Utilization of Magnetic Fields in Electric Machines

An electric machine may include a plurality of stator sections each formed from one or more stator laminations stacked to form a stator. The stator may have windings arranged therein to form magnetic poles. The stator may surround a rotor. A diamagnetic or paramagnetic stator layer may be interposed between at least one adjacent pair of the stator sections.

TECHNICAL FIELD

The present disclosure relates to magnetic field utilization for the stator of an electric machine.

BACKGROUND

Electric machines typically employ a rotor and stator to produce torque. Electric current flows through the stator windings to produce a magnetic field. The magnetic field generated by the stator may cooperate with permanent magnets on the rotor to generate torque.

SUMMARY

The rotor of an electric machine may be formed from a plurality of stacked rotor sections each formed from one or more rotor laminations. The sections may have skewed magnetic poles. A diamagnetic or paramagnetic rotor layer may be interposed between each adjacent pair of the sections that has skewed magnetic poles.

An electric machine stator may include a plurality of sections each formed from one or more stator laminations stacked to form a stator having windings arranged therein to form magnetic poles and surrounding a rotor. A layer may be interposed between an adjacent pair of the stator sections such that magnetic fields associated with the magnetic poles are aligned axially with corresponding magnetic fields from the rotor. The layer may be diamagnetic or paramagnetic.

The layer interposed between an adjacent pair of the stator sections and one of the rotor layers may be coplanar. The thickness of the layer interposed between an adjacent pair of the stator sections and one of the rotor layers may be same. The layer may be polytetrafluoroethylene. The thickness of the layer may be at least twice an airgap distance between the stator and rotor. The thickness may be less than four times the airgap distance.

DETAILED DESCRIPTION

Electric machines are characterized by an undesirable oscillation in the torque which is caused by harmonics present in the airgap flux and in the airgap permeance. Most electric machines, and in particular Permanent Magnet (PM) electric machines, are designed with rotor skew i.e. the laminations of active rotor material may be skewed, or staggered, along the axis of the rotor. Skewing may result in staggered permanent magnets and magnetic poles along the axis of the rotor. Skewed sections may cause an overall reduction in the average torque of the machine at all available speeds because the magnetic components are out of alignment, but skewing helps to minimize the harmonics, as discussed above.

For example, in the case of an 8-pole machine with two rotor sections,48-slot stator, a typical skew angle is 3.75°. The skewing of the rotor is intended to produce a smoother mechanical torque than would otherwise be achieved using a rotor having aligned permanent magnets. Skewing may eliminate undesirable torque ripple caused by harmonics and many different skew angles may be used to achieve this result. Skew, however, does not contemplate two poles that are supposed to be aligned by design but because of manufacturing tolerances are not exactly aligned.

The average torque generated across all speeds of the electric machine may be reduced by skewing, in part, because magnetic field leakage may occur between skewed permanent magnets. This leakage may cause a small reduction in the available torque of the machine, and the leakage may not exist on non-skewed machines.

In addition, skewing may open a path for magnetic flux to leak from one lamination section to the adjacent one, without adding torque. Because magnetic fields generally follow the path of least resistance between opposite poles, the skewing and staggering of permanent magnets to reduce torque ripple may, consequently, cause additional magnetic flux leakage to occur. A section of the rotor may be comprised of one lamination or a plurality of laminations stacked together. The laminations of a section may be skewed relative to other laminations in the section or skewed collectively, relative to other sections of the rotor. This means a section of the rotor may be comprised of any number of laminations stacked together or a single block of composite material.

In order to maximize the magnetic field and resulting torque, the amount of active rotor material is typically maximized. Active rotor material may include a material capable of generating or carrying a magnetic or electric field. Maximization of this material, in theory, generates the most torque. Rotor and stator materials with the highest magnetic permeability are chosen. An introduction of materials without high magnetic permeability would presumably decrease the torque generation of the electric machine because the rotor would have wasted space (i.e., material that does not generate torque). Materials with high magnetic permeability may be generally referred to as ferromagnetic or ferrimagnetic. Presumably, a rotor composed of entirely active rotor material would create a more effective magnetic field than a rotor composed of partially active rotor material.

