JP6217351B2 - PWM control apparatus and PWM control method - Google Patents

Description

  The present invention relates to a PWM control device and a PWM control method.

  In a pulse width modulation (PWM) control system used for control of a power conversion device including an inverter and a converter, a method of dispersing a harmonic spectrum caused by a carrier frequency by changing the carrier frequency to various frequencies without making the carrier frequency constant. Is known (see Patent Document 1).

  In Patent Document 1, a carrier frequency is determined with reference to a predetermined carrier pattern table. A plurality (NN) of carrier pattern tables are prepared, and the carrier frequency in each carrier pattern table changes pseudo-randomly.

JP 2010-130850 A

  However, in Patent Document 1, since the cycle of sequentially referring to the NN carrier pattern tables is repeated, the determined carrier frequency is also repeated according to this cycle. When the total number of carrier frequencies determined with reference to the NN carrier pattern tables, that is, when the data amount of the NN carrier pattern tables is large, the dispersion performance of the harmonic spectrum is improved, but the storage for storing the carrier pattern tables The load on the device increases. On the other hand, when the total number of carrier frequencies is small, the load on the storage device is reduced, but the dispersion performance of the harmonic spectrum is lowered.

  The present invention has been made in view of the above problems, and its object is to reduce the noise amount in the audible range due to the carrier frequency to the white noise sound while suppressing the amount of data in the frequency table in which the carrier frequencies are ordered. It is to provide a PWM control device and a PWM control method that can be approached.

  A PWM control device according to an aspect of the present invention includes a table storage unit that stores at least a frequency table in which a plurality of carrier frequencies used in a PWM control method are ordered as a random number sequence, and sequentially from the frequency table according to the order. A selection cycle for selecting a carrier frequency is repeatedly performed, and carrier waves for a predetermined period used in the PWM control method are generated according to the selected carrier frequency. The PWM control device changes the number of carrier frequencies to be selected for each selection cycle.

  According to the PWM control device and the PWM control method of the present invention, since the number of selected carrier frequencies changes for each selection cycle, the time required for the selection cycle, that is, the cycle differs for each selection cycle. Therefore, when the total number of carrier frequencies constituting the frequency table, that is, the data amount of the frequency table is at least, repetition of the selection cycle is difficult to be detected as noise sound in the audible range. Therefore, it is possible to make the noise sound in the audible range caused by the carrier frequency close to the white noise sound while suppressing the data amount of the frequency table in which the carrier frequencies are ordered.

FIG. 1 is a block diagram showing a motor drive system including an inverter 12 controlled by an inverter control device 10 according to the embodiment, a DC power source 11 and a motor 13. FIG. 2 is a block diagram showing a detailed configuration of the carrier wave calculation unit 28a of FIG. FIG. 3A is a table showing an example of the frequency table 34, FIG. 3B is a diagram showing the state of the repeated selection cycle, and FIG. 3C is an example of the selection number table 35. FIG. 3D is a graph showing changes in the number of selected data for each selection cycle. FIG. 4A is a graph showing changes in the count values (Wna, Wnb, Wnc) of the counter, and FIG. 4B shows a carrier wave generated based on the count values in FIG. It is a graph which shows an example of WCa, WCb, WCc) and a modulation factor (MRa, MRb, MRc). FIG. 5 is a block diagram showing a detailed configuration of the carrier wave calculation unit 28b according to the second embodiment. FIG. 6A is a table showing an example of the frequency table 34, FIG. 6B is a diagram showing a state of repeated selection cycles, and FIG. 6C is data selected for each selection cycle. It is a graph which shows the change of a number. FIG. 7 is a block diagram showing a detailed configuration of the carrier wave calculation unit 28b according to the third embodiment. FIG. 8A is a table showing an example of the frequency table 34, FIG. 8B is a diagram showing a state of repeated selection cycles, and FIG. 8C is an example of the selection number table 35. FIG. 8D is a table showing an example of the start position table 38.

  Embodiments will be described with reference to the drawings. In the description of the drawings, the same parts are denoted by the same reference numerals and description thereof is omitted.

(First embodiment)
The inverter control device 10 according to the embodiment is an example of a PWM control device that controls an inverter 12 that converts DC power supplied from a battery 11 into AC power and supplies the AC power to a motor 13 using a PWM control method.

