Methods and apparatus for a permanent magnet machine with an added air barrier

An internal permanent magnet machine (“IPM machine”) of the type used, for example, with traction motors and hybrid electric vehicles, includes a rotor with an additional air barrier provided above the first magnet barrier in the same rotor slot. Each magnet only fills a portion of each cavity, thereby defining the air barriers. The added air barrier above the permanent magnet of the first layer acts as a barrier to the first layer magnet and lowers the magnet flux.

TECHNICAL FIELD

The present invention generally relates to magnetic devices such as electrical motors, and more particularly relates to interior permanent magnet machines.

BACKGROUND

Interior permanent magnet (IPM) machines are favored for fuel cell and hybrid electric vehicle operations due to their desirable characteristics—i.e., good torque density, good overall efficiency, good constant power range, etc. The rotor field in a permanent magnet machine is obtained by virtue of its structure, unlike other machines such as induction, switched or synchronous reluctance machines, in which the field is generated by a stator current supplied by a source. As a result, permanent magnet machines exhibit superior efficiency as compared to other such machines.

However, as with surface PM machines, an IPM machine is burdened by the fact that the permanent magnet field is present even when the machine is not powered, resulting in losses induced by the rotating permanent magnet field of the rotor. Furthermore, the permanent magnet field induces voltage (“back EMF”) into the stator winding. For a strong permanent magnet machine, this back EMF can be quite significant.

In an IPM machine, a second rotor barrier is sometimes added, and a small magnet (used to saturate the bridge above the rotor barrier) is optionally placed therein. This second layer of magnet or air pocket acts as a barrier to the permanent magnet field of the lower primary magnet layer, reducing the air-gap magnet flux, and also lowering the machine back EMF and losses induced by the permanent magnet field. For some machines, due to limited space, the second barrier can not easily be added. Furthermore, addition of the second barrier may weaken the rotor structure or may not cover the entire lower magnet, resulting in some portion of the lower barrier magnet directly exposed to the air-gap, in turn leading to higher losses and higher back EMF.

Accordingly, it is desirable to provide IPM machines that reduce field-related losses while minimizing back EMF and air-gap magnetic flux. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. The invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For the purposes of conciseness, conventional techniques and systems related to electrical motors, magnetism, and the like are not described in detail herein.

In general, the various embodiments are directed to a permanent magnet machine (“PM machine”), and more specifically an internal permanent magnet machine (“IPM machine”) that includes a rotor with an additional air barrier above the first magnet barrier in the same rotor slot. As a result, no second barrier is needed (i.e., to lower the air-gap flux). The added air barrier above the permanent magnet of the first layer acts as a barrier to the first layer magnet and lowers the magnet flux. Hence, the machine back EMF and the magnet induced losses (e.g., iron loss) are reduced. The added air barrier above the magnet also increases rotor saliency to an extent similar to a two-barrier rotor geometry. This partially compensates for the reduction in torque due to the reduction of the permanent magnet field in the air-gap.

Interior PM machines often incorporate one or more rotor barriers (or simply “barriers”). These barriers introduce resistance (reluctance) to magnetic field thus introducing rotor saliency. This saliency is a source of torque and is commonly well-known as reluctance torque. Higher the number of barriers is usually higher is the reluctance torque.FIGS. 1(a)-(c), for example, illustrate partial cross-sections through various exemplary IPM machines100with single and double barrier rotors106. More particular,FIG. 1(a) illustrates a rotor100with magnets110and rotor slots or cavities (the barrier)125incorporated into the structure at various locations with respect to magnets110. As is conventional, IPM100includes a stator101having a plurality of windings102magnetically interacting with magnets110within rotor106. Various cavities are provided within region104of rotor106, and all or a portion of these cavities are filled with permanent magnets in the conventional manner, depending upon the number of layers incorporated into the structure.

FIG. 1(b), more particularly, depicts a two-barrier PM rotor with the second barrier partially filed with magnets110. Similarly,FIG. 1(c) illustrates a two-barrier PM rotor with no magnets in the second layer—i.e., the second layer comprises only an air-filled cavity. The added second barrier shown inFIGS. 1(b) and1(c) adds resistance to the lower magnet barrier, lowering the air-gap magnet flux. However, as mentioned previously, addition of the second barrier in the rotor can mechanically weaken the rotor. Also, for some machines, addition of any such second barrier is not geometrically feasible due to, for example, limited space (e.g., the rotor ofFIG. 1(a)).

