DESC:FIELD OF INVENTION
[0001] The present invention generally relates to the field of slot-less brushless permanent magnet (PM) electric motors, and more particularly to stator windings in a slot-less brushless direct current (BLDC) motor having improved performance characteristics.
BACKGROUND OF INVENTION
[0002] Brushless direct current (BLDC) permanent magnet (PM) motors have gained popularity in the fields of medical devices, robotics and industrial devices, etc., for its simple construction and long operational lifetime. A BLDC motor typically consists of a stator, wrapped with electro-magnetic coils/windings and a rotor with permanent magnets.
[0003] Depending on the stator construction used, BLDC motors can be broadly classified into two categories: slotted stator BLDC motors and slot-less stator BLDC motors. In a slot-less motor, instead of winding copper wires through slots as in conventional slotted brushless motors, phase coils are spatially oriented around the stator to form the electromagnetic phase relationship required for motor operation, i.e. the stator consists of an axial self-supported coil, which includes one coil body formed by winded electrical conductors, with a head at each distal end. When energized, the coils create a magnetic flux which interact with the PM flux to produce the torque. Such a configuration of the slot-less stator without the stator teeth, reduces cogging torque and increases the torque density of the motors. Accordingly, slot-less brushless DC motors are extensively used in micro torque hand tools requiring high specific torque and efficiency.
[0004] Existing slot-less BLDC motors use full pitch stator windings. These windings are formed by winding a multi stranded insulated conductors in a winding tool. A number of connection tapings may be formed between groups of the turns to form the phase windings. The wound coil is removed from the winding tool and then coil is pressed or flattened to form a cylindrical structure with the head at each distal end. Further, the phase connections outputs extend the external surface of the head.
[0005] Such full pitch windings, while offering maximum motor torque constant (k), also cause greater electrical resistance because of the increased length of conductor, especially at the coil heads at the distal ends. Therefore, the coil presents the disadvantage of requiring a relatively long axial thickness for the heads. Particularly, the heads of the coil do not provide the useful torque and hence, do not contribute towards performance of the motor. Accordingly, active length (La) of the coil is lesser than the total length (Lc) of the coil and this extra length (at the heads) increases weight and costs of the material needed to manufacture the motor. Also, in the existing slot-less BLDC motors which uses a full pitch windings, the conductor filling factor (i.e., the ratio of electrical conductor volume to the total volume of the coil), at the coil heads is poor with reduced active coil length (La) and inferior performance, which hinders realization of a certain motor diameter to length ratio (D/L).
[0006] Fig. 1 illustrates the cross-sectional view of a known single layer full pitch winding of three-phase, 4-pole slot-less BLDC electric motor. The motor comprises of a stator, a rotor (not shown) and a plurality of coils, with each stator coil spanning an angle of 90 mechanical degrees and four coils forming one phase, for example, coils A1, A2, A3 and A4 form one phase for the coil, wherein one coil is represented by A1+ and A1-. Thus, the motor in fig. 1 has four coils per phase. Fig. 2 illustrates a sectional view of a coil along the axial length in a known slot-less BLDC motor. As illustrated, the coil includes coil heads having an axial thickness (AT1, AT2) at the distal ends, one protruding inwardly towards and the other protruding outwardly towards the axial length of the coil. The active length (La) of the coil, having thickness (e) can be defined by the expression:
La = Lc - AT1- AT2;
where Lc is the total length of the coil.
[0007] Fig. 3(a) and Fig. 3(b) illustrate perspective views of a conventional full pitch slot-less BLDC motor, depicting the axial thickness of the coil heads at the distal ends. The axial thickness of the coil heads, in the conventional full pitch windings, causes the active length (La) to decrease and the volume of the coil head to increase. Also, there is cross-over of conductors at the coil heads, which requires more copper and increases the volume and axial thickness of the coil. The coil heads do not produce any useful torque as the conductors are radially/circumferentially protruding at the distal ends of the coil. Therefore, it is advantageous to reduce the length of the coil at distal ends, so as to increase the effective active length of the coil.
