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← 2023 Exam Paper | 🏠 Index | (end)

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Even Semester Examination, 2024

Course No: ECE 2207
Course Title: Electrical Machine - I
Marks: 60
Time: 03 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) Enlist some practical applications of transformer. [Marks: 02, CO: 1]

(b) When a power transformer is excited as a manner shown in following figure, then describe the induced voltage phenomena — [Marks: 04, CO: 2]

Transformer Excitation and Induced Voltage Phenomena

Diagrammatic Details for Text-Only AI & Analytical Understanding

Magnetic Core: A rectangular closed ferromagnetic core consisting of two vertical limbs (left and right) linked by top and bottom yokes.
Mutual Flux ( $Φ_{mutual}$ ): A closed dashed rectangular path inside the core labeled " $Φ mutual$ " with four nodal points at the corners, illustrating the common magnetic flux that couples both windings.
Primary Circuit (Left Limb — "coil1"):
- Wound with $N_{1}$ turns around the left core leg.
- Excited by an electrical source $v$ (illustrated with DC battery polarity: $+$ terminal at the top, $-$ terminal at the bottom) via an inline single-pole switch on the upper conductor.
- The instantaneous primary current is denoted as $i_{1}$ , directed into the upper winding terminal ( $\to i_{1}$ ).
- The induced counter-electromotive force in coil 1 is designated as $e_{1}$ , indicated by an upward vertical arrow ( $↑ e_{1}$ ) across the coil.
Secondary Circuit (Right Limb — "coil2"):
- Wound with $N_{2}$ turns around the right core leg.
- The terminals connect across a passive resistive load element ( $R$ ).
- The induced secondary electromotive force is designated as $e_{2}$ , indicated by an upward vertical arrow ( $↑ e_{2}$ ) across the coil.
- The secondary current $i_{2}$ flows out from the upper terminal into the resistive load as indicated by an arrow ( $\to i_{2}$ ).
Phenomena Addressed:
- Time-varying flux establishment upon closing the switch ( $v \to i_{1} (t) \to Φ (t)$ ).
- Self-induction in coil 1 according to Faraday's Law and Lenz's Law: $e_{1} = - N_{1} \frac{d Φ}{d t}$ .
- Mutual induction in coil 2 linking the mutual flux: $e_{2} = - N_{2} \frac{d Φ}{d t}$ .
- Direction of secondary induced emf, current $i_{2}$ , and the secondary reaction flux opposing the rate of change of core flux.

(c) A 2,200/200 — V transformer draws a no-load primary current of 0.6A and absorbs 400W. Find the magnetizing and iron loss currents.
Now, consider a 2,200/250 — V transformer takes 0.5A at a p.f. of 0.3 on open circuit. Find magnetizing and working components of no-load primary current. [Marks: 04, CO: 2]

Question 2

(a) "The magnetizing current of power transformer is not fully sinusoidal" — justify it. [Marks: 02, CO: 1]

(b) Draw and explain the phasor diagram of a power transformer, when the transformer is loaded with "R-L" load. [Marks: 04, CO: 1]

(c) A 50 KVA, 2200/110 V transformer when tested gave the following results: [Marks: 04, CO: 2]

O.C. test (L. V. side): 400W, 10A, 110V
S. C. test (H. V. side): 808W, 20.5A, 90V

Compute all the parameters of the equivalent circuit referred to the H. V. side and draw the resultant circuit.

Question 3

(a) Define voltage regulation of transformer. [Marks: 02, CO: 1]

(b) Explain with the help of vector diagram, how three 1- $φ$ transformers can be used to design a 3- $φ$ transformer. [Marks: 04, CO: 2]

(c) In no-load test of single-phase transformer, the following test data were obtained: [Marks: 04, CO: 2]

Resistance of primary winding $= 0.6 Ω$ ;
Primary voltage: 220 V;
Secondary voltage: 110 V;
Primary current: 0.5 A;
Power input: 30 W

Find the followings:

i) The turn ratio
ii) The magnetizing component of no-load current
iii) Its working (or loss) component
iv) The iron loss

Question 4

(a) In performing the short circuit test of a transformers, HV side is usually short circuited — explain it. [Marks: 02, CO: 2]

(b) Is it possible to maintain 3- $φ$ power supply when one phase of a 3- $φ$ transformer is burned out? If 'Yes', justify the answer. If 'Not', then also justify your answer. [Marks: 04, CO: 1]

(c) Two T-connected transformers are used to supply a 440 V, 33KVA balanced load from a balanced 3- $φ$ supply of 3300 V. Calculate — [Marks: 04, CO: 2]

i) voltage and current rating of each coil.
ii) KVA rating of the main and teaser transformer.

