JP2024158085A - Drive system and control method - Google Patents

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

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

The drive system drives the electric motor using pulse drive control. The carrier frequency of the pulse drive control (e.g., PWM control) that drives the electric motor can cause resonance in the circuit, resulting in an excessive resonant current flowing. Such resonance in an electric circuit can be caused by the positions of circuit elements arranged in the actual electric circuit, the characteristics of the circuit elements, variations in the characteristics, etc. For example, a resonant current can flow between smoothing capacitors connected in parallel with each other. The resonant frequency can also be affected by the positions of the circuit elements, variations in the characteristics of the circuit elements, etc. It has been desirable to reduce the resonant current in the electric circuit when driving an electric motor.

JP 2014-233178 A

The object of the present invention is to provide a drive system and control method for driving an electric motor so as to reduce the resonant current in an electric circuit.

The drive system of the embodiment includes a smoothing capacitor, an inverter, and a control unit. The smoothing capacitor is connected to a DC bus and smoothes the voltage of the pole of the DC bus to which power is supplied. The inverter includes a switching element, converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to the electric motor via the AC bus. The control unit drives the switching element using a pulse signal (PWM signal) obtained by converting a control amount at a predetermined period among a plurality of different frequencies. A first frequency among the plurality of different frequencies is determined so as to avoid a resonant frequency of an electric circuit including the smoothing capacitor, the inverter, and the electric motor connected via the AC bus. The control unit uses the pulse signal obtained at a period corresponding to the first frequency.

FIG. 1 is a configuration diagram of a drive system according to a first embodiment. FIG. 2 is a configuration diagram of a field weakening control unit according to the first embodiment. 5A and 5B are diagrams for explaining details of the field weakening control according to the first embodiment. FIG. 4 is a diagram for explaining resonance characteristics of an example electric circuit according to the first embodiment. FIG. 4 is a diagram for explaining carrier frequency switching control according to the first embodiment. FIG. 11 is a diagram for explaining carrier frequency switching control according to the second embodiment. FIG. 13 is a diagram for explaining carrier frequency switching control in a modified example of the second embodiment. FIG. 13 is a diagram for explaining carrier frequency switching control according to the third embodiment. FIG. 13 is a diagram for explaining carrier frequency switching control in a modified example of the third embodiment.

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 are denoted by the same reference numerals.

In the specification, "connection" is not limited to physical connection, but also includes electrical connection. In the specification, "rotational rate" refers to a physical quantity corresponding to the rotor angular velocity of the motor. In the specification, "reference rotational rate" refers to a control target value for the rotational speed of the motor, which can be expressed as the angular velocity of the rotor, the rotation speed of the rotor, or the frequency corresponding to the rotation speed of the rotor. In the following explanation, an example is described in which the rotational speed reference defined by the rotor rotation speed is used to control the rotational speed of the motor. For example, "rpm (Revolutions Per Minute)" is used as a unit of the rotational speed and the rotational speed reference of the motor.

First Embodiment
Fig. 1 is a configuration diagram showing a drive system 1 of the first embodiment. Fig. 1 shows the drive system 1, a power conversion device 2 (inverter), an electric motor 3, a mechanical load 4, and an AC power supply PS.

The AC power source PS is a commercial power system or an AC generator, and supplies, for example, three-phase AC power to the power conversion device 2.

The drive system 1 includes, for example, a power conversion device 2 and an electric motor 3.

The electric motor 3 is, for example, a variable speed rotating motor (M) such as an induction motor. When three-phase AC power is supplied from the power conversion device 2, the electric motor 3 outputs a rotational driving force to an output shaft, and the rotational driving force drives a mechanical load 4 connected to the output shaft. The electric motor 3 may be equipped with 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.

The power conversion device 2 generates three-phase AC power and supplies the generated three-phase AC power to the electric motor 3 .
However, when multiple smoothing capacitors are connected to the DC bus in the power conversion device 2, resonance may occur and the resonant current may flow through the smoothing capacitor. The resonant current depends on the switching frequency (period) of the switching element 50S, which is 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 capacity of the smoothing capacitor, and the inductance of the DC bus to which the smoothing capacitor is connected. Therefore, the resonant frequency specific to the electric circuit is determined by the mounting condition of the electric circuit. If the mounting conditions are similar, the resonant frequencies are approximately equal to each other. It is preferable to determine the resonant frequency of the electric circuit in advance experimentally or analytically.

