ASYMMETRIC SALIENT PERMANENT MAGNET SYNCHRONOUS MACHINE

A permanent magnet machine is disclosed, including a stator assembly which includes a housing, a stator backiron, a plurality of windings disposed in the housing coupled to a plurality of electrical connections, and a plurality of stator teeth coupled to the stator backiron. The permanent magnet also includes a rotor assembly which includes a center configured to couple to a mechanical coupling member disposed about the center, an inner core, positioned around the center, an outer core disposed around the inner core, and a plurality of outwardly protruding poles radially located within the stator assembly each outwardly protruding pole having an outer surface adjacent to at least one tooth of the plurality of teeth. Each outer surface of each outwardly protruding pole having a rotor tooth extending from the outer core and a permanent magnet disposed next to the rotor tooth.

TECHNICAL FIELD

The present disclosure generally relates to electric machines and particularly to permanent magnet machines providing high torque density.

BACKGROUND

Currently, there is great social interest in increasing the efficiency of power magnetic devices such as electric generators and motors, which are used in applications such as wind energy turbines and hybrid and/or electric vehicles. For example, there has been an ongoing desire to improve conventional permanent magnet synchronous machines (PMSMs). PMSMs have historically been present in specialty applications of small ratings or in very high speed applications, for example, in excess of 20,000 rpm. Examples include spindle drives and flywheel energy storage machines. Advancements in the late 20th century in high energy permanent magnets increased interest in PMSMs. For example, utilization of PMSMs in full and hybrid electric vehicles has drastically increased in the last several years, where it is believed to have accounted for sixty-five percent of electric machines topologies used. Reasons for this increase are believed to be related to the drawbacks that other machine topologies suffer from. Direct Current (DC) machines, although capable of providing high stall torque, suffer from degradation of carbon brushes, which creates an ongoing maintenance issue. Induction machines exhibit the advantage of low cost and high robustness but need sophisticated control to accommodate wide speed operation. Reluctance machines suffer from low efficiency and relatively low power density. PMSMs on the other hand are typically known for their high torque density for a given loss, high reliability, and high system efficiency.

PMSMs can in general be classified as Surface Mounted Permanent Magnet Synchronous Machines (SM-PMSMs), or Interior Permanent Magnet Synchronous Machines (IPMSMs), with a variety of different forms coming from these general structures. The SM-PMSMs have permanent magnets placed on a surface of a rotor, where the permanent magnets are secured in position by either gluing them or wrapping an inert material around them. In IPMSMs, the permanent magnets are buried in the rotor back iron which provides mechanical protection to the permanent magnets.

In traction applications, one of the main requirements for an electric motor is to be capable of maintaining a wide constant power speed range (CPSR). Numerous ideas have been proposed to improve the efficiency of PMSMs such as enhanced excitation current control methodologies and a structural modification to the machine. Modifying the machine structure to improve the CPSR performance has been considered in a number of published research works.

Increasing saliency increases what is commonly known as saliency or reluctance torque, which contributes to the overall torque production in addition to the torque produced by the permanent magnet. One technique used to increase the saliency in an SM-PMSM is by using flux barriers. Another approach relies on tapering the machine's steel to create an asymmetry. Another approach to create an asymmetry for the purpose of enhancing the performance of a hybrid machine, with both a reluctance rotor and a permanent magnet rotor sharing the same shaft and stator, is by having the reluctance rotor axis shifted with respect to the permanent magnet rotor axis.

In view of the above, there are ongoing efforts to improve the efficiency of PMSMs, and it would be desirable if a PMSM was available that was capable of providing improved operational efficiency and reduced manufacturing cost.

Over the past few years, effort in the prior art has been expanded to develop a reluctance machine with high efficiency and high torque production capability relative to physical size. A reluctance machine is a type of magnetic device, e.g., a motor, where magnetic poles are induced in a rotating non-magnetic member (i.e., a rotor) by at least one winding in a stationary member (i.e., a stator). The rotor is typically provided with a plurality of salient (i.e., outwardly projecting) poles. The poles are induced by applying electrical current to the winding.

