ELECTRIC MACHINE AND METHOD FOR OPERATING SUCH AN ELECTRIC MACHINE

Abstract

Electrical machine, comprising: a stator (11) with at least one armature coil (14); and a rotor (21) having a plurality of rotor teeth (22) with at least one field coil (28) wound around the respective rotor tooth (22), characterized in that the rotor teeth (22) around which the field coil (28) is wound comprise a low coercivity magnet (51), and wherein a superposition of a drive current flowing through the armature coil (14) with a pulse current (PI) causes a pulse current (IC) to flow through the field coil (28) to cause the magnetization of the low coercivity magnet (51) to a predetermined magnetic field strength.

Description

[Technical field]

[0001] The present invention generally relates to an electric machine comprising a rotor having at least one low coercivity magnet and a method for operating such an electric machine. [Previous state of the art]

[0002] Electric machines are used as a power source in various drive devices. For example, embedded permanent magnet (IPM) motors are well known in certain areas of the automotive industry, such as electric vehicles and hybrid electric vehicles.

[0003] To meet the demand for high power (or high energy density) and miniaturization, rare earth permanent magnets such as neodymium magnets are often used in IPM motors due to their high remanent flux density and high heat resistance. However, expensive rare earth elements such as dysprosium (Dy) and terbium (Tb) may need to be added to the neodymium magnet alloys. This makes the magnets even more expensive. Furthermore, supplies of these elements may decrease in the future.

[0004] From EP 2 782 226 A2 and DE 10 2007 000 429 B4 an electrical machine is known which comprises a stator with at least one armature coil and a rotor which has a plurality of rotor teeth with at least one field coil wound around the respective rotor tooth.

[0005] DE 10 2009 057 433 A1 discloses a permanent magnet motor having a rotor equipped with permanent magnets having a coercivity sufficiently low to allow a change in the degree of magnetization.

[0006] From US 2011 / 0084567 A1 a rotating electrical machine system is known, comprising a rotor with magnets with low coercivity.

[0007] In JP 2008- 43 172 A, it was proposed for modern electrical machines to replace expensive neodymium magnets with low-coercivity magnets and to magnetise the low-coercivity magnets embedded in a rotor up to a predetermined magnetic field strength, the so-called magnetisation. [State of the art][Summary of the invention][Technical problem]

[0008] However, when applying the electrical machine described in JP 2008-43172 A in the automotive industry, an inverter must convert a direct current into the alternating current required as the magnetizing current so that the armature coil(s) generate a sufficient magnetic field around the low-coercivity magnet to magnetize the low-coercivity magnet to saturation. For this reason, the inverter must generate a particularly high electric current to magnetize the low-coercivity magnet.

[0009] It follows that a circuit including an inverter, etc., must have high dielectric strength and current carrying capacity, which is impractical because many electronic devices are becoming too large and expensive to be used as components of an on-board electrical machine in a vehicle.

[0010] An object of the present invention is to provide an electrical machine and a method for operating an electrical machine in which a magnetization of a magnet with low coercivity can be carried out by supplying a reduced electrical current. [Solution to the task]

[0011] According to one aspect of the present invention, an electric machine is provided, comprising: a stator having at least one armature coil; and a rotor having a plurality of rotor teeth with at least one field coil wound around each rotor tooth. According to the invention, the rotor teeth around which the field coil is wound comprise a low-coercivity magnet, and wherein a superposition of a drive current flowing through the armature coil with a pulse current causes a pulse current (pulsating current) to flow through the field coil to cause the magnetization of the low-coercivity magnet to a predetermined magnetic field strength.

[0012] According to another aspect of the present embodiment, a method of operating an electric machine comprising a stator having a plurality of rotor teeth with at least one field coil wound around each rotor tooth is provided, comprising: providing the rotor teeth around which the field coil is wound with a low coercivity magnet; providing at least one field coil winding around the rotor; superimposing a pulse current on a drive current flowing through the armature coil to cause a pulse current to flow through the field coil to magnetize the low coercivity magnet to a predetermined magnetic field strength. [Advantageous effect of the invention]

[0013] In this way, a magnet with low coercivity can be magnetized by applying a reduced electric current. [Brief description of the drawings] Fig. 1 is a cross-sectional view of an electric machine according to an embodiment of the present invention. Fig. 2 is a partial enlarged cross-sectional view of a stator and a rotor. Fig. 3 is a simplified block diagram of a supply circuit for supplying an armature coil with a drive current. Fig. Figure 4 is a graphical representation of a rotor rotation angle, expressed in mechanical degrees (deg), versus the magnetic flux density, expressed in Tesla (T), showing the effect on the instantaneous flux density in concentrated wound armature coils and in distributed wound armature coils. Fig. Figure 5 is a graph showing a comparison between the space harmonics in concentrated wound armature coils and the space harmonics in distributed wound armature coils. Fig. Figure 6 is a graph showing flux patterns in three-phase armature coils, induction coil and field coil. Fig. Figure 7 is a simplified circuit comprising diodes connecting the induction coils and the field coils. Fig. Figure 8 is a schematic diagram illustrating the strength of a magnetic flux pf (WF) that the armature coil in a conventional design must generate to achieve the magnetization of a low coercivity magnet. Fig. 9 is a schematic diagram illustrating the strength of a magnetic flux PF that the armature coil in the present embodiment including a field coil must generate to achieve magnetization of a low coercivity magnet. Fig. 10 shows graphs illustrating the course of a pulse current PI required by the armature coil to generate the strength of the magnetic flux PF compared to the course of a pulse current pi required by the armature coil to generate the strength of the magnetic flux pf. Fig. Figure 11 is a flowchart illustrating an algorithm that generates a flux strong enough to achieve magnetization of a low coercivity magnet in response to the application of a reduced amplitude pulse current. Fig. 12 is a graph of rotor speed versus motor torque showing the effect on instantaneous torque at low speeds below 2000 rpm in the present embodiment with a suitable magnetized low coercivity magnet compared to a comparative example without such a low coercivity magnet and to a conventional embedded permanent magnet (IPM) motor. Fig. Figure 13 is a flowchart illustrating another algorithm that generates a flux strong enough to achieve magnetization of a low coercivity magnet in response to the application of a reduced amplitude pulse current. [Description of embodiments]

[0014] In the following, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Fig. 1 to 13 show an electric machine according to an embodiment and a method for operating such an electric machine. STRUCTURE OF THE ELECTRIC MACHINE

[0015] In the Fig. 1 and Fig. 2, the electric machine 100 has a structure that does not require external energy input to a rotor 21 and is preferably installed, for example, in hybrid electric vehicles or electric vehicles.

