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Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination, 2018

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

N.B.

Answer any SIX questions taking THREE from each section.

Figures in the margin indicate full marks.

The questions are of equal value.

Use separate answer script for each section.

The symbols used have as usual meaning.

SECTION - A

Question 1

(a) Classify transformer at a glance. [02]

(b) Describe the effect of frequency and flux in a transformer. [02]

(c) Draw the equivalent circuit of transformer with vector diagram for lagging power factor. [02]

(d) A single phase 50 Hz 20 KVA 11KV/230V transformer whose low voltage side is short circuited and voltage applied on high voltage side until rated current flows. Determine transformer constant referred to the HV winding if the following readings are obtained: [06] $V = 72 V, I = rated, W = 300 W$ Determine the copper loss and voltage regulation.

Question 2

(a) Define all day efficiency of a transformer. [01]

(b) A 10 KVA, 11 KV/230 V, 50 Hz transformer is tested on open and short circuit. If the following readings are obtained, find the half load and full load efficiency for unity power factor and 0.8 lagging power factor. [06]

O.C. test-HS Open: $V = 220 V, I = 1.5 A, W = 200 W$
S.C. test-LS Short: $V = 120 V, I = rated, W = 300 W$

(c) A 100 KVA X-mar was tested and found 200 W core loss and 500 W copper loss. If the transformer was running under the following conditions: 2 hr at five-fourths load; 6 hr at full load; 8 hr at half load; 4 hr at one-quarter load; 4 hr at no load, determine all-day efficiency. [05]

Question 3

(a) Describe about the four wire $Δ$ -connected transformer. [03]

(b) 10 MVA is supplied by an 11 KV supply line to a 230 V load through three transformers connected in Y- $Δ$ . Find the KVA rating of each transformer, the voltage across each coil and the current through each coil. [06]

(c) Prove that, open $Δ$ transformer $KVA = 0.577 \times closed- Δ transformer KVA$ . [03]

Question 4

(a) Explain Scott connections with the help of necessary diagrams. [04]

(b) Two T-connected transformers are used to supply a 440-V, 33-KVA balanced load from a balanced 3-phase supply of 3300 V. Calculate: [04]

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

(c) What are the advantages of transformer bank? Assuming a 10:1 turns ratio, what is the line voltage ratio of the following transformer connections: Y-Y, $Δ$ - $Δ$ , $Δ$ -Y, Y- $Δ$ and open $Δ$ in a table with the following column headings: connection, turns ratio, line voltage ratio. [04]

SECTION - B

Question 5

(a) Write short notes on: [03]

(i) Regenerative braking
(ii) Dynamic braking
(iii) Plugging

(b) Prove that, if some power $P_{2}$ is delivered to a rotor then a portion of $(1 - s) P_{2}$ appears as gross mechanical power in a 3-phase induction motor. Also describe the power stages. [05]

(c) A 440-V, 4-pole, 1470 rpm, 30 KW, 3-phase induction motor is to be used as an asynchronous generator. The rated current of the motor is 40 A and full load power factor is 85%. Calculate: [04]

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

Question 6

(a) What do you mean by single phasing? Explain the effect of single phasing of a 3-phase induction motor. [04]

(b) For 3-phase induction motor, prove that "the magnitude of the resultant flux is constant and is equal to 1.5 times of $Φ_{m}$ , the maximum flux due to either phase". [03]

(c) A 3-phase induction motor having a 6-pole, star connected stator winding runs on 240 V, 50 Hz supply. The rotor resistance and standstill reactance are $0.12 Ω$ and $0.85 Ω$ per phase. The ratio of stator to rotor turns is 1.8. Full load slip is 4%. Calculate the developed torque at full load, maximum torque and speed at maximum torque. [05]

Question 7

(a) Briefly discuss about star-delta starter of a 3-phase squirrel cage induction motor. [04]

(b) Draw the circle diagram for a 5.6 KW, 400 V, 3- $Φ$ , 4-pole, 50-Hz, slip ring induction motor from the following data: [08]

No load test: $400 V, 6 A, cos θ_{0} = 0.087$
Blocked rotor test: $100 V, 12 A, 720 W$

The ratio of primary to secondary turns $= 2.62$ , stator resistance per phase is $0.67 Ω$ and of the rotor is $0.185 Ω$ . Calculate:

(i) full load current,
(ii) full load slip,
(iii) full load power factor,
(iv) maximum power.

