JPS62196098A - Inverter apparatus for controlling ac motor - Google Patents

Abstract

PURPOSE:To suppress current at a restarting state to a low value, by a method wherein when an inverter is restarted, a terminal voltage of AC motor and output of the inverter are made coincident in phase. CONSTITUTION:Period T1 of a terminal voltage of AC motor 10 is detected. Time difference Ta between constraint relief signal of a timer circuit 7 and pulse signal most close to the constraint relief signal is detected, and further inverter output is interrupted from the constraint relief signal by time Tb. Control is performed so that the time relation of Ta and Tb becomes Ta+Tb= T1. In order to select a switching element from this and output of a polarity discrimination circuit 12, the terminal voltage of the AC motor 10 and output voltage of the inverter 2 can be made coincident in phase.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an inverter device for controlling an AC motor that is capable of restarting a coasting AC motor.

[Conventional technology]

FIG. 3 shows an example of an inverter device equipped with a conventional AC motor restart circuit during seven reruns. In the figure, l is an inverter device, 2 is an inverter section composed of switching elements such as transistors or diodes,
3 is a circuit that rectifies the voltage signal obtained from the detected terminal voltage of the AC motor; 4 is a waveform shaping circuit that converts the waveform rectified by this rectifier circuit 3 into a pulse train; 5 is a frequency detection circuit; and 6 is a switching element control circuit. circuit, 7 is a timer circuit, 8 is a DC power supply, 9 is an isolation transformer that is a means for detecting terminal voltage, and 10 is an AC motor. In general, when an AC motor is operated with an inverter, for some reason, free sink The electric motor is often restarted using an inverter, and if the output frequency of the inverter and the free-run rotational frequency of the electric motor do not match, an excessive motor current will flow and the inverter will trip. Therefore, as shown in FIG. 3, the terminal voltage of the AC motor 10 is detected and a frequency corresponding to the number of rotations during free running at the time of restart is output. The outline of the operation will be explained below according to the diagram.

One phase (third phase) of the outputs U, V, and W of the inverter section
In the figure, the terminal voltage of the AC motor 10 is detected through V 2 , W) (the terminal voltage here includes the residual voltage generated by the AC motor 10 during free running). The detected voltage transmitted in isolation by the isolation transformer 9 becomes a sine wave when the output of the inverter section 2 is stopped, and after being full-wave rectified by the rectifier circuit 3, it is converted into a pulse train by the waveform shaping circuit 4. be done.

As shown in FIG. 4, the period T1 of the pulse train is a member of the period T of the detected voltage, and the frequency is detected at a double value. Calculating the frequency from this pulse train is easy if you have a clock and a counter with a cycle that is extremely short compared to the cycle Tl, and it can also be calculated using a microcomputer. This is the frequency detection circuit 5.

The detected frequency is transmitted to the switching element control circuit 6 at the timing when the restart signal of the inverter section 2 is input (at this time, the output voltage gradually increases from zero).

The timer circuit 7 prevents the inverter section 2 from restarting until the terminal voltage decreases to a certain degree in order to reduce the influence of the circulating current due to the terminal voltage of the AC motor 10. This situation is shown in FIG. In other words, the curve in Figure 5 (
stomach).

(B) and (C) are the free-run rotation speed, inverter output frequency, and inverter output voltage of the AC motor 10, respectively, and points a to b indicate a state in which the free-run rotation speed 10 is gradually decreasing, and b Points to e indicate a state in which the inverter output frequency (b) matches the free run rotational speed (a) after one hour of the predetermined timer of the timer circuit 7 has expired.

[Problem that the invention seeks to solve]

Since the conventional inverter device is configured as described above, since the phase of the output voltage of the inverter section 2 and the phase of the terminal voltage of the AC motor 10 do not match, the starting current increases at the time of restart, and the timer circuit 7 needs to be set for a long time, which not only deteriorates the responsiveness of the system, but also causes problems such as overcurrent tripping of the inverter section 2 in the case of a load with large inertia.

This invention was made to solve the above problems, and based on detecting the phase of the terminal voltage of the AC motor, when restarting the AC motor during free running, the starting current can be determined. An object of the present invention is to obtain an inverter device for controlling an AC motor that can reduce the size of the AC motor.

[Means for solving problems]

The inverter device for controlling an AC motor according to the present invention detects the terminal voltage of the AC motor, determines the phase of the terminal voltage from the polarity and frequency of the terminal voltage, and changes the phase of the output voltage of the inverter and the terminal voltage when restarting the inverter. The firing order of the switching elements of the inverter is controlled so that the phases match.

[For production]

The inverter device for controlling an AC motor in this invention includes:
The rotational frequency of the AC motor and the phase of the terminal voltage are detected from the detected terminal voltage of the AC motor, and when the inverter is restarted, the inverter output is generated in accordance with the rotational frequency of the AC motor and the phase of the terminal voltage.

〔Example〕 An embodiment of the present invention will be described below with reference to the drawings.

In Figure 1, 1 to 10 are the same as in Figure 3, and 11
12 is a synchronous circuit, and 12 is a terminal voltage polarity determination circuit of the AC motor 10.

The basic operation of the frequency detection circuit 5''! is the same as the conventional circuit operation shown in FIG. The operation will be explained.