[0016] The electric machine 100 has an approximately cylindrical stator 11 and a rotor 21 within the stator 11. The rotor 21 is attached to a shaft 101 that rotates as a drive shaft, so that the rotor 21 and the shaft 101 are integrally rotatable about the same rotation axis.

[0017] The stator 11 has a longitudinal axis, an inner circumference 12a, and a plurality of spaced-apart inner stator slots 13 extending longitudinally along the inner circumference 12a to define a plurality of stator teeth 12, each stator tooth 12 being defined between two adjacent inner stator slots 13. In other words, the plurality of stator teeth 12 of the stator 11 are circumferentially equidistant and extend radially toward the rotation axis. Each of the stator teeth 12 is formed as a protruding structure having an inner circumference forming part of the inner circumference 12a and facing the outer circumference 22a of the rotor 21 across a calibrated air gap G.A plurality of armature coils 14 are wound around the stator 11 and located in the inner stator slots 13, each of the armature coils being formed between the sides 22b of two adjacent stator teeth 12, so that each of the armature coils 14 for each of the three phases is wound around the corresponding stator tooth 12 by concentrated winding.

[0018] Referring to Fig. 3, a control unit 110 controls an inverter 112 to convert direct current from an onboard battery 111 into three-phase alternating current and controls the supply of the three-phase alternating current to the armature coils 14 as a drive current for rotation control, so that each of the stator teeth 12 functions as an electromagnet, generating a magnetic flux for rotating the rotor 21. Note that the control unit 110 stores control programs for controlling the entire vehicle in a memory 110m. The control unit 110 executes the control programs depending on various parameters to drive the rotation of the electric machine 100.

[0019] The rotor 21 is received in the stator 12 such that its outer circumference 22a is separated from the inner circumference 12a of the stator 12 by the calibrated air gap G. The rotor 21 has a plurality of rotor slots 23 extending longitudinally along the outer circumference 22a to define a plurality of rotor teeth 22, each rotor tooth 22 being defined between two adjacent rotor slots 23. In other words, the plurality of rotor teeth 22 of the rotor 21 are circumferentially equidistant and extend radially away from the axis of rotation. Each of the rotor teeth 22 is formed as a protruding structure having an outer circumference forming part of the outer circumference 22a and facing the inner circumference 12a of the stator 11 across a calibrated air gap G.In particular, the number of rotor teeth 22 and the number of stator teeth 12 are different such that the outer circumference 22a of each rotor tooth 22 approaches and opposes the inner circumferences 12a of the stator teeth 12 one after another during the rotation of the rotor 21 relative to the stator 11.

[0020] When the armature coils 14 of the stator 11 are energized, magnetic fluxes arise in the electric machine 100, which interlink from the inner peripheries 12a of the stator teeth 12 to the outer peripheries 22a of the rotor teeth 22 opposite the stator teeth 12. This electric machine 100 allows rotation of the rotor 21 relative to the stator 11 based on a reluctance torque (or principal torque), which tends to minimize the reluctance within a magnetic circuit. As a result, the electric machine 100 can output electrical energy supplied as input current as mechanical energy via the shaft 101, which is integrally rotatable with the rotor 21 within the stator 11 relative to the stator 11.

[0021] In the electric machine 100, each of the fluxes interlinked with the outer peripheries 22a of the rotor teeth 22 from the inner peripheries 12a of the stator 12 includes space harmonics superimposed on the fundamental frequency. Therefore, the rotor 21 utilizes a change in the magnetic flux density of one or each of the space harmonics contained in the magnetic flux interlinked with the rotor 21 from the stator 11, causing the generation of an induced current in the embedded coils to provide an electromagnetic force.

[0022] However, supplying the armature coils 14 on the stator 11 with current based only on the fundamental frequency causes nothing other than rotation of the rotor 21 (rotor teeth 22) with a flux that varies with the fundamental frequency and does not cause any change in the flux linked to the rotor 21, even if coils are arranged on the rotor 21, so that such coils do not generate an induced current.

[0023] On the other hand, the flux contains space harmonics superimposed on the fundamental frequency, and these space harmonics, which vary in time at frequencies other than the fundamental frequency, concatenate from the outer periphery 22a inwardly with one of the rotor teeth 22. For this reason, the space harmonics superimposed on the fundamental frequency allow for effective generation of induced current by a coil without requiring an external electricity supply when the coil is located inside the rotor tooth 22 near its outer periphery 22a. Consequently, the space harmonics considered to be the cause of iron losses can be recovered as energy for self-excitation.

[0024] In the electric machine 100 according to the present embodiment, the entire plurality of induction coils 27, each wound around a plurality of additional pole cores 25 by concentrated winding, are laid in rotor slots 23, each between two adjacent rotor teeth 22. The plurality of induction coils 27 are arranged in the circumferential direction of the rotor 21. In addition, a plurality of pairs or sets of field coils 28 are wound around the rotor 21 such that each of the plurality of pairs of field coils 28 lies in the two adjacent rotor slots 23. In other words, the field coils 281 and 282 of each pair 28, wound around the associated rotor tooth 22 by concentrated winding and connected in series, form part of the entire rotor tooth 22.

[0025] The legs (or supports) 35 of each additional pole core 25 are supported by opposite sides 22b of the two adjacent rotor teeth 22 and are supportingly connected to its body 31 wound in one of the field coils 27 in order to position and support the field coil 27 within one of the rotor slots 23 between the sides 22b of the two adjacent rotor teeth 22.

