FIELD OF THE INVENTION The invention related to a switched reluctance motor, and the design thereof, which is adapted for supportably rotating a rod string which in turn drives a progressive cavity pump.

BACKGROUND OF THE INVENTION A progressive cavity ("PC") pump is located in a well and is used to pump subterranean fluids to the surface. Well over 12,000 oil wells in Canada use PC pumps. Conventionally the PC pump is driven from a speed-reducing drivehead or top drive located at the surface. A long assembly or string of rods transmit the slow rotation from the top drive down the well to the PC pump. The top end of the rod string is fitted with a polish rod which is driven through a rod clamp which is driven by the top drive. The conventional rod string rotational speed of 30 - 700 rpm is adjusted according to the downhole pumping conditions including the oil characteristics. Lower rotational speeds are not currently practiced due to the large gear reduction required and lack of flexibility to attain greater speeds as pumping conditions improve. Under certain conditions, such as in the case of very viscous oils or low formation influx rates, it desirable to have even lower rotational speeds. These cases are not adequately served with the prior art top drives. When driven, the rod string winds up in torsion like a rubber band. In a 3,000 foot long rod string, the rods could be wound over 100 revolutions. When the top drive ceases to drive the rod string and coupled PC pump (such as during a power failure), the rods try to unwind, spinning the conventional drive and motor backwards, called backspiπ. If unrestrained, backspin speed of the rods can reach upwards of 5,000 rpm and, in combination with gear reduction to the top drive (now acting as a speed increaser), the resulting speed at) the top drive and motor can dangerously destructive - with most injuries being caused by sheave self-destruction. Apparatus is known to control backspin. Some top drives have braking systems which are activated upon backspin, either when the drive shuts down or thθ rod string physically begins to spins backwards. Substantially all modern top drive are fitted with some sort of device which deals with backspin. Older top drives utilize centripetally-actuated mechanical drum- breaking systems. Alternately a hydraulic pump can cause brake pads to engage a disc-braking rotor on the polish rod. Braking shoes, drums, shoes, pads and discs are subject to wear and the possibility of localized hot spots which can be an ignition hazard. Some top driveis are hydraulic motor driven. Upon backspin the drive naturally reverts to a purnp. Hydraulic fluid is directed through a restriction orifice or check valve for releasing pressure. Similarly, in motor-driven gear boxes, a clutch activates on backspin to engage a separate hydraulic pump. At the high torque of backspin conditions, the hydraulic fluid can become very hot, or if the fluid is lost, backspin control is also lost. In another instance, it is known to provide an AC drive motor with an inline shaft. A Variable Frequency Drive or VPD manipulates lin® AC/DC and then DC back to a variable frequency AC to control the shaft speed. No gear reduction is provided thus backspin is not dangerously speed multiplied. Further the VFD can provide regenerative braking. Slip associated with AC induction motor unfortunately limits the motor's ability to reach the low rotational speeds desired in some circumstances, such as with high viscosity oils. Low rotational speeds can result in motor overheating. Further, to start an AC induction motor under load can result in in- rush amperage as much as 5 or 6 times the normal full load operating current. For instance, for a typical 50ι Hp NEMA B motor having a full load current of about 200 A, the in-rush could be 1000 - 1200 amperes. While this is only for a short period of time and is an accepted disadvantage of AC motors, there is a further economic impact. Often, the peak current use of an end user is identified by the utility provider as the basis for setting the per unit charge for all power consumption. Accordingly, even though steady-state use may be only 200 amperes, the price paid for the electricity could be based on a higher, albeit momentary, electrical current use. Thus, there is a need for a more versatile top drive which is capable of a wide range of operating speeds, particularly low rotational speeds and is does not suffer the enumerated backspin disadvantages of the gear-reduced apparatus. 1 SUMMARY OF THE INVENTION 2 Simply, a switched reluctance motor (SRM) is provided as a top 3 drive for a downhole rotary pump. For top drive operations, a typical range of 4 speeds is between 30 and 700 rpm. The embodiments describett herein provide 5 high torque with low rpm output yet avoid the use of gear or belt-reduction. This δ is achieved by combining the switched reluctance motor and a concentric hollow 7 drive shaft through which the rod string's polish rod extends, Further, should 8 there be an interruption in the motor's power or a rod failure deep in the well, any 9 resulting backspin will not produce a dangerous surface reaction. 0 In one embodiment, induced heat, which can build up and cause 1 premature failure of conventional SRM technology under such rigorous operating 2 conditions, is effectively dissipated under another embodiment of the improved 3 top drive, in this preferred embodiment, ambient air is circulated from outside the 4 motor, into the motor and across the stator, stator coils and rotor. In one 5 preferred embodiment, the stator is manufactured having a radially extending 6 outer profile, such as a square, the periphery of which is fit with axial air 7 passages. An air mover conducts air axially through the air passages for cooling 3 the stator. 