DESC:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See Section 10; rule 13]

"OPEN-ENDED STATOR WINDING AND STATOR CORE ARRANGEMENT FOR SLOT-LESS PERMANENT MAGNET SYNCHRONOUS MOTORS"

PORTESCAP INDIA PVT. LTD.
Unit No.2, SDF 1, SEEPZ - SEZ,
Andheri (E), Mumbai - 400096, India

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF INVENTION
The present invention generally relates to stator winding and stator core arrangement for electric motors. More specifically, the present invention relates to stator winding and stator core arrangement for Slot-less permanent magnet synchronous motors.

BACKGROUND OF THE INVENTION
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section are associated with the subject matter of the background section and should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.

In a Permanent Magnet Synchronous (PMS) motor, permanent magnets are present on the rotor while phase windings are present on the stator. A controller is utilized to excite the phase windings sequentially to create a rotating magnetic field. The permanent magnets interact with the rotating magnetic field to generate torque for rotating the rotor.

A stator is typically made of laminated steel which is stamped and stacked to form a cylindrical shape with a central opening to receive a rotor. Steel laminations of the stator have a slotted or a slot-less design. A stator core of annular ring shape and winding placed in the air-gap offers without slots i.e., a slot-less stator has less winding inductance and lower core losses and therefore can operate at very high speeds. Slot-less Permanent Magnet Synchronous (SPMS) motors are generally suitable for small D/L ratio i.e., Outer diameter to axial length ratio, low torque, and high speed applications. Absence of stator core teeth in SPMS motors allows absence of detent/cogging torque as compared to slotted electrical motors in which cogging torque is generated.

Generally, self-supported coil windings are utilized in SPMS motors. Figure 1 illustrates an axial cut-view 100 of a conventional SPMS motor with axial type self-supported coil windings. A stator winding of the SPMS motor has a front head 102 protruding inwards, an active part 104 of the stator winding (also referred as stator active part 104), and a rear head 106 having a flat inner surface and a protrusion on its outer surface. A front balancing ring 108 and a rear balancing ring 110 are present at ends of a rotor magnet 112, and around a rotor shaft 114. The stator active part 104 of the stator winding may be aligned parallelly corresponding a length of the rotor magnet 112.

Figure 2 illustrates another axial cut-view 200 of a conventional SPMS motor. The axial cut-view 200 illustrates a stator outer frame 202, a lamination stack 204 disposed over the stator active part 104, and a lamination holder 206. The lamination holder 206 is disposed on both ends of the lamination stack 204 to hold the lamination stack 204 in its position. Length of the stator active part 104 is responsible for generating torque in the SPMS motor. In conventional configurations, as illustrated in Figure 1 and Figure 2, the length of the rotor magnet 112 gets limited due to inward protrusion of the front head 102 and presence of the rotor front balancing ring 108 under the stator active part 104. Such limited length of the rotor magnet 112 leads to generation of a limited amount of torque in the SBLDC motor when the D/L ratio of the motor is high. Further, inward protrusion of the front head 102 the assembly 100 also obstructs flow of air and thus results in poor heat dissipation, if the forced air-cooling arrangement is considered.

Applications such as medical power drill and saw, require SBLDC motors with better torque density, improved autoclavability, reliability, and better thermal dissipation.

OBJECTS OF THE INVENTION
A general objective of the invention is to increase torque density in Slot-less Permanent Magnet Synchronous (SPMS) motors.

Another objective of the invention is to improve autoclavability, reliability, and thermal dissipation of SPMS motors.

Still another objective of the invention is to develop stator assembly to provide easy assembling of a rotor into a stator assembly.

Yet another objective of the invention is to improve performance and reliability of an SPMS motor.

Still another objective of the invention is to provide a stator core arrangement that reduces cogging torque generated with a proposed stator winding arrangement of SPMS motor.

SUMMARY OF THE INVENTION
This summary is provided to introduce aspects related to a stator winding and a stator core arrangement for Slot-less Permanent Magnet Synchronous (SPMS) motors for increasing torque density, and the aspects are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.

