JPH06296372A - Calculating method for output voltage of pwm inverter - Google Patents

Abstract

PURPOSE:To economically compute an output voltage without using an insulated amplifier by computing an error voltage from a waste time, PWM carrier frequency and an DC voltage of a DC power to be supplied to a voltage type PWM inverter and correcting a conduction ratio using the error voltage as a correction amount after the error voltage is caused to change in proportion to a power factor of the output. CONSTITUTION:In a vector controlled PWM inverter, a power factor is obtained in arithmetic circuits 1, 2 from a torque current command Ij and an excitation current command IM and an amount of compensation is obtained by multiplying the power factor with an error voltage reference amounmt Verr in the arithmetic circuit 3. Here, the error voltage reference amount Verr has a duration of the waste time and an amplitude of the DC voltage and should be equal to the value obtained by averaging the error voltage generated for each switching with the carrier frequency. Next, the amount of compensation is subtracted from the conduction ratio lambda* of the inverter in the arithmetic circuit 4, the lambda-V conversion is carried out in the arithmetic circuit 5 and the result is added with the DC voltage VDC to output an output voltage Vist.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWM inverter output voltage calculation method for calculating the output voltage of a voltage source PWM inverter.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 3, a voltage type PWM inverter 2 is used to detect the output voltage of a voltage type PWM inverter.
The output of 0 was converted into direct current by the rectifier 21, and the voltage was guided to the control device (not shown) and detected by using the insulating amplifier 22. However, for example, a vector-controlled voltage-type PWM inverter uses current as a control element, and it is not necessary to detect the output voltage for control purposes. Here, if the output voltage is to be detected as an additional function, the above-mentioned rectifier, isolation amplifier, and other devices must be newly added. In particular, since the isolation amplifier is expensive, the cost is increased because the additional function is added. Will rise significantly.

In such a case, the output voltage of the voltage-type PWM inverter can be calculated by the following method without using the isolation amplifier. In the voltage-type PWM inverter, the amount of conduction ratio λ ^* exists inside the control device as a control variable for performing PWM modulation. Conduction ratio lambda ^* is a value determined by comparing the peak value of the output voltage command wave and the peak value of the PWM carrier wave, an on-off operation of the semiconductor switching element based on the duty ratio lambda ^* The pulse width of the controlled pulse train is calculated, and the output voltage waveform for control is determined. The effective value of the output voltage fundamental wave for control is calculated from the output voltage waveform derived from the conduction ratio λ ^* .

[0004]

As described above, in order to directly detect the output voltage of the voltage type PWM inverter by using the isolation amplifier, the main circuit of the voltage type PWM inverter is insulated from the control device. This can be achieved by using a high-voltage isolation amplifier that can handle this. However, since an isolation amplifier satisfying such a condition is expensive, there is a problem in that cost is increased if an attempt is made to detect a voltage type PWM inverter output voltage as an additional function.

As described above, the output voltage fundamental wave effective value can be calculated from the control amount without using the isolation amplifier or the like. However, the actual output voltage has a dead time within the waiting time for preventing arm short circuit (the time until the next switching element turns on after the completion of the turn-off operation of the switching element that constitutes the main circuit of the voltage-type PWM inverter). Since the error voltage generated in () is included, there is a problem that a large error that cannot be ignored is generated between the output voltage fundamental wave effective value calculated by the above method and the actual output voltage.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a method for inexpensively calculating the output voltage of a voltage type PWM inverter without using an isolation amplifier.

[0007]

To achieve the above object, in the present invention, an error voltage is calculated from the dead time, the PWM carrier frequency, and the DC voltage of the DC power supplied to the voltage type PWM inverter. Then, the conduction ratio is corrected by making the error voltage proportional to the output power factor and using it as a correction amount.

[0008]

The correct output voltage fundamental wave effective value is calculated by applying a correction to the conduction ratio using a correction amount using the error voltage as a parameter.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the basic principle of the present invention will be described with reference to FIG. 2 by taking PWM modulation of sine wave-triangle wave as an example. FIG. 2 is a waveform comparison diagram for explaining the principle of the present invention.
(A) is a waveform diagram of an output voltage command sine wave 30 and a carrier triangular wave 31, respectively. By comparing the output voltage command sine wave 30 and the carrier triangular wave 31, the PW shown in FIG.
The M pulse waveform is derived, and the voltage-type PWM inverter performs switching according to the PWM pulse waveform to convert DC power into AC power.

Next, referring to FIG.
Assuming that the output current shown in (c) flows, an error voltage having a polarity opposite to the polarity of the output current at each switching is shown in FIG.
As shown in (d), the dead time is generated (the waiting time minus the turn-off time of the switching element).
Since the error voltage is generated in the same polarity as the carrier wave period in the opposite polarity to the output current as shown in FIG. 2 (d), if averaged in the switching period, it will approximate to the error voltage average waveform shown in FIG. 2 (e). can do.

