What is Percentage Impedance?
The impedance of a transformer is the total opposition offered an alternating current. This may be calculated for each winding. However, a rather simple test provides a practical method of measuring the equivalent impedance of a transformer without separating the impedance of the windings. When referring to impedance of a transformer, it is the equivalent impedance that is meant.
The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.
It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions
Calculation of Percentage Impedance
In order to determine equivalent impedance, one winding of the transformer is short circuited, and just enough voltage is applied to the other winding to create full load current to flow in the short circuited winding. This voltage is known as the impedance voltage.
Either winding may be short-circuited for this test, but it is usually more convenient to short circuit the low-voltage winding.
The transformer impedance value is given on the nameplate in percent. This means that the voltage drop due to the impedance is expressed as a percent of rated voltage.
For example, if a 2,400/240-volt transformer has a measured impedance voltage of 72 volts on the high voltage windings, its impedance (Z), expressed as a percent, is:
Z% = (Impedance Voltage / Rated Voltage) x 100
percent Z = (72/2400)*100 = 3 percent
This means there would be a 72-volt drop in the high-voltage winding at full load due to losses in the windings and core. Only 1 or 2% of the losses are due to the core; about 98% is due to the winding impedance.
If the transformer were not operating at full load, the voltage drop would be less. If an actual impedance value in ohms is needed for the high-voltage side:
Z = V/I
where V is the voltage drop or, in this case, 72 volts; and I is the full load current in the primary winding.
If the full load current is 10 amps:
Z = 72V/10A = 7.2 Ohms
Of course, one must remember that impedance is made up of both resistive and reactive components.