[0026] The body 31 of each additional pole core 25 is manufactured by laminating electromagnetic plates or sheets in such a way that it extends longitudinally along and parallel to the shaft 101 and is formed into a plate-like structure that allows the induction coil 27 to be wound opposite the opposite sides 22b of the two adjacent rotor teeth 22 of the rotor 21. The body 31 of the additional pole core 25 is mounted on the rotor 21 such that it extends radially away from the rotation axis or radially in an outward direction of the rotor 21 within the associated rotor slot 23 between the two adjacent rotor teeth 22 to be wound into the induction coil 27, with an outer end surface 32a of its radially outer end portion 32 facing the inner periphery 12a of the adjacent stator tooth 12 of the stator 11.The radially outer end portion 32 has a greater width than the remaining portion of the body 31 in order to limit deflection of the wound induction coil 27 caused by the centrifugal force during rotation of the rotor 21.

[0027] The leg 35 of each additional pole core 25 is manufactured by laminating electromagnetic plates or sheets in such a way that it extends longitudinally along and parallel to the shaft 101. The leg 35 is formed into a plate-like structure so that it extends from the radially inner end of the corresponding body 31 in a lateral direction toward one of the opposite sides 22b of the two adjacent rotor teeth 22 and in the opposite direction toward the other side. The leg 35 supports the associated body 31 by fitting its two farthest lateral edges 36 into two support grooves 39 cut inward from the opposite sides 22b for mounting or coupling to the sides 22b.Thus, each of the additional pole cores 25 can be mounted simply by inserting the lateral edges 36 into two support grooves 39 of one of the rotor grooves 23 from one of two axial ends of the rotor 21 and then pushing the pole core 25 towards the other axial end so that the lateral edges 36 slide along the support grooves 39.

[0028] The leg 35 of each additional pole core 25 is formed by laminating electromagnetic plates in such a way that it provides sufficient support force to support the body 31 and that its width is reduced as much as possible, for example, extending longitudinally along the rotation axis with a width corresponding to at most the thickness of two electromagnetic plates. In other words, the leg 35 is formed as a plate-shaped structure having a small cross-sectional area to restrict the magnetic flux between the body 31 and the adjacent rotor tooth 22 as much as possible to provide a magnetically independent support form in which the additional pole core 25 can operate as a pole separate from the adjacent rotor tooth 22 (additional pole).

[0029] For this reason, the rotor 21 easily reaches a state of magnetic saturation because the flow of magnetic flux through the leg 35 of each additional pole core 25 is restricted, so that the density of the magnetic flux lines flowing therethrough immediately increases. Such a structure prevents magnetic coupling between each of the additional pole cores 25 and the adjacent rotor tooth 22, so that the additional pole core 25 is maintained in a state sufficiently magnetically independent from the adjacent rotor teeth. This allows highly efficient rotation of the rotor 21 using a large torque because a decrease in the efficiency of generating the electromagnetic force and the induced current due to an interaction between the flux interlinked with the additional pole core and the flux interlinked with the rotor teeth 22 is prevented.

[0030] In addition, this structure facilitates the assembly of the rotor 21 by allowing a portion or the entirety of each of the field coils 28 to be wound around a radially inner end portion or around the entirety of one of the rotor teeth 22 before mounting each of the additional pole cores 25 supported by the two adjacent rotor teeth 22, and then inserting the leg 35 of the additional pole core 25 into the support grooves 39 formed in the opposite sides 22b of the two adjacent rotor teeth 22.

[0031] Subsequently, each of the induction coils 27 is simply wound around a body 31 of one of the additional pole cores 25, before or after the leg 35 of each of the additional pole cores 25 is mounted so as to be supported by the two adjacent rotor teeth 22. Furthermore, each of the field coils 28 is wound around one of the rotor teeth 22 so as to be divided into a first field coil 281 and a second field coil 282, which are located radially inward and radially outward from the leg 35 of the additional pole core 25, respectively, the first and second field coils 281 and 282 being connected in series to form the field coil 28.

[0032] Simply winding each of the induction coils 27 around a body 35 of one of the additional pole cores 25 makes it possible to accommodate the induction coil 27 near the outer circumference 22a of the rotor 21 because the additional pole core 25 is supported by the leg 35. In addition, since the leg 25 of each of the additional pole cores 25 is inserted into and supported by the support slots 39, the field coils 28 (consisting of the first and second field coils 281 and 282, respectively) can be wound around the entire rotor teeth 22 without being interrupted by the legs 35 of the additional pole cores 25. Thanks to this structure, the induction coils 27 efficiently generate an induction current by effectively utilizing a distance between each of the rotor slots 23, thus supplying the induction current to the field coils 28 to generate electromagnetic forces.

[0033] The winding of each of the field coils 28, consisting of first and second field coils 281 and 282, around an entirety of one of the rotor teeth 22 (the entirety including both the radially inner portion and the radially outer portion) in one operation requires that each of the additional pole cores 25 remain in a state in which a space into which its leg 35 slips is located to the left in one of the rotor slots 23.The winding of the first field coil 281 around one of the rotor teeth 22 before the one of the rotor teeth 22 and the adjacent rotor tooth 22 supports one of the additional pole cores 25, and the winding of the second field coil 282 around the one rotor tooth 22 after the one and the adjacent rotor tooth 22 supports the one additional pole core 25 requires that after the first field coil 281 has been wound around a radially inner portion of the one rotor tooth 22, the second field coil 281 should be wound around a radially outer portion of the one rotor tooth 22, before or after the winding of the induction coil 27 around the body 31 of the one additional pole core 25.

[0034] By using each of the additional pole cores 25 formed of electromagnetic steel plates (magnetic material) in the induction coils 27, the permeability is increased, allowing the magnetic flux to interlink with high density to the additional pole core 25. Furthermore, by arranging each of the induction coils 27 in a magnetic path at a suitable position facing the inner periphery 12a of an adjacent stator tooth 12 across the calibrated air gap G, a larger magnetic flux of space harmonics can interlink with the additional pole cores 25.To efficiently utilize the third spatial harmonic in the rotating reference frame, which is contained in the magnetic fluxes that link from the inner peripheries 12a of the stator teeth 12 to the outer peripheries 22a of the rotor teeth 22, a magnetic field analysis is performed to precisely determine where in the magnetic paths the induction coils 27 for the third spatial harmonic in the rotating reference frame should be arranged so that they efficiently generate an induction current. Furthermore, the induction coils 27 are each installed between the two adjacent rotor teeth 22 in the rotor slot 23 in such a way that a necessary and sufficient air gap is maintained between each of the induction coils 27 and the adjacent field coils 28.

