How to stop a permanent magnet motor

Abstract

To provide a stop method of an electric motor capable of swiftly starting the electric motor without detecting the initial position of the rotor when starting the electric motor next time.SOLUTION: The method includes the steps of: executing a deceleration operation to reduce the rotation speed of a rotor 15 of a permanent magnet-type electric motor 1 in steady operation; and after the rotation speed of the rotor 15 drops below a threshold, executing a DC excitation operation to fix the position of the rotor 15 by flowing a direct current through a stator winding 17 of the permanent magnet-type electric motor 1.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for stopping a permanent magnet motor, such as an SPM (Surface Permanent Magnet) motor or an IPM (Interior Permanent Magnet) motor, which has a permanent magnet in its rotor. [Background technology]

[0002] When starting a permanent magnet motor from a stopped state, it is necessary to flow an appropriate current based on the rotor's initial position, so it is necessary to detect the initial position.When the motor is stopped, no electromotive force is generated due to the rotation of the rotor, so various technologies have been proposed for detecting the rotor's position.

[0003] In a motor where inductance does not change with rotor angle, the shape of the magnetic path is symmetrical when viewed from the center line of the magnetic path, regardless of the rotor angle. As a result, there is no mutual induction between orthogonal windings. However, if inductance changes with rotor angle, the shape of the magnetic path will not be symmetrical when viewed from the center line of the magnetic path depending on the angle, and as a result, mutual induction will occur between orthogonal windings. If this property is used in a motor with salient poles (where the d-axis inductance and q-axis inductance are different), when the windings are excited to generate an alternating magnetic field, an electromotive force will appear in the winding that is orthogonal to the alternating magnetic field depending on the rotor angle. The magnitude of this electromotive force changes depending on the rotor position.

[0004] When the direction of the alternating magnetic field coincides with the rotor's d-axis, the shape of the magnetic path of each of the two windings that are perpendicular to the d-axis is symmetrical when viewed from the center line of the magnetic path. As a result, no mutual induction occurs, and no electromotive force due to mutual induction is observed. Furthermore, because the polarity of the mutual induction changes at the angle where the direction of the alternating magnetic field coincides with the rotor's d-axis, the phase of the observed electromotive force also reverses at this angle.

[0005] Therefore, by applying an alternating magnetic field at a frequency sufficiently higher than the motor's drive frequency and searching for the angle at which the electromotive force in the orthogonal direction is at a minimum while rotating the alternating magnetic field, the rotor's d-axis position can be estimated. However, since this does not identify the direction of the d-axis magnetic poles, a pulse voltage is then applied in both positive and negative directions in the d-axis direction, and the magnetic pole direction is identified from the difference between the positive and negative response currents caused by magnetic saturation. By performing this operation sequence, the rotor's initial position can be estimated, and the motor can be started properly. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-39227 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned operation sequence requires a certain amount of time to detect the rotor's initial position, resulting in a delay in starting the motor, for example, of about 0.1 to 1 second. Depending on the application of the motor, this delay in starting can cause operational problems. For example, if the motor is connected to a pump in a water supply system, the delay in starting the motor can lead to fluctuations in the water supply pressure.

[0008] Therefore, the present invention provides a method for stopping an electric motor that can quickly start the electric motor without the need to detect the initial position of the rotor the next time the electric motor is started. [Means for solving the problem]

[0009] In one aspect, there is provided a method for stopping a permanent magnet motor during steady-state operation, the method comprising: performing a deceleration operation to reduce the rotational speed of a rotor of the permanent magnet motor during steady-state operation; and, after the rotational speed of the rotor falls below a threshold, performing a DC excitation operation to apply a DC current to a stator winding of the permanent magnet motor to fix the position of the rotor.

[0010] In one embodiment, the DC excitation operation is performed after the rotational speed of the rotor falls below a threshold value and before the rotational speed of the rotor becomes zero. In one aspect, the method further includes, after the rotational speed of the rotor falls below a threshold value and before the DC excitation operation, performing a pre-excitation operation in which a current is passed through the stator windings to continuously or intermittently change the direction of the magnetic field generated by the stator windings. In one aspect, the method further includes, after the DC excitation operation and before the next start of the permanent magnet motor, performing a start preparation operation of passing a DC current through the stator windings. [Effects of the Invention]

[0011] In the process of stopping an electric motor from an operating state, if the rotor's stopping position is identified and the electric motor is stopped, there is no need to estimate the rotor position the next time the electric motor is started. This means that the process of estimating the rotor's initial position can be omitted at start-up, and as a result, the electric motor can be started quickly without delay. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating an embodiment of an electric motor system. [Figure 2] 1 is a cross-sectional view showing an embodiment of a permanent magnet type electric motor. [Figure 3] FIG. 10 is a cross-sectional view illustrating a DC excitation operation. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing one embodiment of an electric motor system. The electric motor system includes a permanent magnet electric motor 1, an inverter 3 that supplies a variable frequency current to the permanent magnet electric motor 1, a current control unit 5 that controls the operation of the inverter 3, and a current measuring device 7 that measures the current flowing from the inverter 3 to the permanent magnet electric motor 1.