  FIG. 1 shows a motor drive system including an inverter 12 controlled by an inverter control device 10 according to an embodiment of the present invention, a battery 11 as a DC power source, and a motor 13. The output terminal of the battery 11 is connected to the input terminal of the inverter 12, and the output terminal of the inverter 12 is connected to the input terminal of the motor 13. The inverter 12 includes one or two or more switching elements, and the inverter control device 10 supplies a PWM signal for switching the on state and the off state of the switching elements to the inverter. Thereby, the inverter 12 can convert the DC voltage from the battery 11 into an AC voltage and supply it to the motor 13 in accordance with the PWM signal.

  The inverter control device 10 includes a phase speed calculation unit 21, a dq conversion calculation unit 22, a PI controller 23, three-phase converters 24a and 24b, a dead time compensation calculation unit 25, a modulation rate calculation unit 26, A carrier wave comparison unit 27, a carrier wave calculation unit 28a, and a target command value calculation unit 29 are provided. The inverter control device 10 can be configured using, for example, a microcontroller including hardware such as a CPU, a ROM, and a RAM.

The phase speed calculator 21 receives the signal of the resolver 15 from the motor 13 and calculates the phase θ and the rotational speed ω of the motor 13. The dq conversion calculation unit 22 receives a current of two or more phases detected by current sensors 14a and 14b provided on a three-phase cable connecting the inverter 12 and the motor 13, and calculates a three-phase current value from the rotational speed ω. (Iu, iv, iw) is converted into a two-phase current value (id, iq). The target command value calculation unit 29 calculates a target current command value (id ^* , iq ^* ) from the torque request value and the rotational speed ω of the motor 13.

The PI controller 23 performs PID control based on the difference between the target current command value (id ^* , iq ^* ) and the two-phase current (id, iq), and outputs the target voltage command value (vd ^* , vq ^* ). To do. The three-phase converters 24a and 24b convert the two-phase target voltage command values (vd ^* , vq ^* ) from the rotation speed ω into the three-phase target voltage command values (vu ^* , vv ^* , vw ^* ). The dead time compensation calculation unit 25 calculates dead time compensation amounts (mu ′, mv ′, mw ′) for preventing a short circuit between the upper arm and the lower arm of the inverter 12.

The modulation factor calculator 26 calculates the ratio of the AC target voltage command values (vu ^* , vv ^* , vw ^* ) to the DC voltage input to the inverter 12 as the modulation factor (mu ^* , mv ^* , mw ^* ). . The modulation factor calculation unit 26 calculates the modulation factor (mu ^* , mv ^* , mw ^* ) from the target voltage command value (vu ^* , vv ^* , vw ^* ) and the dead time compensation amount (mu ′, mv ′, mw ′). Is calculated.

  The carrier wave calculation unit 28a generates a carrier wave used for the PWM control method. The carrier wave calculation unit 28a includes a counter that determines the carrier period. As shown in FIG. 4A, the counter periodically counts up, and when the count value (Wna, Wnb, Wnc) of the counter reaches a predetermined count-up upper limit value (CMa, CMb, CMc), The count value is reset and starts counting from 1 again. The carrier wave calculation unit 28a changes the count-up upper limit values (CMa, CMb, CMc) each time the counter reaches the count-up upper limit value. The change of the count-up upper limit value by the carrier wave calculation unit 28a will be described later with reference to FIG.

  As shown in FIG. 4B, the carrier wave calculation unit 28a generates carrier waves (WCa, WCb, WCc) based on the count value of the counter and the count-up upper limit values (CMa, CMb, CMc). Specifically, the carrier waves (WCa, WCb, WCc) start to increase at the same time as the counter starts counting up. Then, the carrier waves (WCa, WCb, WCc) are turned back at half the height of the count-up upper limit values (CMa, CMb, CMc) and start to decrease. The carrier waves (WCa, WCb, WCc) reach the lower limit simultaneously (Tcu) when the count values (Wna, Wnb, Wnc) reach the count-up upper limit values (CMa, CMb, CMc).

One period (FCa, FCb, FCc) of the sawtooth wave-like count values (Wna, Wnb, Wnc) is the carrier frequency of a triangular wave (carrier waves: WCa, WCb, WCc). The carrier wave cycle (FCa, FCb, FCc) can be changed by changing the count-up upper limit values (CMa, CMb, CMc). Further, the modulation factors (MRa, MRb, MRc) change according to the change of the carrier frequency. Therefore, as shown in FIG. 4B, the modulation factor calculator 26 has a function of correcting the modulation factors (MRa, MRb, MRc) according to the carrier wave periods (FCa, FCb, FCc). Yes. FIG. 4B illustrates a change in one modulation factor, but the modulation factors (mu ^* , mv ^* , mw ^* ) are corrected for each phase.

