JP2624761B2 - AC motor control method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC motor control method, and more particularly, to an AC motor control method suitable for controlling an AC motor by software processing.

<Prior Art> In speed control of an AC motor such as an induction motor or a synchronous motor, a current command (torque command) is generated in accordance with a deviation between the command speed and the actual speed. A phase voltage command is generated based on the difference, and if the phase voltage command is equal to or greater than a preset limit value P _O , the voltage is clamped to the limit value, and a PWM signal is generated based on the clamp output to start the AC motor. Control.

FIG. 7 is a block diagram of a current loop in a conventional AC motor control device, and shows only one phase. In the figure, 1 is synthesizing unit for outputting a difference between the current command (torque command) Tr and the detection current Ir, 2 has an integral gain k _1, synthesis section 1
3 has a proportional gain k ₂ , and k ₂
And amplifier for outputting Ir, 4 combining unit for generating a phase voltage command corresponding to the difference (integrator output) and k ₂ · Ir between the detected current and the current command, 5 PWM signal generating section for generating a PWM signal If the absolute value of the phase voltage command is less than the limit value P _O , the clamp is not performed. If the absolute value of the phase voltage command is more than P _O , the phase voltage command is clamped to the limit value P _O. A pulse width modulation signal (PMW signal) is generated based on the (clamp output). 6 is a servo amplifier 6a and a servo motor
6b, etc.

The PWM signal is generated by comparing the magnitude of the clamp output voltage with the sawtooth wave having a constant frequency and outputting a high-level pulse while the clamp output voltage is higher.

FIG. 8 is a block diagram in the case where the current control of FIG. 7 is realized in a discrete value system by digital processing.

11 and 14 are operation units, 12a is integration gain setting unit, 12b is SUM
An accumulation adder (integrator) that outputs (n), 13 is a proportional gain setting unit, 15 is a PWM signal generator, and has a clamp unit 15a and a signal generator 15b. Reference numeral 16 denotes a hardware unit such as a servo amplifier 16a and a servo motor 16b.

Each unit performs digital processing for servo control every predetermined sampling time T _S and rotates at the command speed of the AC motor 16b. That is, in the n-th cycle (slot) of each T _S , the arithmetic unit 11 sets the torque command T (n) and the detected current
Ir difference (n) calculating the accumulation adder 12b, and the calculation unit 14 shown below, respectively (1), (2) SUM (n) = SUN (n -1) + k 1 '{T (n) −Ir (n)｝ (1) PWM _O (n) = SUM (n) −k ₂ ′ · Ir (n) (2) The error is integrated and the phase voltage command PWM _O
(N) is output.

PWM in the case of ≦ P _O is | when the clamp portion 15a, the phase voltage command PWM _O to (n) is present within a certain limit value P _O ~-P _O, that is | PWM _O (n) (N) = PWM _O (n) (3) and output to the next stage. Incidentally, P _O is the limit value at the time of forward acceleration, -P _O is the limit value at the negative direction acceleration.

When PWM _O (n)> P _O , the clamp unit 15a clamps the phase voltage command to the limit value P _O by setting PWM (n) = P _O (4), and also sets the integral value SUM in the n-th slot. (n) the following equation _{SUM (n) = P O +} k 2 '· Ir (n) (4)' and updated _{by, PWM O (n) <-} PWM in the case of _{P O (n) = - P} O (5) and thereby clamped to limit -P _O phase voltage command, the integral value in the n slot SUM (n) the following equation _{PWM (n) = - P O} k 2 '· Ir (n) (5 ) '.

The signal generation unit 15b generates a PWM signal based on the voltage command PWM (n) output from the clamp unit 15a, and drives the AC motor 16b via the servo amplifier 16a based on the PWM signal,
Rotate at command speed.

Thus, in the conventional digital processing, the phase voltage command PWM _O
(N) with clamps to limit the said phase voltage command when reaches or exceeds the limit value P _O increases (4) ',
The saturation processing is performed by the equation (5) '. For this reason, the pulse width of the PWM signal can be suppressed to a predetermined width or less, and the continuity and stability of rotation of the AC motor can be maintained.

<Problems to be Solved by the Invention> Now, the phase voltage command PWM _O (n) repeats the direction reversal with the rotation of the AC motor, but the phase voltage command has a large value until just before the current command direction reversal of each phase. If the saturated state continues, the integrator output SUM (n) becomes a large value according to the limit value _PO according to the equations (4) 'and (5)'.
Even if the current command (torque command) T (n) is reversed, the phase voltage command PWM _O (n) cannot be reversed immediately, causing a phase delay and an increase in the reactive current component, resulting in heat generation and the like.

From the above, an object of the present invention is to provide an AC motor control method capable of smoothly inverting the integrator output SUM (n), thus reducing the delay in reversing the direction of the phase voltage command PWM _O (n) and reducing the reactive current component. To provide.