To make the explanation easier to understand, examples of basic operation in a conventional circuit and examples of basic operation according to this embodiment are shown in FIG.
B) for comparison. In the figure, 21 is the output signal of the timer circuit 7. The output of the inverter is constrained by the "wH" level, and the constraint is released by the "L level."2
2 is the terminal voltage of the AC motor 10 which has undergone full-wave rectification, and shows how the peak value gradually decreases over time. In addition, the frequency change of the terminal (pressure) was ignored.2
3 indicates a pulse train that is the output of the waveform shaping circuit 4. 24
is the output voltage of the inverter section 2. 25 is the output of the polarity determining circuit 12, which determines the polarity of the terminal voltage of the detection phase.

As is clear from Figure 2, in the conventional circuit, the period of the terminal voltage T! Since the inverter is configured to output an output with a period T1 when the timer circuit detects this and times up, it is inevitable that the phase will shift by the time Ta. The present invention uses a synchronization circuit 11 to eliminate this phase shift.

As shown in FIG. 2(B), the process is the same as before until the period Tl of the terminal voltage of the AC motor 10 is detected. In the present invention, the time difference Ta between the restraint release signal of the timer circuit 7 and the pulse signal closest to this signal is detected, and the inverter output is cut off for a time Tb from this restraint release signal, and the Ta and Tb The time relationship is Ta + T
It is controlled so that b = Tl.

If the phase to be detected at this time (in this circuit, the v-W phase) is determined in advance, it is easy to determine from the output of the polarity discrimination circuit 12 whether the V phase is ten poles or one pole with respect to the W phase. If the phase is 10 poles, select the switching element that should be fired among the 6 switching elements in Figure 1 so that the switching elements on the P side of the V phase and the N side of the W phase are electrically connected. Therefore, it is possible to match the phases of the terminal voltage of the AC motor 10 and the output voltage of the inverter unit 2. The switching elements of the remaining phases are controlled by AC motor 1.
Needless to say, it is uniquely determined by the rotation direction of 0.

In this embodiment, since there is a time lag between detecting the rotation speed from the terminal voltage of the AC motor 10 and restarting it at a frequency commensurate with the detected rotation speed, the AC 't:
If the free run rotational speed of the JJ machine 10 suddenly decelerates, a phase shift will occur, but at low rotational speeds, the generated terminal voltage becomes small, so it is at a level that does not cause any problems in actual use. .

Further, in the above embodiment, the frequency detection circuit 5, the timer circuit 7, and the synchronization circuit 11 are expressed as individual block diagrams, but it is also possible to perform arithmetic processing using a microprocessor. A photocoupler may be used instead of the isolation transformer 9 as the terminal voltage detection means.

Furthermore, in the above embodiment, restarting using an inverter when the AC motor is free-running was explained.
The present invention can also be applied when a commercially operated electric motor is switched to inverter operation and operated at variable speed, or when the electric motor is continuously controlled without being stopped once. Also, if an accident such as a momentary power outage occurs while the inverter is operating, please contact the inverter! When the motor is electrically disconnected and then power is restored, the motor can be quickly restarted.

〔Effect of the invention〕

As described above, according to the present invention, when restarting a free-running AC motor using an inverter, the terminal voltage generated by the AC motor and the phase of the inverter output match, so that when restarting It is possible to keep the starting current low, which shortens the setting time of the timer circuit when restarting, which has the effect of providing a highly responsive system.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an inverter device for controlling an AC motor according to an embodiment of the present invention, and FIG. (B) is an explanatory diagram comparing the basic operations of the conventional example and the present example, FIG. 3 is a block diagram of a conventional inverter device for controlling an AC motor, and FIG. 4 is an illustration of the terminal voltage detection operation in FIG. 3. The explanatory diagram shown in FIG. 5 is an operation diagram when the inverter restarts the motor during free running. ■ is an inverter device, 2 is an inverter section, 3 is a rectifier circuit, 4 is a waveform shaping circuit, 5 is a frequency detection circuit, 6 is a switching element control circuit, 7 is a timer circuit, 8 is a DC 1 source, 9 is an isolation transformer, 10 is an AC motor, 11 is a synchronous circuit, and 12 is a polarity discrimination circuit. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] Terminal voltage means for detecting terminal voltage during free running of an AC motor connected to an inverter section and operated at variable speed;
a rectifier circuit that rectifies the voltage signal obtained by the detection, a waveform shaping circuit that converts the waveform obtained by the rectification into a pulse train, and a frequency circuit that detects the free-run rotation frequency of the AC motor from the pulse train; a switching element control circuit that controls a switching element of the inverter section so that the inverter section generates an output at a frequency corresponding to the free run rotation frequency; and an output of the inverter section until the terminal voltage becomes equal to or less than a predetermined value. In an inverter device for controlling an AC motor, the inverter device includes a timer circuit that restricts generation of the voltage for a predetermined period of time; Synchronization in which an output is generated from the inverter section after a period of time equal to the period of the pulse train has elapsed from the time of generation, and a switching element to be fired is selected based on the polarity determination when the output of the inverter section is generated. An inverter device for controlling an AC motor, characterized in that it is provided with a circuit.