JPH06292385A - Digital current control method for induction motor - Google Patents

Abstract

PURPOSE:To obtain the large gain of a control system and make a detected current value used in digital current control agree with an instructed current value by a method wherein an estimated current value calculated taking the delay of a calculation time into account is used as the detedted current value. CONSTITUTION:The current values of the respective phases of an induction motor 9 are detected by current sensors 8, inputted to the current detectors 7 of a current control part and converted from analog values into digital values. Then, in current estimating devices, deviations between the present digital current values and the previous digital current values are multiplied by a constant and the multiplied values are added to the present digital current values and the resultant values are used as estimated current values. Then, deviations between instructed current values of (u), (v) and (w) phases and the estimated current values of the respective phases are obtained by a deviation calculator 2. The obtained deviations are multiplied by a given gain K by a multiplier 3. Voltage instruction values of (u), (v) and (w) phases which are obtained by voltage output converters 4 in accordance with the multiplication results are outputted to a PWM circuit 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital current control method for an induction motor capable of compensating for current detection delay.

[0002]

2. Description of the Related Art In a conventional digital current control method, the current of each phase (u, v, w) of an induction motor is detected and converted into a digital value, and the deviation between the digital value and each phase current command value is calculated. It is detected, multiplied by a gain (constant), and output to the PWM circuit as a voltage command value for each phase by the voltage output converter, and the current of each phase is controlled to be the same as the current command value.

[0003]

In the above method, the time difference between the current detection time of each phase and the voltage command output time of each phase is not taken into consideration. At the time of voltage command output, the current should have changed from the value at the current detection time. Since the conventional method does not calculate the voltage command value in consideration of the time difference (delay), if the gain (constant) is large, the control system becomes unstable, and as a result, the control becomes impossible. Also,
Even if the gain (constant) is increased up to the limit where the control system becomes unstable, there is a problem in that the followability of the current is poor and, as a result, the normal torque is not output.

In view of the above problems, it is an object of the present invention to provide a digital current control method capable of reducing the time difference and performing stable control.

[0005]

A digital current control method according to the present invention includes a digital current command value provided from a slip frequency vector control device and a digital current value indicating a current supplied to an induction motor under the control of a PWM circuit. Is a digital current control method for calculating a deviation of each sampling timing and instructing a PWM circuit with a voltage command value based on the calculated deviation. When the deviation is multiplied by a constant and the current digital current value is added, and the result is used as the predicted current value, the deviation between the digital current command value and the digital current value is calculated. The predicted current value is used instead.

Preferably, a digital current value one sampling timing before is used as the past digital current value, and the constant is 0 or more and 1 or less.

[0007]

In the digital current controller thus constructed, the currents of the u, v and w phases are read and converted into digital values. The current detection value obtained at the present time and the change amount of the current detection value in the past by one sampling timing are multiplied by an appropriate constant (0 ≦ k _x ≦ 1) and added to the current detection value obtained at the present time. This value is used as a current prediction value to obtain the deviation, calculate the voltage output value of each phase of u, v, w and output it to the PWM circuit so that the time delay is reduced and the current of each phase matches the command value. become.

[0008]

Embodiments of the present invention will now be described with reference to the drawings. 1 is a block diagram showing an embodiment of the digital current control device of the present invention, and FIG. 2 is a graph showing the operation of the embodiment of FIG. Slip frequency vector control device 1
(Hereinafter, abbreviated as the vector control device 1) is the induction motor 9 detected by the torque command and the position detector 10.
The digital current command value for each phase of u, v, and w is output according to the rotor position data of.

The current value of each phase of the induction motor 9 is detected by the current sensor 8 and input to the current detector 7 of the current control section, where it is converted from an analog value to a digital value. The current predictor 6 calculates the predicted current value based on the detected current value supplied from the current detector 7. Here, the calculation of the predicted current value will be described with reference to FIG. Currently, that is, the current value at time t _n and i (n), 1 step previously i.e., the time t _n-1 of a current value i (n-1), the constant k _x is suitably selected from a program or on experimental results
_{(0 ≦ k x ≦ 1)} , at time t _n depends on k _x, the delay of the circuit or the like, actually predicted current values is predicted as a current value at time t 'which command is executed i ^* (n )
Then, the predicted current value i ^* (n) can be obtained by the following equation (1).

[0010] ^{i * (n) = i (} n) + k x · (i (n) -i (n-1)) ··· (1) In other words, to absorb the time delay of the circuit and the like in the k _x, time t _n
As the current value i (n) at, the predicted current value i ^* (n)
By giving, the precise control is intended.

The deviation calculator 2 calculates the deviation between the u, v, and w phase current command values and the predicted current value. The multiplier 3 multiplies the deviation calculated by the deviation calculator 2 by the given gain K. The voltage output converter 4 outputs u, v, w phase voltage command values to the PWM circuit 5 based on the result of the multiplication so that the deviation of the output current of the PWM circuit 5 is canceled. Control the current of each phase.

[0012]

As described above, according to the present invention, the current detection value used in the digital current control is set to the predicted current value in consideration of the delay of the calculation time.
The gain (constant) of the control system can be made large, resulting in the effect that the detected value is in good agreement with the command value.

[Brief description of drawings]

FIG. 1 is a block diagram showing a digital current control device to which an embodiment of a digital current control method of the present invention is applied.

FIG. 2 is a graph showing the operation of the apparatus of FIG.

[Explanation of symbols]

 1 Slip frequency vector control device 2 Deviation calculator 3 Gain (constant) 4 Voltage output converter 5 PWM circuit 6 Current predictor 7 Current detector 8 Current sensor 9 Induction motor 10 Position detector Δt Sampling time

Claims (2)

[Claims] 1. A deviation between a digital current command value given from a slip frequency vector control device and a digital current value showing a current flowing through an induction motor under the control of a PWM circuit is calculated at each sampling timing and calculated. In a digital current control method for instructing a PWM circuit with a voltage command value based on a deviation, a deviation between a current digital current value and a past digital current value multiplied by a constant and a current digital current value Is added, and the result is used as a predicted current value, and when the deviation between the digital current command value and the digital current value is calculated, the predicted current value is used instead of the digital current value. Method. 2. The past digital current value is 1
Using the digital current value before the sampling timing,
The digital current control method according to claim 1, wherein the constant is 0 or more and 1 or less.