JP2013005573A - Control device for ac motor, and refrigeration and air conditioning device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect a DC bus current and highly efficiently drive an AC motor even when a modulation factor is low such as when the AC motor is used at a low load or in a low speed region.SOLUTION: A control device for an AC motor includes: a DC power supply; an inverter converting DC power supplied from the DC power supply to AC power; an inverter control circuit controlling a switching element provided in the inverter; a DC bus current detector detecting a DC bus current flowing in the inverter; a low-pass filter smoothing the DC bus current detected by the DC bus current detector; and a corrector correcting an attenuation amount of the DC bus current smoothed by the low-pass filter.

Description

  The present invention relates to an AC motor control device, and more particularly, to a method for estimating a motor current based on a DC bus current of an inverter.

  If the motor current flowing through the AC motor can be accurately estimated and used for controlling the AC motor, the AC motor can be driven with higher performance.

  In Patent Document 1, a current for two phases is detected from a DC bus current of an inverter, and a motor current is estimated based on the detected value. For this purpose, it is necessary to detect current twice within one carrier cycle, and a high-speed A / D converter is required.

  In Patent Document 2, unlike in Patent Document 1 in which the current for two phases must be detected, the motor current can be estimated using only the detected value for one phase. However, in order to estimate the motor current once, it is necessary to detect the DC bus current a plurality of times in a predetermined period, and the point that a high-speed A / D converter is required is not improved.

JP 2002-95263 A JP 2007-221999 A

  When the AC motor is driven in a low load or low speed region, the energization period of the DC bus current of the inverter is shortened in order to reduce the modulation rate. If the energization period of the DC bus current is short, it is difficult to detect the DC bus current multiple times within the energization period, so the conventional technology has an error in the detection of the DC bus current and can accurately estimate the motor current. Therefore, there arises a problem that the AC motor cannot be driven with high performance.

  An object of the present invention is to provide a technique for accurately estimating a motor current and accurately driving an AC motor by accurately detecting a linear bus current even when an inverter energization period is short.

  DC power supply, inverter for converting DC power supplied from the DC power supply to AC power, inverter control circuit for controlling a switch element provided in the inverter, and DC bus current detection for detecting DC bus current flowing in the inverter AC motor control device comprising: a power supply; a low pass filter that smoothes the DC bus current detected by the DC bus current detector; and a corrector that corrects the attenuation of the DC bus current smoothed by the low pass filter .

  According to the present invention, even when the modulation rate is low, the straight-line bus current can be accurately detected. For this reason, the AC motor can be driven with high performance even in a low load or low speed range.

1 is a configuration diagram of a control device according to Embodiment 1. FIG. FIG. 3 is a voltage / current waveform diagram in the control device of the first embodiment. The vector diagram which shows each component of a voltage and an electric current in the control apparatus of Example 1. FIG. FIG. 4 is a waveform diagram of a logical DC bus current IDC in the control device according to the first embodiment. FIG. 5 is a waveform diagram of a DC bus current when the modulation rate is low in the control device of the first embodiment. FIG. 4 is a waveform diagram of a DC bus current when the modulation rate is high in the control device of the first embodiment. FIG. 6 is a waveform diagram showing switching of the current detection phase according to the third embodiment. The wave form diagram which shows the followability | trackability of the DC bus-current detection of Example 4. FIG. FIG. 6 is a configuration diagram of Example 5. The wave form diagram of the straight line bus current at the time of switching the filter time constant of Example 5. FIG. FIG. 10 is a configuration diagram of Example 6.

  Embodiments of the present invention will be described below with reference to the drawings.

