JP3130440B2 - AC motor current detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting current of an AC motor, and more particularly to a method for detecting current of an AC motor driven by an output signal of a PWM inverter.

[0002]

2. Description of the Related Art In recent years, as the price of microprocessors has been reduced, the control system for electric motors has been being softwareized.
A so-called single-chip microcomputer (hereinafter simply referred to as MPU) comprising a general-purpose CPU, an input / output port, a memory, a timer, an A / D converter, a D / A converter, etc. on one silicon wafer is low in cost because of its good economy. It is often used in control devices for inexpensive AC servomotors.

When the current control system is implemented by software, AD conversion means for converting the output voltage of the current sensor into an amount handled by the microprocessor, that is, a digital amount is required. The AD converter built in the MPU is shown in FIG.
As shown in a successive approximation type mainstream, this sequential in the preceding stage of the comparator type AD converter 17c, several analog voltage signals v _a, multiplexer 17a to select the desired signal from the v _b, during conversion Is generally provided with a sample-and-hold circuit 17b for keeping the input voltage at a constant level.

Now, in a three-phase three-wire AC motor,
In order to accurately control the phase current, it is necessary to detect at least two-phase current amounts. However, when the above-described MPU is used, the current cannot be sampled in two or more phases at the same time because there is only one set of the successive approximation AD converter 17c and the sample hold circuit 17b. Therefore, the current must be sampled and converted in a time sharing manner.

[0005] By the way, the current is sampled when the inverter is a voltage type sub-harmonic system.
As described in Japanese Patent Publication No. 54-54 and the like, a value close to the average value of the current can be sampled by synchronizing with the peak of the carrier wave, which has been generally used conventionally.

Here, the above-mentioned configuration is adopted, for example, 3
An example of a current control device for a phaseless commutator motor is shown below.
4, 5 and 6, FIG. 4 is a functional block diagram of the current control device, FIG. 5 is a hardware configuration thereof, and FIG. 6 is a specific configuration of the AD converter and its peripheral circuits in FIG. 5. Are respectively shown.

In the figure, reference numeral 1 denotes a permanent magnet type synchronous motor, reference numeral 5 denotes an encoder directly connected to the permanent magnet type synchronous motor 1, and reference numeral 6 receives an output pulse of the encoder 5 and sends out a position signal θ (n). The magnetic pole position detecting section is composed of 2a, 2b
Is an A-phase and B-phase current i flowing through the permanent magnet synchronous motor 1
_a, a current sensor for detecting a i _b respectively, the current sensor 2a 7, the voltage signal v _a from 2b, v _b, the position signal θ
(N), an MPU that outputs a voltage command v ^* (n) in response to a torque command T ^* (n) and an interrupt request signal INT from the digital carrier generation circuit 4b.
The MPU 7 has a hardware configuration,
a, input / output ports 7b to 7g, an AD converter and its peripheral circuit 7h (for details, see FIG. 6). On the other hand, as shown in a functional block diagram, a position signal θ (n) and a torque command T ^* ( current command receives n) i ^* (n) and sends the current command production unit 7j, the voltage signal v _a, v _b AD conversion unit 7h for sending the received and current command i (n) to the current command i ^* (N) and i (n) are received and the voltage command v ^* (n)
From the current control unit 7k.

Reference numeral 4 denotes a sub-harmonic PWM for transmitting a switching signal S in accordance with a voltage command v ^* (n).
Reference numeral 3 denotes a voltage source inverter for applying a voltage to the permanent magnet type synchronous motor 1 according to the switching signal S. The sub-harmonic PWM 4 generates a digital carrier and an interrupt request signal INT as shown in FIG. And a PWM circuit 4a that receives the carrier and the voltage command v ^* (n) and sends out a switching signal S. On the other hand, the voltage-source inverter 3 includes transistors 3a to 3f and diodes. 3
g to 3 l. The symbol L indicates an armature winding of the electric motor.

Here, the above i ^* (n), i (n), v ^*
(N), v ^* _a (n) v ^* _b (n), v ^* _c (n),
i and S are respectively represented by the following equations.

