Direct-current tests such as winding resistance measurement can leave residual magnetism in a transformer core, because the DC current saturates part of the magnetic circuit and the core retains flux after the current is removed. Residual magnetism matters because it distorts later electrical tests and can produce high inrush current when the transformer is re-energized.
Demagnetisation after DC testing removes that flux using a controlled alternating or reversing-DC procedure, and verification confirms that the core has returned to a neutral state before the next test or before service.
Why DC Tests Magnetise the Core
When a DC current flows through a transformer winding for a resistance measurement, it establishes a magnetic flux in the core. The core material is ferromagnetic: it retains a portion of that flux when the current is removed, just as a permanent magnet retains its field. The amount of retained flux depends on the test current, the duration of the test, the transformer design and the point in the magnetic cycle at which the current was interrupted. A large power transformer tested at high current can retain significant flux, and the effect is strongest when the test was applied in one direction only.
Not every DC measurement leaves the same level of residual magnetism. Short, low-current tests on small cores may leave little measurable flux, while high-current measurements on large three-phase cores can leave enough to change the behaviour of the transformer noticeably. Because the level cannot be predicted reliably from the test settings alone, the disciplined approach is to treat the core as possibly magnetised after any significant DC test and to verify rather than assume.
What Residual Magnetism Does to Later Tests and Energisation
Residual flux changes the behaviour of tests that depend on the magnetic circuit. Excitation current measurements taken on a magnetised core can show elevated or asymmetric readings that are not representative of the transformer condition, and an insulation test that follows a DC resistance test may be affected by the core state through the same magnetic path. If a diagnostic test produces an unexpected pattern shortly after DC testing, residual magnetism should be one of the first explanations considered before the transformer itself is suspected.
At energisation, residual flux combines with the natural flux of the first cycle. Depending on the phase of the voltage at the instant of closing, the core can saturate on the first half-cycle, producing an inrush current that is much higher than normal. High inrush stresses the windings, the protection system and the connected network, and repeated events can age the transformer. Demagnetisation before re-energization removes that contribution so that the inrush behaviour returns to the design expectation.
Demagnetisation Methods: Decreasing AC and Reversing DC
Two families of demagnetisation methods are in common use. The decreasing-AC method applies an alternating voltage to a winding and progressively reduces its amplitude, so that each cycle drives the core around a smaller hysteresis loop until the flux converges toward zero. The reversing-DC method applies a DC current whose direction reverses while its amplitude is progressively reduced, following a similar converging path. Some instruments automate the process with a programmed sequence of decreasing amplitude and alternating polarity, sometimes described by the acronym DAAPC for the reversing-DC approach.
The choice of method depends on the transformer, the winding accessibility and the instrument available. The procedure should follow the manufacturer’s guidance for the transformer and the instructions of the demagnetisation instrument, because the voltage or current levels and the number of steps are design-specific. A demagnetisation procedure performed at the wrong level or with too few steps can leave the core more magnetised than before, which is why the process should always end with verification.
How to Verify the Core Was Demagnetised
Verification confirms that the procedure achieved its purpose. The practical check is behavioural: repeat the test that is sensitive to residual flux, such as an excitation current measurement, and confirm that the readings return to the values recorded before the DC test. Some instruments provide a direct indication of the residual state or measure the magnetising behaviour during the final steps of the demagnetisation sequence. Whichever method is used, the verification result belongs in the test record together with the demagnetisation settings.
If the verification shows that flux remains, repeat the demagnetisation rather than accepting the state. The cause may be an insufficient number of steps, an amplitude that was too low to reach the saturation level of the core, or a connection problem in the demagnetisation circuit. Recording the first attempt and the repeat allows the procedure to be reviewed and improved for the next unit.
Safety and Sequence Notes
Demagnetisation applies voltage or current to the transformer, so the same safety discipline applies as for any test: isolate the transformer, verify zero energy, control the test area, and discharge and verify before connections are changed. The demagnetisation winding must be connected with the correct polarity and the other windings handled according to the procedure, because an open or wrongly connected circuit can change the flux path and defeat the purpose of the process.
Sequence matters in the wider test programme. Where several DC tests are planned, consider ordering them so that demagnetisation is performed once at the end rather than after every measurement, unless a sensitive test between them demands a neutral core. When an excitation or other magnetic-circuit measurement is planned, perform it before the high-current DC tests or after demagnetisation, and record which sequence was used so that the results remain comparable with earlier tests.
When Demagnetisation Is Essential
| Situation | Why demagnetisation matters |
|---|---|
| After high-current winding resistance testing | Large DC current can leave significant flux in the core. |
| Before excitation or magnetic-balance testing | Residual flux distorts the magnetising readings. |
| Before re-energization after DC testing | Residual flux increases the risk of high inrush current. |
| After repair work that involved DC excitation | Returns the core to a known neutral state before recommissioning. |
| When a diagnostic pattern appears after DC testing | Rules out the core state as the cause before deeper investigation. |
When in doubt, demagnetise and verify. The procedure costs little compared with the cost of misinterpreting a distorted test result or energizing a transformer with an unknown core state.
If the instrument does not offer an automated demagnetisation sequence, the procedure can still be performed manually with care. Apply a current or voltage to the chosen winding, reverse the direction or polarity, and progressively reduce the amplitude through a series of steps until the final steps are near zero. The key is that the amplitude reduction must be gradual enough for the core to follow the converging hysteresis loops; jumping from a large value directly to zero can leave the core with a different residual state. Record the starting level, the number of steps and the final level so that the procedure can be repeated identically next time. Because a manual sequence depends on the operator’s judgement, verification of the core state afterward is even more important than with an automated instrument. When the transformer is large or the residual state affects an important decision, involve the engineer responsible for the unit rather than improvising a procedure on site.
Residual magnetism can also appear as a low-frequency deviation in a frequency-response measurement, because the flux state of the core influences the low-frequency response. If an SFRA trace taken shortly after DC testing shows a change concentrated in the low-frequency region, repeat the measurement after demagnetisation before interpreting the change as core movement. The same rule applies to excitation current and magnetic-balance checks: schedule them before the high-current DC tests or after demagnetisation, and note the sequence in the record so the comparison with earlier tests remains valid.
Frequently Asked Questions
Which tests leave residual magnetism in the core?
Tests that apply DC current through a winding, most commonly winding resistance measurement, can leave residual flux in the core. The effect is strongest with high current on large three-phase cores, but the level cannot be predicted reliably from the test settings, so verification is recommended after significant DC testing.
What effect does residual magnetism have on energisation?
Residual flux can add to the natural flux of the first cycle at energisation, saturating the core and producing higher-than-normal inrush current. Demagnetisation before re-energization removes that contribution and returns the inrush behaviour toward the design expectation.
What does DAAPC mean?
DAAPC refers to a demagnetisation approach that applies a DC current with alternating polarity and decreasing amplitude, driving the core through progressively smaller hysteresis loops until the flux converges toward zero. The acronym describes the method family; the exact procedure should follow the instrument and transformer manufacturer guidance.
For the instruments used in winding and maintenance testing, see the winding test equipment range and the maintenance test equipment range.