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Bachelor of Engineering in Electronic Engineering – Stage 2
(EELXE_7_Y2)
Summer 2008
ELECTROTECHNOLOGY
(Time: 3 Hours)
Answer any five questions All seven questions carry equal marks.
Examiners: Dr. Joe Connell Ms. Mary Looney Mr. David Denieffe Dr. Paula O’Sullivan
Q1.
For the circuit shown below, use AC circuit analysis to calculate: a) The voltage drop across resistor R1 (10 marks) b) The current through resistor R2 (4 marks) c) The voltage drop across inductor L1 (4 marks) d) The current through capacitor C1 (2 marks)
Q2.
a) State both Thevenin’s and Norton’s theorems (2 marks) b) What is the relationship between the Thevenin open circuit voltage and the Norton short circuit current? (2 marks) c) For the circuit below, use Thevenin’s theorem to calculate the load current. (14 marks) d) What impedance should replace L1 for maximum power transfer? (2 marks)
Q3.
a) Draw a plot of the current versus frequency in a series resonant circuit. (3 marks) b) Draw a plot of input impedance versus frequency in a parallel resonant circuit. (3 marks) c) What is the significance of the “half power point” in resonant circuits? (2 marks) d) A series resonant circuit consists of a coil, having an inductance of 159μH and a winding resistance of 10Ω, in parallel with a variable capacitor. The resonant circuit is connected to a supply having a frequency of 1MHz. Calculate:
(i) The value of C to give resonance. (3 marks) (ii) The dynamic impedance and the Q-factor of the circuit (3 marks) (iii) The bandwidth of the circuit (3 marks) (iv) The upper and lower cut off frequencies. (3 marks)
Q7.
a) Why are electromagnetic poles preferred to permanent magnet poles in motors with higher power output? (3 marks) b) Describe an armature loss (briefly) and explain how this loss may be overcome? (4 marks) c) What is “commutation” in a dc generator? (2 marks) d) Why is a squirrel cage winding needed in addition to the normal rotor windings in a synchronous motor? (3 marks) e) A 4-pole, 415V, 50Hz three phase induction motor has a rotor star connected. Calculate (i) The synchronous speed. (2 marks) (ii) The speed of the rotor when the slip is 8%. (3 marks) (iii) The rotor frequency when the speed of the rotor is 1000rpm. (3 marks)