[0008] Several techniques have been suggested in the past to reduce the axial thickness or size of the coils heads. However, no endeavour has been made so far optimize the axial thickness (AT) of the coil head in respect to the active length (La) of the coil and its thickness (e) so as to reduce the motor regulation (R/k2). Motor regulation is the ratio of the motor resistance (R) and square of torque constant (k) and is represented by the slope of the speed torque curve, i.e. the change in speed caused by change in motor torque. The smaller the value of R/k2, the less power is dissipated by the motor to a given torque, thereby ensuring greater efficiency. In the existing full pitch winding, due to the wire cross-over at the heads of the coil, the coil thickness (e) in the active part of the coil (La) is always less than the axial thickness (AT1, AT2). As the result, the overall volume of the coil is increased, thereby making the winding unsuitable for small sized motors. Also, such windings are especially not suitable for motors having a short axial length.
[0009] In view of the foregoing, there is a need of compact stator windings with reduced axial thickness of the coil heads suitable to mitigate the above drawbacks of the existing full pitch windings. Also, it is desirable to increase the torque constant for shorter axial length of the motor, ensuring greater efficiency and superior performance of the motor.
OBJECTS OF THE INVENTION
[0010] An object of the present invention is to improve the performance of four pole slot-less BLDC motor having shorter axial length by using short-pitch winding pattern design.
[0011] Another object of the present invention is to provide a four pole slot-less BLDC motor incorporating reduced number of coils per phase while at the same time improving torque constant and reducing conductor loss.
[0012] Another object of the present invention is to improve the performance of the slot-less BLDC motor.
[0013] Another object of the present invention is to provide a four pole slot-less BLDC motor having coil heads volume of which are reduced to minimize negative impacts on the axial length of the motor.
[0014] Still another embodiment of the present invention is to provide a compact four pole slot-less BLDC motor which does not negatively affect the optimum torque and efficiency of the motor for a given outer diameter to axial length ratio.
SUMMARY OF THE INVENTION
[0015] The present invention envisages to improve the performance of a four pole slot-less brushless DC motor having shorter axial length by using short-pitch winding pattern instead of full pitch winding design. Short pitch winding as per an embodiment of the present invention uses three coils per phase instead of four coils per phase and this results in improved torque constant and reduced conductor loss for the motor having shorter axial length. The present invention provides a short pitch axial winding for a 22mm to 30mm size four pole slot-less BLDC motor, which is compact and does not negatively affect the optimum torque and efficiency of the motor for a given outer diameter to axial length (D/L) ratio.
[0016] An aspect of the present invention relates to a slot-less brushless DC motor having improved performance characteristics and reduced motor regulation. The slot-less BLDC motor includes a stator wound with at least one layer of winding having a plurality of coils spatially oriented around the stator. Each coil of the plurality of coils comprises coil heads disposed at distal ends thereof, and axial thickness of the coil head extending inwardly toward an axial length of the coil and axial thickness of the coil head protruding outwardly from the axial length of the coil are adapted to increase effective axial length of the coil.
[0017] According to an embodiment of the present invention, axial thickness of the coil head extending inwardly toward the axial length of the coil is reduced to increase the effective axial length of the coil. According to an embodiment of the present invention, axial thickness of the coil head protruding outwardly from the axial length of the coil is reduced to increase the effective axial length of the coil.
[0018] According to an embodiment of the present invention, coil spanning of the plurality of coils is 60 mechanical degrees.
[0019] According to an embodiment of the present invention, the at least one layer of winding is formed in a short pitch winding pattern.
[0020] According to an embodiment of the present invention, the at least one layer of winding comprises three coils per phase arrangement.
[0021] According to an embodiment of the present invention, the at least one layer of winding formed in the short pitch winding pattern reduces overlapping of electrical conductors at the coil heads of the plurality of coils.
