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: (210)

a. The magnitude of emf induced in a coil of 30 mH self-inductance when the coil current increase from 0 to 8A in 0.06s is __________.

(A) 4V (B) 6V

(C) 8V (D) 2V

b. The coefficient of coupling between two identical 60 coils having 0.48 mutual inductance is _________.

(A) 0.288 (B) 0.054

(C) 0.012 (D) 0.008

c. The circuit model of a transformer in its simplest form is represented by:

(A) (B)

(C) (D)

d. A 40 kVA transformer has an iron loss of 450W and full-load copper loss of 800W at 0.8 p.f. Its efficiency will be __________.

(A) 96% (B) 98%

(C) 86% (D) 88%

e. When a dc machine carries current, it causes its own mmf distribution called:

(A) compole (B) interpole

(C) armature reaction (D) commutation

f. In the phasor diagram of a synchronous machine shown in Fig.1, the value of is ___________.

(A) 2798.4A,

(B) 2798.4A, -

(C) 1031.3A, -

(D) 1031.3A,

g. A 50 Hz, 1000 rpm alternator drives an induction motor producing a frequency of rotor induced emf of 2 Hz. The speed of the induction motor is _________.

(A) 1000 rpm (B) 860 rpm

(C) 900 rpm (D) 960 rpm

h. Vending and photocopier machines normally prefer to use ___________.

(A) shaded-pole motor (B) reluctance motor

(C) split-phase motor (D) hysteresis motor

i. The solar constant, as the radiant flux density incident on a plane normal to sun’s rays, has a value of __________.

(A) 1867 (B) 1

(C) 365 (D) 1367

j. The major disadvantage of hydrogen energy system refers to hydrogen _______.

(A) production (B) storage

(C) transmission (D) calorific value

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2 a. Draw the hysteresis loop for a ferromagnetic material undergoing cyclic variations of flux density. Explain the importance of its various regions. (8)

b. A ring of magnetic material has rectangular cross-section of 5, mean length of 70.6 cm, and thickness of 2 cm. An air-gap of 1 mm length is cut across the ring. The ring is wound with 500 turns and is carrying a current of 2A. The permeability of the magnetic material is 6000. Find the flux-density in the air-gap and the inductance of the coil taking. (8)

Q.3 a. Write the characteristics of an ideal transformer? If its primary and secondary circuits are linked through the mutual flux, show that the maximum flux is determined by the applied voltage and frequency and is independent of current. (4+4)

b. A 40 kVA single-phase transformer has 500 turns on the primary and 100 turns on the secondary winding. The primary is connected to 50 Hz, 3000V AC supply. Determine (i) the secondary voltage on open circuit (ii) current flowing through the two windings at full load (iii) Maximum value of flux. (2+4+2)

Q.4 a. Derive the condition for maximum efficiency of a dc motor using the power-flow diagram as depicted given in Fig.2. (All the representation have their conventional meaning). (8)

b. A dc shunt motor with an armature resistance of draws a full-load current of 40A. If this motor is supplied directly with 400 V, calculate the starting current drawn by the armature. How this current be reduced to a tolerable limit of 8A? (4+4)

Q.5      a. Draw the combined phasor diagram at unity p.f. and the V-curves to show synchronous machine operation for constant-load variable-excitation. (3+5)

b. A 3-phase synchronous generator feeds into a 22 kV grid. It has a synchronous reactance of /phase and is delivering 12 MW and 6 MVAR into the system. Determine:

(i) The phase-angle of the current,

(ii) The power (torque) angle,

(iii) The generated emf. (2+4+2)

Q.6 a. Explain the methods to control the speed of 3- induction motor. Explain how the direction of rotation of a 3-phase induction motor be reversed? (5+3)

b. A 3-phase induction motor runs at a speed of 940 rpm at full-load when supplied from 50 Hz AC main. If the synchronous speed is 1000 rpm, calculate the no. of poles and % slip. Calculate the corresponding speed of the rotor field w.r.t.: (i) rotor surface & (ii) stator. What would the rotor speed be at twice full-load slip? (24)

Q.7 a. Explain the working principles of (i) reluctance motor (ii) hysteresis motor. (4+4)

b. With the help of a neat connection diagram, explain the functioning of an ac-excited dc series motor (universal motor). (8)

Q.8 a. What is the principle of HVDC Transmission? Write a note on Economics of High voltage DC transmission. (8)

b. How Electrical transmission system is different from the distribution system? What is the importance of electrical grid? (8)

Q.9 a. Explain the importance of renewable energy resources for power generation. Discuss a method to extract electrical energy through wind power. (4+4)

b. Write a short note on:

(i) Solar photovoltaics.

(ii) Magneto Hydro Dynamics (MHD) technique of power generation. (4+4)