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SCHOOL ENGINEERING MODULAR HONOURS DEGREE SEMESTER 2 LEVEL 1 2001/2002
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ELECTRICAL ENERGY AND MACHINES
Examiner: Dr P.A. Howson Attempt FOUR questions. Time allowed : 2 hours Total number of questions = 6 All questions carry equal marks. The figures in brackets indicate the relative weightings of parts of a question. Special requirement Graph Paper (Linear)
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1) a) State five major generation sources of electrical energy in the UK. b) What are the advantages of nuclear generated electrical energy compared to coal fired power stations? (5) c) Why is electrical power usually generated and transmitted as a.c rather than d.c? (5) d) The undersea power link between England and France is operated d.c rather than a.c. State two main reasons why d.c is the preferred transmission method for this particular power link. (5) (5)
2) a) Describe with the aid of sketches the construction of a Core type perfect transformer. (3) b) The primary winding of a step down transformer is rated at 240 V r.m.s. If there are 2000 turns on the primary winding and 100 turns on the secondary, estimated the secondary voltage. (3) c) For the transformer in 2b), if the VA rating of the transformer is 50, calculate the maximum primary and secondary currents. (5) d) Sketch the FULL equivalent circuit of a transformer indicating what parameter each component represents. (9)
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3) a) The following results were obtained from open circuit and short circuit tests performed on a transformer;
Open circuit test Short circuit test
Vp=240 V, Power=60 W, Ip=4 A Vp=20 V, Power=60 W, Ip=62 A
Sketch the simplified approximate equivalent circuit of the transformer showing appropriate component values and all numerical workings. Calculate (10)
b) the maximum efficiently of the transformer (5) c) Calculate the regulation at maximum current. (5)
4) a) Explain with the aid of sketches the operation of a simple ac generator showing what you would expect from the output at four points in a single angular cycle. (7)
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A simple two pole a.c generator armature consists of a loop 10 cm wide and having active sides each 40 cm long, rotates at 2500 rev/min in a uniform field of density of 0.7 T. b) Calculate the maximum value of induced e.m.f. (5) c) The frequency and period. (3) d) The frequency and period if the number of poles is increased to six. (5)
5) The following figures were obtained for the open circuit characteristics of a d.c shunt generator when running at 500 rev/min.
Field Current (A) Armature E.M.F (V)
1 45
2 93
3 140
4 180
5 195
6 205
7 210
a) Determine the voltage to which the machine will excite when run at 300 rev/min the field resistance being 25 ohm. (10) b) When running at 300 r.p.m determine the load current supplied by the generator when the terminal voltage is 100 V. the armature resistance is 0.4 ohm. (10)
6) a) Suggest one application for a series d.c motor. (3) b) Sketch typical speed v Current, and Torque v Current characteristics of a series
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d.c motor. (5) c) A series wound d.c motor produces a torque of 340 Nm at a armature current of 100 A, from a 200 V source. (6) i) If the load on the machine is reduced such that the torque reduces to 170 Nm estimate the new value of armature current. (6) ii) Calculate the efficiency of the motor at the reduced value of torque if the resistance of the armature is 0.2 Ohm and the field resistance is 0.3 Ohm. Ignore bearing and windage losses. (6)