Drive system and control method

Abstract

To drive a motor so as to reduce a resonance current of an electric circuit.SOLUTION: A drive system includes a smoothing capacitor, an inverter, and a control unit. The smoothing capacitor is connected to a DC bus and smooths a voltage of a pole of the DC bus to which electric power is supplied. The inverter includes a switching element, converts DC power of the DC bus into AC power by switching of the switching element, and supplies the AC power to a motor via an AC bus. The control unit drives the switching element by using a pulse signal obtained by converting a control amount at a predetermined frequency among a plurality of frequencies that are different from each other. A first frequency of the plurality of frequencies different from each other is determined so as to avoid a resonance frequency of an electric circuit including the smoothing capacitor, the inverter, and the motor connected via the AC bus. The control unit uses the pulse signal obtained in a frequency corresponding to the first frequency.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a drive system and a control method.

Background Art

[0002] A drive system drives an electric motor by pulse drive control. Resonance may occur in the circuit due to the carrier frequency of the pulse drive control (for example, PWM control) for driving the electric motor, and an excessive resonance current may flow. Such resonance in an electric circuit may occur due to the position of circuit elements arranged in the actual electric circuit, the characteristics of the circuit elements, variations in those characteristics, and the like. For example, a resonance current may flow between smoothing capacitors connected in parallel with each other. The resonance frequency may also be affected by the position of the circuit elements, variations in the characteristics of the circuit elements, and the like. When driving the electric motor, it has been desired to reduce the resonance current in the electric circuit.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a drive system and a control method for driving an electric motor so as to reduce the resonance current in an electric circuit.

Means for Solving the Problems

[0005] The drive system of the embodiment comprises a smoothing capacitor, an inverter, and a control unit. The smoothing capacitor is connected to a DC bus and smooths the voltage at the poles of the DC bus to which power is supplied. The inverter includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to the motor via the AC bus. The control unit drives the switching element using a pulse signal (PWM signal) obtained by converting a control variable at a predetermined period from among a plurality of mutually different frequencies. The above includes a second frequency among the plurality of mutually distinct frequencies that has a smaller difference from the resonant frequency, and a first frequency that has a larger difference from the resonant frequency. before Record number A frequency of 1 is the frequency of an electrical circuit including the smoothing capacitor, the inverter, and the motor connected via the AC bus. The aforementioned The resonant frequency is determined to be avoided. The control unit, The first frequency and the second frequency are switched using either the temperature of the smoothing capacitor or the temperature of the DC bus, and the rotational speed of the motor, and based on the switching result The first frequency and any of the frequencies of the second frequency described above The pulse signal obtained with a corresponding period is used. [Brief explanation of the drawing]

[0006] [Figure 1] Configuration diagram of the drive system according to the first embodiment. [Figure 2] A diagram showing the configuration of the field weakening control unit of the first embodiment. [Figure 3] A diagram illustrating the details of the field weakening control in the first embodiment. [Figure 4] A diagram illustrating the resonance characteristics of an electrical circuit in an example of the first embodiment. [Figure 5] A diagram illustrating the carrier frequency switching control of the first embodiment. [Figure 6A] A diagram illustrating the carrier frequency switching control of the second embodiment. [Figure 6B] A diagram illustrating the carrier frequency switching control of a modified example of the second embodiment. [Figure 7A] A diagram illustrating the carrier frequency switching control of the third embodiment. [Figure 7B] A diagram illustrating the carrier frequency switching control of a modified example of the third embodiment. [Modes for carrying out the invention]

[0007] The drive system and control method of the embodiment will be described below with reference to the drawings. In the following description, components having the same or similar functions will be denoted by the same reference numerals.

[0008] In this specification, "connection" is not limited to physical connections, but also includes electrical connections. In this specification, "rotational rate" refers to the physical quantity corresponding to the rotor angular velocity of the electric motor. In this specification, "reference rotational rate" refers to the control target value for the rotational rate of the electric motor, and can be expressed as the rotor angular velocity, the rotor rotational speed, or the frequency corresponding to the rotor rotational speed. The following explanation describes an example of controlling the rotational rate of an electric motor using a reference rotational rate defined by the rotor rotational speed. For example, the units used for the rotational rate of the electric motor and the reference rotational rate may be "rpm (Revolutions Per Minute)".