Rotors with more than two barriers may also be provided; however, such designs undesirably increase manufacturing complexity. Increasing the number of barriers improves rotor saliency, and thus improves machine torque. Moreover, the second rotor barrier often works as a barrier to the inner magnet layer, consequently lowering the magnet flux in the air-gap. Lowering of magnet flux in the air-gap reduces the magnet torque, but is somewhat compensated by the increased saliency of the rotor as mentioned earlier.

In hybrid applications, when the PM machine is part of a transmission, very often the machine is rotating in conjunction with a different gear-set even though machine is producing no torque or is producing very low torque. If the no-load or light load operation is a substantial portion of the machine drive cycle, the overall efficiency of the transmission is affected. Rotating magnet flux also induces voltage in the stator winding, commonly referred to as back EMF. The high magnet flux of a permanent magnet rotor may induce very high voltage in the stator, especially during high speed operation of the machine. Therefore, lowering of the machine air-gap flux is very desirable for such machines.

FIG. 2depicts an IPM machine200in accordance with one embodiment of the present invention in which an air-barrier is incorporated into the cavity or slot where the magnet is placed, rather than adding an additional barrier. That is, as shown, a pair of magnets110are placed within respective cavities that are configured to be larger than the magnets themselves, thus allowing air barriers to be formed adjacent to the magnets.

In the cross-sectional illustration shown inFIG. 2, the cavity includes the union of the area filled by magnet110and the area defined by the various air pockets adjacent thereto, i.e.: air pockets226,225, and227. The term “cavity” is thus used to refer to a region thus defined prior to insertion of magnet110. WhileFIG. 2illustrates a cross-sectional view of magnets110and air pockets225,226, and227, it will be understood that the cavity extends into region104of rotor106and will define a three-dimensional volume having any suitable shape.

The size, location, and geometry of each air pocket225,226, and227may be selected to achieve the desired design objectives. In the illustrated embodiment, for example, air barriers225are configured adjacent to the “top” of magnets110(i.e., toward the outer surface of the rotor101, radially). These top air barriers225are generally triangular (or trianguloid) and extend substantially the entire length of each magnet110. In this embodiment, pairs of rectangular magnets are configured angled toward each other—i.e., defining an obtuse angle facing outward toward stator101, and the widest portion of air pockets225(corresponding to each apex) is adjacent to that corner of magnet110that is closest to stator101.

In this embodiment, additional “bottom” air barriers227are defined on the opposite side of magnet110from air gaps225, and have a cross-sectional area that is substantially smaller than that of air gaps225. In the illustrated embodiment, bottom air gaps227are also generally triangular and are adjacent to a corner of magnet110.

Further illustrated inFIG. 2are “side” air gaps226which, in this embodiment, are provided adjacent to an edge of magnet110as shown. In one embodiment, side air gaps226extend the full height (radially) of magnets110.

As illustrated, additional conventional air pocket (rotor slot)125(i.e., air pockets that are non-contiguous with the cavities defined above, may also be provided within rotor106.

The structures described above are advantageous in a number of respects. In particular, the added air barrier225above the permanent magnet110acts as a barrier to the first layer magnet and lowers the magnet flux, thereby also reducing machine back EMF and magnet induced losses. At the same time, rotor saliency is increased due to the reduction of the d-axis (magnet axis) inductance.

FIG. 3, for example, is an empirical graph showing the no-load iron loss (spin loss) for the rotor ofFIG. 2compared to that for the rotor ofFIG. 1(a) (no air-barrier). As can be seen, iron loss is greatly reduced. The addition of the air barrier also lowers machine back EMF.FIG. 4depicts the back EMF of the above two rotors. As can be seen, machine back EMF is significantly reduced. The reduction in machine torque due to addition of the air-barrier (lower magnet flux) is minimal as some of the torque loss is compensated by the increased saliency of the rotor (higher reluctance torque).

While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. For example, additional barrier layers may be incorporated in addition to the single layer illustrated. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention and the legal equivalents thereof.