Question

State and explain the two types of transformer core losses. How do you minimise them? You are invited for an interview at Transformer Inc. for the position of a design engineer. In the interview, you are given a 10kVA, 2400/347 V, 50Hz transformer with leakage reactance of X₁ = 4.300 and X₂ = 0.090 on the high voltage (HV) and low voltage (LV) sides,respectively. You are also given the HV and LV resistance values of 5.160 and 0.095 respectively. The magnetising reactance XM, of the transformer and the Core loss, Rc referred to the HV side are given as 18.80 and 0.850 respectively. You are asked to do a quick approximation of various design values including the currents on the LV and HV sides of the transformer.

(i) Figure. Q2(b) shows the equivalent circuit of the transformer with parameters referred to the LV (secondary) side. From the given values, explain why you should neglect the magnetising branch (XM and Rc) in simplifying your circuit for approximate design calculations.For the remaining parts of Q2(b), consider the approximated circuit with the magnetising branch removed. (ii) Short circuit the secondary terminal of the equivalent circuit shown in Figure Q2(b)and calculate, the short circuit current, I2,sc with rated supply voltage. (iii) Comment on the value of 12,sc current obtained in (ii) above and recommend the correct precaution for short circuit transformer test. (iv) If the transformer is now operating at rated load drawing rated full load current at a power factor of 0.85 lagging, determine the rated current flowing in the secondary winding of the transformer. Present your answer in polar form. (v) Determine the no-load and full-load secondary terminal voltage of the transformer? (vi)Compare the two values, no-load and full-load secondary terminal voltage obtained in (v) above. Are they the same? If not, why? (vii) When carrying full load current, as described in (iv) above, the total core loss of the transformer is constant at 800 W. Determine the transformer copper loss on the LV side and hence, the total transformer loss at this operating condition.

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