JP2674768B2 - Digital frequency control device for power converter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a digital frequency control device for a power converter that controls the frequency of AC power output by the power converter. [Prior Art] FIG. 2 is a control block diagram showing a conventional example for digitally controlling the frequency of an alternating current output by an inverter. In FIG. 2, the AC power from the AC power supply 1 is converted into DC power by the rectifier 2. This DC power is converted into AC power again by the inverter 3 as a power converter and is supplied to the induction motor 4. However, in order to drive the induction motor 4 at a desired speed, the inverter 3 is driven.
The frequency of the alternating current output by must be set to an arbitrary value. The frequency of the alternating current output by the inverter 3 is controlled as follows. That is, when a frequency command signal which is a digital amount and a clock pulse output from the clock oscillator 11 are input to the rate multiplier 12, a pulse train having a frequency proportional to the input frequency command signal is input from the rate multiplier 12. Since a signal is output, this pulse train signal is given to the memory 14 via the counter 13. Since this memory 14 stores a logic pattern (generally a sine pattern) of a control signal for turning on / off the switch element which constitutes the inverter 3, the memory 14 is stored in the memory 14 in response to a change in the frequency command signal. The read speed of the stored sine pattern will change. The data read from the memory 14 is the waveform synthesis circuit 15
In, the switching signals for operating the switch element of the inverter 3 are combined, and the switch element is operated via the amplifier circuit 16, whereby the frequency of the alternating current output by the inverter 3 is read from the memory 14 as described above. It corresponds to the reading speed of the sine pattern. [Problems to be Solved by the Invention] However, when the frequency of the alternating current output by the inverter 3 is an integral multiple of the frequency of the alternating current power supply 1, the output current of the inverter 3 resonates with the alternating current power supply 1 and the inverter 3 concerned. The control of (1) is disturbed, which causes a problem that good driving cannot be performed. In particular, when an induction motor 4 as shown in FIG. 2 is connected as the load of the inverter 3, it is generally expected that the induction motor 4 can maintain an arbitrary speed with high accuracy in a wide speed change range. However, the occurrence of the above-mentioned disorder is not preferable. An object of the present invention is to provide a digital frequency control device for a power converter, which can prevent the alternating current output from the power converter from resonating with the alternating current power supply and obtain good control characteristics. [Means for Solving the Problems] In order to achieve the above object, the digital frequency control device of the power converter of the present invention converts AC power from an AC power supply to DC power by a rectifier, and converts this DC power. A power converter for converting into AC power again, a rate multiplier that inputs a clock pulse output from a clock oscillator and a digital frequency command signal, and outputs a pulse train signal having a frequency proportional to the digital frequency command signal. In a digital frequency control device for a power converter, wherein the power converter outputs alternating current having a frequency corresponding to the pulse train signal, a power frequency detecting means for detecting a frequency of the AC power source is provided. The power supply frequency signal detected by the detection means and the digital frequency command signal are input and the digital frequency An arithmetic circuit is provided for adding a predetermined value to the value of the digital frequency command signal and supplying the value to the rate multiplier when the value of the wave number command signal is equal to an integral multiple of the power supply frequency. [Operation] According to the present invention, when the value of the frequency command signal is an integer multiple of the power supply frequency, the value is output from the power converter by the arithmetic circuit that adds a predetermined value to the value of the frequency command signal. The actual frequency of the alternating current is a value deviated from an integral multiple of the power supply frequency. [Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention.
In the figure, the AC power from the AC power supply 1 is converted to DC power by the rectifier 2, and this DC power is converted again to AC power by the inverter 3 as a power converter and supplied to the induction motor 4. Is the same as the case of the conventional example shown in FIG. The frequency of the alternating current output from the inverter 3 is digitally controlled. In the present invention, the frequency command signal S, which is a digital value, is first input to a central processing unit CPU (hereinafter referred to as CPU) which is a kind of arithmetic circuit. To be done. On the other hand, the power supply voltage V is input to the power supply frequency detection circuit 17, the frequency is detected, and the digital frequency signal So is input to the CPU. In the CPU, the frequency command signal S and the frequency signal So of the power supply voltage are compared, and when the value of the frequency command signal S is equal to an integral multiple of the frequency signal So of the power supply voltage, the value of the frequency command signal S is set to a predetermined value, for example 1. The secondary frequency command signal S ₁ is input to the rate multiplier 12 after addition. By the clock pulse from the clock oscillator 11 and the output from the CPU, the rate multiplier 12 outputs a pulse train signal of a frequency corresponding to the output of the CPU, and this pulse train signal is input to the counter 13 and the memory 14 Is determined. Using the data output from the memory 14, the waveform synthesizing circuit 15 synthesizes a switching signal for operating the switching element forming the inverter 3. The waveform synthesizing circuit 15 is composed of, for example, a D-flip-flop, and inputs the data read from the memory 14 and the least significant bit output from the counter 13 to synthesize the switching signal. Then, the switching signal output from the waveform synthesizing circuit 15 is amplified by the amplifier circuit 16 and given to the transistor which is a switching element as an ignition signal. Therefore, the frequency of the alternating current output from the inverter 3 is the frequency input to the CPU. Although it becomes a value corresponding to the command signal S, the frequency of the alternating current output from the inverter 3 is set by the operation of the CPU even if the value of the frequency command signal S is an integer multiple of the frequency of the alternating current power supply 1. The value is different from the above integer multiple. According to the present invention, in the case of setting the frequency of the alternating current output by the power converter to the desired value by digital frequency control, the action of the CPU when the desired value is equal to an integer multiple of the frequency of the alternating current power supply Since the digital frequency control device is configured to be avoided by, when the frequency command signal as a desired value is a value that is an integral multiple of the power supply frequency, the actual frequency of the alternating current output from the power converter is: A value deviating from an integral multiple of the power supply frequency, the output current of the power converter resonates with the power supply and becomes disordered, and it is possible to easily avoid an abnormal state in which control of the power converter becomes difficult.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a control block diagram showing an embodiment of the present invention, and FIG.
The figure is a control block diagram showing a conventional example for digitally controlling the frequency of the alternating current output by the power converter. 1 ... AC power supply, 2 ... Rectifier, 3 ... Power converter, 11
...... Clock oscillator, 12 …… Rate multiplier, 15
...... Waveform synthesis circuit, CPU …… Central processing unit (processing circuit).

Claims (1)

(57) [Claims] A power converter that converts AC power from an AC power supply into DC power by a rectifier, and converts this DC power back into AC power by inputting a clock pulse output from a clock oscillator and a digital frequency command signal. A digital frequency control device for a power converter, comprising a rate multiplier that outputs a pulse train signal having a frequency proportional to the digital frequency command signal, and the power converter outputs alternating current having a frequency corresponding to the pulse train signal. , A power supply frequency detection means for detecting the frequency of the AC power supply, the power supply frequency signal detected by the power supply frequency detection means and the digital frequency command signal are input, and the value of the digital frequency command signal is the power supply frequency. When it is equal to an integral multiple, a predetermined value is added to the value of the digital frequency command signal and the laser frequency Digital frequency converter control apparatus characterized by comprising an arithmetic circuit supplying the multiplier.