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.

The AC power source 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 source 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).

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 may be other types. For example, the switching element 50S of the inverter 50 is ON/OFF controlled by the control unit 60 using pulses of PWM (Pulse Width Modulation) control. The inverter 50 converts DC power supplied from the forward converter 20 or the like via a DC bus into AC power by switching the switching element 50S. The inverter 50 supplies the converted three-phase AC power to the electric motor 3 connected to the output of the inverter 50 via an AC bus or the like to drive the electric motor. The phases of the three-phase AC power output by the inverter 50 are called U-phase, V-phase, and W-phase.

The current detector 70 is provided, for example, on the V phase and W phase of an 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 in the DC bus, and detects the temperature of the smoothing capacitor 30 or in the DC bus.

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 detection 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.

For example, the rotational speed of the electric motor 3 is defined as the rotational speed reference ω_ref. In this embodiment, the rotational speed reference ω_ref is generated based on the required torque from a higher-level controller or the like, and is supplied to the speed control unit 64. The unit of the rotational speed reference ω_ref is "rpm."

The detected speed processing unit 61 generates and outputs the rotation speed ω_fbk and phase θ_fbk based on the rotation speed ωr of the shaft of the electric motor 3 detected by the rotation speed sensor 3A. The rotation speed ω_fbk indicates the rotation speed of the shaft of the electric motor 3, and its unit is "rpm". The phase θ_fbk indicates the electrical angle calculated based on the angle of the shaft of the electric motor 3 and the number of poles of the electric motor 3, and its unit is "radian (rad)".

The speed control unit 64 generates a current reference Idq_ref based on the rotation speed reference ω_ref and the rotation speed ω_fbk output from the detected speed processing unit 61. The current reference Idq_ref is a vector value representing the current reference Id_ref and the current reference Iq_ref in a rotor coordinate system having orthogonal d and q axes. For example, the speed control unit 64 generates the current reference Idq_ref so that the difference between the corrected rotation speed reference ωcor_ref and the rotation speed ω_fbk is zero for each of the d-axis and q-axis components.

The detected current processing unit 65 outputs the current value Iuvw_fbk and the current value 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-phase, V-phase, and W-phase. The current value I_fbk is a scalar value indicating the magnitude of the current value Iuvw_fbk.

The coordinate conversion unit 66 converts the current value Iuvw_fbk in the three-phase coordinate system into a rotor coordinate system having dq axes using the phase θ_fbk to generate a current value Idq_fbk. This is called dq conversion. The rotor coordinate system having dq axes is, for example, a rotating coordinate system that has rotated to a position where the angle between the U-phase axis of the stator coordinate system, which is a stationary coordinate system, and the d axis is equal to the phase θ_fbk.

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 conversion unit 66. For example, the current control unit 67 generates a voltage reference Vdq_ref such that the difference between the current reference Idq_ref and the component of each axis of the current value Idq_fbk becomes zero.
In addition, the current control unit 67 of the embodiment applies the compensation amount by the field weakening control to the current reference Idq_ref and uses the result for the above current control. 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 calculation of the current control. For example, the compensation 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 each axis component of the current value Idq_fbk becomes 0.

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 an inverse transformation of the above-mentioned dq transformation on the voltage reference Vdq_ref to generate the voltage reference Vuvw_ref. This transformation is called the inverse dq transformation.

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

The PWM carrier generation unit 63 generates a carrier signal to be used for PWM control and outputs it to the PWM controller 69. At that time, the PWM carrier generation unit 63 receives the 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.

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 has six switching elements, the PWM controller 69 supplies six gate control signals to the inverter 50 for switching the six switching elements.

Next, the field weakening control unit 62 will be described with reference to FIG. 2. FIG. 2 is a configuration diagram of the field weakening control unit 62 in the embodiment.

The field weakening control unit 62 includes, for example, a memory 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.

