rotate in a clockwise direction as viewed in FIGURES 7 and 8 so that the rotor poles 51 are in the second position in radial alignment with the stator poles 33. When the stator eld 41 is deenengized, means to be hereafter described will return the rotor 22 to the first or initial position.

Rotor 22 inclu-des a central bore 52 which extends axially therethrough, and is adapted to slidably receive an actuator shaft 53. Forward casing wall 28 includes a central openin-g defined by a rearwardly extending annular flange 54 which serves as a irst bearing for the forward end ofshaft 53. The front face of the rotor 22 is provided with a countersunk portion 55 to accommodate the flange 54. The rear casing wall 29 is provide-d with a central opening defined by an annular flange 56 which extends rearwardly from the wall 29, and which forms a second bearing for shaft 53.

A second bore 57 is provided in rotor 22 at the rearward end thereof, and is offset from and eccentric to bore 52. A drive pin 58v is received in bore 57, and extends rearwardly from plate 29 through a rectangular opening 59 (FIGURE 6). Opening 59 is anked by a pair of tabs 61 which extend rearwardly from plate 29 at generally right angles thereto.

A suitable stop member 62 is provided with aligned slots which are impaled by tabs 61 to mount the stop 62 upon plate 29. An elongate opening is provided in stop member 62, and includes a generally circular lower portion 63 which receives flange 56, and an enlarge-d upper portion 64 in which pin 58 is free to move. As can be best seen in FIGURE 6, the sides of the opening portion 64 confine pin 58 to a limited degree of movement, and thus when the current flow to the coil 41 is interrupted, the member 62 will positively locate the lrotor 22 in the Icorrect initial vor first position as the drive pin 58 moves into engagement with the side of stop 62.

Means are provided to interrupt the flow of current to the coil 41, to effect the desired step-by-step operation of the actuator, and include a switch carried by a mounting plate 66. Mounting plate 66 is held in spaced `relation with respect to the rea-r casing wall 29 by a pair of cylindrical spacing members 67, which are received in aper-v tured bosses 68 in the rear casing wall 29. Spacing members 67 may be provided withV an internally threaded bore, as may be best seen at 70 in FIGURE 5, for reception of the threaded end of longitudinally extending tie rods or stud screws 69. As is well known in the art, a plurality of switch sections 71 are mounted on tie rods 69 and contain a plurality of contacts which are connected into circuits to Ibe switched by the rotary actuator. Suitable' sleeve members, such as shown at 72-74, are provided between the Iswitch sections 71 to space the switch sections f-rom one another. A Irotor shaft 75 extends longitudinally rearwardly of the actuator, and includes a portion of cylindrical cross section adjacent casing 20, and a portion 76 of double D cross section which carries the rotor sections corresponding to the switch sections 71. Bearing means are provided for the rotary shaft in the form of an annular ange 77 extending rearwardly from the mounting plate 66. Suitable detent means are associated with the actuator, and may include a -detent member 78 having a plurality of openings, each opening corresponding to one switching station; anda detent spring 79 carrying a ball 80 which is adapted to be seated in the openings in the detent lrotor 78 upon rotation of the shaft 75. Nuts 81 may be provided at the end of stud screws 69 to retain the elements in their assembled relationship.

The means for interrupting the flow of current to the coil 41 includes a rst xed contact member 82 (FIG- URE 3), which is xedly mounted on mounting plate 66 as by rivets 83 (FIGURE 2). Contact member 82 includes a first rearwardly extending contact portion 84, which is adapted to be engaged by the movable contact to be hereafter described. Contact member 82 includes a second rearwardly extending portion 85 which is adapted to be connected in circuit with the coil 41. A movable contact member 86 is pivotally mounted on the mounting plate 66, as by a rivet 87. Contact member 86 includes a pair of forwardly extending sides 88 and `89 (FIGURE 4) which embrace the sides of a -generally V-shaped toggle link 90. Toggle link 90 is formed of a suitable insulating material, and includes an opening adjacent the apex thereof for reception of rivet 87. Contact member 86 includes a contact portion 91, which is movable into and out of engagement with the contact portion 84 of contact member 82 upon the pivotal movement of the toggle link 90.

