When measuring DC resistance of inductive loads such as transformer windings, motor stators, and switch contacts, inconsistent readings at the same measurement point and large deviations from factory values often occur. Field engineers tend to suspect instrument accuracy first, but most deviations do not come from the instrument itself; rather, they result from the combined effects of the constant current source establishment process and the lead connection method.
The equivalent circuit of an inductive load contains a large inductive component, and the test current requires time to rise from zero to the set value. If sampling begins before the current stabilizes, the voltage signal contains inductive back-EMF, and the calculated resistance value will deviate from the true value. Test equipment with a central control unit applies a constant current to the load under test through closed-loop regulation, collects voltage and current signals after the current stabilizes, and obtains the resistance value after processing. Confirming the current stability indication before testing and avoiding reading data during the current ramp-up phase are basic operations to reduce such deviations.
Lead resistance and contact resistance are another common source of error. When measuring low resistances at the 1 mΩ level, ordinary two-wire connections include lead resistance in the measurement. The four-wire connection separates the current loop from the voltage sampling loop, with voltage terminals close to the object under test, effectively eliminating the influence of leads. On site, ensure that voltage clips and current clips do not overlap, that contact surfaces are clean, and that clamping positions avoid oxide layers and paint films. For non-removable winding measurement points, dedicated test clips or probes can be used to improve contact consistency.
The influence of environmental conditions on readings cannot be ignored either. The marked operating temperature range is -20 to 40°C, with corresponding limits on ambient humidity; outside this range, the bias current of electronic components and contact resistance will change. Field measurements should record ambient temperature and, when necessary, convert resistance values according to the conductor temperature coefficient for comparison with historical data. Test leads should avoid being routed parallel to power lines to reduce the impact of electromagnetic interference on voltage sampling.
From a usage perspective, DC resistance measurement of inductive loads relies more on standardized operation than on a single parameter. Establishing fixed wiring positions, waiting for current stabilization before taking readings, and regularly checking test lead continuity are feasible practices for controlling repeat-measurement deviations. When readings are abnormal, prioritize checking connections and stabilization time before evaluating instrument status, which can shorten the on-site troubleshooting path.