The carrier wave comparison unit 27 compares the modulation factor (mu ^* , mv ^* , mw ^* ) of each phase with the carrier wave (WCa, WCb, WCc), and outputs a PWM signal to the inverter 12. The inverter control device 10 uses the PWM signal to control the upper arm switching element when the modulation rate is larger than the carrier wave, and when the modulation rate is larger than the carrier wave, the inverter control device Control to ON state.

  A detailed configuration of the carrier wave calculation unit 28a of FIG. 1 will be described with reference to FIG. The carrier wave calculation unit 28 a includes a table storage unit 31, a carrier frequency selection unit 32, and a carrier wave generation unit 33. The table storage unit 31 stores at least a frequency table 34 in which a plurality of carrier frequencies used in the PWM control method are ordered as a random number sequence. The carrier frequency selection unit 32 repeatedly performs a “selection cycle” for selecting carrier frequencies in order according to the order from the frequency table 34. The carrier wave generation unit 33 generates a carrier wave for a predetermined period (for example, a carrier wave for one period) used in the PWM control method according to the carrier frequency selected by the carrier frequency selection unit 32.

  The carrier frequency selection unit 32 changes the number of carrier frequencies to be selected for each selection cycle by the following method.

  In the first embodiment, the table storage unit 31 further stores a selection number table 35 in which the number of carrier frequencies selected in one selection cycle is ordered as a random number sequence. The carrier frequency selection unit 32 includes a selection number changing unit 36 that selects the number of carrier frequencies in order according to the order from the selection number table 35. The carrier frequency selection unit 32 changes the number of carrier frequencies to be selected for each selection cycle according to the number of carrier frequencies selected by the selection number changing unit 36.

  As shown in FIG. 3A, the frequency table 34 includes a plurality of carrier frequencies (frequency 1, 2, 3, 4,... X,... Y,. .. N) are ordered as a random number sequence. For example, the plurality of carrier frequencies are random numbers in the range of 4000 Hz to 6000 Hz, and the average value is set in advance to be 5000 Hz. As shown in FIG. 3C, in the selection number table 35, the number of carrier frequencies (number of data A, C, D, B,... M) selected in one selection cycle is ordered as a random number sequence. Has been.

  When the control is started, the selection number changing unit 36 selects the number of carrier frequencies from the selection number table 35 in order from the top to the bottom according to the order. The selection number changing unit 36 first selects the data number A. The carrier frequency selection unit 32 selects the carrier frequencies from the frequency table 34 in order from the higher order to the lower order according to the order. The carrier frequency selection unit 32 first selects the frequency 1. Then, the carrier wave generator 33 uses the counter 37 for one cycle according to the carrier frequency (frequency 1) selected by the carrier frequency selector 32 using the method shown in FIGS. 4 (a) and 4 (b). Generate a carrier wave.

  Thereafter, the carrier frequency selection unit 32 selects the carrier frequencies (frequency 2, 3, 4,...) In order according to the order of the frequency table 34, and the carrier wave generation unit 33 is selected by the carrier frequency selection unit 32. The carrier waves for one cycle are generated in order according to the carrier frequency (frequency 2, 3, 4,...). The carrier frequency selection unit 32 performs the “carrier wave generation process”, which is the selection of the carrier frequency by the carrier frequency selection unit 32 and the generation of the carrier wave by the carrier wave generation unit 33, the number of data A selected by the selection number changing unit 36. Just repeat it. A cycle in which this carrier wave generation process is repeatedly performed by the number of data selected from the selection number table 35 (data number A) corresponds to a “selection cycle”.

  Next, the selection number changing unit 36 selects the data number C ordered after the data number A in the selection number table 35. Then, the carrier frequency selection unit 32 repeatedly performs the carrier wave generation process for the number of data C. The carrier frequency selection unit 32 repeatedly performs the selection cycle, and after the number of data M ordered at the end of the selection number table 35 is selected, the number of data A is selected again in the next selection cycle. That is, the selection number changing unit 36 repeatedly selects the number of data ordered in the selection number table 35.

  As described above, the selection number changing unit 36 selects the number of carrier frequencies in order according to the order from the selection number table 35 for each selection cycle. Therefore, as shown in FIGS. 3B and 3D, the carrier frequency selection unit 32 changes the number of carrier wave generation processes repeated in one selection cycle for each selection cycle. Can do. Thereby, the cycle of the selection cycle becomes random, and the repetition of the selection cycle becomes difficult to be detected as an audible noise sound.