[0035] By using such a concentrated wound structure, it is no longer necessary to install the induction coils 27 and the field coils 28 circumferentially with more than two rotor slots, allowing the overall size of the electric machine to be reduced. The concatenation of the third spatial harmonic in the rotating reference frame with the induction coils 27 results in efficient generation of induced current with reduced copper loss on the primary side, resulting in an increase in the recoverable lost energy.

[0036] By using the third spatial harmonic in the rotating reference system instead of the second spatial harmonic in the rotating reference system, which third spatial harmonic changes more frequently in time and has a larger amplitude than the second spatial harmonic in the rotating reference system, a more efficient concatenation for generating an induction current through the induction coils 27 is enabled, which causes an increase in the recoverable lost energy.

[0037] In this way, the induction coils 27 are separated from the field coils 28 such that the magnetic paths of the former do not interfere with the magnetic paths of the latter, and the magnetic flux can flow along the entire lines of each induction coil 27 extending longitudinally over the entire length (width) of the adjacent rotor tooth 22, so that the induction coils 27 efficiently generate an induction current with reduced magnetic interference.

[0038] As an example of a structure that mainly uses the 3f-th space harmonic (f = 1, 2, 3...), an electric machine 100 is manufactured with a structure corresponding to the following relationship: P:S=2:3 where P is the number of salient poles (rotor teeth 22) of the rotor 21; and S is the number of stator slots 13 of the stator 11.

[0039] The third spatial harmonic in the rotating reference frame has a higher frequency than the fundamental frequency of the alternating current supplied to the armature coils 14 and thus pulsates with a shortened cycle. This causes the rotation of the rotor 21 by efficiently recovering energy lost due to the superposition of the fundamental frequency of the magnetic flux with the spatial harmonics. This is because the changing density of the magnetic flux interlinked with each induction coil 27 installed in the rotor slot 23 between two adjacent rotor teeth 22 causes efficient generation of induced current.

[0040] In the electric machine 100, the fulfillment of the above-described ratio P : S = 2 : 3 serves as a parameter for a structure that determines the quality of the magnetic interactions between the rotor 21 and the stator 11, to achieve a rotation of the rotor 21 with a reduced electromagnetic noise by reducing the electromagnetic oscillations.

[0041] A magnetic field analysis of the magnetic flux distribution shows an uneven distribution of the electromagnetic forces applied to the stator 11 because the magnetic flux density is distributed circumferentially over 360 mechanical degrees according to the ratio P : S or P / S.

[0042] In contrast, the use of the structure satisfying the ratio P / S = 2 / 3 in the electric machine 100 allows a concatenation of the magnetic flux that is evenly distributed in the circumferential direction over 360 mechanical degrees, which enables a high-quality rotation of the rotor 21 within the stator 11.

[0043] This enables silent rotation of the electric machine 100 while drastically reducing electromagnetic vibrations because torque is generated based on the magnetic flux of the space harmonics.

[0044] In the electric machine 100, the armature coils 14, the induction coils 27, and the field coils 28, each formed by concentrated winding of a wire, are laid on the stator 11 and the rotor 21, which has a structure satisfying the ratio P / S = 2 / 3, but may also be formed by distributed winding instead of concentrated winding. Fig. 4 shows a waveform of a magnetic flux density linking the inner circumference 12a of the stator teeth 12 and the outer circumference 22a of the rotor teeth 22 in the case of a concentrated winding compared to a distributed winding.

[0045] As in Fig. As can be seen in Figure 5, an electromagnetic analysis of the above-mentioned magnetic flux density waveforms shows that, in the case of a concentrated winding, they exhibit more second spatial harmonics in the stationary frame of reference, i.e., third spatial harmonics in the rotating frame of reference, than in the case of a distributed winding. It follows that, when using a concentrated winding, more spatial harmonics interlink with the induction coils 27 to supply the field coils 28 with an induced current (field current) than when using a distributed winding.

[0046] Referring to Fig. 6, each induction coil 27 on the rotor 21 in the electric machine 100 generates an induction current due to the appearance of a high-frequency magnetic flux corresponding to the changing magnetic flux generated by the armature coils 14 in each of the three phases, in response to a magnetic flux generated by the armature coils 14 in each of the three phases, which flux changes into the waveform shown immediately after or at the beginning of the flow or supply of the three-phase (U-phase, V-phase, W-phase) alternating current to the armature coils 14 on the stator 11. This induction current is generated by a diode 29A or 29B described later (see Fig. 7) and supplied to the associated field coil 28 to cause the generation of a stable magnetic flux, thereby providing an electromagnetic force (magnetic force).

[0047] The electric machine 100 efficiently generates an induction current through each induction coil 27 located on the q-axis of the rotor 21, and supplies this current as a field current to the field coil 28 located on the adjacent d-axis, so that the associated rotor tooth 22 operates as a self-excited electromagnet. Thus, the electric machine 100 enables highly efficient rotation of the rotor 21 by obtaining additional torque (electromagnetic force) to support a main torque derived from the supply of electricity to the armature coils 14.In other words, due to an additional pole located on the q-axis, a magnetic flux derived from the space harmonics on the q-axis is provided as a field energy source, thereby improving the density of the self-excitation-derived magnetic moment by a greater increase in mutual inductance than in a structure without such an additional pole on the d-axis.

[0048] The paired induction coils 27 are wound around the radial directions of the rotor 21 by concentrated winding of an identical wire in the same winding direction and arranged side by side in the circumferential direction of the inner rotor 30. The induction coils 27 belonging to one group are laid in alternating rotor slots 23 and connected in series to form a series circuit, and the induction coils 27 belonging to the other group are laid in the other rotor slots 23 and connected in series to form another series circuit. Two series circuits of induction coils 27 are connected in parallel at both ends. The field coils 28 (281, 282) are wound around the radial directions of the rotor 21 by concentrated winding in opposite winding directions. Each of the field coils 28 consists of series-connected field coils 281 and 282. The field coils 28 wound in one winding direction are connected in series one after the other.Similarly, the field coils 28 wound in the opposite winding direction are connected in series one after the other.