SECTION - B

Question 5

(a) Explain how a rotating field is produced when a balanced 3- $φ$ induction motor is connected to a balanced 3- $φ$ supply. [Marks: 03, CO: 3]

(b) Define slip. Prove that an induction motor cannot run at synchronous speed. [Marks: 03, CO: 3]

(c) A 12 pole, 3- $φ$ alternator driven at a speed of 500 r.p.m supplies power to an 8-pole, 3- $φ$ induction motor. If the slip of the motor, at full load is 3%, calculate the full-load speed of the motor. [Marks: 04, CO: 3]

Question 6

(a) Define starting and running torque of an induction motor (IM). [Marks: 02, CO: 1]

(b) Following step by step process, draw and describe the vector diagram of a 1- $φ$ IM. [Marks: 04, CO: 1]

(c) A 3- $φ$ , slip-ring IM with star-connected rotor has an induced emf of 120 volts between slip-rings at standstill with normal voltage applied to the stator. The rotor winding has a resistance per phase of $0.3 Ω$ and stand-still leakage reactance per phase of $1.5 Ω$ . Determine — [Marks: 04, CO: 2]

i) Rotor current/phase when running short circuited with 4% slip and
ii) The slip and rotor current per phase when the rotor is developing maximum torque.

Question 7

(a) Enlist the test's name used for determining circuit model parameters of an IM. [Marks: 02, CO: 1]

(b) Prove that, [Marks: 04, CO: 1] $T_{m a x} = \frac{3}{2 π N _{s}} \frac{E _{2}^{2}}{2 X _{2}} N-m$ Variables having their usual meanings.

(c) A 50 Hz, 8 pole induction motor has F.L. slip of 4%. The rotor resistance/phase $= 0.01 Ω$ and stand still reactance/phase $= 0.1 Ω$ . Find the ratio of maximum to full-load torque and the speed at which the maximum torque occurs. [Marks: 04, CO: 3]

Question 8

(a) Draw the torque ~ slip of 3- $φ$ induction machine. [Marks: 02, CO: 1]

(b) Describe the process following which an IM can be operated as IG. [Marks: 04, CO: 2]

(c) A 440V, 4-pole, 1470 rpm, 30-kW, 3- $φ$ IM is to be used as IG, the rated current of the motor is 40A and full-load power factor is 85%. Now, calculate — [Marks: 04, CO: 2]

i) Capacitance required per phase if capacitors are connected in delta.
ii) Speed of the driving engine for generating a frequency of 50Hz.

← 2023 Exam Paper | 🏠 Index | (end)

PrevYearQuestions/2024.md

7.4 KB · commit d2f3b6e

← 2023 Exam Paper | 🏠 Index | (end)

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Even Semester Examination, 2024

Course No: ECE 2207
Course Title: Electrical Machine - I
Marks: 60
Time: 03 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) Enlist some practical applications of transformer. [Marks: 02, CO: 1]

(b) When a power transformer is excited as a manner shown in following figure, then describe the induced voltage phenomena — [Marks: 04, CO: 2]

Transformer Excitation and Induced Voltage Phenomena

Diagrammatic Details for Text-Only AI & Analytical Understanding

Magnetic Core: A rectangular closed ferromagnetic core consisting of two vertical limbs (left and right) linked by top and bottom yokes.
Mutual Flux ( $Φ_{mutual}$ ): A closed dashed rectangular path inside the core labeled " $Φ mutual$ " with four nodal points at the corners, illustrating the common magnetic flux that couples both windings.
Primary Circuit (Left Limb — "coil1"):
- Wound with $N_{1}$ turns around the left core leg.
- Excited by an electrical source $v$ (illustrated with DC battery polarity: $+$ terminal at the top, $-$ terminal at the bottom) via an inline single-pole switch on the upper conductor.
- The instantaneous primary current is denoted as $i_{1}$ , directed into the upper winding terminal ( $\to i_{1}$ ).
- The induced counter-electromotive force in coil 1 is designated as $e_{1}$ , indicated by an upward vertical arrow ( $↑ e_{1}$ ) across the coil.
Secondary Circuit (Right Limb — "coil2"):
- Wound with $N_{2}$ turns around the right core leg.
- The terminals connect across a passive resistive load element ( $R$ ).
- The induced secondary electromotive force is designated as $e_{2}$ , indicated by an upward vertical arrow ( $↑ e_{2}$ ) across the coil.
- The secondary current $i_{2}$ flows out from the upper terminal into the resistive load as indicated by an arrow ( $\to i_{2}$ ).
Phenomena Addressed:
- Time-varying flux establishment upon closing the switch ( $v \to i_{1} (t) \to Φ (t)$ ).
- Self-induction in coil 1 according to Faraday's Law and Lenz's Law: $e_{1} = - N_{1} \frac{d Φ}{d t}$ .
- Mutual induction in coil 2 linking the mutual flux: $e_{2} = - N_{2} \frac{d Φ}{d t}$ .
- Direction of secondary induced emf, current $i_{2}$ , and the secondary reaction flux opposing the rate of change of core flux.