  The average values of the carrier frequencies selected in each selection cycle may be equal to each other. That is, the average value from frequency 1 to frequency X, the average value from frequency 1 to frequency Z, the average value from frequency 1 to frequency N, and the average value from frequency 1 to frequency Y in FIG. equal. Here, “the average values are equal to each other” means that the fluctuation range of the average value is, for example, not less than −10% and not more than 10%. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

  Further, the average value of carrier frequencies selected in each selection cycle may be equal to the average value of all carrier frequencies in the frequency table 34. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

  As described above with reference to FIG. 4, the carrier frequency selection unit 32 selects the next carrier frequency for each cycle of the carrier wave, and the carrier wave generation unit 33 selects the carrier selected by the carrier frequency selection unit 32. A carrier wave for one period may be generated according to the frequency. Of course, carrier waves for two cycles or more may be generated according to the carrier frequency selected by the carrier frequency selector 32.

  The cycle period in which the selection number changing unit 36 selects the number of carrier frequencies in order from the selection number table 35 according to the order is preferably 2 seconds or more. That is, as shown in FIG. 3D, the time (Tf) from the selection of the highest data number A to the selection of the highest data number A again after the lowest data number M is selected. It is desirable to set the number of data registered in the frequency table 34 and the selection number table 35 so as to be 2 seconds or longer.

  As described above, according to the first embodiment, the following operational effects can be obtained.

  When the inverter 12 that converts DC power to AC power is controlled by the PWM control method, the current ripple according to the carrier frequency used in the PWM control method is a carrier frequency primary sideband and carrier frequency secondary period, Occurs on current cables that supply DC power. The vibration excited by the current ripple is recognized as an audible noise sound.

  Therefore, in the embodiment, the carrier frequency is not always a fixed value, but is always varied at a random frequency. As a result, the frequency band of the current ripple is diffused and the audible sound is turned into white noise, so that the noise level can be reduced.

  Furthermore, the carrier frequency selection unit 32 changes the number of carrier frequencies selected from the frequency table 34 for each selection cycle. As a result, the time required for the execution of the selection cycle, that is, the cycle changes for each selection cycle. Therefore, at least the total number of carrier frequencies constituting the frequency table 34, that is, the amount of data in the frequency table 34, is less likely to be detected as a noise sound in the audible range. Therefore, the audible noise sound caused by the carrier frequency can be brought close to the white noise sound while suppressing the data amount of the frequency table 34 in which the carrier frequencies are ordered. That is, in the spread spectrum control in which the carrier frequency is varied at a random frequency, it is possible to reduce the memory load required for inverter control.

  The table storage unit 31 further stores a selection number table 35 in which the number of carrier frequencies selected in one selection cycle is ordered as a random number sequence. The selection number changing unit 36 selects the number of carrier frequencies from the selection number table 35 in order according to the order. The carrier frequency selection unit 32 changes the number of carrier frequencies to be selected for each selection cycle according to the number of carrier frequencies selected by the selection number changing unit 36. Thereby, since the number of carrier frequencies changes at every selection cycle, spectrum spreading can be performed without increasing the data amount of the frequency table 34 in which the carrier frequencies are ordered.

  By making the average values of the carrier frequencies selected in each selection cycle equal to each other, switching loss at the time of driving the inverter can be suppressed, and deterioration of responsiveness due to a decrease in switching speed can be suppressed.

  The carrier frequency selection unit 32 selects the next carrier frequency for each cycle of the carrier wave, and the carrier wave generation unit 33 generates a carrier wave for one cycle according to the carrier frequency selected by the carrier frequency selection unit 32. To do. As a result, the sound quality of the audible noise sound due to spread spectrum becomes smooth white noise that is not harsh.

  The cycle number (Tf) in which the selection number changing unit 36 selects the number of carrier frequencies in order from the selection number table 35 in accordance with the order is set to 2 seconds or more. As a result, the periodicity of the audible noise sound loop cannot be recognized by the ear.

(Second Embodiment)
In the second embodiment, an example of a method in which the carrier frequency selection unit 32 changes the number of carrier frequencies to be selected for each selection cycle without using the selection number table 35 and the selection number changing unit 36 will be described.

