advertisement

THE MOMBASA POLYTECHNIC UNIVERSITY COLLEGE Faculty of Engineering and Technology DEPARTMENT OF ELECTRICAL & ELECTRONIC ENGINEERING DIPLOMA IN TECHNOLOGY Electrical Power Engineering EEC 2205 MACHINES UTILIZATION I SEMESTER EXAMINATION SERIES: FEBRUARY 2011 SERIES TIME: 2 HOURS Instructions to Candidates: 1. You are required to have the following for this examination; Answer booklet A non-programmable calculator 2. Answer Question ONE (COMPULSORY) and any other TWO Questions. © 2011 – The Mombasa Polytechnic University College Page 1 (COMPULSORY) Question ONE a) i) ii) iii) b) i) ii) c) i) ii) iii) Explain any THREE significance of back emf in d.c motors. (3 marks) A 440V shunt motor has an armature resistance of 0.8Ω and a field resistance of 200Ω. Determine the back emf when the motor is giving an output of 7.46 kw at 85% efficiency. (4 marks) State any THREE applications of d.c. shunt motors. (3 marks) State any TWO application of the following single phase machine. I) Hysteresis motors II) Universal motor (4 marks) With the aid of a circuit diagram, explain the operation of a capacitor start – capacitor run single phase induction motor. (6 marks) State any TWO types of rotors used in induction motor. (2 marks) A 3-phase induction motor is wound for a pole and is supplied from 50Hz system. Calculate: I) The syndronous speed II) The rotor speed when the slip is 4% III) Rotor frequency when the rotor is at 600vpm. (3 marks) Using a well labelled diagram explain the autotransformer method of starting induction motors. (5 marks) (ANSWER ANY OTHER TWO QUESTIONS) Question TWO a) i) ii) State any THREE tests carried in d.c. machines. (3 marks) Show that the armature torgue developed in a series d.c motor is given by: Ta α Ta2 (7 marks) b) i) State any TWO applications of the following d.c. motor I) Compound motors II) Series Motors (4 marks) A 25kw 250v d.c. slunt generator has an armature field resistance of 0.06Ω and 100Ω respectively. Determine the total armature power developed when working as: I) Generator delivering 25kw II) Motor taking 25kw (6 marks) ii) © 2011 – The Mombasa Polytechnic University College Page 2 Question THREE a) i) ii) iii) Define the term slip as used in induction. (2 marks) Give TWO significance of having skewed slots on squirrel-cage rotor. (2 marks) The input to a 3 phase 4-pole – 50Hz induction motor is 150kw, stator losses are 5kw mechanical losses are 3kw and full load slip 0.05. Find I) Frequency of the rotor emf at standstill II) Frequency of the rotor emf at full load III) Rotor copper loss IV) Efficiency of the motor (6 marks) b) A 415V three-phase 50Hz four pole star connected induction motor operates at 1425rev/min on full load. The rotor resistance and reactance per phase are 0.4Ω and 4Ω respectively and the effective rotor-stator terms ratio is 0.8:1. Calculate: i) The full load torque ii) The power output if the mechanical losses amount to 480W. iii) The maximum torgue iv) The speed at which maxim torque occurs v) The torque at start. (10marks) Question FOUR a) i) ii) State any TWO types of repulsian-type single phase motors. (6 marks) With the aid of circuit diagrams differentiate between a resistor-start and an inductor-start single phase motors. (6 marks) b) i) With the aid of a well labelled diagram, explain the Hopkinson’s test for d.c. motors. (4 marks) A 250V d.c. shunt machine has an armature resistance including inter-pole of 0.5 and shunt field resistance of 125Ω both values at working temperatures. When it is running light as a motor, the current taken from the supply is 5A. Calculate the efficiency of the machine. I) When taking a current of 52A from the supply as a motor II) When delivering a current of 35A as a generator. (8 marks) ii) © 2011 – The Mombasa Polytechnic University College Page 3 Question FIVE a) Draw a well labelled exact equivalent circuit diagram of a 3 phase induction motor.(2marks) b) For a 3-phase induction motor; show that the maximum torque is reached when R S 2 X2 c) (8 marks) A 440V, 3 ,50Hz 8 pole star connected induction motor has the following equivalent circuit parameters per phase. R1 = R12 = 0.1Ω, X1 = X12 = 0.7Ω Rm = 100Ω Xm = 25Ω. Calculate the rotor current referred to the stator current. The input power factor, the torque and the efficiency of the motor at 40% slip and the starting torque. (10marks) © 2011 – The Mombasa Polytechnic University College Page 4