3 In yet another embodiment, forced air flow is achieved using vanes D affixed to the rotating rotor. The flow path of the cooling air is through passages I formed through a combination of unique stator and rotor design features. I Simplistically, the air is directed upwardly through spaces between adjacent coils \ wound on adjacent stator poles and through an array of axially-extending I (vertical) passages formed in the rotor and in the faces of the stator poles. Air is i directed radially past the stator stack and out of the motor. More preferably, the 1 stator stack is formed in two or more sub-stacks of stator laminations. The sub- 2 stacks are spaced axially by a plurality of circumferentially-spaced spacers, Thθ 3 result is a plurality of circumførentlally-spaced air discharge passages extending 4 . radially from the rotor-stator air gap to a location outside the motor. In the case 5 of two stator sub-stacks , an upper and a lower lamination stack, the rotor is fit β with a dividing lamination having a radially protruding barrier, typically formed of 7 laminations of a diameter larger that the axial passages in the rotor, the barrier 8 interrupting the axial passages. Thus, air flowing along the rotor's axially- 9 extending passages is blocked and re-directed radially outwards through the 0 discharge passages. Accordingly, two flow paths are created; a first path flowing 1 upwardly from lower rotor vanes and through the rotor's axially-extendiπg 2 passages to cool the lower lamination stack, coils and rotor, and a second path 3 flowing downwardly from upper rotor vanes and through the rotor's axially- 4 extending passages to cool the upper lamination stack, coils and rotor. Both 5 flows approach the rotor's protruding barrier and are directed out of the motor. 6 Air supplying the second path is obtained from air flowing upwardly between the 7 coils and then is directed downwardly aided by the upper vanes. 8 In another embodiment, the top drive is conveniently provided as a 9 frameless motor which is secured directly to the wellhead. The polish rod 0 supports the rod string. The polish rod is supported by the hollow drive shaft 11 which is in turn rotatably supported by the motor's frame. Unlike conventional 2 frameless motors in which loads are borne by the driven equipment, the present 3 embodiment bears the loads on the motor itself. Accordingly, the frameless 4 motor is capable of bearing the axial loads imposed through the shaft. In another embodiment related to' the frameless motor, as the nature of the stator stack is that there is some tolerance in stack assembly, the dimension or height of the stack can vary circumferentially. Accordingly, the top plane of the motor stator (perpendicular to shaft) may not reside in the same plane as the bottom plane of the motor stator. Thus bearings rotatably supporting the rotor, particularly under load, are compliance with some misalignment. A combination of bearing arrangements ensures reliable operation. More particularly, a spherical roller thrust bearing is employed at the top plane of the motor which has self-aligning capabilities and is forgiving of misalignment of the shaft relative to the bearing. This spherical bearing rotatably supports the suspended load while accepting some deviation from perpendicular between the shaft and the top plane. In another embodiment, the top bearing is provided with a dependable supply of lubricant. In the spherical bearing embodiment disclosed, the bearing has a tendency to throw its own lubricating oil radially outward. Thus, an inner annulus area containing lubricating oil within the spherical bearing is placed in communication with an annular oil reservoir radially outward of the spherical bearing. Oil shed radially outwardly by the bearing is collected in the sump of the oil reservoir and directed back through passages below the spherical bearing so as to replenish the inner annular area. Oil is retained in the upper spherical bearing housing using a standpipβ sealably secured below the spherical bearing and extending upwardly through the bearing and around the shaft. Overall, various improvements and additional capabilities are obtained from the SRM top drive of the present invention include; • Concentric Design: The improved top drive places the drive directly over the center of the well, the concentric design eliminating the overhung movement of a cantilever-mounted motor, reducing the stress on the wellhead and eliminating the need for additional support structures; . • Torque Control; The improved top drive is a programmable torque machine ensuring that the rod string cannot be overtorqued. In addition, the improved top drive system continuously monitors the torque, enabling detection of a pump-off condition. Advanced control algorithms allow the operator to continuously optimize the well production; • Simple Design: The improved top drive is a direct-drive motor eliminating belts, sheaves, gears and other power-robbing paraphernalia. Also, the simple design drastically reduces the parts count and complexity, thereby increasing reliability and robustness; * Safety: The improved top drive features regenerative braking capacity to safely remove the stored energy in the rod string and fluid column when the unit is shut down, or in the event of a trip or power failure. In the event of a more serious failure which prevents the regenerative braking from functioning, the top drive is designed to tolerate a 5000 RPM overspeed without damage; • Performance: o 1 ,000 foot-pounds of torque from 50 to 500 RPM, o 100 HP from 500 RPM to 705 RPM1 o 40,000 pound-force rod load rating, o 20-year bearing life at design load and 500 RPM1 o greater than 90 % power line to rod string efficiency, and o No in-rush current, low power bills; and • Low Maintenance: The improved top drive concentric design simplifies installation and set-up time. There are no belts to tighten or adjust, only periodic check of thθ upper bearing oil level and annual re-greasing of the bottom bearing.

