JP3097709B2 - Inverter control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for an inverter in which the output frequency of an inverter for driving a motor is adjusted so that the rotation speed of the motor is feedback-controlled.

[0002]

2. Description of the Related Art Conventionally, an inverter having a speed feedback control function detects the number of rotations of a motor by a rotation number detector and corrects a difference (slip) between the detected number of rotations (operation frequency) and an output frequency of the inverter. In such a case, the output frequency of the inverter is increased or decreased in such a direction as to perform feedback control of the motor rotation speed.

[0003]

By the way, when performing V / F control for correcting the output voltage (V) and the output frequency (F) of the inverter according to the relationship shown in FIG. 3, when the output voltage reaches the rated voltage, Even if the output frequency is further increased, there is a region where the output voltage does not increase but remains constant. In this constant voltage region, the output torque decreases conversely as the output frequency increases. For this reason, when the motor load increases and the rotational speed decreases in the constant voltage region, if the output frequency is increased to compensate for it, the output torque will decrease, and conversely, the motor rotational speed will increase. In the worst case.

[0004] The present invention has been made in view of such circumstances, and the object thereof is to reduce the rotation speed of the motor further when the rotation speed of the motor decreases even if the output frequency of the inverter is increased. An object of the present invention is to provide an inverter control device capable of suppressing the stall, preventing the stall, and smoothly restoring the rotation speed thereafter.

[0005]

SUMMARY OF THE INVENTION An inverter control device according to the present invention performs feedback control of the motor rotation speed by correcting the inverter output frequency while detecting the rotation speed of the motor by rotation speed detection means. In this configuration, when the rotation speed of the motor falls without increasing even if the output frequency of the inverter is increased in response to the rotation speed increase command, the increase of the output frequency of the inverter is stopped or the output frequency is reduced. After stopping the decrease in the number of revolutions of the motor by lowering
It is configured to return to normal feedback control.

[0006]

When a rotational speed increase command is issued, the output frequency of the inverter is increased accordingly.
Even if the output frequency of the inverter is increased, the number of revolutions of the motor may decrease without increasing due to an increase in the load on the motor. In this case, even if a rotational speed increase command is issued, control is performed so as to stop increasing the output frequency of the inverter or decrease the output frequency.

At this time, for example, if the load is temporarily changed, the load may be reduced for a while. In the constant voltage range of the V / F control described above, if the output frequency of the inverter is decreased, the motor may be changed. The output torque increases. Therefore, when the rotation speed of the motor decreases even if the output frequency of the inverter is increased, as described above, if the increase in the output frequency of the inverter is stopped or the output frequency is decreased, the output torque of the motor will eventually be reduced. When the loads are balanced, the decrease in the rotation speed of the motor stops. Therefore, if it is confirmed that the rotation speed has stopped decreasing and then the control is returned to the normal feedback control and the output frequency of the inverter is increased again, the rotation speed of the motor can be increased smoothly without stalling.

[0008]

An embodiment of the present invention will be described below with reference to the drawings. As shown in FIG. 2, the inverter 1 includes a converter unit 3 that converts a commercial AC power supply 2 into DC power, a smoothing circuit unit 4 that smoothes an output voltage of the converter unit 3,
And an inverter circuit unit 5 for inversely converting the smoothed DC power into AC power of a variable frequency. The inverter circuit unit 5 includes six switching elements Q
1 to Q6 are connected in a three-phase bridge, and these switching elements Q1 to Q6 are turned on and off at a predetermined timing by a base drive circuit 6 to output three-phase AC power to drive the induction motor 7. It has become.

The base drive circuit 6 is controlled by a microcomputer 8 as a control device. The microcomputer 8 is supplied with power from a power supply circuit 9 connected to the output side of the smoothing circuit unit 4, and is supplied with a RAM 8a and a ROM 8b.
The ROM 8b stores a control program shown in FIG.

