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Question Bank
Course: EC Session: 2005-2006
Subject: EE 4301 Principles of Electrical Machines Semester: IV
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DC Machines
1. a) What are the two basic components in a rotating electrical machine? Name the various elements forming i) the magnetic field circuit. ii) the armature circuit.
2. Draw a neat cross sectional view of a 4-pole d.c. machine showing the essential components and prepare a list of the materials used for them.
3. Mention the prime functions of the following:
i) Main magnetic poles and pole shoes
ii) Armature core and armature winding
iii) Commutator
iv) Brushes and brush gears.
4. On what basis armature winding are classified as LAP and WAVE? In what respects the two types differ from each other. Illustrate with a heat sketch the two schemes of winding.
5. What rule is followed for marking the polarity and location of the br4ushes? Which type of winding (Lap/Wave) do you recommend for (i) High voltage. Low current d.c. machine. (ii) Low voltage, high current d.c. machine and why?
6. Define the following terms in context to armature winding. (i) Pole pitch. (ii) Back pitch.(iii) Front pitch. (iv) Progressive and Retrogressive windings. Illustrate each with suitable sketch/ diagram.
7. Mention the characteristic features of the various types of pitches.
8. Test for the feasible type of armature winding with the following arrangements:
i) 28 conductors, 4 poles; ii) 38 conductors, 4 poles;
iii) 26 conductors, 4 poles; iv) 24 conductors, 4 poles with 2 conductors per slot.
9. Why do you consider commutator as a mechanical rectifier? Explain with necessary schematic diagrams. How unidirectional voltage is obtained from the armature coil in a simple d.c. generator?
10. Give the winding table and draw the developed armature-winding diagram for the possible winding pattern (Lap/ wave) and possible pitches (vF & vB).
i) 46 conductors with 4 poles.
ii) 36 conductors with 4 poles.
iii) 26 conductors with 4 poles.
iv) 30 conductors with 4 poles.
Show the polarity and arrangement of the brushes in each case.
11. What is commutation? Explain the process of commutation with the help of neat diagrams.
12. What is reactance voltage? How does it affect the operation of a d.c. machine? Develop an expression for the reactance voltage.
13. Explain the difficulties of commutation in a d.c. generator. What methods are used to overcome these difficulties?
14. What is Armature reaction? Explain with the aid of neat diagram armature reaction in a d.c. machine.
15. Discuss the various effects of armature reaction on working of a d.c. machine.
16. Derive expressions for demagnetizing and cross magnetizing ampere-turns.
17. (a) What for interlopes and compensating winding are provided?
(b) How is the polarity of an interlope marked in case of a d.c. generator and a d.c. motor.
(c) Where are the interlope located and how is the interlope winding connected?
(d) How is the compensating winding connected?
18. Derive the expression for the e.m.f. induced in a d.c. machine. State the compensations made and the symbol is used.
19. A 6 pole 2 circuit wave connected armature has 250 conductor and 1200rom. The electromotive force generated on open circuit is 600V. Find the useful flux per pole. (0.04wb).
20. An 8 pole Lab connected armature has 960 conductors, a flux of 4 mega lines (40mwb) per pole, and a speed of 400 rpm .Calculate the e.m.f. generated on open circuit. (256V).
22. A 6 pole armature winding has 48- conductors. Average e.m.f generated in each conductor is 2.2V and each conductor can carry 100A at full load. Calculate (a) the voltage generated (b) output full load current (c) Total output power developed when the armature is (i) lap connected, (ii) wave connected. (d) 176V, 600A, 105.6KW. (ii) 528V, 200A, 105.6KW.
23. A shunt machine, connected to 250V mains, has an armature resistance (including brushes) of 0.12 ohm, and the resistance of the field circuit is 100 ohms. Find the ratio of the speed as a generator to the speed as a motor the line current in each being 80A. (1.08).
24. A 6 pole armature is wound with 498 conductors. The flux and the speed are such that the average e.m.f generated in each conductor is 2V. The current in each conductor is 120 A, Find the total current and the generated e.m.f of the armature if the winding is connected (a) wave (b) Lap. Also find the total power generated in each case.
{Ans.= 240A, 498V, 166V, 199.5KW}.
25. A 4 pole armature is wound with 564 conductors and driven at 800 rpm, the flux pole being 20mwb. The current in each conductor is 60A. Calculate the total current and the electrical power generated in the armature if the conductors are connected (a) wave, (b) Lap,
{(a) 120A, 36.096kw), (b) 240A, 36.096kw,}.
26. An eight pole Lap-connected armature has 96 slots with 6 conductors per slot and is driven at 500rpm. The useful flux per pole is 0.09wb. Calculate the generated e.m.f. {Ans: 432V}.
27. A 4 pole armature has 624 lap connected conductors and is driven at 1200 rpm. Calculate the useful flux per pole required to generate an emf of 2500V. [ans: 0.02wb].
28 A 6 pole armature has 410 wave connected conductors. The useful flux per pole is 0.025 wb. Find the speed at which the armature must be driven if the generated emf is to be 485V. [ans: 946rpm].
29. The wave connected armature of a 4 pole d.c. generator is required to generate an emf of 250V when driven at 660 rpm. Calculate the flux per pole required if the armature has 144 slote with 2 coil sides per slot, each coil consisting of 3 turns. [Ans: 0.0274wb].
30. A 6 pole dc generator runs at 850 rpm and each pole has a flux of 0.2 ´ 10-2 wb. If there are 150 conductors in series between each pair of brushes. What is the value of the generated emf ? [Ans: 153V].
31. A 4 pole wave wound d.c. machine running at 1500 rpm has a commutator of 30.5 cm diameter. If armature current is 150 amperes, thickness of brush 1.27 cm and self inductance of each armature coil is 0.07mH, Calculate the average emf induced in each coil during commutation. Assume linear communication. [19.8V App.]
32. A 300 KW, 500V, 8-pole d.c. generated has 768 armature conductors, Lap connected. Calculate the number of demagnetizing and cores amp-turn per pole when the brushes are given a lead of 5 electrical degrees from the geometrical neutral. [ Ans: 200 At 3400At].
33. A 4 pole motor has a wave connected armature with 888 conductors. The brushes are displaced backwards through 5(mech.) degree from the geometrical neutral. If the total armature current is 90A, Calculate: (a) The cross and the back ampere turns per pole; and (b) the additional field current to neutralize this demagnetization. If the field winding has 1200 turns per pole. [ Ans: 4440At. 555At, 0.4625A].
34. Calculate the number of turns/pole required for commutating poles of the d.c. generator referred to in Q.(32) assuming the compole ampere turns/pole to be about 1.33 times the armature ampere-turns / pole and the brushes to be in the geometrical neutral. [Ans: 8].
35. An eight pole generator has a lap connected armature with 640 conductors. The ratio of pole arc/pole pitch is 0.7. Calculate the ampere-turns/pole of a compensating winding to give uniform air gap density when the total armature current is 900A. [Ans: 3150At].
36. What are the different methods of excitation used in a d.c. machine? Illustrate them with suitable schematic diagrams.
37. What is meant by (I) magnetization characteristic, (ii) internal characteristics, and (iii) external characteristics of a d.c. generator? Mention their distinguishing features. Sketch the characteristics of series, shunt, compound, and separately excited d.c. generators.
38. What are conditions of building up of voltage in a d.c. shunt and series generator? What is understood by critical resistance and critical speed of a d.c. generator?.
39. Discuss the performance characteristics of d.c. series, shunt and compound generators and motors. Mention their important applications.
40. Enumerate the various losses and derive expression for maximum efficiency of a d.c. shunt machine.
41. Why starter is needed for a d.c. motor? Give a neat illustrative schematic diagram showing the essential parts of a three point and four point d.c. motor starter. Explain their working with particular attention to the function of no load and over load protection schemes.
42. Explain why the terminal voltage of a d.c. shunt generator falls as the current supplied by the machine is increased. The resistance of the field circuit of a d.c. shunt generator is 200 ohms when the output of the generator is 100kw, the terminal voltage is 500V and the generator emf is 525V. Calculate (a) the armature resistance and (b) the value of the generated emf when the output 60kw, if the terminal voltage is then 250V. [Ans: 0.123ohm, 534.5V]. {279.67}.
43. A short shunt compound generator has armature, shunt field, and series field resistance of 0.8 ohm, and 0.6 ohm respectively and supplies a load of 5 kw at 250V. Calculate the wmf generated in the armature {Ans: 282.66V]
44. The following table gives the open circuit voltage for different field currents of a shunt generator driven at a constant speed.