(Equation 1)

(Equation 2)

(Equation 3)

(Equation 4)

(Equation 5)

(Equation 6)

(Equation 7)

A program is written in the CPU 7a of the current control device, and is controlled according to a flowchart shown in FIG. Hereinafter, the operation of this device will be described according to a program. First, when the program starts, in step 1, preprocessing is performed, and the digital carrier generation circuit 4b of the sub-harmonic PWM 4 is executed.
As shown in FIG. 8, when the generated carrier reaches the top, an interrupt request signal INT is output from the digital carrier generation circuit 4b to the input port 7g of the MPU 7, and the process proceeds to an interrupt routine. Then, in step 2, to sample and hold the A-phase current i _a by the holding circuit 17b, and starts the AD conversion by the AD converter 17c, the process proceeds to step 3, in step 3, the voltage from the current control unit 7k command v ^* (N-1) is output, and the process proceeds to step 4. In step 4, the process waits until the A / D conversion is completed, and proceeds to step 5. In step 5, i _a (n)
And input to the current control unit 7k, and the process proceeds to step 6, in step 6, this time hold circuit 1 B-phase current i _b
7b, sample-hold is performed, AD conversion is started by the AD converter 17c, and the process proceeds to step 7. In step 7, the process waits until the AD conversion is completed, and proceeds to step 8.
In step 8, i _b (n) is input to the current control unit 7k, and the process proceeds to step 9. In step 9, the magnetic pole position θ (n) and the torque command T ^* (n) are input to the current command creating unit 7j. Then, proceed to step 10 and step 10
In step 1, the current command i ^* (n) is calculated, and
1 and in step 11, the voltage command v ^* (n)
And returns, and returns the next interrupt request signal INT
I will wait.

[0011]

In the vicinity of the apex of the carrier wave, as shown in FIG. 9, the plus sides 3a, 3c and 3e of the transistor arm of the inverter 3 are provided.
Or, the minus sides 3b, 3d, 3f are all on. Here, for example, when the minus sides 3b, 3d, and 3f are all on, that is, when the carrier is convex, the permanent magnet synchronous motor 1 is in a state where the torque and the rotation direction match. Te, flows a current i _a is the electric motor 1, when the current i _b, i _c is assumed to be flowing out of the motor 1, the equivalent circuit is as shown in Figure 10. Symbols E _a , E _b , and E _c indicate induced voltages of the electric motor 1 and are represented as constant voltages for convenience.

[0012] In this state, current, winding resistance R and the induced voltage E _a of the electric motor 1, E _b, the E _c, 1
As shown in FIG. 1, it decreases with time.

[0013] Thus, the B-phase sampling later than the A-phase is performed, as shown in FIG. 11, that .DELTA.i _b only less than if the A-phase at the same time as sampling is performed is sampled Become. This relationship is different from that of FIG. 11 when the torque and the rotation direction of the electric motor 1 match, even when all the plus sides 3a, 3c, 3e of the transistor arm are on. This also occurs in the case.

Here, assuming that the gain K in Equation 4 is sufficiently large, the following equation is established between the current command i ^* (n) and the fed back i (n).

(Equation 8)

Therefore, the phase B in which the current sampling is always delayed as compared with the phase A has equivalently a smaller feedback amount than the phase A, and as a result, the amplitude of the current actually flowing through the motor 1 becomes larger than the phase A. It will be a large value. Incidentally, C-phase current i _c is, A, B-phase current i _a, in order to become a dependent function of i _b, is expressed by the following equation.

(Equation 9)

Here, the torque constant K _t (θ) is assumed as follows.

(Equation 10) Here, _Kt is a constant. Then, the relational expression between the phase current i and the generated torque T (θ) is as follows.

[Equation 11]

If the amplitude of each phase current is equal to I in equation (11) and there is no distortion, the generated torque T
(Θ) is given by the following equation, which is constant without depending on the electrical angle θ.

(Equation 12)

However, the amplitude of the phase current is i _a , i
_{If b} differs, the generated torque T (θ) is expressed by the following equation, and two cycles of pulsation occur per electrical angle 2π.