[0009] (First Embodiment) Figure 1 is a configuration diagram showing the drive system 1 of the first embodiment. Figure 1 shows the drive system 1, power converter 2 (inverter), electric motor 3, mechanical load 4, and AC power supply PS.

[0010] The AC power source PS is a commercial power grid or an AC generator, for example, and supplies three-phase AC power to the power converter 2.

[0011] The drive system 1 includes, for example, a power converter 2 and an electric motor 3.

[0012] The electric motor 3 is a variable-speed rotary electric motor (M) such as an induction motor, for example. When three-phase AC power is supplied from the power conversion device 2, the electric motor 3 outputs a rotational driving force to the output shaft, and drives a mechanical load 4 connected to the output shaft by the rotational driving force. The electric motor 3 may include a rotational speed sensor 3A that detects the rotational speed of the shaft of the electric motor 3. The rotational speed sensor 3A outputs, for example, the detected rotational speed ωr of the shaft of the electric motor 3.

[0013] The power conversion device 2 generates three-phase AC power and supplies the generated three-phase AC power to the electric motor 3. By the way, resonance may occur when a plurality of smoothing capacitors are connected to the DC bus in the power conversion device 2, and the resonance current may flow through the smoothing capacitor. The resonance current depends on the switching frequency (period) of the switching element 50S serving as the excitation source, the rise time and fall time of the waveform indicating the change over time of the main current that changes due to switching, the capacitance of the smoothing capacitor, and the inductance of the DC bus to which the smoothing capacitor is connected. Therefore, the resonance frequency specific to the electric circuit is determined by the mounting situation of the electric circuit. If the mounting situations are similar, the resonance frequencies are substantially equal to each other. It is preferable to determine the resonance frequency of the electric circuit in advance experimentally or analytically.

[0014] The power conversion device 2 includes a forward converter 20 (rectifier), a smoothing capacitor 30, an inverter 50, a control unit 60, a current detector 70, and a temperature detector 80.

[0015] An AC power supply PS is connected to the AC side of the forward converter 20, and the smoothing capacitor 30 and the inverter 50 are connected to the DC side of the forward converter 20 via a DC bus. The forward converter 20 converts the AC power supplied from the AC power supply PS into DC power, and the smoothing capacitor 30 smoothes the voltage of the DC power (the voltage of the poles of the DC bus).

[0016] The inverter 50 is an example of a power conversion device including, for example, one or more switching elements 50S. The type of the switching element 50S is not limited to IGBT, MOSFET, etc., and other types may also be used. For example, the switching element 50S of the inverter 50 is ON / OFF controlled by pulses of PWM (Pulse Width Modulation) control by the control unit 60. The inverter 50 converts the DC power supplied from the forward converter 20 or the like via the DC bus into AC power by the switching of the switching element 50S. The inverter 50 supplies the converted three-phase AC power to the motor 3 connected to the output of the inverter 50 via an AC bus or the like to drive the motor. Each phase of the three-phase AC power output by the inverter 50 is called the U phase, V phase, and W phase.

[0017] The current detector 70 is provided, for example, in the V phase and W phase of the AC bus connecting the output of the inverter 50 and the motor 3, and detects the load currents Ivs and Iws supplied by the power conversion device 2 to the windings of the motor 3. The temperature detection unit 80 is provided, for example, in the smoothing capacitor 30 or the DC bus, and detects the temperature of the smoothing capacitor 30 or the DC bus.

[0018] The control unit 60 includes, for example, a detection speed processing unit 61, a field weakening control unit 62, a PWM carrier generation unit 63, a speed control unit 64, a detected current processing unit 65, a coordinate conversion unit 66, a current control unit 67, an inverse coordinate conversion unit 68, and a PWM controller 69.

[0019] For example, the rotational speed of the motor 3 is defined as the rotational speed reference ω_ref. The rotational speed reference ω_ref of the present embodiment is generated based on a required torque or the like from a host controller or the like and supplied to the speed control unit 64. The unit of the rotational speed reference ω_ref uses "rpm".

[0020] The speed detection processing unit 61 generates and outputs a rotational speed ω_fbk and a phase θ_fbk based on the rotational speed ωr of the motor 3 shaft detected by the rotational speed sensor 3A. The rotational speed ω_fbk represents the rotational speed of the motor 3 shaft, and its unit is "rpm". The phase θ_fbk represents the electrical angle calculated based on the angle of the motor 3 shaft and the number of poles of the motor 3, and its unit is "radian (rad)".

