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Electrical Lab Experiments

Load test on single phase transformer


To conduct the load test on the given a single phase transformer for finding the efficiency and its regulation.

Apparatus Required:

S.No. Apparatus Range Type Quantity
1 Voltmeter (0-150)V (0-300) V MI MI 1 1
2 Ammeter (0-10)A (0-20) A MI MI 1 1
3 Wattmeter 150V,20A 300V,10A UPF UPF 1 1
4 Auto transformer 240 V, 2.7 KVA,10A 1

Formula Used:

  1. Percentage Regulation = (Vo2-V 2) /V o2*100
  2. Where Vo2= Secondary voltage on no load

    V o= Secondary voltage at a particular load.

  3. Power factor = Pout/V2*I 2
  4. Where Vo2= Secondary voltage on no load

    Where Pout = Secondary wattmeter readings in Watts

    V2= Secondary voltage in Volts

    I2= Secondary current in Amps

  5. Percentage efficiency = Pout/Pin*100
  6. Where Pout = Secondary wattmeter readings in Watts Pin = Primary

    wattmeter readings in Watts.

Model Graph:

Load Test on Single Phase Transformer


  1. Connections are given as per the circuit diagram.
  2. Verify whether the autotransformer is kept at zero voltage position.
  3. By closing the DPST switch, 230V, 1 φ, 50HZ AC supply is given to the transformer.
  4. At no load, the readings from the meters are noted down.
  5. The load is applied to the transformer in steps upto 125% of the rated value of the primary current by using rheostatic load.
  6. The corresponding values from the meters are tabulated for different loads.
  7. Then the load is removed gradually, auto transformer is brought to its minimum position and the supply is switched off.
  8. From the recorded values, the regulation, power factor and efficiency are calculated.


S.No. Primary Voltage V1(V) Primary CurrentI1(A) Primary Wattmeter W1(W) Secondary Voltage V2(V) Secondary Current I2(A) Secondary Wattmeter W2(W) Power Factor Cose θ % Regulation% η%

Model Calculation:




Post lab Questions

  1. What will happen if a DC voltage is given to the transformer primary?
  2. What are the losses in a transformer?
  3. How can we minimize the core losses in a transformer?
  4. What is meant by eddy current losses?
  5. How hysteresis loss can be reduced?


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