Toggle link 90 includes a pair of diverging

legs

92 and 93 which are positioned in alignment with an arcuate slot 94 in the mounting plate 66. A spring 95 is biased between the outer end of link 93, and a suitable tab 96 on a terminal 97, to urge the contact portion 91 of the movable contact 86 into engagement with the fixed contact 84. Terminal 97 is tixedly mounted on mounting plate 66, as by a rivet 98, and by a forwardly extending tab 101 (FIGURE 2) which engages a suitable hole 102 in the mounting plate. Terminal 97 includes a rearwardly extending portion 99, which is adapted to be connected in circuit with the coil 41; and a suitable exible lead, not shown, is connected between a forwardly extending terminal on movable contact 86 and the rearwardly extending portion 99 on terminal 97. Thus, it is clear that when the contact portion 91 of the movable contact 86 is in engagement with the contact portion 84 of the iixed contact 82, a circuit is complete to the coil 41 for energization thereof, and when the contact portion 91 moves out of engagement with the contact portion 84, this circuit is interrupted.

The means for moving the movable contact portion 91 out of engagement with the stationary contact portion 84 includes a generally U-shaped yoke having a central Iportion 103, and rearwardly extending

arms

104 and 105. Arm 105 is longer than arm 104, and extends rearwardly through slot 94 into position for engagement with the

legs

92 and 93 of the toggle link 90. The central yoke portion 103 is provided with an eccentric hole, not shown, which receives the rearward end of drive pin 58 therethrough. Suitable fastening means 106 secure the central portion 103 of the yoke to the shaft 53. Thus as rotor 22 pivots when coil 41 is energized, the drive yoke is pivoted a predetermined angular amount determined by the engagement of stop 62 with pin 58. As the drive yoke pivots in a counterclockwise direction, as viewed in FIGURE 3, the yoke arm 105 will move a certain predetermined amount before moving into contact with the leg 93 of the toggle link. Thus, as the yoke nears the end of its angular movement, the yoke arm 105 will engage the toggle llink leg 93 and pivot the toggle link 90 into an over center position. Pivotal movement of toggle link 90 moves contact portion 9'1 out of engagement with contact portion 84, and thus the flow of current to the coil 41 is interrupted, and the rotor 22 and the yoke are moved back to their initial position by means to be hereafter described. Spring 95 holds the toggle link 90 in the over center position while the drive yoke pivots in a clockwise direction, as viewed in FIGURE 3, a predetermined amount until the arm '5 engages the leg 92 of the toggle link, which then p-ivots the toggle link into the switch closed position, illustrated in FIGURE 3.

A cylindrical post 107 extends forwardly from mounting plate 66, and a rst hooked end 108 of a leaf spring is hooked thereover. The opposite hooked end 109 of the leaf spring is hooked over the arm 104 of the drive yoke, to bias the same in a clockwise direction, as viewed in FIGURE 3. rl'lhus, after the drive yoke has moved the toggle link 90 to an over center position, and the flow of current to coil 41 has been interrupted, the leaf spring will function to return the drive yoke, the toggle link 90, and therotor 22 to their respective initial positions.

Conventional means are provided for converting the pivoting movement of rotor 22 into the desired increment of angular movement for rotor shaft 75, and are set forth in detail in the patent to Dolesh et al., No. 2,820,370. These means include a ratchet disc 110 having slotted end portions engaging

yoke arms

104 and 105, and movable therewith. A second ratchet disc 111 is mounted on the rotor shaft 76, and a spring 112 yieldably biases ratchet disc 110 into engagement with ratchet disc 111 to effect rotation of the rotor shaft upon pivotal movement of the rotor 22. An escapement cam 113 is pivotally mounted on post 107, and is provided with an arcuate cam slot 114 through which yoke arm 104 extends, to provide an overshoot control means, as described in the above mentioned Dolesh et al. patent.

