JPH0121714B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inverter device, and particularly to an inverter device suitable for stable operation of an induction motor.

Conventionally, the speed of an induction motor is controlled by using a voltage-controlled inverter and controlling the ratio of its output voltage V and frequency f to a constant value, using so-called constant V/F ratio control to keep the magnetic flux almost constant. It is well known that disturbances occur when the moment of inertia of the load is small or when the load is light, that is, when the slip is small.

In order to solve this problem, the applicant of the present invention previously proposed an inverter control circuit for driving a motor in Japanese Patent Application No. 11216/1983.

Figure 1 is a block diagram of the inverter device disclosed in the previous application, in which 1 is an AC power supply, 2 is a forward converter connected to the AC power supply 1, and 3 is a capacitor for smoothing the output of the forward converter 2. , 4 is an inverter bridge that converts direct current to alternating current, 5 is a current detector that detects the output current of the inverter bridge 4, 6 is an induction motor supplied with power from the inverter bridge 4, and 7 is the inverter bridge 4 8 is an acceleration/deceleration limiting circuit that receives the reference signal from the frequency setter 7; 9 is an acceleration/deceleration limiting circuit for setting the output frequency of the
PWM (pulse width modulation) circuit, 11 is a V/F (voltage/frequency) converter, 10 is a waveform synthesis circuit that generates a signal to turn on and off the circuit elements forming the inverter bridge 4, 12 is a waveform synthesis circuit 10 is a drive amplifier that amplifies the output signal and supplies it to the inverter bridge 4; 13 is a rectifier circuit that converts the detection signal of the load current of the induction motor 6 by the current detector 5 into a DC signal; and 15 is the output signal of the rectifier circuit 13. This is a change detector that detects the change in and adds it to the input of the V/F converter 11.

In this configuration, the DC output of the forward converter 2 connected to the AC power supply 1 is smoothed by the capacitor 3, converted to AC by the inverter bridge 4, and then supplied to the induction motor 6 via the current detector 5. Supplied as. On the other hand, the reference signal from the frequency setter 7 passes through the acceleration/deceleration limiting circuit 8 and is input as a voltage component to the waveform synthesis circuit 10 by the PWM circuit 9. On the other hand, the frequency component is inputted to the waveform synthesis circuit 10 through the V/F converter 11 as a frequency obtained by multiplying the inverter frequency by an integer. Incidentally, the waveform synthesis circuit 10 constitutes a well-known PWM type inverter control circuit that provides signals for turning on and off the circuit elements of the inverter bridge 4 to the inverter bridge 4 via the drive amplifier 12, and controls the inverter bridge 4. .

On the other hand, the load current to the induction motor 6 is detected by the current detector 5, and this detection signal is sent to the rectifier circuit 13.
This DC output is converted into a DC voltage by inputting it to the DC voltage, but the change is taken out via the change detector 15, and the polarity is changed to the input of the V/F converter 11 so that the frequency increases when the current increases. It is added.

It is also described in detail in the specification of Japanese Patent Application No. 11216/1983 that the unstable phenomenon caused by the inverter device during load operation of the induction motor can be stabilized through the above-described operation.

However, in such a configuration, the output of the load current change detector 15 is fed back as a constant amount corresponding to the inverter frequency, and is unrelated to the load factor, so it is not necessarily optimal in all load conditions, and is ineffective. The drawback was that even if adjustments were made to optimize the load, it would become somewhat unstable during overload.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an inverter device which eliminates the drawbacks of the prior art described above and which makes it possible to always obtain the most ideal stability regardless of the load condition of the induction motor or the like.

Hereinafter, the present invention will be explained in more detail according to the embodiments shown in the drawings.

FIG. 2 is a block diagram of an inverter device according to an embodiment of the present invention, and the differences from the configuration in FIG. 1 are as follows.
This is because a function generator 14 is provided between the rectifier circuit 13 and the change detector 15. This function generator 1
The functional characteristics of No. 4 will be explained in detail later with reference to FIG. 5 and the like.

Next, the principle of operation of this configuration will be explained with reference to FIG. By the way, FIG. 3a shows an equivalent circuit diagram of the induction motor 6, in which r ₁ , r ₂
are the primary and secondary resistances, L ₁ and L ₂ are the primary and secondary inductances, respectively, and L ₀ is the excitation inductance. Also, i ₁ is the primary current, i ₂
is the secondary current, i ₀ is the exciting current, e ₁ is the terminal voltage, e ₀ is the back electromotive force, and S is the slip. Here, if the primary resistor _r1 is omitted to simplify the explanation, the equivalent circuit can be expressed as shown in FIG. 3b. Note that L ₁₂ is the primary side equivalent inductance including the secondary inductance on the primary side.

Further, a vector diagram regarding the operation of the equivalent circuit of FIG. 3b is shown in FIG. 3c. Here, f is the frequency of the inverter bridge 4, and θ is the phase difference between the terminal voltage e ₁ and the back electromotive force e ₀ .

Now, when the induction motor 6 is operated with a light load or low inertia load using a voltage source inverter, it becomes unstable because the operation is performed with the slip S near zero, and in this state the rotor side vibrates transiently. This is because the slip S changes greatly transiently, and the secondary current i ₂ changes including its polarity. In addition, the secondary current i ₂
As is clear from Fig. 3c, when , the phase of the back electromotive force e ₀ changes significantly transiently, which acts to promote the transient vibration of the rotor, which explains the instability phenomenon. You can also do it. That is, if the rotor is now temporarily delayed, the slip S increases,
As the secondary current i ₂ increases and the phase of the primary current i ₁ advances, the phase difference θ between the back electromotive force e ₀ and the terminal voltage e ₁ increases, the position of the magnetic flux lags, and as a result, the rotor further It operates in a direction that causes a delay. The rate of change of this delay angle θ is large at light loads and decreases as the load increases, which explains the unstable phenomenon at light loads.

