JP2016131468A - Axial gap type rotary electric machine - Google Patents

Abstract

An object of the present invention is to obtain an inexpensive and highly reliable rotor yoke that can reduce eddy current in a rotor yoke portion.
A rotor has a rotor and a stator, and the rotor has a rotor back yoke and a rotor magnet. The rotor back yoke has a rotor magnet on a joint surface with the rotor magnet in the circumferential direction of the rotor back yoke. A plurality of grooves are provided, and the plurality of grooves form a plurality of protrusions on the rotor back yoke, the residual magnetic flux density of the rotor magnet is Br, the average magnet circumferential length l per pole of the rotor magnet, and the plurality of protrusions An axial gap type rotating electrical machine satisfying l · Br <4H · R where H is the height of H and R is the ratio of the plurality of protrusions to the plurality of grooves.
[Selection] Figure 1

Description

  The present invention relates to an axial gap type rotating electrical machine.

  Reduction of iron loss is one of the effective means for improving the efficiency of rotating electrical machines. Iron loss occurs mainly in the iron core that serves as an electromagnet. However, in a permanent magnet motor that uses a permanent magnet on the rotor side, iron loss may also occur in the surrounding magnetic part that holds the permanent magnet. Are known. In the radial gap type motor, in order to reduce this iron loss, electromagnetic steel sheets having a thickness of 0.35 mm and 0.5 mm were laminated so that an eddy current does not flow in the axial direction even in the portion holding the magnet. Structures that use things are common.

  Patent Document 1 proposes a structure in which an electromagnetic steel plate or an amorphous metal foil strip is used for the magnet holding member of the rotor of the axial gap motor using the same method as described above.

  On the other hand, in a radial gap type motor, there is an example in which a design in which a magnetic body and a structural body are shared is used for a holding member of a rotor side magnet. For example, it is a structure in which a magnet is adhered and held on the surface of a magnetic shaft such as iron. In the case of this structure, since the magnetic flux generated from the magnet is a direct current magnetic field, no loss occurs in the shaft magnetic body portion. However, the magnetic flux generated by the stator-side coil generates a magnetic flux that changes the magnetic flux beyond the magnet portion to the magnetic body. At this time, an eddy current for canceling the fluctuation flows in the magnetic part according to the change in magnetic flux, causing iron loss called eddy current loss.

  In order to cope with this, Patent Document 2 attempts to reduce the loss by using a method in which the surface of the shaft is spirally processed and an adhesive containing magnetic powder is used. Also in the case of an outer rotor type motor, the method described in Patent Document 3 is used to reduce the loss by providing the same groove as described above.

Japanese Patent No. 5502463 Japanese Patent Laid-Open No. 11-332147 JP 2007-074776 A

  Patent Document 1 discloses a structure in which an amorphous metal foil strip having a thickness of about 0.025 mm is wound up as an iron core as a magnet holding member that serves as a back yoke (a yoke part) of a rotor. In this structure, mechanical strength is required for the rotor yoke to mechanically hold the rotor disk with the rotating shaft that rotates. For this reason, the rotor yoke is made of a high-strength material such as an iron alloy, and the portion requiring magnetism is a wound core (amorphous spiral laminated band), and a permanent magnet for the rotor is bonded to the wound core. The structure is held using a method. In this configuration, in addition to the magnet, a wound iron core and a rotor yoke and three parts are required, and in order to integrate them, a fixing means such as an adhesive is required, which increases the number of parts, Since the stress at the time of winding changes due to the passage of time, external stress, vibration, etc., the reliability such as adhesion peeling is reduced, the man-hours for manufacturing the wound iron core are increased, and the number of parts is also increased Deterioration in assemblability due to an increase is a problem.

  Patent Documents 2 and 3 propose a structure in which the rotor yoke also serves as the rotor back yoke. This method can also be applied to an axial gap type motor. However, when this method is applied, the same problem arises with the radial gap type, but the rotor yoke must be cut using a lathe or the like. Cutting is performed while setting each rotor yoke on a lathe and adjusting the dimensions thereof, which is very time consuming and poor in productivity. In addition, when performing deep groove processing, a narrow processing tool is required, and in this case, there is a problem that the processing speed becomes slow. Furthermore, after the processing, it is necessary to clean the machine oil adhering to the processing, the impression, and the rust prevention, which causes an increase in cost.

