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 the 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.
Z = V/I
If the full load current is 10 amps:
Z = 72V/10A = 7.2 Ohms