JPH026314B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inverter control circuit, and particularly to a stall prevention circuit when an AC motor is driven by an inverter.

In general, a soft start/soft stop method is used for starting and stopping an inverter, and the general configuration of such an inverter will be explained with reference to FIG. In FIG. 1, 1 is a converter that converts AC power into DC power, 2 is a smoothing capacitor, 3 is a diode, 4 is a current detector, 5 is an inverter, 6 is an induction motor as a load, 7 is an inverter control circuit, Reference numeral 8 denotes a frequency command circuit which receives a setting signal from a frequency setter (not shown) and generates a command signal for determining the output frequency of the inverter based on the setting signal. 9 is a frequency/voltage control circuit, and 10 is a base amplifier circuit.

By the way, in the case shown in FIG. 1, if the load inertia is not sufficiently understood, and if the acceleration time or deceleration time is short, the synchronous rotation speed determined by the inverter output frequency and the induction motor 6, which is the load, will be The difference with the rotation speed increases, that is, the slip increases, and the inverter current, which is the load current, increases, resulting in an overcurrent state. As a result, an overcurrent protection circuit (not shown) operates to protect the inverter, and the device stops. Stopping the inverter means stopping the load motor, which is not preferable.

The present invention has been made in order to solve the above-mentioned drawbacks.The present invention detects the main circuit current of the inverter, stops increasing or decreasing the frequency of the inverter when it exceeds a predetermined value, and further increases or decreases the frequency of the inverter after a certain period of time. A timer circuit is used in the control circuit to release the stoppage.

An embodiment of this invention will be described below.

FIG. 2 shows a control circuit according to the present invention. In FIG. 2, If is a DC bus current detection signal, Iref is a stall prevention level reference signal, 11 is a comparator, and 12 is a monostable multivibrator which is a time limit circuit. As for the operation of the circuit, the main circuit current increases and the DC bus current detection signal If becomes the stall prevention level reference signal Iref.
When this happens, the comparator 11 operates, and as a result, the monostable multivibrator 12 outputs a pulse, and this output pulse fixes the inverter output frequency. Here monostable multivibrator 1
The reason for using 2 will be explained. FIG. 3a shows a pulse train indicating an inverter command frequency, and FIG. 3b shows an inverter DC bus current detection signal. When this detection signal and the stall prevention level signal Iref are input to the comparator 11, an output signal as shown in FIG. 4 is outputted from the comparator 11. In other words, a in Figure 3
-b only is output as a pulse signal. Fourth
Since the pulse width of the solid line pulse signal in the figure is narrow, it is difficult to completely control changes in the frequency of the frequency command circuit 8. That is, since the inverter output frequency also increases between b and c in FIG. 3, this increase in frequency must be stopped. Once the current increases, it is necessary to widen the pulse width by more than 1/6f, where f is the minimum frequency of the inverter output. As a means for increasing the pulse width shown in FIG. 4, a retrievable monostable multivibrator may be used, and the inverter frequency is fixed and the inverter operates until the slip becomes small. Furthermore, by selecting the pulse width as 1/6f, it is possible to further shorten the acceleration time. Next, the time of deceleration will be explained with reference to FIG. The solid line in FIG. 5 shows the DC bus current detection signal during deceleration. Here, as the regenerated energy increases, it increases in positive and negative directions as shown by the dotted line in FIG. This increased signal makes it possible to fix the same inverter frequency as at the time of acceleration. From the above, it is possible to fix the inverter frequency immediately before the overcurrent protection circuit operates, thereby preventing stalling during acceleration and deceleration. It is also possible to smooth the DC bus current detection signal shown in FIG. 3, input this smoothed signal to the comparator 11, and directly control the frequency command circuit 9 with the output signal of the comparator 11. In some cases, there is a time lag and a sufficient effect cannot be obtained.

In view of the gist of the invention, it goes without saying that it can be applied whether the system is single-phase or polyphase.

In this way, the present invention is configured to provide a time limit circuit that generates a predetermined time signal based on the output of a comparator that compares the main circuit current of the inverter with a reference signal, and to determine the output frequency of the inverter based on the output of this time limit circuit. Therefore, the inverter can be operated continuously.

Furthermore, in the present invention, an overcurrent of the load is detected from the DC bus and the increase/decrease of the frequency of the inverter is stopped for a certain period of time, so it can be obtained with a simple configuration and at low cost.

[Brief explanation of drawings]

In the figures, Fig. 1 shows the configuration of an inverter device, Fig. 2 shows an embodiment according to the present invention, Fig. 3 shows an inverter frequency command pulse train and a DC bus current detection signal, and Fig. 4 shows a comparator. FIG. 5 is a diagram showing the output signal, and FIG. 5 is a diagram showing the DC bus current detection signal during deceleration. 1 is a converter, 2 is a smoothing capacitor, 3 is a diode, 4 is a current detector, 5 is an inverter, 6
is an induction motor, 7 is an inverter control circuit, 8 is a frequency command circuit, 9 is a frequency/voltage control circuit, 10
1 is a base amplifier circuit, 11 is a comparator, and 12 is a monostable multivibrator. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. An inverter control circuit that converts DC power into variable frequency, variable voltage AC power to drive an AC motor, and stops the AC motor when the current flowing through the AC motor exceeds a predetermined value, A frequency command circuit that generates a command signal that determines the inverter's output frequency based on a setting signal that sets the inverter's output frequency, and a frequency voltage control circuit that inputs the output signal from this frequency command circuit to control the voltage of the inverter. and a comparator that compares the DC bus current detection signal of the inverter with a reference signal, and when the comparator detects that the DC bus current detection signal has exceeded the reference signal comparator, it generates a pulse with a predetermined time period, An inverter control circuit comprising: a time limit circuit that stops the frequency command circuit from increasing or decreasing the output frequency of the inverter for a certain period of time. 2. The inverter control circuit according to claim 1, wherein the timer circuit is composed of a monostable multivibrator. 3. The control circuit for an inverter according to claim 1 or 2, wherein the AC motor is an induction motor.