MAGI (magnetising current) testing
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 B-H 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.
FIGURE 5: Magnetising current equivalent circuit.
- Lm = the magnetisation inductance.
- Im = the magnetising current.
- Ie = the excitation current.
- Ic = the core loss component of the excitation current.
- Rc = the core loss resistance.
Figure 6 shows the parameters required for MAGI on the primary winding of the sample transformer.
The voltage and frequency levels have been selected for U.S line voltage and frequency (RMS) 110V @ 60Hz with the maximum current draw set at 50mA.
Core losses are caused by the magnetising of the core and are always present. The way they are measured is by running full primary voltage with no load on the transformer.