 Electrical Machine-II-Engineering-Mumbai University-Dec2018 - Grad Plus

# Electrical Machine-II-Engineering-Mumbai University-Dec2018

## Semester: 4

[Total Time: 3 Hours]
[Total Marks: 80 M]
Note: i) Question No. 1 is Compulsory
ii) 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*103kg/m3, length of mean flux path=2.2m, gross iron section=10*103m2primary 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/mm2 . [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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