With the increase in load on the transformer, there is a change in its terminal voltage. The voltage falls if the load power factor is lagging. It increases if power is leading. The change in secondary terminal voltage from full load to no load, expressed as a percentage of full load voltage is called the percentage voltage regulation of the transformer
% Regulation = [E- V]/V x 100.
• Connect the circuit diagram as shown in fig (a)
• Apply full load and note down the readings of wattmeter, voltmeter and ammeter.
• Decrease the load and note down the readings.
• Calculate efficiency and regulation.
η = V2 I2 / Wi * 100
% Reg = [ (E – V) / V ]* 100
By calculating the voltage regulation the figure of merit which determines the voltage characteristics of a transformer can be determined. Also the transformer efficiency can’t be determined with high precision since the losses are of order of only 1 to 4%. The best and accurate method of determining the efficiency of a transformer would be to compute losses from open circuit and short circuit test and then determine the efficiency.