[0021] The speed control unit 64 generates a current reference Idq_ref based on the rotational speed reference ω_ref and the rotational speed ω_fbk output from the detected speed processing unit 61. The current reference Idq_ref is a vector representation of the current reference Id_ref and current reference Iq_ref in a rotor coordinate system having orthogonal dq axes. For example, the speed control unit 64 generates the current reference Idq_ref such that the difference between the corrected rotational speed reference ωcor_ref and the rotational speed ω_fbk becomes zero for each component of the d axis and q axis.

[0022] The current detection processing unit 65 outputs current values ​​Iuvw_fbk and I_fbk based on the load current detected by the current detector 70. The current value Iuvw_fbk represents the phase currents Iu_fbk, Iv_fbk, and Iw_fbk of the motor 3 as vector values ​​in a three-phase coordinate space having three axes corresponding to the U, V, and W phases. The current value I_fbk is a scalar value indicating the magnitude of the current value Iuvw_fbk.

[0023] The coordinate transformation unit 66 uses the phase θ_fbk to transform the current value Iuvw_fbk in the three-phase coordinate system into a rotor coordinate system having a dq axis, thereby generating the current value Idq_fbk. This is called the dq transformation. The rotor coordinate system having a dq axis is, for example, a rotating coordinate system in which the stator coordinate system, which is a stationary coordinate system, is rotated to a position where the angle between the U-phase axis and the d-axis is equal to the phase θ_fbk.

[0024] The current control unit 67 generates a voltage reference Vdq_ref based on the current reference Idq_ref generated by the speed control unit 64 and the current value Idq_fbk output from the coordinate transformation unit 66. For example, the current control unit 67 generates a voltage reference Vdq_ref such that the difference between the components of each axis of the current reference Idq_ref and the current value Idq_fbk is zero. In this embodiment, the current control unit 67 applies a compensation amount based on field weakening control to the current reference Idq_ref and uses the result for the current control described above. For example, the current control unit 67 may add the current compensation Id_comp generated by the field weakening control unit 62 to the d-axis component of the current reference Idq_ref and use the result (compensated current reference Idq_ref_comp) for the current control calculation. For example, the compensated current reference Id_ref_comp may be a negative value, and Iq_ref_comp may be 0. In this case, the current control unit 67 generates a voltage reference Vdq_ref such that the difference between the compensated current reference Idq_ref_comp and the components of each axis of the current value Idq_fbk is 0.

[0025] The inverse coordinate transformation unit 68 transforms the voltage reference Vdq_ref generated by the current control unit 67 from a two-phase coordinate system to a three-phase coordinate system using the phase θ_fbk to generate the voltage reference Vuvw_ref. In other words, the inverse coordinate transformation unit 68 performs the inverse transformation of the aforementioned dq transformation on the voltage reference Vdq_ref to generate the voltage reference Vuvw_ref. This transformation is called the inverse dq transformation.

[0026] The field weakening control unit 62 of the control unit 60 configured in this way generates a compensation amount (current compensation Id_comp) for field weakening control of the electric motor 3 using the rotational speed ω_fbk generated by the detected speed processing unit 61, and outputs it to the current control unit 67. The field weakening control unit 62 further generates a switching signal SFS for controlling the carrier frequency of PWM control based on the rotation speed ω_fbk, etc., and outputs it to the PWM carrier generation unit 63.

[0027] The PWM carrier generation unit 63 generates a carrier signal for use in PWM control and outputs it to the PWM controller 69. At that time, the PWM carrier generation unit 63 receives a switching signal SFS generated by the field weakening control unit 62 and determines the frequency (period) of the carrier signal based on the logic state, signal level, etc. of the switching signal SFS.

[0028] The PWM controller 69 compares the voltage reference Vuvw_ref generated by the inverse coordinate transformation unit 68 with the amplitude value of a carrier signal of a predetermined frequency supplied from the PWM carrier generation unit 63 to generate PWM signals for each of the U, V, and W phases. The PWM controller 69 supplies the PWM signals for each of the U, V, and W phases to the inverter 50 to control the switching of the switching elements. For example, if the inverter 50 is equipped with six switching elements, the PWM controller 69 supplies six gate control signals to the inverter 50 to switch the six switching elements.

