Measuring DC resistance is an important item in transformer testing. Through measurement, it is possible to check whether the conductive circuit of the equipment has defects such as poor contact, poor welding, coil failure, and wiring errors. In the actual measurement of medium and small transformers, most of the DC bridge method is used. When the resistance value of the tested coil is more than 1 ohm, it is generally measured with a single-arm bridge, and when the resistance value of the tested coil is less than 1 ohm, it is measured with a double-arm bridge. When using double-arm bridge wiring, the potential pile head of the bridge should be close to the resistance to be measured, and the current pile head should be connected to the potential pile head. Before measuring, you should first estimate the resistance value of the coil under test, place the bridge magnification button in the appropriate position, short-circuit and ground the non-test coil, then turn on the power switch to charge, and press the galvanometer switch after it is fully charged. Quickly adjust the measuring arm to move the pointer of the galvanometer to the zero line in the middle of the scale of the galvanometer, and make fine adjustments. When the pointer stops at the zero position, the resistance value is recorded. At this time, the resistance value of the measured coil = the multiplying factor. ×Measurement arm resistance value. After the measurement is completed, first open the galvanometer button, and then release the power switch.

The inductance of the transformer is relatively large, so the current stabilization time in the DC circuit is relatively long, especially when measuring the DC resistance of the low-voltage delta connection winding of the large-scale three-phase five-leg iron core transformer, it not only has the role of inductance in the circuit and magnetic circuit, but also There are transition processes in each branch of the circuit and each branch of the magnetic circuit, which will cause the test current to reach a stable value for a very long time.

To shorten the time of measuring DC resistance, use the following method: winding series method

When measuring the DC resistance of the low-voltage delta-connected winding, the high-voltage winding and the low-voltage winding can be connected in series to keep the current in the two windings in the same direction of excitation to the core. The number of ampere-turns for excitation is greatly increased to saturate the core to reduce the inductance of the core. , This can shorten the test time of DC resistance.