Question 8

(a) State and explain double field revolving theory. [04]

(b) Draw the phasor diagram of a resistor split-phase motor under maximum starting torque conditions. Hence show: [04] $r_{a} = (\frac{N _{a}}{N _{m}})^{2} (r_{m} + z_{m})$

(c) The following data pertains to a 230 V, 50 Hz, capacitor-start 1- $Φ$ induction motor at standstill: [04]

Main winding excited alone: $100 V, 2 A, 40 W$
Auxiliary winding excited alone: $80 V, 1 A, 50 W$

Determine the value of capacitance for obtaining maximum starting torque.

← 2017 Exam Paper | 🏠 Index | 2019 Exam Paper →

PrevYearQuestions/2018.md

5.9 KB · commit d2f3b6e

← 2017 Exam Paper | 🏠 Index | 2019 Exam Paper →

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination, 2018

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

N.B.

Answer any SIX questions taking THREE from each section.

Figures in the margin indicate full marks.

The questions are of equal value.

Use separate answer script for each section.

The symbols used have as usual meaning.

SECTION - A

Question 1

(a) Classify transformer at a glance. [02]

(b) Describe the effect of frequency and flux in a transformer. [02]

(c) Draw the equivalent circuit of transformer with vector diagram for lagging power factor. [02]

Question 2

(a) Define all day efficiency of a transformer. [01]

O.C. test-HS Open: $V = 220 V, I = 1.5 A, W = 200 W$
S.C. test-LS Short: $V = 120 V, I = rated, W = 300 W$

Question 3

(a) Describe about the four wire $Δ$ -connected transformer. [03]

(c) Prove that, open $Δ$ transformer $KVA = 0.577 \times closed- Δ transformer KVA$ . [03]

Question 4

(a) Explain Scott connections with the help of necessary diagrams. [04]

(b) Two T-connected transformers are used to supply a 440-V, 33-KVA balanced load from a balanced 3-phase supply of 3300 V. Calculate: [04]

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

SECTION - B

Question 5

(a) Write short notes on: [03]

(i) Regenerative braking
(ii) Dynamic braking
(iii) Plugging

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

Question 6

(a) What do you mean by single phasing? Explain the effect of single phasing of a 3-phase induction motor. [04]

(b) For 3-phase induction motor, prove that "the magnitude of the resultant flux is constant and is equal to 1.5 times of $Φ_{m}$ , the maximum flux due to either phase". [03]

Question 7

(a) Briefly discuss about star-delta starter of a 3-phase squirrel cage induction motor. [04]

(b) Draw the circle diagram for a 5.6 KW, 400 V, 3- $Φ$ , 4-pole, 50-Hz, slip ring induction motor from the following data: [08]

No load test: $400 V, 6 A, cos θ_{0} = 0.087$
Blocked rotor test: $100 V, 12 A, 720 W$

The ratio of primary to secondary turns $= 2.62$ , stator resistance per phase is $0.67 Ω$ and of the rotor is $0.185 Ω$ . Calculate:

(i) full load current,
(ii) full load slip,
(iii) full load power factor,
(iv) maximum power.

Question 8

(a) State and explain double field revolving theory. [04]

(b) Draw the phasor diagram of a resistor split-phase motor under maximum starting torque conditions. Hence show: [04] $r_{a} = (\frac{N _{a}}{N _{m}})^{2} (r_{m} + z_{m})$

(c) The following data pertains to a 230 V, 50 Hz, capacitor-start 1- $Φ$ induction motor at standstill: [04]

Main winding excited alone: $100 V, 2 A, 40 W$
Auxiliary winding excited alone: $80 V, 1 A, 50 W$

Determine the value of capacitance for obtaining maximum starting torque.

← 2017 Exam Paper | 🏠 Index | 2019 Exam Paper →

azazarfin60/Electrical_Machine_I_Semester_Final

PrevYearQuestions/2018.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination, 2018

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/2018.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination, 2018

SECTION - A

Question 1

Question 2

Question 3

Question 4

SECTION - B

Question 5

Question 6

Question 7

Question 8

Report a problem