JPS5992800A - Controller for variable voltage and variable frequency inverter - Google Patents

Abstract

PURPOSE:To obtain an adequate torque which is matched to a load in a low speed range by automatically varying V/f according to the magnitude of the load. CONSTITUTION:After the output voltage from an in-phase current detector 20 is added by an adder 16, it is converted to the conduction rate of the pulse by a conduction rate control circuit 17, thereby controlling a transistor which forms each phase inverter by a base drive circuit 14 through a pulse amplifier 13. The output of a switch 28 and the output of a switch 25 are superposed and inputted to a filter circuit. The output of the filter circuit 29 becomes maximum when the input has a power factor of 1, and minimum when it is 0, and the output becomes 0. Accordingly, the output of the filter 29 is varied by the magnitude of the load of a motor. In other words, when a load exists in a low frequency range, the applied voltage of the motor is enhanced to increase the V/f.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for a variable voltage/variable frequency inverter that drives an induction motor.

When operating the motor at variable speed by changing the frequency of the alternating current applied to the induction motor, the voltage ■ applied to the motor and the frequency f
The ratio (hereinafter referred to as V/f) has a large effect on the torque of the electric motor, but in order to keep the generated torque of the electric motor constant, it is sufficient to operate the electric motor so that W is constant. Figure 1 is a conventional graph of voltage and frequency,
The horizontal axis is the frequency f, and the vertical axis is the voltage V. However, V
If the motor is operated, for example, on the straight line ABC in FIG. 1 in order to keep /f constant, the influence of the resistor on the electric fill actance cannot be ignored in the low frequency region, so the maximum torque will decrease. Therefore, in the low frequency region, a method is generally used to secure torque by increasing the voltage. An example of this is the broken line DBC in FIG. 1, in which a function generator is used.

However, if the voltage is increased to increase the value of trade in order to secure torque at low speeds, the excitation becomes too strong (too strong) when there is no load, resulting in problems such as magnetic noise in the motor, torque pulsation, and an increase in excitation current. .

Furthermore, since the excitation current increases, the current may be limited during startup. In the case of a motor load or a load with a square torque characteristic such as a fan or a pump, the load is full load at the rated speed, but it is equivalent to no load in the low frequency range, resulting in the above-mentioned problem.

The present invention has been made to solve the above-mentioned disadvantages, and the present invention is designed to automatically change the W applied to the motor depending on whether the motor is under no load or under load, especially in the low frequency range. It is an object of the present invention to provide a control device for a variable voltage/variable frequency inverter that operates to ensure an appropriate torque commensurate with the torque.

The present invention focuses on the fact that the phase of the motor current changes depending on the size of the load, and by adding a voltage corresponding to the phase to W at no load, the applied voltage is increased when the load is heavy and the power factor is close to 1. The above objective is achieved by lowering the applied voltage when the load is light and the power factor is close to O.

An embodiment of the invention is shown in FIG. The embodiment shown in FIG. 2 illustrates the case of a transistor inverter which controls the voltage and frequency by pulse width modulation to operate a three-phase induction motor at variable speed. In FIG. 2, 1 is a W-phase transistor inverter, 2 is an induction motor, 3 is a rectifier, and 4 is a smoothing capacitor. 9 is a speed setting device, 1Ω is a soft start circuit, 11 is a 1/1 converter that converts voltage to frequency, 12 is a ring counter, 13 is a pulse amplifier, and 14 is a base drive circuit. 15 is a function generator, 16 is an adder, 1
7 is a conductivity control circuit, 20 is an in-phase current detection device, 21 is a current transformer inserted in the W phase, 22 is a resistor, 23 is a polarity inverter, 24 and 27 are switch drive circuits, 2
5 and 28 are switches, 26 is a NOT circuit, 29 is a filter, and 30 is a limiter.

The details of an embodiment of the present invention will be explained below with reference to FIG. When the induction 4m motive 2 is started, the soft start circuit 1
0 outputs a voltage that increases according to a predetermined program, and this voltage is converted into a frequency by a V/f converter 11 and becomes the frequency reference for the inverter, and this signal is distributed to the inverters of each phase by a ring counter 12. After being amplified by the pulse amplifier 13, the base drive circuit 14 drives the bases of the transistors constituting each phase inverter to generate alternating current, and the motor 2 speeds up according to the above program, but ultimately The operation continues at the speed set by the speed setting device 9. On the other hand, soft start circuit 1
Since the output voltage of 0 is proportional to the frequency as described above, the function generator 15 converts the voltage proportional to the frequency into a voltage with characteristics such as aABC in FIG. 1 and uses it as a voltage reference. do. The output voltage of this function generator 15,
The output voltage from the common mode current detection device by 201 is output to adder 1.
In the conductivity control circuit which becomes 17 after being added in 6, the added voltage is converted into a pulse conductivity, and the pulse amplifier 1
3, the base drive circuit 14 controls the transistors forming each phase inverter to control the voltage applied to the motor 2.

