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Unformatted text preview: ELECTRIC POWER TRANSMISSION SINGLE PHASE TRANSFORMER MV 1911 Introduction Experiment 1 : Load test Experiment 2 : Measurement of efficiency and shortcircuit impedance Experiment 3 : Waveform of the noload current INTRODUCTION The singlephase transformer is used, above all, in mains power units for supplying different types of electrical equipment. The transformer is then used to change the mains voltage to other voltages that are required in the equipment. The singlephase transformer consists of two windings on a common iron core. If one of the windings is connected to an AC voltage U 1 , an alternating magnetic field will be set up in the iron core. This field in its turn will induce an AC voltage U 2 in the other winding. The ratio between these voltages is the same as the ratio between the number of turns on the two windings: 2 1 2 1 N N U U = The winding for the higher voltage is called the up winding and the other winding is called the down winding. If the transformer is connected in an electric circuit the input winding is called the primary winding. The other winding, which gives out power, is called the secondary winding. In reality the up and down windings of a transformer are wound on top of each other, and not side by side as shown in the figure. This arrangement results in reduced leakage magnetic flux, and thus a smaller voltage drop in the transformer on load. In transformer MV 1911, however, the windings are laid side by side so that a larger, and hence easier to measure, voltage drop is obtained in the transformer when carrying out the laboratory exercises. EXPERIMENT 1 LOAD TEST Objective: To investigate how the secondary voltage of the transformer changes with different loads. Equipment: Power pack MV 1300 T = Transformer MV 1911 R = Load resistor MV 1100 U 1 = Voltmeter 240 V, MV 1926 U 2 = Voltmeter 130 V, MV 1926 I 1 = Ammeter 6 A, MV 1923 I 2 = Ammeter 10 A, MV 1923 S = Switch MV 1500 for later use: Load reactor MV 1101 Load capacitor MV 1102 Theory: When a transformer is loaded, i.e. current flows through the secondary winding, the secondary voltage will be reduced because of the voltage drop in the winding resistance and because of the magnetic leakage flux. In a model this leakage flux can be simulated by reactances and the following very familiar model for a transformer is obtained, where the noload current has not been taken into consideration. It can be shown that precisely the same effect can be simulated by replacing R2 on the righthand side of the model by a resistor on the lefthand side of the model, with a resistance value of 2 2 2 1 R N N The same applies in the case of X2, which gives us a new model for the transformer: If the two resistances are now combined to gives one single value R k , and the two reactances to give X k , the simple transformed model shown below is obtained....
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 Spring '12
 Terry
 Impedance

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