Since the DC voltage supplied to the voltage type PWM inverter and the dead time are generally set as fixed values, the magnitude of the error voltage average waveform shown in FIG. 2E is proportional to each other. It is determined by time, switching frequency and DC voltage. Further, since the DC voltage and the dead time are fixed values, the number of times of switching is such that when the carrier frequency is constant, the conduction ratio λ ^* is λ ^* ≦ 1.
At the same time, the number of switchings decreases in the overmodulation region where λ ^* > 1. When the number of times of switching decreases, the error voltage generated due to switching also decreases, but the decrease of the error voltage due to the decrease of the number of switching is at the same rate as the decrease of the output voltage due to the decrease of the conduction ratio λ ^*. Therefore, by correcting the conduction ratio λ ^* with the error voltage and then converting it into the output voltage, it is not necessary to consider the number of times the output voltage is switched.

When the carrier frequency is variable, the correction amount by the error voltage is further proportional to the carrier frequency, and then the above-mentioned conduction ratio λ ^* is converted into the output voltage. As described above, the present invention provides a voltage-source PWM inverter
For example, it is used for an inverter that does not directly use the output voltage of a vector control PWM inverter or the like as a control element of the main circuit.

Therefore, an embodiment according to the present invention will be described with reference to FIGS.
Will be explained. Vector control PWM inverter
Since the output voltage is not used as a control element of the main circuit, it has no voltage detecting means. Although an output voltage value is required to add a function such as vector auto-tuning to the vector control PWM inverter, it is as described above that using an isolation amplifier or the like for voltage detection causes a cost increase.

The circuit shown in FIG. 1 shows a configuration for calculating an output voltage. Since the torque current command I _T and the exciting current command I _M are given to the PWM inverter that performs the vector control as control elements of the PWM inverter, the calculation circuits 1 and 2 in FIG. The power factor cosφ can be calculated by The power factor angle φ is shown in FIG. 2 (e).

[0015]

[Equation 1]

When the carrier frequency is fixed, the correction amount is obtained by multiplying the error voltage reference amount V _err in the arithmetic circuit 3 by cos φ obtained by the above equation (1). Here, the error voltage reference amount V _err has a width of dead time and a magnitude of the DC voltage in an arithmetic circuit (not shown), and is calculated by averaging the error voltage generated at each switching in the carrier wave period. It is a value. When the carrier frequency is variable, the correction amount is obtained by multiplying the error voltage reference amount by the carrier frequency.

Then, the arithmetic circuit 4 subtracts the correction amount from the inverter duty ratio λ ^* , and the arithmetic circuit 5 performs λ-V conversion. The λ-V conversion performed in the arithmetic circuit 5 is performed according to the λ-V conversion function that has previously obtained the relationship between the conduction ratio λ and the fundamental wave voltage of the inverter output determined by the value of λ. In the arithmetic circuit 6,
The output voltage V _ist is output by integrating with the DC voltage V _DC .

Of course, the present invention can be implemented by software using parameters in the control device without adding hardware such as an isolation amplifier.

[0019]

According to the present invention, the error voltage reference amount is determined by the dead time, the carrier frequency, and the DC voltage, and the output power factor c
The output voltage can be calculated only from the internal parameters of the control device by converting the correction amount obtained in proportion to osφ into the output voltage after subtracting it from the flow rate command value λ ^* .

Furthermore, in the present invention, since the calculation is performed only by the internal parameters of the control device, it is not necessary to add new hardware such as an insulation amplifier, and the output voltage can be calculated at low cost.

[Brief description of drawings]

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

FIG. 2 is a waveform comparison diagram for explaining the principle of the present invention.

FIG. 3 is a diagram showing a conventional example.

[Explanation of symbols]

 20 PWM inverter 21 Rectifier 22 Insulation amplifier 30 Output voltage command sine wave 31 Carrier triangle wave

Claims (1)

[Claims] 1. A voltage source PW for converting DC power into AC power.
In the method of calculating the output voltage of the M inverter, the turn-off operation of one semiconductor switching element of the plurality of semiconductor switching elements forming the voltage-type PWM inverter is completed and then the turn-on operation of another semiconductor switching element is started. Proportional to the output power factor of the voltage-source PWM inverter from the dead time, the frequency of the PWM carrier wave that determines the switching frequency of the voltage-source PWM inverter, and the DC voltage of the DC power applied to the voltage-source PWM inverter. The correction amount having the above value is calculated, and the correction amount is subtracted from the conduction ratio, which is the ratio of the peak value of the output voltage command wave commanding the output voltage waveform of the voltage-type PWM inverter and the peak value of the PWM carrier wave. Value, the fundamental wave effective of the voltage output from the voltage source PWM inverter Output voltage calculation method of a PWM inverter and calculates a.