[0049] In other words, the field coils 28 are wound such that the magnetization direction is reversed in every other rotor tooth 22 when it acts as an electromagnet in response to the flow of direct current. In other words, one of two adjacent rotor teeth 22 is magnetized to serve as an electromagnet with its S pole facing the inner peripheries 12a of the stator teeth 12, while the other is magnetized to serve as an electromagnet with its N pole facing the inner peripheries of the stator teeth 12.

[0050] As in Fig. As is readily apparent from Figure 7, both ends of the field coils 28, which are connected in series as a whole, are connected to both ends where two series circuits of inductor coils 27 are connected in parallel, via diodes (rectifiers) 29A and 29B. To be more specific, the field coils 28 wound in one direction, i.e., 28A1-28An, are connected in series as a whole, and the field coils 28 wound in the opposite direction, i.e., 28B1-28Bn, are connected in series as a whole, where n: (number of poles) / 2. They are connected in parallel to both ends of the parallel circuits, ie one circuit comprising the paired induction coils 27A1-27An connected in series with the field coils 28A1-28An, the other circuit comprising the paired induction coils 27B1-27Bn connected in series with the field coils 28B1-28Bn.

[0051] Regarding the number of diodes 29A and 29B, if an increase in the number of poles is required, the number of diodes to be used can be limited by connecting the field coils 28 entirely in series. The diodes 29A and 29B are connected to form a star-clamped half-wave rectifier (rectifying element) by connecting elements to provide a 180-degree phase difference between an induction current input to one of the diodes 29A and 29B and the other induction current input to the other diode, to output an output current by half-wave rectification after inverting one induction current.

[0052] As can be seen from the foregoing description, the electric machine 100 enables the generation of induced current and the recovery of generated induced current by reducing the magnetic interference between each of the induction coils 27 and the adjacent field coils 28 as much as possible (i.e., by minimizing any reduction in induced current) and by causing each of the additional pole cores 25 made of high-permeability electromagnetic steel to pass space harmonics contained in the magnetic flux interlinking from the inner periphery 12a of an associated stator tooth 12 to the outer periphery 22a of one of the rotor teeth 22. In addition, the current generated by each of the two series circuits of induction coils is rectified by the associated diode 29A and 29B.After rectification of the induction currents, the rectified induction currents are combined and fed to the series-connected field coils 28. As a result, the electric machine 100 effectively self-excites the field coils 28, thereby generating a strong magnetic flux (electromagnetic force).

[0053] It follows that the electric machine 100 can efficiently recover space harmonics as energy and output them by avoiding mutual interference that could weaken the generated magnetic flux, by dividing the induction coils 27 and the field coils 28 according to whether current generation or electromagnet generation is expected, so that each of the induction coils 27 becomes independent of the adjacent field coils 28, and by effectively utilizing the generated magnetic flux after it is smoothed. Note that in the electric machine 100, each of the field coils 28 cooperates with one of the rotor teeth 22 to form a salient pole, and each of the induction coils 27 cooperates with one of the additional pole cores 25 to form an additional pole.

[0054] As a result, the electric machine 100 causes the rotor teeth 22 to operate as self-excited electromagnets due to the efficient generation of induction current by the induction coils 27 and the supply or flow of this induction current as a field current to the field coils 28. This enables highly efficient rotation of the rotor 21 by obtaining additional torque (electromagnetic force) to support a main torque derived from the supply of electricity to the armature coils 14. In the electric machine 100, the induction current generated by the induction coils 27 is supplied internally as a field current to the field coils 28. This is only one example. The present invention can also be applied to a structure in which the field current is supplied from an external power source.

[0055] At low speeds of the rotor 21, for example, when the vehicle is starting, the frequency at which the flux linked to each induction coil 27 changes is low, so the amount of induced current generated by the induction coil 27 is small and the strength of the magnetic field generated by the associated field coil 28 is weakened. However, in the electric machine 100 according to the present embodiment, another source of a magnetic field is provided to effectively support the torque generated by the armature coils 14 on the stator 11 at such low speeds of the rotor 21.

[0056] As in the Fig. 1 and Fig. 2, the electric machine 100 specifically includes a plurality of low-coercivity permanent magnets 51 embedded in the rotor 21, each located between two adjacent rotor slots 23 and near the outer periphery 22a of the rotor 22. In other words, each low-coercivity magnet 51 is plate-shaped and embedded in one of the rotor teeth 22 such that it faces the inner periphery 12a of the stator 11 for the adjacent stator tooth 12.

[0057] Each low-coercivity magnet 51 is a narrow, longitudinally extending plate. The low-coercivity magnet 51 has approximately the same width as the portion of the outer circumference 22a that forms one of the surfaces of the rotor teeth 22, that is, the portion of the outer circumference 22a that extends between the two adjacent rotor slots 23, and extends in an axial direction of the shaft 101 approximately along the entire length of the rotor 22. As best shown in Fig. As can be seen in Figure 2, the low-coercivity magnet 51 is embedded in one of the rotor teeth 22 near the outer periphery 22a of the rotor 22, with one of two opposing flat magnetic pole faces 51a facing the inner periphery 12a of the stator 11 for the adjacent stator tooth 12 over a large area. It is not necessary for each low-coercivity magnet 51 to have the same width as the portion of the outer periphery 22a forming one of the surfaces of the rotor teeth 22. The width of the low-coercivity magnet 51 is adjustable depending on its ability to resist demagnetization, its magnetic reluctance in a magnetic circuit through which a magnetic flux flows, etc.

[0058] With one of two opposing magnetic pole faces 51a designated as the N pole and the other as the S pole, each low-coercivity magnet 51 is embedded in one of the rotor teeth 22 such that its magnetization direction corresponds to the magnetization direction of the electromagnet formed by the associated field coil 28 wound around the rotor tooth 22. The low-coercivity magnet 51 can be magnetized to generate a magnetic field of a specific strength when exposed to a magnetic field having the same magnetization direction. It can also be magnetized to generate a magnetic field of a specific strength when exposed to a magnetic field having the opposite magnetization direction. Each low-coercivity magnet 51 can be made, for example, from an inexpensive AlNiCo magnet or a cobalt magnet.The material can be selected based on properties such as heat resistance. If it is to be used in an environment exposed to high temperatures, such as a vehicle, a cobalt magnet should preferably be selected as the material for the low-coercivity magnet 51.