[0022] According to an embodiment of the present invention, axial thickness (AT1) of each coil head extending inwardly towards the axial length of the coil (404) satisfies the condition:
AT1 < 1.1e/n, wherein
(e) is the thickness of body of the coil, and
(n) is an integral number denoting half of the number of pole pairs of the motor.
[0023] According to an embodiment of the present invention, axial thickness (AT2) of each coil head protruding outwardly from the axial length of the coil satisfies the condition:
AT2 < 1.7e/n, wherein
(e) is the thickness of the body of the coil, and
(n) is an integral number denoting half of the number of pole pairs of the motor.
[0024] Another aspect of the present invention relates to a stator for a slot-less brushless DC motor, the stator being wound with one or more layers of short pitch windings having a plurality of coils spatially oriented around the stator, wherein each coil of the plurality of coils comprises coil heads disposed at distal ends thereof, and axial thickness of the coil head extending inwardly toward an axial length of the coil and axial thickness of the coil head protruding outwardly from the axial length of the coil are adapted to increase effective axial length of the coil. According to an embodiment of the present invention, the one or more layers of short pitch windings comprise three coils per phase.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0025] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0026] In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.
[0027] Fig. 1 illustrates a cross-sectional view of existing full pitch winding for a slot-less brushless DC (BLDC) motor;
[0028] Fig. 2 illustrates a section view representation of an axial coil of the BLDC motor;
[0029] Figs. 3(a) and 3(b) illustrate three-dimensional representations of conventional full pitch winding for the slot-less BLDC motor;
[0030] Fig. 4 illustrates an exemplary cross-sectional view representation of a stator of a slot-less BLDC motor with short pitch axial windings in accordance with an embodiment of the present invention;
[0031] Fig. 5 illustrates an exemplary sectional view of the axial coil using short pitch winding along axial length of the coil in accordance with an embodiment of the present invention;
[0032] Figs. 6(a) and 6(b) illustrate exemplary three-dimensional representations of a slot-less BLDC motor using short pitch windings in accordance with an embodiment of the present invention;
[0033] Fig. 7 illustrates an exemplary graphical representation showing the relationship of motor regulation (R/k2) versus overall length of the motor for a 22mm size motor in accordance with an embodiment of the present invention; and
[0034] Fig. 8 illustrates an exemplary cross-sectional representation of a stator of an 8-pole BLDC motor with short pitch axial windings in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0035] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0036] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future.
[0037] The present invention relates to a slot-less brushless DC motor, for instance, a four pole slot-less BLDC motor, having shorter axial length by using short-pitch winding pattern having three coils per phase which improves torque constant and reduces conductor loss for the motor having shorter axial length. The objectives of the present invention are realized by using short pitch axial windings which allows optimizing the active length of the coil in respect to the axial thickness of the coil heads, thereby lowering the motor regulation.
[0038] Fig. 4 illustrates the cross-sectional view of a stator (402) of a BLDC motor with short pitch axial windings according to an embodiment of the invention. In one of the embodiments, the stator (402) is wound with single layer short pitch windings which use three coils per phase instead of four coils per phase, and one coil spanning is 60 mechanical degrees instead of 90 mechanical degrees, as in case of the conventional full pitch stator windings. The use of short pitch windings in such a configuration reduces the conductors from overlapping or crossing at the heads of the coil (404), as in case of full pitch windings (Fig. 1), thereby reducing the axial thickness of the coil heads and thus improving the performance of the motor.