In one embodiment, the SPMS motor comprises a shaft present at a center and along a length of the SPMS motor. A stator winding is present around the shaft and has a front coil head and a rear coil head at opposite ends, and an active part is present between the front coil head and the rear coil head. The front coil head and the rear coil head have flat inner surfaces. A rotor magnet is disposed over the shaft and beneath the stator winding. The rotor magnet is present along an entire length of the active part of the stator winding. An open ended stator winding is provided around the rotor magnet. The open ended stator winding has a flat inner surface and a front coil head and a rear coil head as protrusions on ends of outer surface. A lamination stack is disposed in the active part present between the front coil head and the rear coil head. A stator outer frame is positioned outside as an enclosure of the open ended stator winding.

In one embodiment, a front rotor balancing ring is positioned underneath the front coil head and a rear rotor balancing ring positioned underneath the rear coil head, to retain the rotor magnet in its position. The front rotor balancing ring and the rear rotor balancing ring are present outside a bottom region of the active part to increase the active length, thereby increasing the motor torque constant.

In one embodiment, the lamination stack is made of two semi-circular ring shaped stator cores provided with a locking mechanism to lock with each other. The two semi-circular ring shaped stator cores are adjoined using a stepped design or an angled surface to reduce change in magnetic reluctance with respect to rotor position, thereby reducing detent torque of the SPMS motor. The two semi-circular ring shaped stator cores are made of a compacted Soft Magnetic Composite (SMC) material. The two semi-circular ring shaped stator cores are placed on the open ended stator winding and glued together, inserted into a stator housing and glued in an axial position. The open ended stator winding is made of self-supporting, non-hygroscopic thermobond QT type litz wire. The Litz wire allows ease of forming open-ended stator winding shape.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings constitute a part of the description and are used to provide a further understanding of the present invention.

Figure 1 illustrates an axial cut-view of a conventional Slot-less Permanent Magnet Synchronous (SPMS) motor including the stator winding and rotor assembly, in accordance with prior art.

Figure 2 illustrates another axial cut-view a conventional SPMS motor including the stator and rotor assemblies, in accordance with prior art.

Figure 3 illustrates an axial cut-view of a proposed assembly of SPMS motor including the stator winding and rotor assembly, in accordance with an embodiment of the present invention.

Figure 4 illustrates an axial cut-view of a proposed assembly of SPMS motor including the stator and rotor assemblies, in accordance with an embodiment of the present invention.

Figure 5 illustrates a lamination stack arrangement of a two-piece stator core of a SPMS motor, in accordance with an embodiment of the present invention.

Figure 6 illustrates an exemplary representation of a two-piece stator Soft Magnetic Composite (SMC) core of SPMS motor, in accordance with an embodiment of the present invention.

Figure 7 illustrates variation of generated back EMF with position of rotor of SPMS motor, in accordance with another embodiment of the present invention.

Figure 8 illustrates variation of detent torque with position of rotor of the proposed SPMS motor, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

In one embodiment, the present disclosure provides an open-ended stator winding and a stator core arrangement for Slot-less Permanent Magnet Synchronous (SPMS) motor to overcome issues related to performance, reliability, torque density, heat transfer capability, and detent torque of SPMS motor. Structure and arrangement of stator winding and stator core of SPMS motor is explained successively with reference to figures.

Figure 3 illustrates an axial cut-view of an open-ended stator winding 300 and rotor assembly of an SPMS motor. The open-ended stator winding 300 includes a front coil head 302 having a flat inner surface and protrusion on an outer surface. The open-ended stator winding 300 further includes a rear coil head 306 having protrusion on its outer surface. An active part 304 is present between the front coil head 302 and the rear coil head 306. The open-ended stator winding 300 also includes a front rotor balancing ring 308, a rotor magnet 310 present over a shaft 312, and a rear rotor balancing ring 314. The front rotor balancing ring 308 may be positioned underneath the front coil head 302 and the rear rotor balancing ring 314 may be positioned underneath the rear coil head 306. The front rotor balancing ring 308 and the rear rotor balancing ring 314 may be utilized to balance the rotor magnet 310 considering high speed motor operation and retain the rotor magnet 310 in its position. The front rotor balancing ring 308 and the rear rotor balancing ring 314 may be non-magnetic and inactive components. The front rotor balancing ring 308 and the rear rotor balancing ring 314 may not contribute to torque production in an SPMS motor. The front rotor balancing ring 308 and the rear rotor balancing ring 314 are present outside a bottom region of the active part 304 to increase the active length, thereby increasing the motor torque constant.

