JPS6332040B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to an inverter control circuit for controlling an inverter that generates variable frequency alternating current power from a direct current power source, and particularly when an alternating current motor is connected as a load of the inverter. This invention relates to a control circuit with improved braking characteristics.

Figure 1 is a connection diagram showing a general inverter circuit, where 1 is a rectifier for DC power supply that is input to the inverter, 2 is an inverter circuit, and 3 is an inverter circuit 2.
4 is a smoothing capacitor for the output of the rectifier 1, 5 is a transistor of the inverter circuit, 6 is a transistor 5
is a diode connected in antiparallel to .

Fig. 2 is a connection diagram showing an example of a conventional inverter control circuit. 7 is a speed command voltage generator, and in the example shown in the figure, it is composed of a potentiometer connected to a DC voltage, and the output voltage of its variable tap is becomes the speed command voltage. A switch 8 starts and stops the electric motor 3. 9 and 10 are operational amplifiers, 11 is a resistor, 12 is a capacitor, 1
3 is a V/F converter that outputs a pulse train with a frequency proportional to the output voltage V _p of the operational amplifier 10;
4 is a distributor, and the base of each transistor 5 in FIG. 1 is controlled by the output of the distributor 14.

Since the operation of the inverter circuit 2 is already well known, its explanation will be omitted, but in the example shown in FIGS. 1 and 2, the inverter circuit in a steady state is 2, and thus the steady state speed of the electric motor 3. When the electric motor 3 is started and stopped by the switch 8, its starting and braking characteristics are controlled by the operational amplifiers 9 and 10. controlled.

FIG. 3 is a waveform diagram showing an example of a change in the output voltage V _p of the operational amplifier 10 when the switch 8 is turned on and off, in which the horizontal axis shows time t ₁ and the vertical axis shows the voltage V _p . When switch 8 is turned on at point A, the output voltage of potentiometer 7 is amplified by operational amplifier 9 and integrated with a predetermined time constant determined by resistor 11 and capacitor 12, resulting in the output voltage of operational amplifier 10 V _p
However, when V _p gradually increases and reaches point B, V _p becomes the output voltage of potentiometer 7.
Since the output voltage of the operational amplifier 9 becomes 0, the value of V _p =Voref is held. Point C is the point where switch 8 is turned off, and V _p is the point where resistor 1
1 and capacitor 12, and becomes V _p =0 at point D. Since the output frequency of the V/F converter 13 is proportional to V _p and the output frequency of the inverter circuit 2 is also proportional to the output frequency of the inverter circuit 2, the AC motor 3 is started at point A, accelerates between A and B, and accelerates between B and B.
Between C, the output voltage value of potentiometer 7 Voref
is driven at a speed determined by Further C
It decelerates between -D and stops at point D. If C-D
If you try to make the gradient between the two steeper and stop the motor 3 more quickly than it would stop naturally, the inertia energy determined by the moment of inertia of the motor 3 and its load (not shown) and its rotational speed will be It is converted into electrical energy in the motor 3 and stored as electrical energy in the capacitor 4 through six diodes 6 in the inverter circuit 2. Figure 4 corresponds to Figure 3 and shows the terminal voltage of capacitor 4.
This is a waveform diagram showing changes in V _PN , and the horizontal axis is time t.
The vertical axis shows the terminal voltage V _PN of capacitor 4,
Until point C, that is, the point where switch 8 turns off.
V _PN is equal to the output voltage of the rectifier 1, but when it is charged through the inverter circuit 2 during braking of the motor 3 as described above, it increases as shown in the period CD in FIG. 4. If V _PN increases in this way, the capacitor 4, the transistor 5 of the inverter circuit 2, and the diode 6 may become overvoltage and may be destroyed.To avoid such an accident, the
The values of the resistor 11 and capacitor 12 shown in FIG. 2 are selected so that the deceleration pattern indicated by -D is close to the natural stop of the motor 3.

By the way, the load on the electric motor 3 usually changes, and as the moment of inertia of the load increases, the time constant of the natural stop deceleration pattern of the electric motor 3 to which the load is coupled increases, but as the load changes, Since it is difficult to change the time constant determined by the resistor 11 and capacitor 12 in FIG. The value of has been determined. In this case, in order to maintain the deceleration pattern corresponding to the maximum load inertia moment, even though faster deceleration is possible when the moment of inertia of the load becomes smaller or when the load applies reverse torque to the electric motor 3, The electric motor 3 will decelerate while generating drive torque.
This unnecessarily lengthens the time required for stopping (the time indicated by line C-D in FIG. 3), and also consumes electric power during deceleration, which is not desirable in terms of energy conservation.

The present invention was made to eliminate the above-mentioned drawbacks of conventional circuits, and it is an object of the present invention to provide an inverter control circuit that can always operate with a deceleration pattern that matches the load condition.

