Cochin University of Science and Techology (CUST) 2006 B.Tech Electrical and Electronics Engineering electical machines1 - Question Paper

1st TERMINAL exam MARCH 2006
4th SEMESTER
Max marks:50
ELECTRICAL MECHANICS 1

ans any four ques.


1. a) Derive the emf formula of a single phase transformer and discuss 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, compute 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, obtain the new core loss.

3. a) elaborate the various 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. compute the
efficiency on
1. full load
2.half load
Assuming pf of the load to be 0.8 lagging

4. a) explain how parameters of equivalent circuits of a one phase transformer are derived
from suitable tests.

b) find the equivalent ckt of a 200/400V,50Hz,1phase , T/F referred to LV side from
the subsequent 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) discuss 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 one phase T/F on leading pf load and discuss it.

b) A one 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 . compute the primary current and pf when the
secondary current is 280A at a pf of 0.8 lag.



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Earning:   Approval pending.