How to measure the DC winding resistance of a power transformer?

Two primary techniques are commonly used to measure transformer DC resistance: the bridge technique and the voltage drop technique.

The bridge method leverages either a single-arm or double-arm bridge for measurement. It allows for direct data reading and delivers high precision, making it a reliable choice for accurate results—though the associated equipment comes at a higher cost.

The voltage drop technique, by contrast, involves measuring the DC resistance of each phase winding individually before deriving the coil’s overall DC resistance through calculation. This method is typically adopted in settings where bridge equipment is unavailable. Its main drawback lies in the lengthy time required to obtain accurate values. Each phase winding can be modeled as a series circuit of resistance and inductance: when power is applied, the inductor current rises gradually from zero to a steady-state value, while the voltage across the inductor jumps from zero to the supply voltage before gradually declining to a stable level. This transient process is governed by the circuit’s time constant τ = L/R (where L denotes inductance and R represents resistance).

Transformers have cores with extremely high magnetic permeability, which significantly increases the inductance L. Meanwhile, the coil’s DC resistance R is very small—resulting in an unusually large time constant τ. Generally, the current reaches a steady state only after approximately T = 3 to 5 times the time constant, which can take tens of minutes or even longer to complete. This inefficiency conflicts with the fast-paced, high-productivity demands of modern work environments.

The prolonged measurement time of the voltage drop technique stems from the magnetic flux generated in the high-permeability core by the changing coil current, which amplifies inductance L. Reducing this magnetic flux would lower L, thereby shortening the current’s transient period (dependent on the time constant). This goal can be achieved by applying voltage to the transformer’s three-phase windings simultaneously while measuring each phase’s DC resistance.

When voltage is applied across all three phases, the current in each winding increases from zero. Per the right-hand screw rule, the magnetic fluxes generated by the three-phase currents orient in opposite directions within each core limb, effectively canceling each other out—resulting in a nearly zero resultant flux in the core. This significant reduction in inductance L minimizes the time constant τ, drastically shortening the transient period of current variation during measurement. A stable current value can thus be obtained in a short time, allowing for quick calculation of the winding’s DC resistance.

The innovative method of measuring transformer DC resistance via simultaneous voltage application across three-phase windings offers distinct efficiency advantages. In line with Lenz’s law, the magnetic fluxes induced by each phase current cancel each other within the core, leading to a near-zero net flux. This reduces the inductance L, lowering the circuit’s time constant and consequently cutting down the DC resistance measurement time—greatly boosting work efficiency.

During measurement, additional factors must be considered, such as the impact of temperature on winding resistance and the DC resistance unbalance rate, to ensure the accuracy and reliability of results.