Permanent magnet synchronous motor

A structure of a rotor used in a synchronous motor employing both of permanent magnets and a stator using concentrated windings is disclosed. Slits (13) provided in a section of a rotor laminated in a direction of a rotary shaft are shaped like an arc or a bow, and the shape protrudes toward an outside rim of rotor (12). Permanent magnets (14) are inserted into slits (13). This structure produces less magnetic salient poles than a conventional rotor, so that a magnetic flux density can be lowered, and an efficient motor with less loss is obtainable.

TECHNICAL FIELD

The present invention relates to a synchronous motor equipped with both interior magnets and a stator using concentrated windings.

BACKGROUND ART

FIG. 11shows a conventional synchronous motor equipped with interior magnets and a stator using concentrated windings. As shown inFIG. 11, the motor includes a stator1having concentrated windings, a rotor2, slits3provided in the rotor, permanent magnets4located in the slits3. Stator1is formed of a concentrated-winding stator having three phases, four poles and six slots. As shown inFIG. 12, respective teeth are wound with windings independently, and each phase includes two coils 180 degrees apart from each other, i.e., opposite to each other. Slits3shaped like a flat plate are prepared inside rotor2, and permanent magnets having a similar shape to slit3are inserted in slits3respectively. As shown inFIG. 12, a motor using the concentrated windings provides independent windings to respective teeth, so that its coil ends are smaller than those of a distributed-winding stator, where the windings straddle over plural teeth. Thus wire-wound resistance becomes less, and copper loss caused by heat of the windings due to current running through the motor can be reduced. As a result, a highly efficient motor with smaller loss is obtainable.

The concentrated windings as shown inFIG. 11have teeth wound with coils respectively, and the coils of respective phases are adjacent to each other. This structure produces greater inductance. The combination of this stator with a rotor having interior magnets produces magnetic salient poles in the rotor, so that reluctance torque becomes available. However, this structure increases inductance, and a lot of magnetic flux flows into the flux path along axis “q”, and the flux path pulls the magnetic flux into the rotor as shown in FIG.11. Thus a magnetic flux density of the core substantially increases. As a result, iron loss greatly increases although the copper loss decreases, so that efficiency is lowered, which render the advantage of the concentrated windings insignificant. The present invention addresses the foregoing problem, and aims to provide a rotor of a synchronous motor having the concentrated windings. This rotor advantageously lowers the magnetic flux density of the stator core, and yet reduces copper loss and iron loss.

SUMMARY OF THE INVENTION

The present invention provides a rotor of a synchronous motor having both of permanent magnets and a stator using a concentrated winding method. On a section of the rotor laminated in a rotary shaft direction, arc-shaped or bow-shaped slits are provided. The projecting portion of the arc-shape or bow shape face to the outside rim of the rotor, and permanent magnets are inserted into the slits. This structure produces less magnetic salient poles than a conventional rotor, thereby reducing a magnetic flux density of the stator core. This structure can thus provide a highly efficient motor that incurs less loss.

V-shaped slits instead of the arc- or bow-shaped slits can produce a similar advantage. In this case, a vertex of the V-shape faces to the outside rim of the rotor. Further, two sheets of permanent magnets can be inserted into each one of the V-shaped slits, so that a cost of the magnets is reduced. As a result, an efficient and inexpensive motor is obtainable.

Magnets used in the present invention can be any magnets such as ferrite magnet, rare-earth magnet or the like regardless of magnet materials; however, a structure employing the rare-earth magnet, which produces strong magnetic force among others, can reduce iron loss, so that the greatest advantage can be expected. Dividing the rare-earth magnet axially into plural magnets reduces loss due to eddy-current running on magnet surface, so that further efficient motor is achievable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. The embodiments below are exemplary reductions to practice of the present invention, and not to limit the technical scope of the present invention.

FIG. 1illustrates the first exemplary embodiment, a structure is provided that includes a stator11employing concentrated windings1, a rotor12, slits13, and permanent magnets14located in the slits. Stator11has three phases, four poles and six slots. Each one of teeth is wound independently with a winding, and two coils of each phase are apart 180 degrees from each other, i.e., opposite to each other. Stator11is formed by laminating plural electromagnetic steel sheets in a rotary shaft direction, and includes plural teeth. Ends of each one of teeth15slightly encroach into slots as shown in FIG.3.

To be more specific, distance “r” between arc face16, which faces to rotor12, of stator11and the center of rotor12is smaller at a center portion of teeth15than at the end of teeth15. (The broken line tangent to arc face16measures a constant distance “r” from the center of rotor12.) This structure restrains demagnetizing field from flowing to rotor12. Because adjacent teeth of the concentrated windings become different poles from each other, and inductance increases, so that demagnetizing field tends to be applied to rotor12. In order to overcome this phenomenon, both the ends of each one of the teeth slightly encroach into slots for enlarging air-gap at the ends of the teeth.

