【正文】 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 bee 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 TechniqueThe 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 prehensively 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 ~, 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 informationhandling capacity of frequency converter enhance greatly. The plex 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) Selfsetting TechniqueThe selfsetting 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 selfsetting technique divides into offline and the online. The research results in the kind of offline has been applied in the most production, before the system program operate, it operate a block selfsetting 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 selfsetting technique.2 PLC interference analysis With the development of science and technology, the application in industrial control of PLC is more and more extensive. The dependability of PLC control system influences the safety in production and economical operation of the industrial enterprise directly。 On the other hand, require engineering design, installation construct and use of maintaining causing paying close attention to, cooperate and perfect the problem of solving in many ways, strengthen systematic antiinterference performance effectively. Sources of interference and interference with the general classification It is like generally influencing the interference source of the industrial control equipment to influence the interference source of PLC control system, mostly producing the position in the electric current or violent change of voltage, these violent position that move of electric charge is the noise source, namely interfere the source.Interfere type interfere the division different from wave form nature of the noise of the mode according to reason, noise produced to interfere usually. Among them: According to reason that noise produce being different, is it discharge noise, wave well up noise, high frequency shake noise, etc. to divide into。 It is different to interfere the mode according to the noise, is divided into mould interfering and difference mould interference altogether. It is a kind of monly more used categorized method that the mould interferes interfering with the difference mould altogether. It is signal electric potential difference to place for mould not to interfere altogether, bunch enter, ground electric potential difference and space electromagnetic radiation reaction attitude (with direction) voltage is it is it form to add to change altogether at signal line by electric wire netting mainly. Mould voltage sometimes relatively heavy altogether, is it isolate performance mixing electric apparatus supply power in the room bad to adopt, changer output voltage generally relatively high mould altogether of signal, some can up to 130V to be the abovementioned. Mould voltage through asymmetric circuit getting convertible model39。s interfering form altogether by uneven circuit among signal by space electric magnetic field mainly, this kind superposes it on the signal directly, influence measurement and control the precision directly. PLC control system electromagnetic interference the main source1) The radiation from space interference mainly produced by temporary attitude course , thunder and lightning , radio broadcasting , TV,radar, highfrequency reaction heating device of the electric network , electric equipment ,etc., usually called radiation to interfere, it is extremely plicated that it is distributed. Whether if PLC system put on radio frequency in the field is it reaches radiation interfere to retrieve, it influences and mainly passes two routes. First, produced and interferes by the reaction of the circuit to the radiation within PLC directly