  The control apparatus of Example 1 is demonstrated using FIGS. In FIG. 1, an AC motor 1 outputs torque according to three-phase AC currents Iu, Iv, Iw applied from an inverter 2. The inverter 2 includes switch elements Sup, Sun, Svp, Svn, Swp, Swn, applies three-phase AC voltages Vu, Vv, Vw to the AC motor 1 and supplies AC power. DC power supply 3 applies DC voltage VDC to inverter 2 to supply DC power. The DC bus current detector 4 detects the DC bus current IDC of the inverter 2. The low-pass filter 5 smoothes the detection value of the DC bus current detector 4. The corrector 6 corrects the attenuation of the detection value by the low-pass filter 5. The inverter control circuit 7 controls ON / OFF of the switch elements Sup, Sun, Svp, Svn, Swp, Swn. The details of the PWM signal generation unit 7a, the vector control unit 7b, and the current estimation unit 7c in the inverter control circuit 7 will be described in the second embodiment.

  The inverter 2 applies the three-phase AC voltages Vu, Vv, and Vw of (Equation 1) to the AC motor 1, and the three-phase AC currents Iu, Iv, and Iw of (Equation 2) flow through the AC motor 1. The three-phase AC voltage of (Equation 1) is shown in FIG. 2 (a), and the three-phase AC current of (Equation 2) is shown in FIG. 2 (b).

  In Equations 1 and 2, V1 is a motor voltage, I1 is a motor current, θv is a voltage phase based on the U axis, and ψ is a voltage / current phase difference.

  Next, voltage and current components will be described with reference to FIG. In FIG. 3, the U axis represents the U phase coil direction of the stator of the AC motor 1. The components of the motor voltage V1 and the motor current I1 in the U-axis direction are respectively a U-phase voltage Vu and a U-phase current Iu. Similarly, although omitted in FIG. 3, the components in the V-axis direction are V-phase voltage Vv and V-phase current Iv, and the components in the W-axis direction are W-phase voltage Vw and W-phase current Iw. A component of the motor current I1 in the direction of the motor voltage V1 is defined as an effective current Ia, and a component orthogonal thereto is defined as a reactive current Ir.

  The DC bus current detector 4 detects any of the three-phase AC currents Iu, Iv, Iw as the DC bus current IDC. Which of the three-phase alternating currents is detected depends on the combination of ON / OFF of the switch elements Sup, Sun, Svp, Svn, Swp, and Swn. With reference to FIG. 4, the relationship between the ON / OFF combination of the switch elements and the theoretical waveform of the DC bus current IDC will be described in detail.

  FIG. 4A shows a carrier signal having a carrier period Tc. Here, an example using a triangular wave as a carrier signal is shown, but a sawtooth wave may be used.

The inverter control circuit 7 compares the carrier signal and the command values Vu ^* , Vv ^* , Vw ^* of the three-phase AC voltage, and determines on / off of the switch elements Sup, Sun, Svp, Svn, Swp, Swn.

The hatching of Su in FIG. 4B corresponds to a period during which the carrier signal is equal to or lower than Vu ^* and represents a period during which the switch element Sup is turned on. Similarly, the hatching of Sv in FIG. 4C corresponds to a period in which the carrier signal is equal to or lower than Vv ^* , and represents a period in which the switch element Svp is turned on. The hatching of Sw in FIG. This corresponds to a period during which the signal is equal to or lower than Vw ^* and represents a period during which the switch element Swp is turned on.

  The switch elements Sup and Sun shown in FIG. 1 operate in a complementary manner, and when one is on, the other is off. Similarly, the switch elements Svp and Svn, Swp and Swn operate in a complementary manner, and when one is on, the other is off. However, in order to prevent a short circuit of the inverter 2, a dead time during which both are turned off may be provided.

  As shown in FIG. 4, the DC bus current IDC does not flow when all of Sup, Svp, and Swp are on and when all are off. On the other hand, in the period Tu in which only Sup is on, the U-phase current Iu flows in the forward direction, and in the period Tw in which Sup and Svp are on, the W-phase current Iw flows in the reverse direction. As a result, logically, the DC bus current IDC shown in FIG.

  Next, a method for detecting the DC bus current IDC by the DC bus current detector 4 will be specifically described with reference to FIG. The DC bus current detector 4 detects the U-phase current Iu at the timing A Ts after Sup on at the time of falling of the carrier signal. The reason for providing Ts will be described later. Instead of the example shown here, the U-phase current may be detected at the timing B when the carrier signal goes up, or the W-phase current Iw may be detected.

