JPH0750996B2 - Inverter control device - Google Patents

Description

The present invention relates to an inverter control device, and more particularly to prevention of overcurrent.

[Prior Art] FIG. 2 is a block diagram of a well-known inverter control device. In the figure, (1) is an AC power supply,
(2) is a forward converter for rectifying the AC power supply (1), (3) is a capacitor for smoothing the output of the forward converter (2), and (4) is for converting the smoothed DC voltage to send out an AC output. The reverse converter (5) is an induction motor driven by an AC output. Further, (6) is a start command signal of the inverter,
(7) is a frequency setter, (8) is an acceleration / deceleration controller that outputs an output frequency command signal (10) based on an input of a start signal and a set frequency signal, and (9) is an output frequency command signal (10). PWM that outputs the control signal of the inverse converter (4) by
It is a controller.

Next, the operation will be described. The AC power supply (1) is rectified by the forward converter (2) and further smoothed by the capacitor (3). This smoothed voltage is given to the inverse converter (4) as a DC power supply. The inverse converter (4) is composed of a bridge of a transistor and an antiparallel diode, and by performing PWM control of this, an AC output of a desired frequency and voltage is given to the induction motor (5), and the induction motor (5 )
Is operated at variable speed. At this time, the PWM controller (9) receives the output frequency command signal (10) and outputs a PWM-controlled signal to the base of the transistor.

Now, in order to obtain the output frequency command signal (10), this control device is equipped with a start signal (6), a frequency setting device (7) and an acceleration / deceleration controller (8). The acceleration / deceleration controller (8) is composed of an integrator and outputs a ramp signal. Therefore, the output frequency command signal (10) continuously changes even when the start signal is turned on or off, or when the set frequency signal from the frequency setting device (7) suddenly changes.

When the induction motor (5) is driven by such a conventional inverter control device, there is a problem that an overcurrent is likely to occur when an impact load is applied. In particular, when the motor shaft is restrained during operation, for example, when the carrier is stopped by hitting a fixed point stopper, the induction motor (5) slips excessively and causes an overcurrent.

The control method shown in FIG. 3 has been put into practical use as an improvement over these problems. This is called a "slip frequency control method". In Fig. 3, (1
1) is a rotation speed detector for detecting the motor speed, (12) is an amplifier, (13) is a comparator, and (14) is an adder. The other reference numerals are the same as those in the apparatus shown in FIG.

Next, the operation will be described. In this device, the motor speed is detected by the rotation speed detector (11) and compared with the output of the acceleration / deceleration controller (8) by the comparator (13). Then, these differences are amplified by the amplifier (12), added by the motor speed and the adder (14), and sent to the PWM controller (9) as the output frequency command signal (10). Here, the output of the amplifier (12) corresponds to the slip command of the induction motor (5). Therefore, the output frequency command signal (10) is
This slip is added to the motor speed.

Therefore, the maximum slip of the induction motor (5) is appropriately limited by appropriately selecting the saturation value of the amplifier (12), and it is possible to prevent the overcurrent from flowing even when the motor shaft is locked.

[Problems to be Solved by the Invention] Although the conventional control system shown in FIG. 3 can appropriately perform overcurrent protection, it has the following problems.

This control method requires a rotation speed detector (11) for detecting the motor speed, and it is necessary to ensure this accuracy sufficiently. That is, since the slip of an induction motor is generally small, in order to effectively control the slip frequency, it is necessary to detect the rotation speed with an error sufficiently smaller than this slip, and the rotation speed detector ( 11) is generally expensive.

The present invention has been made to solve the above-mentioned problems, and the occurrence of a trip without using a rotation detector and preventing an overcurrent from flowing when an impact load is applied to an electric motor It is an object of the present invention to provide an inverter control device that prevents the above.

[Means for Solving the Problems] An inverter control device according to the present invention includes first comparing means for operating an output frequency at a constant value or less, current detecting means for detecting an output current of an inverter, and current detecting means. The rapid deceleration that operates under the AND condition of the output of the first comparing means and the output of the second comparing means and the second comparing means that operates when the detected value of is equal to or more than a certain value, and rapidly reduces the output frequency signal. And control means.

[Operation] In the present invention, since the set output frequency and the actual rotation speed are usually substantially equal to each other, the state where the rotation speed of the electric motor is equal to or lower than the predetermined speed is detected by the first comparison means. Then, when the motor shaft is restrained, the state is detected by the second comparing means under the condition that the output current is a certain value or more. When the AND conditions of the first comparison means and the second comparison means are satisfied, the output frequency command signal is rapidly reduced by the rapid deceleration control means. By matching the speed of reduction of the output frequency command signal with the speed of speed reduction due to the motor shaft restraint, the motor can be prevented from excessive slipping, and as a result, overcurrent can be suppressed.

[Embodiment] An embodiment of the present invention will be described below with reference to FIG.
In FIG. 1, (21) is an output current detector, (22) is a comparison reference setting device for setting a reference frequency as a control reference,
(23) is a reference setting device that sets an output current that serves as a control reference. (24) is a first comparator which compares the set output frequency from the acceleration / deceleration controller (8) with the set value of the comparison reference setter (22) and sends out the comparison result. (25) is a second comparator which compares the output of the output current detector (21) with the set value of the comparison reference setter (23) and sends out the comparison result.

