JPH0452077B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a control device for an induction motor, and in particular, to prevent damage to semiconductor elements due to overcurrent in a converter without current control.

[Prior art]

FIG. 1 shows the configuration of a control device for an induction motor driven by an inverter.

In the figure, 1 is a commercial three-phase AC power source, 2 is a power running converter that converts the three-phase AC power source 1 to DC, 3 is a regeneration converter that converts DC to AC and regenerates it to the three-phase AC power source 1, 4 is a capacitor that smoothes the rectified output of the converter 2 for power running, 5 is a resistor for detecting the voltage across the capacitor 4, 6 is an inverter that converts the DC voltage across the capacitor 4 into three-phase AC, and 7 is an induction 8 is a motor load detection device that detects the load of the induction motor 7; 9 is a power running converter drive device that drives the power running converter 2; 10 is a regeneration converter drive device that drives the regeneration converter 3; 11 is an inverter; 6 is an inverter drive device, 12 is a microprocessor (CPU) that outputs the result of calculation according to a predetermined program as a command to each drive device 9, 10, 11, and 13 is a microprocessor (CPU) that outputs the program and data of the CPU 12. 14 is a random access memory (RAM) that stores the calculation results of the CPU 12; and 15 is a speed command generator that gives rotation commands to the induction motor 7.

In the above configuration, the three-phase AC from the three-phase AC power supply 1 is rectified by the power running converter 2,
It is smoothed by capacitor 4. CPU12 is
Calculations are performed according to the program stored in the ROM 13, and when a signal to start starting is received from the speed command generator 15, the load amount of the induction motor 7 is determined based on the output from the motor load detector 18. Based on the judgment, a current command necessary to generate a torque commensurate with the load to the inverter 6 is output.

For example, when the induction motor 7 is used as a hoisting motor for an elevator, an electromagnetic brake (not shown) is applied to the induction motor 7 by the load until it starts.
The rotation of the elevator is mechanically suppressed, and when the CPU 12 receives a start start signal from the speed command generator 15 and releases the mechanical braking force of the induction motor 7 by releasing the electromagnetic brake, the elevator stops due to the load. In order to avoid this, the CPU 12 takes in the output of a weighing device (not shown), which corresponds to the motor load detection device 8 installed in the elevator car etc. in advance, so that the motor jumps out or moves in the opposite direction. This corresponds to a case where it is necessary to generate torque.

However, in the above method, since the motor load detection device 8 has already outputted a value commensurate with the load, the CPU 12 directly outputs this value to the inverter 6 via the inverter drive device 11. When a large torque current command is instantaneously issued in steps from the CPU 12 due to a failure of the load detection device 8, etc., a voltage drop across the capacitor 4 occurs due to rapid discharge of the capacitor 4. The voltage of the capacitor 4 is fed back through the resistor 5 to the converter drive devices 9 and 10 for power running and regeneration. At this time, the power running converter drive device 9 operates to maintain the voltage value across the capacitor 4 as per the command value of the CPU 12, but since the converter current control is not performed, if the voltage drop is large, the power running converter drive device 9 is operated. Therefore, a large current flows to the power running converter 2. As a result, semiconductor elements such as thyristors that make up the converter may be destroyed,
There is a risk that the overcurrent detection device of the converter (not shown) will operate and the entire drive system of the induction motor device 7 will stop. This state will be further explained with reference to FIG.

In FIG. 2, A is the output 16 of the motor load detection device 8. Output 17 in figure B is CPU1
2 corresponds to the motor load amount used when calculating the current command of the inverter 6, and the level is the same as the output 16 of A in the figure, and the speed command generator 1
When a signal to start activation is received from 5, the output signal 17 rises steeply. Same figure C
18 is a current command signal to the inverter 6 corresponding to the output signal 17 in FIG. As this current command signal 18 rises in a stepwise manner, a rapid current flows through the inverter 6. Therefore, the voltage across the capacitor 4 drops rapidly. This condition is represented by the reference numeral 19 in FIG. 2D. As a result, the power running converter drive device 9 operates to maintain the voltage across the capacitor 4 at its original value, but since the voltage drop is large, the current command signal increases accordingly, and the power running converter drive device 9 operates to maintain the voltage across the capacitor 4 at its original value. A large current flows. This current command signal is represented by 20 in FIG.

If a large current flows through the power running converter 2 as described above, it is undesirable because there is a risk that the semiconductor elements constituting the converter 2 will be destroyed or the system will go down due to the operation of the converter's overcurrent detection device.

[Summary of the invention]

This invention was made in order to eliminate the above-mentioned drawbacks. Even when the output of the motor load detection device 8 is large, it is possible to prevent converter element damage by changing the calculation method of the CPU without adding a new circuit. It is an object of the present invention to provide a control device for an induction motor that does not cause problems such as stopping the drive system of the motor 7 due to the operation of an overcurrent detection circuit.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 3 and 4.

First, in the present invention, the configuration of the control device itself is the same as that of the conventional example, but the arithmetic function of the CPU 12 is unique. That is, the CPU 12 determines the amount of load on the induction motor 7 based on the output from the motor load detection device 8 based on the program stored in the ROM 13, and issues a current command necessary to cause the inverter 6 to generate a torque commensurate with the load. Calculation is performed to output a signal, and this calculation is performed so that the output does not rise steeply but with a constant slope.

