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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering - Stage 3
(NFQ - Level 8)
Summer 2006
Electrical/Electronic Engineering
(Time: 3 Hours)
Instructions Answer FIVE questions with at least TWO questions from each section. Use separate answer books for each section.
Examiners: Dr. R.A. Guinee Dr. N.M. Barry Mr. J.E. Hegarty Prof. M. Gilchrist
Section A
Q1.(a) Describe how LabView works and discuss the function of the following items in such a programming environment. (i) Block Diagram (ii) Front Panel (4 Marks)
(b) It is required to simulate the statistical temperature variation of an office complex due to solar gain. A uniform random number generator (RNG) is employed for this purpose to model the temperature excursion between a low temperature value of 18°C and a high of 30 °C. The RNG supplied with LabView runs with a clock speed of 1ms. Implement a LabView virtual instrument, inclusive of block diagram and front panel, to accomplish this task with the following design features: (i) The RNG is run continuously upon start up. (ii) A graphical display with a digital readout of temperature variation in degrees Centigrade. (iii) An input delay feature that retards the RNG output update to 5 minute intervals to monitor sunlight variation (iv) a display of the running average ambient temperature. ( Marks)
(c) What changes are necessary in the VI of part (b) to monitor the room temperature for fixed period of 3 hours? Redraw the block diagram configuration for this loop count. (5 Marks)
Q2 (a) Discuss the operational features of the following functions used in LabView VI design with the aid of sketches: (i) Case Structure (ii) Shift Register (5 Marks)
(b) Design a virtual instrument (VI) to monitor the average hydrostatic pressure at a location in an estuary with water depth varying between 10 to 12.5 m, due to the effects of wind action on surface wave motion, with the following features: (i) a random number generator (RNG) is used to model the pressure variations at 1. minute intervals. Note the supplied LabView RNG operates with an internal clock speed of 1ms. (ii) 4 successive RNG outputs are used in the averaging process with an initialization value of 11.25.
Note: that hydrostatic pressure P is given by P = g ρ h where g = 9.81m/s^2 is the acceleration due to gravity, ρ is the density of water at 1kg//litre and h is the water depth. (10 Marks)
(c) Modify the VI block diagram in part (b) to output the maximum pressure level over a 75 minute period. (5 Marks)
Q3 (a) Discuss the following measurement characteristics that pertain to sensor accuracy: (i) response time (ii) linearity error (iii) stability (iv) precision (5 Marks)
t 1 t 2
b 1 b 2
Strain Gauges
F =mg
Fig. Q3: Force Transducer
(c) Fig. Q4 illustrates the block diagram of a LabView data acquisition system for data capture and display along with a spectral analysis module for determination of the essential harmonic content of the input signal.
Fig. Q
Discuss briefly the operation of the sub VIs used. If this VI is used in a dynamical system where the dominant resonant mode is 10Hz, with amplitude excursion –3volts ≤ Vm ≤ +3volts, and is accompanied with additional harmonic disturbances at 15Hz and 35Hz of lower amplitude values discuss the settings that should be employed in the analog_to_digital converter section and the spectrum express VI for an accurate gauge of the input data characteristics. (8 Marks)
Section B
Q5. Consider the magnetic structure shown below in Fig. Q (5) g
g
height, h r
I
N
turns Fig. Q (a) Show that the flux which flows in the structure is given as g
NI rh 20 Φ= μ^ θ where θ is the overlap angle. [8 marks]
(b) Show that the torque on the centre piece, acting about its centre is given as T = B μ^2 o rhg [8 marks]
(c) For an air gap of 2.54mm what is the maximum possible torque on the rotor, if the saturation flux density of the material is 2.02T? Assume the radius and the height are 25.4mm respectively. [4 marks]
Q6. (a) Show how a three phase voltage source is produced with a common neutral in the stator. [5 marks] (b) Write down sinusoidal expressions for the instantaneous voltages of the three supplies produced, including the phase angles. [5 marks] (c) From your results in part b (or otherwise) show that the instantaneous line-to-line voltage is 3 greater than that of the line-to-neutral voltage, at the same frequency and leading it by 30 degrees. [7 marks] (d) What is the peak value of the line-to-line voltage available between two sockets on different benches in our laboratories at College? [3 marks]
Useful Formula s s ω sr = ω −ω ω (^) e = p 2 ω s F = Q ( E + v × B ),^ F = BlI
e=Blv, e av = K Φ ω
T J ω^ B
T K i
e Ri L K
ddt
ma
a adidta b
P = T ω
s
2 R Pg = qI 2 2
s
22 R T = ω^1 s qI^2
N^ s
N Z (^) p (^) s Z =( p^ )^2 μ 0 = 4 π× 10 −^7 A / m 1hp=746W
