JPH055835Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to a control device for a pulse width modulation (PWM) type inverter, and in particular to an open loop control device.

(Prior art and problems) In open-loop control of PWM inverters, fluctuations in the power supply voltage directly appear as fluctuations in the inverter output voltage, and changes in the load of an inverter-driven motor result in fluctuations in the inverter output voltage. appear. Such fluctuations in output voltage cause the following problems.

(1) Due to the drop in inverter output voltage, the acceleration/deceleration torque of the motor becomes insufficient, making it impossible to secure accurate acceleration/deceleration times.

(2) When the inverter output voltage decreases, in order to ensure the rated torque of the motor, it is necessary to increase the output current by the voltage drop, so it is necessary to design the motor and inverter to have a large overload capacity.

(3) Increase the carrier wave frequency to reduce motor vibration and noise, and correct the output voltage in the low range of the inverter output frequency so that the increased frequency eliminates unstable operation during acceleration and deceleration. In an inverter that corrects the output frequency in a high range (for example, Japanese Patent Application No. 58-218875),
Unless the switching point (switching frequency) and correction amount between output voltage correction and output frequency correction are adjusted as the inverter output voltage fluctuates, unstable operation will occur.

(4) Overvoltage may occur when resistance regeneration of the motor stops due to a rise in inverter output voltage.

(5) For inverters equipped with an automatic voltage control system that detects the inverter output voltage, the control range is wide (for example, 3Hz to 180Hz) or special F/V characteristics are provided to increase acceleration torque. This requires a special transformer to detect the inverter output voltage, resulting in an expensive device.

(Purpose of the invention) An object of the invention is to provide a control device that eliminates fluctuations in the inverter output voltage due to fluctuations in power supply voltage and load fluctuations, and which can be realized by adding a small amount of circuit to the control circuit.

(Summary of the invention) The invention corrects the inverter output voltage setting signal according to fluctuations in power supply voltage and load current,
It is characterized by keeping the output voltage constant.

(Embodiment) Fig. 1 is an overall circuit diagram showing an embodiment of the present invention, and as shown in the above-mentioned item (3), control is performed to switch between output voltage correction and output frequency correction according to the output frequency range of the inverter. This is the case when applied to a device. First, the voltage and frequency correction switching circuit will be explained.

In a main circuit configuration that obtains DC power from an AC power supply 1 through a rectifier 2, converts this DC power into AC power through a PWM type inverter 3, and controls the speed of an induction motor 4 as a load, the control circuit is as follows: Basically, the inverter output frequency and voltage commands are sent from the speed setter 5 through the cushion circuit 6.
V _N is obtained, this command V _N is used as the output frequency control signal of the voltage controlled oscillator 7 and the output voltage control signal of the voltage controlled amplifier 8, and the output voltage of the oscillator 7 is controlled by the output of the amplifier 8. A sine wave signal with a predetermined ratio of voltage and frequency is obtained from the output of 8, and this sine wave signal is used as a pulse width modulation signal to obtain a PWM waveform gate signal to the pulse width modulation circuit 9 to drive each switch element of the inverter 3. do.

In such an open-loop PWM type inverter, voltage correction or frequency correction is performed depending on the control range of the inverter. For this correction, the inverter control range is determined from the frequency/voltage command _VN , and the frequency/voltage correction circuit 10 adds an output voltage correction signal ΔV to the input of the voltage control amplifier 8 in the low range of the inverter output frequency. The output frequency correction signal ΔF is added to the input of the voltage controlled oscillator 7 in a high frequency range.

Such voltage and frequency correction can obtain favorable results when the power supply voltage is constant and the load condition is constant, and when the inverter output voltage and load change due to fluctuations in these conditions, correction switching is necessary for stable operation. The point changes, and an unstable region exists at a fixed switching point, and the amount of correction is not optimal.

Therefore, in this embodiment, in order to eliminate fluctuations in the inverter output voltage, the voltage command V _N voltage control amplifier 8
The multiplier 11 corrects the voltage given to the voltage, and the output voltage correction circuit 12 provides a correction coefficient as a multiplier as the correction amount. This correction circuit 12 obtains a correction coefficient from fluctuations in the power supply voltage and load current, and uses the voltage detection signal V _AC of the power supply 1 from the transformer 12A (or the output voltage of the rectifier 2) and the DC current detector 12B. Obtain the inverter DC side current (corresponding to load current) detection signal I _DC to obtain the voltage setting correction coefficient signal. This output voltage correction circuit 12 is realized by the embodiment shown in FIG.

In FIG. 2, a rectifier circuit 13 rectifies the power supply voltage detection signal V _AC and converts it into a DC signal. The limiter adder 14 detects the deviation between the reference power supply voltage signal set in the setting device 14A and the output of the rectifier 13, and obtains a correction signal proportional to the variation in the power supply voltage.
The adder 14 is composed of an operational amplifier _A1 , an operational resistor, etc., and its output has a limiter value set by variable resistors 14B and 14C, and the limiter value is set when the power supply voltage fluctuates by ±20% or more from the rated voltage. Restrict.

This limiter operation is performed by variable resistors 14B, 14
The voltage set by C (the voltage division ratio for the output voltage of operational amplifier _A1 and the voltage difference of ±15V) is the transistor
When the threshold level of TR ₁ and TR ₂ is exceeded, the operational amplifier is turned on by turning on the transistor.
This is done by negative feedback operation that suppresses the output of _A1 . Furthermore, the reason why the adder 14 has a limiter function is as follows.
Taking into account the expected range of voltage fluctuations in the power supply voltage, it is set to ±20% of the rated voltage, and any voltage fluctuations beyond this are treated as an abnormality and correction of the inverter output voltage is suppressed.

