LAB REPORT

ELECTRICAL POWER

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Experiment 1

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

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Aim

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

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Apparatus

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.

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Theory

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

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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.

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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.

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Where

R₁:       Primary winding resistance

R₂:       Secondary winding resistance

X₁:       Primary leakage reactance

X₂:       Secondary leakage reactance

R_C:       Equivalent resistance to represent Iron loss

X_m:      Magnetizing reactance

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Procedure

Part A: Open circuit test

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

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

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 (I_OC), voltmeter (V_OC) and the wattmeter (W_OC) readings.

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

If:

V_OC       Primary input voltage when the secondary is open circuited.

I_OC        Primary current when the secondary is open circuited.

W_OC      Iron loss when the secondary is open circuited.

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For rated values note down the following observations.