On the other hand, the induction motor 7 is provided with a rotation speed detector 10 such as a tacho generator as rotation speed detecting means for detecting a rotation speed, and a rotation speed signal of a voltage proportional to the rotation speed is output from the rotation speed detector 10. Is output. The microcomputer 8 calculates the rotation speed (operation frequency value) of the induction motor 7 based on the rotation speed signal, and determines that the rotation speed of the induction motor 7 has decreased while the output frequency of the inverter 1 has increased. Rise and calculation (PID processing)
Is stopped, an alarm signal is output via an alarm signal output driver 11, and an alarm unit 12 such as an alarm lamp or an alarm buzzer installed outside is driven to generate an alarm.

Hereinafter, the contents of control of the inverter 1 by the microcomputer 8 will be specifically described. Induction motor 7
During the operation, the microcomputer 8 calculates the rotation speed (operation frequency value) of the induction motor 7 based on the rotation speed signal input from the rotation speed detector 10, and calculates the difference between the operation frequency value and the frequency command value. (Slip) and calculate this difference as PI
Each of the switching elements Q1 to Q6 of the inverter 1 is switched in a direction to be corrected by the D processing to correct the output frequency, and the number of revolutions of the induction motor 7 is feedback-controlled. This feedback control is executed by a main program (not shown), and the main program executes a control program for countermeasures to decrease the rotational speed during the output increase command shown in FIG.

When the load on the induction motor 7 is rapidly increased during operation of the induction motor 7 due to, for example, loading of the load, the rotation speed of the induction motor 7 decreases, and the rotation speed output from the rotation speed detector 10 is reduced. The voltage of several signals decreases. The microcomputer 8 that detects this increases the rotation frequency of the induction motor 7 by increasing the output frequency of the inverter 1. At this time, V shown in FIG.
If the induction motor 7 is operated in the constant voltage region by the / F control, the output torque of the induction motor 7 will decrease with an increase in the output frequency of the inverter 1. Due to such a torque characteristic, despite the increase in the output frequency of the inverter 1, the rotation speed of the induction motor 7 reversely decreases due to the lack of torque, and the voltage of the rotation speed signal output from the rotation speed detector 10 is reduced. It will be even lower. Therefore, if the output frequency of the inverter 1 is continuously increased, the induction motor 7 may stall.

In order to cope with such a situation, the microcomputer 8 uses a main program (not shown) for executing the feedback control to execute a control program for reducing the rotational speed during the output increase command shown in FIG.
M8b will be read and executed. In this control program, first, it is determined whether or not a “rotation speed decrease countermeasure control flag during output increase command” described later is set (step S1). Here, when it is determined to be "NO", it is determined whether or not the rotation speed of the induction motor 7 is decreasing (step S2). In the case of "NO", that is, when the rotation speed is not decreasing, Feedback control is performed by normal PID processing (step S6).

On the other hand, if the number of revolutions is decreasing, the process proceeds from step S2 to S3, where it is determined whether or not the output frequency is increasing. If "NO", that is, the output frequency is not increasing. In this case, feedback control is performed by ordinary PID processing (step S6).

On the other hand, if the determinations in steps S2 and S3 are both "YES", that is, the rotation of the induction motor 7 is increased despite the output frequency of the inverter 1 being increased in response to the rotation speed increase command. On the other hand, if the number is decreasing, the "rotational speed decrease countermeasure control flag during output increase command" is set (step S4), and the increase of the output frequency is stopped (step S5). Return.
While the "output increase command medium rotation speed decrease countermeasure control flag" is set, an alarm signal is output via the alarm signal output driver 11, and an externally provided alarm means 1 is provided.
Drive 2 to give an alarm.