  An object of the present invention is to obtain an inexpensive and reliable rotor yoke that can reduce eddy currents in a rotor yoke portion.

  The features of the present invention for solving the above problems are as follows, for example.

  The rotor has a rotor and a stator, and the rotor has a rotor back yoke and a rotor magnet. In the circumferential direction of the rotor back yoke, a plurality of grooves are formed on the joint surface of the rotor back yoke with the rotor magnet. An axial gap type rotating electrical machine provided.

  According to the present invention, since the rotor yoke is composed of one component (material) except for the magnet, the number of manufacturing steps can be reduced, and the number of bonded components can be reduced. Improvement in reliability can be expected. Moreover, the reduction of the eddy current of the initial purpose and securing of the induced voltage can be achieved at the same time, and the motor efficiency can be increased. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is a perspective view of the rotor of the axial gap type rotary electric machine which concerns on one Embodiment of this invention. The structure of the rotary electric machine which concerns on one Embodiment of this invention is shown. It is a graph which shows the experimental result for demonstrating the effect of the groove | channel structure which concerns on one Embodiment of this invention. FIG. 4 is a cross-sectional comparison diagram of structures illustrating various structures described in FIG. 3. It is a conceptual diagram explaining the forge manufacturing method which concerns on one Embodiment of this invention. It is a conceptual diagram which shows the transfer press manufacturing method which concerns on one Embodiment of this invention. It is a conceptual diagram which shows the manufacturing method by the powder metallurgy technique which concerns on one Embodiment of this invention. The various variation shape of the groove shape which concerns on one Embodiment of this invention is shown. It is a perspective view which shows the application example of the axial gap type motor to which the structure of this invention is applied.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

  FIG. 1 is a perspective view of a rotor of an axial gap type rotating electrical machine according to an embodiment of the present invention. FIG. 1A is a perspective view of a rotor, and FIG. 1B is a perspective view of a rotor back yoke.

  A rotor 2 of the axial gap type rotating electrical machine shown in FIG. 1A is a rotor having a permanent magnet. The rotor 2 includes a rotor back yoke 11 and a rotor magnet 20, and is configured by attaching the rotor magnet 20 to the rotor back yoke 11. The rotor magnet 20 and the rotor back yoke 11 are integrated by a method such as adhesion. The rotor back yoke 11 is made of a soft magnetic material such as iron.

  The rotor magnet 20 includes a rotor magnet N pole 21 and a rotor magnet S pole 22. A rotor magnet N pole 21 and a rotor magnet S pole 22 paired with the N pole and the S pole are periodically arranged in the rotation direction. A rotor shaft hole 12 for fastening with a shaft is formed at the center of the rotor 2.

  In FIG. 1A, the ring-shaped disk-shaped magnet has a structure in which 10 magnetic poles are arranged in the circumferential direction. One magnetic pole has a fan shape and is arranged on the circumference at an equiangular pitch. This magnetic pole is assembled as a rotor 2 in the form shown in FIG. 1A, and is then magnetized by an electromagnet called a magnetized yoke to constitute such a magnetic pole. A similar rotor structure can be obtained by attaching one magnet or a plurality of divided magnets.

  In one embodiment of the present invention, as shown in FIG. 1B, a plurality of grooves 13 are provided on the surface (joint surface) of the disc-like rotor back yoke 11 with the rotor magnet 20. Yes. In the figure, a case where a plurality of concentric grooves 13 are provided on the surface of the disk-like rotor back yoke 11 to which the rotor magnet 20 is attached is shown. By forming the concentric groove 13, a protrusion 113 is formed on the rotor back yoke 11. The grooves 13 can reduce eddy currents generated by changes in magnetic flux density generated by energization of the stator coils.

  In FIG. 2, the structure of the rotary electric machine which concerns on one Embodiment of this invention is shown. FIG. 2A shows the configuration of an axial gap type rotating electrical machine. The axial gap type rotating electrical machine 1 has two rotors 2 in the axial direction. A rotor magnet 20 is disposed on each rotor 2.

