Load Test on Single-Phase Transformer

Load Test on single phase transformer

Before starting the Load Test on Single-Phase Transformer , let’s define the voltage regulation.

Voltage Regulation

When a transformer loaded with a constant primary voltage, the secondary voltage varies because of the internal resistance and leakage reactance of the winding.

The regulation of transformer defined as “the change in secondary terminal voltage from no load to full load”

Percentage voltage regulation is expressed as below equation,

    \[ \% \, Voltage \, Regulation = \frac{No \, Load \, Secondary \, voltage \, - \, Secondary \, Voltage \, at \, Full \, Load }{No \, Load \, Secondary \, Voltage} \times 100 \]

    \[ \% \, Voltage \, Regulation = \frac{E_2-V_2}{E_2} \times 100 \]

Direct Load Test on Single-phase Transformer

Now the direct load test performs on the single-phase transformer to determine the voltage regulation and efficiency of transformer.

The circuit diagram for direct load test is as shown in below figure.

load test on single phase transformer
load test on single phase transformer

Direct load test used to determine the performance of transformer at various loads. The lamp load connected at the secondary winding of transformer as varying load.

The voltmeter, ammeter, and wattmeter connected at primary side and secondary side as shown in above circuit diagram.

And V1, I1, and W1 shows reading of primary side and V2, I2, and W2 shows reading from the secondary side.


  • First of all, set up all measuring instrument as shown in circuit diagram.
  • For first reading, keep the secondary side open or in no load condition.
  • Next switch ON the AC power and note the no load secondary voltage of transformer by voltmeter V2. And note this value as E2.
  • Now connect the lamp load and increase the load with the step of 25%, 50%, 75%, and 100% till the load current reaches to the full load value.
  • At each step, note the value of V1, I2 and W1 from primary side and V2, I2, and W2 from secondary side.
  • At each step, the input voltage of primary winding remains constant and use single-phase variac to make the input voltage constant.
  • After all steps, switch of the lamp load and switch OFF the AC power.

Now you have the data as per following table. We will use this data to find the efficiency and voltage regulation of single-phase transformer.

Sr. No. Input Output % effi % VR
V1 Volts I1 Ampere W1 Watts V2 Volts I2 Ampere W2 Watts



1) Efficiency:

The efficiency of transformer calculates from the input power from W1 and output power from W2.

Input power to the transformer = W1 = V1 I1
Output power to the transformer = W2 = V2 I2

For the lamp load, we will assume the power factor is one.


    \[ Transformer \, Efficiency = \frac{Output Power}{Input Power} \]

    \[ Transformer \, Efficiency = \frac{W_2}{W_1} \times 100 \% \]

2) Voltage Regulation

No load secondary voltage = E2
Secondary voltage at a particular load = V2


    \[ \% Voltage \, Regulation = \frac{E_2-V_2}{E_2} \times 100 \% \]

The below graph is for efficiency versus load for a transformer and it drawn from the data of load test.

Load vs efficiency of single phase transformer
Load vs efficiency of single phase transformer

Load test also gives information for the temperature rise of the transformer under loaded condition.

This test used to find the efficiency and voltage regulation of a low rating transformer.

This test is not advisable for high rating transformers because of non-availability of large capacity load in the laboratory and expensive power consumption for testing transformers.

Open circuit test of Transformer

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