  Next, an example of detecting the actually observed DC bus current IDC will be described with reference to FIGS. FIG. 5 is a waveform diagram of the DC bus current IDC when the modulation rate is low and the period Tu is short. FIG. 6 is a waveform diagram of the DC bus current IDC when the modulation rate is high and the period Tu is long.

  First, the case where the modulation rate is low will be described with reference to FIG. In an actual circuit in which the switching element has a delay or non-linear characteristic, the waveform of the DC bus current IDC does not have the ideal shape shown in FIG. 4 (e), but as shown by the solid line in FIG. 5 (b). It pulsates. For this reason, in order to accurately detect the U-phase current Iu, it is necessary to detect the current after the pulsation has subsided. However, when the modulation rate is low, the three-phase voltages Vu, Vv, and Vw are near zero, so the period Tu is short, and the next switching is performed before ringing is settled, and the U-phase current Iu cannot be accurately detected.

  Therefore, in this embodiment, as shown in FIG. 1, a low-pass filter 5 having the characteristic shown in (Equation 3) is provided at the subsequent stage of the DC bus current detector 4. Here, in (Equation 3), Tf is a time constant. By using the low pass filter 5 having the characteristic of (Equation 3), the DC bus current IDC indicated by the solid line is smoothed to the filter value IDC ′ indicated by the dotted line. The characteristic of the low-pass filter 5 shown in (Equation 3) is an example, and a higher-order low-pass filter may be used.

  When the modulation rate is low as shown in FIG. 5, since the period Tu is short with respect to the carrier period Tc, the pulsation converges early, and the filter value IDC ′ at timing C, which is the timing when the waveform of the DC bus current IDC rises, is zero. Can be approximated. Note that the timing C is ideally when Sup is turned on when the carrier signal falls in FIG. At this time, the detected value Iu ′ of the U-phase current Iu can be obtained by (Equation 4).

The correction coefficient ε in (Equation 4) is (Equation 5).

  Next, the case where the modulation rate is high will be described with reference to FIG. In FIG. 5, the filter value IDC ′ at the timing C can be approximated to zero. However, in FIG. 6, the pulsation does not converge at the timing C because the period Tu is longer than the carrier cycle Tc, and the filter value IDC ′ is also Does not converge to zero. In this case, it is desirable to accurately calculate the filter value IDC ′ at timing C without approximating it to zero.

  Therefore, in this embodiment, as shown in FIG. 1, the corrector 6 is provided after the low-pass filter 5. The corrector 6 substitutes the correction coefficient ε into (Equation 4), and reversely calculates the U-phase current Iu from the U-phase current detection value Iu ′. Thereby, the attenuation of the low-pass filter 5 can be corrected. The inverter control circuit 7 controls on / off of the switch elements Sup, Sun, Svp, Svn, Swp, Swn based on the three-phase AC currents Iu, Iv, Iw thus obtained. Can be driven stably.

  As described above, the configuration of the present embodiment using the low-pass filter 5 and the corrector 6 can correctly detect the current both when the modulation rate is low and when the modulation rate is high. For this reason, the AC motor 1 can be driven with high performance even in a low load or low speed range, and the drive range can be widened.

  Example 2 will be described with reference to FIG. Note that the description of the same points as in the first embodiment will be omitted.

  In this embodiment, as shown in FIG. 1, the inverter control circuit 7 includes a PWM signal generation unit 7a, a vector control unit 7b, and a current estimation unit 7c. Is vector controlled. Each will be described in detail below.

  The current estimation unit 7c estimates the reactive current Ir and the effective current Ia described with reference to FIG. 3 based on the output from the corrector 6, and estimates the amplitude and phase of the motor current I1. This estimation principle is described below.

  As is apparent from FIG. 3, the reactive current Ir and the effective current Ia are expressed by (Equation 6) and (Equation 7), respectively.

  Substituting (Equation 6) and (Equation 7) into (Equation 2) yields (Equation 8).

