6.2 Magnetizing Current (MAGI)
A magnetising current test is usually performed on transformers using laminate cores, which are designed to operate over the full extent of the B-H curve. The BH curve shows the characteristics of a magnetic material, in terms of magnetising force (H) and resulting flux density (B). Magnetising current is the current needed to establish core flux, which results in the combination of current required to magnetise the core and current required to supply the losses in the core comprising of hysteresis and eddy currents. Hysteresis is the energy used up by changing the magnetic state of the core during each cycle and eddy currents are currents induced in the core by time varying fluxes.
Magnetizing current is the term used to denote the total current that flows into the primary of a transformer when the transformer is energized at a specific voltage and frequency, with the secondaries open circuited. Although known as magnetizing current it is actually the combination of the current required to magnetize the core (I1) and the current required to supply the losses in the core (I2).
The magnetizing current test is used to confirm that a transformer has been assembled properly, with the appropriate number of turns, the right grade of magnetic material for the core, and the correct air gap if required. The magnetizing current test is the preferred test for line frequency transformers constructed using laminated iron cores which are designed to operate over the full extent of the B-H curve