Adder 15 outputs DC current detection signal I _DC and setter 1
The deviation from the no-load current I ₀ minute cancel signal set to 5A is detected only with positive polarity, and a signal with a correction coefficient proportional to the load fluctuation is obtained. This adder 15 is composed of an operational amplifier _A2 , an operational resistor, etc., and has a load current I _DC.
When the no-load current I ₀ or less, the output becomes a reference potential of 0 volts, and when it exceeds I ₀ , the output voltage becomes proportional to the load current.

The operation of the adder 15 is such that when the output of the operational amplifier _A2 becomes negative, the diode _D1 conducts and suppresses the output to zero volts, and when the output of _A2 becomes positive, the diode _D1 conducts. becomes non-conductive and amplifies with a predetermined gain, while diode _D2 becomes conductive and takes out the output.

Adder 16 adds the correction signals of adders 14 and 15, and inverting amplifier 17 inverts the output of adder 16 to obtain a correction coefficient signal for multiplier 11.
These circuits 16 and 17 are composed of operational amplifiers A ₃ and A ₄ and operational resistors.

In the above configuration, when the power supply voltage is at the rated voltage and the load current is at a certain value, adder 1
The output of adder 15 becomes a voltage signal proportional to the load current minus the no-load current. The voltage signal in this state becomes the multiplier of multiplier 11 with a correction coefficient of 1.

Here, in response to fluctuations in the power supply voltage V _AC , a voltage output proportional to the fluctuation is obtained in the limiter adder 14 with the opposite polarity, and this fluctuation is added to the output of the adder 15, and the voltage output is outputted by the adder 16 and the amplifier 17. The multiplier of the multiplier 11 is corrected through this process. For example, if the power supply voltage V _AC increases by 10%, a voltage proportional to this will appear at the output of the adder 14 with the opposite polarity, and this output will be added to the output of the adder 15 in the adder 16. The multiplier of the multiplier 11 becomes an amount corresponding to the load current reduced by 10% by the correction of the power supply voltage, and the voltage command of the voltage control amplifier 8 is corrected to decrease by the increase in the power supply voltage.

As a result, even though the DC voltage of the inverter 3 fluctuates due to voltage fluctuations in the power supply voltage V _AC , the voltage command V _N is corrected and the output voltage of the inverter 3 is adjusted to the power supply voltage V _AC
so that the voltage is the same as when it is at the rated voltage. At this time, the load current due to fluctuations in the power supply voltage
No I _DC fluctuation occurs.

On the other hand, when the power supply voltage V _AC is at the rated voltage and the load current I _DC fluctuates, an output proportional to the load current including this fluctuation is obtained from the adder 15, and this output is passed through the adder 16 and the amplifier 17. A proportional voltage signal is obtained and the voltage command V _N is corrected by using this voltage signal as a correction coefficient of the multiplier 11 . for example,
When the load current increases by 10%, the voltage command V _N
We get a correction factor of 1.1 which increases it by 10%. Thereby, when the inverter output voltage is about to fluctuate due to a fluctuation in the load current, the voltage command V _N is amplified and corrected, thereby suppressing the fluctuation in the inverter output voltage and supplying the necessary load current.

Next, when fluctuations in the power supply voltage and fluctuations in the load current occur simultaneously, the outputs of the adders 14 and 15 are added together by the adder 16 to correct both fluctuations together.

In this way, by correcting the inverter output voltage command according to fluctuations in the power supply voltage and load current, the inverter output voltage fluctuations due to fluctuations in the power supply voltage and load can be corrected and commanded.
The relationship between the inverter output voltage and output frequency can be kept constant by _VN , and the correction switching points of voltage correction ΔV and frequency correction ΔF to eliminate unstable operation can also be kept constant to ensure stable switching. can be made to do so.

(Effects of the invention) As described above, according to the invention, in an open-loop PWM inverter, it is possible to supply the necessary load current while eliminating fluctuations in the inverter output voltage due to power supply voltage fluctuations and load fluctuations. It is possible to solve the problem of fluctuation in voltage, overload capacity of inverter and motor, switching point fluctuation of voltage and frequency correction, and deviation of voltage and frequency ratio. It has the effect of being realized with a low cost and stable circuit.

[Brief explanation of drawings]

FIG. 1 is an overall circuit diagram showing an embodiment of the present invention;
FIG. 2 is a circuit diagram showing an embodiment of the output voltage correction circuit 12 in FIG. 1. 2... Rectifier, 3... Inverter, 5... Speed setting device, 6... Cushion circuit, 7... Voltage controlled oscillator, 8... Voltage controlled amplifier, 9... Pulse width modulation circuit, 10... Frequency/ Voltage correction circuit, 11
... Multiplier, 12 ... Output voltage correction circuit, 13 ...
... Rectifier circuit, 14 ... Limiter adder, 15,
16... Adder, 17... Inverting amplifier.

Claims (1)

[Scope of claim for utility model registration] In an open-loop pulse width modulation type inverter that pulse width modulates the inverter main circuit in accordance with voltage and frequency commands, a voltage proportional to the deviation between the inverter's power supply voltage detection signal and the rated voltage signal. A first adder with a limiter that obtains a fluctuation signal, a second adder that obtains a load current signal obtained by subtracting a no-load current from the load current detection signal of the inverter, and a signal obtained by adding the outputs of both adders. and a multiplier that uses the output signal of this adder as a multiplier to correct the voltage command of the voltage/frequency command, suppressing fluctuations in the inverter output voltage due to fluctuations in the power supply voltage and load current. A control device for a pulse width modulation type inverter, characterized in that: