Electric motor

Abstract

To reduce copper loss by combination of a movable rotor and field-weakening control.SOLUTION: An electric motor has a rotor, a stator, and a control circuit. The rotor has: a shaft; a rotor core; and a movable member which relatively rotates to the rotor core centering on the shaft between a first position and a second position according to rotational speed of the rotor. There are more leakage fluxes of the rotor core in the case that the movable member is at the second position than in the case that the movable member is at the first position. The movable member is configured to be arranged at the first position when the rotational speed is smaller than first rotational speed and to be arranged at the second position when the rotational speed is larger than the first rotational speed. The control circuit executes field-weakening control when the rotational speed is larger than second rotational speed. The first rotational speed is smaller than the second rotational speed.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The technology disclosed in this specification relates to an electric motor.

[0002] The rotor disclosed in Patent Document 1 has a fixed core and a movable core. The movable core is rotatable relative to the fixed core. The movable core is provided with a guide groove extending obliquely to the radial direction. A weight is disposed in the guide groove. When the rotation speed of the rotor increases, the weight moves radially outward due to centrifugal force. Then, the inner surface of the guide groove is pushed by the weight, and the movable core rotates relative to the fixed core. By the movable core rotating relative to the fixed core in this manner, the amount of magnetic flux extending from the rotor to the stator (hereinafter referred to as the magnetic flux amount of the rotor) decreases. By reducing the magnetic flux amount of the rotor when the rotation speed of the rotor is high, the back electromotive voltage generated in the stator is reduced. Therefore, an electric motor having this rotor can operate at a high rotation speed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-242461 A Summary of the Invention [Problem to be solved by the invention]

[0004] Another technique for reducing the back electromotive force generated in the stator is known as field-weakening control. In field-weakening control, the current flowing through the stator is controlled so that the magnetic field of the rotor is cancelled by the magnetic field generated in the stator, thereby weakening the magnetic field of the rotor compared to normal times (i.e., when field-weakening control is not performed). This reduces the back electromotive force generated in the stator, making it possible to rotate the rotor at high speed. Figure 3(b) shows conventional field-weakening control. As shown in Figure 3(b), the back electromotive force generated in the stator increases as the rotor rotation speed W increases. By performing field-weakening control at a rotation speed W3 at which the back electromotive force reaches its upper limit, the back electromotive force is reduced. Therefore, it becomes possible to rotate the rotor at a rotation speed higher than the rotation speed W3. As shown in the figure, in the rotation speed range Wa immediately before the field-weakening control is performed (i.e., immediately before the back electromotive force reaches its upper limit), the current flowing through the stator becomes high, causing copper loss.

[0005] This specification proposes a technique for reducing copper loss by combining a moving rotor and field weakening control. [Means for solving the problem]

[0006] The electric motor disclosed in this specification includes a rotor, a stator, and a control circuit that controls power supplied to the stator. The rotor includes a shaft, a rotor core fixed to the shaft, and a movable member that is attached to the shaft and rotates around the shaft between a first position and a second position relative to the rotor core depending on the rotation speed of the rotor. When the movable member is in the second position, the rotor core has more leakage magnetic flux than when the movable member is in the first position. The movable member is configured to be disposed at the first position when the rotation speed is smaller than the first rotation speed, and to be disposed at the second position when the rotation speed is larger than the first rotation speed. The control circuit performs field weakening control when the rotation speed is larger than the second rotation speed. The first rotation speed is smaller than the second rotation speed.

[0007] The leakage flux refers to the magnetic flux generated from the rotor core that returns to the rotor core via the movable member.

[0008] In this motor, the first rotation speed at which the movable member operates is lower than the second rotation speed at which the field-weakening control is executed. Therefore, when the rotation speed of the rotor increases, the movable member operates before the field-weakening control. That is, when the rotation speed of the rotor increases, the movable member operates, and then the field-weakening control is executed. When the rotation speed of the rotor increases, the movable member moves from the first position to the second position. This reduces the magnetic field of the rotor (i.e., the magnetic field reaching the stator from the rotor) and reduces the back electromotive voltage generated in the stator. This suppresses an increase in the current flowing through the stator, and suppresses copper loss. As described above, in this motor, when the rotation speed of the rotor increases, the movable member operates before the field-weakening control, thereby suppressing copper loss before the execution of the field-weakening control. [Brief description of the drawings]

[0009] [Figure 1] FIG. [Diagram 2] FIG. 4 is an explanatory diagram of the position of the movable member 40 when viewed along a direction parallel to the rotation axis. [Diagram 3] 3 is a graph showing the relationship between rotation speed, back electromotive force, and stator current, in which Fig. 3(a) shows an embodiment and Fig. 3(b) shows a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The electric motor 10 shown in Fig. 1 is mounted on an electric vehicle. The electric motor 10 includes a rotor 20, a stator 50, and a control circuit 90. The control circuit 90 operates the electric motor 10 by supplying AC power to the stator 50. The electric motor 10 drives the wheels of the electric vehicle. The stator 50 has a cylindrical shape. The rotor 20 is disposed radially inside the stator 50. The rotor 20 is disposed rotatably about a rotation axis AX.

[0011] The rotor 20 has a shaft 22, a rotor core 24, and two movable members 40. The shaft 22 is arranged so that its central axis coincides with the rotation axis AX. The rotor core 24 has a cylindrical shape. The rotor core 24 is made of a material having high magnetic permeability. The shaft 22 is inserted into a central hole of the rotor core 24. The rotor core 24 is fixed to the shaft 22. Although not shown, a plurality of magnetic poles are arranged inside the rotor core 24. The magnetic poles are, for example, permanent magnets. The multiple magnetic poles are arranged rotationally symmetrically with respect to the rotation axis AX. Non-magnetic pole portions exist between the multiple magnetic poles.

[0012] The movable members 40 are disposed on both sides of the rotor core 24 in the direction along the rotation axis AX. In other words, the rotor core 24 is disposed between the two movable members 40 in the direction along the rotation axis AX. Since each movable member 40 has the same structure, only one of the movable members 40 will be described below.

[0013] The movable member 40 has a cylindrical shape. The movable member 40 is made of a material having high magnetic permeability. The shaft 22 is inserted into a central hole of the movable member 40. The movable member 40 can rotate relatively to the shaft 22 about the rotation axis AX. As shown in FIG. 2(a), the movable member 40 has a plurality of protrusions 40a on its outer circumferential surface. In the circumferential direction, the plurality of protrusions 40a are provided at the same intervals as the plurality of magnetic poles of the rotor core 24.

[0014] As shown in FIG. 1 , a housing 42 and a frame 44 are disposed on the opposite side of the movable member 40 from the rotor core 24. The frame 44 is disposed between the movable member 40 and the housing 42. The frame 44 is fixed to a side surface of the movable member 40. Therefore, the frame 44 rotates integrally with the movable member 40 relative to the shaft 22. The housing 42 is fixed to the shaft 22. In other words, the housing 42 is fixed to the rotor core 24 via the shaft 22. Therefore, the frame 44 rotates relative to the housing 42.

[0015] A plurality of guide grooves 42a are provided on the side surface of the housing 42 facing the frame 44. As shown in FIG. 2(a), each guide groove 42a extends along the radial direction. In addition, a plurality of guide holes 44a are provided in the frame 44. As shown in FIG. 1, each guide hole 44a penetrates the frame 44 in a direction parallel to the rotation axis AX. As shown in FIG. 2(a), each guide hole 44a extends along a direction inclined with respect to the radial direction. That is, each guide hole 44a extends so as to be displaced in the circumferential direction as it moves radially outward. As shown in FIG. 1, a rod-shaped weight 46 is provided inside the rotor 20, extending from the guide groove 42a to the guide hole 44a. The weight 46 can slide radially inside the guide groove 42a. A spring 48 is provided inside each guide groove 42a. The spring 48 biases the weight 46 radially inward (i.e., toward the rotation axis AX). Therefore, when the rotor 20 is at rest, as shown in FIG. 2(a), each weight 46 is located at the end of the guide groove 42a on the inner circumferential side.