  Further, the U-phase current detection value Iu ′ represented by (Equation 4) is integrated in the section Q1 (θv1 ≦ θv ≦ θv2) shown in FIG. 2 to obtain the integrated value S1 of (Equation 9). Similarly, integration is performed with Q2 (θv2 ≦ θv ≦ θv3) to obtain an integral value S2 of (Equation 10).

  When (Equation 4) and (Equation 8) are substituted into (Equation 9) and (Equation 10), (Equation 11) and (Equation 12) are obtained.

Note that _{k r1, Δk r1, k a1} , Δk a1 of (11) in defined in equation (13), is _{k r2, Δk r2, k a2} , Δk a2 (Equation 12) in equation (14) Defined by

  Then, (Equation 15) is obtained from (Equation 11) and (Equation 12).

The current estimation unit 7c can estimate the reactive current Ir and the effective current Ia from (Equation 15) obtained above. Note that the coefficients Δk _r1 , Δk _r2 , Δk _a1 , Δk _a2 in (Equation 15) correspond to the amount of attenuation of the DC bus current IDC by the low-pass filter 5, and the corrector 6 corrects the influence of attenuation. Therefore, the DC bus current IDC is corrected using the coefficients Δk _r1 , Δk _r2 , Δk _a1 , Δk _a2 stored in advance or calculated using (Equation 13) and (Equation 14).

  In the above description, the U-phase current detection value Iu ′ integral value in the continuous sections Q1 and Q2 has been described. However, the integral value in the non-continuous sections Q1 and Q2 may be obtained. In the case of performing the calculation, it is possible to distribute the timing of performing the calculation processing, and to prevent the calculation load from being concentrated. Further, the integral values of three or more sections Q1, Q2,..., Qn may be obtained, and the current is estimated using more integral values, so that a suitable current can be obtained even when noise is mixed. Estimation can be performed. When current estimation is performed by these methods, it is possible to cope with the problem by simply changing the integration intervals of Equations 13 and 14 to the actual current detection intervals.

A vector control unit 7b provided in the subsequent stage of the current estimation unit 7c calculates voltage commands Vu ^* , Vv ^* , Vw ^* from the amplitude and phase of the motor current I1 based on vector control. Based on the voltage commands Vu ^* , Vv ^* , Vw ^* from the PWM signal generator 7a and the vector controller 7b, control signals for the switching elements Sup, Sun, Svp, Svn, Swp, Swn are output. Thereby, the inverter 2 is PWM-controlled and the AC motor 1 is driven.

  As described above, according to the configuration of this embodiment, the reactive current Ir and the effective current Ia can be estimated by correcting the attenuation of the DC bus current IDC by the low-pass filter 5. Thereby, compared with the structure which does not correct | amend attenuation by the low-pass filter 5, the AC motor 1 can be driven with high performance.

  Example 3 will be described with reference to FIG. Note that the description of the same points as in the second embodiment will be omitted.

(Number 13), as can be seen from equation (14), the coefficient _{Δk r1, Δk r2, Δk a1} , Δk a2 is voltage phase θv1 shown in FIG. 2, Shitabui2, it depends on Shitabui3. Since the voltage phase θv advances as the AC motor 1 is driven, correction is required when the coefficients Δk _r1 , Δk _r2 , Δk _a1 , and Δk _a2 are recalculated using (Equation 13) and (Equation 14) for each current estimation. A large calculation load is applied to the device 6. Further, when the coefficients Δk _r1 , Δk _r2 , Δk _a1 , Δk _a2 are stored in advance, the corrector 6 must have a large capacity memory storing all the coefficients.

Therefore, in this embodiment, the coefficients Δk _r1 , Δk _r2 , Δk _a1 , and Δka ₂ of (Equation 15) are unified to reduce the calculation load or the storage capacity of the corrector 6.

In order to unify the coefficients Δk _r1 , Δk _r2 , Δka ₁ , and Δka ₂ , the current detection value has periodicity, and the current may be estimated in synchronization with this. If the current phase to be detected is appropriately switched according to the voltage phase θv, the current detection value has periodicity.

