JP6019904B2 - Control device for controlling power conversion device - Google Patents

Description

  The present invention relates to a control technique for a power converter such as a PWM inverter that converts a direct current into an alternating current by pulse width modulation (PWM) using a switching element and supplies the alternating current to an alternating current motor.

As a conventional example of this type of technology, the technology disclosed in Non-Patent Document 1 can be cited. FIG. 7 is a block diagram illustrating a configuration example of the control device 3 that controls the PWM inverter 2 that supplies three-phase AC power to the AC motor 1. The control device 3 shown in FIG. 7 controls the PWM inverter 2 based on the position and rotational speed of the rotating shaft of the AC motor 1 and the current value of each phase supplied from the PWM inverter 2 to the AC motor 1. Thus, the drive control of the AC motor 1 is performed. In the control device 3 shown in FIG. 7, the position / speed detection unit 10 detects the position and rotation speed of the rotating shaft of the AC motor 1 and gives a position / speed detection signal indicating the detection result to the motor controller 20. The motor controller 20 is given information indicating the current value of the current supplied to the AC motor 1. More specifically, the current values (i _u , i _v , i _w ) of each phase supplied from the PWM inverter 2 to the AC motor 1 are two-phase current values by three-phase two-phase conversion by the current coordinate conversion unit 30. These are converted to (i _1d , i _1q ), and these current values (i _1d , i _1q ) are supplied from the current coordinate conversion unit 30 to the motor controller 20.

The motor controller 20 calculates and outputs various commands for drive control of the PWM inverter 2 based on the current detection values i _1d and i _1q and the position / speed detection signal. Examples of commands calculated by the motor controller 20 include a phase angle command value θ ^* , a voltage command ν _1d ^*, and ν _1q ^* . The voltage coordinate conversion unit 40 generates a three-phase voltage command (ν _u ^* , ν _v ^* , ν _w ^* ) based on the voltage commands ν _1d ^* and ν _1q ^* and the phase angle command value θ ^*, and performs PWM calculation. Part 70 is given. The three-phase voltage command (ν _u ^* , ν _v ^* , ν _w ^* ) refers to a phase voltage command for controlling the voltage of each phase of the three-phase AC current applied to the AC motor 1. The carrier generation unit 50 generates a triangular wave having a frequency set by the carrier frequency setting unit 60 and supplies the triangular wave to the PWM calculation unit 70 as a carrier (carrier wave). The carrier frequency is a carrier frequency generated by the carrier generation unit 50 (that is, a frequency set by the carrier frequency setting unit 60).

The PWM calculation unit 70 performs pulse width modulation by inputting the voltage command (ν _u ^* , ν _v ^* , ν _w ^* ) of each phase and the carrier, and gives the PWM inverter 2 a three-phase PWM command. FIG. 8 is a diagram showing the relationship between the voltage command and carrier given to the PWM calculation unit 70 and the PWM command generated by the PWM calculation unit 70. In FIG. 8, one of the three-phase voltage commands given to the PWM calculation unit 70 is illustrated. As shown in FIG. 8, the PWM calculation unit 70 compares the voltage command and the carrier waveform, and when the former is larger than the latter, the PWM command is at a high level, and conversely, the latter is greater than the former. Executes a process for outputting a PWM command at a low level for each phase. The PWM inverter 2 performs switching of the switching element based on the PWM command of each phase given from the PWM calculation unit 70, and applies the three-phase AC voltage generated thereby to the AC motor 1.