[0016] FIG. 2(a) shows a state where each weight 46 is located at the end of the inner circumference side of the guide groove 42a. FIG. 2(b) shows a state where each weight 46 is located at the end of the outer circumference side of the guide groove 42a. When each weight 46 moves radially outward in the guide groove 42a from the state shown in FIG. 2(a), each weight 46 presses the inner surface of the guide hole 44a, causing the frame 44 to rotate relative to the housing 42. Therefore, as shown in FIG. 2(b), when each weight 46 moves to the end of the outer circumference side of the guide groove 42a, the angle of the frame 44 with respect to the housing 42 changes. In this way, the frame 44 rotates relative to the housing 42 as a result of the weight 46 moving in the guide groove 42a. As described above, the frame 44 is fixed to the movable member 40, and the housing 42 is fixed to the rotor core 24. Therefore, when the frame 44 rotates relative to the housing 42, the movable member 40 rotates relative to the rotor core 24.

[0017] When the movable member 40 is in the position shown in FIG. 2(a), the protruding portions 40a do not overlap with the magnetic poles of the rotor core 24 in the axial direction. Therefore, no active short circuit occurs between the magnetic poles, and almost no leakage magnetic flux occurs. Therefore, in the state shown in FIG. 2(a), a lot of magnetic flux reaches the stator 50 from the rotor core 24. When the movable member 40 is in the position shown in FIG. 2(b), the protruding portions 40a overlap with the magnetic poles of the rotor core 24 in the axial direction. Therefore, a magnetic path is formed through the protruding portions 40a, and a short circuit occurs between the magnetic poles. Therefore, in the state shown in FIG. 2(b), a lot of leakage magnetic flux occurs, and a little magnetic flux reaches the stator 50 from the rotor core 24. Hereinafter, the position shown in FIG. 2(a) is referred to as a non-shorting position, and the position shown in FIG. 2(b) is referred to as a shorting position.

[0018] When the electric motor 10 is driven, the entire rotor 20 rotates around the rotation axis AX. When the rotation speed of the rotor 20 is low, the centrifugal force acting on each weight 46 is small, so that each weight 46 is supported by the spring 48 at the position shown in FIG. 2(a). Therefore, the movable member 40 is disposed in a non-shorting position. Therefore, the leakage magnetic flux of the rotor 20 is small, and the magnetic flux reaching the stator 50 from the rotor 20 is large.

[0019] When the rotation speed of the rotor 20 increases, the centrifugal force acting on each weight 46 increases. When the rotation speed of the rotor 20 exceeds a certain value, each weight 46 moves toward the outer periphery in the guide groove 42a against the spring 48, as shown in FIG. 2(b). Therefore, the frame 44 and the movable member 40 rotate relative to the housing 42 and the rotor core 24. This causes the movable member 40 to move to the short-circuited position. As a result, the leakage flux of the rotor 20 increases, and the magnetic flux reaching the stator 50 from the rotor 20 decreases. Thereafter, when the rotation speed of the rotor 20 decreases, the spring 48 returns each weight 46 to the position shown in FIG. 2(a), and the movable member 40 returns to the non-shorting position.

[0020] As described above, in the rotor 20 of the embodiment, the leakage flux increases during high speed rotation and decreases during low speed rotation. During high speed rotation, the leakage flux increases and the magnetic flux reaching the stator 50 decreases, so that the back electromotive force generated in the stator 50 can be reduced. Therefore, the maximum rotation speed of the rotor 20 can be improved.

[0021] 3(a) indicates the rotation speed of the rotor 20 when the movable member 40 rotates between the non-shorting position and the short-circuiting position. When the rotation speed W of the rotor 20 is lower than the first rotation speed W1, the movable member 40 is disposed in the non-shorting position. When the rotation speed W of the rotor 20 is equal to or higher than the first rotation speed W1, the movable member 40 is disposed in the short-circuiting position. The first rotation speed W1 can be adjusted by the mass of the weight 46 and the spring constant of the spring 48, etc.

