BTS (C) — IV — 10 — 015 — P
B. Tech Degree IV Semester Examination, April 2010
EE 405 ELECTRICAL MACHINES I
(2002 Scheme)
Time : 3 Hours
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(b)
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Maximum Marks : 100
Explain the main parts and their functions of a D.C. generator. A 30 KW, 300 V d.c. shunt generator has armature and field resistance of
0.050 and 1000 respectively. Calculate the power developed by the armature
when it delivers full load output.
OR
Derive the equation for emf generated in a d.c. generator. An 8 pole 240 V, simplex lap wound armature delivers 125 A. Determine (i) the voltage per path (ii) the current per path (iii) the current per brush. Explain the effects of armature reaction in a d.c. machine. Explain voltage build up in a self excited generator. Draw the open circuit characteristic
and define initial field resistance for a shunt generator. Explain the necessity of connecting small generators in parallel instead of a single large
generator. Also explain the process of paralleling two shunt generators.
OR
Draw the internal and external characteristics of a series generator. Why does the
terminal voltage decreases with increase in load current? A 220 V compound generator is supplying a load of 100 A at 220 V. The resistance
of its armature, shunt and series windings are 0.10, 500 and 0.060 respectively.
Find the induced emf and armature current when the machine is connected (i) short
shunt (ii) long shunt.
(a)
(b)
(a)
(b)
(a)
(b)
(10)
(6)
(6)
(8)
(10)
(10)
(10)
(10)
Explain the uses of different types of d.c motors with the support of their characteristics. (10)
A d.c series motor operates at 800 rpm with a line current of 100 A from 230 V mains.
Its armature circuit resistance is 0.150 and field resistance is 0.10. Find the speed
at which the motor runs at a line current of 25A, assuming the flux at this current is
(10)
45% of flux at 100A.
OR
(10)
Explain Swinbume's test for determining efficiency of a d.c machine. A 220V series motor takes 50A. The armature resistance is 0.10 and the series field
resistance is 0.080. If the iron and friction losses are equal to copper losses at this
load, find (i) the B.H.P (ii) the efficiency. VII.
(10)
(10)
(10)
Draw and explain the equivalent circuit of a transformer referred to primary. The voltage on the secondary of a single phase transformer is 200 V when supplying
a load of 8 KW at a p.f of 0.8 lagging. The secondary resistance is 0.040 and
(10)
secondary leakage reactance is 0.80. Calculate the induced emf in the secondary. OR
(Turn Over)
2
(a)
(b)
(a)
(b)
Describe the various losses that occur in a transformer and methods to reduce them in
practice.
A 40 KVA transformer has iron losses of 450 W and full-load copper loss of 850 W.
If the power factor of the load is 0.8 lagging, calculate (i) full load efficiency
(ii)
the KVA load at which maximum efficiency occurs and (iii) the maximum
efficiency.
Explain Sumpner's test of transformers. A 20 KVA single phase, 50 Hz, 2200/200V transformer gave the following test results
(10)
(10)
(10)
0.0 test : 2200 V applied to primary, power taken 220 W
S.0 test : Power required to circulate full load current in short circuited secondary 240 W.
Calculate the efficiency at full load and half full load at p.f 0.8 lagging. (10)
X.
(a)
(b)
OR
Explain (i) three phase transformer connections (ii) all day efficiency. A balanced 3 phase load of 30 KVA at a power factor of 0.866 lagging is connected
to two transformers connected in open delta to give 230 V three-phase system. Find the
power delivered by each transformer. ***
(10)
(10)