Primary current
The current drawn from the supply network (I1).
Definition
The primary current is the current drawn by the transformer from the supply network when it delivers its rated power to the load. It is denoted I1 and is calculated from the apparent power (S) and the primary voltage (U1): I1 = S / (U1 × √3) for a three-phase transformer, or I1 = S / U1 for single-phase.
At switch-on, the primary current can reach 8 to 15 times the rated current for a few milliseconds: this is the inrush current, caused by transient saturation of the magnetic core. This phenomenon must be taken into account when sizing the upstream protections (fuses, type-D or time-delayed circuit breakers).
In steady state, the primary current depends on the load connected to the secondary. A transformer operating at 50 % load draws about 50 % of the rated primary current.
The ABL tip
If your protections trip when the transformer is energised for no apparent reason, the inrush current is often the cause. ABL Transfo can guide you towards soft-start devices or advise you on the choice of a suitable tripping curve.
