Residual magnetism can also be introduced during routine electrical testing. One common example is winding resistance testing. Because winding resistance testing uses direct current, the transformer core can be driven into saturation. When the test current is removed, the winding may be discharged, but the core may still retain magnetism.

This remaining magnetism matters because transformers are designed to operate with alternating magnetic flux. When a transformer is re-energized with residual magnetism already present, the normal flux waveform can be offset. If the polarity of the applied voltage reinforces the existing residual magnetism, the transformer core may saturate heavily during the first few cycles.

The result is transformer inrush current.

Inrush current is a high transient current that occurs during transformer energization. It can be several times the normal current and may last for a few cycles. In some situations, it can stress transformer windings, affect power quality, or cause protective relays to operate unnecessarily.

Remnant magnetism is not visible from the outside, and it may not be obvious from basic inspection. However, its effects may appear during energization, excitation-current testing, and sweep-frequency response analysis.

Tech-Tip Takeaway:
Residual magnetism is magnetic flux left in the transformer core after de-energization, fault events, or DC testing. If not removed, it can affect energization and diagnostic testing.