(Equation 13) Here, alpha; (amplitude i _a) (i amplitude _b) / I; amplitude i _a

As described above, in the conventional method for detecting the current of the electric motor, the current sampling is always delayed from the A phase by the B sampling.
The phase has a problem that the amplitude of the current becomes larger than that of the A phase, and the influence of the current pulsation due to the PWM voltage becomes large. As a result, the current control cannot be performed with high accuracy, and the torque ripple becomes large.

Here, a proposal for solving the above problem has been made in Japanese Patent Application Laid-Open No. 6-245538. The current detection method for an AC motor described in Japanese Patent Application Laid-Open No. 6-245538 is such that the order of sampling of each phase shown in FIGS. However, even in this case, as is clear from FIG. 11, on average, the value to be actually sampled can be made closer to the value to be sampled as compared with the above-mentioned value. Can not.

Accordingly, the present invention provides a current detection method for an AC motor which can reduce the influence of current pulsation due to the PWM voltage, perform high-precision current control, and reduce torque ripple. The purpose is to do.

[0022]

In order to achieve the above object, a method for detecting a current of an AC motor according to the present invention includes the steps of driving an AC motor with an output signal of a PWM inverter in which a modulated wave signal and a carrier wave signal are superimposed. At least two-phase inverter output currents are sampled in order, A / D converted and fed back to the voltage command unit, the current amount is set and output as the modulated wave signal, and the first phase of the multi-phase sampling is output. Sampling is performed at a timing that is approximately 1/2 of the conversion time of the AD converter before the maximum amplitude value of the carrier, and sampling of the last phase is performed based on the conversion time of the AD converter based on the maximum amplitude value of the carrier. , And the sampling order of the polyphase is sequentially changed every time.

[0023]

According to the method for detecting the current of an AC motor in such a means, the sampling of the first phase among the samplings of the multi-phases is performed based on the maximum amplitude value of the carrier wave and about 1/1 / the conversion time of the AD converter. Sampling of the last phase is performed at a timing that is approximately two-half before the conversion time of the AD converter from the maximum amplitude value of the carrier, and the order of sampling of the polyphase is one. Since the values are sequentially replaced each time, on average, the values to be actually sampled are approximately equal to the values to be sampled.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram schematically showing current sampling and its timing in a method for detecting a current of an AC motor according to an embodiment of the present invention. FIG. 2 is a flowchart of a program stored in a CPU of the AC motor. This current detection method is applied to the current control devices described with reference to FIGS. 4, 5 and 6 of the prior art, and the device configuration is the same, so that the description here is omitted to avoid duplication.

The difference between the current detection method of the present embodiment and that of the prior art is that the sampling of the first phase of the two-phase sampling, for example, the sampling of phase A, is performed based on the maximum amplitude value of the carrier wave. carried out at about 1/2 before the timing of the conversion time T _conv of the next (last) the sampling of the B phase sampling, conversion time of the AD converter 17c than the maximum amplitude of the carrier wave T
performed at timing after substantially half a _conv, if swapping the output current i _a of the inverter 3, the order of the sample and hold the i _b reversed every time, for example, A phase and sampled in the order of B-phase For example, next, sampling is performed in the order of the B phase and the A phase. A program for executing such an operation procedure is written in the CPU 7a of the current control device, and is as shown in FIG.

The operation will now be described with reference to the flowchart shown in FIG. First, when the program starts, preprocessing is performed in step 1 and then the process proceeds to step 2 where X = 1 is set. Here, X is a variable for determining the order of sampling and holding the currents of the A-phase and the B-phase. In this embodiment, the X-phase current is set when X = 1, and the X-phase current is set when X = -1. Is set to sample and hold the B-phase current.

As shown in FIG. 1, before the carrier wave generated by the digital carrier wave generating circuit 4b of the sub-harmonic system PWM 4 reaches the peak, that is, the conversion time T _conv of the AD converter 17c from the peak of the carrier wave is approximately one. / 2
On the front side, the digital carrier generation circuit 4b sends the MPU
7, an interrupt request signal INT is output to the input port 7g, and the process proceeds to an interrupt routine. And step 3
It is determined whether or not X = 1. If X = 1, the process proceeds to step 4. On the other hand, if X = 1, that is, if X = -1, the process proceeds to step 15. here,
In the present embodiment, since X = 1 is set in step 2, the process proceeds to step 4.

