Complete Specification Homopolar axial flux hub stepper motor

Field of Invention:

The present invention relates to homopolar machines and more particularly to axial flux hybrid stepper motor with stationary permanent magnets.

Background of the Invention:

Homopolar refers to the absence of polarity change and hence the homopolar machines are characterized by unidirectional field flux in the rotor. They have been conventionally used to generate DC power with disc type construction. The homopolar electric generation process involves a moving electric conductor enclosed by a unidirectional and constant magnetic field along the axis of rotation. Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This restricts the practical application of this type of motor.

Axial Flux motors differ from the other types of motors for the flux direction and for the shape of the motor. In axial flux machines flux goes through the axial direction and also the shape of the motor is disc type. These axial flux motors are widely used in low torque servo applications. To obtain high energy density disc type axial flux machines are preferred over radial flux machines. Axial flux machines are generally constructed either as induction motors or Brushless DC (BLDC) motors. BLDC motors are the most preferred to be used as hub motors for electric vehicles. Bipolar field of these machines in the rotor requires sensing of rotor position either directly or indirectly for its operation. Main drawback of axial flux machines are the presence of two air gaps and its complex fabrication and assembly process.

The stepper motors which converts electrical pulses into discrete mechanical movements provides several advantages such as good holding torque; precise positioning and repeatability of movement; excellent response to start and stop; highly reliable because of no contact brushes; can achieve low speed synchronous rotation; can operate in a wide range of rotational speed, etc. The most significant advantage of the stepper motor is its ability to operate in open loop without the need for rotor position sensing. They are available in both radial and linear forms. In spite of offering several advantages stepper motors are not easy to operate at extremely high speeds and there exists a possibility for the occurrence of resonance in the absence of adequate control.

Normally permanent magnets are housed in the rotor of the stepper motor to provide unipolar flux. In a radial stepper motor the rotor flux passes through axially, whereas stator wound coils produce radial flux. The cylindrical configuration of the rotor limits the area available for housing the permanent magnet in an inner rotor machine. These magnets can be either stationary or rotating. Outer magnets will have non-symmetric flux distribution around the magnets. The locating of permanent magnets in the moving part causes demagnetization due to temperature rise. High torque application requires more volume of permanent magnets. Further direct drive application requires high torque than high speed.

The present invention overcomes all the drawbacks reported above by providing a homopolar axial flux hub stepper motor which is a disc type machine with stationary permanent magnets that can produce high torque directly at low speed and works without the need for rotor position sensing.

Objective of the Invention:

The main objective of the present invention is to develop a homopolar axial flux hub stepper motor in which the radial type stepper motor is configured as an axial flux hub machine.

The second objective of the present invention is to construct a homopolar axial flux hub stepper motor with concentric disc type ring stator and rotor.

The third objective of the present invention is to construct a homopolar axial flux hub stepper motor with Stationary permanent magnets or electromagnets.

Another objective of the present invention is to construct a homopolar axial flux hub stepper motor with light weight rotor with no windings.

Brief description of the prior art:

US7719154B2 discloses a single field rotor motor with a stator which has plurality of stator poles having coils for creating magnetic poles and a rotor mounted for rotation with plurality of circumferentially spaced salient rotor poles. Donut type magnets are provided for creating a non-alternating, unchanging single polarity field on all rotor poles. The interaction of alternating flux field at stator poles and non-alternating flux fields at rotor poles produces motor torque for the rotation of rotor. This non-reversing flux field produced in the major portion of stator and rotor parts creates less loss and produces greater efficiency.

US4234808 discloses a stepping motor of a homopolar design in a cubic form. The stepping motor utilizes a layered stator with only 4 stator poles extending radially inward facing the working air gap and a concentric rotor which is secured to a shaft. The rotor includes an axially disposed disc shaped permanent magnet of high remanent energy density. The magnet is sandwiched between two toothed steel rotors and secured to the same shaft with a pair of bearing bells. The stepping motor of this type suffers from low power and energy density, demagnetization of permanent magnets and availability of area for housing of permanent magnets.