Turning now to the interrupter switch form shown in FIGURE 9, a movable contact member 136 is pivotally mounted on a mounting plate 166 by a pivot pin 187. Movable contact member 186 includes a contact portion 191, which is movable into and out of engagement with the upstanding contact portion 184 of a contact member 1182 which is fixed on the mounting plate 166. A toggle link 190 includes a pair of diverging

legs

192 and 193 which are positioned in alignment with an arcuate slot 194 in the mounting plate 166, Toggle link 190 is pivotally mounted on pivot pin 187, and is free to pivot relative to the movable contact member 186. The movable contact member 186 includes a second arm 188 having a threaded hole in which a screw 120' is received. A leaf spring 121 is carried by a pin 122 that is fixed on the mounting plate 166, and engages the outer end of the arm 188 of the movable contact 186. A compression spring 195 is biased between the outer end of the leg 193 of the toggle link 190i and a stop 197 that is fixed upon the mounting plate 166 to bias the toggle link 190 in a counterclockwise direction, as viewed in FIGURE 9, It will be understood, of course, that when the coil of the step motor is deenergized, spring 195 forces the leg 192 of the toggle link 190 into engagement with the movable contact portion 191 to maintain the interrupter switch closed. As the drive yoke begins to rotate, the drive yoke arm 205 will engage the toggle link leg 193 to pivot the toggle link leg 192 out of engagement with the movable contact portion 191, as is shown in FIGURE 9. The interrupter switch will not open until the toggle link leg 193 engages the screw 120 to initiate pivotal movement of the contact member 186, and it will be appreciated that as the drive yoke completes its movement toward its energized position, the toggle link will be moved to an Aover center position by the yoke arm 205. In this position, the leaf spring 121 will be held against a stop pin 123 which is fixed on the mounting plate 166. The compression spring 195 is stronger than the leaf spring 121 so that the toggle link 190 will remain in its over center position until yoke arm 205 engages the leg 192 of the toggle link 190 to move it back to its initial position. It will be readily appreciated that by varying the position of screw 120 in the arrn 188 of the movable contact, the angle at which the interrupter switch opens and closes may be adjusted and controlled.

I claim:

1. In a rotary actuator including a casing: an electromagnet comprising first and second spaced stator members, each having an opening therethrough with a plurality of poles being provided at circumferentially spaced points around each opening, each pole on said first member being positioned in axial alignment with a corresponding pole on said second member to define a plurality of pairs of axially aligned stator poles; a third stator member between said first and second stator members and carrying an annular coil having a central opening aligned with the openings in said first and second stator members, said coil being wound of a length of wire having opposite ends; a terminal block carried by said third stator member and having a terminal connected to each end of the wire, said terminals being adapted to be connected to a source of direct current; a generally cylindrical rotor mounted in the openings in said rst and second stator members and said coil; bearing means on said casing rotatably supporting said rotor; a plurality of axially extending poles on said rotor, each rotor pole corresponding to one of said pairs of axially aligned stator poles; stop means engageable with said rotor for establishing a first position wherein each rotor pole is positioned between adjacent pairs of stator poles when said coil is deenergized, said stop means also establishing a second position wherein each rotor pole is positioned in radial alignment with one of'said pairs of stator poles when said coil is energized; means for returning said rotor from said second position to said rst position; and means fxedly mounting said rst, second and third stator sections 'in said casing.

2. In a rotary actuator, a casing having front and rear walls; an electromagnet comprising first and second spaced stator members, each having an opening therethrough with a plurality of poles being provided at circumferentially spaced points around each opening, each pole on said first member being positioned in axial alignment with a corresponding pole on said second member to define a plurality of pairs of axially aligned stator poles; a third stator member between said first and second stator members and carrying an annular coil having a central opening aligned with the openings in said first and second stator members, said coil being wound of a length of wire having opposite ends; a terminal block carried by said third stator member and having a terminal connected to each end of the wire, said terminals being adapted to be connected to a source of direct current; means fixedly mounting ysaid first, second and third stator sections in said casing; a generally cylindrical rotor mounted in the openings in said first and second stator members and said coil; bearing means on said front and rear casing walls rotatably supporting said rotor; a plurality of axially extending poles on said rotor, each rotor pole corresponding to one of said pairs of axially aligned stator poles; stop means engageable with said rotor for establishing a first position wherein each rotor pole is position between adjacent pairs of stator poles when said coil is deenergized, said stop means also establishing a second position wherein each rotor pole is positioned in radial alignment with one of said pairs of stator poles when said coil is energized; and means for returning said rotor from said second position to said first position.