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

Each of 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 is realized by, for example, a hardware processor such as a CPU (Central Processing Unit) 620 executing a program (software). In addition, some or all of these components may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. The storage unit 621 is realized by, for example, a HDD (Hard Disk Drive), a flash memory, an EEPROM (Electrically Erasable Programmable Read Only Memory), a ROM (Read Only Memory), or a RAM (Random Access Memory).

The current value acquisition unit 622 acquires the current value I_fbk from the detected current processing unit 65, and adds the acquired data of the current value I_fbk to the memory unit 621 as time series data.

The rotation speed acquisition unit 623 acquires the rotation speed ω_fbk from the detected speed processing unit 61, and adds the acquired data of the rotation speed ω_fbk 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 data of the detected temperature T_fbk to the storage unit 621 as time-series data.

The determination unit 625 determines the rotation speed ω_fbk acquired by the rotation speed acquisition unit 623 based on a predetermined threshold (rotation speed threshold ωTH). For example, the rotation speed threshold ωTH may be set to a magnitude equivalent to a base speed. The magnitude equivalent to the base speed means that the rotation speed is within a predetermined speed range based on the base speed.
The determination unit 625 may make the determination based on a combination of the current value I_fbk and the detected temperature T_fbk in addition to the rotation speed ω_fbk, as will be described later.

The carrier frequency control unit 626 outputs the result of the judgment made by the judgment unit 625 to the PWM controller 69. For example, when the judgment unit 625 detects that the rotation speed ω_fbk is less than a predetermined threshold value, the carrier frequency control unit 626 generates a signal for adjusting the carrier frequency accordingly.

The field weakening control unit 62 configured in this way performs the original field weakening control and adjusts the carrier frequency.

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 for explaining the details of the field weakening control of the first embodiment.

The graph shown in FIG. 3 shows the relationship between the terminal voltage V and the current I with respect to the rotation speed ω of the motor 3.
An example of the relationship between the terminal voltage V and the current I when the rotation speed ω of the motor 3 is in the range from 0 when the motor is stopped to the so-called base speed and up to the so-called top speed is shown. In the region where the rotation speed ω is equal to or higher than the base speed, field-weakening control is implemented. In the region where the rotation speed ω is from 0 to the base speed, the terminal voltage V is monotonically increased in accordance with the rotation speed ω. In the region of field-weakening control (called the field-weakening region) that 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 output capacity of the motor 3 to be constant.

The electric circuit of this control system includes capacitance 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 electric circuit due to switching of the switching element 50S, etc. In this case, when the electric motor 3 is being controlled, resonance occurs in this electric circuit, and a current that is the sum of the resonant current due to this resonance flows.
In other words, the current flowing through the capacitor (called the capacitor current) has a magnitude equal to the original load current plus the resonant current.
In the weak-field region, the control unit 60 limits the main current by using the weak-field control. This is expected to make the main current in the weak-field region smaller than the main current in a region where the weak-field control is not applied. Since the main current in the weak-field region is limited, the capacitor current in the region is also expected to decrease. 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 electric circuit, so it does not change according to the magnitude of the output current set by the control.

In general, to improve the responsiveness of control during high-speed rotation of the motor 3, it is desirable to increase the carrier frequency of the PWM control, but increasing the carrier frequency tends to increase the switching loss of the switching element 50S.

When the motor 3 is driven at a speed at which the field-weakening control is activated, the current is limited by the field-weakening control performed by the control unit 60, and the switching loss of the switching element 50S is expected to be limited. In contrast, when the motor 3 is driven at a speed at which the field-weakening control is not activated, the current cannot be limited by this.

Therefore, in this embodiment, such a phenomenon is dealt with by focusing on the following viewpoints.
The demand for responsiveness of the motor 3 during low speed rotation is not as high as during high speed rotation (in the field weakening region). Therefore, even if the carrier frequency is set to a relatively low frequency during low speed rotation, no problems arise with the responsiveness of the motor 3.

The current (current value Idq_fbk) when outside the field-weakening region is larger than when inside the field-weakening region. Therefore, it is recommended to lower the current (current value Idq_fbk) when outside the field-weakening region by using the method described below.

