### EE 2251 — ELECTRICAL MACHINES – I NOVEMBER/DECEMBER 2010 FOURTH SEMESTER ANNA UNIVERSITY

Reg. No. :
B.E./B.Tech. DEGREE EXAMINATION, NOVEMBER/DECEMBER 2010
Fourth Semester
Electrical and Electronics Engineering
EE 2251 — ELECTRICAL MACHINES – I
(Regulation 2008)
Time : Three hours  Maximum : 100 Marks
PART A — (10 × 2 = 20 Marks)
1. Give the analogy between electric circuit and magnetic circuit.
2. Distinguish between statically and dynamically induced electromotive force.
3. What are the no load losses in a two winding transformer and state the
reasons for such losses.
4. Mention the conditions to be satisfied for parallel operation of two winding
transformers.
5. Draw the power low diagram for motor and generator operation.
6. In a magnetic circuit with a small air gap, in which part the maximum energy
is stored and why?
7. Explain the concept of electrical degree. How is the electrical angle of the
voltage in a rotor conductor related to the mechanical angle of the machines
shaft?
8. Why does curving the pole faces in a D.C. machine contribute to a smoother
D.C. output voltage from it?
9. State the conditions under which a D.C. shunt generator fails to excite.
10. What is the precaution to be taken during starting of a D.C. series motor?
Why?
Question Paper Code : 53136
132  132  132 2 53136
PART B — (5 × 16 = 80 Marks)
11. (a) (i) Define inductance of a coil.  (4)
(ii) For the magnetic circuit shown in Fig. 11.a (ii) determine the
current required to establish a flux density of 0.5 T in the air gap.
(12)

Fig. 11 (a) (ii)
Or
(b) (i) Define permeability of a magnetic material and the factors on
which it depends.    (4)
(ii) Explain the operation of a magnetic circuit when A.C. current is
applied to the coil wound on iron core. Draw the B-H curve and
obtain an expression for hysteresis loss. (12)
12. (a) (i) Define “Voltage Regulation” of a two  winding transformer and
explain its significance.   (4)
(ii) A 100 kVA, 6600 V/ 330 V, 50 Hz single phase transformer took
10 A and 436 W at 100 V in a short circuit test, the figures referring
to the high voltage side. Calculate the voltage to be applied to the
high voltage side on full load at power factor 0.8 lagging when the
secondary terminal voltage is 330 V. (12)
Or
Iron  core :
thickness = 2 cm
µ core = 5000 µ0
8 cm
1 cm
2 cm
I
10 cm
N = 1000
132  132  132 3 53136
(b) (i) Explain the reasons for ‘tap changing’ in transformers. State on
which winding the taps are provided and why? (4)
(ii) A transformer has its maximum efficiency of 0.98  at 15 kVA at
unity power factor. During the day it is loaded as follows :
12 Hours   2 kW  at power factor of 0.5
6 Hours   12 kW  at power factor of 0.8
4 Hours   18 kW  at power factor of 0.9
Find the ‘All Day Efficiency’?   (12)
13. (a) (i) Derive an expression for the magnetic energy stored in a singly
excited electromagnetic relay.  (8)
(ii) The relay shown in Fig. Q.13.a (ii) is made from infinitely
permeable magnetic material with a movable plunger also of
infinitely permeable material. The height of the plunger is much
greater than the air gap length (h >> g). Calculate the  magnetic
energy stored as a function of plunger position (0 < x < d) for
N=1000 turns, g = 2.0 mm, d=0. 5 m, l = 0.1 m and I = 10 A.   (8)
Fig. Q. 13. (a) (ii)
Or
(b) Two windings one mounted on the stator and the other mounted
on a rotor have self and mutual inductances of, L11 = 4.5  H,
L 22 = 2.5 H and L12 = 2.8 cos θ H,  where ‘θ ’ is the angle between the
axes of the windings. The resistances of the windings may be neglected.
Winding 2 is short circuited and the current in Winding 1 as a function of
time is i1 = 10sin wt A. Derive an expression for the numerical value of
the instantaneous torque on the rotor in N-m in terms of the angle θ . (16)
plunger
µ → ∞
g
g
d
h
I
x
λ
Coil of
N Turns
Core  µ → ∞
132  132  132 4 53136
14. (a) (i) Show the arrangement of a distributed stator winding with
appropriate number of conductors in the slots designed to produce a
sinusoidally varying air gap flux density. (6)
(ii) Prove that a three phase set of currents, each of equal magnitude
and differing in space by 120° applied to a three phase
winding spaced 120 electrical degrees apart around the  surface of
the machine will produce a rotating magnetic field of  constant
magnitude.   (10)
Or
(b) (i) A D.C. machine has ‘P’ number of poles with curved pole faces
having  ‘Z’ number of conductors around the rotor armature of
radius ‘r’ and the flux per pole is given as, ϕ . The rotor rotates at a
speed of ‘n’ rpm. Obtain the induced e.m.f. of the  D.C. machine
assuming a number of parallel paths.  (8)
(ii) A 12 pole D.C. generator has a simplex wave wound armature
containing l44 coils of 10 turns each. The resistance of each turn is
0.011Ω . Its flux per pole is 0.05 Wb and it is running at a speed of
200 rpm. Obtain the induced armature voltage and the effective
armature resistance.    (8)
15. (a) (i) Draw the load characteristics of D.C. shunt and compound
(cumulative and differential) generators and explain. (6)
(ii) In a 110 V, compound generator the resistances of the armature
shunt and series field windings are 0.06Ω , 25Ω and 0.04 Ω
respectively. The load consists of 200 lamps each rated at  55 W,
110 V. Find the total electro motive force and armature current
when the machine is connected long shunt and short shunt.  (10)
Or
(b) (i) Give the reasons for using ‘starters’ to start D.C. motors. (3)
(ii) Draw the circuit of any one type of starter and explain its operation.
(5)
(iii) A series motor of resistance 1 Ω between terminals runs at 800 rpm
at 200 V with a current of 15 A. Find the speed at which it will run
when connected in series with a 5Ω resistance and taking the same
current at the same supply voltage. (8)
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