PrevYearQuestions/2023.md
5.3 KB · commit d2f3b6e
← 2021 Exam Paper | 🏠 Index | 2024 Exam Paper →
Rajshahi University of Engineering & Technology
Department of Electrical & Computer Engineering
B.Sc. Engineering 2nd Year Even Semester Examination, 2023
Course No: ECE 2207
Course Title: Electrical Machines I
Full Marks: 60
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.
- All the symbols have their usual meanings.
SECTION - A
Question 1
(a) Show that the rms value of the induced emf in the whole of primary winding is E1=4.44fN1BmA. [CO1, Marks: 03]
(b) Briefly describe the effect of variation of load on core flux and primary current of a transformer. [CO1, Marks: 03]
(c) Define voltage transformation ratio. A transformer has a primary winding of 800 turns and a secondary winding of 200 turns. When the load current on the secondary is 80 A at 0.8 P.F. lagging, the primary current is 25 A at 0.707 P.F. lagging. Determine the no-load current of the transformer and its phase with respect to the voltage. [CO3, Marks: 04]
Question 2
(a) Define power transformer. Prove that, the efficiency of a transformer will be maximum when copper loss is equal to iron loss. [CO2, Marks: 03]
(b) The corrected instrument readings obtained from open and short-circuit tests on 10-kVA, 450/120 V, 50-Hz transformer are: [CO3, Marks: 04]
- O.C. test: V1=120 V;I1=4.2 A;W1=80 W; (V1,I1, and W1 were read on the low voltage side).
- S.C. test: V1=9.65 V;I1=22.2 A;W1=120 W; (with low-voltage winding short circuited).
Compute:
- (i) Equivalent circuit constants,
- (ii) Efficiency and voltage regulation for an 80% lagging p.f. load.
(c) Prove that less copper is used in auto-transformer than in an ordinary transformer. [CO2, Marks: 03]
Question 3
(a) Is it possible to continue 3−φ power supply when one phase is burn out? If "yes" then explain one method. [CO1, Marks: 04]
(b) Define all day efficiency of a transformer. A 100-kVA lighting transformer has a full-load loss of 3 kW, the losses being equally divided between iron and copper. During a day, the transformer operates on full load for 3 hours, one-half load for 4 hours, the output being negligible for the remainder of the day. Calculate the all day efficiency. [CO3, Marks: 04]
(c) Mention the limitations of a Y-Y connected transformer. [CO1, Marks: 02]
Question 4
(a) Write down the conditions of parallel operation of two 3−φ transformers. [CO1, Marks: 03]
(b) Prove that closed-Δ kVA is 3 times higher than that open-Δ kVA. [CO1, Marks: 03]
(c) A 3−φ transformer, ratio 33/6.6 kV,Δ/Y 2-MVA has a primary resistance of 8 Ω per phase and a secondary resistance of 0.08 Ω per phase. The percentage impedance is 7%. Calculate the secondary voltage with rated primary voltage for full-load 0.75 p.f lagging conditions. [CO1, Marks: 04]
SECTION - B
Question 5
(a) Draw the electrical equivalent circuit of an induction motor. Also draw the complete torque-speed curve of a 3-phase induction motor. [CO3, Marks: 03]
(b) Define slip. Prove that an induction motor can not run at synchronous speed. [CO3, Marks: 03]
(c) Prove that, the magnitude of resultant flux is constant and equal to 23Φm, due to any phase in an induction motor of 3−φ stator supply system with necessary figures. [CO2, Marks: 04]
Question 6
(a) Define synchronous watt. Derive an expression of rotor efficiency of a 3−φ induction motor. [CO2, Marks: 04]
(b) Draw the equivalent circuit of an induction motor as a generalized transformer. [CO1, Marks: 03]
(c) Determine the average torque of an induction motor if the rotor is assumed to be fully inductive. [CO3, Marks: 03]
Question 7
(a) Explain the Y−Δ starter to start 3−φ induction motor with the help of neat sketch. [CO3, Marks: 03]
(b) A 15 Hp, 3−φ, 6-pole, 50 Hz, 400 V, Δ−connected induction motor runs at 960 rpm on full load. If it takes 84.6 A on direct starting, find the ratio of starting torque to full load torque with a star-delta starter. Full load efficiency and power factor are 88% and 0.85, respectively. [CO3, Marks: 04]
(c) Explain the double field revolving theory for the operation of 1−φ induction motor. [CO1, Marks: 03]
Question 8
(a) Briefly explain crawling and cogging of an induction motor. Also, mention one method of speed control of an induction motor. [CO3, Marks: 03]
(b) With the help of schematic arrangements, describe how IM can be operated as IG? [CO3, Marks: 03]
(c) How is a single phase induction motor is made self-starting? Describe two methods of making a single phase induction motor self-starting. [CO3, Marks: 04]