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Posted By: arya Member Level: Gold Posted Date: 29 Jun 2008
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2006 Cochin University of Science & Technology B.Tech Electrical and Electronics Engineering electical machines1 Question paper
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FIRST TERMINAL EXAMINATION MARCH 2006
FOURTH SEMESTER
Max marks:50
ELECTRICAL MECHANICS 1
Answer any 4 questions
1. a) Derive the emf equation of a single phase transformer and explain voltage
transformation ratio.
b) a single phase transformer has 400 primary and 100 secondary turns. The net cross-
sectional area of the core is 60 sq.cm. If the primary winding be connected to a
50Hz supply at 500V, calculate the peak value of flux density in the core, and the
voltage induced in the secondary winding.
2. a) Write short notes on 1. conservator
2. breather
3. commutator
4. brushes
5. armature
b) When a T/F is connected to 1000V, 50Hz, 1phase supply, the core loss is 1000W, of
which 650W is hysteresis and 350W is eddy current loss. If the applied voltage is
raised to 2000V and frequency to 100Hz, find the new core loss.
3. a) What are the different losses in a transformer? Derive the condition for maximum
efficiency of the transformer.
b) The primary and secondary windings of a 500KVA, 1phase transformer have
resistance of 0.4ohm and 0.0015ohm respectively. The primary & secondary
voltages are 6000V& 400V respectively& the core loss is 3.2KW. Calculate the
efficiency on
1. full load
2.half load
Assuming pf of the load to be 0.8 lagging
4. a) Discuss how parameters of equivalent circuits of a 1 phase transformer are derived
from suitable tests.
b) Obtain the equivalent ckt of a 200/400V,50Hz,1phase , T/F referred to LV side from
the following test data.
O.C test: 200V,0.7 A,70W-on LV side
S.C test: 15V,10A,85W-on the HV side
5. a) Explain the principle of operation of a d.c generator.
b) A1100/230V,15KVA,1phase,50Hz,T/F has core loss of 1.4 kW and full load Cu
copper loss of 1.6kW.Determine
1. the kVA load for maximum efficiency and value of maximum efficiency at unity
pf load
2. the efficiency at half full load, 0.8 pf leading
6. a) Draw the phasor diagram of a 1 phase T/F on leading pf load and explain it.
b) A 1 phase T/F has 1000 turns on primary and 2000 turns on the secondary. The no
load current is 3A at of pf 0.2 lag . Calculate the primary current and pf when the
secondary current is 280A at a pf of 0.8 lag.
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