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Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Examination, 2021

Course No: ECE 2107
Course Title: Electrical Machines-I
Full Marks: 72
Time: 3 Hours

N.B.

Answer SIX questions taking any THREE from each section.

Figures in the margin indicate full marks.

Use separate answer script for each section.

SECTION - A

Question 1

(a) What are the characteristics of an ideal transformer? [03]

(b) Derive the equation for the rms e.m.f induced in both primary and secondary winding. From this equation prove that e.m.f/ turn is the same in both primary and secondary windings. [05]

(c) A 25 kVA transformer has 500 turns on the primary and 50 turns on the secondary windings. The primary is connected to 3000 V, 50 Hz supply. Find the full load primary and secondary currents, the secondary e.m.f and the maximum flux in the core. (Leakage drops and no-load primary currents are negligible). [04]

Question 2

(a) Draw the phasor diagram of transformer by considering winding resistance and leakage reactance. [04]

(b) Explain the procedure of no-load test of a transformer. Why this test is performed? [04]

(c) It is desired to obtain the equivalent transformer constants of a 20 kVA, 2400/240 V, 50 Hz transformer. The low-voltage side is short circuited and voltage is varied on the high voltage side until rated current flows. Determine the transformer constants referred to the high voltage winding if the following readings are obtained: [04] $V = 72 V; W = 275 W; I = rated current .$ Also determine the regulation of the transformer at a lagging power factor of 0.80.

Question 3

(a) What are the limitations of a Y-Y connected transformer? How you can overcome these? [03]

(b) Is it possible to continue 3- $φ$ power transmission if one phase of the transformer is damaged? If your answer is "YES" then justify the answer by describing any one technique. [06]

(c) Draw the complete torque/speed curve of induction motor. [03]

Question 4

(a) What are the conditions for parallel operation of two 3- $φ$ transformers? [04]

(b) What are the methods that can be used for generating magnetic field? [04]

(c) The rotor of a 4-pole, 50 Hz slip ring induction motor has a resistance of $0.30 Ω/ phase$ and runs at 1440 rpm at full load. Calculate the external resistance/phase which must be added to lower the speed to 1320 rpm, the torque being the same as before. [04]

SECTION - B

Question 5

(a) For an induction motor (IM) define the followings: [03]

(i) synchronous speed
(ii) slip
(iii) slip speed

(b) Determine the starting torque of an IM. [04]

(c) Show that the 2- $φ$ supply produces rotating magnetic field having magnitude of the maximum flux due to either phase and in synchronous speed. [05]

Question 6

(a) What will happen to the torque and speed of an IM if the supply frequency is increased suddenly? [04]

(b) A 3- $φ$ IM is driving full load torque which is independent of speed. If line voltage drops to 90% of the rated value, find the increase in motor cu losses. [04]

(c) Determine the rotor efficiency of an IM. [04]

Question 7

(a) Explain the double field revolving theory of single phase IM. [04] (Note: mark omitted in original paper margin; deduced from question total of 12)

(b) How does a capacitor start and run single phase IM operate? [04]

(c) A 250-W, 230 V, 50 Hz, capacitor start motor has the following constants for the main and auxiliary windings: [04]

Main winding, $Z_{m} = (4.5 + j 3.7) Ω$
Auxiliary winding, $Z_{a} = (9.5 + j 3.5) Ω$ (Note: printed as $Z_{m} = (9.5 + j 3.5) Ω$ in the original paper)

Determine the value of starting capacitor that will place the main and auxiliary winding currents in quadrature at starting.

Question 8

(a) With an example define synchronous watt. [03]

(b) What information does the vector group of transformer bear? The winding configuration "Dyn5" represents what? [03]

(c) Draw the circle diagram from no-load and short circuit test of a 3- $φ$ , 14.92 KW, 400-V, 6-pole IM from the following test results (line values): [06]

No-load: 400-V, 11 A, pf = 0.2
Short-circuit: 100-V, 25 A, pf = 0.4

Rotor cu loss at standstill is half the total cu loss. From the diagram, find:

(i) line current, slip, efficiency and pf at full load
(ii) the maximum torque

← 2020 Exam Paper | 🏠 Index | 2023 Exam Paper →

PrevYearQuestions/2021.md

4.9 KB · commit d2f3b6e

← 2020 Exam Paper | 🏠 Index | 2023 Exam Paper →

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Examination, 2021

Course No: ECE 2107
Course Title: Electrical Machines-I
Full Marks: 72
Time: 3 Hours

N.B.