Terminal voltage(V) 120 240 334 400 444 470
Field Current ( C ) 0.5 1.0 1.5 2.0 2.5 3.0
Plot a graph showing the variation of generated emf with exciting current and from this graph determine the value of the generated emf when the shunt circuit has a resistance of (a) 160 ohms (b) 210 ohms © 300 ohms. Also find the value of the critical resistance of the shunt circuit. [Ans: 467V, 373V, 0.240ohms]
45. The following table relates to the open circuit curve of a shunt generator running at 750 rpm.
Generated emf (v) 10 172 300 360 385 395
Field Current (A) 0 1.0 2.0 3.0 4.0 5.0
Determine the no load terminal voltage if the field circuit resistance is 125 ohms. If the speed is now halved, what is the resultant terminal voltage? At the reduced speed, what value of field circuit resistance will give a no load terminal voltage of 175V? [Ans: 345V, 5V, 64.5ohms].
46. The open circuit characteristics of a shunt generator when separately excited and running at 1000rpm is given by:
Open circuit voltage(V) 56 112 150 180 200 216 230
Field current (A) 0.5 1.0 1.5 2.0 2.5 3.0 3.5
If the generator is shunt connected and runs at 1100 rpm with a total field resistance of 80 ohms, determine : (a) the no load emf (b) the output current when the terminal voltage is 200V if the armature resistance is 0.1 ohm © the terminal voltage of the generator when giving the maximum output current. Neglect the effect of armature reaction and of brush contact drop. [Ans: 236V 100A……………]
47. A shunt machine has armature and field resistance of 0.04 ohm and 100 ohm respectively. When connected to a 460V d.c. supply and driven as a generated at 600rpm, it delivers 50 kw from the same supply. Show that the direction of rotation of the machine as a generator and as a motor under these conditions remains unchanged. [Ans: 589 rpm].
48. A 100 kw , 500V, 750rpm, d.c. shunt generator, connected to constant voltage bus, has field and armature resistances of 100 ohms and the machine continues to run taking 50 A from the bus bars, Calculate its speed. Neglect brush drop and armature reaction effects. [Ans: 714 rpm].
49. A 4-pole d.c. motor is connected to a 500V d.c. supply and takes an armature current of 80A. The resistance of the armature circuit is 0.4 ohm. The armature is wave wound with 522 conductors and the useful flux per pole is 0.025 wB. Calculate (a) the back emf of the motor (b) the speed of the motor © The torque in Nw-m developed by he motor. [ans: 468V, 1075rpm, 333Nw-m].
50. A d.c. shunt generator delivers 5 kw at 250 V when driven at 1500 rpm. The shunt circuit resistance is 250 ohms and the armature circuit resistance is 0.4 ohm. The iron, friction and windage losses are 250W. Determine the torque in Nw-m required to drive the machine at the above load. [Ans: 36.2Nw-m]
51. A shunt wound motor has a field resistance of 350 ohms and an armature resistance of 0.2 ohm and runs of a 250V supply. The armature current is 55A and the motor speed is 1000 rpm. Assuming a straight line magnetization curve, calculate (a) an additional resistance required in the field circuit to increase the speed of 1100 rpm for the same armature current, and (b) the speed with the original field current and an armature current of 100A. [Ans; 35 ohms, 962 rpm].
52. Why a d.c. series motor should never be started at no load? A d.c. series motor, connected to a 440V supply, runs at 600 rpm when taking current 0f 50A. Calculate the value of a resistor which, when inserted in series with the motor, will reduce the speed to 400 rpm, the gross torque being then half its previous value. Resistance of motor = 0.2 ohm. Assume the flux to be proportional to the field current [Ans: 6.53 ohms].
53. A series motor runs at 900 rpm when taking 30 A at 230V. The total resistance of the armature and field circuits is 0.8 ohm. Calculate the values of the additional resistance required in series with the machine to reduce the speed to 500 rpm if the gross torque is (a) constant; (b) proportional to speed; © proportional to the square of the speed. Assume the magnetic circuit to be unsaturated. [Ans: 3.05ohms, 5.67ohms, 9.2 ohms].