12:15 AM PG 4G 'WA (45) 157 B/S UNIVERSITY OF ABUJA, ABUJA FACULTY OF ENGINEERING DEPARTMENT OF ELECTRICAL/ELECTRONIC ENGINEERING 300 LEVEL SECOND SEMESTER EXAMINATIONS, EEE316: ELECTROMECHANICAL DEVICES & MACHINES I 23th December, 2022 INSTRUCTIONS: Answer FIVE (5) QUESTIONS, DURATION: 3 HOURS QUESTION ONE: 1a) 1b) What is electromechanical energy conversion. (3 Marks) Differentiate between a generator and motor and give two examples of household electromechanical devices. (5 Marks) 1c) A 230-V DC machine has an armature resistance of 2.00 and a full-load armature current is 20A. With the aid of appropriate sketches, find the induced EMF when the machine acts as (i) generator (ii) motor. QUESTION TWO: (6 Marks) 2a) 2b) List and explain the two drum type of armature windings mostly used in DC machines. Discuss the vital role of air-gap in an electromechanical system. (4 Marks) (5 Marks) 2c) A short shunt generator delivers 450A at 230V and the resistance of the shunt field and armature are 500 and 0.030 respectively. Calculated the generated emf. (5Marks) QUESTION THREE: 3a) зь) 3c) Enumerate the types of generators and discuss them briefly. (4 Marks) Name the two types of brushes attached to slip rings and therefore, differentiate between slip rings and split rings. (3 Marks) A long-shunt compound generator is rated: 300kW, 675V, Shunt field resistance 750, armature resistance including brush resistance 0.03, commutating field winding resistance 0.0110, series field resistance 0.012, divertor resistance 0.360. When the machine is delivering full load, calculate the voltage and power generated by the armature. QUESTION FOUR: 4a) 4b) 4c) Derive the equation of induced emf in a generator. Using the derived equation, state the constant parameters and explain. (7Marks) (4 marks) (4 marks) An 8-pole DC generator has 500 armature conductors and a useful flux of 0.05Wb per pole. What will be the EMF generated if it is lap-connected and runs at 1200 rpm? What mut be the speed at which it is to be driven produce the same EMF if it is wave-wound? (6 marks) QUESTION FIVE: 5a) 5b) 5c) State the principle of operation of a motor. Discuss the significance of back EMF in DC motors. (3 Marks) (4 Marks) A DC motor takes an armature current of 110A at 480V. The 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 1) The speed and. ii) the gross torque developed by the armature. (7 Marks) QUESTION SIX: 6a) What is torque in relation to DC motors. (3 Marks) бы) Compare a generator and a motor action. (4 Marks) 60) A 25kW, 250V, DC shunt generator has armature and field resistances of 0.060 and 1000 respectively. Determine the total armature power developed when working 1) As a generator delivering 25kW output and. ii) As a motor taking 25kW input. (7 Marks) QUESTION SEVEN: 7a) Discuss the two main designs for transformer cores. (4 Marks) 7b) Real transformer cores are covered with primary windings while the larger secondary winding surrounds the primary windings. Why? (4 Marks) 7b) An ideal transformer has 3000 and 6000 turns for the primary and secondary coils respectively which source voltage is 500V at 50Hz. A load of 120 draws current at a power factor of 0.8 lagging. Calculate the primary current, secondary current, secondary voltage and power output. (6 Marks) ||| = O Մ כ