Semester 4 th ( Back/2008 AB))
Sub & Code EM EE403
Branch MECHANICAL
SPRING END SEMESTER EXAMINATION -2015
ELECTRICAL MACHINES
[EE-403]
Full Marks: 60 Time: 3 Hours
Answer any six questions including question No. 1 which is compulsory.
The figures in the margin indicate full marks.
Candidates are required to give their answers in their own words as far as practicable and all
parts of a question should be answered at one place only.
Q. No Marks
1. a) Write four application of a d.c generator [2x10]
b) Draw neatly the different types of generator.
c) Draw & explain the electrical & mechanical characteristics of a shunt motor
d) Why a d.c series motor is never started without any mechanical load?
e) The no-load current of a transformer is 5Aat 0.3p.f lagging when supplied at
230V,50Hz.The number of turns of primary winding is 200.Calculate (i)the
maximum value of flux in the core (ii) the core loss (iii) the magnetizing current.
f) Name the different methods for controlling the speed of a d.c motor.0
g) Can an Induction motor rotates at synchronous speed?Justify your answer.
h) Explain briefly why a synchronous motor rotates at synchronous speed only or not
at all.
i) The stator of a 3-ph alternator has 9 slots/pole & carries a balanced three phase
double layer winding.The coils are short pitched & the coil pitch is 7 slots .Find the
distribution factor & pitch factor.
j) Derive the condition for maximum torque under running condition for a 3-ph
Induction motor.
Q2 a) Explain with neat diagram the process of voltage build up of a shunt generator. 4
b) A 4-pole long shunt lap-wound generator supplies 25kW at a terminal voltage of 4
500V.The armature resistance is 0.03Ω , series field resistance of 0.04Ω & shunt
field resistance is 200Ω.The brush drop is taken as 1V per brush. Determine the
generated e.m.f. Calculate the number of conductors if the speed is 1200r.p.m 7
flux per pole is 0.02Wb.Neglect armature reaction.
Q3 a) A 220V shunt motor takes a total current of 80A& runs at 800r.p.m.Shunt field 4
resistance & armature resistance are 50Ω & 0.1Ω respectively.If iron & friction
loss amount to 1600W,find (i) copper loss (ii) armature torque (iii)shaft torque (v)
efficiency.
b) A 200V shunt motor has an armature current of 15A when running at 4
800r.p.m.against full-load torque.The armature resistance is 0.6Ω.What resistance
must be inserted in series with the armature to reduce the speed to 400r.p.m.at the
same torque.?What will be the speed if the load torque with the resistance in the
citcuit ?Assume theflux to remain constant throughout.
Q4 a) Draw the exact equivalent circuit & phasor diagram of a loaded transformer with 4
lagging p.f.
b) A 500 KVA ,3800/400V ,50Hz single ph transformer has the following data:
R1-0.114Ω , X1=0.459Ω, R2=0.00102Ω , X2=0.00416Ω. 4
Find the percentage voltage regulation of the transformer when supplying full load-
current at a p.f of 0.8 lagging
Q5 a) . Explain the consruction & principle of operation of 3φ-alternator. 4
b) A 3-phase ,50Hz ,star connected ,2000KVA ,2300V alternator gives a short circuit 4
current of 600A for a certain field excitation. With the same excitation ,the open
circuit voltage was 900V(line value).The resistance between a pair of terminals was
0.12Ω.Find the voltage regulation at 0.8 p.f lagging.
Q6 a) With neat diagram explain the torque-slip characteristics of a 3-ph Induction motor. 4
b) A 3-ph Induction motor has a synchronous speed of 250 r.p.m & 4% slip at full 4
load .The rotor has a resistance of 0.02Ω/ph & a standstill reactance of 0.15Ω/ph
.Calculate (i)the ratio of maximum & full-load torque (ii) the speed at which
maximum torque develops.Neglect resistance & leakage reactance of stator.
Q7) (a) Why single phase Induction motor is not self starting? Explain the different method 4
of starting of it
(b) Describe the construction & working principle of Universal motor. 4
Q8) Write short notes(any two) 4x2
a) Three-point starter
b) V-curves
c) Star-delta & Delta-delta connection of 3φ transformer
Paper setter…………………..
Moderator…………………..
KIIT-U/-2012/Autumn End Semester Examination-2012