<Means for Solving the Problems> FIG. 1 is a block diagram of AC motor current control according to the present invention realized in a discrete value system.

11, 14 are operation units, 12b is an accumulation and addition unit (integrator) that outputs SUM (n), 15 is a PWM signal generation unit, 15a is a clamp unit,
15b is a signal generator, and 16b is a servomotor.

<Operation> The difference between the current command T (n) and the detected current Ir (n) is integrated by the accumulation adder (integrator) 12b, and the phase voltage command PWM _O (n) corresponding to the integrated output SUM (n) is obtained. Generated by the arithmetic unit 14.

The clamp unit 15a sets the limit value _VL to the current command T (n).
For a predetermined period immediately before the direction is reversed, the current is reduced according to the current command transfer, and when the phase voltage command PWM _O (n) is equal to or larger than the limit value V _L (n), the phase voltage command is changed to the limit value. Clamp and integrate output SUM (n) to limit value V
_The value is updated to a value according to _L (n).

The signal generator 15b controls the AC motor 16b by generating a PWM signal based on the clamp output. The above digital processing is performed every predetermined sampling time, and the AC motor rotates at the command speed.

Therefore, even if the phase voltage command PWM _O (n) is saturated just before the direction reversal of the current command T (n), the integral output SUM (n) is small at the time of the direction reversal, so that the phase output is smoothly reversed. The reversal delay of the voltage command also decreases, and the reactive current decreases.

<Embodiment> FIG. 1 is a current control block diagram of an AC motor according to the present invention realized in a discrete value system.

Numeral 11 denotes a calculation unit for outputting the difference between the torque command T (n) and the detected current Ir (n) in the n-th slot for each predetermined sampling time T _S , and 12a sets the integral gain k ₁ ′ in the discrete value system An integration gain setting unit 12b is an accumulation addition unit (integrator), and adds the integration output SUM (n-1) in the immediately preceding slot (n-1) and the difference in the current slot n to calculate the result. SUM (n) is output and stored. 13 is a proportional gain setting unit that sets a proportional gain k ₂ ′ in a discrete value system, 14 is a calculation unit that outputs a phase voltage command PWM _O (n) using the output of the integrator 12b and the proportional gain setting unit 13, and 15 Denotes a PWM signal generation unit, and 16 denotes a hardware unit such as a servo amplifier 16a and a servo motor 16b.

The PWM signal generator 15 has a clamp unit 15a and a clamp output PWM.
The signal generator 15b generates a PWM signal based on (n). The clamp unit 15a decreases the limit value V _L (n) with the lapse of time immediately before the direction of the torque command T (n) is reversed, and the phase voltage command PWM _O (n) reduces the limit value V _L (n). In the above case, the current value is clamped to the limit value V _L (n). Note that the current command T (n) changes from positive to negative and from negative to positive every 180 °, and this change is called direction reversal.

FIG. 2 is an explanatory diagram of a method of changing the limit value V _L (n) immediately before reversing the direction of the current command T (n). In FIG. 2, the limit value V _L is shown immediately before and immediately after the current command T (n).
_L (n) is smaller than the predetermined value P _O. In FIG. 2, V _L (n) is a limit value during normal rotation,
_L (n) 'limit value at the reversal, T (n)' is the current command T (n) and the phase reference signal, S _B is the reference signal absolute value signal (= | T (n) ' | ). LA is a region immediately before the direction reversal of the current command T (n) ', LB is a region immediately after the reversal of the direction of the current command, and both are limit value change regions, and LC is a limit value constant region.

As can be seen from FIG. 2, S _B ≧ a (a is 0 <a <
The limit value V _L (n) is controlled to be constant P _{O in the} constant limit value LC where the constant value is 1, for example, 0.5)), and S _B
<Controlled to be decreased from P _O with the passage direction immediately before the inversion in the region LA time a to _{a · P O, S B <} P O from a · P _O over time in the direction reversed immediately region LB of a It is controlled so as to increase until. That, (i) S if _{B ≧ a V L (n)} = P O (ii) S B> If _{a V L (n) = P} O · {(1-a) · S B / a + a The limit value V _L (n) is controlled so as to satisfy と (6).

 Next, the overall operation of FIG. 1 will be described.

Obtains a difference between the current command T (n) and the detected current Ir (n) by the calculation unit 11, the following equation SUN (n) by the integrator 12b = SUN (n-1) + k 1 '{T (n) -Ir The error is integrated by (n)｝ (7), and the calculation unit 14 calculates the following formula: PWM _O (n) = SUN (n) −k ₂ ′ · Ir (n) (8) to calculate the phase voltage command PWM _O (n) is output.