Because the front coil head 302 has a flat inner surface and protruded externally, the front rotor balancing ring 308 is shifted out of a bottom region of the active part 304. By shifting the front rotor balancing ring 308, length of the active part 304 gets increased. Due to increase in length of the active part 304 of the open-ended stator winding, torque generated by an SPMS motor also increases. Thus, the torque density and performance of an SPMS motor improves upon usage of the open-ended stator winding 300. Further, the open-ended stator winding may comprise a straight conductor in the active part 304 to increase torque constant of an SPMS motor. The open-ended stator winding may also allow for better autoclavability of an SPMS motor by transfer molding the stator assembly.

In accordance with the present embodiment, ball bearings may be press-fitted onto the shaft 312 and the rotor assembly could pass through the open-ended stator winding assembly. This allows for better assembly of the ball bearings, and therefore results in improved life of the motor.

The open-ended stator winding may use self-supporting, non-hygroscopic thermobond QT type litz wire to provide improved transfer molding capability of stator and autoclavability to an SPMS motor with higher working temperature of approximately 200°C. Due to implementation of the thermobond QT type litz wire, reliability of an SPMS motor may be improved. Further, the Litz wire allows ease of forming open-ended stator winding shape.

Figure 4 illustrates an axial cut-view of a proposed assembly 400 of SPMS motor. In addition to the components of the assembly 300, the assembly 400 comprises a stator outer frame 402 and a lamination stack 404. The stator outer frame 402 acts as an enclosure of the open-ended stator winding 300. The lamination stack 404 is disposed in the region defined between the front coil head 302 and the rear coil head 306 and interposed between the active part 304 and the outer frame 402.

Miniature motors have in-built Printed Circuit Board (PCB) assemblies. Hall sensors are mounted on such in-built PCB assemblies. These Hall sensors are used to find rotor position for phase commutation. Failure of these Hall sensor is predominant in sterilizable SPMS motors. To avoid such failure of Hall sensors, PCBs can be transfer molded along with the stator assembly to improve reliability of the SPMS motors.

Figure 5 illustrates a lamination stack arrangement of a two-piece stator core 500 for the SPMS motor. The two-piece stator core 500 may include two semi-circular ring shaped stator cores 502 and 504. The two semi-circular ring shaped stator cores 502 and 504 may be made of lamination stacks, and provided with a locking mechanism to lock with each other when assembled to form a stator winding. The two-piece stator core 500 may be manufactured as two semi-circular ring shaped stator cores 502 and 504 to mount over the active part 304 having lesser outer diameter than an outer diameter the front coil head 302 and the rear coil head 306 which defines the shape of the winding. The laminations are stacked, and laser welded on the outer surface, or riveted together. The outer diameter of the stacked lamination core is higher than the front coil head 302 and the rear coil head 306.

Figure 6 illustrates an exemplary representation of two-piece stator core 600 for SPMS motor. In one implementation, to eliminate the generation of cogging torque due to joining surfaces, semi-circular ring shaped stator cores 602 and 604 may be adjoined using a stepped design or an angled surface to reduce magnetic reluctance variation with respect to rotor position, as illustrated in Figure 6. The reduction in magnetic reluctance reduces detent torque of the SPMS motor. The two semi-circular ring shaped stepped stator cores 602 and 604 may be made up of a compacted Soft Magnetic Composite (SMC) material.

As a first step in the assembly process, the open-ended coil is formed using forming tools. Subsequently, stator cores are placed on stator winding and glued together. Thereupon, the subassembly is inserted together into stator housing and glued in a required axial position. Because of the open-ended winding arrangement, the rotor assembly could be inserted from either side of the stator housing. Such feature would allow modularity of the assembly and motor failure mode analysis during manufacturing stage.

Figure 7 illustrates variation of generated back Electromotive Force (EMF) with position of rotor of SPMS motor for two different stator winding implementations. Back EMF is indicates a generated open-circuit voltage in the stator windings when the rotor is rotating. Also, the back-EMF relates to the torque constant of the motor. Higher the motor torque constant, lower the current required to meet the load torque requirement. Curve 702 represents variation of back EMF in a traditional stator winding implementation in a SPMS motor. Curve 704 represents variation of back EMF in the proposed open-ended stator winding implementation in the SPMS motor. By implementing the open-ended stator winding in the SPMS motor, performance of the SPMS motor may increase at least up to 8% as compared to the performance of other SPMS motors implemented with a traditional stator winding. Furthermore, the increase in stator winding resistance is negligible as compared to the existing coil design.