Exemplary prior art reluctance machines with distributed windings is depicted inFIG. 6. A cross-sectional view of a typical reluctance machine configuration with distributed windings is shown inFIG. 6. The reluctance machine10includes a stator assembly12and a rotor assembly14. The stator assembly12includes a stator body20(also referred to as housing) with a plurality of stator teeth22coupled thereto. Windings (not shown) are provided around and about the stator teeth22in a distributed fashion. The rotor assembly14is centrally mounted within the stator assembly12. The rotor assembly14includes a plurality of protruded poles30each having a pole face32that is substantially parallel with an interior surface defined by adjacent stator teeth22. The rotor assembly14further includes a rotor core26and a shaft24centrally located about the rotor core26. The reluctance machine10depicted inFIG. 6Aincludes a large number of stator teeth where many teeth are associated with a winding (not shown), and the associated phase. In the exemplary embodiment of the prior art depicted inFIG. 6A, the conventional reluctance machine may include three windings, each associated with a phase, where the windings are distributed around the teeth22(where a large number of teeth22are provided around the stator body). The rotor assembly14, as depicted inFIG. 6A, includes four protruded poles30, although it may include less or more poles.

Referring back toFIG. 6, it should be noted that flux characteristics of the air gap between the pole face32of each pole30and the adjacent stator teeth22remains substantially constant. The consistency in the flux characteristics results in the same torque output capability in either a clockwise38or a counter clockwise36rotational direction. In particular, as a leading edge34bof a rotor pole30approaches the next stator tooth22when the rotor assembly14is rotating in the direction depicted by arrow38(i.e., clockwise directions), the output torque remains substantially the same as if a trailing edge34aof a rotor pole30approaches the next stator tooth22when the rotor assembly14is rotating in the direction depicted by arrow36(i.e., counter clockwise) corresponding with a reversal of the rotational direction of the stator MMF.

Attempts to improve the performance of the synchronous reluctance machines are typically associated with design of the rotor assembly14of the reluctance machine10such that it will result in improved performance One category of performance is torque density which is the amount of torque that is generated relative to the physical size or mass of the machine for a given amount of loss. The rotor assembly14depicted inFIG. 6, although simple and can be manufactured at a relatively low cost, has relatively poor performance in terms of torque density, since flux density (i.e., the MMF resulting in output torque) varies considerably over the pole faces32of the pole30, as discussed further below. Therefore the spatial region of high flux density is limited (which is a function of position within the rotor assembly14), if a high degree of saturation (which leads to high loss) is to be avoided.

One approach to decrease power loss in one rotational direction is to employ an asymmetric reluctance machine (i.e., as compared to symmetric reluctance machines). Referring toFIG. 7and as seen in U.S. Pat. No. 9,000,648 to Harianto et al., a cross sectional schematic view of a stator assembly110and a rotor assembly140of an asymmetric reluctance machine (ARM)100is depicted. The stator assembly110is a stationary member of the ARM100while the rotor assembly140is the portion of the A-RM100that moves (i.e., rotates about the stator assembly110). The stator assembly110is cylindrical in shape including a housing116which transitions into a plurality of teeth120inwardly protruding toward center of the housing116along the radial direction. The teeth120are formed at intervals130along a circumferential direction.

The rotor assembly140includes a rotor core150and a plurality of outwardly protruding poles160. Each of the plurality of outwardly protruding poles160has an asymmetrical shape, pointed out by the shape of pole tapers164. The rotor assembly140also includes a shaft170positioned at the center of the rotor core150.

The asymmetrical nature of the rotor assembly140improves the power loss of the reluctance machine in one direction (the main rotational direction of the reluctance machine). While this improvement is advantageous, additional improvement is needed.

Therefore, there is a need for a to power magnetic machine that improves output torque density based on the relationship between the rotor shape and the stator.

