AMIETE – ET (NEW SCHEME) – Code: AE55
Subject: PRINCIPLES OF ELECTRICAL ENGINEERING
Time: 3
Hours
Max. Marks: 100
NOTE: There are 9 Questions in all.
· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.
· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.
· Any required data not explicitly given, may be suitably assumed and stated.
Q.1 Choose
the correct or the best alternative in the following: (2
10)
a. The magnetic intensity H of a 200 turn 80 cm coil carrying 1.5 A current is:
(A)
375
(B)
0.6
(C)
240 (D)
107
b.
The secondary impedance 
transforms in an ideal transformer
with 
to
the primary side as:
(A) 
(B)
(C) 
(D)
c. With a variable voltage dc source available, speed control of a dc shunt motor needs no starting resistance if ideally we have:
(A) zero field current
(B) starting current of 100 times the rated current
(C) low-voltage starting
(D) field excited at start with half the rated voltage
d. A variable condenser or inductor may be obtained by varying the excitation of an unloaded
(A) synchronous motor (B) induction motor
(C) dc motor (D) hysteresis motor
e. The slip s of a 3-phase 50 Hz 4-pole induction motor running at 1440 rpm is
(A) 0.417 (B) 0.014
(C) 0.04 (D) 0.139
f. If the rotor of a single-phase induction motor running at N rpm has a slip s w.r.t. forward field, then the rotor slip at N rpm w.r.t. backward field is:
(A) 1-s (B) 2-s
(C) 
(D)
g. To have an efficiency comparable to that of a steam turbine, high temperature and pressure are generated in a:
(A) hydraulic turbine (B) coal fired plant
(C) oil fired plant (D) combined cycle gas turbine
h. Although the gas fuel combustion efficiency is doubled, a disadvantage of the compressed air storage scheme is that much of the energy used in compressing air appears in the form of:
(A) heat that gets wasted (B) Wasteful air volume
(C) High pressure developed (D) Avoidable energy for air retrieval
i. The self-inductance of a 1000 turn coil carrying 5A current to establish a flux of 0.3 mWb is:
(A) 300 mH (B) 150 mH
(C) 15 mH (D) 60 mH
j. The speed of a 4 pole 888 wave connected conductor dc shunt motor with armature voltage of 229 V and no-load flux of 5.41 mWb is:
(A) 1430 rpm (B) 1400 rpm
(C) 1500 rpm (D) 1374 rpm
Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.
Q.2 a. Derive an expression for the magnetic energy W stored in a coil of inductance L Henry due to a current i Ampere flowing through it. (4)
b. Find
the value of relative permeability of iron given that a 350 turn coil wound on
an iron ring of mean diameter of 12 cm establishes a magnetic flux density of
1.4 
when
2A current is passed through the coil. Take 
. (6)
c. Calculate the
magnitude of the dynamically induced emf when a 100 cm conductor moves at (i)
right angles, (ii) 
, to a uniform magnetic field of flux-density
1.5 with
a velocity of 30 m/s. (6)
Q.3 a. Define voltage regulation and efficiency of a transformer. (4)
b. Derive the condition for maximum efficiency at any fraction n of full-load of a transformer in terms of its losses. (6)
c. Equivalent
resistance and reactance referred to the H V side of a 10 kVA, 50 Hz,
2300/230 V transformer are 
and 
respectively.
(i) Find the leading pf for zero voltage regulation.
(ii) The transformer delivers rated kVA at 0.8 pf lagging to a load on the LV side. Find the HV-side voltage necessary to maintain 230 V across load terminals. Also find the percentage voltage regulation. (2+4)
Q.4 a. Derive the emf equation of a dc machine using usual notations. Deduce the torque equation from the emf equation. (6+2)
b. A
wave wound 6-pole dc generator has 36 slots, 2 coil-sides/slot, 8 turns/coil.
The pole-shoe is 18 cm long and the mean air-gap diameter is 25 cm. The
average flux density over one pole pitch is 0.8
Find the induced emf, power and
torque developed when the machine is operating as a motor at 1200 rpm with an
armature input current of 10 A. (3+2+3)
Q.5 a. With reference to the circuit model and power-angle characteristic of a synchronous machine, explain the conditions for generating and motoring modes of operation. (8)
b. A 3-phase synchronous machine at a terminal voltage of 12.5 kV and having synchronous reactance of is run as a generator to deliver 10 MW to the bus-bars, while it is overexcited by 20%. Find the power angle current, pf and MVAR delivered by the machine. (8)
Q.6 a. Sketch the torque-slip characteristics of an induction motor at fixed terminal voltage. Indicate corresponding speeds, the various operating modes w.r.t slip s, Tstart and full-load operating point. (5)
b. Prove
that in a 3-phase induction motor the ratio of maximum to starting torque is
, where K is the
ratio of rotor resistance to rotor reactance. Neglect stator impedance. (5)
c. The efficiency of a 3-phase 400 V 50 Hz 6 pole induction motor drawing a line current of 80 A at 0.75 pf with 4% slip is 85%. Calculate the output power and torque. (6)
Q.7 a. Explain the working principle of a two-value capacitor motor. (5)
b. How is starting torque produced in a shaded-pole motor? How is reversal of direction of rotation achieved? (3+2)
c. What happens when a dc series motor is excited by ac, as is done in a universal motor? (6)
Q.8 a. Explain how biofuels can be used to produce electricity. (4)
b. Explain the principle of direct conversion of sunlight into electricity. (6)
c. Describe briefly how electric energy is directly generated using Magneto Hydro Dynamics (MHD) technology. (6)
Q.9 a. What is the purpose of interconnection or grid formation in a power system? (4)
b. Explain the hydrogen energy system. (4)
c. List the advantages and disadvantages of HVDC transmission system. (8)