  The stator 3 is arranged at the center in the axial direction, and is composed of a stator core 15 having a substantially fan-shaped cross section like the rotor magnet 20 and a stator coil 18 wound around the stator core 15. . The magnetic flux is based on the principle that an induced voltage is generated in the stator coil 18 by flowing into the stator core 15 through the gap on the axial surface and interlinking with the stator coil 18.

  The housing 14 holds the outer peripheral portion of the stator 3. The shaft 17 connects the rotors 2 on both sides in the axial direction and is arranged to output the shaft. The lead wire 19 forms a terminal portion by three-phase stator coils 18 being electrically delta-connected or star-connected.

  FIG. 2B schematically shows the flow of magnetic flux on a plane. The magnetic flux emitted from the rotor magnet N pole 21 of the rotor 2 passes through the stator core 15, flows in the axial direction, and flows into the rotor magnet S pole 22 arranged on the opposite side. In the rotor back yoke 11, the magnetic flux of the magnet flows from the rotor magnet N pole 21 in the circumferential direction and flows into the rotor magnet S pole 22 adjacent in the circumferential direction. Since the magnetic flux generated by the magnet is a constant magnetic field, the magnetic field fluctuation is basically small in the rotor back yoke 11, but due to the change in magnetic flux density due to the slots of the stator 3 and the influence of the stator winding current, It is known that a minute change in magnetic flux density occurs near the interface between the rotor magnet 20 and the rotor back yoke 11. For this reason, the rotor back yoke 11 generally employs a laminated iron core such as an electromagnetic steel plate in a radial type motor.

  In the axial gap type rotating electrical machine, when a laminated iron core of electromagnetic steel sheets is adopted for the rotor back yoke 11, a structure in which the laminated structure is laminated in a direction perpendicular to the flow of magnetic flux is required. For this reason, as shown in Patent Document 1 shown above, a magnetic steel sheet iron core wound around the circumference and a rotor back yoke such as an amorphous foil band wound iron core are required. Since these structures cause an increase in the number of parts, an increase in man-hours, and a decrease in reliability, a simpler structure is required. For this reason, the eddy current is reduced by the groove structure described above.

  FIG. 2C shows a magnet shape for one pole of the axial gap type rotating electric machine. According to the magnetic flux lines shown in FIG. 2B, the magnetic flux flows in half from the center of the rotor magnet 20 and is divided in the left-right direction. Therefore, the magnetic flux of the rotor magnet 20 has half of the magnetic pole surface area. You can see that it flows to the next. For this reason, since the total amount of the magnetic flux can be expressed by the product of the magnetic flux density and the surface area, the residual magnetic flux density Br of the rotor magnet 20, the average magnet circumference l per pole of the rotor magnet 20, and the rotor From the width W of the magnet 20, it can be considered to be about W · l / 2 · Br.

  FIG. 2D shows a cross-sectional shape of the groove portion of the rotor back yoke. Since it is considered that eddy current can be prevented by passing only the protrusion 113 as a magnetic path for passing the magnetic flux between adjacent magnets, the height H of the protrusion 113, the protrusion 113, and the gap (groove 13). The ratio of the cross-sectional area of the protrusion 113 is H · W · R. When the material of the rotor back yoke 11 is iron and the magnetic flux density is 2T, the amount of magnetic flux that can be flowed can be expressed by 2H · W · R. Considering that the magnetic flux of the previous rotor magnet 20 flows only to the protrusion 113, it can be seen that this is satisfied when the relationship of l · Br <4H · R is satisfied.

  In the example of the rotating electrical machine shown in FIG. 2A, the rotor magnet 20 has an outer diameter of φ63 and an inner diameter of φ29, which is a 10-pole magnet. At this time, the average magnet circumference l per pole is about 14 mm. Since this magnet uses a ferrite magnet, assuming that the residual magnetic flux density Br is 0.45 T and the distance between the projection 113 and the groove 13 is equal, R = 0.5. The length is about 3 mm or more. In the dimensional relationship of FIG. 2A, eddy current can be prevented by setting the protrusion 113 to 3 mm or more.

