PrevYearQuestions/2019.md
5.2 KB · commit d2f3b6e
← 2018 Exam Paper | 🏠 Index | 2020 Exam Paper →
Rajshahi University of Engineering & Technology
Department of Electrical & Computer Engineering
B.Sc. Engineering 2nd Year Odd Semester Examination, 2019
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) Define transformer. How is the energy transferred from primary to secondary winding of a transformer? Distinguish between primary and secondary windings. [04]
(b) Derive an expression for the EMF induced in a transformer winding. Also show that EMF per turn is the same in both the primary and secondary windings. [04]
(c) Why current flows in the primary side of transformer when secondary side is open circuited? Explain how primary current increases as the current on the secondary side of the transformer is increased. [04]
Question 2
(a) Discuss about O.C and S.C tests of a single phase transformer in brief. [03]
(b) Define voltage regulation. Find the voltage regulation for: [04]
- (i) unity pf,
- (ii) lagging pf and
- (iii) leading pf.
(c) A 10kVA 2200/220V, 60Hz transformer is tested on open and short circuit. If the following readings are obtained, find half load and full load efficiencies for unity pf and 0.8 pf lagging. [05]
| Test | Voltage (V) | Current (A) | Power (W) |
|---|---|---|---|
| O.C. (High side open) | 220 | 1.5 | 153 |
| S.C. (low side shorted) | 115 | Rated | 224 |
Question 3
(a) What is transformer breathing? Show that maximum efficiency of a transformer occurs when core loss equals to copper loss. [04]
(b) "A considerable economy is achieved in core material if the middle phase winding of a 3-phase shell type transformer is wound in the reverse direction as compared with the outer two phase winding." Justify the statement. [04]
(c) Is it possible to operate a three phase transformer Yd11 of vector group four, in parallel with another transformer Dy1 of vector group number 3? If your answer is affirmative then explain how? [04]
Question 4
(a) Define single phasing of a three phase transformer. Show that the secondary line voltages remain balanced if one of the phase of a delta-delta transformer get damaged and supply voltages are balanced. [04]
(b) Is it possible to convert a three phase system into two phase or vice-versa? If your answer is affirmative then explain the procedure with proper justification. [04]
(c) A 100kVA distribution transformer has a full load loss of 6kW, the losses being equally divided between iron and copper. The transformer operates on full load for 3 hours, one half load for 4 hours during a day. Calculate the commercial efficiency of the transformer. [04]
SECTION - B
Question 5
(a) What is meant by electrical machine? Describe the principle of operation of a 3-Φ induction motor. [04]
(b) Why an induction motor is called a rotating transformer? Also list the advantages and disadvantages of induction motor. [04]
(c) A 3-Φ IM is wounded for 4 poles and is supplied from 50Hz system. Calculate: [04]
- (i) the synchronous speed,
- (ii) the rotor speed when slip is 4% and
- (iii) rotor frequency when rotor runs at 600 rpm.
Question 6
(a) A 415V, 29.84kW, 50Hz delta connected motor gave the following test data: [08]
| Test | Voltage (V) | Current (A) | Power (W) |
|---|---|---|---|
| No Load Test | 415 | 21 | 1250 |
| Locked Rotor Test | 100 | 45 | 2730 |
Construct the circle diagram and determine the line current and power factor for rated output and the maximum torque. Assume stator and rotor Cu losses are equal at stand still.
(b) Prove that Xc=Xa+Zm+Xmrarm for a capacitor split phase motor. [Symbols have their usual meaning]. [04]
Question 7
(a) Define plugging of induction motor. Describe blocked rotor test and no load test of induction motor in brief. [04]
(b) What are the effects of direct line starting of induction motor of rating more than 25kW? How will it be minimized? [04]
(c) Define braking of induction motor. How can the speed of induction motor be controlled? [04]
Question 8
(a) What is pull out torque? Derive the equation for maximum starting torque of a 3-Φ IM. [05]
(b) Explain why a 1-Φ IM is not self starting with the help of double field revolving theory. [03]
(c) A 3-Φ, 50Hz, 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: [04]
- (i) the ratio of the maximum to full load torque and
- (ii) the speed at which the maximum torque occurs.