Study On Magnetic Shielding Effectiveness Of Heavy Direct Current Sensor

The heavy direct current is applied in the metallurgic plant and the chemical plant commonly. The metallurgic plant and the chemical plant are important industries in our country. They consume a great deal of electricity. It is important to measure the heavy direct current precisely for energy conversation. For a long time the indication value of instrument is regarded as referenced value only because the online metrical instrument applying in monitoring and metering is short of reliability. It will affect the energy conversation and economic returns of these industries directly. It is important to collect the virtue and overcome the shortcoming of measurement theory in existence in order to change the situation. Then we search for a new kind of direct current sensing theory. The instrument based on this new kind of direct current sensor theory has high stability and high dependability. This paper establishes a kind of dc sensor based on self-balance of magnetic potential and feed-back compensation. The direct current sensor is divided into two types of principle based on open loop and closed loop measurement theories. The sensor based on open loop theory has many advantages such as simple construction, small volume, and lightweight. It doesn't exist the oscillation and heavy driving power either. That based on closed loop theory has many advantages such as high accuracy, good linearity, and strong anti-interference and so on. A model machine is made in order to validate the correctness of theory. The experiment results have proved that the most of magnetic potential is balanced by the ac voltage source e. The system composed of the detecting and compensation coils and electronics module A is an automatic dc compensation system whose open loop gain is small. The zero ampere-turns compensation of the detecting core is realized by this circuit.When the heavy direct current sensor is applied in industry spot, it is inevitable to be disturbed by magnetic field. This disturbance will affect the heavy direct current sensor work naturally. It is very important to design the magnetic shielding body. Three ways of problem about magnetic shielding will studied in this paper.At first the magnetic shielding technology of heavy direct current sensor is discussed. The heavy direct current sensor applies magnetic shielding layer and copper shielding layer at the same time. The magnetic shielding effectiveness is the sum of every layer's magnetic shielding effectiveness.The copper shielding layer can shield electric field and high-frequency magnetic field at the same time. The silicon-steel sheet layer can shield low-frequency magnetic field. When the magnetic shielding body is made of these two materials the satisfactory magnetic shielding effectiveness for low-frequency magnetic field and high-frequency magnetic field can be obtained. The magnetic shielding technology can be divided into electric shielding and magnetic shielding. The calculated formula of magnetic shielding effectiveness can be deduced by equivalent transmission line method. When the magnetic shielding body is designed the mutichannel technology is applied.Secondly the calculation of magnetic shielding effectiveness applied magnetic circuit method is discussed. Since Moore has put forward the magnetic shielding of dc sensor, the calculation of magnetic shielding effectiveness is concerned by the designer of dc sensor all the while. It is important to apply more precise method to calculate reliably. The calculation formula is deduced applying magnetic circuit method when adding radial magnetic field and axis magnetic field. In order to prove that the calculated result is truthfulness, the experiment is carry on to test the magnetic shielding effectiveness in this paper. The permeabilityμr isn't constant in iron core. It is the intensity of magnetic field's function. When intensity of magnetic field is 20GS, the permeability is tested. The tested permeability is substituted into formula deduce by magnetic circuit method. Compared the calculated result and the tested result we can see that the calculated result is close to factual situation. The conclusion can be obtained that magnetic circuit method can estimate the dc sensor's magnetic shielding effectiveness.Finally the problem of multichannel comparator's closed channel is studied. When the iron core of dc sensor is integral, the iron core has remnant magnetic potential. When the measured current and disturbed current operate on iron core at the same time, the magnetization of iron core is uneven. It will lead to closed channel of multichannel comparator. A new kind of multichannel comparator is designed in this paper in order to solve the problem of closed channel of multichannel comparator. An air gap between two channels can counteract remnant magnetic field in iron core. Every channel based on double magnetic detector sensing theory is an absolute dc comparator. The magnetic characteristic of annular iron core which has air gap and the open-loop characteristic of multichannel comparator for heavy direct current have studied. At first the length of air gap and equivalent intensity of magnetic field are calculated. Then the math model of multichannel comparator for heavy direct current is established. In the end the open-loop characteristic curve of multichannel comparator for heavy direct current is described. When the iron core of dc sensor has gas gap, the reluctance is increase. Compared the open-loop characteristic calculated curve the conclusion can be obtained that the output direct voltage of the dc sensor which has gas gap is lower than the dc sensor which has integral iron core when input ampere-turn is same.When the multichannel comparator has gas gap, it can reduce the effect of iron core's non-linear. But it brings negative effect also. It can increase leak flux. It will reduce the veracity of ampere-turns ratio. The equivalent permeability of iron core is decrease also. Thus the sensitivity of mutichannel heavy direct current comparator is reduced. The stability of multichannel comparator for direct current sensor is analyzed applying Routh criterion. Whether the roots of character equation are all negative real part is judged by the relation of character equation's root and every coefficient. The interferential signal will affect system's stable error. The feed forward compensation is put forward to counteract the effect...