[0059] The Fig. 8 and Fig. 9 show two different magnetization methods for generating a magnetizing flux WF strong enough to magnetize the low-coercivity magnet 51. Both magnetization methods are similar in that, in addition to a basic flux BF generated by each armature coil 14 when the armature coils of the stator 11 are supplied with a drive current, the application of a pulse current (magnetizing current) superimposed on the drive current causes the generation of a magnetic pulse to generate the magnetic flux WF, which flows through the low-coercivity magnet 51 and sufficiently magnetizes it.

[0060] Referring to Fig. 8, the magnetization of the low-coercivity magnet 51 requires the passage of a magnetic pulse (or pulsating flux) pf as the magnetizing flux WF (which is approximately equal to pf), wherein the magnetic pulse pf has a magnetic field strength large enough to induce the required suitable magnetization. When the low-coercivity magnet 51, as in Fig. 8, is to be magnetized only by this magnetic pulse pf, which is generated only by the associated armature coil 14, it is necessary to superimpose the drive current with a pulse current whose maximum value has a very large absolute value. As a result, the electronic devices, including an inverter 112 and connecting cables (not shown), become too large and expensive to withstand such a large current load. Feeding such a high current into the armature coils 14 can cause the torque of the drive rotor 21 to pulsate, thus increasing the occurrence of vibrations and / or noise caused by such torque pulsations.

[0061] In the electric machine 100 according to the present embodiment, each of the field coils 28 is wound around one of the rotor teeth 22 in which the low coercivity magnets 51 are embedded. As shown in Fig. 9, a magnetic pulse or pulsating flux PF having a smaller maximum value than the magnetic pulse pf (see Fig. 8), by feeding into each of the armature coils 14 of the stator 11 a pulse current whose maximum value is a much lower absolute value than that in the conventional method of Fig. 8 is derived and tends to flow through the associated field coil 28 of the rotor 21. To prevent the pulsating flux PF from flowing through the field coil 28, the field coil 28 generates an induced magnetic pulse or induced pulsating flux (or reaction flux) IF directed toward the stator 11. As a result, the armature coil 14 generates a reaction flux RF in one direction, so that the change in the flux derived from the pulsating flux IF induced by the field coil is offset by the armature coil. This reaction flux RF is applied as magnetizing flux WF to or flows through the low-coercivity magnet 51 to magnetize it to the saturation region.

[0062] It follows that this electrical machine 100 can induce a reaction flux RF as a magnetizing flux WF for magnetizing the magnet 51 with low coercivity by the armature coil 14 of the stator 11 to generate a Fig. 9 is supplied with only a pulse current whose maximum amplitude has an absolute value lower than that required to generate a Fig. 8 shown pulsating flow is sufficiently strong pulse current.

[0063] Fig. 10 also shows that in the electric machine 100 according to the present embodiment, when supplying the armature coils 14 of the stator 11 with a pulse current PI having a lower maximum amplitude than a pulse current pi causing the generation of a strong pulsating flux pf (see Fig. 8) causes, after the superposition of a drive current with the pulse current in order to cause the armature coil 14 to generate a pulsating flux (self-induced flux) PF having a low magnetic field strength through its self-induction, the pulsating flux PF causes an induction current IC to flow through the associated field coil 28 in order to induce an induced pulsating flux IF (mutual inductance) (see. Fig. 9). Furthermore, the interaction with this induced pulsating flux IF (mutual inductance) causes the generation of a reaction flux (induction flux) RF through the armature coil 14. This reaction flux RF is introduced into the low-coercivity magnet 51 as a strong magnetizing flux WF, which has an increased magnetic field strength compared to the less strong pulsating flux PF.

[0064] In the foregoing description of the present embodiment, an example was described in which a pulse current is supplied to the armature coil 14 as a magnetizing current for a low-coercivity magnet 51. However, the number of pulse currents is not limited to one. As long as the time interval for supplying a pulse current to an armature coil 14 is within a range allowable for rotation control or magnetization control, it may be divided into a plurality of periods or extended. However, since copper loss caused by the supply of the pulse current occurs in the armature coil 14, a one-time supply of a pulse current is preferred as in the present embodiment. METHOD FOR OPERATING THE ELECTRIC MACHINE 100

[0065] From the foregoing description, the electric machine 100 uses a direct current from an onboard battery 111 and allows the control unit 110 to supply a drive current for rotation control and a pulse current to the armature coils 14 via the inverter 112. To achieve this goal, the control unit 110 executes a control method for magnetizing low-coercivity magnets 51, that is, a current supply control method to apply a direct current from the onboard battery 111 as a pulse current to the armature coil 14 via the inverter 112 at an appropriate time by executing control programs stored in a memory 110m. That is, the control unit 110 forms a pulse control unit.

[0066] In particular, the control unit 110 starts, as shown in the flow chart of Fig. 11, when the vehicle starts moving, the control unit 110 controls the supply of the alternating current drive current to the armature coil 14 by driving the inverter 112 when starting the drive to rotate the rotor 201, i.e., for example, the shaft 101. At the same time, the control unit 110, following instructions from the control program stored in the memory 110, executes a control process for magnetizing the low-coercivity magnet 51 by causing the inverter 112 to superimpose a pulse current having a maximum amplitude, the absolute value of which is lower than a pulse current conventionally used for magnetization by a pulsating flux pf, onto the drive current in the form of alternating current, in order to supply the pulse current to the armature coil 14 at an appropriate time (step S11).

[0067] In response to the input of the pulse current superimposed on the drive current flowing through the armature coil 14, the armature coil 14 generates a pulsating flux (self-induced flux) PF having a small amplitude corresponding to the amplitude of the previously input pulse current, allowing the pulsating flux PF to interlink with the associated rotor tooth 22 of the rotor 21 (step S12). When this pulsating flux PF interlinks with the associated rotor tooth 22, the associated field coil 28 on the rotor 21 generates an induced pulsating flux IF and allows the induced flux IF to interlink with the associated stator tooth 12 (step S13). Due to the interlinkage of this induced pulsating flux IF with the stator tooth 12, the armature coil 14 generates a reaction flux (mutual inductance flux) RF on the stator 11 and applies the reaction flux RF to the low coercivity magnet 51 (step S14).