  As shown in FIG. 5, the carrier wave calculation unit 28b according to the second embodiment includes a table storage unit 31 that stores a frequency table 34, and a selection cycle that sequentially selects carrier frequencies from the frequency table 34 according to an order. The carrier frequency selection unit 32 that repeatedly performs the above and the carrier wave generation unit 33 that generates a carrier wave for a predetermined period used in the PWM control method according to the carrier frequency selected by the carrier frequency selection unit 32. The table storage unit 31 may not store the selection number table 35 of FIG. 3C, and the carrier frequency selection unit 32 may not have the function of the selection number changing unit 36 of FIG.

  The carrier frequency selection unit 32 determines a value obtained by adding a predetermined number to the number of carrier frequencies selected in the previous selection cycle as the number of carrier frequencies selected in the next selection cycle. Thereby, the carrier frequency selection unit 32 changes the number of carrier frequencies to be selected for each selection cycle.

  FIG. 6A shows an example of the same frequency table 34 as in FIG. An initial value (number of data A) of the number of carrier frequencies to be selected in the first selection cycle is predetermined. As shown in FIG. 6B, the carrier frequency selection unit 32 sequentially selects carrier frequencies by the initial value (number of data A) in the first selection cycle. In the next selection cycle, carrier frequencies are sequentially selected by a numerical value (data number B) obtained by adding a predetermined value (X) to an initial value (data number A). In this way, the carrier frequency selection unit 32 selects, in order from the frequency table 34, the carrier frequency that is higher by the predetermined value (X) than the previous time for each selection cycle.

  Then, as shown in FIG. 6C, the number of data registered in the frequency table 34 and the number of data registered in the frequency table 34 so that the cycle (Tf) of the selection cycle with the number of selected carrier frequencies as an initial value is 2 seconds or more. It is desirable to set a predetermined value (X).

  As described above, the carrier frequency selection unit 32 selects a value obtained by adding a predetermined number (X) to the number of carrier frequencies selected in the previous selection cycle in the next selection cycle. The number of carrier frequencies to be selected is changed for each selection cycle. Thereby, the noise sound in the audible range caused by the carrier frequency can be brought closer to the white noise sound while further suppressing the amount of data stored in the table storage unit 31.

  By setting the cycle of the selection cycle with the number of selected carrier frequencies as the initial value (data number A) to 2 seconds or more, the periodicity due to the loop of the audible noise sound cannot be recognized by the ear.

  The average values of the carrier frequencies selected in each selection cycle may be equal to each other. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

  Further, the average value of carrier frequencies selected in each selection cycle may be equal to the average value of all carrier frequencies in the frequency table 34. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

(Third embodiment)
The carrier frequency selection unit 32 according to the third embodiment changes the position of the carrier frequency at which selection starts from the frequency table 34 for each selection cycle.

  Specifically, the carrier wave calculation unit 28c according to the third embodiment is different from the carrier wave calculation unit 28a of FIG. 2 in the following points. That is, as shown in FIG. 7, the table storage unit 31 further stores a start position table 38 in which positions of carrier frequencies to be selected from the frequency table 34 are randomly arranged. The carrier frequency selection unit 32 further includes a start position changing unit 39 that sequentially selects the position of the carrier frequency from the start position table 38 according to the order.

  The carrier frequency selection unit 32 changes the position of the carrier frequency that starts to be selected from the frequency table 34 for each selection cycle according to the position of the carrier frequency selected by the start position changing unit 39.

  In the first selection cycle, the selection number changing unit 36 selects the uppermost data number A from the selection number table 35 in FIG. 8C, and the start position changing unit 39 is in FIG. 8D. From the start position table 38, the highest start frequency 3 is selected. The carrier frequency selection unit 32 repeatedly performs the “carrier wave generation process” for the data number A selected by the selection number changing unit 36 from the start frequency 3 selected by the start position changing unit 39. In the next selection cycle, the selection number changing unit 36 selects the next data number C, and the start position changing unit 39 selects the next start frequency 1. The carrier frequency selection unit 32 repeatedly performs “carrier wave generation processing” for the number of data C from the start frequency 1.

  In this way, the carrier frequency selection unit 32 changes the position of the carrier frequency at which selection starts from the frequency table 34 for each selection cycle. Thereby, the cycle of the carrier frequency selected repeatedly becomes further random, and it becomes difficult to detect it as an audible noise sound.