BRIEF DESCRIPTION OF THE DRAWINGS 1 Figure 1 is cross-sectional view of a well, a rotary pump and a wellhead with a rod string extending therethrough and having a switched reluctance motor top drive mounted above the wellhead; Figure 2 is a panel for an SRM controller; Figure 3 is a side view of a model of the SRM with a polish rod extending therethrough; Figure 4 is a side view of the SRM of Fig* 3 with a polish rod 8 extending therethrough; 9 Figure 5 is a cross-sectional view of the SRM of Fig. 3 with the 0 polish rod removed and illustrating cooling air flow therethrough; 1 Figure 6 is a detailed cross-sectional view of the SRM along lines Z B-B of Fig. 4 with the polish rod removed; 3 Figure 7 is a perspective view of the hollow shaft and upper \ bearing; 1 Figure 8 is a cross-sectional view of a spherical roller bearing Z embodiment of the upper bearing; 3 Figure 9 is a partial cross-sectional view of the bottom bearing 4 illustrating the full side view of the wave spring; 5 Figure 10 is a perspective view of a partial cross-section of the 6 bottom bearing according to Fig. 9; 7 Figure 11 is an underside perspective view of the stator illustrating 8 the upper and bottom stator lamination stacks and cooling shroud discharge; θ Figure 12 is a cross-sectional view of the stator cut with the upper 0 stator lamination stack removed to illustrate the stack spacers and Illustrate 1 cooling air discharge paths; 2 Figure 13 is a perspective view of the rotor illustrating the cooling 3 air vanes, axially-extending passageways and axial flow barrier; 4 Figures 14a and 14b are top views and perspective views 5 respectively of the assembled stator, hollow shaft, upper bearing and lower 6 bearing; 7 Figure 15 is a cross-sectional view of the rotor along A-A of Fig. 14a 8 with a side view of the hollow shaft extending therethrough, the axially-extending 9 passages not being shown; 0 Figure 16 is a partial cross-sectional view along D-D of Fig. 4 1 illustrating the rotor inserted through the stator; 2 Figure 17 is a partial perspective cross-section of the rotor and 3 stator and illustrating air cooling flow paths; 4 Figure 18 is a partial perspective view of the upper plate, upper 5 bearing housing and cooling air discharge shroud; Figure 19 is a partial cross-section perspective view of the upper plate, and cooling air discharge shroud of Fig. 18; Figure 20 is an underside perspective view of the SRM of Fig. 3 illustrating the cooling air intakes with the protective filter screen removed; Figure 21 is a cross-sectional perspective view of the SRM of Fig. 20 with the upper plate removed; Figure 22 is a perspective view of the SRM of Fig. 3; Figure 23 is a cross-sectional perspective view of the SRM of Fig. 22 with the upper plate removed; Figure 24 is a partial underside perspective view of the SRM of Fig. 21; Figure 25 is a cross-sectional view of another embodiment of the stator to illustrate the alternate body and cooling passages; I Figure 26 is a cross-sectional view of another embodiment of the stator of Fig. 25 illustrating additional cooling passages; Figure 27 is a detailed cross-sectional view of the SRM of Fig. 25 along lines E-E illustrating an embodiment of an air mover and cooling air flow though the stator; and Figure 28 is an alternate embodiment of the detailed cross-sectional view of the SRM of Fig. 25 illustrating an alternate arrangement of air mover and cooling air flow though the stator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In on© embodiment, an improved switched reluctance motor