Jadavpur University 2007 B.E Mechanical Engineering ELECTRICAL TECHNOLOGY-II - exam paper

EX/ML12E/76/2006
FIRST ENGG. MECHANICAL EXAMINATION, 2006
(2nd Semester)
ELECTRICAL TECHNOLOGY - II
Time : 3 Hours Full Marks - 100
Use separate answer-scripts for every part
50 marks for every part
PART - I
Q. No.1 is compulsory and
attempt other 2 from the rest
1. a) describe stability in a drive system. discuss with
help of sketches TL. and TM with respect to angular
speed. 3
b) A motor having a suitable control circuit develops a
torque provided by the relationship TM =c?+d where
c and d are positive constants the motor is used to
drive a load whose torque is expressed as TL =?2 + b
where a and b are also other positive constants. The
polar moment of inertia of the rotating masses is J.
i) Determine the relation amongst the constants
a, b, c and d in order that the motor can begin
together with the load and have an equilibrium
operating speed.
ii) Determine the initial acceleration of the drive.
iii) compute the equilibrium operating speed.
iv) Determine the maximum acceleration of the
drive.
[ Turn over ]
-2 -
v) Determine whether the drive will be stable at the
speed. TL=Load Torque, TM = Motor Torque
? = r.p.s. = Angular Velocity. 15
2. A 250V 25KW d.c. shunt motor running at its rated speed
of 1200 rpm is to be braked electrically. The armature
resistance is 0.1O. and the rated efficiency of the motor is
90%. If the braking is counter current braking compute.
a) Resistance to be connected in series with the
armature to limit the initial braking current to twice
rated current
b) Initial braking torque and
c) The torque when the speed of the motor falls to 600
rpm.
d) If the dynamic braking would have been applied to the
motor instead of counter current braking what
resistance would have been connected in series with
the armatures at begin of the braking. All parameters
may be presumed identical. 16
3. a) Why series parallel control is favoured in case of train
movement with more than 1 motors. provide sketch
also. 8
b) A worn out shaft 10 cm diameter 25 cm long is to be
coated with two mm thick layer of nickel. Determine
the volume of electricity needed and time taken if
current density of 160 amp./sq. meter is adopted.
presume current efficiency of 90%. Density of
Nickel may be taken as 8.9 gm/cc. ECE of Nickel is
0.3043 mgm/coulomb.
-3 -
4. a) Develop the equivalent diagram of a single phase
induction motor with the help of double revolving
field theory. 4
b) What do you understand by co-efficient of Adhesion
and tractive effort. 4
c) Normally how power is fed to the driving motors of
a train and why higher voltage is preferred? 4
d) In case of series motor while rehostatic brakings are
used do you need any precaution? discuss. 4
PART - II
ans ques. No.5 and 2 from the rest
5. a) discuss the basis of design of feeder and
distributor. 4
b) A uniformly loaded distributor is fed at the centre.
Show that maximum voltage drop = where I is
the total current fed to the distributor and R is the
total resistance of the distributor. 6
c) A 800 metres 2-wire d.c. distributor AB fed from
both ends is uniformly loaded at the rate of 1.25 A/
metre run. compute the voltage at the feeding points
A and B if the minimum potential of 220V occurs at a
point C at a distance of 450 metres from the end A.
Resistance of every conductor is 0.05 ohm/km. 8
6. a) What factors are usually to be considered for selecting
a suitable site for a hydro-electric power station?
discuss the function of surge tank in such a plant. 8
[ Turn over ]
- four -
b) A hydro-electric power station is supplied from a
reservoir having an area 50Km2 and a head of 50
metre. If the overall efficiency of the plant is 60%,
obtain the rate at which the water level will fall when
the station is generating 30,000 kW. 8
7. a) discuss clearly the advantages of high transmission
voltage, with derivation of formula needed to
support your ans. explain its limitations also. 10
b) explain the advantages of overhead system which
supersedes the merit of underground system for
transmission and distribution of electric power. 6
8. a) explain the advantages of combined operation of
power plants. 10
b) A generating station has a maximum demand of
500 MW. The annual load factor is 50% and
capacity factor is 40%. obtain the reserve capacity of
the plant. 6
9. a) Write short notes on :
i) The core-less induction furnace.
ii) Dielectric heating.
b) A plywood board 0.5 × 0.25 × 0.02 metre is to be
heated from 25°C to 125°C in 10 minutes by
dielectric heating employing a frequency of 30 MHz.
Determine the power needed in this heating
process. presume specific heat of wood 1500J / Kg /
°C; weight of wood 600 Kg/m3 and efficiency of
process is 50%. 8

Earning:   Approval pending.