  The start position changing unit 39 selects the position of the carrier frequency from the start position table 38 in order according to the order. Therefore, as shown in FIG. 8B, the carrier frequency selection unit 32 can change the position of the carrier frequency to start selecting from the frequency table 34 for each selection cycle. Thereby, the cycle of the carrier frequency selected repeatedly becomes further random, and it becomes difficult to detect it as an audible noise sound.

  The average values of the carrier frequencies selected in each selection cycle may be equal to each other. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

  Further, the average value of carrier frequencies selected in each selection cycle may be equal to the average value of all carrier frequencies in the frequency table 34. Thereby, the switching loss at the time of inverter drive can be suppressed, and the deterioration of the responsiveness by the fall of switching speed can be suppressed.

  The cycle period in which the selection number changing unit 36 selects the number of carrier frequencies in order from the selection number table 35 according to the order is preferably 2 seconds or more.

  As described above, the first to third embodiments of the present invention have been described. However, it should not be understood that the descriptions and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.

  In the embodiment, the inverter 12 that converts DC power to AC power is illustrated as an example of a power conversion device that converts DC power. However, the power conversion device is not limited to this, and includes a DCDC converter that converts DC power into DC power.

10 Inverter controller (PWM controller)
28a, 28b, 28c Carrier wave calculation unit 31 Table storage unit 32 Carrier frequency selection unit 33 Carrier wave generation unit 34 Frequency table 35 Selection number table 36 Selection number change unit 38 Start position table 39 Start position change unit

Claims (10)

A PWM control device that controls a power conversion device that converts DC power by a PWM control method,
A table storage unit for storing at least a frequency table in which a plurality of carrier frequencies used in the PWM control method are arranged as a random number sequence;
A carrier frequency selection unit that repeatedly performs a selection cycle for selecting carrier frequencies in order according to the order from the frequency table;
A carrier wave generation unit that generates a carrier wave for a predetermined period used in the PWM control method according to the carrier frequency selected by the carrier frequency selection unit,
The carrier frequency selection unit changes the number of carrier frequencies to be selected for each selection cycle. The table storage unit further stores a selection number table in which the number of carrier frequencies selected in one selection cycle is ordered as a random number sequence,
The carrier frequency selection unit includes a selection number changing unit that selects the number of carrier frequencies in order according to the order from the selection number table,
2. The PWM control according to claim 1, wherein the carrier frequency selection unit changes the number of carrier frequencies to be selected for each of the selection cycles according to the number of carrier frequencies selected by the selection number changing unit. apparatus.   The carrier frequency selection unit determines the value by adding a predetermined number to the number of carrier frequencies selected in the previous selection cycle as the number of carrier frequencies to be selected in the next selection cycle. 2. The PWM control device according to claim 1, wherein the number of carrier frequencies to be selected is changed for each cycle.   The PWM control device according to claim 1, wherein the carrier frequency selection unit changes a position of a carrier frequency that starts to be selected from the frequency table for each selection cycle. The table storage unit further stores a start position table in which positions of carrier frequencies to be selected from the frequency table are randomly arranged;
The carrier frequency selection unit includes a start position change unit that sequentially selects a position of the carrier frequency from the start position table according to the order,
The carrier frequency selection unit changes the position of the carrier frequency that starts to be selected from the frequency table for each selection cycle according to the position of the carrier frequency selected by the start position change unit. Item 3. The PWM control device according to Item 2.   The PWM control device according to claim 1, wherein average values of carrier frequencies selected in each selection cycle are equal to each other. The carrier frequency selection unit selects the next carrier frequency for each cycle of the carrier wave,
The PWM control device according to claim 1, wherein the carrier wave generation unit generates a carrier wave for one period according to a carrier frequency selected by the carrier frequency selection unit. .   3. The PWM control device according to claim 2, wherein a cycle period in which the selection number changing unit selects the number of carrier frequencies in order from the selection number table is 2 seconds or more. . The carrier frequency selection unit, after selecting the carrier frequency located at the end of the frequency table, returns the number of carrier frequencies to be selected in the next selection cycle to the initial value,
4. The PWM control device according to claim 3, wherein the cycle of the selection cycle having the number of selected carrier frequencies as an initial value is 2 seconds or more. A PWM control method for controlling a power converter for converting DC power by a PWM control method,
From a frequency table in which a plurality of carrier frequencies used in the PWM control method are arranged as a random number sequence, a selection cycle for selecting carrier frequencies in order according to the order is repeatedly performed,
According to the selected carrier frequency, a carrier wave used for the PWM control method is generated,
A PWM control method, characterized in that the number of selected carrier frequencies is changed for each selection cycle.

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