B.E. /B.Tech. (Part Time) DEGREE END SEMESTER EXAMINATIONS, APRIL/MAY 2014
ELECTRICAL AND ELECTRONICS ENGINEERING BRANCH
THIRD SEMESTER
Duration: 3 Hr
PTEE8301 ELECTRICAL MACHINES - I
Max. Marks: 100
REGULATIONS 2013
Answer All Questions
PART - A (10 x 2 = 20 Marks)
1. Explain why saturation of flux occurs in an electrical machine.
2. How the use of laminations help in reducing eddy current loss?
3. Using Lenz's law explain the relationship between applied load torque and developed torque,
when full load is suddenly thrown on an electrical motor.
4. Why the air gap of rotating machines have to be as small as possible?
5. Why the armature winding is kept in the rotating part of the d.c. machine?
6. The current through one of the parallel paths of a 4 pole wave connected dc generator is 100
A. What is the total current supplied to the load?.
7. A 220 V, 5kW, 26A, 1460 RPM d.c separately excited motor runs at rated load conditions. The
armature resistance is 0.5 Q. Calculate the speed of the motor if the load is halved.
8. If the flux of a d.c. shunt motor is weakened why it fails to drive a constant torque load?
9. A 1000 VA, 230/115V single phase transformer has constant loss = 25 W and full load copper
loss = 30 W. Compute the efficiency at 80% full load and 0.8 power factor lagging.
10. Which three phase transformer is employed in the sub-stations?
P A R T - B ( 5 x 1 6 = 80 Marks)
11.(a). An electromagnetic system shown in Figure 1 has the following dimensions: Ac = 1.0 cm ,
2
-3r
l = 15 cm, g = 0.8 cm, N = 480 turns. Neglecting leakage and fringing and assuming relative
c
permeability of iron core, u = 1000, calculate the inductance of the coil.
[6]
r
,
t
> - J/
_J
Figure 1
11 (b). Derive the expression of the energy stored in the air gap of a permanent magnet shown in
Figure 2, if the maximum magnetic field intensity is H (AT/m) and maximum magnetic flux density is
m
B (Wb/m ). Assume that the reluctance of the permanent magnet and soft iron is negligible and the
2
m
fringing effect is absent.
[10]
A
—'
rue£-
Figure 2
Of 2
[16]
12 (a). Deduce the expression of developed torque of a reluctance motor.
(Or)
12 (b). Show a doubly excited system and derive the expression for stored energy in the air gap. [16]
13 (a) A four pole d.c. machine has a wave winding of 300 turns. The flux per pole is 0.025 Wb and
the d.c. machine rotates at 1000 rpm. Determine the generated voltage and kW rating of the
generator if the rated current through the turn is 25 A.
[16]
[Or]
13 (b) Show that two oppositely rotating magnetic fields are produced in the air gap of a single phase
motor excited with single phase supply.
[16]
14 (a). A separately excited d.c. generator has the following magnetization characteristics at 1200
rpm.
If (A)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.8
1.0
1.2
Ea(V)
5
20
40
60
79
93
102
114
120
125
The machine parameters are r =0.2 O, Rfw = 100Q. The field winding is separately excited with 120 V
a
and the field current is adjusted at l = 0.8 A. A load resistance R = 2 Q is connected to the terminals.
f
L
Neglect armature reaction effect. If the generator is made to run at 1500 rpm and delivers rated
current at rated voltage then determine the corresponding field current and external field circuit
resistance.
[16]
(Or)
14 (b) (i) Write short notes on the armature reaction.
[8]
14 (b) (ii) Explain the steps to be followed to predetermine the efficiency of a d.c. generator using
Swinburne's test.
[8]
15.(a).(i).Explain the principle of operation of the transformers with help of the phasor diagram.
[8]
I
15 (a) (ii). A single phase, 25 kVA, 220/440 V , 60 Hz, transformer gave the following test results.
Open circuit test (440 V side open): 220 V, 9.5 A , 650 W; Short circuit test (220 V side shorted): 37.5
V, 55 A, 950 W. Determine the voltage regulation at full load and 0.8 power factor lagging. Draw the
respective phasor diagram also.
[8]
[Or]
15 (b) (i). A two winding transformer is converted into an autotransformer. Prove that the latter can
handle more power with the same amount of copper of the former?
[8]
15 (b) (ii). Write short notes on Y / A three-phase transformer connection.
[8]
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