Surface groove patterns for permanent magnet machine rotors

A permanent magnet electric machine includes a rotor configured to rotate about an axis. The rotor is comprised of axially stacked sections. Each section is comprised of axially stacked laminations that define a pattern of axial grooves on an outer surface of the rotor to reduce torque ripple. The pattern is different for at least two of the sections. The pattern for each section may alternate with respect to poles of the rotor.

TECHNICAL FIELD

This application generally relates to surface groove patterns for permanent magnet machine rotors.

BACKGROUND

Hybrid-electric and electric vehicles utilize one or more electric machines to provide propulsion for the vehicle. A variety of electric machine technologies are available for such applications. Permanent magnet machines are a typical choice for vehicle applications. The permanent magnet machine includes a stator and a rotor. The rotor is constructed with permanent magnets. Coils in the stator are energized to create an electromagnetic flux that interacts with electromagnetic flux created by the permanent magnets of the rotor. The interaction of the fluxes causes the rotor to rotate. Due to various motor design characteristics, the interacting electromagnetic fluxes create a torque that is comprised of harmonic components. The torque may be described as a summation of components having different frequencies. This is observed as a ripple or oscillation in the torque. The torque ripple or torque oscillation causes vibration and noise.

SUMMARY

A permanent magnet machine includes a rotor configured to rotate about an axis and comprising a plurality of sections arranged along the axis, each of the sections comprising a plurality of axially stacked laminations that define a pattern of axial grooves on a circumferential surface of each of the sections such that the pattern is different for at least two of the sections.

For at least one of the sections, the pattern may repeat on the circumferential surface at an arc length corresponding to one pole of the rotor. For at least one of the sections, the pattern may repeat on the circumferential surface at an arc length corresponding to two poles of the rotor. For at least one of the sections, the pattern may repeat on the circumferential surface at an arc length corresponding to three poles of the rotor. The sections may be offset at a predetermined angle from adjacent sections such that pole locations defined by each section are offset from corresponding pole locations of adjacent sections. For each of the sections, a subset of the axial grooves may be aligned such that the subset of the axial grooves extends across an axial length of the rotor. The subset may include at least one of the axial grooves within each arc length corresponding to poles of the rotor. The pattern may alternate between adjacent sections. The rotor may further comprise a smooth section having no axial grooves.

A permanent magnet machine includes a rotor comprising a plurality of sections arranged along an axis of rotation, each of the sections comprising a plurality of axially stacked laminations that define a pattern of axial grooves on an outer surface of each of the sections such that, for an arc length of the outer surface corresponding to a pole of the rotor, the pattern is different for at least two of the sections.

The pole may be one of a plurality of poles of the rotor and, for at least one of the sections, the pattern may repeat for each of the poles. The pole may be one of a plurality of poles of the rotor and, for at least one of the sections, the pattern may alternate between poles that are adjacent. For each of the sections, a subset of axial grooves may be aligned such that the subset of axial grooves extends across an axial length of the rotor. The pole may be one of a plurality of poles of the rotor, and the poles of each of the sections may be offset by a predetermined angle from the poles of adjacent sections.

A permanent magnet machine includes a rotor comprising a plurality of poles arranged about an axis, each of the poles corresponding to a predetermined arc length of a circumferential surface of the rotor formed by a plurality of axially stacked laminations that define a pattern of axial grooves on the circumferential surface of the rotor such that, for each of the poles, the pattern is different for at least two of the poles.

The pattern of axial grooves for each of the poles may include at least one axial groove. The pattern may be different for each of the poles. The rotor may further comprise one pole without axial grooves. The pattern of axial grooves for each of the poles may include two axial grooves. The pattern of axial grooves for each of the poles may be defined by an angle between the axial grooves of each of the poles and the angle may be different for each of the poles.

DETAILED DESCRIPTION

FIG. 1depicts a typical plug-in hybrid-electric vehicle (PHEV). A PHEV12may comprise one or more electric machines14mechanically coupled to a hybrid transmission16. The electric machines14may be capable of operating as a motor or a generator. In addition, the hybrid transmission16is mechanically coupled to an engine18. The hybrid transmission16is also mechanically coupled to a drive shaft20that is mechanically coupled to the wheels22. The electric machines14can provide propulsion and deceleration capability when the engine18is turned on or off. The electric machines14also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in a friction braking system. The electric machines14may also reduce vehicle emissions by allowing the engine18to operate at more efficient speeds and allowing the hybrid-electric vehicle12to be operated in electric mode with the engine18off under certain conditions.