[0029] Next, the field weakening control unit 62 will be described with reference to Figure 2. Figure 2 is a configuration diagram of the field weakening control unit 62 according to this embodiment.

[0030] The field weakening control unit 62 includes, for example, a storage unit 621, a current value acquisition unit 622, a rotation speed acquisition unit 623, a temperature detection value acquisition unit 624, a determination unit 625, and a carrier frequency control unit 626.

[0031] The memory unit 621 stores data such as current value I_fbk based on the load current detected by the current value acquisition unit 622, rotation speed ω_fbk generated by the rotation speed acquisition unit 623, detected temperature T_fbk generated by the temperature detection value acquisition unit 624, programs for field weakening control and current correction processing, and threshold values ​​(variables) for determination. The memory unit 621 stores each of the above detection results as time-series data. Details of each of the above pieces of information will be described later.

[0032] The current value acquisition unit 622, the rotation speed acquisition unit 623, the determination unit 625, the carrier frequency control unit 626, and the flag data generation control unit 627 are each implemented, for example, by a hardware processor such as a CPU (Central Processing Unit) 620 executing a program (software). Furthermore, some or all of these components may be implemented by hardware (including circuitry) such as LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), and GPU (Graphics Processing Unit), or by the cooperation of software and hardware. The storage unit 621 is implemented by, for example, an HDD (Hard Disk Drive), flash memory, EEPROM (Electrically Erasable Programmable Read Only Memory), ROM (Read Only Memory), or RAM (Random Access Memory).

[0033] The current value acquisition unit 622 acquires the current value I_fbk from the detected current processing unit 65 and adds the acquired current value I_fbk data to the storage unit 621 as time-series data.

[0034] The rotation speed acquisition unit 623 acquires the rotation speed ω_fbk from the detected speed processing unit 61 and adds the acquired rotation speed ω_fbk data to the storage unit 621 as time-series data. The temperature detection value acquisition unit 624 acquires the detected temperature T_fbk from the temperature detection unit 80 and adds the acquired detected temperature T_fbk data to the storage unit 621 as time-series data.

[0035] The determination unit 625 determines the rotational speed ω_fbk obtained by the rotational speed acquisition unit 623 based on a predetermined threshold (rotational speed threshold ωTH). For example, the rotational speed threshold ωTH may be set to a magnitude equivalent to the base speed. A magnitude equivalent to the base speed means that it is within a predetermined speed range based on the base speed. The determination unit 625 may also make a determination using a combination of the current value I_fbk and the detected temperature T_fbk, in addition to the rotation speed ω_fbk. This will be explained later.

[0036] The carrier frequency control unit 626 outputs the result of the determination by the determination unit 625 to the PWM controller 69. For example, if the determination unit 625 detects that the rotation speed ω_fbk is less than a predetermined threshold, the carrier frequency control unit 626 generates a signal to adjust the carrier frequency accordingly.

[0037] The field weakening control unit 62, configured in this way, performs the original field weakening control and adjusts the carrier frequency through its control.

[0038] Next, the details of the field weakening control of the embodiment will be described with reference to Figures 3 to 5. Figure 3 is a diagram illustrating the details of the field weakening control of the first embodiment.

[0039] The graph shown in Figure 3 illustrates the relationship between the terminal voltage V and current I with respect to the rotational speed ω of the electric motor 3. This shows an example of the relationship between terminal voltage V and current I for the rotational speed ω of the motor 3, from a stationary state of 0 to the so-called base speed and beyond to the so-called top speed. In the region where the rotational speed ω is above the base speed, field weakening control is implemented. In the region where the rotational speed ω is from 0 to the base speed, the terminal voltage V is increased monotonically in accordance with the rotational speed ω. In the region where field weakening control is performed (called the field weakening region), which is above the base speed, the terminal voltage V is kept constant, and the main current component of the current flowing through the motor 3 is reduced, thereby controlling the motor 3 to maintain a constant output capacity.

