UNIVERSITY OF ABUJA, FACULTY OF ENGINEERING DEPARTMENT OF ELECTRICAL/ELECTRONIC ENGINEERING 300 LEVEL SECOND SEMESTER EXAMINATIONS, EEE316: ELECTROMECHANICAL DEVICES & MACHINES I 23rd April, 2024. INSTRUCTIONS: Answer FIVE (5) QUESTIONS, QUESTION ONE: DURATION: 2:30 HOURS 1a) What is electromechanical energy conversion. (3 Marks) 1b) With the help of sketches, circuits diagrams, equations and examples, compare generator and motor action. (11 Marks) QUESTION TWO: 2a) List any six essential parts of a practical generator. (4 Marks) 2b) A direct current motor rated 240A, 480V has armature circuit resistance is 0.20. The machine has 6 poles and the armature is lap connected with 864 conductors. The flux per pole is 0.05Wb. Calculate i) The speed and. ii) the gross torque developed by the armature. (10 Marks) QUESTION THREE: 3a) Why is the air-gap very important in electromechanical systems. (6 Marks) 3b) An 8-pole DC generator has 500 armature conductors and a useful flux of 0.1Wb per pole. What will be the EMF generated if it is lap-connected and runs at 2400 rpm? What must be the speed at which it is to be driven produce the same EMF if it is wave-wound? (8 marks) QUESTION FOUR: 4a) Compare and contrast electric circuits and magnetic circuits. (5 Marks) 4b) A 220 V D.C machine has an armature resistance of 0.5 Q. If the full-load armature current is 20 A, find the induced E.M.F. when the machine acts as: i) ii) Generator Motor (9 Marks) QUESTION FIVE: 5a) Derive the equation of induced emf in a generator. (6.5 marks) 5b) A long-shunt compound generator rated 300 kW, 800 V has a shunt resistance of 75 Q, armature resistance 0.03 2, winding resistance 0.011 0, series resistance 0.012 02, divertor resistance 0.036 Q. Calculate the voltage and power generated by the armature when the machine delivers power at full load. (7.5 marks) QUESTION SIX: 6a) What is the advantage of wave windings over lap windings and vice versa? (4 Marks) 6b) A 30kW, 250V, DC shunt generator has armature and field resistances of 0.060 and 1000 respectively. Determine the total armature power developed when working i) As a generator delivering 30kW output and. ii) As a motor taking 30kW input. (10 Marks) QUESTION SEVEN: Real transformer cores are covered with primary windings while the larger secondary winding surrounds the primary windings. Why? (4.5 Marks) A fire alarm system is powered by a small transformer that steps down 240 V ac to 12 V ac. If such a transformer has a primary winding with 2000 turns and the current in the bell is 500 mA, 7a) 7b) i) How many turns are there on the secondary winding? ii) What is the current in the primary windings? iii) What is the power output iv) Sketch the transformer circuit (9.5 Marks)