A traction battery or battery pack24stores energy that can be used by the electric machines14. A vehicle battery pack24typically provides a high-voltage direct current (DC) output. One or more contactors42may isolate the traction battery24from a high-voltage bus when opened and couple the traction battery24to the high-voltage bus when closed. The traction battery24is electrically coupled to one or more power electronics modules26via the high-voltage bus. The power electronics module26is also electrically coupled to the electric machines14and provides the ability to bi-directionally transfer energy between high-voltage bus and the electric machines14. For example, a traction battery24may provide a DC voltage while the electric machines14may operate with a three-phase alternating current (AC) to function. The power electronics module26may convert the DC voltage to a three-phase AC current to operate the electric machines14. In a regenerative mode, the power electronics module26may convert the three-phase AC current from the electric machines14acting as generators to the DC voltage compatible with the traction battery24. The description herein is equally applicable to a pure electric vehicle. For a pure electric vehicle, the hybrid transmission16may be a gear box connected to an electric machine14and the engine18may not be present.

In addition to providing energy for propulsion, the traction battery24may provide energy for other vehicle electrical systems. A vehicle12may include a DC/DC converter module28that is electrically coupled to the high-voltage bus. The DC/DC converter module28may be electrically coupled to a low-voltage bus56. The DC/DC converter module28may convert the high voltage DC output of the traction battery24to a low voltage DC supply that is compatible with low-voltage vehicle loads52. The low-voltage bus56may be electrically coupled to an auxiliary battery30(e.g., 12V battery). The low-voltage systems52may be electrically coupled to the low-voltage bus56.

The vehicle12may be an electric vehicle or a plug-in hybrid vehicle in which the traction battery24may be recharged by an external power source36. The external power source36may be a connection to an electrical outlet. The external power source36may be electrically coupled to a charger or electric vehicle supply equipment (EVSE)38. The external power source36may be an electrical power distribution network or grid as provided by an electric utility company. The EVSE38may provide circuitry and controls to regulate and manage the transfer of energy between the power source36and the vehicle12. The external power source36may provide DC or AC electric power to the EVSE38. The EVSE38may have a charge connector40for plugging into a charge port34of the vehicle12. The charge port34may be any type of port configured to transfer power from the EVSE38to the vehicle12. The charge port34may be electrically coupled to a charger or on-board power conversion module32. The power conversion module32may condition the power supplied from the EVSE38to provide the proper voltage and current levels to the traction battery24. The power conversion module32may interface with the EVSE38to coordinate the delivery of power to the vehicle12. The EVSE connector40may have pins that mate with corresponding recesses of the charge port34. Alternatively, various components described as being electrically coupled or connected may transfer power using a wireless inductive coupling.

One or more wheel brakes44may be provided for decelerating the vehicle12and preventing motion of the vehicle12. The wheel brakes44may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes44may be a part of a brake system50. The brake system50may include other components to operate the wheel brakes44. For simplicity, the figure depicts a single connection between the brake system50and one of the wheel brakes44. A connection between the brake system50and the other wheel brakes44is implied. The brake system50may include a controller to monitor and coordinate the brake system50. The brake system50may monitor the brake components and control the wheel brakes44for vehicle deceleration. The brake system50may respond to driver commands via a brake pedal and may also operate autonomously to implement features such as stability control. The controller of the brake system50may implement a method of applying a requested brake force when requested by another controller or sub-function.

One or more electrical loads46may be coupled to the high-voltage bus. The electrical loads46may have an associated controller that operates and controls the electrical loads46when appropriate. The high-voltage loads46may include compressors and electric heaters.

Electronic modules in the vehicle12may communicate via one or more vehicle networks. The vehicle network may include a plurality of channels for communication. One channel of the vehicle network may be a serial bus such as a Controller Area Network (CAN). One of the channels of the vehicle network may include an Ethernet network defined by Institute of Electrical and Electronics Engineers (IEEE) 802 family of standards. Additional channels of the vehicle network may include discrete connections between modules and may include power signals from the auxiliary battery30. Different signals may be transferred over different channels of the vehicle network. For example, video signals may be transferred over a high-speed channel (e.g., Ethernet) while control signals may be transferred over CAN or discrete signals. The vehicle network may include any hardware and software components that aid in transferring signals and data between modules. The vehicle network is not shown inFIG. 1but it may be implied that the vehicle network may connect to any electronic module that is present in the vehicle12. A vehicle system controller (VSC)48may be present to coordinate the operation of the various components.

