04,28,2026 80 views
Transformer DC resistance testermeasurement method
1、 Current methods for measuring DC resistance and existing problems
At present, there are two methods for measuring DC resistance: bridge method and voltage drop method. The bridge method uses a single arm bridge or a double arm bridge for measurement, which can directly read data with high accuracy. The voltage drop method measures the DC resistance of each phase winding, and then uses the measurement data to calculate the DC resistance of the coil. In places where there is no bridge, this measurement method is generally used. The main disadvantage of this method is that it takes a long time to measure accurate values.
Due to the high magnetic permeability of transformer cores, it takes several tens of minutes or even longer to accurately measure the DC resistance. This undoubtedly does not conform to today's fast-paced and efficient work style.
2、 Measurement of DC resistance using three-phase winding simultaneous pressurization method
It takes a long time to obtain an accurate value for measuring DC resistance using the voltage drop method, mainly due to the magnetic flux generated in the high permeability iron core during the change of the current passing through the coil, resulting in an increase in L. If the magnetic flux is reduced, the L value is also lowered, and the time for the current to change is reduced. Simultaneously applying voltage to the three-phase windings of the transformer and measuring the DC resistance of each phase can achieve this goal. When voltage is applied to the three-phase windings simultaneously, the current flowing into each phase winding increases from zero. According to the right-hand screw rule, the magnetic flux direction generated by the three-phase current in each iron core column is different, and their effects cancel each other out, resulting in the synthesized magnetic flux in the iron core being approximately zero. This greatly reduces the inductance value L, thus reducing the time constant τ to its lowest value. The transition process of current change during testing is greatly shortened, and a stable current value can be obtained in a short period of time, which can then be used to calculate the DC resistance value of the winding.
IIITransformer DC resistance testerConclusion
Simultaneously applying voltage to the three-phase winding to measure the DC resistance of a transformer is based on Lenz's law, which causes the magnetic fluxes generated by the currents of each phase to cancel each other out in the iron core, resulting in zero synthesized magnetic flux. This reduces the inductance L value and decreases the time constant of the circuit, thereby reducing the time required to measure DC resistance and improving work efficiency. When measuring, factors such as the influence of temperature on the size of winding resistance and the imbalance rate of DC resistance should also be considered.
