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Unformatted text preview: Electrical Machines I Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao Indian Institute of Technology Madras 7 Testing of Transformers The structure of the circuit equivalent of a practical transformer is developed earlier. The performance parameters of interest can be obtained by solving that circuit for any load conditions. The equivalent circuit parameters are available to the designer of the transformers from the various expressions that he uses for designing the transformers. But for a user these are not available most of the times. Also when a transformer is rewound with different primary and secondary windings the equivalent circuit also changes. In order to get the equivalent circuit parameters test methods are heavily depended upon. From the analysis of the equivalent circuit one can determine the electrical parameters. But if the temperature rise of the transformer is required, then test method is the most dependable one. There are several tests that can be done on the transformer; however a few common ones are discussed here. 7.1 Winding resistance test This is nothing but the resistance measurement of the windings by applying a small d.c voltage to the winding and measuring the current through the same. The ratio gives the winding resistance, more commonly feasible with high voltage windings. For low voltage windings a resistance-bridge method can be used. From the d.c resistance one can get the a.c. resistance by applying skin effect corrections. 44 Electrical Machines I Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao Indian Institute of Technology Madras V 1 V 2 V 3 V s ~ A 2 A 1 a 2 a 1 V S +- + A 2 A 1 a 2 a 1 (a)A.C.test (b)D.C.test Figure 18: Polarity Test 7.2 Polarity Test This is needed for identifying the primary and secondary phasor polarities. It is a must for poly phase connections. Both a.c. and d.c methods can be used for detecting the polarities of the induced emfs. The dot method discussed earlier is used to indicate the polarities. The transformer is connected to a low voltage a.c. source with the connections made as shown in the fig. 18(a). A supply voltage V s is applied to the primary and the readings of the voltmeters V 1 , V 2 and V 3 are noted. V 1 : V 2 gives the turns ratio. If V 3 reads V 1 − V 2 then assumed dot locations are correct (for the connection shown). The beginning and end of the primary and secondary may then be marked by A 1 − A 2 and a 1 − a 2 respectively. If the voltage rises from A 1 to A 2 in the primary, at any instant it does so from a 1 to a 2 in the secondary. If more secondary terminals are present due to taps taken from the windings they can be labeled as a 3 ,a 4 ,a 5 ,a 6 . It is the voltage rising from smaller number towards larger ones in each winding. The same thing holds good if more secondaries are present....
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This note was uploaded on 04/02/2010 for the course ECE 403 taught by Professor K.k.singh during the Spring '10 term at Birla Institute of Technology & Science.
- Spring '10