[0039] Fig. 5 depicts the sectional view of an axial coil using short pitch winding along the axial length of the coil (404). As illustrated, the axial thickness (AT1, AT2) of the coil (404) in short pitch windings is less in comparison to the axial thickness (AT1, AT2) of full pitch windings, as can be seen in Fig. 2. Therefore, by the virtue of the relationship of the active length
La = Lc - AT1 - AT2;
the active length of the coil (404) having short pitch windings is greater than the active length of the coil (404) having full pitch windings. The reduced axial head thickness and greater active length of the coil (404) in case of short pitch windings leads to an improved torque constant (k) due to increase in magnet length for the shorter axial length motor. Further, the electrical resistance (R) of the motor is also reduced due to short spanning of the coil (404). Consequently, by optimizing the active length of the coil (404) in respect to the axial thickness (AT) of the coil heads, a reduced electrical resistance (R) and an increased torque constant (k) are achieved. This further leads to a reduced (R/k2), resulting in a better motor performance i.e., less heat generation for a given output torque. Further, since there is reduced crossing or overlapping of the conductors at the coil heads, less volume of the coil is used, which further reduces the overall weight and cost of the motor.
[0040] According to an embodiment of the present invention, use of short pitch winding instead of full pitch winding results in reduced torque constant because the flux density experienced by the coil (404) is not maximum. The torque constant is increased by extending the active length due to the decreased coil head thickness at both the ends. Further, the electrical resistance is lowered due to short spanning of the coil (404). This results in increased efficiency and specific torque of the motor.
[0041] According to an embodiment of the present invention, axial thickness (AT1, AT2) of the coil heads can be reduced by modifying the winding pattern from full pitch winding to short pitch winding pattern.
[0042] In Figs. 6(a) and 6(b), the perspective three-dimensional views of a slot-less BLDC motor using short pitch windings according to the above embodiment are illustrated. As shown, the axial thickness (AT1, AT2) of the coil heads at the distal ends is reduced in comparison to that in case of full pitch windings as illustrated in Figs. 3 (a) and 3 (b).
[0043] In an embodiment, the present invention provides a winding technique for a slot-less BLDC motor to optimize the length of the coil head versus coil active length, in the case of coil having electric wires arranged in parallel to the rotor axis. Optimization of the windings is dependent on the active length of the coil versus coil thickness and coil diameter. This winding technique allow to build the coil faster with each phase having three consecutive coils for a 4-pole BLDC motor.
[0044] In an embodiment, the short pitch axial winding is used in a 22mm to 30mm external diameter 4-pole slot-less brushless permanent magnet motor, which is compact and does not negatively affect the optimum torque and efficiency of the motor for a given outer diameter to axial length (D/L) ratio. Also, when the D/L ratio of the motor is more than 0.47, the use of full pitch winding pattern exhibits a higher ratio of axial thickness of coil heads to the active length (La) of the coil (404).
[0045] In another embodiment, the invention seeks to optimize the thickness (e) of the coil body in respect to the axial thickness (AT) of each coil head to reduce the motor regulation (R/k2). In one of the embodiments, the coil body having a thickness (e), the electrical motor having a number of pole pair (p) = 2n, where n is an integral number and the axial thickness (AT2) of each coil head extending outwardly satisfies the condition (1):
AT2 < 1.7e/n…. (1)
In another embodiment, the coil body has a thickness (e) and the electric motor has a number of pole pair (p) = 2n, and the axial thickness (AT1) of each coil head extending inwardly satisfies the condition (2):
AT1 < 1.1e/n….. (2)
The aforesaid conditions (1) and (2), indicate the relationships which are satisfied for short pitch axial winding as per an embodiment of the invention.
[0046] In yet another embodiment, the 4-pole motor advantageously has an outer diameter (D) of 30mm or more, such that the short pitch winding motor has improved performance even at the longer motor length (L) of about 60-64mm.
[0047] In an embodiment, the winding may be in the form of any of faulhaber winding, honeycomb winding and fractional winding.
[0048] Fig. 7 is a graph illustrating the relationship of motor regulation (R/k2) versus overall length of the motor (L) for a 22mm size motor. As illustrated, the use of short pitch windings exhibits a lower motor regulation in comparison with full pitch windings, particularly at smaller lengths of the motor. The performance of axial self-supported short-pitched winding for different lengths of a motor is carried out using finite element analysis (FEA) based simulation tool. By simulation, it has been found that the motor regulation is reduced up to 22% by using short pitch windings for the 35mm length motor. Further, the initial feasibility of the axial self-supported short-pitched winding pattern is experimentally validated.
