Code No: R05220204
Set No. 1
II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007
(Electrical & Electronic Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
(a) Explain the various types of material used in construction of core of trans-
former? Briefly explain all the properties. Explain how quality of core material
is related with core losses in transformer.
(b) A single phase transformer has 400: 1000 turns ratio. The net cross sectional
area of the core is 60 cm2. The primary winding be connected to 50 Hz supply
& 500 V. Calculate the peak value of flux density in the core. The voltage
induced in the secondary winding.
(a) Why it is essential to calculate all day efficiency for distribution transformers.
How it is different from full load efficiency.
(b) A 20 kVA transformer has maximum efficiency of 98% when delivering 75%
load at 0.95 pf. If during the day transformer is delivering load as follows, find
the all day efficiency of transformer.
12 kW, 0.8 pf
20 kW at upf.
(a) Explain the various simple tests conducted on a single transformer to find the
approximate equivalent circuit of transformer.
(b) OC test is preferred to conduct on LV side & SC test is preferred to conduct
on HV side. Explain the reasons.
(a) What is the three winding transformer? What is the third winding called as?
Why third winding is needed?
(b) A Scott connected transformer is fed from a 6600 V, 3-Φ network and supplies
2-Φ power at 500 V/Ph. Calculate the line current on 3-Φ system, if the load
on the 2-Φ sides are 500 A at 0.8 pf lagging (teaser) and 400 A at 0.9 lagging
(a) Deduce the expression for (rotor side) starting current, starting power factor,
standstill frequency and standstill EMF of squirrel cage IM.
(b) A 4-pole IM is fed from 50 Hz supply and has rotor speed of 1425 RPM find
slip speed & slip.
(c) A 12 pole, 3-Φ alternator driven at a speed of