US20080100169 discloses a direct-current homopolar generator which comprises of plurality of stationary electrically conductive plates, a plurality of rotary magnetic plates. Each of the conductive plate is sandwiched between two magnetic plates forming a homopolar generator unit. Further the magnetic polarity of the magnetic plates is aligned in the same direction to generate a magnetic flux perpendicular to the conductive plates. The homopolar generator also includes a drive device for effecting the rotation of the rotary magnetic plates and an electrical circuit assembly for extracting electrical current from the generator. The rotary magnetic plate is an annular disk comprising an outer permanent magnet segment and an inner bearing segment to allow free rotation of the outer magnet segment. This rotating magnet may get demagnetized due to heat generation through continuous rotation. The drive further comprises a cylindrical housing to protect the homopolar generator. The cylindrical housing may limit the area for lodging the magnet.

US7446441B2 discloses a cylindrical outer rotor type hybrid stepping motor which uses new design parameters for the optimum motor design with reduced size, high output and high resolution. The hybrid stepping motor employs a stator winding disposed in an inner circumferential side of a gap and a rotor disposed in the outside of the gap with axially disposed magnets sandwiched between either the stator or rotor cores. Further the flux distribution around the magnet is non-symmetric and not symmetric. The design of hybrid stepping motor is optimized using a simplified calculation with the following parameters magnet axial direction surface area (A), diameter (D), lamination thickness of the stator core (L), residual magnetic flux density (Br) and average magnetic flux density (Bg).

US6847135B2 discloses an improved unipolar transverse flux machine with one rotor module and one stator module. The rotor module comprises two co-axial, axially spaced apart, ferromagnetic rotor rings toothed over their outer circumference with a constant tooth pitch and a unipolar permanent magnet ring placed between the two rotor rings. The stator module consists of concentric yokelike stator poles facing the two rotor rings, with an air gap between them and an annular coil. This machine is a conventional radial type cylindrical machine which lacks high power density, concentricity and high torque.

US7362026B2 discloses an improved homopolar motor-generator with a hollow rotor and a stator where one magnetic pole of the stator is located inside the hollow rotor. The homopolar motor-generator can also accommodate a plurality of nested hollow rotors connected in series, which increase the torque produced by the machine and reduces the working current thereby increasing the working voltage. Another embodiment of the invention comprises a stationary hollow magnet with annular slot through which current carrying rods are free to rotate. These current carrying rods can also be connected in series to increase the torque produced by the machine and reduce the working current.

The present invention eliminates the drawbacks of the existing system by providing a homopolar axial flux stepper motor which is a disc type machine with stationary permanent magnets that can produce high torque directly at low speed and works without the need for rotor position sensing.

Further the homopolar axial flux hub stepper motor is constructed with light weight rotor and no windings.

Summary of the invention:

The homopolar axial flux hub stepper motor according to the present invention comprises of concentric disc type stator and rotor ring sections. Stator ring section includes an outer stator ring and an inner stator ring; and the rotor ring section includes an outer rotor ring and an inner rotor ring. The two concentric stator and rotor ring sections are assembled in the machined slot of non-conductive discs. Torroidal bipolar copper windings are wound around the outer and inner stator core rings in their corresponding insulated slots. Stationary permanent magnets are housed in-between the two concentric outer and inner stator sections. The stationary permanent magnets are polarized in the radial direction to provide unidirectional field flux in the rotor. The entire assembly including stator rings, rotor rings, permanent magnet along with supporting non-conductive discs are enclosed in an outer casing frame. The frame is a cylindrical ring which houses a shaft; a shaft end bell and a terminal end bell. The shaft end bell and terminal end bells are provided with provisions for holding the metal bearings. The homopolar axial flux stepper motor of the present invention is a disc type machine with stationary permanent magnets that can produce high torque directly at low speed and works without the need for rotor position sensing. Further the homopolar motor is constructed with light weight rotors that are devoid of windings or magnets making the motor a light weight machine.

Brief description of the drawing:

Fig 1 shows the construction of the homopolar axial flux stepper motor.

Fig 2 shows the outer casing frame of the homopolar axial flux stepper motor with shaft, shaft end bell and terminal end bell.

Fig 3 shows the homopolar axial flux stepper motor inside the casing frame.

Fig 4 shows the outer and inner stator ring sections along with its poles.

