বৈজ্ঞানিক যন্ত্রপাতির নাম ও তার ব্যবহার...

DC MACHINES AND TRANSFORMERS ( SET - 2)

Max. Marks: 100                                                                                                     Duration: 3 Hours        PART A

Answer all questions.

1. Compare lap and wave windings used for DC machine armature                    (5)
2. Draw the magnetization characteristic of self excited DC shunt generator and explain.        (5)
3. How back emf is generated in a DC motor? Explain its significance.                  (5)
4. Draw and explain the detailed phasor diagram of a practical transformer supplying
lagging power factor load.                                                                   (5)
5. Define all-day efficiency of a transformer. What is done to improve the all-day
efficiency of a distribution transformer?                            (5)
6. List out and explain the necessary and desirable conditions for parallel operation of
transformers.                                                            (5)
7. Enumerate the purposes which dictate the use of tertiary winding in a three winding
transformer. (5)
8. Draw the diagram for V-V connection of transformers and explain the voltage and
current relations of line and phase values. Derive the capacity ratio as a fraction of
Δ-Δ capacity. (5)

PART B

Answer any 2 questions.

9. a. Derive the electro-dynamic equation of rotating electrical machines and explain the
principle of energy conversion. (5)
b. Draw the developed winding layout of a lap connected simplex double layer DC
armature with 16 slots and 4 poles. Furnish the winding table and show connections to
4 equalizer rings. (5)
10. a. Derive the EMF equation of a DC generator, from first principles (5)
b. A shunt generator delivers 195 A at a terminal voltage of 250 V. The armature and
shunt field resistances are 0.02 Ω and 50 Ω respectively. The iron and frictional losses
are equal to 950 W. Find (i) emf generated (ii) Copper losses (iii) output of prime
mover (iv) commercial, mechanical and electrical efficiencies. (5)
11. a. What is armature reaction and explain its effects? Derive expressions for cross
magnetizing and demagnetizing ampere turns per pole (5)
b. A 4 pole, wave wound armature of a DC machine has 880 conductors and delivers
120 A. The brushes are displaced through 3 angular degrees from the geometrical
axis. Calculate (i) demagnetizing ampere turns per pole (ii) cross-magnetizing ampere
turns per pole and (iii) the additional field current for neutralizing the demagnetization
if the field winding has 110 turns per pole. (5)

PART C

Answer any 2 questions.

12. a. Draw and explain the electrical and mechanical characteristics of DC shunt motors. (5)
b. A 250 V DC shunt motor has an armature resistance of 0.5 Ω and a field resistance
of 250 Ω. The motor draws 21 A when driving a constant torque load at 600 rpm.
What will be the new speed of the motor if an additional 250 Ω resistance is inserted
in the field circuit? (5)
13. a. Enumerate the losses in a loaded transformer. Derive the condition for maximum
efficiency in a transformer. (5)
b. A 200 / 2000 V transformer is fed from a 200 V supply. The total winding
resistance and leakage reactance as referred to the LV side is 0.15 Ω and 0.6 Ω
respectively. The resistance representing core loss is 450 Ω and magnetizing
reactance is 250 Ω. A load of impedance (600+j400) Ω is connected across the
secondary terminals. Calculate (i) input current (ii) secondary terminal voltage and
(iii) primary power factor. (5)
14. a. With neat diagrams, explain the speed control methods in separately excited DC
motors. (5)
b. The efficiency of a 200 KVA, single phase transformer is 98.75% when delivering
full load at 0.8 pf and 99% at 80% of full load at 0.9 pf. Calculate (i) the iron loss and
(ii) the full load copper loss. (5)

PART D

Answer any 2 questions.

15. a. With a neat diagram, describe the Sumpner’s method of testing transformers. How
can the voltage regulation be predetermined using this test? (5)
b. A 3 ϕ step down transformer is connected to 6.6 kV supply mains and takes 80A.
Calculate its secondary line voltage and line current for the following connections if
the ratio of turns per phase is 16 (i) Y-Y (ii) Y- Δ (iii) Δ – Y (iv) Δ – Δ. (5)
16. a. Derive expression for saving in copper effected by using an autotransformer instead
of a two winding transformer. (5)
b. A load of 6 kW is supplied by an autotransformer at 120 V and upf. If the primary
voltage is 240 V, determine (i) transformation ratio (ii) secondary current (iii) primary
current (iv) number of secondary turns if the total number of turns is 280 (v) power
transformed and (vi) power conducted directly from supply mains to load. (5)
17. a. Explain with neat circuit diagram and phasors, how a 2-phase supply can be
obtained from a 3-phase supply. (5)
b. Explain the vector groupings Yy0, Dd0, Yd1, Dy1, Yd11 and Dy11 in three phase
transformers. (5)

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