DC resistance tester working principle
The DC resistance measurement of the transformer winding is an important item in its daily test. Through the measurement of the DC resistance, the quality of the coil, the contact position of the tap switch, the breakage of the coil or the lead, the correctness of the parallel branch, and the presence or absence of the coil can be checked. Short-circuit phenomenon is important data for determining the short-circuit loss. Therefore, this test is required after handover, pre-test, overhaul and exchange of tap changer. In recent years, with the increasing capacity of power systems, the capacity of transformers has also increased. The greater the transformer's capacity, the higher the voltage level, and the greater the ratio of inductance to resistance. Therefore, the stability of the large-scale transformer's winding DC loop may be as long as several tens of minutes or even longer. How to quickly and accurately measure the DC resistance of the power transformer winding becomes the main target of research and pursuit. DC resistance measurement plays an important role in the production, operation and maintenance of power transformers. In the production and maintenance department, the DC resistance measurements at the intermediate testing stage can check the quality problems of the transformer in the winding, welding, and lead wires; as an important part of the factory test, the DC resistance is not only a good judgement of the transformer. A key indicator, but also a performance parameter that loads the resume, provides the basis for operation and maintenance. In the use department, regular measurement of the DC resistance can grasp the operation status of the transformer and eliminate hidden dangers. At present, electronic instruments have been used in resistance measurement, but due to the characteristics of power transformers, accuracy and automatic testing cannot meet the requirements. Therefore, it is of great practical significance to develop a precise, fast and automatic measuring device. Transformer DC resistance measurement is a must-measure item in semi-finished products and finished product ex-factory test, installation, transfer testing and power department preventive testing in transformer manufacturing. It can effectively discover the selection of transformer coils, welding, loose connections, missing strands, and disconnections. Defects and hidden dangers after running. DC resistance tester is a new generation DC resistance tester for transformers. It can automatically select test current according to different types of power transformers and display test results at a fast speed. The DC resistance tester has the functions of storage, printing and discharge indication. It has built-in non-power-failure memory and can store measurement data for a long time. The adoption of the liquid crystal display makes the instrument have a good man-machine interface and is the preferred device for DC resistance testing. The DC resistance of the transformer winding is measured by voltage drop method or bridge method. Although the measurement method is simple, there are many factors that affect the accuracy of the measurement. It is necessary to select an appropriate instrument and measure according to the relevant regulations in order to obtain more accurate results. In order to ensure the accuracy of measurement, safety of measurement equipment and personnel, and speed up the testing process. The following items must be noted when measuring: (1) Transformers with voltage taps, measurements should be made at all tap positions. (2) When the three-phase transformer has a midpoint lead-out line, the resistance of each phase winding should be measured; when there is no midpoint lead-out line, the resistance between the lines can be measured, and then the resistance of each phase can be calculated. (3) The measurement must be performed with the temperature of the winding stable. The difference between the winding and ambient temperature must not exceed 3°C. In the case of stable temperature, the upper layer oil temperature of the transformer can generally be used as the winding temperature and should be recorded when measuring. (4) Due to the large inductance of the transformer, it takes a long time for the current to stabilize. In order to measure accurately, you must wait for the meter indication to stabilize before reading, and if necessary, take measures to shorten the settling time. (5) Considering that there are many factors that affect the accuracy of DC resistance measurement, such as the accuracy level of the instrument, the test wiring, the accuracy of temperature measurement, the contact status of the connection, and the current stability, etc., it is necessary to review it after the measurement. In case of suspicion, retesting is required to obtain accurate measurement results. (6) During the measurement, the non-tested windings shall be open circuited and not shorted. When measuring the low-voltage winding, a high voltage is induced in the high-voltage winding at the moment of power switching, and attention should be paid to personal safety. (7) Due to the large transformer inductance, the self-induction electromotive force is very high when the power supply is turned on or off. Therefore, in order to prevent damage to the instrument, special attention must be paid to the operation sequence. When turning on the power, first turn on the power circuit, turn on the voltmeter or galvanometer, and then disconnect the power circuit. (8) Measuring the resistance value should correct the influence of the lead wire.
Investment casting technical flow process
The silica sol casting process also called precision
investment casting process. It is a less cutting or no cutting casting
process. It is an excellent technique in foundry
industry. Its application is very extensive. It not only applicable to various
types, various kinds of alloy and casting, and produce the casting dimension
accuracy and surface quality is higher than other casting method, Even other
casting method can not do the complex, high temperature resistant, not easy
machining castings, all can be used for investment precision casting process.
The investment casting process, it is a process that use fusible
materials make soluble model, and coated with several layers of tailor-made
refractory coating on the surface, after drying and hardening become a whole
shell, melt the shell model by steam or hot water. Then put the shells in the
sand, filling dry sand in all round and modeling. Finally roasting it based on
high-temperature in the furnace, the shell after roasting, and getting the
casting part after pouring molten metal.
The investment casting process can produce High precision
dimensional parts, generally can reach to CT4-6(sand casting is CT10-13, die
casting is CT5-7). Due to the complexity of investment casting process, there
are many factors influencing the casting dimension accuracy. Such as shrinkage
of mold material, deformation of the casting, Shell heating and cooling line
quantity change in the process, alloy shrinkage and deformation of castings in
the solidification process. The surface finish of investment castings is higher
than the ordinary casting, can reach to Ra1.6~3.2.
The greatest advantages of investment casting is casting parts
have high dimensional accuracy and surface finish, so it can reduce the
mechanical processing. The another advantage of investment casting method is
that it can cast all kinds of alloy and complex casting part, special casting
high temperature alloy casting part.
The precision of the Steel Casting Part: ±0.15mm~
Smoothness of surface of rough part: Ra6.4;
Shrinkage rate of steel investment casting products: 0.6%;
Material wastage rate: 5%;
General stainless steel casting part machining allowance: 0.7mm;
Surface preparation methods: Sand blasted, Shot blasting, Powder coating,
Painting, Plating, Electrophoresis, Polishing.
Our
company's production equipment and management system
CNC machining center: 8 sets AIGEMA A-8;
CNC lathe machine: 6 Guangzhou Machines and 7 Shenyang Machines;
Work days of one week: From Monday to Saturday;
Shift work: 2 shift per day;
Working hours: From AM 8:00 to PM 5:00;
2D and 3D drawing Software: AutoCAD, UG, Pro/E, SolidWorks 2013;
QC control: New type NANO CMM, FPI M5000 desktop metal analyzer, Easson
projector, Mechanical properties tester.
The steel casting process and machining
equipment:
Steel Casting Part Stainless Steel Lost Wax Casting, Steel Casting Part, Steel Investment Casting Part, Steel Silica Sol Casting Part NINGBO BEILUN LEMA MACHINERY TECHNOLOGY CO.,LTD , http://www.china-lema.com
