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Cork Institute of Technology
Higher Certificate in Engineering in Electronic Engineering – Award
(National Certificate in Engineering in Electronic Engineering – Award)
(NFQ – Level 6)
Summer 2005
Electrotechnology
(Time: 3 Hours)
Instructions Answer any FIVE questions.
All questions carry equal marks.
Examiners: Dr. J. Connell Dr. R. O Dubhghaill Mr. J. Berry
Q1.
For the circuit below, use AC circuit analysis to find:
(a) The voltage drop across R1 (11 marks) (b) The current through L2 (5 marks) (c) The voltage drop across C1 (4 marks)
R1 L1^ C
L
R
15 Ω 0.75mH 10 μF
500 μH 22 Ω
V AC
5V
2kHz
For the circuit below:
(a) Determine the Norton equivalent circuit (15 marks) (b) Use (a) to determine the Thevenin equivalent circuit (3 marks) (c) Use either equivalent circuit to find the current in RLOAD (2 marks)
Q3.
(a) Draw a plot of the current versus frequency in a parallel resonant circuit. (3 marks) (b) Draw a plot of input impedance versus frequency in a series resonant circuit. (3 marks) (c) What is the significance of the Q-factor in resonant circuits? (2 marks) (d) A series resonant circuit consists of a coil having an inductance of 500μH and a winding resistance of 10Ω in parallel with a variable capacitor. The resonant circuit is connected to a supply having a frequency of 1.5MHz. 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)
R1 C
C
R
RLOAD
27 Ω (^) 33nF
10nF
5mH
V AC
20V
15kHz
L
(a) What is “ripple” at the output of a DC generator and what steps can be taken in the construction of the generator to reduce ripple? (4 marks)
(b) What is “counter emf”? (4 marks)
(c) Why is a squirrel cage winding needed in addition to the normal rotor windings in a synchronous motor? (2 marks)
(d) What is “slip” in an induction motor? (2 marks)
(e) A three-phase induction motor is wound for four poles and is supplied from a 50Hz system with a line voltage of 400V. If the rotor is star connected, calculate (i) The synchronous speed. (^) (2 marks)
(ii) The speed of the rotor when the slip is 6%. (3 marks) (iii) The rotor frequency when the speed of the rotor is 1200rpm. (3 marks)