The introduction of a magnetically reluctant rotor layer or layers that is not active rotor material unexpectedly increases the utilization of permanent magnets in the rotor and increases the torque output of the electric machine. For example, the introduction of a reluctant layer with a thickness twice that of the airgap thickness between the stator and rotor may provide a specific torque increase greater than 0.25%. This amount, while seemingly nominal, can justifiably decrease the cost of electric machines because the improved utilization of permanent magnets may allow the size of the permanent magnets to be reduced. The increase in specific torque of the electric machine may depend on the thickness of the layer relative to the airgap and the electric current flowing through the stator.

A reluctant layer with low magnetic permeability may be inserted between adjacent sections having skewed magnetic poles. The layer may have a solid, liquid, or gas phase. The layer may redirect the magnetic field of the permanent magnets to a more desirable course and reduce leakage between permanent magnets. The layer may be a diamagnetic or paramagnetic material (e.g., water, copper, bismuth, superconductors, wood, air, polytetrafluoroethylene, or vacuum). Many different types of matter are capable of obtaining similar results and may fall into these designations. Materials with low magnetic permeability may be able to reduce the field leakage between sections with skewed poles or redirect the field into a more desirable course. Properly directed magnetic flux paths may increase the generated torque of the machine.

Permanent magnets may have multiple orientations when disposed on or within the sections. For example, permanent magnets may be arranged in a V-shape position providing poles at each V. Permanent magnets may also be oriented such that one of the magnetic poles is directed radially outward. The orientation and position of the magnets may have a direct effect on the electric machine's efficiency, and any skewed orientation or position may cause magnetic field leakage between the permanent magnets.

The poles of the permanent magnets may individually or cooperatively form magnetic poles of the rotor. Many rotors have a plurality of permanent magnets arranged to cooperate with the stator' s magnetic field in order to generate torque. The poles may be generated using permanent magnets, induced fields, excited coils, or a combination thereof.

Laminations are generally made of materials with high magnetic permeability. This high magnetic permeability allows magnetic flux to flow through the laminations without losing strength. Materials with high magnetic permeability may include iron, electrical steel, ferrite, or many other alloys. Rotors with laminations may also support an electrically conductive cage or winding to create an induced magnetic field. A rotor having four laminations or sections of laminations may have the sections configured in an ABBA orientation. The ABBA orientation means that the “A” sections are skewed to the same degree relative to the “B” sections. The rotor may have other lamination configurations (e.g., ABC or ABAB). In an ABBA configuration, the “A” sections may be referred to as outer sections. The “B” sections may be referred to as inner sections. The “A” sections may be skewed at the same degree and have aligned poles. The “B” sections may be skewed at the same degree and have aligned poles.

Introduction of a magnetically reluctant layer on the rotor reduces magnetic leakage between the skewed magnetic poles of the rotor. The rotor layer may, however, result in the corresponding stator material being underutilized. The amount of active stator material is also typically maximized to increase flux generated from the stator windings. With the introduction of a rotor layer, the underutilized stator material unnecessarily increases the weight of the electric machine. A stator layer may be introduced to match the separator layers of the rotor to ensure alignment between the active material of the stator and the active material of the rotor. Meaning, the rotor sections may be axially aligned and coplanar with corresponding stator sections. The layers of both the rotor and stator may increase the overall volume or displacement of the electric machine but reduce its weight by removing heavy underutilized magnetic material. The stator layer may be made of a material similar to the rotor layer. The stator layer may also have similar material properties as the rotor layer.

Referring now toFIG. 1A, a rotor section10for a rotor is shown. The rotor section10may define a plurality of pockets or cavities12adapted to hold permanent magnets. The center of the rotor section10may define a circular central opening14for accommodating a driveshaft with a keyway16that may receive a drive key (not shown). The cavities may be oriented such that the permanent magnets (not shown) housed in the pockets or cavities12form eight alternating magnetic poles30,32. It is well known in the art that an electric machine may have various numbers of poles. The magnetic poles30may be configured to be north poles. The magnetic poles32may be configured to be south poles. The permanent magnets may also be arranged with different patterns. As shown inFIG. 1A, the pockets or cavities12, which hold permanent magnets, are arranged with a V-shape34. Referring now toFIG. 1B, a plurality of rotor sections10may form a rotor8. The rotor has a circular central opening14for accommodating a driveshaft (not shown).

