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Transformers lab report

Updated on September 25, 2009





Experiment 1

Open circuit and short circuit tests of a single-phase transformer



1.                  To study the variation of iron loss and no load current with the applied voltage

2.                  To determine the equivalent circuit parameters of the single phase transformer



1.                  (100VA, 240/110) single phase transformer               1 no.

2.                  (0-250V, AC) voltmeter                                             1 no.

3.                  (0-150V, AC) voltmeter                                             1 no.

4.                  (0-1A, AC) ammeter                                                   1 no.

5.                  Wattmeter                                                                   1 no.

6.                  230/(0-250V) variac                                                    1 no.

7.                  Leads                                                                          8 no.



For a practical transformer the following features should be taken in to account:


1.      Finite winding resistance

2.      Flux is not entirely confined to the core. Some portion appears as a leakage.

3.      It draws a finite current from the source on no load.

4.      It contains hysterisis and eddy current losses.

5.      The magnetic circuit of the transformer is not linear.


By taking suitable steps, we can separate out those deviations in the practicle transformer and derive some relationship. We will use them to predict the performance of the transformer.


The exact equivalent circuit of a transformer is as shown in Figure 1



R1:       Primary winding resistance

R2:       Secondary winding resistance

X1:       Primary leakage reactance

X2:       Secondary leakage reactance

RC:       Equivalent resistance to represent Iron loss

Xm:      Magnetizing reactance



Part A: Open circuit test


(a)                Examine the transformer and note down the rated voltage, KVA rating and operating frequency.


(b)               Identify the HV side and LV side and calculate the rated currents for each side.

(c) Connect the circuit as shown in Figure 2.

Note that the open circuit test is done on the LV side while HV side is open circuited.

(d)   Keep the variac at it's minimum position.

(e)    Switch on the power supply and raise the primary voltage to it's rated voltage.

(f)    Note down the ammeter (IOC), voltmeter (VOC) and the wattmeter (WOC) readings.

(g)   Decrease the primary voltage in steps of 10V and observe the meter readings on the given table below.

Table of results

Result-1 Plot the wattmeter reading (Iron Loss) vs. applied voltage and applied voltage vs. no Load Current.



VOC       Primary input voltage when the secondary is open circuited.

IOC        Primary current when the secondary is open circuited.

WOC      Iron loss when the secondary is open circuited.


For rated values note down the following observations.


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    • profile image

      John 5 years ago

      Why is a transformer rated in volt-ampere (VA)?

    • profile image

      sarang titare 5 years ago

      Thanks sir for providing us detail about eqvt ckt of transformer.

      but i have a one doubt.

      why transformer works on only AC why not work on DC?

    • profile image

      Rakesh 5 years ago

      I got informatiom about oc test bt one doubt why we use LV side as primary.

    • profile image

      salem 6 years ago

      thanks i got a lot of information from this report

    • profile image

      kavan raj urs 7 years ago

      dear sir i want full explantion about oc and sc test .and why it like.

    • profile image

      Charles Rawlins 7 years ago

      Did you measure the DC resistance of the primary?

    • profile image

      D.Sathiyakumr 8 years ago

      discussion this open circuited practical

    • profile image

      Lukasz Kocewiak 8 years ago

      I like it. For no load losses test there might be included some calculations related with magnetizing impedance.