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OpenCircuit and ShortCircuit Tests, Exciting Admittance, and Equivalent Impedance
The constants of the transformer for the approximate equivalent circuit can be determined from the opencircuit and shortcircuit tests. The opencircuit test is made to obtain the values of the excitingadmittance, conductance, and susceptance. In the case of power transformers or constantvoltage transformers operating at one specified frequency, the opencircuit test consists in the application of rated voltage at rated frequency usually to the lowvoltage winding with the highvoltage winding open circuited. Measurements are made by means of indicating instruments (voltmeter, ammeter, and wattmeter) of voltage, current, and power. Actually, the opencircuit test can be made by applying rated voltage at rated frequency to the highvoltage winding with the lowvoltage winding open circuited. However, it is usually more convenient to work on the lowvoltage side because the lower voltage is easier to handle.
The quantities involved in most communication transformers are generally too small for wattmeter and ammeter measurements. Methods involving the use of ac bridges or other suitable devices are employed. In Fig. 616 let I_{exc} = the exciting current as read by the ammeter, A^{(1)} Then the exciting admittance is
and the exciting conductance is
from which the exciting susceptance is found to be
The shortcircuit test yields results from which the equivalent impedance, equivalent resistance, and equivalent leakage reactance of the transformer can be evaluated. In Fig. 614(b) the equivalent impedance of the transformer referred to the primary is
in which the equivalent resistance referred to the primary of the transformer is
and the equivalent leakage reactance referred to the primary is
The equivalent impedance referred to the secondary is
from which
and
It is the general practice, in the shortcircuit test, to short circuit the lowvoltage winding and to apply voltage, at rated frequency, such that rated current flows in the transformer. Measurements are made of input current, power, and voltage, using indicating instruments, i.e., ammeter, wattmeter, and voltmeter as shown in Fig. 617. Again, in the case of communication transformers, an ac bridge method or an arrangement adapted for measuring the smaller quantities associated with such transformers is used.
The shortcircuit test could also be made by short circuiting the highvoltage side and making the measurements on the lowvoltage side. This, however, is more inconvenient since the applied voltage is low and the measured current high. These quantities are handled and measured more readily on the highvoltage side. For the shortcircuit test let V_{sc} = applied voltage as read by the voltmeter, V (in Fig. 617) The voltage V_{sc} is low in comparison with the rated voltage. It can be shown that, in the case of the transformer of Example 62, the shortcircuit voltage is less than 3 percent of rated value, On that basis, the exciting current can be neglected and the shortcircuit impedance Z_{sc} assumed to equal the equivalent series impedance of the transformer. Hence, the equivalent series impedance is
and the equivalent series resistance is practically equal to the shortcircuit resistance
Also, the equivalent leakage reactance of the transformer is


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