[0028] Here, the flow from step 4 to step 13 are similar to steps 2 through Step 11 described in FIG. 8 of the prior art, the sample-hold in step 4, the A-phase current i _a by the holding circuit 17b And AD
The A / D conversion is started by the converter 17c, and the process proceeds to step 5. In step 5, the voltage command v ^* (n-1) is output from the current control unit 7k, and the process proceeds to step 6. In step 6, the A / D conversion is performed. There proceeds to step 7 wait until the end, in step 7, type i _a (n) to the current control unit 7k, the process proceeds to step 8, in step 8, now holds the B-phase current i _b circuit 17b , And the AD converter 17c starts AD conversion.

As shown in FIG. 1, the sampling is started from the top of the carrier wave by the conversion time T of the AD converter 17c.
It is a time point approximately half after _conv .

Then, the process proceeds to step 9, and in step 9, waits until the A / D conversion is completed, and proceeds to step 10. In step 10, i _b (n) is input to the current control unit 7 k, and the process proceeds to step 11. In step 11, the magnetic pole position θ (n) and the torque command T ^* (n) are input to the current command creating unit 7j, and the process proceeds to step 12.
In step 12, the current command i ^* (n) is calculated, and the process proceeds to step 13. In step 13, the voltage command v ^* (n) is calculated, and the process proceeds to step 14.

Then, in step 14, the sign of X is reversed, that is, X = X · (−1) is reset, and
The process returns and waits for the next interrupt request signal INT.

Here, when the next interrupt request signal INT is input, it is determined in step 3 whether X = 1 or not. This time, the sign is reversed from the previous time, and X = −1 is set. Go to step 15.

The flow from step 15 to step 21 is obtained by the step 4 to flow A from step 10, the current i _a of B-phase, the order of the sample and hold the i _b reversed. That is, in step 15, samples and holds the B-phase current i _b in the hold circuit 17b, A
The A / D conversion is started by the D converter 17c, and the process proceeds to step 16. In step 16, the voltage command v ^* (n-1) is output from the current control unit 7k, and the process proceeds to step 17.
In step 17, the process waits until the A / D conversion is completed, and proceeds to step 18. In step 18, i _b
(N) is input to the current control unit 7k, and the process proceeds to step 19, in step 19, this time is sampled and held in the hold circuit 17b a A-phase current i _a, AD converter 17
Start the AD conversion in c, the process proceeds to step 20, in step 20, the process proceeds to step 21 to wait until AD conversion is completed, in step 21, by entering i _a (n) to the current control unit 7k, Proceeding to step 11, the same flow from step 11 to step 14 follows.

In step 14, since the sign of X is reversed again, the next interrupt request signal INT
Is input, the sample and hold order is now A
Phase, then B phase.

FIG. 1 shows the sampling of the current and its timing. Incidentally, black circle on the inverter output current i _a in FIG. 1 is the value to be sampled, i.e. the value of the maximum amplitude value of the carrier wave, triangle are respectively actual values for sampling.

As described above, in the present embodiment, the phase A,
The sampling of the first phase among the samplings of the B phase is performed by using the maximum amplitude value of the carrier wave as approximately 1 conversion time T _conv of the AD converter 17c.
/ 2 before the timing, sampling of the next phase is performed based on the maximum amplitude value of the carrier wave, and the conversion time T _{conv of the} AD converter 17c.
This is performed at a timing approximately half of the above, and the sampling order of the A-phase and the B-phase is changed every time in order. Therefore, it is clear from the comparison between the black circle and the triangle in FIG. Meanwhile, on average, the value to be actually sampled is approximately equal to the value to be sampled. Therefore, PW
The influence of the current pulsation due to the M voltage is reduced, so that the current control can be performed with high accuracy and the torque ripple can be reduced.