3. In a rotary actuator, a casing having front and rear walls; an electromagnet comprising first and second spaced :stator members, each having parallel front and rear faces with an opening therethrough at right angles to said faces, a plurality of poles being provided at circumferentially spaced points around each opening, each pole on said first member being positioned in axial alignment with a corresponding pole on said second member to define a plurality of pairs of axially aligned stator poles, the front face of said first stator members being positioned against said casing front wall, and the rear face of said second stator member being positioned against said casing rear Wall; a third stator member having parallel front and rear faces, with said front face being positioned against the rear face of said rst stator member, and said rear face positioned against the front face of said second stator member; an annular coil carried by said third stator member and having a central opening aligned with the openings in said first and second stator members, said coil being formed of a length of wire having opposite ends; a terminal block carried by said third stator member and having a terminal connected to each end of the wire, said terminals being adapted to be connected to a source of direct current; means fixedly mounting said first, second and third stator sections in said casing; a generally cylindrical rotor mounted in the openings in said first and second stator members and said coil, bearing means on said front and rear casing walls rotatably supporting said rotor, a plurality of axially extending poles on said rotor, each rotor pole corresponding to one of said pairs of axially aligned stator poles; stop means engageable with said rotor for establishing a first position wherein each rotor pole is positioned between adjacent pairs of stator poles when said coil is deenergized, said stop means also establishing a second position wherein each rotor pole is positioned in radial alignment with `one of said pairs of stator poles when said coil is energized; and means for returning said rotor from said second position to said first position.

4. In a rotary actuator, a casing having front and rear walls; an electromagnet comprising first and second spaced stator members, each having parallel front and rear faces with an opening therethrough at right angles to said faces, a plurality of poles being provided at circumferentially spaced points around each opening, each pole on said first member being positioned in axial alignment with a corresponding pole on said second member to define a plurality of pairs of axially aligned stator poles, the front face lof said first stator member being positioned against said casing front wall, and the rear face of said second stator member being positioned against said casing rear wall; la third stator member having parallel front and rear faces, with said front face being positioned against the rear face of said first stator member, and said rear face positioned against the front face of said second stator member, said third stator member having a discontinuous portion defined by spaced apart facing surfaces, aligned notches in each surface; an annular coil carried by said third stator .member and having a central opening aligned with the openings in said first and second stator members, said coil being formed of a length of wire having opposite ends; a terminal block formed of insulating material and having a thickness substantially the same yas the thickness of said third stator member, said terminal block having opposite edges, each being slidably received in one of said notches and retained therein by the engagement of the rear face of said first stator section with one end of said terminal block, and the engagement of the front face of said second stator sec-tion with the other end of said terminal block; a pair of terminals carried by said terminal block, each connected to one end of said wire, said terminals being adapted to -be connected to a source of direct current; means fixedly mounting said first, second and third stator sections in said casing; a generally cylindrical rotor mounted in the openings in said first and second stator members and said coil; bearing means on said casing rotatably supporting said rotor; a plurality of axially extending poles on said rotor, each rotor pole corresponding to one of said pairs of axially aligned poles; stop means engageable with said rotor for establishing a first position wherein each rotor pole is positioned -between adjacent pairs of stator poles when said coil is deenergized, said stop means also establishing a second position wherein each rotor pole is positioned in radial alignment with one of said pairs of stator poles when said coil is energized; and means for returning said rotor from said second position to said first position.