[0040] Furthermore, the electrical circuit of this control system includes capacitive components such as the smoothing capacitor 30, and inductance components such as the windings of the motor 3, the DC bus, and the AC bus. Resonance may occur in this electrical circuit due to the switching of the switching element 50S. In this case, when the motor 3 is being controlled, resonance occurs in this electrical circuit, and a current flows that includes the resonant current caused by this resonance. In other words, the current flowing through the capacitor (called the capacitor current) is the sum of the original load current and the resonant current. In the field weakening region, the control unit 60 limits the main current through field weakening control. As a result, the main current in the field weakening region is expected to be smaller than the main current in the region where field weakening control is not applied. Because the main current in the field weakening region is limited, the capacitor current in that region is also expected to be reduced. On the other hand, the resonant current is generated according to the switching frequency of the switching element 50S, and its magnitude is determined by the constants of the electrical circuit, so it does not change according to the magnitude of the output current set by the control.

[0041] Generally, to improve the responsiveness of the control of the electric motor 3 at high rotational speeds, it is desirable to increase the carrier frequency of the PWM control, and increasing the carrier frequency tends to increase the switching loss of the switching element 50S.

[0042] When the motor 3 is driven at a speed at which field weakening control is activated, the current is expected to be limited by the field weakening control by the control unit 60, thereby limiting the switching loss of the switching element 50S. Conversely, when the motor 3 is driven at a speed at which field weakening control is not activated, this current limitation cannot be expected.

[0043] Therefore, in this embodiment, we address such issues by focusing on the following points. The responsiveness requirements for the motor 3 at low rotational speeds are not as high as those at high rotational speeds (within the field weakening region). Therefore, at low rotational speeds, setting the carrier frequency to a relatively low frequency does not cause any problems with the responsiveness of the motor 3.

[0044] Note that the current (current value Idq_fbk) outside the field weakening region is larger than that inside the field weakening region. Therefore, it is advisable to lower the current (current value Idq_fbk) outside the field weakening region using the method shown below.

[0045] Let's assume that the electrical circuit in the drive system 1, shown as an example, has the resonance characteristics shown in Figure 4. Figure 4 is a diagram illustrating the resonance characteristics of an electrical circuit in an example embodiment. As shown in Figure 4, this electrical circuit has two resonance points. The frequencies corresponding to each resonance point are called resonance frequencies fr1 and fr2. Resonance frequency fr1 is a lower frequency than resonance frequency fr2.

[0046] We assume that the carrier frequency fc1 is used as the primary carrier frequency for PWM control by the control unit 60. As shown in this figure, the relationship between the resonant frequency fr1 and the carrier frequency fc1 related to the first resonance point is within a range where their frequencies can be judged to be relatively close.

[0047] When the resonant frequency fr1 and the carrier frequency fc1 have the relationship described above, it is desirable to increase the impedance of this electrical circuit and reduce the current by resetting the carrier frequency to a lower frequency, such as carrier frequency fc2.

[0048] Figure 5 is a diagram illustrating the carrier frequency switching control of the embodiment. This explanation will use the rotational speed ω_fbk of electric motor 3. The threshold for determining the rotational speed ω_fbk is defined as the rotational speed threshold ω_th. As shown in Figure 6A, the control state of the electric motor 3 is divided into the following two states. First state: Rotational speed ω_fbk is less than the base speed. Second state: Rotation speed ω_fbk is greater than or equal to the base speed.

[0049] By selecting an appropriate carrier frequency based on each condition, the magnitude of the current flowing through the motor 3 can be adjusted. An example of this is shown in Figure 5.

[0050] For example, if the goal is to suppress the current flowing through a capacitor, it is advisable to set the carrier frequency of the first state to carrier frequency fc3 and the carrier frequency of the second state to carrier frequency fc1.

[0051] As described above, the smoothing capacitor 30 of the drive system 1 in this embodiment is connected to a DC bus and smooths the voltage at the poles of the DC bus to which power is supplied. The inverter 50 includes a switching element 50S, which converts the DC power of the DC bus to AC power by switching the switching element 50S, and supplies the AC power to the motor 3 via the AC bus. The control unit 60 drives the switching element 50S using a pulse signal obtained by converting a control variable at a predetermined period from among a plurality of mutually different frequencies. The first frequency among the plurality of mutually different frequencies is determined so as to avoid the resonant frequency of the electrical circuit including the smoothing capacitor 30, the inverter 50, and the motor 3 connected via the AC bus. The control unit 60 uses a pulse signal obtained at a period corresponding to the first frequency. This makes it possible to drive the motor 3 in a way that reduces the resonant current of the electrical circuit.

