Electronically commutated motor having skewed magnetics

• US 4,933,584 A
• Filed: 12/22/1988
• Issued: 06/12/1990
• Est. Priority Date: 12/22/1988
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Krefta, Ronald J., Oldenkamp, John L., Harms, Harold B.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Rebsch, D. L.
• Application Number:
  US07/288,474
• Time in Patent Office: 537 Days
• Field of Search: 310/49 R, 310/156, 310/162, 310/168, 310/186, 310/187, 310/254, 310/269, 318/138
• US Class Current: 310/162
• CPC Class Codes: H02K 1/16 Stator cores with slots for windings
• CPC Class Codes: H02K 1/278 Surface mounted magnets; Inset magnets
• CPC Class Codes: H02K 1/2791 Surface mounted magnets; Inset magnets
• CPC Class Codes: H02K 2201/06 Magnetic cores, or permanent magnets characterised by their skew
• CPC Class Codes: H02K 29/12 using detecting coils using the machine windings as detecting coil

Abstract

An electronically commutated motor has substantially constant air gap energy to reduce cogging. A rotatable assembly has permanent magnet elements which rotate about an axis of rotation. The elements are oriented with respect to said axis of rotation to provide a magnetic field with lines of flux along a plane forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation, wherein s₁ is not equal to zero. The elements may be provided with an unmagnetized portion referred to as a phantom skew. A stationary assembly is in magnetic coupling relation with the permanent magnet elements of the rotatable assembly and has t spaced teeth with adjacent teeth defining a slot therebetween, wherein t is positive integer. Each tooth has a surface adjacent the rotatable assembly having one or more notches forming a skew angle of s₂ mechanical degrees with the axis of rotation. Each tooth has winding stages adapted for commutation in at least one preselected sequence. The skewed magnetic field, skewed notches and phantom magnetic skews reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates, while maintaining a back EMF waveform having a maximized flat top width.

Claims

1. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein of length L, tranverse width W and radial depth D, each said notch being along a helical path which traverses a skew angle of s mechanical degrees with respect to the axis of rotation, said stationary assembly having distributed winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein n and t are positive integers and s is not equal to zero; and

means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly whereby the skewed notches reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates.

3. The motor of claim 1 wherein at least one of said teeth has a laterally transversely extending tip which forms part of the surface adjacent said rotatable assembly;

wherein a portion of one of the notches in said one tooth is in the tip with the remaining portion of the notch in said one tooth;

wherein the portion of the one notch has a first surface substantially parallel to the surface of said one tooth adjacent said rotatable assembly; and

wherein the remaining portion of the one notch has a second surface substantially parallel to a surface of said tip opposite the surface of said tip adjacent said rotatable assembly, said first and second surfaces intersecting to form an angle.

4. The motor of claim 1 wherein at least one of said teeth has a laterally transversely extending tip which forms part of the surface adjacent said rotatable assembly;

wherein a portion of one of the notches in said one tooth is in the tip with the remaining portion of the notch in said one tooth;

wherein at least one of said teeth has a laterally transversely extending tip which forms part of the surface adjacent said rotatable assembly;

wherein at least part of one of the notches in said one tooth is in the tip;

wherein the tip has a second radially inwardly tapering surface opposite the surface of said tip adjacent said rotatable assembly; and

wherein the notch has a surface parallel to the second surface.

6. The motor of claim 1 wherein the skew angle s substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top width of at least approximately 120 electrical degrees.

7. The motor of claim 6 wherein the n notches in the surface of each tooth define n+1 lands, each said land having a surface area substantially equal to W times L.

8. The motor of claim 7 wherein each said notch has a substantially rectangular shape when viewed in a transverse cross-section.

9. The motor of claim 8 wherein W=2D.

10. The motor of claim 1 wherein n=1 and t=36.

11. The motor of claim 1 wherein n=2 and t=24.

12. The motor of claim 1 wherein n=1 and t=24.

13. The motor of claim 1 wherein n=2 and t=12.

14. The motor of claim 1 wherein the slots are parallel to the notches.

2. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein of length L, transverse width W and radial depth D, each said notch being along a helical path which traverses a skew angle of substantially 360/2(n+1)t mechanical degrees with respect to the axis of rotation, said stationary assembly having a winding stage on each tooth adapted for commutation in at least one preselected sequence, wherein n is a positive even integer, and t is a positive integer and the skew angle is not equal to zero; and

means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly.

5. The motor of claim 2 wherein the skew angle s substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top width of at least approximately 120 electrical degrees.

15. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates, said elements being oriented with respect to said axis of rotation to provide a magnetic field with lines of flux along a plane forming a skew angle of s mechanical degrees with respect to the axis of rotation, wherein s is not equal to zero, each said permanent magnet element comprising two circumferentially adjacent permanently magnetized segments having the same radial polarization and having a space between the segments comprising an axially elongate substantially unmagnetized portion forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly, and having winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein t is positive integer whereby the skewed magnetic field reduces cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates.

17. The motor of claim 15 wherein the n notches in the surface of each tooth define n+1 lands, each said land having a surface area substantially equal to W times L.

