Inrush Current
A high transient current when a transformer is first energized, caused by core saturation, reaching 10-30x rated.
Inrush current flows when a transformer is first energized, caused by temporary core saturation. Flux may reach 2x normal peak, driving core deep into saturation.
During saturation, magnetizing inductance drops dramatically. Current: 8-15x rated for distribution, up to 30x for power transformers. Contains DC offset and 2nd harmonic content, decaying over cycles to seconds.
Can cause nuisance tripping, voltage dips, mechanical stress. Modern relays use 2nd harmonic restraint to distinguish from fault current.
QDTB provides energization guidance and controlled switching options for large installations.
Related Products
Related Terms
Magnetizing Current
The reactive component of no-load current establishing magnetic flux in the transformer core.
Excitation Current
Current drawn by transformer primary when energized with no load, consisting of magnetizing and core-loss components.
Fault Current
The extremely high current during a short circuit that transformers must withstand mechanically.
Magnetic Core
The laminated silicon steel assembly providing a low-reluctance path for magnetic flux, backbone of transformer operation.
More Terms Starting with “I”
Impedance (%Z)
Total opposition to current flow in a transformer as a percentage, determining fault current and voltage regulation.
Insulation Class (A/E/B/F/H)
Standardized temperature ratings for transformer insulation materials defining maximum operating temperatures.
IEC 60076
The IEC standard series defining requirements for power transformers, the global reference standard.
IEEE C57
The IEEE standard series for transformers, the primary reference for the North American market.
More in “Electrical Fundamentals”
Apparent Power (kVA)
The total power in an AC circuit combining real (active) and reactive power, measured in kilovolt-amperes (kVA).
Eddy Current
Circulating currents induced in conductive materials by changing magnetic fields, causing energy losses in transformers.
EMF (Electromotive Force)
The voltage generated by electromagnetic induction in a transformer winding, proportional to flux change and turns.
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