Question 2

(a) "The magnetizing current of power transformer is not fully sinusoidal" — justify it. [Marks: 02, CO: 1]

(b) Draw and explain the phasor diagram of a power transformer, when the transformer is loaded with "R-L" load. [Marks: 04, CO: 1]

(c) A 50 KVA, 2200/110 V transformer when tested gave the following results: [Marks: 04, CO: 2]

O.C. test (L. V. side): 400W, 10A, 110V
S. C. test (H. V. side): 808W, 20.5A, 90V

Compute all the parameters of the equivalent circuit referred to the H. V. side and draw the resultant circuit.

Question 3

(a) Define voltage regulation of transformer. [Marks: 02, CO: 1]

(b) Explain with the help of vector diagram, how three 1- $φ$ transformers can be used to design a 3- $φ$ transformer. [Marks: 04, CO: 2]

(c) In no-load test of single-phase transformer, the following test data were obtained: [Marks: 04, CO: 2]

Resistance of primary winding $= 0.6 Ω$ ;
Primary voltage: 220 V;
Secondary voltage: 110 V;
Primary current: 0.5 A;
Power input: 30 W

Find the followings:

i) The turn ratio
ii) The magnetizing component of no-load current
iii) Its working (or loss) component
iv) The iron loss

Question 4

(a) In performing the short circuit test of a transformers, HV side is usually short circuited — explain it. [Marks: 02, CO: 2]

(c) Two T-connected transformers are used to supply a 440 V, 33KVA balanced load from a balanced 3- $φ$ supply of 3300 V. Calculate — [Marks: 04, CO: 2]

i) voltage and current rating of each coil.
ii) KVA rating of the main and teaser transformer.

SECTION - B

Question 5

(a) Explain how a rotating field is produced when a balanced 3- $φ$ induction motor is connected to a balanced 3- $φ$ supply. [Marks: 03, CO: 3]

(b) Define slip. Prove that an induction motor cannot run at synchronous speed. [Marks: 03, CO: 3]

Question 6

(a) Define starting and running torque of an induction motor (IM). [Marks: 02, CO: 1]

(b) Following step by step process, draw and describe the vector diagram of a 1- $φ$ IM. [Marks: 04, CO: 1]

i) Rotor current/phase when running short circuited with 4% slip and
ii) The slip and rotor current per phase when the rotor is developing maximum torque.

Question 7

(a) Enlist the test's name used for determining circuit model parameters of an IM. [Marks: 02, CO: 1]

(b) Prove that, [Marks: 04, CO: 1] $T_{m a x} = \frac{3}{2 π N _{s}} \frac{E _{2}^{2}}{2 X _{2}} N-m$ Variables having their usual meanings.

Question 8

(a) Draw the torque ~ slip of 3- $φ$ induction machine. [Marks: 02, CO: 1]

(b) Describe the process following which an IM can be operated as IG. [Marks: 04, CO: 2]

(c) A 440V, 4-pole, 1470 rpm, 30-kW, 3- $φ$ IM is to be used as IG, the rated current of the motor is 40A and full-load power factor is 85%. Now, calculate — [Marks: 04, CO: 2]

i) Capacitance required per phase if capacitors are connected in delta.
ii) Speed of the driving engine for generating a frequency of 50Hz.

← 2023 Exam Paper | 🏠 Index | (end)

azazarfin60/Electrical_Machine_I_Semester_Final

PrevYearQuestions/2024.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Even Semester Examination, 2024

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/2024.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Even Semester Examination, 2024

SECTION - A

Question 1

Question 2

Question 3

Question 4

SECTION - B

Question 5

Question 6

Question 7

Question 8

Report a problem