[0022] The control circuit 90 supplies an AC current to the stator 50 to rotate the rotor 20. The control circuit 90 controls the frequency and amplitude of the AC current supplied to the stator 50 to control the torque applied to the rotor 20 and the rotation speed of the rotor 20. The control circuit 90 can also perform normal control and field-weakening control. In normal control, the electrical angle is controlled so that a high torque is applied to the rotor 20. In field-weakening control, the electrical angle is controlled so that the magnetic field of the rotor 20 is canceled by the magnetic field generated by the stator 50. Therefore, in field-weakening control, the back electromotive force generated in the stator 50 is smaller than in normal control. Therefore, when field-weakening control is performed, the torque applied to the rotor 20 is reduced, while the rotor 20 can be rotated at a higher speed.

[0023] 3(a) indicates the rotation speed of the rotor 20 when control is switched between normal control and field-weakening control. When the rotation speed W of the rotor 20 is lower than the second rotation speed W2, the control circuit 90 executes normal control. When the rotation speed W of the rotor 20 is equal to or higher than the second rotation speed W2, the control circuit 90 executes field-weakening control.

[0024] FIG. 3(a) shows the operation of the electric motor 10 of the embodiment. FIG. 3(b) shows the operation of an electric motor in which the rotor does not have a movable member 40. First, as a comparative example, the operation of the electric motor shown in FIG. 3(b) will be described. In FIG. 3(b), the field weakening control is executed when the rotation speed W of the rotor is equal to or higher than the rotation speed W3. As shown in FIG. 3(b), as the rotation speed W of the rotor increases, the back electromotive voltage generated in the stator increases. When the back electromotive voltage reaches an upper limit, the rotation speed of the rotor cannot be increased any more. Therefore, in FIG. 3(b), the field weakening control is started at the timing when the back electromotive voltage reaches an upper limit. When the field weakening control is executed, the back electromotive voltage decreases, and it becomes possible to increase the rotation speed of the rotor. The current flowing through the stator increases in the rotation speed zone Wa immediately before the back electromotive voltage reaches the upper limit. For this reason, high copper loss occurs in the rotation speed zone Wa. Thus, in the comparative example, a problem occurs in that high copper loss occurs before the field weakening control is started.

[0025] As shown in FIG. 3(a), in the electric motor 10 of the embodiment, the first rotation speed W1 at which the movable member 40 operates is set to a value smaller than the second rotation speed W2 at which the field-weakening control is disclosed. Therefore, when the rotation speed W of the rotor 20 increases, the movable member 40 moves from the non-shorted position to the shorted position at an earlier stage (i.e., the first rotation speed W1) than the stage at which the back electromotive voltage reaches the upper limit value, and the magnetic flux of the rotor 20 decreases. Therefore, the back electromotive voltage decreases at the first rotation speed W1. Therefore, even if the rotation speed W of the rotor 20 is further increased from the first rotation speed W1, the current flowing through the stator 50 does not increase. This suppresses copper loss. When the rotation speed W of the rotor 20 reaches the second rotation speed W2, the field-weakening control is executed by the control circuit 90. Therefore, it is possible to rotate the rotor 20 at a higher rotation speed than the first rotation speed W1.

[0026] As described above, in the electric motor 10 of the embodiment, the increase in the current flowing through the stator 50 can be suppressed before the execution of the field weakening control. Therefore, the copper loss can be suppressed. Therefore, the electric motor 10 of the embodiment can operate with high efficiency.

[0027] Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or drawings achieves multiple objectives simultaneously, and achieving one of the objectives itself has technical utility. [Explanation of symbols]

[0028] 10: Electric motor 20: Rotor 22: Shaft 24: Rotor core 40: Movable parts 42: Housing 44: Frame 46: Weight 48: Spring 50: Stator 90: Control circuit

Claims

[Claim 1] An electric motor, A rotor, A stator; A control circuit that controls the power supplied to the stator; having The rotor is A shaft, A rotor core fixed to the shaft; a movable member attached to the shaft and configured to rotate about the shaft relative to the rotor core between a first position and a second position in response to a rotational speed of the rotor; having When the movable member is in the second position, the leakage magnetic flux of the rotor core is greater than when the movable member is in the first position, the movable member is configured to be disposed at the first position when the rotational speed is less than a first rotational speed, and to be disposed at the second position when the rotational speed is greater than the first rotational speed; The control circuit performs a field weakening control when the rotation speed is greater than a second rotation speed, The first rotation speed is smaller than the second rotation speed. Electric motor.

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