15/02/2021

Direct Loading Test on 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.
direct-loading-test-of-transformer.png


Primary Voltage

Primary Current

Input Power

Secondary Voltage

Load Current

Output Power

 

V1

I1

WI

V2

I2

W0

 

 

 

 

 

 



Efficiency

% 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 %

Where

NLSV = No load secondary voltage

FLSV = Full load secondary voltage

Advantages

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

Disadvantages

  • 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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