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Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination 2017

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

N.B.

Answer any 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) Why induction motor is called a rotating transformer? [03]

(b) Show how a uniformly rotating magnetic flux of constant value is produced in the stationary coils of 3- $Φ$ induction motor? [04]

(c) The rotor of a 6-pole, 50 Hz induction motor is rotated by some means at 1000 r.p.m. Compute: [04]

i) Rotor voltage
ii) Rotor frequency
iii) Rotor slip &
iv) Torque developed.

Can the rotor rotate at this speed by itself?

(d) What will happen if the slip of a 3- $Φ$ induction motor becomes negative? [01]

Question 2

(a) Define: [02]

i) Plugging
ii) Pull out torque of induction motor.

(b) For full load and maximum torque show: [03] $\frac{T _{f}}{T _{m a x}} = \frac{2 a S _{f}}{a ^{2} + S _{f}^{2}}$ Where the symbols have their usual meaning.

(c) Mention the names of speed control methods and discuss any one of them. [04]

(d) A three phase, 50 Hz, 8 pole induction motor has full load slip of 2%. The rotor resistance and stand still rotor reactance per phase are $0.001 Ω$ and $0.005 Ω$ respectively. Find: [03]

(i) The ratio of the maximum to full load torque and
(ii) The speed at which the maximum torque occurs.

Question 3

(a) Draw the step by step equivalent circuit diagram of 3- $Φ$ induction motor. [03]

(b) A 415V, 29.84 KW, 50 Hz delta connected motor gave the following test data: [09]

No load test: 415V, 21A, 1250W
Locked rotor test: 100V, 45A, 2730W

Construct the circle diagram and determine:

(i) The line current and power factor for rated output
(ii) The maximum torque. Assume stator and rotor cu losses are equal at standstill.

Question 4

(a) Briefly explain why the single phase induction motor inherently produces no starting torque. [04]

(b) Briefly explain why the permanent-split capacitor motor runs more quietly than the capacitor-start motor. [04]

(c) Explain how an auxiliary winding provides a starting torque for single phase induction motors. [04]

SECTION - B

Question 5

(a) What is transformer? Write down its advantages. [04]

(b) Explain the operating principle of an ideal transformer. [04]

(c) In a 25 KVA, 2000/200V, single phase transformer, the iron and full load cu losses are 350W and 400W respectively. Calculate the efficiency at unity power factor and 0.8 lagging p.f. on: [04]

(i) full load
(ii) half load.

Question 6

(a) Draw the full load phasor diagram of a single phase transformer. [03]

(b) Write short notes on: [04]

(i) Hysteresis loss
(ii) Eddy current loss

(c) A 2300/208 V, 500 KVA, 50 Hz transformer has the following O.C. and S.C. test data: [05]

O.C. test: 208V, 85A, 1800W (L.V. side used)
S.C. test: 95V, 217.4A, 8200W (H.V. side used)

Find the:

(i) Equivalent resistance referred to the low voltage side ( $R_{02}$ ) &
(ii) Other equivalent transformer parameters.

Question 7

(a) Explain why the open $Δ$ transformer connection is limited to supplying 57.7 percent of a normal $Δ$ - $Δ$ transformer bank. [04]

(b) Write down the conditions of parallel operations of 3- $Φ$ transformers. [04]

(c) A distribution transformer is rated at 18 kVA, 20000/480V and 60 Hz. Can this transformer safely supply 15 KVA to 415 load at 50 Hz? (Note: printed as "415 load" without "V" in original paper). Why or why not? [04]

Question 8

(a) What is Instrument transformer? Explain the operation of Potential Transformer in brief. [03]

(b) What happens to a transformer when it is first connected to a power line? (Note: printed as "corrected" in the original paper). Can anything be done to mitigate this problem? [04]

(c) The equivalent circuit for a 200/400V step-up transformer has the following parameters referred to the low voltage side: [05]

Equivalent resistance $= 0.15 Ω$
Equivalent reactance $= 0.37 Ω$
Core loss component resistance $= 600 Ω$
Magnetizing reactance $= 300 Ω$

When transformer is supplying a load at 10A at a power factor of 0.8 lag, calculate:

(i) The primary current and
(ii) Secondary terminal voltage.

