Direct Loading Test of Single Phase Transformer

  • The circuit diagram for the direct loading test is shown in the figure. 
  • As the transformer is loaded, this test is known as direct test. The rated voltage is given to transformer primary side with the help of single phase auto-transformer and secondary side is loaded. 
  • The load is increased step-by-step till ammeter A2 shows the full load secondary current. Note down the reading of wattmeter, voltmeters and and ammeters.

Primary Voltage

Primary Current

Input Power

Secondary Voltage

Load Current

Output Power















% Efficiency = [ Output Power / Input Power ] × 100%

                      = [ W0 WI ] × 100%

Voltage Regulation

Let us consider that no load secondary voltage is 0V2 and full load secondary voltage is V2

% Voltage Regulation = { NLSV – FLSV / NLSV } × 100 %

                                    = { 0V2  V2 / 0V2 }× 100 %


NLSV = No load secondary voltage

FLSV = Full load secondary voltage


  • It is accurate method.
  • As the transformer is loaded, heating condition is known at different load current.


  • This test is applicable only for small kVA transformer because it is very difficult to arrange large load for medium and high capacity transformers.
  • Wastage of power is done in the external load circuit

 What is effect of load current on efficiency and voltage regulation of transformer?

  • The efficiency increases up to full load current and its value slightly decrease for overloaded condition. 
  • As the load current increases, the voltage regulation is also increased. 
  • It should be noted that the voltage regulation of the transformer depends on both the load current and power factor of the load.

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