[0052] Furthermore, when the motor 3 is driven at a speed lower than the reference speed that determines whether or not field weakening control is appropriate for the motor 3, the control unit 60 may use the first frequency, which has a larger difference from the resonant frequency, to drive the switching element 50S, instead of the second frequency, which has a smaller difference from the resonant frequency, among a plurality of different frequencies.

[0053] Furthermore, when the motor is driven at a speed lower than the reference speed, the control unit 60 may use a first frequency lower than the second frequency to drive the switching element 50S.

[0054] (Modified version of the first embodiment) A modified example of the first embodiment will now be described. The example shown in the first embodiment illustrates a method of lowering the carrier frequency to move it away from the resonant frequency fr1 of the electrical circuit when the rotational speed ω_fbk is less than the base speed. In this modification, a method of raising the carrier frequency to move it away from the resonant frequency of the electrical circuit when the rotational speed ω_fbk is greater than or equal to the base speed will be described.

[0055] As described above, even if the current flowing to motor 3 is suppressed by the field weakening control implemented at speeds above the base speed, the capacitor current may become excessive if the carrier frequency is relatively close to the resonance point. In such cases, it is advisable to raise the carrier frequency to carrier frequency fc2 when the speed is above the base speed. By adjusting the carrier frequency in this way, the carrier frequency can be moved out of the resonant frequency band across the entire range of the rotational speed ω of the electric motor 3, thereby reducing the resonant current flowing through the smoothing capacitor 30.

[0056] At top speed, the main current is reduced by field weakening control, which provides leeway in the capacitor current. Therefore, operation at carrier frequency fc1 is acceptable unless there are specific issues. If the carrier frequency is shifted to fc2, the capacitor current can be suppressed.

[0057] (Second embodiment) A second embodiment will be described with reference to Figure 6A. In the first embodiment, an example of adjusting the carrier frequency based on the rotational speed of the electric motor 3 was described. In this embodiment, an example of adjusting the carrier frequency using the detected value of the current flowing through the electric motor 3 will be described.

[0058] Figure 6A is a diagram illustrating the carrier frequency switching control of an embodiment. This explanation will use the current value I_fbk, which is the detected value of the current flowing through the electric motor 3. The threshold value used to determine the magnitude of that current is defined as the current threshold I_th. As shown in Figure 6A, the control state of the electric motor 3 can be divided into the following four categories. First state: Current value I_fbk is less than current threshold I_th, and rotational speed ω_fbk is less than base speed. Second state: Current value I_fbk is greater than or equal to current threshold I_th, and rotational speed ω_fbk is less than base speed. Third state: Current value I_fbk is less than current threshold I_th, and rotational speed ω_fbk is greater than or equal to the base speed. Fourth state: Current value I_fbk is greater than or equal to current threshold I_th, and rotational speed ω_fbk is greater than or equal to base speed.

[0059] It is advisable to adjust the magnitude of the current flowing through the motor 3 by selecting an appropriate carrier frequency based on each condition. An example of this is shown in Figure 6A.

[0060] For example, if the goal is to suppress the current flowing through a capacitor, it is advisable to set the carrier frequency of the first state to carrier frequency fc2, and the carrier frequencies of the second, third, and fourth states to carrier frequency fc1.

[0061] By adjusting the carrier frequency of PWM control in this way, it is possible to suppress the excessively large current during resonance in the electrical circuit.

[0062] (Modified version of the second embodiment) A modified example of the second embodiment will now be described. In the second embodiment, when the rotational speed ω_fbk is equal to or greater than the base speed, an example was given in which the carrier frequency fc1 is used regardless of the magnitude of the current value I_fbk. In this modification, an example is described in which the carrier frequency is adjusted according to the magnitude of the current value I_fbk when the rotational speed ω_fbk is equal to or greater than the base speed.

[0063] Figure 6B is a diagram illustrating a modified example of the carrier frequency switching control of the embodiment. For example, if the objective is to similarly suppress the current flowing through the smoothing capacitor 30, the carrier frequency of the fourth state is set to carrier frequency fc3. As mentioned earlier, the carrier frequencies of the first and third states are set to carrier frequency fc1, and the carrier frequency of the second state is set to carrier frequency fc3.