  A case where the current detection phase is switched as shown in Table 1 will be described. Here, −U indicates that −Iu ′ is detected by reversing the positive / negative of the U-phase current detection value Iu ′. The same applies to the V phase and the W phase. Table 1 shows one aspect of the present embodiment. When the current detection value has periodicity and current estimation can be performed in synchronization therewith, instead of the example shown in Table 1, various sections and voltage phases are used. , A combination of detection phases can be used.

The current detection value at this time is shown by a solid line in FIG. The detected current value has periodicity. For example, detecting the U phase in the section D is equivalent to detecting the −V phase in the section C. The same applies to the other sections, and the coefficients Δk _r1 , Δk _r2 , Δk _a1 , and Δka ₂ may be calculated based on any one of the sections.

In the present embodiment described above, the coefficients Δk _r1 , Δk _r2 , Δk _a1 , and Δk _a2 can be unified, so that the calculation load of the current estimation unit 7 c can be reduced.

  Example 4 will be described with reference to FIG. It should be noted that description of points that are the same as in the previously described embodiments will be omitted.

  In general, the greater the time constant Tf, the lower the response of the filter value IDC ′ to the linear bus current IDC. At this time, the response of the estimated current value to the actual current is reduced, and the control of the AC motor 1 is also deteriorated.

  Therefore, in the present embodiment, the control response is obtained by defining the time constant Tf of (Equation 3) in relation to the carrier cycle Tc so that the filter value IDC ′ follows the linear bus current IDC at a sufficient response speed. To prevent deterioration of sex.

  FIG. 8 shows waveform diagrams of the DC bus currents IDC and IDC ′. It is assumed that the filter value IDC ′ is detected once every carrier period Tc. Here, IDC0 represents the rated value of the DC bus current IDC, and the DC bus current IDC is not more than the rated value IDC0. Further, the detection value of the filter value IDC ′ in a certain energization period is IDC1, and the filter value IDC ′ immediately before the next energization period is IDC2. IDCm represents the minimum resolution of the DC bus current detector 4, and when the filter value IDC 'is equal to or less than IDCm, zero is detected.

  At this time, if the filter value IDC ′ can vary from the rated value IDC0 to the minimum resolution IDCm or less within the carrier period Tc, the delay of the low-pass filter 5 can be ignored. This condition is expressed by (Expression 16).

  Here, with respect to IDC1 and IDC2, (Equation 17) holds.

  (Equation 18) is obtained from (Equation 16) and (Equation 17).

  In this embodiment, by using the time constant Tf that satisfies (Equation 18), a filter value IDC ′ having a sufficient response speed can be obtained, and the AC motor 1 is controlled based on this filter value IDC ′. A decrease in control response of the motor 1 can be prevented.

  Embodiment 5 will be described with reference to FIGS. It should be noted that description of points that are the same as in the previously described embodiments will be omitted.

  In this embodiment, the accuracy of current estimation is improved by adjusting the time constant Tf according to the modulation rate of the inverter 2.

  FIG. 9 is a configuration diagram of the low-pass filter 5 used in the fifth embodiment. As shown here, the low-pass filter 5 includes a plurality of low-pass filters 5a, 5b, and 5c, and the low-pass filter to be used can be switched by the multiplexer 5d. The low-pass filter 5a constitutes a filter having a predetermined time constant by combining a predetermined resistor and a capacitor. The other low-pass filters 5b and 5c have the same configuration, but resistors and capacitors are selected so as to form filters with different time constants. Here, it is assumed that the time constant decreases in the order of the low-pass filters 5a, 5b, and 5c.

  The period Tu described in FIG. 4 becomes longer as the modulation rate is higher, and it becomes unnecessary to smooth the DC bus current IDC by the low-pass filter 5. Therefore, as shown in FIG. 10, the multiplexer 5d is switched so that the time constant Tf decreases as the modulation rate increases. Thus, the accuracy of current estimation can be improved by using an appropriate filter corresponding to the modulation rate, such as using the low-pass filter 5a when the modulation rate is the highest and using the low-pass filter 5c when the modulation rate is the lowest. it can.