On the other hand, the inverter device of the present invention is configured to suppress the instability phenomenon by performing control such that the phase change of the back electromotive force e ₀ is small due to load changes.

By the way, in a voltage controlled inverter, the back electromotive force
Since it is desirable to control the ratio between e ₀ and frequency f to be constant, it can be generally expressed as e ₀ =K ₁ ·f (K ₁ is a constant) (1). Therefore, the vector diagram shown in Figure 3c becomes as shown in the vector diagram in Figure 4a, and the delay angle θ is independent of frequency as shown in the vector diagrams in Figures 4a to 4b. It is determined only by the excitation current i ₀ , the secondary current i ₂ and the primary equivalent inductance L ₁₂ . Now, if the load motor is constant, the primary equivalent inductance L ₁₂
is constant, and the excitation current i ₀ is also constant, so the delay angle θ is related only to the secondary control i ₂ , and this relationship is as shown in the characteristic diagram of FIG. 4c.

Furthermore, how to select the characteristics of the function generator 14 in FIG. 2 will be explained with reference to FIG. The relationship between the secondary current i ₂ and the primary current i ₁ is as shown in the characteristic diagram of FIG. 5a. Therefore, Figures 4c and 5
Since the relationship shown in FIG. 5b is determined from each characteristic diagram in FIG. 5, the characteristics of the function generator 14 in FIG.
It is sufficient to use a function that has saturation type characteristics with respect to the primary current i ₁ as shown in FIG. That is, the function generator 14 operates in a small current range, typically 20 to
Saturation is a smooth curve connecting two straight lines that have a steep, almost constant slope in the no-load current range equivalent to 30%, and a gentle, almost constant slope in the large current range above the rated current. Outputs the shape function.

On the other hand, the change detection circuit 15 is a differentiation circuit,
Since the V/F converter 11 is an integrator circuit, the output of the function generator 14 and the delay angle θ are proportional to each other in the voltage _phase of the inverter. This can be considered to represent the relationship between the angle θ. (However, the part to the left of i ₀ does not exist.) Therefore, by inserting the function generator 14 with the characteristics shown in FIG. 5c, the primary order of the induction motor 6 as shown in FIG. Even if the current i ₁ changes, the terminal voltage e 1 of the induction motor 6 can be compensated by moving the inverter output voltage phase by the delay angle θ so that the phase of the back electromotive force _{e 0 of} the motor 6 does not change. .

Thus, by providing the saturation characteristic as shown in FIG. 5c as the characteristic of the function generator 14 in FIG. By adjusting, unstable phenomena are suppressed,
Furthermore, since control can be performed such that the phase of the back electromotive force does not change, the operation is close to vector control, and characteristics with fast speed response can be obtained.

In the embodiment shown in FIG. 2, the load current is detected from the AC output of the inverter, but it is also possible to adopt a configuration in which the peak value on the DC side of the inverter bridge is detected.

Furthermore, in the embodiment shown in FIG. 2, the inverter bridge 4 shows the symbol of a transistor as a circuit element, but other elements such as a thyristor may be used, and even an inverter other than a PWM type may have a different effect. Needless to say, there is no such thing.

Furthermore, in the embodiment shown in FIG. 2, the current of the induction motor 6 is detected and inputted to the function generator 14, but in a control circuit provided with a current minor loop, the current of the induction motor 6 is detected instead of being input to the function generator 14. The same effect can be obtained by substituting the current reference of the current minor loop.

As described above, according to the present invention, the load motor becomes unstable by detecting the current (or current reference) of the load motor and feeding back the variation to the frequency of the voltage source inverter through the saturation type function generator. It is possible to realize an inverter device that stabilizes the phenomenon over the entire load range and at the same time has high-speed response similar to vector control.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional inverter device, Fig. 2 is a block diagram of an inverter device according to an embodiment of the present invention, Figs. 3a and b are equivalent circuit diagrams of an induction motor, and Fig. 3c is an equivalent circuit diagram of an induction motor. A vector diagram related to the operation of the circuit in FIG. 3b, FIGS. 4a and b are vector diagrams to explain the operation of the configuration in FIG. 2, and FIG. 4c is a characteristic diagram to explain the operation of the configuration in FIG. 2. 5A, 5B, and 5C are characteristic diagrams for explaining the operation of the configuration shown in FIG. 2. 2... Forward converter, 4... Inverter bridge,
5... Current detector, 6... Induction motor, 7... Frequency setter, 11... V/F converter, 14... Function generator, 15... Change amount detector.

Claims (1)

[Claims] 1. An inverter that converts DC power into variable frequency AC power and supplies it to a motor, a frequency setting means for setting the output frequency of the inverter, and controlling the output voltage of the inverter according to the set frequency of the frequency setting means. a voltage control means for controlling the output frequency of the inverter according to a set frequency of the frequency setting means;
A current detector detects either the output current or the current reference of the inverter, and the output of this current detector is used as an input, and has a steep and almost constant slope in the small current range of no-load current, and the current is about the rated current. In the above large current range, there is a function generating means that outputs a saturated function in the form of a smooth curve connecting two straight lines with a gentle, almost constant slope, and a function generating means that outputs a saturated function in the form of a smooth curve connecting two straight lines with a gentle, almost constant slope. An inverter device comprising: a change detecting means for detecting a change in frequency, and a correcting means for correcting a set frequency signal given from the frequency setting means to the frequency control means by the output of the change detecting means.