  FIG. 3 is a graph showing experimental results for explaining the effect of the groove structure according to the embodiment of the present invention. 3 (a) to 3 (d) show an induced voltage (FIG. 3 (b)) in which the rotating electrical machine shown in FIG. 2 (a) is actually manufactured and the material and shape of the rotor back yoke are changed. )), No-load loss (FIG. 3 (a)), and the results of measuring the motor efficiency (FIG. 3 (d)) when the rotating electrical machine is driven at a constant voltage. FIG. 3C shows the relationship between torque and rotational speed when the rotating electrical machine is driven with a constant voltage.

  Each dimensional relationship has a shape shown in FIG. FIG. 4 is a cross-sectional comparison diagram of structures for explaining various structures described in FIG. FIG. 4A shows a magnetic steel sheet or amorphous steel using a yoke of a spiral iron core 10, FIG. 4B shows a solid iron used as a yoke, FIG. 4C and FIG. A groove structure according to an embodiment of the invention is employed.

  As a rotor yoke of an axial gap type rotating electric machine, all of those using solid iron as a yoke, those using a yoke of a magnetic steel sheet wound iron core, and those employing a groove structure according to an embodiment of the present invention are obtained. There was no significant change in the effective value of the induced voltage and the shape of the waveform. However, in the measurement result of the no-load loss, the loss of the iron solid yoke and the grooved yoke is larger than that of the magnetic steel sheet wound core yoke. This is considered to be an increase in yoke loss due to a change in magnetic flux density due to slot harmonics.

  As for the efficiency when the rotating electrical machine is driven, the magnetic steel sheet-wound iron core has the highest efficiency, and the solid iron yoke has the lowest efficiency. This is an effect of yoke loss due to magnetic flux change due to the stator winding current. It has been confirmed that the grooved rotor yoke according to the embodiment of the present invention has improved efficiency as compared with the solid iron yoke, and has an effect of reducing the yoke loss. Further, in relation to the projection dimensions, it was confirmed that the smaller the sectional area of the projection, the more effective. As a result of this experiment, the cross-sectional area of the projection is small relative to the surface area of the magnet, and the magnetic flux also passes through the solid part of the rotor yoke, so the loss reduction effect is small. Therefore, it is considered that the loss reduction effect is equivalent to that of the magnetic steel sheet wound iron core.

  An example of a method for manufacturing this protrusion structure is shown in FIG. FIG. 5 is a conceptual diagram illustrating a forging manufacturing method according to an embodiment of the present invention. As described above, the method using a wound iron core such as an electromagnetic steel sheet causes an increase in manufacturing cost and a decrease in reliability. Therefore, the rotor yoke portion is desired to be formed of an integral body of the same material. However, since post-processing increases the number of man-hours, it is necessary to create it by a single process. FIG. 5 shows a means for obtaining a structure according to an embodiment of the present invention from a blank by forging by pressing.

  FIG. 5A shows a blank 31 as a material. Prepare an iron-based material that has a substantially disk shape and can be a magnetic material. This material can be prepared inexpensively by pressing or cutting out from a bar. In the following description, iron is used as an example, but alloys such as magnetic stainless steel, nickel alloy, and cobalt alloy may be used in addition to iron. .

  As shown in FIG. 5B, this material is placed in a mold for plastic forming and subjected to forging, and a desired shape is formed by applying a forming force to the mold. In FIG.5 (b), 32 is a base part of a press installation. Reference numeral 33 denotes an operating die of the press facility. Reference numeral 34 denotes a punch for forming a central hole of the upper mold attached to the operating die 33. Reference numeral 35 denotes a punch for forming the upper mold groove 13. At this time, since the upper punch 35 for forming the groove becomes thin, it is necessary to give strength to buckling or the like. For this reason, when forming by such a method, in order to improve the strength of the base portion of the punch, a structure with an R added to the base portion, or as shown in FIG. It is desirable to take measures such as providing a taper angle (10 ° in this figure). Therefore, the groove shape when formed by this method is as shown in FIG. In this way, a plurality of grooves are formed by applying stress in the thickness direction of the rotor back yoke.

  FIG. 6 shows an example of manufacturing from a flat plate. FIG. 6 is a conceptual diagram showing a transfer press manufacturing method according to an embodiment of the present invention. FIG. 6 shows a manufacturing method in which the final shape is obtained at the third processing station 44 by repeating the forming from the hoop-like material through the presses of the first forming station 42 and the second processing station 43. Since this method can be configured to undergo a plurality of pressing steps, there is an advantage that punching of the center hole and adjustment of the final accuracy can be performed.

  In FIG. 6, reference numeral 41 denotes a hoop-shaped iron material. Reference numeral 42 denotes a first molding station. In the first molding station 42, a center hole is formed. Reference numeral 43 denotes a second processing station. In the second processing station 43, processing for attaching a seating surface for securing dimensions for holding the magnet is performed. Reference numeral 44 denotes a third processing station. The third processing station 44 performs groove processing by plastic processing (forging processing). In this way, it is possible to obtain a high quality product shape (high accuracy) by dividing each processing step.

  FIG. 7 shows still another manufacturing method. FIG. 7 is a conceptual diagram showing a manufacturing method using a powder metallurgy technique according to an embodiment of the present invention. 51 shows the iron powder arrange | positioned in the metal mold | die for compression molding. Reference numeral 52 denotes a base plate of a compression molding press facility. Reference numeral 53 denotes an operation die of a compression molding press facility. Reference numeral 54 denotes a compression molding punch attached to the operating die 53.

  An iron powder that can be a magnetic material is inserted into a lower mold having a cylindrical cylinder, and compression molding is performed with an upper punch that can be molded into a grooved rotor yoke shape according to an embodiment of the present invention. Thereafter, firing is performed to obtain a rotor yoke as a sintered part. In this method, it is possible to obtain a part by molding with a relatively low compressive force as compared with the method of obtaining a shape by plastic deformation described above. For the time being, it is possible to reduce the compressive force by molding it to a density that is not so high, but since iron is bonded and contracted by sintering, a molded body of high density is finally obtained. . In this method, the depth and shape of the groove can be configured relatively freely.

  Since the manufacturing method shown above is an axial gap type rotor yoke, it can be manufactured by molding. This is because the magnet attachment surface is on the wide side of the disk. In the case of a radial type motor, both the inner rotor type and the outer rotor type have a cylindrical surface. .

  FIG. 8 shows various variations of the groove shape according to the embodiment of the present invention.

  FIG. 8A shows an example in which the groove 13 is also inserted in the radial direction. In FIG. 8A, a plurality of grooves 13 are provided on the joint surface of the rotor back yoke 11 with the rotor magnet 12 in the radial direction of the rotor back yoke 11. In order to suppress the generation of eddy currents, the effect is higher as it is finely divided. Therefore, it is effective to make the shape of the protrusion 113 fine. Therefore, according to the flow of the magnetic flux shown in FIG. 2B, the center of the magnetic pole of the rotor magnet 20 is a place where the magnetic flux does not flow. By dividing 113, the projection 113 can be subdivided without hindering the flow of magnetic flux.

  FIG. 8B shows a form in which the width of the protrusion 113 is changed. Previously, the amount of magnetic flux flowing was described as an average circumference, but in reality, the larger the diameter, the greater the magnetic flux. For this reason, the ratio R between the protrusion 113 and the groove 13 is not set to one ratio, and where the diameter is large, the width of the protrusion 113 is increased, or the length of the protrusion 113 is increased (the groove 13 is deepened). The structure which increases the quantity of the magnetic flux which flows into the protrusion 113 is shown.

  In FIG. 8 (c), the guide portion 60 holding the rotor magnet 20 is simultaneously used by utilizing the characteristics that can be manufactured by forging or powder metallurgy, as shown in FIG. Indicates making. According to this structure, effects such as maintaining the anti-centrifugal strength of the rotor magnet 20 and improving the positioning accuracy of the rotor magnet 20 can be expected.

  FIG. 8D shows a structure having protrusions 213 also on the rotor magnet 20 side. In FIG. 8D, the rotor magnet 20 is provided with a plurality of protrusions 213 at portions corresponding to the plurality of grooves 13 in the circumferential direction of the rotor back yoke 11. In the case where the rotor magnet 20 is also a sintered magnet, it is manufactured with a mold, so that the protrusion 213 can be configured relatively easily. Generally, the surface of a magnet is polished to adjust the dimensions. Since this processing takes man-hours, the portion with the protrusion is used without being processed. The rotor back yoke 11 according to the embodiment of the present invention is also assembled with the gap opposing surface of the rotor magnet 20 that has been polished only on one surface to ensure accuracy, without post-processing and with no post-processing, and rotated. A member for fixing such as an adhesive is poured between the child back yoke 11 and the rotor magnet 20. Since the surface of the rotor magnet 20 and the surface of the rotor back yoke 11 are rugged, a large bonding area can be secured and strength can be improved.

  Next, FIG. 8E shows an example of a bonded magnet in which the rotor magnet 20 is formed by injection molding. In FIG. 8E, the material of the bond magnet is also disposed in the groove 13 of the rotor back yoke 11 by injection molding. Since the bonded magnet is formed by direct injection molding on the rotor back yoke 11, man-hours and losses can be reduced. Since the bond magnet has a large specific resistance value of the magnet, magnet loss such as eddy current can be reduced.

  FIG. 8F shows that the grooves 13 and the protrusions 113 are also formed on the surface opposite to the surface of the rotor back yoke 11 that is attached to the rotor magnet 20 when the manufacturing method according to the embodiment of the present invention is used. Can be formed. In other words, in the circumferential direction of the rotor back yoke 11, a plurality of grooves 13 are provided on the surface of the rotor back yoke 11 opposite to the joint surface with the rotor magnet 20. For this reason, since a wind is received as a rotary body, it is also possible to obtain a rotor with a high heat dissipation effect as a heat dissipation fin.

  FIG. 9 shows a system usage example of an axial gap type rotating electrical machine using a rotor shape according to an embodiment of the present invention. It can be incorporated into a mechanical device such as a pump or a fan, or applied to a flywheel device. Since the rotor loss is small, a product with high motor efficiency, in other words, energy saving can be realized.

DESCRIPTION OF SYMBOLS 1 Axial gap type rotary electric machine, 2 Rotor, 3 Stator, 10 Spiral iron core, 11 Rotor back yoke, 12 Rotor shaft hole, 13 Groove, 14 Housing, 15 Stator iron core, 17 Shaft, 18 Stator coil, 19 Lead wire, 20 Rotor magnet, 21 Rotor magnet N pole, 22 Rotor magnet S pole, 31 Blank, 32 Base part, 33 Working die, 34 35 Punch, 41 Iron material, 42 First forming station, 43 2nd processing station, 44 3rd processing station, 51 iron powder, 52 base plate, 53 working die, 54 compression molding punch, 60 guide part, 113 213 protrusion

Claims (8)

Having a rotor and a stator,
The rotor has a rotor back yoke and a rotor magnet;
An axial gap type rotating electrical machine in which a plurality of grooves are provided in a joint surface of the rotor back yoke with the rotor magnet in a circumferential direction of the rotor back yoke. In Claim 1, The said rotor back yoke is disk shape,
An axial gap type rotating electrical machine in which the plurality of grooves are formed by applying a stress in the thickness direction of the rotor back yoke. The axial gap type rotating electrical machine according to claim 1, wherein the rotor back yoke is made of a soft magnetic material. A plurality of projections are formed on the rotor back yoke by the plurality of grooves according to any one of claims 1 to 3.
The residual magnetic flux density of the rotor magnet is Br, the average magnet circumference l per pole of the rotor magnet, the height of the plurality of protrusions is H, and the ratio of the plurality of protrusions to the plurality of grooves is R. Axial gap type rotating electrical machine satisfying l · Br <4H · R. The axial gap type rotating electrical machine according to any one of claims 1 to 4, wherein a plurality of grooves are provided in a joint surface of the rotor back yoke with the rotor magnet in a radial direction of the rotor back yoke. The axial gap type rotating electrical machine according to any one of claims 1 to 4, wherein in the rotor magnet, a plurality of protrusions are provided in portions corresponding to a plurality of grooves in a circumferential direction of the rotor back yoke. In any one of Claim 1 thru | or 4, The said rotor magnet is a bond magnet,
An axial gap type rotating electrical machine in which the bond magnet is disposed in a plurality of grooves in the circumferential direction of the rotor back yoke. 8. The axial gap type rotation according to claim 1, wherein in the circumferential direction of the rotor back yoke, a plurality of grooves are provided on a surface of the rotor back yoke opposite to the joint surface with the rotor magnet. Electric.