Method of measuring transformer DC resistance

Measurement method 1. Current-voltage method The principle is to pass an appropriate DC current in the winding under test, then measure the current in the winding and the voltage drop across the winding, and then calculate the DC resistance of the winding according to Ohm's law. When measuring, the instrument used should not be less than 0.5, the ammeter should be selected with a smaller internal resistance, the voltmeter should be selected with a higher internal resistance, and the lead should have a measurement method 1. The principle of the current-voltage method is in the measured winding , Pass an appropriate DC current, measure the current in the winding and the voltage drop across the winding, and then calculate the DC resistance of the winding according to Ohm's law. When measuring, the instrument used should be no less than 0.5, the ammeter should be selected with a smaller internal resistance, the voltmeter should be selected with a higher internal resistance, and the lead should have sufficient cross section. When measuring windings with large inductance, sufficient charging time is also required. The current through the winding should be limited to less than 20% of the rated current of the winding. The main disadvantage of this method is that it takes a long time to measure an accurate value. Because each phase winding can be equivalent to a series circuit of resistance and inductance, after the power is turned on, the current in the inductor gradually increases from zero to the power supply voltage, and then gradually decreases to the steady-state value. A transition process is required, and the length of the transition time depends on The time constant of the circuit tu003dL/R. Because the magnetic permeability of the transformer core is very high, the value of L is greatly increased, and the value of the DC resistance of the coil is very small, so the value of the time constant t is very large. Generally speaking, the internal resistance of the ammeter and voltmeter has a certain influence on the measurement results, and the current can reach the steady-state value after a time of about Tu003d3 to 5 times the time constant, that is, it takes tens of minutes or even longer to measure The accurate value of the DC resistance. 2. Balanced bridge method The balanced bridge method uses the principle of bridge balance to measure DC resistance. The commonly used balanced bridge method has two types: single-arm bridge or double-arm bridge. This method can directly read the data with high accuracy. In the actual measurement of medium and small transformers, the DC bridge method is mostly used. When the resistance value of the tested coil is above 1Ω, it is generally measured by a single-arm bridge, 1Ω The following are 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, first estimate the resistance value of the coil under test, set the bridge magnification knob to the proper position, short-circuit and ground the non-measured coil, then turn on the power switch to charge, and press the galvanometer switch after it is fully charged. 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 u003d the number of magnifications × Measure the arm resistance value. After the measurement is completed, first release the galvanometer button, and then release the power switch. 3. Three-phase winding simultaneous pressurization method Three-phase winding simultaneously adds voltage to measure the DC resistance of the transformer. According to Lenz's law, the magnetic flux generated by each phase current cancels each other in the iron core, and the resultant magnetic flux is zero. Reducing the value of inductance L reduces the time constant of the circuit, that is, it reduces the time for measuring DC resistance and improves work efficiency. In the measurement, the factors such as the temperature-affected factor of the winding resistance and the unbalance rate of the DC resistance should also be considered. It takes a long time to measure the DC resistance with the voltage drop method to obtain an accurate value, mainly because the current flowing in the coil produces a magnetic flux in the high permeability iron core during the change process, which causes L to increase. If the magnetic flux is reduced, the value of L is also reduced, and the time for the current change (depending on the time constant) is reduced. This can be achieved by simultaneously applying voltage to the three-phase windings of the transformer and measuring the DC resistance of each phase at the same time. When voltage is applied to the three-phase windings at the same time, the current in each phase winding increases from zero. According to the right-hand spiral rule, the three-phase currents produce different magnetic flux directions in each iron core leg, and their effects are mutually exclusive. Cancellation, the result is that the resultant magnetic flux in the iron core is approximately zero. This greatly reduces the inductance value L, so the time constant t is also minimized, and the transient process of current changes during the test is greatly shortened, and a stable current value can be obtained in a short time, and then the DC resistance value of the winding can be obtained. . Purpose of measurement 1. Check whether there is a short circuit, open circuit or wrong wiring in the conductive circuit; 2. Check whether the welding point of the winding wire, the connection between the lead and the bushing is good, whether the tap switch has bad contact, etc. 3. It can also check whether the wire specification used in the winding meets the design requirements.