Fig 5 shows the outer and inner rotor ring sections.

Fig 6 shows the offset of teeth between the outer and inner rotor ring sections.

Fig 7 shows the two phases of outer stator ring.

Fig 8 shows the two phases of inner stator ring.

Fig 9 shows the construction of the homopolar axial flux stepper motor with second set of permanent magnets.

Fig 10 shows the construction of the homopolar axial flux stepper motor with ring shaped permanent magnets.

Fig 11 shows the magnetic field flux produced during the working of the homopolar axial flux stepper motor

Fig 12 shows the "Aligned" position of rotor ring teeth and the stator ring teeth.

Fig 13 shows the "Unaligned" position of rotor ring teeth and the stator ring teeth.

Fig 14 shows the "Half-aligned" position of rotor ring teeth and the stator ring teeth.

Detailed description of the invention with respect to drawing:

The present invention provides a homopolar axial flux hub stepper motor which is simple in construction and aimed to be used as an inwheel motor for electric vehicles.

Construction of homopolar axial flux hub stepper motor:

The homopolar axial flux hub stepper motor is made of four sections with two concentric stator ring sections (1, 2) and two concentric rotor ring sections (3,4). Two concentric stator ring sections include an outer stator ring (1) and an inner stator ring (2) with their mean diameter ratio of 19:11. The Outer stator ring (1) has its outer diameter to inner diameter at a ratio of 10:9 and the inner stator ring (2) has its outer diameter to inner diameter at a ratio of 6:5. Two concentric rotor ring section are made of an outer rotor ring (3) and an inner rotor ring (4) with their mean diameter ratio of 19:11. The Outer rotor ring (3) has its outer diameter to inner diameter at a ratio of 10:9 and the inner rotor ring (4) has its outer diameter to inner diameter at a ratio of 6:5 The two concentric stator and rotor sections are assembled in the machined slot of non-conductive discs (5, 6) respectively. Torroidal bipolar copper windings (7) are wound around the stator core rings (1, 2) in their corresponding insulated slots. Multiple segments of rectangular shaped permanent magnets (8) are placed in the stator housing between the two concentric outer and inner stator sections (1, 2). These multiple segments of rectangular shaped permanent magnets (8) are polarized in the radial direction and are arranged in a ring form between the concentric stator sections. The two concentric outer and inner stator ring sections consist of a plurality of axially extending poles provided with teeth. The outer stator ring section (1) comprises of eight poles (9a, 9b, 9c, 9d, 9e, 9f, 9g, 9h) with five teeth per pole. The outer stator ring section in total contains 40 teeth (10). The inner stator ring section (2) comprises of eight poles (11a, lib, lie, lid, lie, llf, llg, llh) with five teeth per pole. The inner stator ring section in total contains 40 teeth (12). The two concentric outer and inner rotor ring sections (3, 4) consist of plurality of axially extending poles. The outer rotor ring section is provided with 50 teeth (13) facing the working air gap region and the inner rotor ring section is provided with 50 teeth (14) facing the working air gap region. The outer and inner rotor section teeth (13,14) have the same pitch as the teeth (10, 12) on the stator poles. The teeth (13, 14) at the outer and inner rotor ring sections are offset such that the centerline of a tooth (15) at the outer rotor ring (3) coincides with the slot (16) in the inner rotor ring (4).

The number of stator phases and the number of rotor poles decide the stepping angle of the motor. The outer stator ring (1) of the present embodiment exhibit two phases (17, 18) with four stator poles per phase. Phase A (17) of the outer stator ring (1) comprises of four stator poles (9a, 9c, 9e, 9g). Phase B (18) of the outer stator ring (1) comprises of four stator poles (9b, 9d, 9f, 9h). The inner stator ring (2) also has two phases (19,20) with four stator poles per phase. Phase C (19) of the inner stator ring (2) comprises of four stator poles (11a, lie, lie, llg). Phase D (20) of the inner stator ring (2) comprises of four stator poles (lib, lid, llf, llh).). Phase A (17) and Phase C