SUMMARY

A permanent magnet machine is disclosed. The machine includes a stator assembly which includes a housing, a stator backiron, a plurality of windings disposed in the housing coupled to a plurality of electrical connections, and a plurality of stator teeth coupled to the stator backiron. The permanent magnet also includes a rotor assembly which includes a center configured to couple to a mechanical coupling member disposed about the center, an inner core, positioned around the center, an outer core disposed around the inner core, and a plurality of outwardly protruding poles radially located within the stator assembly each outwardly protruding pole having an outer surface adjacent to at least one tooth of the plurality of teeth. Each outer surface of each outwardly protruding pole having a rotor tooth extending from the outer core and a permanent magnet disposed next to the rotor tooth forming the outer surface that is substantially continuous.

A drive system is disclosed. The drive system includes a voltage source, and a permanent magnet machine as described above and structured to be coupled to a mechanical load.

DETAILED DESCRIPTION

A novel electric machine has been developed which improves torques output in one direction as compared to known prior art power magnet machines. In particular, aspects of the present disclosure provide an asymmetrical permanent magnet synchronous machine (A-PMSM) architecture that employs rotational asymmetry to reduce machine mass, cost, and power loss in constant power speed range (CPSR) applications.

Referring toFIG. 1, a block diagram of a drive system70is depicted. The drive system70in an exemplary embodiment which includes a voltage source72and a power converter74coupled to the voltage source72. The voltage source72is typically a single phase direct current (DC) source; however, single and multi-phase alternating current (AC) outputs are also possible. The voltage source72may represent power available at an electrical outlet. In such a configuration, an electrical conductor73arepresents a power output and an electrical conductor73brepresents a return (or commonly referred to as the neutral). Alternatively, the conductors73aand73bmay represent conductors of a DC voltage source.

The power converter74includes power inputs which are connected to the conductors73aand73bto receive one of a DC power, a single-phase electrical current or a multi-phase electrical current (wherein, in a multi-phase AC configuration there are corresponding conductors). Additionally, the power converter74includes an input which is coupled to an output79of a converter controller78, described further below. The Power converter74also includes three outputs representing three phases with currents that are each separated by 120 electrical degrees. Each phase is provided on a conductor75a,75b, and75c. It should be noted that a common neutral line for return of each phase of the electrical currents is not shown and may or may not be present. It should also be appreciated that the power converter74may produce more or less number of phases (i.e., more or less than three phases).

The drive system70also includes an A-PMSM76which is coupled to the power converter74. The A-PMSM76may include a plurality of inputs which are connected to the conductors75a,75b, and75c. The inputs are coupled to respective windings, described further below (seeFIG. 2) which are distributed about a stator. The A-PMSM76includes a signal output77a, which in one embodiment represents position of a rotor assembly240(seeFIG. 2) with respect to a stator assembly210(seeFIG. 2). The A-PMSM76also includes a mechanical output77bwhich can be an interface for a mechanical coupling between the A-PMSM76and a mechanical load80.

The drive system70also includes the converter controller78which is coupled to the A-PMSM76and the power converter74. The converter controller78includes an input which is coupled to the signal output77aof the A-PMSM76. The signal output77arepresents a feedback signal from the A-PMSM76that can be used to control the power converter74. In one embodiment, this feedback signal is the position of the rotor assembly240(seeFIG. 2). In such an embodiment, the feedback signal (i.e., the signal output77a) can be output of a variable reluctance (VR) sensor, an optical sensor, a hall-effect sensor, or other position determining sensors known to a person having ordinary skill in the art.

These sensors may be positioned on the rotor assembly240(seeFIG. 2), on the stator assembly210(seeFIG. 2), or positioned on both. Circuitry for conditioning the signal output77acan be placed in the A-PMSM76or in the converter controller78. Additionally, the converter controller78includes the output79which is coupled to the power converter74. The output79, therefore, represents the control signal from the converter controller78which is used to control the power converter74. The combination of proper winding distribution and current waveform generate a desired stator magnetomotive force (MMF) distribution relative to the rotor assembly240(seeFIG. 2).

It should be appreciated that in an alternative embodiment the power converter74may be avoided and the A-PMSM76powered directly by an appropriate voltage source72.

