In1964, Germany's A. Schonung and some people took the lead to propose the pulse-duration modulation (PWM -pulse width modulation) frequency conversion thought, opened the recent development domain for the modern alternating velocity modulation system. The PWM control technique controls the inversion output alternating fundamental wave voltage peak-to-peak value by changing the rectangle pulse width, controls its output frequency through changing the modulation cycle, in order to carry on the output voltage peak-to-peak value and the frequency control on the inversion at the same time. The PWM technique simplified the inversion structure, can distinctly change the output wave shape of the frequency converter, reduces the overtone loss of electric motor, and reduces the torque pulsation, at the same time enhances the performance of dynamic response system. The PWM technique also may use in the rectifier controlling, can realize the input current which approaches the sine extremely. And may cause the electrical network power factor is 1. The PWM rectifier thus is called “the green converter”. At present, PWM technique has become the most widespread control technique applied in the frequency converter. The unceasingly velocity modulation performance of alternating current motor is as a result of the PWM technique unceasing progress enhancement in the very great degree. At present the widely applies is the optimizes PWM law which developing based on the regular sampling PWM foundation, that is Thrice Harmonic Superpose theory and voltage space vector PWM theory.
5) Numerical Control Technique
The digitalize in control technique is the key technique of static frequency conversion installment, also will be the next development tendency, at present in the market frequency conversion installment nearly comprehensively has realized the numerical control. Has realized the fast operation and the high accuracy control by using the DSP and the ASIC technique, and could gain a fine current wave, reduce the noise of the frequency converter in a large scale, and the scanning time also be reduced in a large scale, at present the electric current response is 0.1~0.7ms, the speed response is 2~4 ms, could satisfy the control request sufficiently in the transmission domain. At the same time, on account of the application of the micro electron and ASIC technique, the installment primary device quantity can be reduced in a large scale, thus, could make the frequency conversion equipment volume to be reduced, the reliability obtains the large scale enhancement. The entire numerical control way can make the information-handling capacity of frequency converter enhance greatly. The complex control has been realized in nowadays which could not be realized by simulation control mode, so that, the RAS which mean Reliability, Availability, Serviceability could be realized.
6) Self-setting Technique
The self-setting technique is widespread used in the frequency conversion velocity modulation system day by day, it may accord to the speed and the load to auto adjust the parameter of the control system, enable the system with rapid dynamic response. The self-setting technique divides into off-line and the on-line. The research results in the kind of off-line has been applied in the most production, before the system program operate, it operate a block self-setting program, recognize the correlation data, and amend the parameter of the system program that related, in order to gain a approved system control performance. Moreover, improve the control technique, enhance the robustness of the system, also have close relatives with self-setting technique.
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