(19) are connected in parallel to form phase one. Phase B (18) and Phase D

(20) are connected in parallel to form phase two. The outer and inner stator ring sections (1, 2) are wound using torroidal bipolar copper winding (7) to have a phase difference of 180° between them. To maintain the same flux density in both the outer and inner stator core sections the ring length of the inner stator ring section (2) has been doubled to accommodate more coil turns. The multiple segments of the rectangular shaped permanent magnets (8) are housed in the stator ring section to provide unipolar flux to the rotor sections. The entire assembly comprising of stator rings (1, 2), rotor rings (3, 4), permanent magnet (8) along with supporting non-conductive discs (5, 6) are enclosed in an outer casing frame (21). The frame (21) is substantially a cylindrical ring which houses a shaft (22); a shaft end bell (23) and a terminal end bell (24). The shaft end bell and terminal end bells are provided with provisions for holding the metal bearings (25).

In the preferred embodiment the stator ring sections (1, 2) are made of solid or laminated steel.

In the preferred embodiment the rotor ring sections (3, 4) are made of solid or laminated steel.

In the preferred embodiment the non-conductive discs (5, 6) are made with high-density polyethylene plastic material.

In the preferred embodiment the frame (21) is be made of non magnetic steel or cast iron material.

In the preferred embodiment the shaft end bell (23) and the terminal end bell (24) are made of high-density polyethylene plastic material with provisions for holding the metal bearings (25).

In the preferred embodiment the rotor section (3, 4) are devoid of any windings or magnets thus making the homopolar axial flux hub stepper motor a light weight machine. Further the flat cylindrical shaped rings of stator section (1, 2) provides a very less stator core mass which also accounts to the light weight of the homopolar axial flux stepper motor.

In the preferred embodiment the rotor rings have low moment of inertia. This accounts for the resonant frequency of the homopolar motor to be shifted to a very high value avoiding the problem of resonance during low frequency operations.

In another embodiment the stator can be either core wound or tooth wound.

In another embodiment to increase the field strength of the homopolar motor second set of rectangular shaped multiple permanent magnets (26) are arranged in a ring fashion and can be housed in-between the rotor sections (3, 4) with the opposite polarity, but fixed to the stationary part of the non conductive disc (5) to aid the unipolar flux.

In another embodiment, a single ring type permanent magnet (27) is placed in the stator housing between the two concentric outer and inner stator sections (1,2). The single ring type magnet is polarized in the radial direction. To increase the field strength of the motor second set of ring shaped permanent magnet (28) can be housed in-between the rotor ring sections (3,4) fixed to the stationary part of the non conductive disc (5) with the opposite polarity, to aid the unipolar flux.

In another embodiment the said construction for the homopolar axial flux hub stepper motor can be extended diametrically or axially with similar modules to increase the power output of the machine.

Working of homopolar axial flux hub stepper motor:

In the preferred embodiment the homopolar axial flux stepper machine works on the principle of operation of magnets. The unipolar rotor ring sections (3,4) with teeth align themselves with the magnetic field produced by the torroidal bipolar copper winding in the stator ring section (1, 2) to produce torque. Since there are no conductors on the rotor ring sections (3,4) to carry current, the torque of the homopolar axial flux stepper machine is produced by the reluctance of the rotor ring sections (3,4) and synchronous action of the stator ring section (1,2).

The permanent magnet (8) polarized in the radial direction makes the primary flux of the permanent magnets to be unidirectional. The radial primary flux setup from the stationary permanent magnet (8) meets the inner toothed stator (2) which gets turned up axially and crosses the small air gap twice to return to the outer stator poles (1) through inner rotor poles (4) and outer rotor poles (3). The field flux thus traverses through two air gaps in the axial direction to achieve a unipolar flux which keeps the inner rotor teeth (14) to be always all south and the outer rotor teeth (13) to be always all north.

In the preferred embodiment when there is no current flow in the two phases (17, 18) of the outer stator ring (1) and in the stator windings, the permanent magnet (8) acts as an only source of magnetic flux across the working airgap. This flux makes the rotor teeth to detentor align with the stator making the homopolar axial flux motor to be held in its step position by the detent torque.

