Code No: R05220204
Set No. 1
II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007
ELECTRICAL MACHINES-II
(Electrical & Electronic Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1.
(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.
[8+8]
2.
(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.
[6+10]
12 hours
No load
6 hours
12 kW, 0.8 pf
6 hours
20 kW at upf.
3.
(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.
[10+6]
4.
(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
(main).
[6+10]
5.
(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