How to measure the DC winding resistance of a power transformer?
1 Method and existing questions of measuring DC resistance
There are two methods of measuring DC resistance: the bridge method and the voltage drop method. The bridge method is to measure with a single-arm bridge or a double-arm bridge. This method can directly read the data and has high accuracy, but the equipment is more expensive. The voltage drop method is to measure the DC resistance of each phase winding, and then use the measurement data to calculate the DC resistance of the coil. This measurement method is generally used in places where there is no bridge. The main disadvantage of this method is that it takes a long time to measure the accurate value. Since 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, and finally reaches a stable value, and the voltage across the inductor suddenly increases from zero to the power supply voltage, and then gradually To drop to the steady state value, a transition process is required, and the length of the process depends on the time constant t=L/R of the circuit.
Because the magnetic permeability of the transformer core is very high, the L value is greatly increased, and the DC resistance value of the coil is very small, so the time constant t value is very large. Generally speaking, after about time T = 3 to 5 times the time constant, the current can reach the steady-state value, that is, it takes tens of minutes or even longer to measure the accurate value of the DC resistance. This is certainly not in line with today;s fast-paced, high-efficiency work style.
2. Measurement of DC resistance by pressing three-phase windings together
It takes a long time to measure the DC resistance with the voltage drop method to obtain an accurate value. The main reason is that the current flowing in the coil generates a magnetic flux in the iron core with high magnetic permeability during the change process, resulting in an increase in L. If the magnetic flux is reduced, the L value is also reduced, and the current change time (depending on the time constant) is reduced. This purpose can be achieved by 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 together, the current flowing into each phase winding increases from zero. It can be seen from the right-hand spiral rule that the three-phase currents generate different magnetic flux directions in each core column, and their effects are mutually exclusive. The result is that the composition flux in the core is approximately zero. This greatly reduces the inductance value L, so the time constant τ is also minimized, the transition process of the current change during inspection 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. .
3 Conclusion
The three-phase windings are applied with voltage to measure the DC resistance of the transformer. According to Lenzs law, the magnetic fluxes generated by the currents of each phase cancel each other out in the iron core, and the magnetic flux is zero, and then the inductance L value is reduced to make the circuit time The constant is reduced, that is, the time for measuring the DC resistance is reduced, and the work efficiency is improved. When measuring, the factors that the size of the winding resistance is affected by temperature and the unbalance rate of the DC resistance should also be considered.
