1. Measurement Method of Transformer DC Resistance Tester
Measuring DC resistance is an important item in transformer testing. Through measurement, defects such as poor contact in the conductive circuit, poor welding, coil faults, and wiring errors can be detected. In actual measurements of medium and small transformers, the DC bridge method is mostly used. When the resistance value of the tested coil is above 1 ohm, a single-arm bridge is generally used; for values below 1 ohm, a double-arm bridge is used. When wiring with a double-arm bridge, the potential terminals of the bridge should be close to the measured resistance, and the current terminals should be connected above the potential terminals. Before measurement, estimate the resistance value of the tested coil, set the bridge multiplier knob to an appropriate position, short-circuit and ground the non-tested coils, then turn on the power switch to charge. After full charging, press the galvanometer switch, quickly adjust the measuring arm to move the galvanometer pointer toward the zero line in the middle of the galvanometer scale, fine-tune until the pointer stops steadily at zero, and record the resistance value. At this time, the tested coil resistance = multiplier × measuring arm resistance. After measurement, first release the galvanometer button, then turn off the power switch.
2. Precautions for Transformer DC Resistance Tester
During the measurement process, in addition to strictly complying with electrical safety regulations and equipment testing regulations, special attention should be paid to:
1) Perform measurement when the coil temperature is stable, requiring that the temperature difference between the upper and lower parts of the transformer oil tank does not exceed 3°C;
2) Due to the inductance of transformer coils, the charging current during measurement is not stable. Be sure to count after the current is stable, and if necessary, take measures to shorten the charging time.
3) Minimize the contact resistance of wires in the test circuit. The tap changers of transformers in operation are often affected by contaminants such as oil films, causing poor contact. Generally, switch several times before measuring to avoid misjudgment.
4) When measuring the low-voltage side of the transformer, all personnel should maintain an appropriate safe distance from the high-voltage side.
5) The instrument should avoid strong vibration, direct sunlight, and magnetic field effects during transportation, storage, and operation. When storing and keeping the instrument, pay attention to ambient temperature and humidity. It should be placed in a dry, ventilated place without corrosive gases, and should not be damp, rained on, or exposed to sunlight. Handle with care during use, and do not drop it.