is provided. In another embodiment, an improved top drive for a rotary pump utilizing the improved switched reluctance motor is provided. Having reference to Fig. 1, a top drive 1 is used to rotate the rod string 2 of a rotary pump 3 anchored downhole in a well 4. The well 4 is sealed with a wellhead 5 through which the rod string 2 extends. A polish rod θ at the upper end of the rod string 2 extends through the wellhead 5 and through a packing 7. The top drive 1 iεt located above the wellhead S. The top drive 1 is secured to the wellhead 5 to absorb reaction torque and thus prevent rotation. The top drive 1 is secured to the wellhead 5 to arrest reactive rotation, preferably using a flanged connection {not detailed). A hollow shaft 14 extends through the top drive 1 guiding the polish rod 6 concentrically therethrough. The polish rod 6 extends through a hollow shaft 14 through the top drive 1. A rod clamp 9, secured to the polish rod 6, bears against the top of the hollow shaft 14 of top drive 1 to suspend itself and the rod string 2 therefrom. Optionally a taper-lock bushing (not shown) locks the hollow shaft 14 to the polish rod 6. The top drive 1 is usually spaced above the wellhead packing 7 using a housing or frame 10 so as to provide ease of access to the polish rod 6 and packing 7. Accordingly, a length of polish rod 6 is exposed between the wellhead 5 and the top drive 1. The polish rod 6 is shown extending above the 1. 1 The top drive 1 comprises an embodiment of a switched reluctance 2 motor SRM 11 mounted to the wellhead 5. On one embodiment, the frame 10 3 comprises the SRM 11 itself in a frameless configuration. 4 As shown in Fig. 6, the SRM 11 has a stator 12 and a rotor 13. The 5 principles of switched reluctance motors are known to those of ordinary skill in 6 the art. An SRM stator 12 is a laminate stack of electrical steel formed with a 7 plurality of slots wound with coils for forming phase windings - typically three 8 phases are provided. Typically, pairs of diametrically opposing stator poles or 8 sets of poles (12 poles shown as 4 sets of 3 poles) conventionally wired in series 0 for forming each independent phase of the multiphased switched reluctance 1 motor. A nine-pole stator would have 3 sets of 3 poles orientated a 120 degrees. 2 The coils are electronically switched (electronically commutated) in a 3 predetermined sequence so as to form a moving magnetic field. The rotor 13 4 also forms one or more poles and has no phase windings but has a plurality of 5 teeth which are closely radially spaced by an air gap to the stator poles. 6 The stator 12 produces a moving magnetic field which induces 7 torque through the teeth of the rotor 13. The rotor rotates to move the teeth inline 18 with the energized stator poles for minimizing the flux path (minimum reluctance). 19 Using predetermined switching of the appropriate stator poles, the desired rotor >Q speed is achieved, as is forward or reverse rotation. >1 The SRM 11, and hence the top drive 1, produces high starting !2 torque, a relatively flat torque curve and thus is capable of a wide range of !3 operational speeds. Accordingly, the top drive is fully functional and operable at !4 low rotational speeds approaching zero rpm. As shown in Figs. 1 and 2, systems to control the SRM 11 include electronics 11c to control high frequency switching of the stater's poles and means for determining the position of the rotor 12. These controls are known in the industry including those provided by Ideal Electric, USA and Emotron AB1 in Sweden.

