Direct Loading Test on Transformer – Experimental Set-up & Procedure

The direct loading test is used to find the efficiency and regulation of the transformer.

Experimental Set-up of Direct Loading Test on Transformer

In this test, the secondary winding is connected directly across the load for which regulation and efficiency are to be obtained. The experimental setup for this is shown in Fig 1.

Figure 1: Experimental setup for Direct Loading Test on Transformer.

A variac is connected across the single-phase ac supply in order to keep the primary voltage constant to its rated value. Ammeters A₁ and A₂ are used to measure the primary and secondary currents respectively. Two wattmeters W₁ and W₂ are used to measure the active powers on the primary and secondary sides respectively. The voltmeters V₁ and V₂ are connected to measure the primary voltage and load voltage respectively. The load for which regulation and efficiency are to be obtain is connected across the secondary as shown in Fig. 1.

Procedure and Observations for Efficiency Measurement:

Connect the desired load across the secondary as shown in Fig. 1. We obtain the values of efficiency and regulation at this load current. Switch on the single-phase AC supply connected across the variac and adjust the variac to get the rated primary voltage. This is measured by voltmeter V₁. Measure the primary and load active powers by means of wattmeter W₁ and W₂ respectively, and calculate the percent efficiency as follows,

\[\%\eta =\frac{\text{Output power delivered to load}}{\text{Input power to the primary}}\times 100\]

\[\%\eta =\frac{{{W}_{2}}}{{{W}_{1}}}\times 100\]

Note that,

\[{{W}_{1}}={{V}_{1}}{{I}_{1}}\cos {{\phi }_{1}}\] And \[{{W}_{2}}={{V}_{2}}{{I}_{2}}\cos {{\phi }_{2}}\]

Procedure and Observations for the Regulation Measurement:

Do not connect any load. This is the no load condition I₂ = 0 and V₂ = E₂. Switch on the AC power supply and adjust the variac to obtain the rated primary voltage V₁. Measure the voltage V₂. This voltage is called as the no load voltage. V₂ = E₂ under the no load condition.  Switch off the ac supply and connect the desired load. Turn on the power supply again and measure V₂. This voltage is the full load secondary voltage. Calculate the regulation as,

\[\%\text{Regulation = }\frac{{{V}_{2NL}}-{{V}_{2FL}}}{{{V}_{2NL}}}\times 100\]

Also

\[\%\text{Regulation =}\frac{{{E}_{2}}-{{V}_{2}}}{{{E}_{2}}}\times 100\]

Regulation characteristics:

Using this table we can plot the regulation characteristics of the transformer. To do so, we have to change the load from 0 to full load, measure the corresponding values of load current I₂ and load voltage V₂. Then plot the load regulation characteristics of load current (I₂) versus the load voltage V₂.