JPH0984363A - Dead-time compensating method for inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform stable compensation in the total area of an inverter without using extra hardware for dead-time compensation by setting the dead-time compensation value to 0 for the preset frequency of the inverter and setting the value closer to 0 for the maximum value as much as possible. SOLUTION: R, S and T are three-phase AC power supplies. The AC power supplies are converted to DC by a forward rectifier 20. The DC is converted to the AC with a smoothing capacitor 21 and a reverse inverter 22 by PWM control. An induction motor load 23 and a one-chip CPU 24 having a PWM generating means 25 and a dead-time correcting means 26 are provided. A frequency command device 27 is provided. An inverter, which controls the speed of the induction motor 23, is constituted of these parts. In the vicinity, where the output voltage of the inverter is 0, it is possible that the output voltage becomes unstable by the excessive correction. In the vicinity, where the output voltage V has the maximum value, the compensation at the dead-time compensation value cannot be performed. Then, the coefficient is set so that the PWM compensation value is 0 at the output voltage and becomes small in the vicinity of the maximum value. The PWM compensation value is made to be the value obtained by multiplying the coefficient on the dead-time compensation value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter dead time compensation method for preventing a short circuit between upper and lower switching elements of an inverter in a speed control of an induction motor by a PWM control output of the inverter.

[0002]

2. Description of the Related Art In the conventional inverter, as shown in FIG. 1, R, S, T are three-phase AC power supplies, 1 is a forward converter for converting the three-phase AC power to DC, and 2 is an inverse converter. DC to PW
Converted to AC by M control, and 3 is an induction motor load. When controlling the speed of an induction motor with this conventional inverter,
It is necessary to increase the carrier frequency in order to prevent noise due to the PWM control carrier frequency, but the upper and lower switching elements U + and U-, V + and V-, W + shown in FIG.
The waveform distortion component caused by the arm short circuit prevention period (dead time) of W and W-caused an increase in the ripple of the output torque and an unstable operation for a light load.

As shown in FIG. 2, the PWM command voltage U _ON
On the other hand, when the current i + flows out to the induction motor 3 as shown in FIG. 1, the output voltage U ₊ for one time is the dead time T _d.
When the current i- flows in from the induction motor 3 as shown in FIG. 1, the output voltage U- increases by the dead time _Td . As shown in FIG. 3, the phase order of the inverter output is W, V, U in the order of the phase currents iU, iV, i.
When W is an outflow, it is "1", and when it is an inflow, it is "0". The average loss values ΔU _U , ΔU _V , and ΔU _W of the output voltage due to the current vector I and the dead time Td are as shown in FIG. If I is known, the direction of the output voltage loss ΔV can be known, and if the loss ΔV is added to the command voltage V, the output voltage loss can be compensated.

As a concrete example of the conventional dead time compensation, the one shown in FIG. 4 is a hardware method, in which the dead time compensation value T _d , is added to the pulse command value T _ON by the adders 4 and 5.
A method of performing PWM control of the inverse converter 2 by adding −T _d ,
In the method using software shown in FIG. 5, the output voltage command value of the inverter is calculated in step 100, and the step 10
In step 1, calculate the PWM pulse command value T _ON , and
In step 02, the pulse command value T _ON ± dead time T _d is calculated according to the direction of the output current of the inverter.
In step 3, the upper and lower limits of the pulse command value T _ON ± dead time T _d are calculated, and in step 104, the dead time is compensated by T _ON ± T _d .

[0005]

SUMMARY OF THE INVENTION The present invention requires an extra control circuit in a hardware method as a dead time compensating method, and requires a program execution time at a high carrier frequency in a software compensating method. There was a problem that had to be addressed.

[0006]

According to the present invention, the dead time compensation value of the pulse command value of the upper and lower switching elements of the inverter is set to 0 and the maximum value of the set frequency of the inverter so that the dead time compensation value becomes zero. I compensated the time.

[0007]

The dead time compensation value can be made variable without using extra hardware for dead time compensation, so that stable compensation can be performed in the entire area of the inverter. You can speed it up.

[0008]

EXAMPLE An explanation will be given below with reference to FIGS. 6 to 10.
R, S, T are three-phase AC power supplies, 20 is a forward converter for converting the AC power supply to DC, 21 is a smoothing capacitor, 22 is an inverse converter for converting DC to AC by PWM control, and 23 is Induction motor load, 24 is a 1-chip CPU, PWM generating means 2
5 and dead time correction means 26, and 27 is a frequency commander, which constitutes an inverter for controlling the speed of the induction motor 23.