The operation of the common-mode current detecting device 20 in FIG. 2 will be explained with reference to the current graph of the common-mode current detecting device in FIG. 3 and FIG. In FIG. 2, the combination of a current transformer 21 and a resistor 22 that short-circuits this current transformer 21 creates an electric! The W-phase current of lA2 is converted into a voltage proportional to this current. On the other hand, due to the combination of the switch drive circuit 24 and the switch 25, the switch 25 is closed during the period when the W-phase voltage is at a positive potential, which is shown at T1 in FIG. There is. Furthermore, in the combination of the switch drive circuit 27 and the switch 28, since the NOT circuit 26 is provided in the preceding stage of the switch drive circuit 27, the switch 28 is closed during the period when the Wa voltage is at a negative potential. This is shown at T2 in FIG. When the W-phase current is in the same phase as the W-phase voltage, that is, the power factor is 1, the output voltage waveform of the device that converts the W-phase current into voltage, ie, the resistor 22 in FIG. 2, is shown by A1 in FIG. The switch 28 switches the output voltage of the resistor 22, which is the W-phase current, with its polarity inverted by the polarity inverter 23, so the output of the switch 28 and the switch 2
5, that is, the input waveform of the filter 29 is as shown in FIG. 3A2. The waveform when the W-phase current is delayed by 45 degrees from the W-phase voltage is as shown in FIG. 3B1, and the input waveform of the filter 29 at this time is as shown in FIG. 3B2. Similarly, the waveforms when the W-phase current lags the W-phase voltage by 90 degrees, that is, when the power factor is O, are shown by C1 and C2 in FIG. 3, respectively. Therefore, the output of the filter 29 in FIG. 2 is maximum when the input is A2 in FIG. 3, that is, when the power factor is 1;
The power factor in FIG. 02 is the minimum when the power factor is O, and the output is zero.

When the induction motor is under no load, the only current flowing through the motor is the excitation current, so its power factor is approximately O. When a load is applied to the motor, the power factor approaches 1. As is well known, the phase of the motor current can be detected by the magnitude of the output of the filter 29 shown in FIG. When there is a load during low-speed operation, the motor tries to increase the applied voltage to increase V/f, and near the rated speed, vμ is set by the function generator 1.
There is no need to make it particularly larger than the value determined in 5.

Therefore, in FIG. 2, the output of the filter 29 is the limiter 3
It is added to the output voltage of the function generator 15 after being limited by 0 so that it does not exceed a certain value.

4 and 5 are graphs of voltage and frequency showing an embodiment of the present invention, in which the horizontal axis shows the frequency f and the vertical axis shows the voltage V. Direct fiEF in FIG. 4 is a characteristic of the function generator 15 in FIG. The straight line GH in FIG. 4 is the sum of the outputs. Furthermore, if a load with a square-law Trulli characteristic such as a fan is coupled to the induction motor 2, the load at low speeds is almost 0, so the output of the function generator 15 plus the output of the common-mode current detection device 20 Although not shown, is the connection between E and H in Figure 4.

As mentioned above, in the past, when the induction motor 2 was at low speed, the applied voltage was increased regardless of the load size, and the voltage was increased (1), but in the present invention, V/f is increased depending on the load size. By making the change automatic and making the change particularly large in the low speed region, the inconvenience caused by the conventional method during low speed no-load operation is resolved.Furthermore, the output characteristic of the function generator 15 is Broken line DBC in the figure and straight iE in Figure 4
As is clear from the comparison with F, the embodiment of the present invention is simpler. Note that the output of the function generator 15 may have a characteristic such as curve JK in FIG. 5, or another curve not shown in FIG. 1, depending on the characteristics of the load.

Although the above embodiment is for a transistor inverter with pulse width modulation, other types of variable voltage/variable frequency innovations, such as square wave inverters, may use transistors or other semiconductors, or may be It is also included in the present invention that the switches 25 and 28 for detecting current may be transistor switches, other semiconductor switch devices, or contact type switches.

[Brief explanation of the drawing]

FIG. 1 is a conventional voltage and frequency graph. FIG. 2 is a circuit diagram that is an embodiment of the present invention, FIG. 3 is a current graph of a common-mode current detection device that is an embodiment of the present invention, and FIG. 4 is an example of a voltage and frequency graph of the present invention. FIG. 5 is also another example of the voltage and frequency graph of the present invention. 1... Transistor inverter (W phase), 2... Induction motor, 3... Rectifier, 4... Smoothing capacitor,
9...Speed constant meter, 10...Soft start circuit, 1
1... ■01 converter, 12... ring counter, 1
3... Pulse amplifier, 14... Base drive circuit, 1
5...Function generator, 16...Adder, 17...Conductivity control circuit, 20...Dosage current detection device, 21...
・Current transformer (W phase), 22...Resistor, 23...Polarity inverter, 24.27...Switch drive circuit, 25, 2
8... Switch, 26... NOT circuit, 29-... Filter, 30... Limiter. Agent Patent Attorney 10 Yamaguchi Isai 1 Koichi 1 = 40 sai 5 darksai? Senichi';l'3 Ward

Claims (1)

[Claims] 1) A control device for a variable frequency inverter that supplies power to an induction motor to be driven at variable speed, comprising: speed setting means; means for converting a signal from the speed setting means into a frequency command; means for converting the signal from the speed setting means into a voltage command of an arbitrary function; means for converting and detecting a current in phase with the inverter output voltage into a voltage; and means for controlling the voltage and frequency of an inverter based on the added voltage command and the converted frequency command.
Control device for variable frequency inverter. 2. In the control device for a variable voltage/variable frequency inverter according to claim 1, the means for converting and detecting a current in phase with the inverter output voltage into a voltage includes means for detecting an inverter output current; means for converting the detected current into a voltage; means for intermittently connecting the current converted to the voltage by being closed when the inverter output voltage is at a positive potential and open during the negative potential period; a means for inverting the polarity; a means for closing during the period when the inverter output voltage is at a negative potential; and being open during the period for the positive potential; and for intermittent current that has been converted into the voltage and whose polarity has been reversed; and an output of the two above-mentioned intermittent means. A control device for a variable voltage/variable frequency inverter, comprising: means for superimposing and averaging the outputs; and means for limiting the averaged output.