[0014] The current measuring device 7 is connected to the current control unit 5, and the measured current value is sent to the current control unit 5. The current control unit 5 is configured to generate a current command value for the current to be supplied to the permanent magnet electric motor 1 based on the measured current value and a speed command value from a higher-level operation control unit (not shown), and to provide the current command value to the inverter 3. An example of the current control unit 5 is a vector control unit. The inverter 3 generates a current (in this embodiment, a three-phase AC current) in accordance with the current command value, and supplies the current to the permanent magnet electric motor 1.

[0015] Fig. 2 is a cross-sectional view showing one embodiment of the permanent magnet electric motor 1 shown in Fig. 1. As shown in Fig. 2, the permanent magnet electric motor 1 includes a rotor 15 having permanent magnets 12 and a stator 16 for generating a rotating magnetic field. The permanent magnets 12 are fixed to the rotor 15 and rotate integrally with the rotor 15. The stator 16 has a plurality of stator windings 17 and a stator core 18 arranged to surround the rotor 15. The stator windings 17 are attached to the plurality of teeth 18a of the stator core 18, respectively.

[0016] The stator windings 17 are electrically connected to the inverter 3 shown in Figure 1. When current supplied from the inverter 3 flows through the stator windings 17, the stator 16 generates a rotating magnetic field. The rotor 15, which has permanent magnets 12, is rotated by the rotating magnetic field. The permanent magnet type electric motor 1 is connected to a load such as a pump.

[0017] The permanent magnet electric motor 1 of this embodiment is an interior permanent magnet (IPM) motor in which permanent magnets 12 are embedded in a rotor 15. However, the present invention is not limited to IPM motors, and may be, for example, a surface permanent magnet (SPM) motor in which permanent magnets are arranged on the surface of the rotor. In the following description, the permanent magnet electric motor 1 may be simply referred to as the electric motor 1.

[0018] The operation of stopping the electric motor 1 will be described below. The current control unit 5 controls the operation of the electric motor 1 via the inverter 3. More specifically, when the current control unit 5 receives a stop command from a higher-level operation control unit (not shown), it issues a command to the inverter 3 to execute a deceleration operation that reduces the rotational speed of the rotor 15 of the electric motor 1 during steady operation. Furthermore, after the rotational speed of the rotor 15 falls below a threshold as a result of the deceleration operation, the current control unit 5 issues a command to the inverter 3 to pass a DC current through the stator windings 17 and execute a DC excitation operation that fixes the position of the rotor 15. Here, the position of the rotor 15 refers to the relative angle of the rotor 15 with respect to the stator 16.

[0019] Steady-state operation of motor 1 refers to the operating state when a load (e.g., a pump) coupled to motor 1 is operated in a manner that achieves its intended purpose. For example, steady-state operation of motor 1 coupled to a water pump refers to the operating state of motor 1 when the pump is transporting liquid to a target point at a target pressure. In one example, during steady-state operation, motor 1 is operated at its rated speed.

[0020] In a sequence in which rotor 15 is stopped from a rotating state (deceleration and stopping sequence), after the rotation speed of rotor 15 falls below a threshold, stator winding 17 is excited with a DC current in a fixed direction (i.e., DC excitation operation is performed) to stop rotor 15. If DC excitation is continued, rotor 15 will stop with its d-axis attracted in the direction of magnetic field M generated by stator winding 17, as shown in Figure 3. In other words, rotor 15 stops with the direction of its d-axis identified. Therefore, the next time motor 1 is started, there is no need to estimate the initial position of rotor 15.

[0021] Known techniques can be used to determine when the rotation speed of the rotor 15 falls below the threshold value. For example, the rotation speed of the rotor 15 can be detected by monitoring the magnitude of the terminal voltage of the electric motor 1. A voltage sensor may be used to detect the terminal voltage. Alternatively, the terminal voltage of the electric motor 1 may be detected from the modulation degree (conduction rate) of the inverter 3. In another example, the rotation speed of the rotor 15 can also be detected by detecting the terminal voltage of the electric motor 1 or the frequency of the winding current.

[0022] The DC excitation operation described above is desirably started after the rotation speed of the rotor 15 falls below the threshold value and before it reaches 0. If DC excitation is performed in a certain direction when the rotation speed of the rotor 15 is not completely 0, the rotor 15 will stop after rotating a little when its d-axis faces the direction of the DC excitation.

[0023] Even during DC excitation, information about the rotation speed of the rotor 15 can be obtained from the detected value of the voltage sensor, or the modulation factor (duty ratio) during operation of the current control unit 5, or the frequency of the voltage and current. Therefore, the rotation speed of the rotor 15 may be monitored, and DC excitation may be stopped when it is confirmed that the rotor 15 has stopped. However, detecting zero speed may be difficult or impractical. Therefore, in one embodiment, the current control unit 5 may start DC excitation when a set time has elapsed since the rotation speed reached a threshold value, and stop DC excitation when a preset excitation time has elapsed.