The electric machines14may be Interior Permanent Magnet (IPM) machines that include a stator122and a rotor120.FIG. 2Adepicts an example rotor lamination138andFIG. 2Bdepicts a side view of a stator122and rotor120configuration having multiple rotor laminations138and multiple stator laminations136arranged in an axially stacked relationship. The rotor laminations138may define a circular central opening160for accommodating a drive shaft with a keyway that may receive a drive key162. The rotor laminations138may define a plurality of magnet openings142that are symmetrically disposed with respect to adjacent pairs of magnet openings142.

A plurality of rotor sectors124corresponding to poles of the rotor may be defined by a plurality of inter-polar axes (e.g.,180,184) emanating from a central axis170of rotation to an outer surface150of the rotor lamination138. Each of the sectors124may include a pair of magnet openings142. The inter-polar axes (e.g.,180,184) may be positioned to be midway between adjacent magnet openings142. Note thatFIG. 2Aonly shows two of the possible inter-polar axes180,184and does not show all possible inter-polar axes.FIG. 2Bdepicts a series of axially stacked rotor laminations138that are stacked along the central axis170about which the rotor120is configured to rotate.

FIG. 3depicts a partial radial cross-sectional view of a possible construction of the rotor120and the stator122. A partial stator lamination136and a partial rotor lamination138are depicted inFIG. 3. The rotor laminations138and the stator laminations136may be comprised of a ferrous alloy. A small air gap140is located between the inner periphery of the stator laminations136and the outer periphery150of the rotor laminations138. The stator laminations136may define radially extending openings134.

The rotor laminations138may define symmetrically positioned magnet openings142near the outer periphery150of each rotor lamination138. Each magnet opening142may be configured to receive a magnet144. Any number of laminations in a given design may be used, depending on design choice. The rotor laminations138and the stator laminations136may be arranged in a stack along the axis170of rotation. The axially stacked rotor laminations138and the magnets144may define a plurality of magnetic poles distributed about the axis170.

The stator136may have conductors disposed in the radially extending openings134to form windings. The stator122may be comprised of an iron core made of a stack of stator laminations136and a winding arrangement for conductors that carry an excitation current. Current flowing through the stator winding generates a stator electromagnetic flux. The stator flux may be controlled by adjusting the magnitude and frequency of the current flowing through the stator windings. Because the stator windings are contained in openings134rather than a uniform sinusoidal distribution along the inner circumference of the stator, there may be harmonic fluxes in the stator flux.

The rotor120may be comprised of an iron core made of a stack of rotor laminations138and sets of permanent magnets144inserted within holes or cavities142that are defined by the iron core. The permanent magnets144in the rotor120may generate a rotor electromagnetic flux. The rotor flux may include harmonic fluxes due to shapes and sizes of the discrete permanent magnets. The stator flux and the rotor flux may be distributed in the air-gap140. Interaction between the stator flux and the rotor flux causes the rotor120to rotate about the axis170.

Poles of the rotor120may be geometrically defined to correspond to the sectors124defined by the rotor laminations138. Each of the poles may be represented by a sector124. A pole location may be generally defined by a center-pole axis182that extends radially from the axis170toward the outer surface150of the rotor138along a midpoint between adjacent magnet openings142. The inter-polar axes (e.g,180,184) may extend radially from the axis170toward the outer surface150of the rotor138between adjacent poles. An angular distance between two adjacent poles may define a pole pitch parameter. The arc length on the circumferential rotor surface150between two adjacent poles of the rotor may be referred to as the pole pitch. The pole pitch may be measured circumferentially around the outer rotor surface150between adjacent center-pole axes182. Each pole may have an associated surface area on the outer circumferential surface150of the rotor120. Each pole may be represented by the arc length on the surface between adjacent inter-polar axes180,184.

An electromagnetic field or signal may be composed of a summation of harmonic components having different frequencies and magnitudes. Each harmonic component may be represented as a frequency and a magnitude. The signal may include a fundamental component. The fundamental component may be the frequency component having the greatest magnitude.