Takano Nakano "Vector Control of AC Motor", Nikkan Kogyo Shimbun

  When a three-phase AC voltage is generated by a PWM inverter, the ratio R of the carrier frequency to the frequency of the voltage command for each phase (hereinafter referred to as the primary frequency) (ratio R = carrier frequency divided by primary frequency) is 9 to 10 times or more. It is necessary to. This is because if the ratio R is not sufficiently high, voltage distortion occurs, which may hinder drive control of the AC motor. From the viewpoint of suppressing the occurrence of voltage distortion, the ratio R of the carrier frequency to the primary frequency is preferably as high as possible, but there is an upper limit to the carrier frequency in relation to the magnitude of the current flowing through the AC motor. The reason is as follows. The higher the carrier frequency and the larger the current flowing through the AC motor, the higher the switching loss in the PWM inverter, and the increased switching loss appears as a temperature rise of the switching element. If the temperature of the switching element becomes too high, the characteristics of the element change or the switching element is damaged. Therefore, the switching loss must be within a range that matches the cooling capacity of the switching element. In other words, the upper limit of the carrier frequency when a predetermined current is supplied to the AC motor is determined according to the cooling capacity of the switching element included in the PWM inverter that supplies three-phase AC power to the AC motor. Hereinafter, the upper limit determined according to the cooling capacity of the switching element or the like is referred to as “the upper limit of the carrier frequency due to current”.

  As a design method of an AC motor, a predetermined rated speed is generally provided, and as a rated load condition, a torque is a constant value below the rated speed, and an output is a constant value above the rated speed. In this case, the current flowing through the AC motor tends to be smaller in the constant output region where the output is constant than in the constant torque region where the torque is constant. In particular, when only the magnet torque is used in the permanent magnet synchronous motor, the current flowing through the AC motor decreases in inverse proportion to the speed in the constant output region (see FIG. 9), and this tendency becomes remarkable. The example shown in FIG. 9 shows the primary frequency / current characteristics of the AC motor. However, since the speed of the AC motor is determined according to the primary frequency, the primary frequency / current characteristics are equivalent to the speed / current characteristics. When the AC motor that is subject to drive control by the PWM inverter has the speed / current characteristics shown in FIG. 9, the magnitude of the current flowing through the AC motor is shown in FIG. 9 according to the speed (primary frequency). The carrier frequency must be set appropriately in consideration of such changes.

  For example, as shown in “low set value” in FIG. 10, if the carrier frequency is too low, the ratio R is lower than the predetermined value n at the primary frequency exceeding fA. As described above, when the ratio R is less than the predetermined value n, a large voltage distortion may occur, and the drive control of the AC motor may be disabled. Conversely, as shown in “high setting value” in FIG. 10, if the carrier frequency is too high, the carrier frequency exceeds the “upper limit of the carrier frequency due to current” at the primary frequency below fB, and the switching loss increases. For this reason, in the region where the primary frequency is fB or less, it is necessary to reduce the current flowing to the AC motor so that the amount of heat generated due to the switching loss is commensurate with the cooling capacity of the element. Will not be able to.

  The present invention has been made in view of the above problems, and a first object is to determine a current that is determined according to the speed of the AC motor when controlling the power converter that supplies three-phase AC power to the AC motor. The second purpose is to ensure that the current can flow, and the second purpose is to control the voltage over the entire speed range up to a predetermined maximum speed when controlling the power converter that supplies three-phase AC power to the AC motor. It is to reliably suppress the generation of strain.

  In order to solve the above problems, the present invention provides a control device for controlling a power conversion device such as a PWM inverter that converts a direct current into a three-phase alternating current using pulse width modulation by a switching element and supplies the alternating current to an alternating current motor. A carrier frequency setting unit that sets a carrier frequency, which is a frequency of a carrier used for modulation, according to the primary frequency is provided. Then, the carrier frequency setting unit is caused to execute processing for setting the carrier frequency so as not to exceed the upper limit value determined according to the cooling capacity of the switching element and the magnitude of the current flowing through the AC motor. As is well known, the speed of the AC motor is determined according to the frequency (that is, the primary frequency) of the three-phase AC voltage applied to the AC motor. According to such an aspect, setting a carrier frequency that is too high is avoided, and even if a predetermined current determined according to the speed is supplied to the AC motor, the temperature rise of the switching element due to the switching loss is not affected by the switching. It will be within the range suitable for the cooling capacity of the element.

  In a more preferred aspect, the carrier frequency setting unit sets the carrier frequency so that the ratio R to the primary frequency does not fall below a predetermined value. According to such an aspect, setting a carrier frequency that is too low can be avoided, and the occurrence of voltage distortion can be reliably suppressed over the entire speed range up to the maximum speed.

