Magnetising current
The no-load current that establishes the flux in the core.
Definition
The magnetising current is the current a transformer draws at no load, i.e. with no load connected to the secondary. Its role is to establish and maintain the magnetic flux in the core. It is generally small — a few percent of the rated current.
This current is largely reactive (shifted 90° from the voltage) because it serves to magnetise the core, but it also contains an active component that covers the iron losses. Its value depends on the quality of the magnetic circuit: a core with low reluctance and high permeability minimises the magnetising current.
It is not sinusoidal: the non-linearity of the core gives it a waveform rich in harmonics.
The ABL tip
An abnormally high no-load current often signals a poorly assembled core or degraded laminations. If you observe this symptom on an existing transformer, our teams can diagnose and redesign the unit.