During operation, the stator and rotor fundamental component fluxes may align and rotate in the same direction at the same frequency. The interaction between the fundamental component of the stator flux and the rotor flux generates a torque. The stator and rotor harmonic fluxes may have different pole numbers, rotation speeds and directions. As a result, the interaction between the harmonic fluxes generates torque fluctuations, referred to as torque ripple. The torque ripple may have harmonic components having different frequencies. The order of a torque ripple component may be defined as the ratio of the frequency of the torque ripple component to the speed of the rotor in revolutions per second.

One effect of the torque ripple is that it may cause speed oscillations of the rotor. Further, the torque ripple may impact noise and vibration of the motor and components coupled to the electric machine. Higher order torque ripple frequencies may be filtered out by the limited bandwidth of the coupled mechanical system. Lower harmonic frequencies of the torque ripple may give rise to mechanical oscillations in the coupled system. It is desirable to reduce the torque ripple in order to reduce vibration and noise in systems that incorporate electric machines.

A typical outer circumferential surface150of the rotor120is rounded or smooth. In some applications, the outer surface150of the rotor laminations138may define a pattern of axial grooves. The grooves may be channels that are oriented parallel to the axis170. The grooves may span an axial length of the outer surface150of the rotor138. The effect of the grooves is to reduce the magnitude of a selected harmonic component of the torque while not affecting other harmonic components. In many vehicle applications, it may be desirable to reduce the magnitude of several harmonic components. The grooves may be of a rounded shape having a predetermined depth from the outer surface150. In other configurations, the grooves may have alternate shapes such as rectangular or trapezoidal. The shape of the grooves may be configured to minimize a particular harmonic component.

Laminations that are bonded together that define the same pattern of grooves on the rotor surface150may be referred to as a section. In some rotor configurations, the rotor120may be constructed of a single section. A subset of one or more axial grooves may correspond to the poles of the rotor120. In some configurations, the axial grooves associated with each pole may be the same pattern. For example, an axial groove may be located at a midpoint of each pole. As another example, axial grooves may be defined at a predetermined circumferential distance on either side of the midpoint of the pole. Each rotor lamination138may be configured to define the same groove pattern for each of the poles. The groove pattern defined for the poles may repeat as the outer circumferential surface150is followed around the axis170.

In some configurations, the rotor may be comprised of more than one section.FIG. 4Adepicts one pole of a two-section rotor212. In the two-section rotor212, a first section200may be comprised of a plurality of first rotor laminations204having a first outer circumference groove pattern208as shown inFIG. 4B. A second section202may be comprised of a plurality of second rotor laminations206having a second outer circumference groove pattern210as shown inFIG. 4C. The first section200and the second section202may be bonded together to comprise the rotor212having two sections. The first outer circumference groove pattern208may define one or more grooves at a first set of predetermined locations on the outer circumferential surface of the first rotor lamination204relative to a midpoint214of each of the poles. The second outer circumference groove pattern210may define one or more grooves on the outer circumferential surface of the second rotor lamination206at a second set of predetermined locations relative to the midpoint214of each of the poles. The first and second set of predetermined locations may be different such that when the first section200and the second section202are bonded together that the grooves do not go across the entire axial length of the two-segment rotor212.

In some configurations, the first groove pattern208may repeat for each of the poles. In some configurations, the first groove pattern208may repeat every two poles or three poles. In some configurations, the first groove pattern208may be different for each of the poles. Similar configurations are possible for the second groove pattern210. In some configurations, a subset of the axial grooves may be defined across the entire axial length of the outer circumferential surface of the rotor212. The first set of predetermined locations and the second set of predetermined locations may include a subset of axial grooves at a same location relative to the midpoint214of each pole.

An advantage of the multiple-section rotor configuration is that the magnitude of multiple harmonic components may be reduced. The groove pattern of each section may be configured to reduce a particular harmonic frequency component. For example, the first section200may be configured to reduce the magnitude of a first harmonic frequency component and the second section202may be configured to reduce the magnitude of a second harmonic frequency component. By combining segments with different patterns of grooves, torque ripple created by multiple harmonic frequencies may be reduced.

FIG. 5depicts one pole of an alternative two-section rotor300configuration. A first section302may be comprised of rotor laminations having a smooth outer circumferential surface. That is, the first outer circumference groove pattern does not define any grooves on the surface of the first section302. The second section304may be comprised of rotor laminations that define a single groove306per pole. In some configurations, the single groove may306be at the same position relative to the midpoint214of each of the poles. In some configurations, the position of the single groove306relative to the midpoint214of each of the poles may be different for two or more poles.