Embodiments of the invention will be described below with reference to the drawings. FIG. 5 is a connection diagram showing an embodiment of the present invention, where the same reference numerals as in FIG. 17 is a reference voltage source that provides a reference voltage, 18 is a comparator, the output of which becomes negative when the output of the voltage detection circuit 16 exceeds the voltage of the reference voltage source 17; In other cases, the polarity is positive, and 19 is a diode.
In the case of FIG. 5, the time constant determined by the resistor 11 and capacitor 12 is set regardless of the deceleration pattern of the motor 3 when it comes to a natural stop.

In the circuit shown in FIG. 5, if the value of V _PN in the circuit shown in FIG. The operation of the circuit is similar to that of the circuit shown in FIG. FIG. 6 is a waveform diagram showing changes in the output voltage V _p of the operational amplifier 10 in the circuit of FIG. 5, and is expressed using the same symbols as in FIG. 3.
The operation between A-B-C is the same as in the case of FIG. FIG. 7 is a waveform diagram corresponding to FIG. 6 and showing changes in the terminal voltage V _PN of the capacitor 4, and is represented using the same symbols as in FIG. 4.

When the switch 8 is turned off at point C, the output V _p of the operational amplifier 10 begins to decrease with a time constant determined by the circuit constants, as described above. Here, when the terminal voltage V _PN of the capacitor 4 rises and the output voltage of the voltage detection circuit 16 becomes equal to or higher than the voltage of the reference voltage source 17, the comparator 18 operates, and the positive polarity output of the operational amplifier 11 is connected to the resistor 15 and the diode 19. Connect to zero potential through. As a result, the operational amplifier 10 stops the integrating operation and holds the output at that time. Therefore, the output frequency of the inverter circuit 2 stops decreasing and the same frequency is maintained.
The rotational speed of is kept constant and its inertial energy is no longer converted into electrical energy. Furthermore, in this state, energy commensurate with the load of the motor 3 is supplied from the capacitor 4, the voltage V _PN of the capacitor 4 decreases, and the comparator 1
The output voltage of 8 becomes a positive potential, and the connection between the comparator 18 and the resistor 11 is cut off by the diode 19. As a result, the output of the operational amplifier 9 is applied to the operational amplifier 10 through the resistors 5 and 11, and the operational amplifier 10 starts integrating again, its output voltage V _p decreases, and the motor 3 decelerates. When the comparator 18 operates again during this deceleration, the same operation is repeated, and the output voltage V _p of the operational amplifier 10
The voltage V PN of the capacitor 4 follows the course as shown in CD in FIG. 6, and the voltage V _PN of the capacitor 4 follows the course as shown in CD in FIG. 7, and the motor stops at point D.

Although FIG. 1 shows an inverter circuit 2 using a transistor 5, it is clear that the inverter control circuit of the present invention can be used in an inverter circuit using any semiconductor element other than transistors.

As described above, according to the present invention, no matter how the inertia and/or torque of the load changes, the inverter is controlled so that the deceleration pattern suitable for the load is always automatically determined and the operation is performed. energy and time losses can be minimized.

[Brief explanation of the drawing]

Figure 1 is a connection diagram showing a general inverter circuit.
Fig. 2 is a connection diagram showing an example of a conventional inverter control circuit, Fig. 3 is a waveform diagram showing an example of changes in the output voltage of the operational amplifier shown in Fig. 2, and Fig. 4 corresponds to Fig. 3. FIG. 5 is a connection diagram showing an embodiment of the present invention; FIG. 6 is a waveform diagram showing an example of changes in the output voltage of the operational amplifier shown in FIG. 5; FIG. 7 is a waveform diagram corresponding to FIG. 6 and showing changes in the terminal voltage of the capacitor shown in FIG. 1. 1... Rectifier, 2... Inverter circuit, 3...
Motor, 4... Capacitor, 7... Speed command voltage generator, 8... Switch, 9, 10... Operational amplifier (integrating circuit), 11, 15... Resistor, 12...
...Capacitor, 13...V/F converter, 16
...Voltage detection circuit, 17...Reference voltage source, 18...
...Comparator, 19...Diode. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. In an inverter control circuit that controls a variable frequency inverter connected as a power source for an AC motor, a speed command voltage generator that outputs a speed command voltage indicating a reference speed to the motor; an integrating means that integrates with a predetermined time constant and holds the integrated output when the integrated output becomes equal to the speed command voltage; the integrating means is interposed between the integrating means and the speed command voltage generator; a switch that connects or disconnects the input of the speed command voltage to the inverter, a voltage detection circuit that detects the voltage value of the input DC power source of the inverter, and the integration at that point in time when the output of the voltage detection circuit exceeds a predetermined value. means for clamping the integrating means to maintain the output voltage of the means at a constant voltage, and releasing the clamp when the output of the voltage detection circuit becomes equal to or less than the predetermined value; An inverter control circuit comprising: a V/F converter that generates a frequency; and means for controlling the output frequency of the inverter based on the output frequency of the V/F converter.