  When the AC motor subject to drive control by the power converter has an output characteristic (see FIG. 9) with constant torque in the speed range slower than the rated speed and constant output in the speed range from the rated speed to the maximum speed. Therefore, the carrier frequency setting unit may be caused to execute a process of gradually raising the carrier period as the primary frequency increases.

  For example, when two types of carrier frequencies (1) and a carrier frequency (2) higher than the carrier frequency (1) are used as carrier frequencies, the carrier frequency may be set in the following manner. First, a switching frequency is determined between a primary frequency value (hereinafter, rated frequency) corresponding to the rated speed of the AC motor and a primary frequency value (hereinafter, maximum frequency) corresponding to the maximum speed of the AC motor. Then, the carrier frequency (1) is set as the carrier frequency at the primary frequency below the switching frequency, and the carrier frequency (2) is set as the carrier frequency at the primary frequency higher than the switching frequency. However, the carrier frequency (1) is “below the limit of the carrier frequency due to the current” and “the ratio R is a predetermined value n (n = 9 to 10) or more” for all the primary frequencies below the switching frequency. It needs to be defined. Similarly, for the carrier frequency (2), for all primary frequencies that are higher than the switching frequency and lower than the maximum frequency, “below the limit of the carrier frequency due to current” and “the ratio R is a predetermined value n (n = 9 to 10). ) Or more ”.

  In a further preferred aspect, each of the control devices includes a current amplitude detection unit that detects an amplitude of a current flowing through the AC motor, and the carrier frequency setting unit detects an amplitude detected by the current amplitude detection unit if the primary frequency is the same. The larger the value is, the lower the carrier frequency is set. If the carrier frequency is the same, the switching loss increases as the amplitude of the current flowing from the power converter to the AC motor increases. According to such an aspect, since the carrier frequency is set lower as the amplitude of the current flowing from the power converter to the AC motor is larger, the switching loss can be further reduced.

It is a block diagram which shows the structural example of 3 A of control apparatuses of 1st Embodiment of this invention. It is a figure which shows the relationship between the primary frequency and the electric current which flows into the alternating current motor 1 in this embodiment. It is a figure which shows the relationship between the primary frequency and carrier frequency in this embodiment. It is a block diagram which shows the structural example of the control apparatus 3B of 2nd Embodiment of this invention. It is a figure for demonstrating operation | movement of the carrier frequency setting part 60B of the same control apparatus 3B. It is a figure which shows the pattern of the carrier frequency in this embodiment. It is a block diagram which shows the structural example of the conventional control apparatus 3 which controls the PWM inverter 2 which supplies three-phase alternating current power to the alternating current motor. It is a figure which shows the relationship between the voltage command in the same control apparatus 3, a carrier, and a PWM command. It is a figure which shows an example of the speed and electric current characteristic of an AC motor. It is a figure for demonstrating the problem of a prior art.

Embodiments of the present invention will be described below with reference to the drawings.
(A: 1st Embodiment)
FIG. 1 is a block diagram illustrating a configuration example of a control device 3A according to the first embodiment of the present invention.
The control device 3A shown in FIG. 1 is a device that controls the PWM inverter 2 that supplies three-phase AC power to the AC motor 1 as in the control device 3 of FIG. The AC motor 1 is a permanent magnet synchronous motor. As shown in FIG. 2, the AC motor 1 has primary torque / current characteristics (speed / current characteristics) that are constant torque up to the rated frequency and constant output from the rated frequency to the maximum frequency. The control device 3A shown in FIG. 1 detects the detection result of the position and rotational speed of the rotating shaft of the AC motor 1 and the current of each phase flowing into the AC motor 1 from the PWM inverter 2, as in the control device 3 of FIG. Based on the result, the PWM inverter 2 is controlled. In FIG. 1, the same components as those in FIG. The configuration of the control device 3A is that the motor controller 20A is provided instead of the motor controller 20, and the carrier frequency setting unit 60A is provided instead of the carrier frequency setting unit 60. Different.