[0049] The invention as described above can be used in a 4-pole slot-less BLDC electric motor. However, the invention is not limited to 4 pole motors and can also be used for motors having higher number of poles. For instance, Fig. 8 illustrates the cross-sectional view of a stator of a 8-pole BLDC motor with short pitch axial windings according to an embodiment of the invention.
[0050] In view of the aforesaid short pitch axial windings, slot-less brushless DC motors with reduced size, more torque, lower power consumption and less cost can be produced. Such motors would be most suitable for battery operated applications, particularly for small sized motor applications.
[0051] Various modifications to these embodiments are apparent to those skilled in the art from the description and drawings herein. The principles associated with the various embodiment defined herein may be applied to other embodiments. Therefore, the description is not intended to be limited to the embodiments shown along with the accompanying drawings but is to be provided broadest scope consistent with the principles and novel and inventive features describe/disclosed or suggested herein. Any modifications, equivalent substitutions, improvements etc. within the scope and principle of the present invention shall all be included in the scope of protection of the present invention.
,CLAIMS:We Claim:
1. A slot-less brushless DC motor comprising:
a stator (402) wound with at least one layer of winding having a plurality of coils (404) spatially oriented around the stator (402),
wherein each coil of the plurality of coils (404) comprises coil heads disposed at distal ends thereof, and axial thickness (AT1) of the coil head extending inwardly toward an axial length of the coil (404) and axial thickness (AT2) of the coil head protruding outwardly from the axial length of the coil (404) are adapted to increase effective axial length (La) of the coil.
2. The motor as claimed in claim 1, wherein axial thickness (AT1) of the coil head extending inwardly toward the axial length of the coil (404) is reduced to increase the effective axial length (La) of the coil.
3. The motor as claimed in claim 1, wherein axial thickness (AT2) of the coil head protruding outwardly from the axial length of the coil (404) is reduced to increase the effective axial length (La) of the coil.
4. The motor as claimed in claim 1, wherein coil spanning of the plurality of coils (404) is 60 mechanical degrees.
5. The motor as claimed in claim 1, wherein the at least one layer of winding is formed in a short pitch winding pattern.
6. The motor as claimed in claim 3, wherein the at least one layer of winding comprises three coils per phase arrangement.
7. The motor as claimed in claim 3, wherein the at least one layer of winding formed in the short pitch winding pattern reduces overlapping of electrical conductors at the coil heads of the plurality of coils (404).
8. The motor as claimed in claim 1, wherein axial thickness (AT1) of each coil head extending inwardly towards the axial length of the coil (404) satisfies the condition:
AT1 < 1.1e/n, wherein
(e) is the thickness of body of the coil, and
(n) is an integral number denoting half of the number of pole pairs of the motor.
9. The motor as claimed in claim 1, wherein axial thickness (AT2) of each coil head protruding outwardly from the axial length of the coil (404) satisfies the condition:
AT2 < 1.7e/n, wherein
(e) is the thickness of the body of the coil, and
(n) is an integral number denoting half of the number of pole pairs of the motor.
10. A stator (402) for a slot-less brushless DC motor, the stator (402) being wound with one or more layers of short pitch windings having a plurality of coils (404) spatially oriented around the stator (402), wherein each coil of the plurality of coils (404) comprises coil heads disposed at distal ends thereof, and axial thickness (AT1) of the coil head extending inwardly toward an axial length of the coil (404) and axial thickness (AT2) of the coil head protruding outwardly from the axial length of the coil (404) are adapted to increase effective axial length (La) of the coil.
11. The stator (402) as claimed in claim 10, wherein the one or more layers of short pitch windings comprise three coils per phase.
Dated this 13.06.2018
[JAYANTA PAL]
IN/PA NO. 172
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]