Referring now toFIG. 2A, a portion of the rotor section10is shown within a stator40. The rotor section10defines pockets or cavities12adapted to hold permanent magnets20. The permanent magnets20are arranged in a V-shape, collectively forming poles. Flux lines24emanating from the permanent magnets20are shown. The flux lines24may permeate through the rotor section10and across the airgap22into the stator40. In general, magnetic flux has greater field density when the flux lines24are closer together. Redirection of the flux lines24may cause an increased magnetic field density in certain locations as shown inFIG. 2A. The stator40has windings42that are not energized.

Referring toFIG. 2B, a portion of the rotor section10is shown within the stator40. The stator40may have windings42that are energized. Flux lines44may emanate from the windings42. The flux lines44may permeate through the stator40and across the airgap22into the rotor section10. A three-phase motor may have windings A, B, and C. The flux lines44and flux lines24may at least partially interact at position46in known fashion to produce torque.

Referring toFIG. 3A, a skewed, adjacent pair of rotor sections10,80may have cavities12,84adapted to hold permanent magnets20,82. The permanent magnets20,82may be magnetized such that the north poles26face a radially outward direction with respect to the rotor. The permanent magnets20,82may be magnetized such that the south pole28faces a generally inward direction. The permanent magnets20,82may be arranged to form magnetic poles30,88. The magnetic poles30,88may be skewed or staggered. A rotor layer86having low magnetic permeability may be disposed between the rotor sections10,80. The rotor layer's outer diameter may fit flush with the outer diameter of the rotor sections10,80or the rotor layer's outer diameter may stop short of the outer diameter of the rotor sections10,80. As shown inFIG. 3B, the permanent magnets20may be offset from the permanent magnets82to form a skewed rotor. A rotor layer86having low magnetic permeability may be placed between the rotor sections10,80.

Referring toFIG. 4, a skewed rotor8may have a plurality of rotor sections10,80. The plurality of rotor sections may be skewed in an ABBA pattern, wherein the letters reference the rotor sections relative skewing and position in the rotor8stack. Rotor layers86may be interposed between the adjacent AB rotor sections.

Referring now toFIG. 5, a stator section41has a generally annular shape and may be formed by stacking at least one lamination. The laminations may be made of electric steel or other material having low magnetic reluctance. The stator section41may have teeth43that define stator winding cavities45. The stator cavities may house windings (as shown inFIG. 2B). The stator section may define fastening cavities48configured to enable a fastener to join a stack of stator sections to form a stator.

Now referring toFIG. 6, a portion of an electric machine is shown. A stator layer47has a generally annular shape similar to the stator section41(not shown). The layer may be made of a material having high magnetic reluctance. The stator layer47may include fastening cavities49configured to enable the fastener to include the stator layer within the stack of stator sections. The inner diameter or outer diameter of the stator layer47may be dissimilar to the stator section41to further reduce weight or alter the magnetic field generated. The stator layer47may have a thickness similar to the rotor layer86. The stator layer47thickness may vary depending on the desired magnetic field generated. The thickness and type of the stator layer47may have a direct impact on the magnetic field. The stator section41and stator layer47may be stacked to form a stator.

Now referring toFIG. 7, a plurality of stator sections41is stacked to form a stator40. Each stator section41has teeth43and stator winding cavities45to support a set of stator windings. The stator sections may be aligned, as shown. The stator layers47may be interposed between stator sections41to form the stator40.

Now referring toFIG. 8, a plurality of stator sections41are stacked to form a stator40. Each stator section41has aligned teeth43and stator winding cavities45to support a set of stator windings. The stator layers47may be interposed between stator sections41to form the stator40. The stator40may surround a rotor8having a plurality of rotor sections10,80(10not shown) having permanent magnets20,82(20not shown) arranged therein. Some of the sections are not shown. Each of the rotor sections10,80(10not shown) may be axially aligned with a corresponding one of the stator sections41. The rotor layers86may be axially aligned with a corresponding stator layer47.

Now referring toFIG. 9, a rotor8having rotor sections10,80may be stacked in an ABBA fashion. The adjacent rotor sections10,80having skewed magnetic poles may have rotor layers86therein. The rotor8may be surrounded by a stator40. The stator40may include stator sections41and stator layers47. Each of the stator sections41may be axially aligned and paired with a corresponding one of the rotor sections10,80. The stator layers47may only be disposed between stator sections41having corresponding rotor sections10,80having skewed magnetic poles. Meaning, the stator layers47may also have corresponding rotor layers86.