21 to 25 in FIGS. 3A to 3E indicate waveforms of portions corresponding to 16 to 20 in FIGS. 2A to 2E, respectively. That is, 21 in FIG. 3A is the output of the motor load detection device 8, and 22 in FIG. 3B is the input to the CPU 12, which are the same as in the conventional example. Figure C-2
3 is the current command signal to the inverter 6 calculated by the CPU 12 based on the input 22 of the CPU 12, 24 in D in the figure is the voltage across the capacitor 4, and 25 in E in the figure
is the current flowing through the power running converter 2.

In other words, even if the input to the CPU 12 has a waveform that rises steeply as shown in B22 in FIG.
Based on the program stored in the internal memory, the current command output to the inverter drive device 11 is changed to a current value that generates a torque corresponding to the load of the induction motor 7 based on the output of the motor load detection device 8, as shown in FIG. 23
CPU12 so that it rises at a constant slope like
Calculate by By doing this, the voltage across the capacitor 4 becomes like 24 in D in the figure, and since the drop in voltage is small, the voltage across the capacitor 4 is maintained at the voltage command value from the CPU 12. The command value of the power running converter drive device 9 to the power running converter 2 can be small, and the maximum value of the current flowing through the power running converter 2 can be reduced to 20 in Figure 2 E, which is the conventional example, as shown at 25 in Figure E. Compared to this, it can be significantly reduced. This prevents damage to semiconductor elements such as thyristors that constitute the power running converter 2, and prevents the overcurrent detection device (not shown) of the converter from operating and stopping the entire drive system of the induction motor 7. can. still,
The slope of increasing the current value mentioned above is appropriate considering the current capacity of the elements such as thyristors that make up the converter and the allowable dead time from the generation of the start signal to the time when the brake is actually released and the induction motor starts moving. You just have to choose the value that is right for you.

A flowchart for calculating this current command is shown in FIG.

That is, in FIG. 4-1, it is determined whether there is a start command from the speed command generator 15, and if it is determined that there is a start command, then in 4-2, the inverter current command value is calculated based on the program stored in the ROM 13. Ru.

In 4-3, it is determined whether the inverter command value calculated in 4-2 is larger than the motor load,
If it is determined that the inverter current command value is large, the inverter current command value is calculated in step 4-4 until the output of the motor load detection device 8 generates a torque corresponding to the load of the induction motor 7.

Incidentally, if there is no activation command from the speed command generation device 15, the inverter current command value is calculated in 4-5.

The above effects can be obtained by repeating the arithmetic processing according to the above flowchart at regular intervals.

Note that X at 4-2 in FIG. 4 determines the slope of the current command.

Further, in the above embodiment, the motor load detection device 8
It goes without saying that the same effect can be obtained by making the current command have a gradient in response to the input from the speed command generator 15. .

〔Effect of the invention〕

As explained above, the present invention provides an induction motor control device using an inverter device without a converter current control device, in which a torque current corresponding to the load of the induction motor is caused to flow in advance before the mechanical braking force of the induction motor is eliminated. Then, the influence of the load on starting the induction motor is eliminated by eliminating the mechanical control force, by outputting a command to increase the induction motor current from zero to a torque current corresponding to the load at a constant gradient. , destruction of the semiconductor elements and smoothing capacitors that make up the converter, and damage to the system due to the operation of the converter's overcurrent detection device.
Can prevent down. Another advantage is that there is almost no increase in manufacturing costs, since the control device itself remains the same as before, and only the CPU calculation method needs to be changed.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of an induction motor control device using an inverter device, and Fig. 2 A to E are waveforms showing CPU input data, motor current command, capacitor voltage, and converter current corresponding to the motor load according to the conventional example. 3A to 3E are waveform diagrams obtained by the induction motor control device according to the present invention similar to FIGS. 2A to E, and FIG. 4 is a waveform diagram obtained from the motor load detection device of the induction motor control device according to the present invention 7 is a flowchart showing a method of calculating a current command for an input. 2... Power running converter, 4... Smoothing capacitor, 5... Voltage feedback resistor, 6... Inverter, 7... Induction motor, 8... Motor load detection device, 11... Inverter drive device, 12 ...Microprocessor, 15...Speed command generation device.

Claims (1)

[Scope of Claims] 1. A converter using a rectifier that converts alternating current to direct current, a capacitor that smoothes direct current, and an inverter that converts the direct current converted by this converter into alternating current and supplies it to an induction motor. A motor load detection device that detects the load of the induction motor, a speed command generation device that determines the rotational speed of the induction motor, and a microprocessor that calculates control command values for the converter and inverter and outputs them to the respective drive devices. A control device for an induction motor having a setter, comprising: means for determining whether or not a signal output from the motor load detection device when starting the induction motor is equal to or greater than a certain value; In the case of an induction motor, the microprocessor is provided with means for calculating a control command value for driving the inverter that increases at a constant gradient until a torque current corresponding to the load of the induction motor is generated. Electric motor control device.