On the other hand, as shown in FIG. 2, the clamp unit 15a varies the limit value V _L (n) in the section between LA and LB, and controls it to be constant in the section between LC and the phase voltage command PWM _O (n). Compare the limit value V _L (n) and | PWM _O (n) | ≦ V
_L in the case of (n) is output to the next stage without clamping as _{PWM (n) = PWM O (} n), PWM O (n)
If> V _L (n), PWM (n) = V _L (n) (9) and the phase voltage command is clamped to V _L (n) and output, and the integral value SUN ( n) is updated by the following equation: SUM (n) = V _L (n) + k ₂ ′ · Ir (n) (9) ′ Further, PWM _O (n) <- when V _L of (n) is PWM (n) = - while clamped to V _L (n) (10) and to the phase voltage command -V _L (n),
The integrated value SUM (n) the following equation SUM in the n slots (n) = - updated by _{V L (n) + k 2} '· Ir (n) (10)'.

The signal generator 15b generates a PWM signal based on the voltage command PWM (n) output from the clamp
control b.

Thereafter, the above-described digital processing is performed every sampling time T _S , and the AC motor rotates at the command speed.

As described above, even when the phase voltage command PWM _O (n) is saturated at the time of reversing the direction of the current command T (n), the integral output SUM (n) is reduced by the small limit value V by the equations (9) 'and (10)'. _L (n)
, The direction is smoothly reversed without time delay, and the phase voltage command PWM _O (n) is also reversed in direction.
The phase lag also decreases, and the reactive current decreases.

FIG. 3 shows the case where the limit value is changed according to the present invention (a = 0.5), and FIG. 4 shows the integral output SUN of the clamp output PWM (n) when the limit value is kept constant by the conventional method.
(N) is a waveform diagram, where the upper row is PWM (n) and the lower row is SUM
(N).

As apparent from FIGS. 3 and 4, PWM (n) in the present invention is the time which is clamped to the limit value P _O of the maximum level is shorter than the conventional methods, moreover step number at the time of direction reversal And the direction is smoothly reversed. In addition, the integrated output SUM (n) of the present invention also has a larger number of stages at the time of direction inversion as indicated by a circle than the conventional method,
It turns out that it is turning smoothly. Further, the phase voltage command amount (shaded line) according to the method of the present invention in FIG. 3 is smaller than the phase voltage command amount (shaded line) according to the conventional method in FIG. 4, so that the same speed and the same torque can be obtained. It is understood that the present invention requires less applied power to rotate the AC motor.

FIG. 5 shows the case where the limit value is changed according to the present invention (a = 0.5), and FIG. 6 shows the clamp output PWM (n) and the actual current Ir when the limit value is made constant by the conventional method.
It is a waveform diagram of (n), upper stage is PWM (n), lower stage is Ir
(N).

As is apparent from FIGS. 5 and 6, the actual current (shaded line) according to the method of the present invention is smaller than the actual current (shaded line) according to the conventional method, and the AC motor is driven at the same speed and the same torque. It can be seen that the applied power required for rotation can be small.

<Effect of the Invention> As described above, according to the present invention, a predetermined period immediately before the direction of the current command is reversed is identified, and the PW in that period is determined.
Since the limit value of the M signal generation unit is reduced according to the current command phase, even if the phase voltage command is saturated at the time of current command reversal, the integral output SUM (n) is small, so the smoothness is obtained. In the same manner, the direction is reversed with no time delay. Similarly, the direction of the phase voltage command is also smoothly reversed, the phase delay is reduced, the reactive current is reduced, and the heat generation is reduced.

In addition, even when the torque command changes rapidly, the phase voltage command can be changed smoothly without time delay, so that the current loop can be operated more stably.

[Brief description of the drawings]

1 is a current control block diagram of an AC motor according to the present invention when realized by a discrete value system, FIG. 2 is an explanatory waveform diagram of variable limit value control, and FIGS. 3 and 4 are a method of the present invention and a conventional method. PW in the way
FIG. 5 and FIG. 6 show the waveforms of the M signal and the integration output.
FIG. 7 is a block diagram of a conventional AC motor control, and FIG. 8 is a conventional block diagram of controlling an AC motor by digital processing. 11, 14 arithmetic unit, 12b accumulating and adding unit (integrator), 15 PWM signal generating unit, 15a clamp unit, 15b signal generating unit, 16b servo motor

Claims (2)

(57) [Claims] A voltage command for each phase is generated in accordance with a difference between a current command for each phase and a detected current for each phase. If the voltage command is equal to or greater than a limit value, the voltage command is clamped to the limit value. An AC motor control method for controlling an AC motor by generating a PWM signal based on a clamp output, wherein a predetermined period immediately before the direction of the sine wave-shaped current command is reversed is identified, and the limit value in the period is determined. An AC motor control method characterized by gradually decreasing as time passes. 2. When the magnitude of a reference signal obtained by converting a signal having the same phase as the sinusoidal current command and having a predetermined amplitude into an absolute value is equal to or smaller than a set value, the limit value is changed to the reference signal value. The AC motor control method according to claim 1, wherein the AC motor control method changes in accordance with the following.