[0064] Thus, when the rotational speed ω_fbk is greater than or equal to the base speed, the carrier frequency is determined by the magnitude of the current value I_fbk, making it possible to control the motor 3 using the magnitude of the current value I_fbk across the entire range of rotational speeds.

[0065] (Third embodiment) A third embodiment will be described with reference to Figure 7A. In the first embodiment, an example of adjusting the carrier frequency based on the rotational speed of the electric motor 3 was described. In the second embodiment, an example of adjusting the carrier frequency using a detected value of the current flowing through the electric motor 3 was described. In this embodiment, an example of adjusting the carrier frequency using a detected value of temperature in addition to the conditions of the first embodiment will be described.

[0066] Figure 7A is a diagram illustrating the carrier frequency switching control of an embodiment. The explanation will use the detected temperature T_fbk, which is the temperature value detected. The temperature used as an example here is, for example, the temperature of the smoothing capacitor 30 or the DC bus. The threshold for determining the temperature level is defined as the temperature threshold T_th. As shown in Figure 7A, the control state of the electric motor 3 can be divided into the following four categories. First state: Detected temperature T_fbk is less than the temperature threshold T_th, and rotational speed ω_fbk is less than the base speed. Second state: Detected temperature T_fbk is greater than or equal to the temperature threshold T_th, and rotational speed ω_fbk is less than the base speed. Third state: Detected temperature T_fbk is less than the temperature threshold T_th, and rotational speed ω_fbk is greater than or equal to the base speed. Fourth state: Detected temperature T_fbk is greater than or equal to the temperature threshold T_th, and rotational speed ω_fbk is greater than or equal to the base speed.

[0067] It is advisable to adjust the magnitude of the current flowing through the motor 3 by selecting an appropriate carrier frequency based on each condition. An example of this is shown in Figure 7A.

[0068] For example, if the goal is to suppress the current flowing through a capacitor, it is advisable to set the carrier frequency of the first state to carrier frequency fc2, and the carrier frequencies of the second, third, and fourth states to carrier frequency fc1.

[0069] By adjusting the carrier frequency of PWM control in this way, it is possible to suppress the excessively large current during resonance in the electrical circuit.

[0070] (Modified version of the third embodiment) A modified example of the third embodiment will now be described. In the third embodiment, an example was given in which the carrier frequency is adjusted using the detected temperature when the rotational speed ω_fbk is equal to or greater than the base speed. In this modification, an example is described in which the carrier frequency is adjusted according to the height of the detected temperature T_fbk when the rotational speed ω_fbk is equal to or greater than the base speed.

[0071] Figure 7B is a diagram illustrating the carrier frequency switching control of an embodiment. For example, if the objective is to similarly suppress the current flowing through the smoothing capacitor 30, the carrier frequency of the fourth state is set to carrier frequency fc3. As mentioned earlier, the carrier frequencies of the first and third states are set to carrier frequency fc1, and the carrier frequency of the second state is set to carrier frequency fc3.

[0072] Thus, when the rotational speed ω_fbk is greater than or equal to the base speed, the carrier frequency is determined by the height of the detected temperature T_fbk, making it possible to control the motor 3 using the height of the detected temperature T_fbk across the entire range of rotational speeds.

[0073] According to at least one embodiment described above, the drive system comprises a smoothing capacitor, an inverter, and a control unit. The smoothing capacitor smooths the voltage at the poles of a DC bus to which power is supplied. The inverter includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to the motor via the AC bus. The control unit drives the switching element using a pulse signal obtained by converting a control variable at a predetermined period from a plurality of mutually different frequencies. The first frequency from the plurality of mutually different frequencies is determined so as to avoid the resonant frequency of an electrical circuit including the smoothing capacitor, the inverter, and the motor connected via the AC bus. The control unit uses the pulse signal obtained at a period corresponding to the first frequency. This allows the motor 3 to be driven in a manner that reduces the resonant current of the electrical circuit.