5. In a rotary actuator including a casing: an electromagnet comprising first and second spaced stator members, each having an opening therethrough with a plurality of poles being provided at circumferentially spaced points around each opening, each pole on said first member being positioned in axial alignment with a corresponding pole on said second member to define a plurality of pairs of axially aligned stator poles; a third stator member between said first land second stator members and carrying an annular coil having an opening aligned with the openings in said first and second stator members, said coil being adapted to be' connected to a source of direct current; a rotor mounted in the openings in said first and second stator members tand said coil; a plurality of axially extending poles on said rotor, each rotor pole corresponding to one of said pairs of axially aligned stator poles; stop means engageable with said rotor for establishing a first position wherein each rotor pole is positioned between adjacent pairs of stator poles when said coil is deenergized, said stop means also establishing a second position wherein each rotor pole is positioned in radial alignment with one of said pairs of stator poles when said coil is energized; means for returning said rotor from said second position to said first position; and means fixedly mounting said first, second and third stator sections in said casing.

References Cited UNITED STATES PATENTS 3,119,941 l1/1964 Guiot 310-49 3,153,157 10/1964 Rabe 310-156 3,204,136 8/1965 K'aiwa 310-49 3,275,861 9/1966 Goroszko 310-156 MILTON O. HIRSHFIELD, Primary Examiner.

J. W. GIBBS, Assistant Examiner.

Claims

1. IN A ROTARY ACTUATOR INCLUDING A CASING: AN ELECTROMAGNET COMPRISING FIRST AND SECOND SPACED STATOR MEMBERS, EACH HAVING AN OPENING THERETHROUGH WITH A PLURALITY OF POLES BEING PROVIDED AT CIRCUMFERENTIALLY SPACED POINTS AROUND EACH OPENING, EACH POLE ON SAID FIRST MEMBER BEING POSITIONED AN AXIAL ALIGNMENT WITH A CORRESPONDING POLE ON SAID SECOND MEMBER TO DEFINE A PLURALITY OF PAIRS OF AXIALLY ALIGNED STATOR POLES; A THIRD STATOR MEMBER BETWEEN SAID FIRST AND SECOND STATOR MEMBERS AND CARRYING AN ANNULAR COIL HAVING A CENTRAL OPENING ALIGNED WITH THE OPENINGS IN SAID FIRST AND SECOND STATOR MEMBERS, SAID COIL BEING WOUND OF A LENGTH OF WIRE HAVING OPPOSITE ENDS; A TERMINAL BLOCK CARRIED BY SAID THIRD STATOR MEMBER AND HAVING A TERMINAL CONECTED TO EACH END OF THE WIRE, SAID TERMINALS BEING ADAPTED TO BE CONNECTED TO A SOURCE OF DIRECT CURRENT; A GENERALLY CYLINDRICAL ROTOR MOUNTED IN THE OPENINGS IN SAID FIRST AND SECOND STATOR MEMBERS AND SAID COIL; BEARING MEANS ON SAID CASING ROTATABLY SUPPORTING SAID ROTOR; A PLURALITY OF AXIALLY EXTENDING POLES ON SAID ROTOR, EACH ROTOR POLE CORRESPONDING TO ONE OF SAID PAIRS OF AXIALLY ALIGNED STATOR POLES; STOP MEANS ENGAGEABLE WITH SAID ROTOR FOR ESTABLISHING A FIRST POSITION WHEREIN EACH ROTOR POLE IS POSITIONED BETWEEN ADJACENT PAIRS OF STATOR POLES WHEN SAID COIL IS DEENERGIZED, SAID STOP MEANS ALSO ESTABLISHING A SECOND POSITION WHEREIN EACH ROTOR POLE IS POSITIONED IN RADIAL ALIGNMENT WITH ONE OF SAID PAIRS OF STATOR POLES WHEN SAID COIL IS ENERGIZED; MEANS FOR RETURING SAID ROTOR FROM SAID SECOND POSITION TO SAID FIRST POSITION; AND MEANS FIXEDLY MOUNTING SAID FIRST, SECOND AND THIRD STATOR SECTIONS IN SAID CASING.