It should also be appreciated that a synchronous reluctance machine is different than the A-PMSM76, in that windings positioned in the rotor are short circuited to assist in startup (often referred to as damper windings). In such a configuration, the reluctance machine can be operated directly from a polyphase voltage source; thereby eliminating the need for power electronics or controls. However, the drive system70depicted inFIG. 1, includes the signal output77a(i.e., control and rotor position feedback), thereby the aforementioned damper windings are not necessary. Addition of such a damper winding to the rotor of the A-PMSM76, would have the advantage of being able to be operated without the power converter74or control scheme/electronics.

It should be appreciated that while the A-PMSM76ofFIG. 1is depicted as a reluctance machine that can receive electrical power to produce mechanical power, it can also be used such that it receives mechanical power and thereby converts that to electrical power. In such a configuration, the power converter74is utilized to excite the winding using a proper control and thereafter extract electrical power from the A-PMSM76while receiving mechanical power.

Referring toFIG. 2, a cross sectional schematic view of a stator assembly210and a rotor assembly240of an A-PMSM200, according to one embodiment of the present disclosure is depicted. The stator assembly210is a stationary member of the A-PMSM200while the rotor assembly240is the portion of the A-PMSM200that moves (i.e., rotates about the stator assembly210). The stator assembly210is cylindrical in shape including a a housing (not shown), a stator backiron216which transitions into a plurality of teeth220inwardly protruding toward center of the stator backiron216along the radial direction. The teeth220are formed at intervals230along a circumferential direction. The number of teeth220, which is a function of the intervals230, is a design parameter that can affect torque ripple and other electrical and mechanical characteristics of the A-PMSM200as is known to a person having ordinary skill in the art. The stator assembly210is configured to have a single or multi-phase distributed winding (seeFIG. 3) and corresponding electrical connections (not shown) that can be placed about the stator teeth220.

The rotor assembly240includes an inner core250which could be made from a magnetically inert material and an outer core255which terminates in a plurality of outwardly protruding poles258(which can also be made from magnetic steel). Each of the plurality of outwardly protruding poles258forms an asymmetric arrangement, pointed out by a rotor tooth260and a permanent magnet263. The permanent magnets can be made from ferrite, samariam cobolt, neodynium iron boron, or other magnetic material known to a person having ordinary skill in the art. The rotor assembly240also includes a center configured to receive a shaft270(or also referred to as a mechanical coupling member) positioned at the center of the inner core250. The shaft270is configured to be coupled to a mechanical load (e.g., the mechanical load80depicted inFIG. 1). The outwardly protruding poles258are formed at intervals262along a circumferential direction. The number of outwardly protruding poles258, which is a function of the intervals262, is a design parameter that can affect torque ripple and other electrical and mechanical characteristics of the A-PMSM200as is known to a person having ordinary skill in the art.

The ratio of circumferential portions defined by the rotor tooth and the accompanying permanent magnet263define the asymmetrical nature of the A-PMSM.

While a curved surface is depicted inFIG. 2for the rotor tooth260and magnet263, other possible surfaces can be realized with the goal being to maximize flux density substantially over the entire the outward protruding pole258. The asymmetry between these pole (i.e., the rotor tooth260and the permanent magnet263) on the outer surface of the outwardly protruding poles258is such that the flux density profile can be manipulated as the operating point is changed to yield improved performance. The maximum allowed flux density is defined by an amount of flux that saturates the material of the outwardly protruding poles258or corresponding stator teeth based on its shape. Therefore, the asymmetry depicted inFIG. 2is influenced by shapes and configuration of the stator assembly210and the rotor assembly240. Other asymmetrical arrangements may result in differing flux density profiles.

It should be appreciated that it is the flux density profile on the stator teeth220over the outwardly protruding poles258that define the pole asymmetry. It should be noted that the flux density in the stator teeth220and the outwardly protruding poles258are correlated. However, the flux density in the stator teeth220becomes higher as the teeth conduct the flux over the slots between the teeth220.

One goal is to cause the flux profile of the stator teeth220that are over the outwardly protruding poles258to be such as to be favorable from torque production and loss viewpoints, particular as operating conditions (speed, required torque) change.