[0074] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0075] 1…Drive System 2…Power converter 3...Electric motor 30…Smoothing Capacitor 50…Inverter 60... Control Unit 61...Detection speed processing unit 62...Field weakening control unit 63...PWM carrier generation unit 64... Speed ​​control unit 65...Detection current processing unit 67...Current control unit 68...Inverse coordinate transformation section 69...PWM controller 70...Current detector 80...Temperature detection unit 621...Storage section 622...Current value acquisition unit 623...Rotation speed acquisition unit 624...Temperature detection value acquisition unit 625...Judgment section 626... Carrier frequency control unit

Claims

1. A smoothing capacitor connected to a DC bus, which smooths the voltage at the poles of the DC bus to which power is supplied, An inverter that includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to an electric motor via the AC bus, A control unit that drives the switching element using a pulse signal obtained by converting a control variable at a predetermined period among multiple mutually different frequencies, Equipped with, The above includes a second frequency among the plurality of mutually distinct frequencies whose difference from the resonant frequency is smaller, and a first frequency whose difference from the resonant frequency is larger. The first frequency is determined to avoid the resonant frequency of the electrical circuit, which includes the smoothing capacitor, the inverter, and the motor connected via the AC bus. The control unit, The first frequency and the second frequency are switched using either the temperature of the smoothing capacitor or the temperature of the DC bus, and the rotational speed of the motor. The pulse signal obtained at a period corresponding to either the first frequency or the second frequency based on the switching result is used. Drive system.

2. The control unit, When the motor is driven at a speed lower than the reference speed that determines whether or not field weakening control is appropriate for the motor, the first frequency, which has a larger difference from the resonant frequency, is used to drive the switching element instead of the second frequency, which has a smaller difference from the resonant frequency, among the multiple different frequencies. The drive system according to claim 1.

3. The control unit, When the motor is driven at a speed lower than the reference speed, the first frequency, which is lower than the second frequency, is used to drive the switching element. The drive system according to claim 2.

4. A smoothing capacitor connected to a DC bus, which smooths the voltage at the poles of the DC bus to which power is supplied, An inverter that includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to an electric motor via the AC bus, A control unit that drives the switching element using a pulse signal obtained by converting a control variable at a predetermined period among multiple mutually different frequencies, Equipped with, The above includes a second frequency among the plurality of mutually distinct frequencies whose difference from the resonant frequency is smaller, and a first frequency whose difference from the resonant frequency is larger. The first frequency is determined to avoid the resonant frequency of the electrical circuit, which includes the smoothing capacitor, the inverter, and the motor connected via the AC bus. The control unit, The combination of the temperature of the smoothing capacitor and the rotational speed of the motor, The combination of the temperature of the DC bus and the rotational speed of the motor. The first frequency and the second frequency are switched using the result of a determination process using any combination of the above. The pulse signal obtained at a period corresponding to either the first frequency or the second frequency based on the switching result is used. Drive system.

5. A smoothing capacitor connected to a DC bus, which smooths the voltage at the poles of the DC bus to which power is supplied, An inverter that includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to an electric motor via the AC bus, A control unit that drives the switching element using a pulse signal obtained by converting a control variable at a predetermined period among multiple mutually different frequencies, A control method for a drive system comprising, The above includes a second frequency among the plurality of mutually distinct frequencies whose difference from the resonant frequency is smaller, and a first frequency whose difference from the resonant frequency is larger. The first frequency is determined to avoid the resonant frequency of the electrical circuit, which includes the smoothing capacitor, the inverter, and the motor connected via the AC bus. The first frequency and the second frequency are switched using either the temperature of the smoothing capacitor or the temperature of the DC bus, and the rotational speed of the motor. A process using the pulse signal obtained at a period corresponding to either the first frequency or the second frequency based on the switching result. A control method including

6. A smoothing capacitor connected to a DC bus, which smooths the voltage at the poles of the DC bus to which power is supplied, An inverter that includes a switching element, which converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to an electric motor via the AC bus, A control unit that drives the switching element using a pulse signal obtained by converting a control variable at a predetermined period among multiple mutually different frequencies, A control method for a drive system comprising, The above includes a second frequency among the plurality of mutually distinct frequencies whose difference from the resonant frequency is smaller, and a first frequency whose difference from the resonant frequency is larger. The first frequency is determined to avoid the resonant frequency of the electrical circuit, which includes the smoothing capacitor, the inverter, and the motor connected via the AC bus. The combination of the temperature of the smoothing capacitor and the rotational speed of the motor, The combination of the temperature of the DC bus and the rotational speed of the motor. The first frequency and the second frequency are switched using the result of a determination process using any combination of the above. A process using the pulse signal obtained at a period corresponding to either the first frequency or the second frequency based on the switching result. A control method including

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