Similarly to the motor controller 20, the motor controller 20A calculates and outputs various commands based on the current detection values i _1d and i _1q and the position / speed detection signal. The motor controller 20A is different from the motor controller 20 in that the primary frequency command value ω ₁ ^* for designating the primary frequency value among the above commands is given to the carrier frequency setting unit 60A. The carrier frequency setting unit 60 </ b> A sets the carrier frequency in the carrier generation unit 50 similarly to the carrier frequency setting unit 60. However, the carrier frequency setting unit 60A is different from the carrier frequency setting unit 60 in that the carrier frequency is switched according to the primary frequency value indicated by the primary frequency command value ω ₁ ^* given from the motor controller 20A.

  The carrier frequency setting unit 60A stores data representing the primary frequency / current characteristics (see FIG. 2) of the AC motor 1 in advance. Here, as an example of data representing the primary frequency / current characteristics of the AC motor 1, a table storing current values corresponding to the respective primary frequencies, and a functional relationship between the primary frequency and the current shown in FIG. Data representing the mathematical formula in the case. The carrier frequency setting unit 60A sets the switching frequency, which is a threshold value for the primary frequency for determining whether or not to switch the carrier frequency, using the above data as follows.

That is, the carrier frequency setting unit 60A performs the switching so that the current flowing through the AC motor 1 is equal to or less than a predetermined current value in a region where the primary frequency indicated by the primary frequency command value ω ₁ ^* is between the switching frequency and the maximum frequency. Set the frequency. More specifically, the carrier frequency setting unit 60A obtains a primary frequency when a current of a predetermined current value flows through the AC motor 1 from the data, and sets this frequency as a switching frequency. In the present embodiment, data indicating the primary frequency / current characteristics of the AC motor 1 is stored in advance in the carrier frequency setting unit 60A, and the switching frequency is calculated by the carrier frequency setting unit 60A based on this data. However, data representing the switching frequency calculated in advance as described above may be stored in the carrier frequency setting unit 60A.

Then, as shown in FIG. 3, when the primary frequency indicated by the primary frequency command value ω ₁ ^* is equal to or lower than the switching frequency, the carrier frequency setting unit 60A sets the carrier frequency (1) as the carrier frequency, and the primary frequency is When the switching frequency is exceeded, the carrier frequency (2) higher than the carrier frequency (1) is set as the carrier frequency. Here, as shown in FIG. 3, the carrier frequency (1) and the carrier frequency (2) are both lower than the “limit of carrier due to current” and the ratio R is a predetermined value n (n = 9 to 10). ) Is higher than the frequency on the boundary line (illustrated by a dotted line in FIG. 3). Various modes are conceivable as to how the carrier frequency (1) and the carrier frequency (2) are specifically determined.

  For example, for the carrier frequency (2), either “n × maximum frequency” or “limit of carrier frequency due to current when primary frequency = switching frequency” or an intermediate value between the two may be adopted. It is done. When the value of “limit of carrier frequency due to current when primary frequency = switching frequency” is used as carrier frequency (2), data representing “upper limit of carrier frequency due to current” for each primary frequency is set as carrier frequency. The carrier frequency (2) may be stored in the unit 60A in advance, and the carrier frequency (2) may be calculated based on the switching frequency obtained from the primary frequency / current characteristics of the AC motor 1 and the data. When a value between “n × maximum frequency” and “limit of carrier frequency due to current when primary frequency = switching frequency” is used as the carrier frequency (2), the former value and the latter value are set to 1. A value obtained by internally dividing −m vs. m (0 <m <1) may be used as the carrier frequency (2), and the above value m is appropriately determined through experiments. Just do it. Similarly, for the carrier frequency (1), either “n × switching frequency” or “limit of carrier frequency due to current in the constant torque region” or an intermediate value between them may be adopted. In addition, instead of causing the carrier frequency setting unit 60A to calculate the carrier frequency (1) and the carrier frequency (2), data indicating each of the carrier frequency (1) and the carrier frequency (2) determined in the above manner, It may be stored in advance in the carrier frequency setting unit 60A together with the data indicating the switching frequency.

