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What are the problems with field test of conventional loop resistance tester?
The principle of most loop resistance testers on the market today is to use a typical four-wire measurement method, which provides a test current greater than 100A from a high-frequency switching power supply. When the measurement key is pressed, the output current of the high-frequency switching power supply is greater than 100A. At the same time, the sampling circuit starts sampling, and the obtained signal is amplified by an amplifier. After the analog signal is converted into a digital signal by the A/D converter, the data is filtered, calculated, processed by the microprocessor, and finally sent to the display to show the measurement. The current and resistance values. When the current test circuit is broken or in poor contact, the instrument will judge the current loop contact fault or open circuit according to the voltage on the current shunt. The DC resistance fast tester adopts a new power supply technology and has It is an ideal device for measuring the DC resistance of transformer windings and high-power inductive devices due to its rapid measurement, compact size, ease of use, and high measurement accuracy.
1, the existing test method problems
The loop resistance tester designed in accordance with the conventional design principle found in field tests that there is a common problem: When the tester's voltage wiring loop is poorly connected or opens, the tester will also display a numerical value. Happening:
(1) The voltage loop is open and there is no strong electric field interference on the test site. In this case, the differential mode voltage of the amplifier input is basically 0, so the test value displayed by the instrument is close to 0. If the tester has enough on-site test experience, It can be judged that the instrument voltage loop test line is abnormal. After the abnormality of the instrument voltage loop test line is eliminated, the final correct test result can be obtained; if the tester does not have enough on-site test experience, it may misjudge the tester to have a problem and interrupt. Testing, replacing or repairing instruments delays power outages and brings unnecessary trouble to the test.
(2) Poor contact of the voltage circuit. In most cases, after the long-term operation of the terminal block of the circuit breaker, an oxide film or an oil film will be generated on the outer surface of the terminal block. When the voltage test clamp of the circuit resistance meter is clamped to such a terminal block, Poor contact occurs, and the voltage test wire clamp itself must also generate a certain contact resistance. When the contact resistance value reaches the internal resistance of the voltage sampling loop, it will have a serious impact on the test results.
(3) The voltage circuit is open or poorly contacted (visual contact resistance R1 is infinite when open). There is strong electromagnetic interference at the test site. If the bus is energized, the live bus passes the air-based capacitor and interferes with the tester. The voltage test line, due to the effect of interference, causes differential voltage across the voltage tester of the loop tester.
If the interference is large, the loop resistance tester will show a much larger value than the resistance value of the test object. At this time, if the tester has enough on-site test experience, it may be able to determine the abnormality of the test result, attract attention and eventually Get the correct test results. However, if the tester does not have enough experience, it may misjudge the switch circuit resistance value exceeds the standard. It may use the method of power failure maintenance to deal with defects and cause unnecessary losses to the power production.
If the intensity of the interference is not very large, the instrument display value is exactly within the acceptable resistance range of the circuit breaker. This situation is the same as the result of "(2) Poor voltage circuit contact", which also causes misjudgment.
2. Use the improved four-wire measurement method to solve the above problems.
YTC5501 loop resistance tester based on the analysis of the above issues, proposed improved four-wire measurement method, the use of the latest power technology, continuous output of large current for a long time, to overcome the shortcomings of pulsed power supply transient current, can effectively break through the switch touch Oxide film, to solve the current problems of traditional loop resistance tester.
Power test of the sample through a large current constant current source, collecting the value of voltage and current, and calculating the resistance value of the sample. At this time, the measured value must be the true resistance value of the sample, eliminating the generation of false data. There is a strong electric field interference at the site. Since the resistance in the voltage loop is very small, the voltage at the voltage test terminal is sensed as a common-mode voltage. This product will reduce this AC common-mode interference to a very low level through capacitive grounding, and will not test accuracy. Have an impact.
The above indicates that the automatic judgment and prompting function greatly improves the test accuracy and improves the production efficiency. It is a reliable way for field test personnel to quickly solve problems.