Frameless Embodiment As shown in Fig. 1 and 3 - β, generally, in another embodiment, thθ top drive 1 is conveniently provided as a frameless motor which is secured directly to the wellhead, such as through frame 10 or other connection to the wellhead 5 such as through a flange to the wellhead components. The polish rod 6 supports the rod string 2. The polish rod 6 is supported by the hollow drive shaft 14 which is in turn rotatably supported by the SRM 11. Unlike conventional frameless motors in which loads are borne by the driven equipment, the present embodiment bears the loads on the motor itself. Accordingly, the SRM 11 is provided in a frameless configuration such as that complying with NEMA C-flange standard dimensions for convenience of mounting and shaft sizes. In other words, the motor stator itself forms the frame for resisting axial loads and reactive torque. The C-flange is mounted to the frame 10 and to the wellhead 5 or the frameless SRM C-flange is secured directly to the wellhead 5 to restrain reactive rotation. The SRM 11 comprises a structural assembly of the stator 12, an first or upper housing 18 at a first end of the stato^s lamination stack and a second or bottom housing 19 at a second end of the stator's lamination stack. Axial loads, in a top drive embodiment being vertical loads for suspending rod string 2 from the hollow shaft 14, are directed or transferred through the upper housing 18, through the statαr 12 and through the bottom housing 19 into the wellhead 5, In typical installations the axial loads can be as high as about 4Q1OQQ pounds. The polish rod 6 extends upwardly and out of the top drive's hollow shaft 14. The polish rod clamp 9 secures to the rod 6 above the top drive 1 for preventing the polish rod from passing downwardly through the hollow shaft 14. The clamp 9 and shaft 14 cooperate to enable rotational drive. As shown in Figs. 6 and 15, a dog and notch arrangement is conventionally employed to enable dogs extending from the shaft 14 to engage notches in the clamp 9 or vice-versa and thereby transmit torque from the SRM shaft 14 to the polish rod 6. In greater detail and referring to Figs. 2 - 24, the SRM 11 comprises a stator 12 having an upper lamination stack 12u and a bottom lamination stack 12b. The frameless SRM is achieved by sandwiching the stator 12 between the upper and bottom housings 18,19. The upper housing 18 comprises an upper plate 20 spaced from the stator 12 and supported by a structural member or cylindrical wall from a peripheral first flange or flanged end ring 2Or and forming annular space therein. The bottom housing 19 comprises a bottom plate 21 spaced from the stator 12 and supported on a structural member or cylindrical wall from a peripheral first flange or flanged end ring 21 r. The hollow shaft 14 supporting the rotor 13 extends axially through the stator 12,12u,12b and the upper and bottom plates 20,21. The shaft 14 is rotationally and axially supported using a first or upper radial thrust bearing 22 supported on the upper plate 20, The shaft 14 is 1 further radially supported with a second or bottom ball bearing 23 supported in 2 the bottom plate 21. 3 Axial loads from suspending the hollow shaft 14 and rod string 2 are 4 therefore directed or transferred through the upper plate 20, the end ring 2Or1 5 through the stator 12, through end ring 21 r, and through the bottom plate into the 6 wellhead 5. 7 The upper plate 20 is strengthened in part by a bearing housing 24 8 enclosing the upper radial thrust bearing 22. The bearing housing 24 is further θ strengthened with gussets 25. The bearing housing 24 is sealed from ambient Q conditions through a top seal 26 and top seal cover 27. 1 2 Shaft Misalignment Compensation 3 With reference to Figs. 6 - 10, and in another embodiment, there is 4 some tolerance in the rotational arrangement which is related to the frameless 5 motor and in particular due to the nature of an SRM stator 12 in which there can 6 be significant variation in the lamination stack assembly of the stator 12. The 7 dimension or height of multiple laminations in the stator stack can vary about the 8 circumference which could be as much as a % inch height variation on a stack of 9 about 12 inches or so. Accordingly, a top plane at the first end of the motor stator 0 12 (perpendicular to shaft 14) may not reside parallel to a bottom plane of the 1 second end of the motor stator 12. Thus the upper plate 20 and bottom plate 21 2 supporting the upper and bottom bearings 22,23 can be misaligned. 3 Conventional rigid mounting of the upper and bottom bearings 22,23 could result 4 in misalignment on the shaft axis and bearing axes. Normally, an axis through the bearings Ie aligned with th© axis of the shaft. Normally, the axis of the shaft is perpendicular to a plane of the upper plate 20. A combination of the bearings and of their mounting arrangements ensures reliable operation and alignment of the respective axes. More particularly, with reference to Figs. 7 and 8, in a first mounting assembly, the upper bearing 22 is a spherical roller thrust bearing (see Fig. 8) which has self- aligning capabilities and is forgiving of misalignment of the axis of shaft 14 relative to the default axis of the bearing despite a deviation from a perpendicular from the upper plate 20. This spherical bearing 22 rotatably supports the axial load while accepting some deviation from perpendicular between the shaft 14 and the upper plate 20. With reference to Figs. 9 and 10, in a second mounting assembly, the bottom bearing 23 is also provided with some compliance aided with some radial or transverse tolerance being provided and being fit with circumferential O- rings 30 to retain sealing without restricting movement. The bottom bearing Is a ball bearing which supports radial loads. The outer race 31 of the bottom ball bearing 23 has some radial clearance to a bearing mount 32 supported in the bottom plate 21 so as to enable some radial or transverse misalignment of the shaft 14 and bottom plate 21. The outer race 31 is therefore not conventionally firmly restrained and thus is additionally pre-loaded with a wave spring 33. With reference once again to Fig. 7, a rotor position encoder 80 is provided in the bearing housing 24 for sensing rotor position for feed back to the SRM controller 11c.