Assume that the electric circuit in the drive system 1 shown as an example has the resonance characteristics as shown in FIG. 4. FIG. 4 is a diagram for explaining the resonance characteristics of an electric circuit in one embodiment. As shown in FIG. 4, this electric circuit has two resonance points. The frequencies corresponding to the respective resonance points are called resonance frequencies fr1 and fr2. Resonance frequency fr1 is a lower frequency than resonance frequency fr2.

It is assumed that the carrier frequency fc1 is used as the main carrier frequency for PWM control by the control unit 60.
As shown in this figure, the relationship between the resonance frequency fr1 related to the first resonance point and the carrier frequency fc1 is within a range in which it can be determined that the two frequencies are relatively close to each other.

When the resonant frequency fr1 and the carrier frequency fc1 have the above relationship, it is advisable to reset the carrier frequency to a lower frequency, fc2, to increase the impedance of this electrical circuit and reduce the current.

FIG. 5 is a diagram for explaining carrier frequency switching control according to the embodiment.
The following description will be given using the rotation speed ω_fbk of the electric motor 3.
A threshold value for determining the rotation speed ω_fbk is defined as a rotation speed threshold value ω_th.
As shown in FIG. 6A, the state relating to the control of the electric motor 3 is divided into the following two states.
First state: Rotation speed ω_fbk is less than the base speed. Second state: Rotation speed ω_fbk is equal to or greater than the base speed.

The magnitude of the current flowing through the motor 3 can be adjusted by selecting the appropriate carrier frequency for each state. An example of this is shown in Figure 5.

For example, if the goal is to suppress the current flowing through the capacitor, the carrier frequency in the first state can be set to carrier frequency fc3, and the carrier frequency in the second state can be set to carrier frequency fc1.

As described above, the smoothing capacitor 30 of the drive system 1 of this embodiment is connected to the DC bus and smoothes the voltage of the pole of the DC bus to which power is supplied. The inverter 50 includes a switching element 50S, converts the DC power of the DC bus into AC power by switching the switching element 50S, and supplies the AC power to the electric motor 3 via the AC bus. The control unit 60 drives the switching element 50S using a pulse signal obtained by converting a control amount at a predetermined period among a plurality of different frequencies. A first frequency among the plurality of different frequencies is determined so as to avoid a resonant frequency of an electric circuit including the smoothing capacitor 30, the inverter 50, and the electric 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 electric motor 3 so as to reduce the resonant current of the electric circuit.

When the motor 3 is driven at a speed slower 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 the multiple different frequencies.

In addition, when the electric motor is driven at a speed slower than the reference speed, the control unit 60 may use a first frequency, which is lower than the second frequency, to drive the switching element 50S.

(Modification of the first embodiment)
A modification of the first embodiment will 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 electric circuit when the rotation 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 electric circuit when the rotation speed ω_fbk is equal to or greater than the base speed will be described.

As described above, even if the current flowing through the motor 3 is suppressed by the effect of the field weakening control performed 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 a case, it is advisable to increase the carrier frequency at speeds above the base speed to carrier frequency fc2.
By adjusting the carrier frequency in this manner, the carrier frequency falls outside the resonant frequency band over the entire range of the rotation speed ω of the motor 3, and the resonant current flowing through the smoothing capacitor 30 can be reduced.

At top speed, the main current is reduced by the field weakening control, which allows for some margin in the capacitor current, so it is OK to operate at carrier frequency fc1 if no particular problems arise. If the carrier frequency is shifted up to fc2, the capacitor current can be suppressed.

Second Embodiment
A second embodiment will be described with reference to FIG. 6A.
In the first embodiment, a case has been described in which the carrier frequency is adjusted based on the rotation speed of the electric motor 3. In the present embodiment, a case will be described in which the carrier frequency is adjusted further using a detection value of a current flowing through the electric motor 3.

