JPH05103471A - Control method for inverter - Google Patents

Abstract

PURPOSE:To reduce noise by decreasing a PWM modulation frequency by a quadratic function of an output voltage command value of an inverter according to a special equation if the command value is 1/2 or less of a highest voltage. CONSTITUTION:A voltage command value generator 1 generates a voltage command value V corresponding to a frequency command value F. The value V is given approximately in proportion to the value F. However, since it cannot output a highest voltage or more controllable by a sine wave PWM in a high frequency range, the value V coincides with the highest voltage, exhibiting saturated characteristics. A modulation frequency subtracter 2 generates a modulation wave number command value F'c corresponding to the value V. Here, if it generates an output voltage waveform without synchronization with the PWM modulation wave and the value V is 1/2 or less of the highest voltage controllable by the PWM, the modulation frequency Fc of the PWM is reduced by the output voltage command value of an inverter according to the equation (K1, K2: positive constants), the quadratic function of the V. Thus, noise can be reduced.

Description

Detailed Description of the Invention

 [Object of the Invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter control method for controlling an inverter that drives an inductive load by voltage type PWM control.

[0002]

2. Description of the Related Art Conventionally, in this type of inverter control method, control is often performed by an asynchronous PWM system in which an output voltage waveform is generated without being synchronized with a PWM modulation wave at a low frequency. This is because in the synchronous PWM system that generates the output voltage waveform in synchronization with the PWM modulation wave, it is necessary to frequently switch the number of pulses, and the ripples of torque and current are significantly different before and after the switching, resulting in variations in characteristics. This is because it has a drawback that there are many. Conventionally, the modulation frequency when performing asynchronous PWM control during such a low frequency operation is constant regardless of voltage or frequency.

[0003]

By the way, as described above, when operating at a constant modulation frequency, especially when the output voltage is very low, a dead period (hereinafter referred to as dead time) for preventing a short circuit between the upper and lower arms of the inverter. The error voltage included in the output PWM waveform cannot be ignored due to the influence of (1). This is because the error voltage due to the dead time is generated in proportion to the modulation frequency, and when the output voltage is low, the ratio of the error included in the output voltage increases, so the distortion of the output current increases, and in the case of a motor load, Pulsation occurs in torque.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an inverter control method for controlling an inverter for driving an inductive load by PWM control.
It is another object of the present invention to provide an inverter control method that minimizes output current distortion due to dead time. [Constitution of Invention]

[0005]

To achieve the above object, the present invention provides an inverter control method for controlling an inverter for driving an inductive load by PWM control, wherein P
When the output voltage waveform is generated without being synchronized with the WM modulation wave and the output voltage command value is 1/2 or less of the maximum voltage that can be PWM controlled, the PWM modulation frequency is a quadratic function of the output voltage command value. An inverter control method for reducing the number is provided.

[0006]

In the inverter control method of the present invention configured as described above, the modulation frequency is reduced while the inverter output current ripple accompanying the PWM control is kept at a constant level, and the output PWM waveform is changed due to the dead time. Since the included error voltage is proportional to the modulation frequency, the distortion of the output current due to the dead time is reduced, the torque pulsation is reduced when the motor is driven, and the instability that is observed during medium to low speed driving is also reduced. Furthermore, when a PWM waveform is generated using a digital circuit, the accuracy is significantly reduced when the output voltage command value is low due to the restriction of resolution determined by the clock frequency, but the present invention does not require increasing the clock frequency. The system does not decline significantly. Further, in the case of an inverter having a modulation frequency in the audible frequency range, when the output voltage is low, the modulation frequency can be reduced to a region with a relatively low audibility of several hundred Hz, so that noise is reduced.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings. In this embodiment, the three-phase induction motor is a three-phase P motor.
The case of driving with a WM inverter will be described.

FIG. 1 shows a block diagram for generating a PWM voltage when a frequency command value is given. In the figure, the voltage command value generator 1 has a frequency command value F ^*. Voltage command value V ^{* according to} Is generated, and this characteristic is shown in FIG. In the figure, the voltage command value V ^* Is the frequency command value F ^* However, in the low frequency region, it has a slightly raised characteristic to compensate for the voltage drop due to the primary resistance, and in the high frequency region, it has a sine wave PWM controllable maximum voltage V _MAX or more. Cannot be output, the voltage command value V ^* _Corresponds to the maximum voltage V _MAX and exhibits saturation characteristics. The modulation frequency reducer 2 has a voltage command value V ^*. Modulation frequency command value F _c ^{* according to} Is generated, and this characteristic is shown in FIG. In the figure, the modulation frequency command value F _c ^*
Is given by the following equation.