Lubrication Embodiment Returning to Figs. 6, 7 and 8 in another embodiment, the upper bearing 22 is provided with a dependable supply of lubricant such as oil. In the spherical bearing embodiment disclosed, dynamic operation of the bearing 22 has a tendency to shed its own lubricating oil radially outwardly. The upper bearing has an outer and lower race supported on the upper plate 20. The upper bearing has an upper race supportably and coupled for co-rotation with the shaft 14. An inner annulus area 35 within the tower race of the spherical bearing is placed in communication with an annular oil reservoir 36 radially outward of the spherical bearing. Typically the oil reservoir 36 is formed between the bearing housing 24 and the upper bearing 22. Oil shed radially outwardly by the bearing 22 is collected in a sump of the oil reservoir 36 and directed through passages 37 below the outer race, such as through a race mounting block, and back to replenish the inner annular area 35. Oil is retained in the upper spherical bearing housing 34, and loss down the hollow shaft is prevented, using a standpipe 33 sealably secured to the upper plate 20 below the spherical bearing 22 and extending upwardly through the bearing and around the shaft 14. The upper race of the upper bearing 22 is coupled to the shaft above the standpipe 38. A bearing hold down retainer 40 is secured by a nut 41 to the shaft 14. The retainer 40 is fit with passages 42 for release of potential pressure buildup or flow of gases.

Cooling Embodiment A Induced heat, which can build up and cause premature failure of conventional SRM technology under such rigorous operating conditions, is effectively dissipated under another embodiment of the improved top drive 1. In this preferred embodiment, ambient air is circulated from outside the SRM 11 , into the motor and across the stator 12 and rotor 13. With reference to Fig. 6 - 24, the flow path of the cooling air is through cooling air passages formed through a combination of unique stator and rotor design features. Simplistically, air is conducted through passages in the stator 12 or laminate stack to remove heat generated by the motor. The air can be directed axially through the stator or radially therethrough. In one embodiment, the air is directed upwardly through spaces between adjacent coils wound on adjacent stator poles and through an array of axially-exteπding passages formed in the rotor 13. With reference to Figs. 11 and 12, the stator 12 is fit with axially-extending passages 50 formed in the radially inward faces of the poles 51 adjacent the air gap between the rotor and the stator. Coils 52 are wound about each pole 51 leaving some clearance therebetween forming additional pole-to-pole axially-extending passages 53 between poles 52. The passages 53 isolated from the air gap. With reference to Figs. 13 - 15, the lamination stack of the rotor 13 is fit with axially-extending passages 60 open to the air gap. Upper and lower air movers or vanes 61,62 aid in forcing air flow along the axially-extending passages 60 of the rotor 13 and the passages 50 the stator 12. With reference to an assembly shown in Fig. 16, the rotor 13 and stator provide a plurality of cooling, axially-extending passages 50, 60 and 53. Cooling air is drawn inwardly through ports 75 in the bottom plate 21 and is directed radially past the stator 12 and out of the motor 11. 1 With reference to Fig. 17, and as also shown in Figs. 6,12,19,21 2 and 23, more preferably, the stator 12 is formed in two or more sub-stacks of 3 stator laminations. The sub-stacks, or upper and bottom stators 12u,12b, are 4 spaced axially by a plurality of circumfereπtially-spaced spacers 70. The result is 5 a plurality of circumferential ly-spaced air discharge passages 71 extending 6 radially from the rotor-statαr air gap 72 through a shroud 76 to a location outside 7 the motor. In the case of the upper and a bottom lamination stack or stators 8 I2u,i2b, the lamination of the rotor 13 is also conveniently differentiated using a 9 radially protruding barrier 73, typically formed of rotor laminations having a 0 diameter larger than the axially-extending passages in the rotor 13. Air flowing 1 along the rotor's axially-extending passages 60 are blocked and re-directed 2 radially outwards through the discharge passages 71 between the stators 3 12u,12b. 4 Thus, in the spaced stator