(26) is an AND circuit, which is the output of the first comparator (24) and the second
Find the AND logic with the output of the comparator (25). A rapid reducer (27) reduces the output of the acceleration / deceleration controller (8) at a constant deceleration and sends a frequency signal. (28) is a low speed selector which inputs the output of the acceleration / deceleration controller (8) and the output of the rapid speed reducer (27) and selects a lower speed signal from the two to select the output frequency command value (10 ) To the PWM controller (9). Other parts are the same as those of the conventional device shown in FIG.

Next, the operation will be described. The set output frequency signal which is the output of the acceleration / deceleration controller (8) is compared with the set value of the comparison reference unit (22) by the first comparator (24), and when the set output frequency signal is small, the AND circuit (26 ) Is output. On the other hand, the output current detector (21) detects the output current of the inverter and supplies it to the second comparator (25). The second comparator (25) compares the output of the output current detector (21) with the set value of the comparison reference setter (23), and outputs the output current detector (21).
When the output of is large, the output is sent to the ND circuit (26).

Thus, when the current is below a certain speed (detected by the first comparator (24)) and above a certain value (detected by the second comparator (25)) (for example, restraint of the motor shaft). The rapid deceleration circuit (27) operates. The rapid deceleration circuit (27) is composed of an integrator, and outputs the output of the acceleration / deceleration controller (8) which is rapidly reduced at a constant deceleration.

The output of the rapid deceleration circuit (27) is given to the output frequency signal selector (28) together with the output of the acceleration / deceleration controller (8).
The output frequency signal selector (28) gives a low speed signal from these inputs to the PWM controller (9) as an output frequency signal command value (10). Therefore, when the rapid deceleration circuit (27) operates, the output of the rapid deceleration circuit (27) has priority and the output frequency command signal (10) is rapidly reduced. By combining this deceleration with the speed of the decrease in the motor speed due to the motor shaft restraint, the output frequency command signal and the actual motor speed can be made to substantially match, and the motor slip can be prevented from becoming excessive.

Next, the necessity of detection by the first comparator (22), that is, detection of the set output frequency will be described. Induction motor (5) speed reduction due to motor shaft restraint and rapid deceleration circuit (27)
As long as the deceleration commands in (1) and (2) match, the motor should not slip excessively, that is, become overcurrent, regardless of the speed. However, in reality, there is no constant deceleration when hitting the stopper with the carrier,
This variation in deceleration is also large. Therefore, when the speed is rapidly decelerated from the high speed state, the error between the actual speed of the induction motor (5) and the output frequency command signal (10) increases, and overcurrent is likely to occur. Therefore, it is necessary to limit the start of deceleration to a certain speed or less to reduce the error.

On the other hand, when stopping against a stopper, it is necessary to stop after deceleration because the impact will be greater if it hits more than full speed, and it is practically acceptable to limit the rapid deceleration start point to a certain speed or less. Further, in order to perform the overcurrent protection more reliably, it is desirable to reduce the comparison value of the output current so that the rapid deceleration circuit (27) operates with smaller slip. However, if the set output frequency is not limited, the rapid deceleration circuit (27) will operate at all speeds, and even if there is a slight load fluctuation during normal operation, this will cause unnecessary rapid deceleration operation. Will be done. For this reason, this control device needs to be in an operable state only during deceleration before stop, and the first comparator (22) is provided.

In the above embodiment, the hardware configuration using the integrator as the acceleration / deceleration controller (8) and the rapid deceleration circuit (27) is shown, but these functions are provided by the program of the microprocessor, that is, software. Even if it is realized by the configuration, the same effect can be obtained.

[Advantages of the Invention] As described above, according to the present invention, the output frequency is equal to or lower than a constant value, that is, the speed is equal to or lower than a constant value, as when the motor is restrained.
In addition, when the output current is above a certain value, the output frequency command signal is rapidly reduced to cause the motor to perform a rapid deceleration operation, which prevents the occurrence of excessive slippage, and consequently prevents the occurrence of overcurrent. , The output current is suppressed and the occurrence of trip is avoided. Therefore, an expensive high-precision rotation speed detector is not required in the control system for protection against overcurrent, and an inexpensive control device can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram of an inverter device according to an embodiment of the present invention, FIG. 2 is a block diagram of a conventional general inverter device, and FIG. 3 is a block diagram of an inverter device of a conventional slip frequency control system. (1) AC power supply, (2) forward converter, (3) capacitor, (4) inverse converter, (5) induction motor, (6)
Is an inverter start signal, (7) is a frequency setter, (8)
Is an acceleration / deceleration controller, (9) is a PWM controller, (10) is an output frequency command signal, (11) is a rotation speed detector, (12) is an amplifier, (13) is a comparator, (14). Is an adder, (21) is an output current detector, (22) and (23) are comparison reference setters, (24) is a first comparator, (25) is a second comparator, and (26) is an AND circuit. ,
(27) is a rapid deceleration controller, and (28) is an output frequency command signal selector. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A first comparing means for operating an output frequency below a fixed value, a current detecting means for detecting an output current of an inverter, and a second operating for a detected value of the current detecting means above a fixed value.
And a rapid deceleration control unit that operates under the AND condition of the output of the first comparing unit and the output of the second comparing unit to rapidly reduce the output frequency signal. Inverter device.