[0068] According to this method, adjusting the direction, amplitude, and timing of the pulse current applied to the armature coil 14 on the stator 11 is sufficient to magnetize the low-coercivity magnet 51, since even a pulsating flux PF induced by the application of the low-amplitude pulse current allows the magnetic field strength to be increased to provide a reaction flux RF with a necessary and sufficient magnetic field strength. In short, by superimposing a drive current with a low-amplitude pulse current, it is possible to maintain the magnet's magnetic force strong until saturation by magnetizing the low-coercivity magnet 51.

[0069] As a result, the electric machine 100, as shown in Fig. 12, at low speeds of the rotor 21, such as when the vehicle is starting, when the field coil 28 acts as an electromagnet (dotted line), the magnitude of both the magnetic force and the electromagnetic force can be increased to the magnitude of the magnetic force (two-dotted line) by adding the magnetic force of the low-coercivity magnet 51. This causes the electric machine 100 to drive the rotation of the shaft 101 with a torque assisted by the magnetic torque applied to the rotor 21, which is as strong as a torque generated by a synchronous motor with neodymium magnets embedded in its rotor.

[0070] This low-coercivity magnet 51 should be magnetized to the required magnetic force if the field coil 28 is to operate as an electromagnet. If magnetization to the saturation range is not necessary, it is possible, for example, to adjust the amplitude of the pulse current to correspond to an amplitude required to generate a reaction flux strong enough to achieve magnetization to a desired magnetic force and supply the set pulse current to the armature coil 14. If inexpensive AlNiCo or cobalt magnets are used, the magnet is likely to be inadvertently demagnetized if the phase of the drive current becomes out of phase with the magnet when advancing the phase of the drive current to generate a desired torque.For this reason, it is necessary to magnetize the magnet in order to maintain an appropriate coercive force in the event of demagnetization caused by the action of an opposing magnetic field.

[0071] If the electromagnetic force generated by the field coil 28 is amplified to the saturation region by increasing the rotational speed of the rotor 21 immediately after increasing the strength of the magnetic force of the low-coercivity magnet 51, it is possible to perform field weakening control to attenuate the electromagnetic force generated by the field coil 28 in order to adjust the magnetic moment to the desired value. However, performing field weakening control can also be considered a waste of energy because it consumes energy to attenuate the magnetic force.

[0072] Therefore, when the electromagnetic force generated by the field coil 28 increases, the control unit 110 executes a control program stored in the memory 110m to demagnetize the low-coercivity magnet 51 without performing such field weakening control. Performing this control allows the execution of a control method, that is, a power supply control method, in which a direct current from the onboard battery 111 is processed by the inverter 112 and supplied to the armature coil 14 as a pulse current at an appropriate time.

[0073] As shown in the flow chart of the Fig.Specifically, as described in Figure 13, the control unit 110 executes a control process following instructions from a control program stored in the memory 110m to demagnetize the low-coercivity magnet 51 to increase the electromagnetic force generated by the field coil 28. This allows a demagnetization process of the low-coercivity magnet 51 by causing the inverter 112 to superimpose a pulse current whose maximum amplitude has a small absolute value on the drive current in the form of an alternating current and to supply the pulse current to the armature coil 14 at an appropriate time (step S21) to allow the generation of a reaction flux 15 in a direction opposite to the magnetization direction and the application or flow through the low-coercivity magnet 51.

[0074] In response to the input of the pulse current flowing in a direction opposite to the magnetization direction, the armature coil 14 generates a pulsating flux PF on the stator 11. The pulsating flux PF has a small amplitude corresponding to the amplitude of the aforementioned input pulse current and is oriented in a direction opposite to the magnetization direction to allow the interlinking of the pulsating flux PF with the associated rotor tooth 22 of the rotor 21 (step S22). When this pulsating flux PF interlinks with the associated rotor tooth 22, the associated field coil 28 on the rotor 21 generates an induced pulsating flux IF oriented in a direction opposite to the magnetization direction, allowing the interlinking of the induced flux IF with the associated stator tooth 12 (step S23).Since this induced pulsating flux IF interlinks with the stator tooth 12, the armature coil 14 generates a reaction flux RF on the stator 11 and applies the reaction flux RF, which is oriented in a direction opposite to the magnetization direction, to the low coercivity magnet 51 (step S24).

[0075] According to this method, adjusting the direction, amplitude, and timing of the pulse current applied to the armature coil 14 on the stator 11 is sufficient to magnetize the low-coercivity magnet 51, since even a pulsating flux PF induced by the application of the low-amplitude pulse current allows the magnetic field strength to be increased to provide a reaction flux RF with a necessary and sufficient magnetic field strength. In short, by superimposing a drive current with a pulse current having a low amplitude and oriented in a direction opposite to the magnetization direction, it is possible to demagnetize the low-coercivity magnet 51 from the saturation range to a desired magnetic force.

[0076] Thus, the electric machine 100 can appropriately adjust the magnetic force of the low-coercivity magnet 51, whose magnetic force strength is increased to the saturation range. In other words, the electric machine 100 according to the present embodiment can provide high efficiency and high drive torque for rotating the shaft 101 over the range from low speeds because of the magnetic torque assist derived from the magnetic force amplification generated by the field coil 28 acting as an electromagnet based on the magnetic force of the low-coercivity magnet 51.

[0077] In the electric machine 100, the low-coercivity magnet 51, magnetized to the saturation range, can be magnetized by supplying the armature coil 14 of the stator 11 with only a small pulse current having a maximum amplitude with a lower absolute value than a pulse current required for magnetization with a pulsating flux pf generated only by the armature coil 14. This eliminates the need to increase the capacitance of the electronic equipment, including the inverter 112 and the wiring. In other words, the size of the electronic equipment, including the inverter 12 and the wiring devices, can be reduced, thus allowing the size of the motor to be reduced.In addition, the vibrations and noises caused by supplying a pulse current with a large amplitude to the armature coils 14 can be reduced, enabling rotational drive of the shaft 101 by stable rotation of the rotor 21.

