SEPARATION OF CORE (OR IRON) LOSSES IN A TRANSFORMER

Hysteresis loss and eddy current loss are the components of the iron losses. For the applied fl ux density B_max to the core, we have

Hysteresis loss = Af

and Eddy current loss = Bf²

The no load loss can be expressed as

W_c=A_f+B_f² (1.36)

where A and B are constants.

Therefore,

Figure 1.38 shows the graph, which is a straight line when and f are plotted along the y-axis and x-axis, respectively. The intercept on the y-axis gives the value of A, whereas the slope of the line gives the value of B. Now the hysteresis and eddy current loss can be determined at any desired frequency.

The experimental circuit arrangement for determining and f is shown in Figure 1.39.

In Figure 1.39, a variable frequency alternator supplies to the transformer under the test, which is driven by DC shunt motor whose speed can be varied over a wide range. The switches S₁ and S₂ are opened and the alternator is started with the help of the DC shunt motor. The speed is adjusted to the

Figure 1.38 Plot of and f

Figure 1.39 Experimental Circuit for Determining and f

value of the required frequency. The excitation of the field coil (X-XX) is varied until the voltmeter on the secondary side of the transformer achieves the rated value. If E₂ is the transformer emf on the secondary, we have

E₂=4.44Φ_mfN

i.e.

For constant , the flux density in the transformer remains constant. To achieve this, the frequency of the alternator emf is varied so that remains constant. The necessary f can be adjusted to vary E₂ so that is kept constant. For different values of frequencies above and below the rated value, the reading of wattmeter (W) is noted. The graph and f is drawn to get the constants A and B. After getting the value of A and B, the hysteresis loss and eddy current loss is obtained.