Transformer winding resistance must be interpreted at a known conductor temperature. Copper and aluminum resistance increase as temperature rises, so two electrically identical windings can produce different readings when measured in different environments. Correcting results to a reference temperature makes design calculations, production limits, supplier comparisons, and heat-run data meaningful.
Why resistance changes with temperature
Metal resistivity has a positive temperature coefficient over the normal transformer operating range. For copper, a commonly used correction relationship is based on a material constant near 234.5 degrees Celsius; aluminum uses a different constant. The governing standard, customer specification, and conductor material should define the exact method.
The correction can be expressed as R2 = R1 脳 (K + T2) / (K + T1), where R1 is resistance measured at temperature T1, R2 is the equivalent resistance at reference temperature T2, and K is the conductor constant. Temperatures must use the same Celsius scale.
Conductor temperature may not equal room temperature
A transformer moved from storage, handled after soldering, or tested soon after a heat run may not be in equilibrium with the room. Large windings can take hours to stabilize. Directly attached temperature sensors, adequate soak time, and documented handling reduce uncertainty.
Fine windings can also be affected by measuring current. A four-wire milliohm test minimizes lead error, but the test current and duration should not warm the winding enough to shift the result.
Use four-wire measurement for low resistance
High-current secondary windings may have resistance comparable to test leads and contact interfaces. Kelvin connections separate current and voltage paths so lead resistance does not dominate. Clean terminals, repeatable contact pressure, stable current, and instrument zeroing remain important.
Connect resistance to copper loss
DC copper loss is current squared times resistance at operating temperature. Using cold resistance underestimates hot loss. AC winding loss from skin and proximity effects is additional and cannot be obtained from DC resistance alone, but corrected DCR remains an essential baseline.
Resistance can estimate average winding temperature
After a heat run, the change in winding resistance can estimate average conductor temperature. Measurement should begin quickly after shutdown, with multiple readings extrapolated back to the power-off time when required. This average is not necessarily the internal hot-spot temperature.
Production control checklist
- Conductor material and reference temperature
- Ambient and winding stabilization method
- Two-wire or four-wire connection and fixture definition
- Test current, settling time, and instrument accuracy
- Raw resistance and temperature-corrected resistance
- Terminal contact and lead compensation
- Trend limits that distinguish temperature from real process drift
BaoHui Tech recommends stating winding-resistance limits and their reference temperature on the controlled transformer specification. Whether evaluating a power transformer or high frequency transformer, this prevents the customer and transformer manufacturer from comparing uncorrected readings taken under different conditions.
Frequently asked questions
Can room temperature be used as winding temperature?
Only after the transformer has reached thermal equilibrium and no recent process or test has warmed it.
Why can resistance pass cold but fail after correction?
The raw value may look low because the winding is colder than the reference. Correction reveals the equivalent value at the specified temperature.
Does corrected resistance prove the winding has the correct turns?
No. Resistance helps detect conductor length, size, joint, or material changes, but turns ratio and inductance tests are still required.