  As described above, the carrier frequency (1) and the carrier frequency (2) are both frequencies lower than the limit of the carrier frequency due to the current and the ratio R to the primary frequency exceeds the predetermined value n. For this reason, according to this embodiment, it becomes possible to flow the electric current according to speed to AC motor 1 over the whole speed range up to the maximum speed, and to suppress generation of voltage distortion. In this embodiment, the case where the carrier frequency is raised in a two-step staircase according to the increase in the primary frequency has been described. Also, according to the value of the primary frequency, the carrier frequency is calculated and set so that it is below the “limit of carrier due to current” and above the “frequency on the boundary line where the ratio R = n”. You may make it do. For example, at each primary frequency, a value obtained by internally dividing “limit of carrier due to current” and “frequency on the boundary line where ratio R = n” into 1-m vs. m (where 0 <m <1). May be set as the carrier frequency.

(B: Second embodiment)
FIG. 4 is a block diagram illustrating a configuration example of the control device 3B according to the second embodiment of the present invention. This control device 3B is also a device that controls the PWM inverter 2 that supplies three-phase AC power to the AC motor 1 in the same manner as the control device 3A in FIG. 1 and the control device 3 in FIG. In FIG. 4, the same components as those in FIG. 1 are denoted by the same reference numerals. The configuration of the control device 3B is different from the configuration of the control device 3A in that a current amplitude calculation unit 80 is provided and a carrier frequency setting unit 60B is provided instead of the carrier frequency setting unit 60A.

The current amplitude calculation unit 80 is for calculating the amplitude value of the three-phase AC current supplied from the PWM inverter 2 to the AC motor 1. The current amplitude calculation unit 80 is provided with the current detection values i _1d and i _1q from the current coordinate conversion unit 30. Current amplitude calculator 80, the current amplitude from the current detection value _{i 1d} and _{i 1q | i} 1 _{| (i.e.,} the square root of the square sum of _{i 1d} and _{i 1q)} by calculating gives to the carrier frequency setting unit 60B. The carrier frequency setting unit 60B sets the carrier frequency in the carrier generation unit 50 and switches the carrier frequency according to the primary frequency command value ω ₁ ^* given from the motor controller 20A. This is the same as the carrier frequency setting unit 60A. However, the carrier frequency setting unit 60B adjusts the carrier frequency (1) when the primary frequency is equal to or lower than the switching frequency as shown in FIG. 5 according to the current amplitude | i ₁ | given from the current amplitude calculation unit 80. This is different from the carrier frequency setting unit 60A.

As shown in FIG. 5, when the current amplitude value | i ₁ | is small, the carrier frequency setting unit 60B increases the carrier frequency (1) even if the current primary value | i ₁ | is small, and the current amplitude value | i ₁ | is large. When the carrier frequency (1) is lowered. The reason for this is as follows. As shown by the one-dot chain line and two-dot chain line graph curves in FIG. 6, the “carrier frequency limit due to current” decreases as the current amplitude increases. The carrier frequency (1) is lowered when the current amplitude value | i ₁ | is large, as shown in FIG. 6, below the “limit of the carrier frequency when the current amplitude is large”. This is so that is set. Conversely, when the current amplitude | i ₁ | is small, the carrier frequency (1) is set high because the “carrier frequency limit due to current” increases as the current amplitude value | i ₁ | decreases. This is because the carrier frequency (1) is set as high as possible within a range that does not exceed, and the occurrence of voltage distortion is reliably suppressed.