embodiment, two flow paths F1,F2 are 5 created; a first path F1 flowing upwardly from lower rotor vanes 62 and through 6 the rotor's axially-extending passages to cool the lower lamination stack 12d, 7 coils 52 and rotor 13, and a second path F2 flowing downwardly from upper rotor 8 vaneβ 61 and through the rotor's axially-extending passages to coo! the upper 9 lamination stack 12u, coils and rotor. Both flows F1.F2 approach the rotor's .0 protruding barrier 73 and are directed out of the motor 11 Air supplying the .1 second path F2 is obtained from air flowing upwardly between the coils 52 and 12 then is directed downwardly aided by the upper vanes 61 »3 1 Cooling Embodiment B 2 While the stator 12 may be constructed of low-loss or loss-less 3 material to minimize heat generation, excessive heat may yet result, In this 4 embodiment, a combination of low-loss electrical steel, a modified cross-section 5 for the stator having cooling passages, and forced-cooling is implemented. Q Wrth reference to Figs. 25 and 26, a stator for the SRM can 7 optionally comprise a laminate stack of rectangular electrical steel. Preferably, 8 heating due to energy loss in the magnetization cycles is minimized through the Q use of non-oriented, high silica electrical steels such as 0.014 inch gauge ASTM 0 36F155 / AISI 29 M1S. The nominal properties of M19 electrical steel includes a 1 maximum core loss Pc of 1.74 VWIb and resistivity of 45-55 mW cm. This is 2 contrast jto conventional motors having 0.025 inch gauge ASTM 64F275 / AISI 24 3 M25 electrical steel having a maximum core loss Pc of 2.75 W/lb and resistivity of 4 20-3O mW cm. 5 As shown in Fig. 25 the stator 12 oan have a radially-extending 6 outer profile which exceeds that required from electrical operation of the motor. 7 In one embodiment, the profile can be square. Using a square profile, economic 3 savings can be realized due to the user of a full or smaller mill sheets for 9 obtaining multiple stator core plates with less manufacturing waste and cutting. 0 The resulting laminate stack sheets have comer mass 100 which do not 1 participate substantially in the magnetic properties of performance of the SRM 2 but do provide significant cooling mass without increasing the cross-sectional 3 space requirements for the motor overall. An air mover 101 , such as a fan driven 4 by the hollow shaft 14 and shroud 102, directs cooling air though cooling 5 passages 103 positioned through the corner mass. As shown in Figs. 25 and 26, a plurality of axial air passages 103 are formed axiatly through tha stator 12, limited to the corners 100 or distributed about the stator respectively, With reference to Fig. 27, the air mover 101 is coupled for co- rotation with the shaft 14 adjacent and above the upper housing 18. The shroud 102 is positioned to sandwich the air mover 101 between the shroud 102 and the upper housing 18. The air mover 101 draws cooling air into the shroud 102 for flow through the air passages 103 and to a location external to the motor 11. In another embodiment shown in Fig. 28, the air mover 101 is sandwiched between the upper housing 18 and the stator. The upper housing 18 forms the shroud 102. The air mover 101 positioned within a first annular space 104 within the upper housing and is coupled for co-rotation with the shaft 14. The fan draws cooling air into the shroud 18,102 for flow through the air passages 103 and to a location external to the motor 11. It is preferable to protect the motor 11 from accidental damage by recessing the shaft 14 within the lower housing 19. Should the shaft 14 protrude (as shown in Fig. 28), then a further or additional plate 110 is installed or the lower housing 19 is modified to recess the shaft 14 therein.

As a result of the novel combination of the present invention, certain advantages are obtained: • very low rotational speed capability - including less than 100 rpm • high starting torque and a substantially flat torque curve; • minimal components; • use of a concentric shaft and rod string and elimination of speed-reduction for providing passive and safe response to backspin situations; * use of a framθless motor configuration capable of supporting full axial loads; • a bearing arrangement is tolerant of misalignment in the motor or the polish rod; and • elimination of the need for additional mechanical or hydraulic backspin prevention devices.