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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering
– Stage 3
(NFQ – Level 8)
Autumn 2006
Electrical/Electronic Engineering
(Time: 3 hours)
Answer six questions – at least three from each section.
Use separate answer books for each Section.
Please note: all questions carry equal marks.
Internal Examiners: Dr R.A. Guinee Dr M.N. Barry Prof. M. Gilchrist Mr. J. Hegarty
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) Control Panel (4%)
(b) It is required to simulate the statistical temperature variation of a reading room in a library due to student occupancy. A uniform random number generator (RNG) is employed for this purpose to model the temperature excursion between a low temperature value of 15°C and a high of 25°C. The RNG supplied with LabView runs with a clock speed of 1ms. Design a LabView virtual instrument to accomplish this task with the following features: (i) The RNG is run continuously upon activation. (ii) A graphical display with a digital readout of temperature variation (iii) An input delay feature that retards the RNG output update to 2.5 minute intervals in order to simulate student average arrival to and from the reading room. (iv) a display of the running mean temperature value (8%) (c) What changes are necessary in the VI of part (b) to monitor the room temperature for a fixed period of 2.5 hours? Redraw the block diagram configuration to accommodate this fixed time period. (4.66%) [16.67 Marks]
Q2 (a) Explain the following terms that pertain to LabView VI design layout. (i) Charts and the different types with available features (ii) For Loop (iii) Case Structure (6%) (b) Design a VI that checks an input number to determine if it is positive. If the number is positive the VI determines the square root and returns an error if the number is negative. The VI should contain the following features: (i) A Case structure (ii) A comparison device (iii) A dialog function that outputs the error message “ERROR … Negative Number” (6.66%)
(c) Repeat exercise (b) using a Formula Node structure which outputs the value –9999 for the error condition and comment. (4%) [16.67 Marks]
Q3 (a) Explain the features of the following functions used in LabView VI design: (i) Formula Node (ii) Shift Register (5.66%) (b) Design a VI that computes and displays the running average o/p from a pressure sensor with measurements ranging from 5 to 20 psi with the following in-built features: (i) a random number generator (RNG) is used to model the pressure variations at 2 sec 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. (11%) [16.67 Marks] Q4 (a) Discuss the following measurement characteristics that pertain to sensor accuracy: (i) Sensitivity (ii) Linearity (iii) Accuracy Error (iv) Stability (4%) (b) Discuss the principle of operation of the following measurement sensors that are in sampled data acquisition systems (i) The Bonded resistance strain gauge (ii) Silicon Photodiode (8%) (c) Discuss the significance of the sampling rate in analog-to-digital conversion. A signal has the following frequency content
V m ( t )= 2 sin ω 0 t + 1. 5 sin 3 ω 0 t + 3 sin 6 ω 0 t
where f 0 is 150Hz. Determine the Nyquist sampling rate. If the signal is sampled at
10 ω 0 explains what happens using a spectral diagram and comment.
[16.67 Marks]
Q6. A 3 phase star connected 127V, 50Hz, 6 pole/phase induction motor has the equivalent circuit per phase as shown in Fig. Q6. When the slip is 2%, the stator current works out to be 18.8A, the rotor equivalent current to 12.2A and the power factor is 0.84.
Figure Q
There is a windage loss of 403W. Determine; (a) Rotor speed [5 marks] (b) The mechanical output power [5 marks] (c) The motor torque [5 marks] (d) The motor efficiency. [5 marks] (Use any formulae attached if you wish)
Q7 Consider the structure shown in the fig. Q7 below.
Widthand height d
Width d and Height d/
g g Width d and Height d/2x
N turns
I
Fig. Q
(a) Carefully show the flux paths. (5 marks) (b) Show that the inductance of the structure is given as (^) L xdNg μ 0 2 = from first principles. (5 marks) (c) From the result in (1) show that the force on the central member is given as
g F dNI 2
μ 0 22 =. (5 marks) (d) If the maximum flux density the metal has is 1.2T, work out the force on the central member when the air gap is 0.5mm and the width/height is 10mm. (5 marks)
V 1
= Ω R^ 2 / s =^0.^144 / s X ϕ 13. 25
- 294 Ω^0.^503 Ω^0.^209 Ω
Q8. Show that the voltage developed in a conductor, of length l , moving across a
magnetic field of strength B and at a velocity v , is given as
e = Blv [8 marks]
(You can start with either the Lorenz’s force law, F = Q ( E + v × B )or Faraday’s law)
A jumbo jet having a wing span of 60m is flying at 800km/hr at a height where the
vertical component of the earth’s magnetic field is 40μT. What is the voltage induced from end to end of its wings? [8 marks]
Does the answer depend upon whether the wings are of a ferromagnetic or non-
forromagnetic material? Explain. [4 marks]
Useful formulae
s s s r ω = ω^ −ω s
p ω (^) e = 2 ω F = Q ( E + v × B ), F = BlI
e=Blv, e av = K Φ ω
ω
ω
T J ω^ B
T K i
e Ri L K
dt
d
ma
dt b
di a a
a
P = T ω
s
2 R g 2 P = qI^2
s
2 R ω 2
1 2 s
T = qI
N^ s
N Z (^) p (^) s Z =( p^ )^2
μ 0 = 4 π× 10 −^7 A / m
1hp=746W