The following modes are conceivable as specific examples of modes in which the magnitude of the carrier frequency (1) is adjusted according to the value of the current amplitude value | i ₁ |. First, a reference value for the current amplitude value | i ₁ | is determined in advance, and data indicating the reference value is stored in the carrier frequency setting unit 60B. Furthermore, the carrier frequency (1-1) set when the current amplitude value is equal to or less than the reference value, and the carrier frequency (1-2) set when the current amplitude value exceeds the reference value (however, the carrier Frequency (1-1)> carrier frequency (1-2)) is determined separately, and data indicating each of the carrier frequency (1-1) and the carrier frequency (1-2) is stored in the carrier frequency setting unit 60B. Remember. Here, it goes without saying that each of the carrier frequencies (1-1) and (1-2) needs to satisfy the requirements regarding the carrier frequency (1) described above. The carrier frequency setting unit 60B sets _{one of} the carrier frequencies (1-1) and (1-2) as the carrier frequency according to the comparison result of the comparison between the reference value and the current amplitude value | i ₁ |. It is set as. FIG. 5 shows an example in which the carrier frequency is not changed suddenly, but is gradually changed by a linear function for a certain period. The period range for this change and the mode of change of the carrier frequency in this period are as follows. The switching is not limited to the linear function as shown in FIG. 5, and various modes are employed. As another specific example of the aspect of adjusting the magnitude of the carrier frequency (1) according to the value of the current amplitude value | i ₁ |, the value of the carrier frequency to be applied when the primary frequency is equal to or lower than the switching frequency. There may be a mode in which only one type is determined and a value obtained by multiplying the value by dividing the reference value by the current amplitude value | i ₁ | is used as the carrier frequency (1).

  Also according to the present embodiment, it is possible to flow a current corresponding to the speed to the AC motor 1 over the entire speed range up to the maximum speed, and to suppress the occurrence of voltage distortion.

(C: deformation)
Although the first and second embodiments of the present invention have been described above, it is needless to say that the following modifications may be added to these embodiments.
(1) In the first and second embodiments, the position and speed of the rotating shaft of the AC motor 1 are actually detected by the position / speed detector 10, and the operation control of the PWM inverter 2 is performed based on the detection result. Explained when to do. However, an estimation unit that estimates the position and rotation speed of the rotating shaft of the AC motor 1 based on the current flowing from the PWM inverter 2 to the AC motor 1 is provided instead of the position / speed detection unit 10. Of course, a sensorless configuration for controlling the operation of the PWM inverter 2 on the basis thereof may be used.

(2) In the first and second embodiments, the carrier frequency is set such that the carrier frequency is lower than the upper limit of the carrier frequency due to current and the ratio R to the primary frequency is equal to or greater than the predetermined value n. However, only one of the requirements is satisfied. The carrier frequency may be set so as to satisfy. If the former requirement is satisfied, the AC motor 1 can be made to reliably flow a current determined according to the speed. If the latter requirement is satisfied, voltage distortion can be generated over the entire speed range. This is because it can be suppressed.

(3) In the first and second embodiments, the AC motor 1 to which the three-phase AC power is supplied by the PWM inverter 2 is a permanent magnet synchronous motor having the speed / current characteristics shown in FIG. Needless to say, the present invention may be applied to operation control of a power converter that supplies three-phase AC power to an AC motor having speed and current characteristics.

  DESCRIPTION OF SYMBOLS 1 ... AC motor, 2 ... PWM inverter, 3, 3A, 3B ... Control apparatus, 10 ... Position / speed detection part, 20, 20A ... Motor controller, 30 ... Current coordinate conversion part, 40 ... Voltage coordinate conversion part, 50 ... carrier generation unit, 60, 60A, 60B ... carrier frequency setting unit, 70 ... PWM calculation unit, 80 ... current amplitude calculation unit.

Claims (2)

Converts direct current to three-phase alternating current using pulse width modulation by a switching element, and supplies it to an AC motor that has constant output characteristics in the speed range below the rated speed and constant output in the speed range from the rated speed to the maximum speed. In the control device that controls the power conversion device to
A carrier frequency setting unit that sets a carrier frequency, which is a carrier frequency used for the pulse width modulation, according to a primary frequency, which is a frequency of each phase voltage applied to the AC motor, and the cooling capacity of the switching element A carrier frequency setting unit for setting a carrier frequency so as not to exceed an upper limit value determined according to the magnitude of the current flowing through the AC motor ;
A current amplitude calculation unit that calculates the amplitude of the current flowing through the AC motor,
The carrier frequency setting unit
As the primary frequency increases, the carrier frequency is raised step by step,
A control device that adjusts a carrier frequency when a primary frequency is equal to or lower than a switching frequency in accordance with an amplitude calculated by the current amplitude calculation unit .   The control device according to claim 1, wherein the carrier frequency setting unit sets the carrier frequency so that a ratio to the primary frequency does not fall below a predetermined value.

Images