Figure 8 illustrates variation of detent torque with position of rotor of SPMS motor for three different stator core implementations. Curve 802 represents variation of detent torque in a traditional or existing annular ring stator core and existing stator winding implementation in SPMS motor. Curve 804 represents variation of detent torque in an open-ended stator winding with two semi-circular ring shaped stator cores having flat design. Curve 806 represents variation of detent torque in an open-ended stator winding with two semi-circular ring shaped stator cores having stepped design. By implementing the flat design in both of semi-circular ring shaped stator cores (i.e. in two-piece stator core), detent torque may be increased as compared to that of the traditional circular ring shaped stator cores. By implementing a stepped design/profile in both of semi-circular ring shaped stator cores (i.e. in two-piece stator core), peak detent torque may be reduced to 1.5 mNm and is negligible. Thus, the stepped core design reduces the detent torque generated by the SPMS motor.

In an embodiment, the open-ended stator winding and stator core/assembly can be implemented in any of two-pole, four-pole, and eight-pole Slot-less Permanent Magnet (PM) Synchronous motors.

Exemplary embodiments for stator winding and stator core arrangement used for SPMS motors, as discussed above, may provide certain advantages. Though not required to practice aspects of the disclosure, these advantages may include those provided by the following features.
,CLAIMS:We Claim:
1. A Slot-less Permanent Magnet Synchronous (SPMS) motor comprising:
a shaft (312) present at a center and along a length of the SPMS motor;
a stator winding present around the shaft (312) and having a front coil head (302) and a rear coil head (306) at opposite ends, and an active part (304) present between the front coil head (302) and the rear coil head (306), wherein the front coil head (302) and the rear coil head (306) have flat inner surfaces;
a rotor magnet (310) disposed over the shaft (312) and beneath the stator winding, wherein the rotor magnet (310) is present along an entire length of the active part (304) of the stator winding;
an open ended stator winding (300) provided around the rotor magnet (310), wherein the open ended stator winding (300) has a flat inner surface and a front coil head (302) and a rear coil head (306) as protrusions on ends of outer surface;
a lamination stack (404) disposed in the active part (304) present between the front coil head (302) and the rear coil head (306); and
a stator outer frame (402) positioned outside as an enclosure of the open ended stator winding (300).

2. The SPMS motor as claimed in claim 1, further comprising a front rotor balancing ring (308) positioned underneath the front coil head (302) and a rear rotor balancing ring (314) positioned underneath the rear coil head (306), to retain the rotor magnet (310) in its position, wherein the front rotor balancing ring (308) and the rear rotor balancing ring (314) are present outside a bottom region of the active part (304) to increase the active length, thereby increasing the motor torque constant.

3. The SPMS motor as claimed in claim 1, wherein the lamination stack (404) is made of two semi-circular ring shaped stator cores (502, 504; 602, 604) provided with a locking mechanism to lock with each other.

4. The SPMS motor as claimed in claim 3, wherein the two semi-circular ring shaped stator cores (502, 504; 602, 604) are adjoined using a stepped design or an angled surface to reduce change in magnetic reluctance with respect to rotor position, thereby reducing detent torque of the SPMS motor.

5. The SPMS motor as claimed in claim 3, wherein the two semi-circular ring shaped stator cores (502, 504; 602, 604) are made of a compacted Soft Magnetic Composite (SMC) material.

6. The SPMS motor as claimed in claim 1, wherein the active part (304) of the winding has lesser outer diameter than an outer diameter of the front coil head (302) and the rear coil head (306) which defines the shape of the stator winding.

7. The SPMS motor as claimed in claim 1, wherein the two semi-circular ring shaped stator cores (502, 504; 602, 604) are placed on the open ended stator winding (300) inserted together, into a stator housing and glued in an axial position.

8. The SPMS motor as claimed in claim 1, wherein the open ended stator winding (300) is made of self-supporting, non-hygroscopic thermobond QT type litz wire, and wherein the Litz wire allows ease of forming open-ended stator winding shape.

Dated this 28th day of Mays, 2021

[JAYANTA PAL]
IN/PA 172
Of REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]