FIG. 6depicts one pole of an alternative two-section rotor configuration350. The first section352may be comprised of rotor laminations that define two grooves356,358per pole. The second section354may be comprised of rotor laminations that define three grooves360,362,364per pole. For each pole, the grooves may be located at the same position relative to the midpoint214of the pole. The first section352and the second section354may be configured such that no grooves traverse the entire axial length of the outer circumferential surface.

In some configurations, an axial length of each section of the rotor may be equal. In some configurations, the axial length of the sections may be different. The axial stack length may be varied by the number of laminations used for each section. The axial length of each section may impact the effectiveness at reducing a particular harmonic component. The axial length of each section may be tuned to achieve the desired harmonic component reduction.

In some configurations, more than two sections may be utilized.FIG. 7depicts a four-section rotor configuration400. In this configuration, a first rotor lamination and a second rotor lamination may be defined. Sections may be formed from the first rotor lamination and the second rotor laminations and arranged such that the rotor has sections with alternating groove patterns. For example, the four-section rotor400may comprise a first section402, a second section404, a third section406, and a fourth section408. The first section402and the third section406may be comprised of the first rotor lamination. The second section404and the fourth rotor section408may be comprised of the second rotor lamination. This configuration defines a circumferential rotor surface in which the axial groove patterns alternate such that adjacent sections have a different groove pattern. In other configurations, four different rotor laminations may be defined such that each section has a different groove pattern.

In some configurations, the rotor laminations may define a subset of grooves that extends for the entire axial length of the rotor surface. In some configurations, the rotor laminations may define a subset of grooves that extend for more than one consecutive section but not across the entire axial length of the rotor. In some configurations, there may be no grooves that extend the entire axial length of the rotor surface.

FIG. 8depicts a five-section rotor configuration450that includes five sections in which the sections are not all of the same axial length. For example, the rotor450may comprise a first section452, a second section454, a third section456, a fourth section458, and a fifth section460. In some configurations, the first section452and the fifth section460may be half the length of the sections between. The sections may be assembled such that the groove pattern for adjacent sections is different while the groove pattern for every second section is the same. The sections located at distal ends of the rotor axis may be of reduced axial length and may have the same groove pattern.

Another technique to adjust the torque ripple may be by skewing the rotor. A skewed rotor may be described as a rotor having at least two sections in which the magnet openings are offset from one another. The skewed rotor may be combined with the various groove patterns to further reduce torque ripple.

FIG. 9Adepicts a rotor500comprised of two sections that are not skewed relative to one another. The rotor500may be comprised of a first section506and a second section508. The two sections are arranged such that a first section pole midpoint504is aligned with a second section pole midpoint510.FIG. 9Bdepicts a skewed rotor configuration502. In the skewed rotor configuration502, the first section506is rotated relative to the second section508such that the first section pole midpoint504is at an angle relative to the second section pole midpoint510. The skewing of the rotor sections may also apply to rotor configurations that include more than two sections. The rotor sections may be aligned such that the pole midpoints for each section are rotated relative to the others. The skewing described is relative to the pole midpoint but the skewing may be described relative to a different reference point on the sections. The pole locations as defined by each of the sections may be rotated or offset by a predetermined angle from one another.

In some configurations, the rotor may be comprised of a single section. However, there may be at least two poles that have a different groove pattern.FIG. 10depicts a single section rotor550comprised of laminations that define a different groove pattern for adjacent poles. For example, a first pole552may have a first associated groove pattern556and a second pole554may have a second associated groove pattern558. The second groove pattern558may define grooves at different positions relative to a second pole midpoint562than the first groove pattern556defines relative to the first pole midpoint560. In this configuration, the first groove pattern556and the second groove pattern558may repeat every other pole. That is, the groove pattern for the rotor may repeat on the circumferential surface at an arc length corresponding to two poles of the rotor.

The axial groove patterns may be defined such that adjacent poles have different groove patterns. The groove patterns may alternate between poles about the axis. In some configurations, the groove pattern may differ for three consecutive poles. That is, none of three consecutive poles may exhibit the same groove pattern. The groove pattern may repeat for each set of three poles. In some configurations, the axial groove pattern for the rotor may repeat on the circumferential surface at an arc length corresponding to one pole of the rotor.