Inverter output voltage vectors V _{1 to} V ₆
The relationship between the current vectors I _{1 to} I ₆ is as shown in FIG. 7. For example, when the voltage vector V ₁ is set to (001) and the current vector I ₁ becomes (001), the output voltage of the U phase Is ON, the output voltage of the V phase is OFF, the output voltage of the W phase is OFF, the U phase current i _{U +} flows in the direction of the induction motor 23, and the V phase current i _V − and the W phase current i _W − are the induction motor. As shown in FIG. 6, the current flows from 23 in the direct current direction. U-phase current is i
_{When U} +, the voltage loss due to the dead time _Td is −V DC · T _d , so the voltage V _d to be compensated is V _d = V DC · T _d , and the compensation voltage of each phase of U, V, W. Is as shown in Table 1.

[0010]

[Table 1]

Here, when the output voltage of the inverter is near 0, and when the compensation voltage V _d consisting of the voltage command value V _OUt before dead time compensation is reached, correction is performed as shown in Table 1,
There is a possibility that the output voltage becomes unstable due to overcorrection, and when the output voltage V is near the maximum value, the dead time compensation value V _d
Will not be able to compensate. In addition, the voltage command value V _OUT +
In the case of V _d size becomes 100%, completely can not be the correction in the _{V d.} Therefore, the PWM compensation value V _d is as shown in FIG.
The coefficient h is set so that the compensation value V _d becomes small in the vicinity of 0 and the maximum value of the output voltage. When the output command voltage V _OUt is _close to 0 and the maximum value, the compensation value V _d ′ = h · V _d is set.

Next, referring to FIG. 9, a program for calculating each carrier period will be described. In step 200, V (output voltage) ± V of the inverter is calculated by the current vector.
_d (compensation value) is calculated, and in step 201 the output voltage V
Calculate the PWM pulse command value T _ON of
The PWM command pulse compensated in 2 is output.

Based on FIG. 10, the compensation value V _d and the inverter output current are calculated at regular intervals of the carrier period _d .
Describing the program for calculating the direction of the vector, to calculate the V _d value by h value of the command voltage V _out and 8 in step 300. In step 301, the direction of the current vector is calculated according to the phase of the output current of the inverter and the result is stored in RAM for use in the flow chart of FIG. Since this portion is not calculated for each carrier cycle, the calculation time of FIG. 9 performed for each carrier cycle is shortened.

[0014]

According to the present invention, the program control of the dead time compensation can be speeded up without adding hardware, and the waveform distortion of the output voltage due to the dead time can be stably compensated over the entire output frequency range of the inverter. It was

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a conventional inverter.

FIG. 2 is an explanatory diagram of a conventional inverter pulse command.

FIG. 3 is an explanatory diagram of a conventional inverter output waveform.

FIG. 4 is an explanatory view of a conventional hardware dead time compensation method.

FIG. 5 is a flowchart of a dead time compensation method using conventional software.

FIG. 6 is a block circuit diagram of an inverter according to the present invention.

FIG. 7 is a vector diagram of the output voltage and output current of the inverter.

FIG. 8 is a diagram showing a dead time compensation coefficient with respect to the frequency of the inverter.

FIG. 9 is a software flowchart of the dead time compensation method of the present invention.

FIG. 10 is a software flowchart of the dead time compensation method of the present invention.

[Description of Reference Signs] 1 forward converter 2 inverse converter 3 induction motor load 4 adder 5 adder 20 forward converter 22 inverse converter 23 induction motor 24 1-chip CPU 25 RWM generation means 26 dead time correction means 27 frequency command vessel

Claims (1)

[Claims] 1. A forward converter for converting an AC power supply into a direct current, an inverse converter for converting an output of the forward converter into an alternating current by PWM control, and an inverter for controlling a speed of an induction motor by an output of the inverse converter. In the inverter, the pulse command value for turning on and off without shorting the upper and lower switching elements of the inverse converter, the dead time compensation value for compensating the pulse command value of the upper and lower switching elements, and the dead time compensation value The dead time compensation method of the inverter in which the set frequency is set to 0 and the maximum value is set to 0.