Each one of slits13prepared inside rotor12is shaped like an arc and protruded toward an outside rim of rotor12. A distance between slit13and the outside rim of rotor12is narrower at the center of slit13and becomes gradually wider toward both the ends of the slit. At the outer most end, the distance becomes narrow again.

Permanent magnets14are located in slits13, and the most outer both ends of each one of slits13remain void as non-magnetic portions, which work as leakage flux preventing sections that can prevent leakage flux from occurring between the adjacent permanent magnets. The ends of each of the slits13which act as the non-magnetic portions are not necessarily required to be voids, but resin can be filled with a resin.

As shown inFIG. 2, permanent magnet14protrudes its center portion A toward the outside rim of rotor12from the line running through both ends B of the magnet. This shape of permanent magnet14prepares narrower space “a” between magnet14and rotor12at its center than space “b” at both its ends. This structure substantially narrows the flux-path width along axis “q” of the first embodiment than that of the conventional rotor. The inductance of axis “q” thus decreases, so that an amount of magnetic flux along axis “q” running inside the rotor is reduced. As a result, the magnetic flux density of the stator core when it is loaded can be lowered. FIG.4andFIG. 5compare the magnetic flux density of the rotor of the present invention when it is loaded with the magnetic flux density of a conventional rotor. The rotor of the present invention has lower magnetic flux densities at its teeth and yoke of the stator core than those of the conventional rotor. Since the iron loss increases as the frequency and the magnetic flux density increase, the motor of the present invention obtains a great advantage particularly when the motor is highly loaded or spun at a high speed.

FIG. 6compares iron loss produced in the first embodiment with that produced by a conventional motor. This graph shows the number of rotations of the motor on the X-axis and the iron loss produced by the motor on the Y-axis. As shown inFIG. 6, the rotor of the present invention produces less iron loss than the conventional one, thus a highly efficient motor producing a little loss can be provided. The graph shows that the iron loss decreases in a greater amount as the number of rotations increases, in particular, the loss decreases advantageously at a high speed rotations faster than 3000 r/min.

In the first embodiment, one piece of permanent magnet is buried in each one of the slits. However, the stator having concentrated windings tends to produce an eddy current, so that as shown inFIG. 7the permanent magnet is divided into plural pieces19along the axial direction of the rotary shaft before they are buried. Thus the path length of the eddy current running on the magnet surface can be shortened, whereby loss due to the eddy current is substantially reduced. The rare-earth magnet advantageously reduces the eddy-current loss.

Japanese Patent Application Non-Examined Publication No. H05-304737 discloses a motor with permanent magnets shown inFIG. 8, apparently similar to the motor in accordance with this embodiment; however, the disclosed motor uses a stator with distributed windings. Since the stator used in this first embodiment employs the concentrated windings, adjacent poles have different polarities. The inductance thus becomes greater, which invited the present invention. However, the stator employing distributed windings does not have such a problem, thus the foregoing publication cannot anticipate the present invention.

FIG. 9illustrates the second embodiment of the present invention.FIG. 9illustrates a structure that includes a stator21that employs the concentrated windings1, wherein V-shaped slits23are formed in a rotor22, and wherein permanent magnets24are located in the slits23. The vertex of each one of the V-shaped magnets faces toward the outside rim of the rotor. This structure also can narrow the flux path along axis “q” as discussed in the first embodiment, and thus can lower the iron loss. This structure produces a similar advantage to the first embodiment. Further, as shown inFIG. 9, the permanent magnet to be buried in each slit is divided in half, and two sheets of magnet shaped like a flat plate can be inserted into the slit. Thus an efficient motor of a lower cost, which uses inexpensive and flat magnets instead of expensive and arc-shaped magnets shown inFIG. 1, can be obtained.

The V-shaped magnet shown inFIG. 9uses two sheets of permanent magnet in one slit, so that a void is formed between the magnets forming the V-shape. The two sheets of magnet form one magnetic pole.

FIG. 10shows sectional view of a compressor which employs the synchronous motor having permanent magnets in accordance with the first exemplary embodiment. As shown inFIG. 10, the compressor comprises the stator11using the concentrated windings, rotor12, the permanent magnets14, an accumulator31and a compressing mechanism32. The motor of this compressor has a shorter length including its coil end, and works efficiently, so that the compressor is best fit for a place where the power or the storage area is limited, such as an air-conditioner compressor C for an air-conditioning unit104for a vehicle100, in particular an electric vehicle, for cooling a passenger compartment102, as schematically illustrated in FIG.10A.

INDUSTRIAL APPLICABILITY

In a motor having interior magnets and a stator employing the concentrated windings, a distance between the outside rim of the rotor and a slit, in which a permanent magnet is buried, is narrower at a center portion of the slit than at both the ends of the slit. This structure reduces a magnetic flux density of the stator core, so that a more efficient motor with less iron loss than conventional motors is obtainable.