In the preferred embodiment when phase one (17) is supplied with the positive current the outer stator ring poles 9a and 9e are magnetized as 'south' and the outer stator ring poles 9c and 9g are magnetized as 'north'. The outer rotor ring teeth (13) which are always 'north' due to unipolar field flux gets attracted to the outer stator poles 9a and 9e. This renders the teeth of the outer rotor ring section facing the outer stator poles 9a & 9e and the teeth of the corresponding outer stator poles 9a & 9e to be in an "Aligned" (29) position. The outer rotor ring teeth facing the outer stator poles 9c & 9g and the teeth of the corresponding outer stator poles 9c & 9g are in "Unaligned" (30) position. Concurrently the inner stator ring poles 11a and lie are magnetized as 'north' and the inner stator ring poles lie and 11g are magnetized as 'south'. The inner rotor ring teeth (14) which are always 'south' due to unipolar field flux gets attracted to the inner stator ring poles 11a and lie. This makes the teeth of the inner rotor ring section facing the inner stator poles 11a & lie and the teeth of the corresponding inner stator poles 11a & lie to be in an "Aligned" (29) position. The inner rotor ring teeth facing the inner stator poles lie & 11g and the teeth of the corresponding inner stator poles lie & 11g are in "Unaligned" (30) position. To reach the 'Aligned' or 'Unaligned' position the rotor has to move by VA of a rotor slot-pitch.

In the preferred embodiment single phase excitation of phase one (17) results in a half aligned position (31). When phase one (17) is excited, the teeth of the poles (9b, 9d, 9f, 9h) corresponding to phase two (18) of the stator ring gets arranged in a half aligned position (31).

In the preferred embodiment when the current flow is shifted to the phase two (18) of the torroidal copper winding of the stator ring, the stator poles shift in space by 45°. To reach the 'Aligned' or 'Unaligned' position the rotor again moves by V* of a rotor slot-pitch. Phase one (17) and phase two (18) are excited sequentially to move the rotor through 200 steps to complete one rotation.

In the preferred embodiment both phase one (17) and phase two (18) are excited concurrently to achieve a doubled torque.

In another embodiment which uses two sets of permanent magnets (8, 26) to increase the field strength of the homopolar motor, the radial primary flux from the first set of stationary permanent magnet (8) meets the inner toothed stator (2) which gets turned up axially and crosses a small air gap to reach the inner rotor (4) poles. The second set of stationary permanent magnets (26) placed in-between the rotor rings directs the flux to the outer rotor ring (3) poles. The salient outer rotor ring (3) poles force the flux into the air gap to reach the outer stator (1) poles to complete its path. The field flux thus traverses through two air gaps in the axial direction to achieve a unipolar flux which keeps the inner rotor teeth (14) to be always all south and the outer rotor teeth (13) to be always all north.

We Claim:

1. A homopolar axial flux hub stepper motor comprising:

a) two concentric stator ring section including an outer stator ring and an inner stator ring;

b) two concentric rotor ring section including an outer rotor ring and an inner rotor ring;

c) two non-conductive discs to house the concentric stator and rotor ring sections;

d) torroidal bipolar copper windings wound around the outer and inner stator rings;

e) permanent magnets housed between the two concentric outer and inner stator sections;

f) a shaft which connects the shaft end bell and terminal end bell;

g) metal bearings which connects the shaft with the shaft end bell; and

h) a cylindrical outer casting frame which encloses the homopolar axial flux hub stepper motor;

2. The homopolar axial flux hub stepper motor of claim 1, wherein the stator and rotor are constructed as a concentric disc type ring structures.

3. The homopolar axial flux hub stepper motor of claim 1, wherein the stator and rotor ring sections consist of plurality of axially extending poles provided with teeth.

4. The homopolar axial flux hub stepper motor of claim 1, wherein the permanent magnets are stationary.

5. The homopolar axial flux hub stepper motor of claim 1, wherein the permanent magnets are rectangular shaped and arranged in a ring form.

6. The homopolar axial flux hub stepper motor of claim 1, wherein the permanent magnets are ring shaped.

7. The homopolar axial flux hub stepper motor of claim 1, wherein the rotor section is devoid of magnets and winding.

8. The homopolar axial flux hub stepper motor of claim 1, wherein second set of permanent magnets are housed in-between the rotor sections but fixed to the stationary part of the non conductive disc to aid the unipolar flux and increase the field strength of the motor.