In this way, once the "power increase command rotation speed reduction countermeasure control flag" is set, the next time this output increase command rotation speed reduction countermeasure control program is executed, the program proceeds to step S1. The determination becomes "YES", and the routine shifts to step S7. In this step S7, it is determined whether or not the rotational speed of the induction motor 7 is decreasing. If "YES", that is, if the rotational speed is still decreasing, the output frequency of the inverter 1 is decreased (step S8). ).

If the output frequency at this time is in the constant voltage region shown in FIG. 3, the output torque of the induction motor 7 increases with a decrease in the output frequency.
And the load is balanced, so that the rotation speed of the induction motor 7 stops decreasing. In this case, the next time the control program for reducing the number of revolutions during output increase command is executed, the determination in step S7 becomes "NO", and
The “rotation speed decrease countermeasure control flag during output increase command” is reset (step S9), and the process returns to the feedback control by the normal PID process (step S10). Thereby, the output frequency of the inverter 1 is increased again, and the rotation speed of the induction motor 7 is smoothly returned.

If the load fluctuation that causes the rotation speed of the induction motor 7 to decrease is temporary, the load may decrease after a while. In such a case, the inverter 1 Even if the output frequency is not reduced, the rotation speed of the induction motor 7 may stop decreasing. Also in this case, the determination in step S7 is "NO", as described above.
And the `` rotation speed decrease countermeasure control flag during output increase command ''
Is reset (step S9), and the process returns to the feedback control by the normal PID process (step S9).
10).

According to this embodiment described above, even if the rotation frequency of the induction motor 7 decreases due to an increase in load or the like while the output frequency of the inverter 1 is increasing, the increase in the output frequency of the inverter 1 is stopped or the output is stopped. By lowering the frequency, it is possible to suppress a further decrease in the rotation speed of the induction motor 7, prevent stall, and smoothly recover the rotation speed thereafter.

In addition, when the rotation speed of the induction motor 7 decreases while the output frequency of the inverter 1 increases, a warning signal is output to the outside to drive the warning means 12, so that surrounding workers can be operated. There is also an advantage that an alarm of an increase in load can be issued and an overload state can be prevented from occurring.

In this embodiment, a tacho generator is used as the rotation speed detector 10 (rotation speed detecting means) to output a rotation speed signal (analog signal) having a voltage proportional to the rotation speed of the induction motor 7. However, a pulse signal having a frequency proportional to the number of revolutions of the induction motor 7 is output using, for example, a rotary encoder, and the number of pulses is counted by the microcomputer 8 to determine the number of revolutions of the induction motor 7. May be. In addition, the present invention can be variously modified without departing from the gist, such as omitting a function of outputting an alarm signal to the outside.

[0022]

As is apparent from the above description, the present invention stops increasing the output frequency of the inverter or reducing the output even if the rotation speed of the motor decreases due to an increase in load while the output frequency of the inverter increases. By lowering the frequency, it is possible to suppress a further decrease in the number of rotations of the motor, thereby preventing stall, and at the same time, providing an excellent effect that the number of rotations can be smoothly restored.

[Brief description of the drawings]

FIG. 1 is a flowchart of a control program showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an electrical configuration.

FIG. 3 is a characteristic diagram showing a relationship between an output frequency of an inverter, an output voltage, and an output torque.

[Explanation of symbols]

1 is an inverter, 7 is an induction motor, 8 is a microcomputer, 10 is a rotation speed detector (rotation speed detecting means), 12
Is an alarm means, and Q1 to Q6 are switching elements.

Claims (1)

(57) [Claims] An inverter for driving a motor is controlled, and the output frequency of the inverter is corrected while the number of rotations of the motor is detected by a number-of-rotations detecting means, thereby feedback-controlling the number of rotations of the motor. In such a configuration, even if the output frequency of the inverter is increased in response to a rotation speed increase command and the rotation speed of the motor decreases without increasing, the increase in the output frequency of the inverter is stopped or A control device for an inverter, wherein the control is returned to normal feedback control after stopping the decrease in the number of revolutions of the motor by decreasing the output frequency.