Additionally, the shape of the rotor assembly240and in particular the shape and characteristics of the asymmetry defined by the rotor tooth260and permanent magnet263of the outwardly protruding poles258in relationship to the stator assembly210and in particular to its teeth220, results in a flux density profile over the surface of the outwardly protruding poles258and in particular over the outer surfaces of the rotor tooth260and the permanent magnet263so as to be favorable from a torque production and loss viewpoints.

Therefore, the asymmetry is designed1) to generate a flux density profile over the poles (i.e., the outer surfaces of the rotor tooth260and the permanent magnet263) of the outwardly protruding poles258which is favorable for torque production; and 2) to have this profile be adjustable with operating conditions so as to facilitate a wide speed range.

Referring toFIG. 3, a close-up of portions of the stator assembly210and the rotor assembly240is provided. The stator assembly210includes a single or multi-phase distributed winding280shown between the stator teeth220.

It should be observed that the rotor shape of a conventional PMSM substantially achieves the same flux density over the poles irrespective of the direction of the desired torque. Thus, the induced field is substantially the same with the rotor assembly14rotating in the direction of arrow36or38(seeFIG. 6).

In comparison, the rotor assembly240(seeFIG. 2) of the present disclosure provides a novel rotor designed to induce a tailored field in the outwardly protruding poles258over the surfaces of164when the rotor is rotating in one direction (e.g., designated by the arrow242).

Since a higher amount of output torque is produced, the A-PMSM of the present disclosure can be smaller, lighter, and less costly as compared to a conventional PMSM producing the same output torque. In contrast, for the same size PMSM, the A-PMSM of the present disclosure can generate a higher level of output torque in one direction (e.g., direction142bas shown inFIG. 2), which is an acceptable limitation in many applications where the output torque needs to be high only in one principal direction. In addition, due to the uniformity of the induced field, the A-PMSM of the present disclosure produces smaller amounts of ripple in the output torque.

Lower torque ripple can result in a smoother operation of the A-PMSM even in lower speeds.

Referring toFIG. 4, a graph of torque and power vs. RPM is provided for the A-PMSM of the present disclosure. The torque graph is divided into a substantially constant portion (17.8 Nm according to one embodiment) followed by a drop in the torque. The power graph is also divided into two corresponding portions wherein the first corresponding portion output power increases from zero to a maximum (e.g., to about 1.86 KW) corresponding to a substantially constant torque and then remains substantially constant at that maximum as corresponding to the declining toque.

Referring toFIG. 5a weighted energy loss (measured in Watts) vs. cost (measured in $$) is provided to compare a conventional surface mount PMSM with an A-PMSM of the present disclosure. As can be seen the energy loss is lower for the A-PMSM of the present disclosure providing a showing that arrangement of the present disclosure provides an advantage over the conventional PMSM. Note that for the same weighted power loss, the A-PMSM of the present disclosure can provide a total cost reduction of around 18% compared to the SM-PMSM. This improvement is important in applications that use PMSM equipped with expensive rare-earth permanent magnets, and that are unidirectional, such as traction and spindle drive applications.

In operation, with respect toFIG. 1, electrical power can be provided from the voltage source72in DC, single-phase AC, or polyphase AC form. The electrical power can then be optionally converted to a three-phase output by the power converter74and provided to the A-PMSM76. The A-PMSM76in this configuration is configured to receive electrical power and convert it to mechanical power to thereby apply the mechanical power to the mechanical load80. A position signal can be placed on the signal output77aand provided to converter controller78to control the power converter74.

Alternatively, the A-PMSM76can be configured to convert mechanical power to electrical power. In this configuration, the mechanical load80is providing mechanical power to the A-PMSM76and in turn, the A-PMSM76converts the mechanical power to electrical power which is provided to the power converter74or directly to the voltage source72.

While the asymmetrical rotor concept described here has been applied to a permanent magnet machine, it is recognized that the same concept could be applied to other types of machines which use a continuously rotating magnetic field, wherein the rotor rotates in synchronism with the field, including wound rotor synchronous machines.

Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. Therefore, the following claims are not to be limited to the specific embodiments illustrated and described above. The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.