22. The motor of claim 15 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise salient windings and n is even; and

wherein the skew angle s₁ substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

23. The motor of claim 15 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise distributed windings; and

wherein the skew angle s₁ and substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

24. The motor of claim 15 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise salient windings and n is even;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle st equals the sum of s₁ and s₂ and substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

25. The motor of claim 15 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise distributed windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle st equals the sum of s₁ and s₂ and substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

26. The motor of claim 17 wherein each said permanent magnet element is radially polarized and includes an axially elongate, substantially unmagnetized portion forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation.

27. The motor of claim 26 wherein each said permanent magnet element comprises two permanently magnetized segments having the same polarization and having a space between the segments comprising an axially elongate opening forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation.

16. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates, said elements being oriented with respect to said axis of rotation to provide said magnetic field with lines of flux along a plane forming a skew angle of substantially s₁ mechanical degrees with the axis of rotation; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein along the axis of length L, transverse width W and radial depth D forming a skew angle of S₂ mechanical degrees, said stationary assembly having a winding stage on each tooth adapted for commutation in at least one preselected sequence, wherein n is a positive even integer, t is a positive integer, and a total skew angle s_t w is equal to s₁ plus s₂ substantially equals 360/4(n+1)t; and

means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly whereby the notches and the skewed magnetic field reduce cogging of the electronically commutated motor.

18. The motor of claim 16 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise salient windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation, where n is an even positive integer; and

wherein a total skew angle s_t equals the sum of s₁ and s₂ and substantially equals 360/4(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

19. The motor of claim 16 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise salient windings and n is even; and

wherein the skew angle s₁ substantially equals 360/4(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

20. The motor of claim 16 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise distributed windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle st equals the sum of s₁ and s₂ and substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

21. The motor of claim 16 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise distributed windings; and

wherein the skew angle s₁ and substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

28. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates, said elements being oriented with respect to said axis of rotation to provide a magnetic field with lines of flux along a plane forming a skew angle of s mechanical degrees with respect to the axis of rotation, wherein s is not equal to zero, each said permanent magnet element having an elongate, substantially unmagnetized portion forming a skew angle of s mechanical degrees with respect to the axis of rotation; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly, said stationary assembly having winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein t is a positive integer whereby the skewed magnetic field and the space reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates.

33. The motor of claim 28 wherein each said permanent magnet element comprises two circumferentially adjacent permanently magnetized segments having the same radial polarization and a space therebetween.

35. The motor of claim 33 wherein the winding stages comprise distributed windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle s_t equals the sum of s₁ and s₂ and substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

49. The motor of claim 28 wherein the space between the segments comprises an elongate opening having an elongate axis forming a skew angle of s mechanical degrees with respect to the axis of rotation, where s is not equal to zero;

wherein said winding stages comprise distributed windings; and

wherein the surface of each tooth adjacent said rotatable assembly has n elongate notches therein, wherein n is zero or a positive integer; and

further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly; and

wherein the skew angle s substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

50. The motor of claim 28 wherein the space between the segments comprises an elongate opening having an elongate axis forming a skew angle of s mechanical degrees with respect to the axis of rotation, where s is not equal to zero;

wherein said winding stages comprise salient windings; and

wherein the surface of each tooth adjacent said rotatable assembly has n elongate notches therein, wherein n is zero or a positive, even integer; and

further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly; and

wherein the skew angle s substantially equals 360/4(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

29. A stationary assembly for reducing cogging in an electronically commutated motor having a rotatable assembly having a plurality of permanent magnet elements adapted to rotate about an axis of rotation and having a magnetic field in magnetic coupling relation with said stationary assembly, said motor further having means for sensing the back electromotive force (back EMF) induced in the stationary assembly by the permanent magnet elements and for commutating the stationary assembly in response thereto to rotate the rotatable assembly;

said stationary assembly comprising;

a stator core having t spaced teeth with adjacent teeth defining a slot therebetween, each tooth having a surface adjacent said rotatable assembly with n elongate notches therein of length L, transverse width W and radial depth D, each said notch being along a helical path which traverses a skew angle of s mechanical degrees with respect to the axis of rotation, where n and t are positive integers and s is not equal to zero; and

a plurality of distributed winding stages on the teeth adapted for commutation in at least one preselected sequence whereby the notches reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates.

30. The motor of claim 29 wherein at least one of said teeth has a laterally transversely extending tip which forms part of the surface adjacent said rotatable assembly, and wherein a portion of one of the notches in said one tooth is in the tip with the remaining portion of the notch in said one tooth;

wherein the tip has a second radially inwardly tapering surface opposite the surface of said tip adjacent said rotatable assembly;

wherein the portion of the one notch has a surface substantially parallel to the second surface; and

wherein the remaining portion of the one notch has a surface substantially parallel to the surface of said one tooth adjacent said rotatable assembly.

31. The motor of claim 29 wherein the skew angle s substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top width of at least approximately 120 electrical degrees.