  Although the low-pass filter is always used here, when the modulation factor is sufficiently high and exceeds the threshold, the low-pass filter 5 and the corrector 6 may be avoided and the direct current bus current may be observed directly. In the present embodiment, a configuration other than that shown in FIG. 9 may be used, and the time constant Tf may be changed according to the modulation rate. For example, the time constant Tf may be continuously changed according to the modulation rate by providing a short circuit in parallel with the capacitor of the low-pass filter 5 and controlling the discharge amount.

  Example 6 will be described with reference to FIG. The sixth embodiment is an embodiment in which any one of the control devices described in the first to fifth embodiments is applied to a driving device for a refrigeration air conditioner.

  These drive devices are often driven under operating conditions where the load decreases as the speed decreases. The induced voltage of AC motor 1 decreases as the speed decreases, and the motor current I1 decreases as the load decreases. For this reason, the modulation rate at extremely low speed is extremely small. Therefore, the drive range can be widened by applying the control device described in any one of the first to fifth embodiments to the drive device of the refrigeration air conditioner such as a fan drive device or a compressor drive device. it can. By applying the present invention to the corresponding driving device, it is possible to detect current even when the modulation rate is extremely small, and it is possible to control the driving device from an extremely low speed.

DESCRIPTION OF SYMBOLS 1 AC motor 2 Inverter 3 DC power supply 4 DC bus current detector 5 Low pass filter 5d Multiplexer 6 Corrector 7 Inverter control circuit 7a PWM signal generation unit 7b Vector control unit 7c Current estimation unit VDC DC voltage IDC DC bus current Sup, Sun, Svp, Svn, Swp, Swn Switch elements Vu, Vv, Vw U phase voltage, V phase voltage, W phase voltage Vu ^* , Vv ^* , Vw ^* U phase voltage command, V phase voltage command, W phase voltage command V1 Motor voltage I1 Motor current Ir Reactive current Ia Effective current θv Voltage phase ψ Voltage / current phase difference

Claims (7)

DC power supply,
An inverter that converts DC power supplied from the DC power source into AC power;
An inverter control circuit for controlling a switch element provided in the inverter;
A DC bus current detector for detecting a DC bus current flowing in the inverter;
A low-pass filter for smoothing the DC bus current detected by the DC bus current detector;
A controller for an AC motor, comprising: a corrector that corrects the attenuation of the DC bus current smoothed by the low-pass filter. The control device according to claim 1,
The inverter control circuit is
Current estimation means for estimating the amplitude and phase of an alternating current flowing through the alternating current motor from a plurality of direct current bus currents detected by the direct current bus current detector;
Vector control means for calculating a voltage command of the inverter control circuit based on the output of the current estimation means;
PWM signal generating means for PWM controlling the switch element based on the output of the vector control means;
An AC motor control apparatus comprising: The control device according to claim 2.
The control apparatus for an AC motor, wherein the current estimation means estimates the amplitude and phase of an AC current flowing through the AC motor in synchronization with the periodicity of the DC bus current detected by the DC bus current detector . The control device according to claim 2.
The time constant Tf of the low-pass filter is
A control apparatus for an AC motor, characterized in that
Where Tf: time constant, Tc: carrier cycle of the inverter control circuit
IDC0: Rated value of linear bus current, minimum resolution of IDCm DC bus current detector The control device according to claim 2.
An AC motor control device, wherein a time constant of the low-pass filter is changed in accordance with a modulation factor of the inverter. The control device according to claim 2.
The DC bus current detector detects the linear bus current flowing in the inverter directly, avoiding the low-pass filter and the corrector when the modulation factor of the inverter is greater than or equal to a threshold value. apparatus.   A refrigerating and air-conditioning apparatus, wherein the AC motor control device according to any one of claims 1 to 6 is a control device for an AC motor used in a fan or a compressor.