Therefore, the reading of the wattmeter (W) shows the core or iron losses only. Also, the no-load current (I 0) is very small (about 2% to 5% of the rated primary current), hence, the I 2R loss in the primary winding at no-load can be neglected. I 2 = 0, thus there is no I 2R loss in the Since the secondary winding is open circuited i.e. The total power loss in the transformer at no-load is due to the core loss and a very small I 2R loss in the resistance of the Therefore, the reading of the ammeter (A) shows the no-load current (I 0) of the transformer. Thus, the reading of the voltmeter (V) shows the rated voltage V 1 of the primary winding.Īs the transformer is on no-load, i.e., the secondary winding is open circuited so a very smallĬurrent I 0 (called no-load current), flows in the primary winding. Now, in the connection diagram, a voltmeter (V), an ammeter (A) and a wattmeter (W) areĬonnected in the primary side (which is low voltage (l v) winding of the given transformer) and the low-voltage winding is supplied at rated primary voltage and the frequency of the transformer. The open-circuit test is conducted to determine the core or iron losses and the no-load circuit parameters R 0 and X m of the transformer. In this test the high voltage side of the transformer is left open, i.e., the open circuit test is to be performer on the low-voltage side of the transformer.
The circuit arrangement for the open-circuit test of a transformer is shown in the figure. The principle of advantage of these tests is the power consumption is very small as compared to the full-load output of the transformer. The open-circuit and short-circuit tests give more accurate results than those are obtained by performing the measurements on a fully loaded transformer. The open-circuit test and the short-circuit test being performed on a transformer to determine the circuit parameters, efficiency and the voltage regulation without actual loaded of the transformer.
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