FIG. 6A is a diagram for explaining carrier frequency switching control according to the embodiment.
The explanation will be given using the current value I_fbk which is the detected value of the current flowing through the electric motor 3.
The threshold value for determining the magnitude of the current is defined as the current threshold value I_th.
As shown in FIG. 6A, the state relating to the control of the electric motor 3 is divided into the following four states.
First state: the current value I_fbk is less than the current threshold I_th, and the rotation speed ω_fbk is less than the base speed. Second state: the current value I_fbk is equal to or greater than the current threshold I_th, and the rotation speed ω_fbk is less than the base speed. Third state: the current value I_fbk is less than the current threshold I_th, and the rotation speed ω_fbk is equal to or greater than the base speed. Fourth state: the current value I_fbk is equal to or greater than the current threshold I_th, and the rotation speed ω_fbk is equal to or greater than the base speed.

The magnitude of the current flowing through the motor 3 can be adjusted by selecting the appropriate carrier frequency for each state. An example of this is shown in Figure 6A.

For example, if the goal is to suppress the current flowing through the capacitor, the carrier frequency in the first state can be set to carrier frequency fc2, and the carrier frequencies in the second, third, and fourth states can be set to carrier frequency fc1.

By adjusting the carrier frequency of the PWM control in this way, it is possible to prevent the current from becoming excessively large when the electric circuit resonates.

(Modification of the second embodiment)
A modification of the second embodiment will be described.
In the second embodiment, a case has been described in which the carrier frequency fc1 is used regardless of the magnitude of the current value I_fbk when the rotation speed ω_fbk is equal to or higher than the base speed. In this modified example, a case will be described in which the carrier frequency is adjusted depending on the magnitude of the current value I_fbk when the rotation speed ω_fbk is equal to or higher than the base speed.

FIG. 6B is a diagram for explaining carrier frequency switching control in the modified example of the embodiment.
For example, similarly, when the objective is to suppress the current flowing through the smoothing capacitor 30, the carrier frequency in the fourth state is set to carrier frequency fc3. As described above, the carrier frequencies in the first and third states are set to carrier frequency fc1, and the carrier frequency in the second state is set to carrier frequency fc3.

In this way, when the rotation speed ω_fbk is equal to or greater than the base speed, the carrier frequency is determined based on the magnitude of the current value I_fbk, making it possible to control the entire range of the rotation speed of the motor 3 using the magnitude of the current value I_fbk.

Third Embodiment
A third embodiment will be described with reference to FIG. 7A.
In the first embodiment, a case has been described in which the carrier frequency is adjusted based on the rotation speed of the electric motor 3. In the second embodiment, a case has been described in which the carrier frequency is adjusted further using a detected value of the current flowing through the electric motor 3. In the present embodiment, a case will be described in which the carrier frequency is adjusted further using a detected value of temperature in addition to the conditions in the first embodiment.

FIG. 7A is a diagram for explaining carrier frequency switching control according to the embodiment.
The following description will be given using a detected temperature T_fbk, which is a detected value of the temperature. The temperature exemplified here is, for example, the temperature of the smoothing capacitor 30 or the DC bus.
The threshold value for determining the level of the temperature is defined as the temperature threshold value T_th.
As shown in FIG. 7A, the state relating to the control of the electric motor 3 is divided into the following four states.
First state: the detected temperature T_fbk is less than the temperature threshold T_th, and the rotation speed ω_fbk is less than the base speed. Second state: the detected temperature T_fbk is equal to or greater than the temperature threshold T_th, and the rotation speed ω_fbk is less than the base speed. Third state: the detected temperature T_fbk is less than the temperature threshold T_th, and the rotation speed ω_fbk is equal to or greater than the base speed. Fourth state: the detected temperature T_fbk is equal to or greater than the temperature threshold T_th, and the rotation speed ω_fbk is equal to or greater than the base speed.

The magnitude of the current flowing through the motor 3 can be adjusted by selecting the appropriate carrier frequency for each state. An example of this is shown in Figure 7A.

For example, if the goal is to suppress the current flowing through the capacitor, the carrier frequency in the first state can be set to carrier frequency fc2, and the carrier frequencies in the second, third, and fourth states can be set to carrier frequency fc1.

By adjusting the carrier frequency of the PWM control in this way, it is possible to prevent the current from becoming excessively large when the electric circuit resonates.