Answer SIX questions taking any THREE from each section.

Figures in the margin indicate full marks.

Use separate answer script for each section.

SECTION - A

Question 1

(a) What are the characteristics of an ideal transformer? [03]

(b) Derive the equation for the rms e.m.f induced in both primary and secondary winding. From this equation prove that e.m.f/ turn is the same in both primary and secondary windings. [05]

Question 2

(a) Draw the phasor diagram of transformer by considering winding resistance and leakage reactance. [04]

(b) Explain the procedure of no-load test of a transformer. Why this test is performed? [04]

Question 3

(a) What are the limitations of a Y-Y connected transformer? How you can overcome these? [03]

(b) Is it possible to continue 3- $φ$ power transmission if one phase of the transformer is damaged? If your answer is "YES" then justify the answer by describing any one technique. [06]

(c) Draw the complete torque/speed curve of induction motor. [03]

Question 4

(a) What are the conditions for parallel operation of two 3- $φ$ transformers? [04]

(b) What are the methods that can be used for generating magnetic field? [04]

SECTION - B

Question 5

(a) For an induction motor (IM) define the followings: [03]

(i) synchronous speed
(ii) slip
(iii) slip speed

(b) Determine the starting torque of an IM. [04]

(c) Show that the 2- $φ$ supply produces rotating magnetic field having magnitude of the maximum flux due to either phase and in synchronous speed. [05]

Question 6

(a) What will happen to the torque and speed of an IM if the supply frequency is increased suddenly? [04]

(b) A 3- $φ$ IM is driving full load torque which is independent of speed. If line voltage drops to 90% of the rated value, find the increase in motor cu losses. [04]

(c) Determine the rotor efficiency of an IM. [04]

Question 7

(a) Explain the double field revolving theory of single phase IM. [04] (Note: mark omitted in original paper margin; deduced from question total of 12)

(b) How does a capacitor start and run single phase IM operate? [04]

(c) A 250-W, 230 V, 50 Hz, capacitor start motor has the following constants for the main and auxiliary windings: [04]

Main winding, $Z_{m} = (4.5 + j 3.7) Ω$
Auxiliary winding, $Z_{a} = (9.5 + j 3.5) Ω$ (Note: printed as $Z_{m} = (9.5 + j 3.5) Ω$ in the original paper)

Determine the value of starting capacitor that will place the main and auxiliary winding currents in quadrature at starting.

Question 8

(a) With an example define synchronous watt. [03]

(b) What information does the vector group of transformer bear? The winding configuration "Dyn5" represents what? [03]

(c) Draw the circle diagram from no-load and short circuit test of a 3- $φ$ , 14.92 KW, 400-V, 6-pole IM from the following test results (line values): [06]

No-load: 400-V, 11 A, pf = 0.2
Short-circuit: 100-V, 25 A, pf = 0.4

Rotor cu loss at standstill is half the total cu loss. From the diagram, find:

(i) line current, slip, efficiency and pf at full load
(ii) the maximum torque

← 2020 Exam Paper | 🏠 Index | 2023 Exam Paper →

azazarfin60/Electrical_Machine_I_Semester_Final

PrevYearQuestions/2021.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Examination, 2021

SECTION - A

Question 1

Question 2

Question 3

Question 4

SECTION - B

Question 5

Question 6

Question 7

Question 8

Report a problem

azazarfin60/Electrical_Machine_I_Semester_Final

PrevYearQuestions/2021.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Examination, 2021

SECTION - A

Question 1

Question 2

Question 3

Question 4

SECTION - B

Question 5

Question 6

Question 7

Question 8

Report a problem