32. A rotatable assembly for reducing cogging in an electronically commutated motor having a stationary assembly having t spaced teeth with adjacent teeth defining a slot therebetween, each tooth having a surface adjacent said rotatable assembly, and having winding stages on the teeth adapted for commutation in at least one preselected sequence, where n and t are positive integers, the motor also having means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the rotatable assembly and for commutating the winding stages in response thereto to rotate the rotatable assembly;

said rotatable assembly comprising;

an armature having a plurality of permanent magnet elements adapted to rotate about an axis of rotation so that said elements are adjacent the surface of each tooth and a magnetic field of said elements is in magnetic coupling relation with said stationary assembly, said elements being oriented to provide said magnetic field with lines of flux along a plane forming an angle of s mechanical degrees with the axis of rotation, each said permanent magnet element being radially polarized and including an axially elongate, substantially unmagnetized portion forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation, wherein s is not equal to zero whereby the notches and the skewed magnetic field reduce cogging of an electronically commutated motor including said rotatable assembly.

34. The motor of claim 32 wherein the winding stages comprise salient windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation n being a positive, even integer; and

wherein a total skew angle s_t equals the sum of s₁ and s₂ and substantially equals 360/4(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

36. The motor of claim 32 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise salient windings and n is even;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle s_t equals the sum of s₁ and s₂ and substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

37. The motor of claim 32 further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly;

wherein the winding stages comprise distributed windings;

wherein each surface of each tooth has n elongate notches therein along the axis of length L and width W forming a skew angle of s₂ mechanical degrees with respect to the axis of rotation; and

wherein a total skew angle st equals the sum of s₁ and s₂ and substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

38. The motor of claim 32 wherein the n notches in the surface of each tooth define n+1 lands, each said land having a surface area substantially equal to W times L.

39. The motor of claim 38 wherein each said permanent magnet is radially polarized includes an axially elongate, substantially unmagnetized portion forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation.

40. In an electronically commutated motor having substantially constant air gap energy to reduce cogging comprising a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates;

and a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein of length L, transverse width W and radial depth D, and having distributed winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein n and t are positive integers; and

means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly, the improvement comprising;

each said notch being along a helical path which traverses a skew angle of s mechanical degrees with respect to the axis of rotation, wherein s is not equal to zero.

41. The motor of claim 40 wherein the skew angle s substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

42. The motor of claim 40 wherein at least one of said teeth has a laterally transversely extending tip which forms part of the surface adjacent said rotatable assembly, and wherein a portion of one of the notches in said one tooth is in the tip with the remaining portion of the notch in said one tooth;

wherein the portion of the one notch has a surface substantially parallel to a surface of said tip opposite the surface of said tip adjacent said rotatable assembly; and

wherein the remaining portion of the one notch has a surface substantially parallel to the surface of said one tooth adjacent said rotatable assembly.

43. In an electronically commutated motor having substantially constant air gap energy to reduce cogging comprising a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates;

and a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein along the axis of length L, transverse width W and radial depth D, and having winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein n and t are positive integers; and

means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly, the improvement comprising;

said permanent magnet elements being oriented with respect to said axis of rotation to provide said magnetic field with lines of flux along a plane forming an angle of s mechanical degrees with the axis of rotation, wherein s is not equal to zero.

44. The motor of claim 43 wherein the winding stages comprise salient windings;

wherein n is even; and

wherein the skew angle s₁ substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.

45. The motor of claim 43 wherein the winding stages comprise distributed windings;

and wherein the skew angle s₁ and substantially equals 360/(n+)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

46. An electronically commutated motor having substantially constant air gap energy to reduce cogging comprising:

a rotatable assembly having a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates; and

a stationary assembly in magnetic coupling relation with the permanent magnet elements of said rotatable assembly and having t spaced teeth with adjacent teeth defining a slot therebetween, each said tooth having a surface adjacent said rotatable assembly with n elongate notches therein along the axis, said stationary assembly having winding stages on the teeth adapted for commutation in at least one preselected sequence, wherein n and t are positive integers;

a tip laterally transversely extending from at least one of said teeth and forming part of the surface adjacent said rotatable assembly;

wherein a portion of one of the notches in said one tooth is in the tip and forms an angle with the remaining portion of the notch in said one tooth; and

wherein the tip has a second radially inwardly tapering surface opposite the surface of said tip adjacent said rotatable assembly with the portion of the one notch having a surface substantially parallel to the second surface, and the remaining portion of the one notch has a surface substantially parallel to the surface of said one tooth adjacent said rotatable assembly whereby the notches reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates.

47. The motor of claim 46 wherein the elongate axis of said one notch forms a skew angle of s mechanical degrees with respect to the axis of rotation, where s is not equal to zero;

wherein said winding stages comprise distributed windings; and

further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly and wherein the skew angle s substantially equals 360/(n+1)t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees.

48. The motor of claim 46 wherein the elongate axis of said one notch forms a skew angle of s mechanical degrees with respect to the axis of rotation, where s is not equal to zero;

wherein said winding stages comprise salient windings and n is even; and

further comprising means for sensing the back electromotive force (back EMF) induced in each of the winding stages by the permanent magnet elements and for commutating the winding stages in response thereto to rotate the rotatable assembly and wherein the skew angle s substantially equals 360/2(n+1)t thereby resulting in a back EMF signal having a maximized flat top width.