FIG. 11depicts a rotor600comprised of a single section. The single section602is comprised of rotor laminations that define a different axial groove pattern for three consecutive poles604,606,608. The first pole604may be associated with a first groove pattern, the second pole606may be associated with a second groove pattern, and the third pole608may be associated with a third groove pattern. The pattern defined by the three poles604,606,608may repeat such that the groove pattern is repeated around the circumference of the rotor600. In this configuration, the next pole (not shown) adjacent to the third pole608may have the same groove pattern as the first pole604. The axial groove pattern for the rotor may repeat on the circumferential surface at an arc length corresponding to three poles of the rotor.

The configurations described herein may be combined. A rotor may be comprised of multiple sections that define a different surface groove pattern. Each section may define a different surface groove pattern for each of the poles. The groove pattern defined by the sections may repeat over a number of poles.

FIG. 12depicts a two-section rotor configuration650in which each of the sections define a different groove pattern for adjacent poles. A first section660may include a first groove pattern for a first pole652and a second groove pattern for a second pole654. A second section662may define a third groove pattern for the first pole652and a fourth groove pattern for the second pole654. In some configurations, the same rotor lamination may be used for each of the sections. However, the rotor laminations may be shifted by one pole for each section so that each pole has a different groove pattern across the axial length of the rotor.

FIG. 13depicts a three-section rotor700in which each section defines a groove pattern for every third pole. Depicted is a first pole702, a second pole704and a third pole706. Also depicted are a first section708, a second section710, and a third section712. The first section708may be comprised of rotor laminations that define a first groove pattern for the first pole702while the surface is smooth for the second pole704and the third pole706. The second section710may be comprised of rotor laminations that define a second groove pattern for the second pole704while the surface is smooth for the first pole702and the third pole706. The third section712may be comprised of rotor laminations that define a third groove pattern for the third pole706while the surface is smooth for the second pole704and the third pole706. The groove pattern for each of the sections may repeat every third pole.

FIG. 14depicts a single-section rotor lamination800for a rotor that may comprise a single section in which the groove pattern for each pole is different. Depicted is an eight-pole rotor in which each pole has a different axial groove pattern. The number of axial grooves defined for the rotor poles802-816may not be the same for each of the poles. For example, a first rotor pole802, may define four axial grooves on the circumferential surface. A third rotor pole806and a seventh rotor pole814may define three axial grooves on the circumferential surface. A fifth rotor pole810and a sixth rotor pole812may define two axial grooves on the circumferential surface. A second rotor pole804and an eighth rotor pole816may define one axial groove on the circumferential surface. A fourth rotor pole808may define a smooth circumferential surface without any axial grooves. The axial groove pattern may be different based on the number of axial grooves defined and the arrangement of the axial grooves relative to the midpoint of the pole.

In some configurations, each pole of the rotor may include the same number of axial grooves.FIG. 15depicts a single section rotor lamination900in which each of the poles defines two axial grooves. The pattern of the two axial grooves for each of the poles may be defined by an angle between each of the axial grooves. In some configurations, the axial grooves may be equidistant from the midpoint of the pole. For example, a first rotor pole906may define axial grooves that are separated by an angle α1918. A second rotor pole908may define axial grooves that are separated by an angle α2920. A third rotor pole910may define axial grooves that are separated by an angle α3922. A fourth rotor pole912may define axial grooves that are separated by an angle α4924. A fifth rotor pole914may define axial grooves that are separated by an angle α5926. A sixth rotor pole916may define axial grooves that are separated by an angle α6928. A seventh rotor pole902may define axial grooves that are separated by an angle α7930. An eighth rotor pole904may define axial grooves that are separated by an angle α8932. In some configurations, the angles918-932may be different for each of the poles. In some configurations, the angles918-932may alternate values.

In some configurations, the rotor may comprise multiple sections with an alternative groove pattern over a predetermined number of poles. For example, the first section may define a first groove pattern that repeats every second pole. The second section may define a second groove pattern that repeats every second pole. The groove pattern for the first and second sections may be different for each pole.

The placement of the axial grooves within each pole and the number of sections may be determined to reduce selected harmonics. The figures herein depict the axial grooves but it is expected that the number of grooves and positioning of the axial grooves may be varied based on a particular motor design. The number of sections that are used may also vary based on the particular motor design.