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

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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 
Answer ALL questions 
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
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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
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(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 
  2 Hours   No load 
  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 → ∞
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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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