(Modification of the third embodiment)
A modification of the third embodiment will now be described.
In the third embodiment, a case was exemplified in which the carrier frequency was adjusted using a detected temperature value when the rotation speed ω_fbk was equal to or higher than the base speed. In this modified example, a case will be described in which the carrier frequency is adjusted depending on the level of the detected temperature T_fbk when the rotation speed ω_fbk is equal to or higher than the base speed.

FIG. 7B is a diagram for explaining carrier frequency switching control according to the embodiment.
For example, similarly, when the objective is to suppress the current flowing through the smoothing capacitor 30, the carrier frequency in the fourth state is set to carrier frequency fc3. As described above, the carrier frequencies in the first and third states are set to carrier frequency fc1, and the carrier frequency in the second state is set to carrier frequency fc3.

In this way, when the rotation speed ω_fbk is equal to or higher than the base speed, the carrier frequency is determined based on the magnitude of the detected temperature T_fbk, making it possible to control using the magnitude of the detected temperature T_fbk over the entire range of the rotation speed of the electric motor 3.

According to at least one embodiment described above, the drive system includes a smoothing capacitor, an inverter, and a control unit. The smoothing capacitor smoothes the voltage of the pole of the DC bus to which power is supplied. The inverter includes a switching element, converts the DC power of the DC bus into AC power by switching the switching element, and supplies the AC power to the electric motor via the AC bus. The control unit drives the switching element using a pulse signal obtained by converting a control amount at a predetermined period among a plurality of different frequencies. A first frequency among the plurality of different frequencies is determined so as to avoid a resonant frequency of an electric circuit including the smoothing capacitor, the inverter, and the electric motor connected via the AC bus. The control unit uses the pulse signal obtained at a period corresponding to the first frequency. This makes it possible to drive the electric motor 3 so as to reduce the resonant current of the electric circuit.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and gist of the invention.

1...Drive system 2...Power conversion device 3...Electric motor 30...Smoothing capacitor 50...Inverter 60...Control unit 61...Detected speed processing unit 62...Field weakening control unit 63...PWM carrier generation unit 64...Speed control unit 65...Detected current processing unit 67...Current control unit 68...Inverse coordinate transformation unit 69...PWM controller 70...Current detector 80...Temperature detection unit 621...Memory unit 622...Current value acquisition unit 623...Rotational speed acquisition unit 624...Temperature detection value acquisition unit 625...Determination unit 626...Carrier frequency control unit

Claims (6)

a smoothing capacitor connected to the DC bus for smoothing a voltage of a pole of the DC bus to which power is supplied;
an inverter including a switching element, which converts 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 amount at a predetermined cycle among a plurality of frequencies different from each other;
Equipped with
a first frequency of the plurality of different frequencies is determined so as to avoid a resonance frequency of an electric circuit including the smoothing capacitor, the inverter, and an electric motor connected via the AC bus;
The control unit is
using the pulse signal obtained at a period corresponding to the first frequency;
Drive system. The control unit is
2. The drive system according to claim 1, wherein when the motor is driven at a speed slower than a 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 a second frequency, which has a smaller difference from the resonant frequency, among the plurality of different frequencies. The control unit is
The drive system according to claim 2 , wherein when the electric motor is driven at a speed slower than the reference speed, the first frequency, which is lower than the second frequency, is used to drive the switching element. The control unit is
switching between the first frequency and the second frequency using a magnitude of a current flowing through the motor, a temperature of the smoothing capacitor, or a temperature of the DC bus, and a rotation speed of the motor;
3. The drive system according to claim 2. The control unit is
A combination of the magnitude of the current flowing through the electric motor and the rotation speed of the electric motor;
A combination of the temperature of the smoothing capacitor and the rotation speed of the motor;
a combination of the temperature of the DC bus and the rotation speed of the motor.
3. The drive system according to claim 2. a smoothing capacitor connected to the DC bus for smoothing a voltage of a pole of the DC bus to which power is supplied;
an inverter including a switching element, which converts 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 amount at a predetermined cycle among a plurality of frequencies different from each other,
a first frequency of the plurality of different frequencies is determined so as to avoid a resonance frequency of an electric circuit including the smoothing capacitor, the inverter, and an electric motor connected via the AC bus;
using the pulse signal obtained at a period corresponding to the first frequency.

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