[0024] On the other hand, after the rotation speed of the rotor 15 falls below the threshold, the rotor 15 may have already stopped before the DC excitation operation is started. For example, this may occur when the load connected to the electric motor 1 temporarily increases. In this case, unlike the above, the stopping position of the rotor 15 cannot be determined, so even if DC excitation is performed in a certain direction, torque may not be generated depending on the angle of the rotor 15, and the d-axis of the rotor 15 may not be attracted and captured.

[0025] Therefore, in one embodiment, after the rotational speed of the rotor 15 falls below the threshold value and before the DC excitation operation, the current control unit 5 issues a command to the inverter 3 to pass current through the stator windings 17 and perform a pre-excitation operation in which the direction of the magnetic field generated by the stator windings 17 is changed continuously or intermittently. Specifically, the current control unit 5 issues a command to the inverter 3 to rotate (change) the direction of the DC excitation continuously or intermittently to generate torque in an open loop and perform a pre-excitation operation in which the rotor 15 rotates slightly, and then performs DC excitation in a fixed direction to end the excitation. In this way, the rotor 15 can be stopped at a specified stop position.

[0026] If the application does not require the rotor 15 to be rotated by the load side when the motor 1 is stopped, stopping the motor 1 using the above operation will prevent the position of the rotor 15 from being lost. However, the rotor 15 may be rotated slightly when the motor 1 is stopped. For example, a fan connected to the motor 1 may be rotated slightly by the air flow, causing the rotor 15 to shift position.

[0027] Therefore, in one embodiment, after the DC excitation operation and before the next start of the motor 1, the current control unit 5 further executes a start preparation operation in which the inverter 3 issues a command to pass a DC current through the stator windings 17. For example, by executing the start preparation operation in which the DC excitation is repeatedly performed at appropriate time intervals, it is possible to correct the magnetic pole position of the rotor 15 that has rotated slightly and maintain a state in which the position of the rotor 15 can be completely identified. However, during this short start preparation operation, a start command for the motor 1 may be issued. If the current control unit 5 receives a start command for the motor 1 during this start preparation operation, the current control unit 5 must immediately cancel this start preparation operation and must continue to monitor the torque command and the like during this start preparation operation.

[0028] According to each of the above-described embodiments, the d-axis position of the rotor 15 can be determined when the electric motor 1 is stopped. Therefore, when the electric motor 1 is next started, it is not necessary to estimate the initial position of the rotor 15 based on, for example, measuring inductance by passing a high-frequency current. Since initial position estimation does not require time, the electric motor 1 can be started quickly by generating a magnetic field on the q-axis that is orthogonal to the d-axis. Another advantage is that the initial position can be determined even in a permanent magnet electric motor that does not have salient poles.

[0029] After the motor 1 is stopped, the position of the rotor 15 may shift for some reason before the next start of the motor 1. Therefore, after performing the DC excitation operation to stop the motor 1 and before the next start of the motor 1, the current control unit 5 may further execute a start preparation operation in which a DC current flows through the stator windings 17. That is, before starting the motor 1, the current control unit 5 issues a command to the inverter 3 to flow a DC current through the stator windings 17, thereby reliably determining the d-axis position of the rotor 15, and then executes a sequence to start excitation on the q-axis. This start preparation operation ensures that the q-axis of the rotor 15 is accurately captured. While this start preparation operation requires a short time, it is shorter than the conventional initial position estimation. Therefore, the motor 1 can be started quickly.

[0030] The above-described embodiments have been described for the purpose of enabling a person of ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments would be obvious to a person skilled in the art, and the technical concept of the present invention may be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical concept defined by the claims. [Explanation of symbols]

[0031] 1 Permanent magnet electric motor 3 inverters 5 Current control section 7 Current measuring device 12 Permanent magnets 15 rotors 16 Stator 17 Stator Winding 18 stator core

Claims

1. A method for stopping a permanent magnet motor during steady operation, comprising: performing a deceleration operation to reduce the rotational speed of the rotor of the permanent magnet electric motor during steady operation; After the rotational speed of the rotor falls below a threshold value, a DC excitation operation is performed to fix the position of the rotor by passing a DC current through a stator winding of the permanent magnet motor; a start preparation operation that repeats DC excitation by passing a DC current through the stator winding at predetermined time intervals after the DC excitation operation and before the next start of the permanent magnet motor.

2. The method of claim 1 , wherein the DC excitation operation is performed after the rotational speed of the rotor falls below a threshold value and before the rotational speed of the rotor becomes zero.

3. 3. The method according to claim 1, further comprising: performing a pre-excitation operation after the rotational speed of the rotor falls below a threshold value and before the DC excitation operation, in which a current is passed through the stator windings to continuously or intermittently change the direction of a magnetic field generated by the stator windings.

4. A method described in any one of claims 1 to 3, wherein if the current control unit receives a start command for the permanent magnet type motor during the start preparation operation, the current control unit stops the start preparation operation.

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