**[Total Time: 3 Hours]****[Total Marks: 80 M]Note: i) Question No. 1 is Compulsoryii) Attempt Any Three questions from remaining five questions.(iii) Make any suitable assumption wherever required.**

Q.1 Answer any four.

(a) Explain properties of magnetic materials required for electrical machine design.** [5M]**

(b) Explain saving of copper in autotransformer over two winding transformer. ** [5M]**

(e) Explain the conditions for parallel operation of three phase transformer. ** [5M]**

* (d) Explain disadvantages of harmonics in transformer.* ** [5M]**

* (e) Explain Burden of potential transformer.* ** [5M]**

Q.2 (a) Draw and explain back to back test. **[10M]**

b) Two single phase transformer A and B rated at 600KVA and 500KVA res. are operated in parallel to supply a load of 1000KVA at 0.8 lagging power factor. The* resistance and reactance of transformer A are 3% and 6.5% while that of transformer B are 1.5% and 8%. Calculate the KVA loading and the power factor at which * *each transformer operate. ***[10M]**

Q.3 (a) Explain excitation phenomenon in three phase transformer. ** [10M]**

(b) Calculate the no load current of a 400V 50Hz, single phase core type transformer having the following data.

Stacking factor=0.9, density=7.8*10^{3}kg/m^{3}, length of mean flux path=2.2m, gross iron section=10*10^{3}m^{2}primary turns=200 joints equivalent to 0.2 mm air gap. Assume mmf/meter=210 A/m.Iron loss per kg=1.3kg for the corresponding flux density of 1 wb/mm^{2} . ** [10M]**

Q.4 (a) Derive an output equation of a three phase core transformer.** [10M]**

(b) Determine the dimensions of core and yoke for a 200KVA, 50Hz single phase core type transformer. A cruciform crre is used with distance between adjacent limbs equal to 1.6 times the width of core aminations. Assume voltage per tum 14V, maximum flux density 1.1Wb/m, widow space factor 0.32, current density 3A/mm2 and stacking factor -0.9. The net iron rea is 0.56in a cruciform core where d is the diameter of circumscribing circle. Also the width of largest stamping is 0.85d. ** [10M]**

Q.5 (a) Explain “Oscillating Neutral** [10M]**

(b) A 300 KVA, 6600/400V, 50 Hz, delta/star three phase core type transformer has the following data: Width of hy winding = 25mm, Width of lv winding = 16mm, height of S coils=0.5m, length of mean turn=0.9m, hv winding turns=830, width of duct between hy and ly winding =15mm, calculate the leakage reactance of the transformer referred to the hy side. If the ly coil is split into two parts with one part on each side of the v coil, calculate leakage reactance referred to hv side. Assume that there is a duct 15mm wide between hy winding and each part of lv winding. ** [10M]**

Q.6 (a) Draw and explain Scott connection. What are the applications of Scott connection?** [10M]**

(b) Explain various cooling methods in transformer. ** [10M]**

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