FIG. 3 is a diagram schematically showing current sampling and its timing in a current detection method according to another embodiment of the present invention. The difference between the current detection method of this embodiment and that of the previous embodiment is that the current is sampled before and after the maximum positive amplitude value of the carrier wave only (in the previous embodiment, the maximum value of both the positive and negative carrier waves). The sampling of the first phase of the sampling of the A phase and the B phase is performed
From the maximum amplitude value of the carrier, the conversion time T of the AD converter 17c is calculated.
Sampling of the next phase is performed at a timing that is about 1/2 before the _conv , and sampling of the next phase is performed at about a half of the conversion time T _conv of the AD converter 17c from the maximum amplitude value of the carrier.
It is the same as the previous embodiment in that the order of sampling of the phase and the phase B is changed every time.

Therefore, it is needless to say that the same effect as in the previous embodiment can be obtained in this case.

Although the invention made by the inventor has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the invention. Needless to say, for example, in the above embodiment, the current is sampled before and after both the positive and negative maximum amplitude values of the carrier wave or before and after the positive only maximum amplitude value. It is also possible to perform before and after only the maximum amplitude value.

Further, in the above embodiment, the carrier is set to 3
Although it is a square wave, it is not limited to a triangular wave,
A saw wave or the like may be used.

The motor 1 is not limited to a three-phase non-commutator motor, but may be an induction motor, a reluctance AC motor, or the like. The present invention is also applicable to AC motors other than three-phase.

[0042]

As described above, according to the AC motor current detection method of the present invention, the sampling of the first phase of the sampling of the polyphase is performed by the conversion of the AD converter based on the maximum amplitude value of the carrier. Sampling of the last phase is performed at a timing approximately 1/2 of the time before, and sampling of the last phase is performed at a timing approximately 1/2 of the conversion time of the AD converter from the maximum amplitude value of the carrier wave. Since the order is changed every time and averaged, the value to be actually sampled is approximately equal to the value to be sampled. Therefore, the influence of the current pulsation due to the PWM voltage is reduced, and the accuracy is improved. Current control can be performed, and the torque ripple can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating current sampling and its timing in a method for detecting a current of an AC motor according to an embodiment of the present invention.

FIG. 2 is a flowchart of a program stored in a CPU of the AC motor.

FIG. 3 is a diagram schematically illustrating current sampling and its timing in a current detection method for an AC motor according to another embodiment of the present invention.

FIG. 4 is a functional block diagram of a current control device to which a current detection method for an AC motor according to the related art is applied.

FIG. 5 is a hardware configuration diagram of a current control device to which a current detection method for an AC motor according to the related art is applied.

FIG. 6 is a configuration diagram specifically showing an AD converter and its peripheral circuits in FIG. 5;

FIG. 7 is a flowchart of a program stored in a CPU in FIG. 5;

FIG. 8 is a diagram schematically illustrating current sampling and its timing in a current detection method for an AC motor according to the related art.

FIG. 9 is a diagram showing a relationship between ON / OFF of each transistor arm and a carrier wave.

FIG. 10 is an equivalent circuit diagram of a main part of FIG.

FIG. 11 is a diagram showing a temporal change of a current of each phase and a carrier wave.

[Explanation of symbols]

1 AC motor 3 inverter 4 subharmonic, PWM circuit 7 MPU 17b sample and hold circuit 17c AD converter i _a, i _b, i _c inverter output current

Claims (1)

(57) [Claims] 1. A P signal obtained by superimposing a modulated signal and a carrier signal.
In driving the AC motor by the output signal of the WM inverter, at least two-phase inverter output current is sampled in order,
A / D-converted and fed back to the voltage command unit, set the current amount and output as the modulated wave signal, and sampled the first phase of the multi-phase sampling from the maximum amplitude value of the carrier wave. Approximately 1/2 of conversion time of converter
In addition to performing at the timing before, sampling of the last phase,
A method for detecting current of an AC motor, comprising: performing the sampling at approximately half the conversion time of the AD converter from the maximum amplitude value of the carrier wave, and changing the order of sampling of the polyphase one by one. .