Here, V is a standardized value of the voltage command (voltage command value V ^* / Maximum voltage V _MAX ), F _co is the maximum value of this function, V = 1/3 ^1/2 The value at the time of. This expression holds under the condition that the maximum ripple width of the current waveform does not change with V when the load is regarded as pure inductance in the modulation frequency band by performing the three-phase sinusoidal PWM control, but the actual motor load Cutoff frequency is tens of Hz
Since the modulation frequency is several hundred Hz, it can be regarded as a pure inductance. Therefore, by reducing the modulation frequency based on the above equation, the maximum ripple width of the current waveform can be kept constant, and the influence of harmonics does not change.

Next, the integrator 3 outputs the frequency command value F ^*. Phase command value θ ^* Is what occurs. The reciprocal calculator 4 determines the modulation frequency command value F _c ^* Of the modulation cycle command value T _sw ^* To occur. Also,
The PWM voltage generator 5 is, for example, Japanese Patent Application No. Sho 62-22488.
A PWM waveform with an arbitrary voltage command value, an arbitrary phase angle command value, and an arbitrary modulation cycle command value such as 9 is formed.

FIG. 4 is a motor current simulation waveform when the modulation frequency is reduced based on the above configuration. In the figure, (a) shows V = 0.49, F _c ^*
= 2930 Hz, (b) shows V = 0.112, F _c
^* = 1050 Hz, (c) shows V = 0.038, F
_c ^* = 380 Hz is shown. It can be seen that the maximum ripple width of the current waveform does not decrease even if the modulation frequency decreases.

As described above, in the first embodiment, the modulation cycle command value T
_sw ^* Is provided to the PWM voltage generator 5,
As shown in, the modulation frequency command value F _c ^{* is} calculated using the VF converter 6 ^. It can also be realized by generating a pulse train F _CK proportional to and inputting it to the PWM voltage generator 5 as a clock. Further, although the hardware configuration is shown in the first embodiment, all or part of them may be configured by software.

[0013]

As described above, according to the present invention, when the output voltage command value is 1/2 or less of the maximum voltage, the PWM
The modulation frequency is reduced by a quadratic function of the inverter output voltage command value, which reduces the modulation frequency while keeping the inverter output current ripple at a constant level, so the effect of dead time can be reduced. When driving, the torque pulsation can be reduced, and the instability seen during medium-low speed driving can be reduced. Furthermore, when a PWM waveform is generated using a digital circuit, it is possible to prevent the accuracy from degrading when the voltage is low without increasing the clock frequency. Further, in the case of an inverter having a modulation frequency in the audible frequency range, when the output voltage is low, the modulation frequency can be reduced to a region with a relatively low audibility of several hundred Hz, so that an excellent effect of reducing noise can be obtained. is there.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing characteristics of a voltage command value generator in the embodiment shown in FIG.

FIG. 3 is a characteristic diagram showing the characteristics of the modulation frequency reducer in the embodiment shown in FIG.

FIG. 4 is a simulation waveform diagram showing the operation of the embodiment shown in FIG.

FIG. 5 is a circuit block diagram showing another embodiment of the present invention.

[Explanation of symbols]

1 ... Voltage command value generator, 2 ... Modulation frequency reducer,
3 ... integrator, 4 ... reciprocal calculator, 5 ... PWM voltage generator, 6 ... VF converter.

Claims (1)

[Claims] 1. An inverter control method for controlling an inverter that drives an inductive load by PWM control, comprising:
When the output voltage waveform is generated without being synchronized with the WM modulation wave, and the output voltage command value is ½ or less of the maximum voltage that can be PWM-controlled, the PWM modulation frequency is a quadratic function F _c of the output voltage command value. = K ₁ · (1−K ₂ · V) · VF _c : Modulation frequency K ₁ , K ₂ : Positive constant V: Output voltage command value is used for reduction.