[0078] In the electric machine 100 according to the present embodiment, since the field coil 28 is provided within each rotor tooth 22 on the rotor 21, when a pulse current PI is supplied to the armature coil 14, the armature coil 14 generates a pulsating flux (self-induced flux) PF by self-induction, whereby the field coil 28 generates a mutual inductance flux or induced pulsating flux IF. Furthermore, when the field coil 28 generates the induced pulsating flux IF responsive to the aforementioned pulsating flux PF, this induced pulsating flux IF causes the armature coil 14 to generate a reaction flux RF responsive to the induced pulsating flux IF, which allows the magnetization of the low-coercivity magnet 51 located in each rotor tooth 22.

[0079] This enables the generation of a sufficiently large reaction flux RF for magnetizing the low-coercivity magnet 51 with only a weak pulse current, the amplitude of which is lower than the strong pulse current typically required to generate the pulsating flux pf when magnetizing the low-coercivity magnet 51. In other words, a high-quality rotational drive of the rotor 21 (shaft 101) is enabled while simultaneously reducing the size and cost of the electric machine 100.

[0080] In the present description of the present embodiment, the induction coils 27 are located on additional pole cores 24 arranged in rotor slots 25. However, this is only an example. In another example, a two-stage structure may be used, in which each of the induction coils 27 may be located on the associated rotor tooth 22 near the outer periphery 22a near the stator 11, and its associated field coil 28 may be located on the rotor tooth 22 near the shaft 101 (rotation axis).

[0081] Furthermore, the present embodiment is not limited to the structure having a diametrically spaced calibrated air gap G as in the electric machine 100, but may also have a structure having an axial calibrated gap aligned in the direction of the rotation axis.

[0082] The structures of the stator 11 and the rotor 21 are not limited to laminated structures made by lamination of electromagnetic steel plates or steel sheets. For example, the structures can use so-called soft magnetic composite cores, which can be described as powder magnetic cores produced by compression molding iron powder and heat-treating soft magnetic composite materials (SMCs) made of ferromagnetic powder particles, such as iron powder particles, surrounded by an electrically insulating film. The SMC core is suitable for a structure with axial gaps because its compression molding is simple.

[0083] Diodes 29A and 29B can rectify the induced current, but are not limited to this. Other semiconductor elements, such as switching elements, can also be mounted. Additionally, the present embodiment can be applied to an electric machine using a stator with a so-called claw-pole structure.

[0084] The applications of the electric machine 100 are not limited to the automotive sector. The electric machine 100 can also be used as a generator in wind turbines and / or as a motor in machine tools.

[0085] Although embodiments of the present invention have been described, it will be apparent to those skilled in the art that changes may be made without departing from the scope of the present invention. All such modifications and equivalents are to be considered covered by the following claims. [Description of reference symbols] 11 Stator 12 stator teeth 13 Stator slot 14 Armature coil 21 Rotor 22 rotor teeth (salient pole) 23 Rotor groove 25 Additional pole core (additional pole) 27 Induction coil (additional pole) 28, 281, 282 field coil 29A, 29B Diode (rectifier element) 51 Low coercivity magnet 51a Pole face 100 Electric Machine 101 Shaft (rotating shaft) 110 Control unit (pulse controller) 110m storage 111 On-board battery 112 inverters BF base flow G calibrated air gap IF induced pulsatile flow PF pulsating flow pf pulsating river RF reaction flow WF magnetization flux

Claims

[1] Electrical machine, comprising: a stator (11) with at least one armature coil (14); and a rotor (21) having a plurality of rotor teeth (22) with at least one field coil (28) wound around the respective rotor tooth (22), characterized by , that the rotor teeth (22) around which the field coil (28) is wound comprise a low coercivity magnet (51), and wherein a superposition of a drive current flowing through the armature coil (14) with a pulse current (PI) causes a pulse current (IC) to flow through the field coil (28) to cause the magnetization of the low coercivity magnet (51) to a predetermined magnetic field strength. [2] Electrical machine according to claim 1, wherein the rotor (21) has at least one induction coil (27) arranged to generate an induction current in response to a change in the magnetic flux derived from the flow of the drive current through the armature coil (14), and the induction coil (27) is electrically coupled to the field coil (28) such that a field current flows through the field coil (28) in response to the induction current. [3] An electric machine according to claim 1 or 2, wherein the low coercivity magnet (51) is magnetized to saturation by a magnetic field generated in response to a pulsating flux (PF) generated during the flow of the pulse current (PI) through the armature coil (14) and to an induced pulsating flux (IF) generated in response to a change in the pulsating flux (PF) through the field coil (28). [4] Electrical machine according to claim 1, further comprising: a control unit (110), wherein the control unit (110) is configured to adjust the pulse current (PI) such that the low coercivity magnet (51) is magnetized to a predetermined magnetic field strength in response to a self-induced flux (PF) generated by self-induction at the armature coil (14) from the pulse current (PI) and in response to a mutual induction flux (IF) generated by mutual inductance at the field coil (28) from the self-induced flux (PF). [5] Electrical machines according to claim 4, wherein the control unit (110) is adapted to adjust the pulse current (PI) such that the low coercivity magnet (51) is demagnetized in response to the self-induced flux (PF) and the mutual induction flux (IF) up to a predetermined magnetic field strength. [6] Method for operating an electrical machine, comprising a stator (11) with at least one armature coil (14) and a rotor (21) which has a plurality of rotor teeth (22) with a field coil (28) wound around the respective rotor tooth (22), characterized by providing the rotor teeth (22) around which the field coil (28) is wound with a low coercivity magnet (51); the provision of at least one field coil winding (21) around the rotor (28); and superimposing a drive current flowing through the armature coil (14) with a pulse current (PI) to cause a pulse current (IC) to flow through the field coil (28) to magnetize the low coercivity magnet (51) to a predetermined magnetic field strength. [7] The method of claim 6, wherein the pulse current (PI) is adjusted such that the low coercivity magnet (51) is magnetized to saturation in response to a self-induced magnetic flux (PF) due to self-induction at the armature coil (14) caused by the pulse current (PI) and in response to a counter-induction magnetic flux (IF) due to counter-induction at the field coil (28) caused by the self-induced flux (PF). [8] Method according to claim 6, wherein the superposition of the pulse current (PI) is carried out only at low speeds of the rotor (21). [9] A method according to claim 6, wherein the low coercivity magnet (51) is magnetized to saturation when the pulse current (IC) flows through the field coil (28) in response to the pulse current (PI) superimposed on the drive current.

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