(start) | 🏠 Index | 2018 Exam Paper →

PrevYearQuestions/2017.md

4.9 KB · commit d2f3b6e

(start) | 🏠 Index | 2018 Exam Paper →

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination 2017

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

N.B.

Answer any 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) Why induction motor is called a rotating transformer? [03]

(b) Show how a uniformly rotating magnetic flux of constant value is produced in the stationary coils of 3- $Φ$ induction motor? [04]

(c) The rotor of a 6-pole, 50 Hz induction motor is rotated by some means at 1000 r.p.m. Compute: [04]

i) Rotor voltage
ii) Rotor frequency
iii) Rotor slip &
iv) Torque developed.

Can the rotor rotate at this speed by itself?

(d) What will happen if the slip of a 3- $Φ$ induction motor becomes negative? [01]

Question 2

(a) Define: [02]

i) Plugging
ii) Pull out torque of induction motor.

(b) For full load and maximum torque show: [03] $\frac{T _{f}}{T _{m a x}} = \frac{2 a S _{f}}{a ^{2} + S _{f}^{2}}$ Where the symbols have their usual meaning.

(c) Mention the names of speed control methods and discuss any one of them. [04]

(i) The ratio of the maximum to full load torque and
(ii) The speed at which the maximum torque occurs.

Question 3

(a) Draw the step by step equivalent circuit diagram of 3- $Φ$ induction motor. [03]

(b) A 415V, 29.84 KW, 50 Hz delta connected motor gave the following test data: [09]

No load test: 415V, 21A, 1250W
Locked rotor test: 100V, 45A, 2730W

Construct the circle diagram and determine:

(i) The line current and power factor for rated output
(ii) The maximum torque. Assume stator and rotor cu losses are equal at standstill.

Question 4

(a) Briefly explain why the single phase induction motor inherently produces no starting torque. [04]

(b) Briefly explain why the permanent-split capacitor motor runs more quietly than the capacitor-start motor. [04]

(c) Explain how an auxiliary winding provides a starting torque for single phase induction motors. [04]

SECTION - B

Question 5

(a) What is transformer? Write down its advantages. [04]

(b) Explain the operating principle of an ideal transformer. [04]

(i) full load
(ii) half load.

Question 6

(a) Draw the full load phasor diagram of a single phase transformer. [03]

(b) Write short notes on: [04]

(i) Hysteresis loss
(ii) Eddy current loss

(c) A 2300/208 V, 500 KVA, 50 Hz transformer has the following O.C. and S.C. test data: [05]

O.C. test: 208V, 85A, 1800W (L.V. side used)
S.C. test: 95V, 217.4A, 8200W (H.V. side used)

Find the:

(i) Equivalent resistance referred to the low voltage side ( $R_{02}$ ) &
(ii) Other equivalent transformer parameters.

Question 7

(a) Explain why the open $Δ$ transformer connection is limited to supplying 57.7 percent of a normal $Δ$ - $Δ$ transformer bank. [04]

(b) Write down the conditions of parallel operations of 3- $Φ$ transformers. [04]

Question 8

(a) What is Instrument transformer? Explain the operation of Potential Transformer in brief. [03]

(b) What happens to a transformer when it is first connected to a power line? (Note: printed as "corrected" in the original paper). Can anything be done to mitigate this problem? [04]

(c) The equivalent circuit for a 200/400V step-up transformer has the following parameters referred to the low voltage side: [05]

Equivalent resistance $= 0.15 Ω$
Equivalent reactance $= 0.37 Ω$
Core loss component resistance $= 600 Ω$
Magnetizing reactance $= 300 Ω$

When transformer is supplying a load at 10A at a power factor of 0.8 lag, calculate:

(i) The primary current and
(ii) Secondary terminal voltage.

(start) | 🏠 Index | 2018 Exam Paper →

azazarfin60/Electrical_Machine_I_Semester_Final

PrevYearQuestions/2017.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination 2017

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/2017.md

Rajshahi University of Engineering & Technology

Department of Electrical & Computer Engineering

B.Sc. Engineering 2nd Year Odd Semester Examination 2017

SECTION - A

Question 1

Question 2

Question 3

Question 4

SECTION - B

Question 5

Question 6

Question 7

Question 8

Report a problem